Properties and applications of metastable precious

Transcription

Properties and applications
of metastable precious metal
intermetallic compounds
Supitcha Supansomboon
B.Sc. (Materials Science) and M.Eng (Materials Technology)
A thesis submitted in fulfilment of the requirements for
the degree of Doctor of Philosophy in Science
Institute for Nanoscale Technology
School of Physics and Advanced Materials
University of Technology, Sydney
2014
Certificate of original authorship
I certify that the work in this thesis has not previously been submitted for a degree nor has it
been submitted as part of requirements for a degree except as fully acknowledged within the
text.
I also certify that the thesis has been written by me. Any help that I have received in my
research work and the preparation of the thesis itself has been acknowledged. In addition, I
certify that all information sources and literature used are indicated in the thesis.
Supitcha Supansomboon
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Acknowledgements
First and foremost, I would like to express my sincere gratitude to my principle supervisor,
Professor Michael Cortie. My precious journey began when he gave me the opportunity in
November 2007 to explore precious metals during a visit to UTS. I have not been able to find
the best words to show my sincere gratitude and to thank him for his support, encouragement,
motivation, enthusiasm, patience and immense knowledge that he offered me along this
journey, but I am truly grateful for his ongoing confidence in my work and abilities. I also
could not have asked for better role models. I would like to say thank you to my cosupervisor, Dr Annette Dowd, who has advised, inspired me but also has been patient in
assisting me on TEM. I would like to thank all my internal assessors, Assoc. Prof. Kendal
McGuffie, Dr Gregory Heness and Dr Matthew Arnold, for their comments and kind
suggestions. I would like to thank Dr Vicki Keast from the University of Newcastle in
regards to the brilliant discussions, suggestions and the knowledge regarding coloured
intermetallic compounds and Professor Candace Lang from Macquarie University for sharing
her knowledge and expertise regarding platinum alloys and compounds. They have greatly
fulfilled my precious journey.
Along this precious journey, I could not have achieved this without the assistance of a special
community at the University of Technology Sydney (UTS) and numerous external
individuals who offered me priceless contributions. I would like to thank each and everyone
who assisted over the past few years. Dr Richard Wuhrer not only taught and showed me how
to use SEM efficiently, but also encouraged me with his kindness. Geoff McCredie spent
many hours patiently teaching and supporting me to produce precious samples. He also
modified instruments whenever required. Dr Angus Gentle shared his knowledge and support
on magnetron sputtering, optical property measurements and the fitting model and I thank
him for his understanding and patience. Mark Berkahn advised me on XRD and its analysis.
Katie McBean assisted in the operation of SEM and facilitated the use of instruments at
MAU. Dr Norman Booth assisted in the metallographic specimen preparation and also with
the optical microscope. Dr Ronald Shimmon, Jean-Pierre Guerbois and Greg Dalsanto
assisted in the set up of furnaces. Adam Sikorski taught and gave me such a wonderful
technique for TEM preparations. Shaun Bulcock and Dr Hongwei Liu taught and assisted me
to operate the TEM effectively. Dr Qinfen Gu and Dr Justin Kimpton, who are powder
diffraction beamline scientists, assisted in the set- up and operation at the Australian
Synchrotron.
I would like to extend my sincere thanks to the other staff within the; School of Physics and
Advanced Materials (PAM) at UTS, Microstructural Analysis Unit (MAU) at UTS,
Australian Centre for Microscopy and Microanalysis (ACMM) at the University of Sydney,
and the Australian Synchrotron for facilitating the use of the instruments and all of the
support throughout the years. I also extend my thanks to Mintek, South Africa for donating
bulk samples of coloured precious metals.
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Most importantly, my journey could not have happened without the financial support given
by a Royal Thai government scholarship, offered by the Ministry of Science and Technology.
It has been an excellent opportunity to have the opportunity to gain more knowledge and
experiences abroad in Australia.
Since I have been away from my home country, I have never felt that I have had to walk this
journey alone. There were dozens of people who have helped and encouraged me along the
way. All INT people present and past, Dr Dakrong Pissuwan, Dr Abbas Maaroof, Dr Martin
Blader, Dr Burak Cankurtaran, Dr Jonathan Edgar, Dr Dylan Riessen, Dr Jonathan Mak, Dr
Alex Porkovitch, Dr Vijay Bhatia, Dr Tim Lucey, Valerio Taraschi, Jose Aguilar, Nikta
Shahcheraghi, Daniel Golestan, Fadi Bonnie, David McPherson, Shirin-Rose King, Daniel
King, Carsten Steinel, Aaron Colusso, Angelo Garruzzo, Dr Sujeewa De Silva and Dr Shaoli
Zhu. Thank you for all the discussion, support and friendship you offered me. I would like to
thank all fellow colleagues, Dr Barry Liu, Innocent Macha and Elisabeth Meijer, whom I had
the opportunity of working with as demonstrators in chemistry and in the materials
laboratory. It has been such an enriching experience for my future career. I would like to say
special thanks to Professor Tony Moon who has supported and encouraged my friends and I
through countless and varied issues since the very beginning. I would like to thank the
Minister Counsellor for Education at the Royal Thai Embassy, Canberra; Ms. Thanida
Techachokvivat, Ms.Kaewta Srisung and Mrs.Kamonwan Sattayayut, as well as the staff
from Office of Educational Affairs for all the assistance and facilitation regarding the
financial support as well as all documentation processes required in Thailand.
Even though I lived away from my immediate family in Thailand, I had such a huge and
magnificent family in Sydney. Settling down in Sydney for the first time would not have been
as smooth as it was without the assistance from my friends. I would like to say a special
thank you to Chanthakorn Ketwong and Sopita Thientospol. I also thank you for your
constant assistance and friendship all along the journey. My accommodation in Sydney made
me feel like I was at home. Thank you for being lovely housemates and being helpful in
everything;Arin Tjintana, Kevin Tan, Albert Saputra and Karina Saputra.
Friends who I have known from Insearch since my first year, Daiki Hagino, Jaeseok Ahn,
Siriwat Sakarin and Thanh Hung Nguyen, have also spent time and showed support to each
other along our respective journeys. We are like a part of each other’s families. I would like
to say thank you to Assist.Prof. Watcharin Jinwuth, Dr Pholchai Chotiprayanakul, Sakkaphan
Ritjan, Chayapol Moemeng, Woraporn Kanjanawong, Duangkaew Theerasin, Jakkrit
Sintunava, Chotika Jindaapirat, Ruamporn Jitjurjun, Dr Akitomo Kawasaki, Pornwan
Pornprasitpol, Piti Roglertjanya, Dr Suwin Sandu, Pakawat Pupatwibul, Suphinya Panyasi,
Chalakorn Karupongsiri, Busayasachee Puang-Ngern, Suranan Anantachaisilp, Dr Chanick
Wangphanich, Shakuntala Anuruang, Tipajin Thaipisutikul, Sanya Khruahong, Sumavalee
Chindapol, Nantira Pookhao, Songsin Teerakunpisut, Arpar Nateprapai, Pareena Lertsurawat,
Dee Le and Dessie Wanda for your interesting, useful and discussions, as well as for your
encouragement and friendship. I would like to thank to Aunty Ying-Utumporn Jaturawong
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and Uncle Odd-Pornchai Dechsri for your kindness and the delicious food for you provide
my friends and I. Last but not least, I would like to thank my close supporters in Sydney. I
would like to especially thank you for your support, your encouragement, for caring for each
other’s mental wellbeing and for the friendship through my journey to Dr Khanokon
Amprayn, Dr Sutinee Sinutok, Ponlachart Chotikarn, Dr Sirinut Sawatdeenarunat, Supannada
Chotipant, and Angelica Casado. I also would like to thank all the people I have met and who
I shared experiences along my journey including; fellow PhD students either at UTS or other
universities in Australia, fellow Thai government scholarship students, UTS Thai society and
UTS PoLSA.
Before starting this precious journey, I could not forget all my teachers and friends from
Sainumtip School, Satriwithaya School, Srinakharinwirot University and King Mongkut's
University of Technology Thonburi who built my background and passion to pursue my
studies. I would like to say a special thank you to Professor Narongrit Sombatsompop, and
Assist.Prof. Thongdee Leksophee. I also would like to thank my colleagues and former
students from the Faculty of Science, Srinakharinwirot University as well for all your
ongoing understanding and encouragement, especially Assoc. Prof. Sawat Pannau, Assist.
Prof. Natthapong Phinichka, Supinya Wongsriruksa and Janraem Plangsakron.
I am very grateful to Yani Andrutsopulos, Dr Massimiliano Cannalire, Michael Binder, Wray
Menzies, Max Doerfler and especially Peter Tulii for your support, inspiration and guidance
especially in improving my English along with your patience. I unfortunately cannot
mention all my friends in this section, however, I would also like to extend my thanks to all
of my friends from all over Sydney and also the world, who have been right there beside me
throughout this challenging experience.
I would not have completed this journey without the understanding, encouragement,
wholehearted and generous support and patience from my lovely family. I cannot express
enough how much I would like to acknowledge all of you. I would not be who I am today
without all of you. Thank you so much to my dad, mom, aunty Daengnoi and sisters-P’Pong
and P’Prang.
The purpose of my precious journey was to investigate precious metal alloys and compounds.
Throughout my journey, not only did I achieve this purpose, but also I gained far greater
precious experiences and friendships here in Australia.
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Table of Contents
Certificate of original authorship _______________________________________________ i
Acknowledgements __________________________________________________________ii
Table of Contents ___________________________________________________________ v
Abstract _________________________________________________________________ viii
Publications and conference presentations arising from this work ______________________ x
List of Figures _____________________________________________________________xii
List of Tables ____________________________________________________________xxii
Chapter 1: Introduction _______________________________________________________ 1
Chapter 2: Literature review ___________________________________________________ 6
2.1 General: precious metal alloys and intermetallic compounds _____________________ 7
2.1.1 Definition of precious metal alloys and intermetallic compounds ______________ 7
2.1.2 Applications of precious metals and their alloys and intermetallic compounds ___ 8
2.2 Optical properties of materials ___________________________________________ 13
2.2.1 The colour of metallic materials _______________________________________ 13
2.2.2 The CIE-L*-a*-b* colour coordinate system _____________________________ 14
2.2.3 Dielectric function _________________________________________________ 15
2.2.4 Plasmon resonances in precious metal nanoparticles _______________________ 17
2.3 Specific precious metal alloys ____________________________________________ 18
2.4 Specific precious metal intermetallic compounds _____________________________ 22
2.4.1 Colour of pure phases _______________________________________________ 22
2.4.2 Alloying effects ___________________________________________________ 27
2.5 Nanoporous precious metal sponges _______________________________________ 29
2.5.1 Nanoporous gold (np-Au) ____________________________________________ 32
2.5.2 Nanoporous silver (np-Ag) ___________________________________________ 33
2.5.3 Nanoporous platinum (np-Pt) _________________________________________ 34
2.5.4 Nanoporous palladium (np-Pd) _______________________________________ 35
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Chapter 3: General Experimental ______________________________________________ 37
3.1
Overview _________________________________________________________ 38
3.2
Materials preparation ________________________________________________ 39
3.2.1
Magnetron sputtering ____________________________________________ 39
3.2.2
Heat treatment __________________________________________________ 41
3.3
Materials characterization ____________________________________________ 42
3.3.1 X-ray diffraction ___________________________________________________ 42
3.3.2 Scanning Electron Microscopy (SEM) __________________________________ 46
3.3.3 Transmission Electron Microscopy (TEM) ______________________________ 47
3.3.4 Determination of optical properties ____________________________________ 50
Chapter 4: The AuAl2-PtAl2 system ____________________________________________ 53
4.1 Background __________________________________________________________ 54
4.1.1 Review of the fabrication and applications of AuAl2 and PtAl2 ______________ 54
4.1.2 Review of the optical properties of AuAl2 and PtAl2 _______________________ 59
4.2 Objective of this chapter ________________________________________________ 63
4.3 Experimental details specific to this chapter _________________________________ 63
4.4 Results and discussion__________________________________________________ 65
4.4.1 Single layer films of coloured intermetallic compounds ____________________ 65
4.4.2 Bi-layers of coloured intermetallic compounds ___________________________ 85
4.4.3 Multi-layer films of coloured intermetallic compounds _____________________ 88
4.5 Conclusion__________________________________________________________ 117
Chapter 5: Nanoporous platinum sponges ______________________________________ 119
5.1 Background _________________________________________________________ 120
5.2 Objectives of this chapter ______________________________________________ 127
5.3 Experimental detail specific to this chapter ________________________________ 127
5.4 Results and discussion_________________________________________________ 130
5.4.1 Effect of composition ______________________________________________ 130
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5.4.2 Effect of temperature ______________________________________________ 137
5.4.3 Effect of deposition time ___________________________________________ 144
5.4.4 Effect of deposition rate ____________________________________________ 145
5.4.5 Effect of de-alloying parameters _____________________________________ 149
5.4.6 Comparison between my nanoporous Pt sponges and those in the literature ____ 150
5.5 Conclusion__________________________________________________________ 157
Chapter 6: Conclusions and future work _______________________________________ 158
6.1 The AuAl2-PtAl2 system _______________________________________________ 159
6.2 Nanoporous platinum sponges __________________________________________ 161
References _______________________________________________________________ 163
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Abstract
Precious metal alloys and compounds have myriad applications in the fast-expanding
horizons of the commercial and industrial worlds. They are also fascinating topics for
scientific research. These materials have a long history, with gold and silver amongst the very
earliest metals used by humans. Over the past millennia, the primary applications of the
precious metals and their alloys have been in the ever-lucrative jewellery manufacturing
industry. The traditional alloys have been perfected in over three thousand years of
experience. However, in the recent past, precious metal alloys and compounds have also
found themselves a crucial place of pride in the burgeoning ‘advanced materials’ sector.
Gold-based and platinum-based alloys and compounds are amongst the candidates being
investigated for serving in those applications. In the present project I sought to explore how
gold aluminide and platinum aluminide could be developed for further innovative
applications. In particular, I initially became interested in the optical properties of these
materials, with a view to developing their application in the jewellery industry. The PtxAl
alloys are, however, also useful as precursors for producing nanoporous metal sponges. The
availability of such samples from the first part of the project encouraged me to consider
technological applications of the aluminides in the chemical catalysis industry in the second
part of the project. The two parts are linked by virtue of starting with the same materials,
which are fabricated and mostly characterized the same way. In both cases the samples are
fabricated as thin films by direct-current magnetron sputtering and then various techniques
are used to characterize their chemical composition, structures, morphologies and specific
properties. The main difference comes only at the very end of each part, with the first group
of materials being evaluated on their optical properties and the second on their spongeforming properties.
My work is developed around two hypotheses. First, I hypothesized that the compounds
PtAl2 (brassy yellow) and AuAl2 (metallic purple) can be alloyed to yield a range of
intermediate colours. It is generally stated that these compounds would be immiscible but I
proposed that a series of metastable solid solutions could be formed by means of magnetron
sputtering. Secondly, I hypothesised that the preparation of nanoporous platinum sponges
from metastable (PtxAl) precursors would produce a different result than producing them
from well-crystallized precursors, and that this could be exploited to provide a new way to
control the morphology of such sponges.
The work has showed that the attractive colours of the intermetallic compounds AuAl2
(‘purple gold’) and PtAl2 (‘golden platinum’) can be combined or mixed to produce an
interesting colour spectrum. This may be of interest to the jewellery industry. A series of
metastable solid solutions could be formed by using the magnetron sputtering technique,
which enables users to produce any desired stoichiometry. In addition, procedures to reliably
viii
produce pure AuAl2 and PtAl2 thin films have been established. These have lattice parameters
of 0.599 nm and 0.594 nm respectively, which are similar to those of bulk samples produced
by vacuum arc melting. Addition control may be obtained by designing multilayer stacks of
these intermetallic compound films, with both bi-layer and multi-layer films being produced
in the present project. It was also shown that a metastable solid solution of Au and Pt could
be formed by sputtering, with a co-deposited film of 54 at.%Au- 46 at.%Pt film forming a
solid solution with a lattice parameter of 0.401 nm, which lies between that of pure Au films
(0.408 nm) and pure Pt films (0.394 nm). This metastable solid solution could be reacted with
a pure Al film to form a metastable solid solution of (Au,Pt)Al2 after annealing. However,
thin film stacks of AuAl2 and PtAl2 may be a better choice to tune colours of these two
compounds as they are easier to control.
Next I showed that Pt-Al alloys and intermetallic compounds can be de-alloyed in alkaline
solutions to produce nanoporous platinum sponges. These nanoscale sponges can be used as
chemical catalysts although I did not pursue this aspect myself. Rather, in this part of the
project I considered how the microstructure of the precursor alloys could control the
morphology of subsequent sponges. Once again, metastable precursors could be prepared by
using magnetron sputtering, and produced a different morphology of sponges compared to
those produced from well-crystallized precursors. Other processing parameters have also
been studied. It was found that mole fraction (FAl) of Al in the precursor and the deposition
temperature are the two most important factors. Precursors with ȤAl < 0.60 did not form
sponges after either deposition at elevated or room temperature. 'Mud-cracked' mesoporous
sponges could be formed by preparing precursors with ȤAl =0.67 and deposited at elevated
temperature. The Pt8Al21 and meta-stable phase (H-phase) were formed in precursors with
0.67< ȤAl < 0.90 that had been deposited at elevated temperature. In this case de-alloying
produced classic isotropic fibrous sponges. Disordered and fragile masses were obtained
when precursors with ȤAl > 0.90 were de-alloyed. These had originally consisted of a mixture
of PtAl6 and pure Al. It was also found that precursors that had been deposited at room
temperature produced very different sponge morphologies to those that had been deposited at
elevated temperature: in this case the amorphous precursors with 0.67 < ȤAl <0.96 produced
sponge morphologies ranging from pinhole to unusual isotropic foamy. This work has shown
that different morphologies of nanoporous platinum sponges can be produced by controlling
the processing parameters. These sponges might be considered for use in specific catalytic or
sensor applications because they can be fabricated using simple and cost-effective production
techniques.
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Publications and conference presentations arising from
this work
Publications
1. Supansomboon, S., Bhatia, V., Thorogood, G., Dowd, A., and Cortie, M.B., Advanced
precious metal alloys, Materials Australia, 2011. 44 (4): p.41-46.
2. Keast, V.J., Birt, K., Koch, C.T., Supansomboon, S., and Cortie, M.B., The role of
plasmons and interband transitions in the color of AuAl2, AuIn2, and AuGa2. Applied
Physics Letters, 2011. 99(11): p. 111908.
3. Keast, V.J., Zwan, B., Supansomboon, S., Cortie, M.B., and Person, P.O.Å., AuAl2
and PtAl2 as potential plasmonic materials. Journal of Alloys and Compounds, 2013.
577: p. 581-586.
4. McPherson, D.J., Supansomboon, S., Zwan, B., Keast, V.J., Cortie, D.L., Gentle, A.,
Dowd, A., and Cortie, M.B., Strategies to control the spectral properties of Au–Ni thin
films. Thin Solid Films, 2014. 551: p. 200-204.
5. Supansomboon, S., Porkovich, A., Dowd, A. Arnold, M.D., and Cortie, M.B., Effect
of precursor stoichiometry on the morphology of nanoporous platinum sponges. ACS
Applied Materials & Interfaces, 2014. 6(12): p. 9411-9417.
6. Supansomboon, S., Dowd, A., Lingen, E. van der, Keast, V.J., and Cortie, M.B.,
Coatings of coloured intermetallic compounds for decorative and technological
applications, Materials Forum (accepted, 2014).
7. Supansomboon, S., Dowd, A., Gentle, A., Keast, V.J., Lingen, E. van der, and M.B.
Cortie, Thin films of PtAl2 and AuAl2 by solid-state reactive synthesis (being prepared
for submission).
Conference presentations
1. Supansomboon, S., Dowd, A., and Cortie, M.B., Phase relationships in the PtAl2AuAl2 system, 35th Condensed Matter and Materials Meeting, 1-4 February, 2011
Charles Sturt University, Wagga Wagga, New South Wales, Australia. (Poster
presentation)
2.
Supansomboon, S., Dowd, A., and Cortie, M.B., Optical properties of nanoscale bilayers of the coloured intermetallic compounds Al2Pt and Al2Au, the 10th Asia-Pacific
Microscopy Conference (APMC10), the 2012 International Conference on
Nanoscience and Nanotechnology (ICONN2012) and the 22nd Australian Conference
on Microscopy and Microanalysis (ACMM22), 5 – 9 February 2012, the Perth
Convention & Exhibition Centre, Western Australia, Australia. (Poster presentation)
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3. Supansomboon, S., Dowd, A., and Cortie, M.B., Thin film stacks of the colored
intermetallic compounds Al2Au and Al2Pt, Gold 2012, 5-8 September 2012, Keio
Plaza Hotel Tokyo, Tokyo, Japan. (Poster presentation)
4. Supansomboon, S., Dowd, A., and Cortie, M.B., Colored intermetallic compounds for
gems and jewelry application, The 3rd International Gem and Jewelry Conference, 1216 December, 2012, the Imperial Queen's Park Hotel, Bangkok, Thailand (Oral
presentation)
5. Supansomboon, S., Dowd, A., and Cortie, M.B., Coatings of coloured intermetallic
compounds: AuAl2 and PtAl2 for decorative and technological applications, Materials
Innovation in Surface Engineering, 19-21 November 2013, University of South
Australia, Adelaide, South Australia, Australia (Oral presentation)
6. Supansomboon, S., Gentle, A., Dowd, A., and Cortie, M.B., Nanoscale bi-layers of
coloured intermetallic compounds, Australian Nanotechnology Network Early Career
Workshop, 10-11 July 2014, University of Technology, Sydney, New South Wales,
Australia. (Poster presentation)
7. Supansomboon, S., Dowd, A., and Cortie, M.B., Formation of nanoporous platinum
sponges by de-alloying AlxPt, The 3rd Biennial Conference of the Combined
Australian Materials Societies (CAMS 2014), 26-28 November 2014, Charles Perkins
Centre, University of Sydney, New South Wales, Australia. (Oral presentation)
xi
List of Figures
Figure 1.1 Periodic table of elements ........................................................................................ 2
Figure 1.2 Crystal structure of precious metals (a) face-centered cubic (FCC) and (b) closepacked hexagonal (HCP) [4] ...................................................................................................... 3
Figure 2.1 Schematic representation of the mechanism of photon absorption (a) and emission
(b) for metallic materials [35] ................................................................................................. 14
Figure 2.2 CIE L*a*b* colour space [38] ................................................................................ 15
Figure 2.3 Imaginary part of the interband dielectric constant as function of energy (a) Au-Ag
and (b) Au-Cu [39]................................................................................................................... 16
Figure 2.4 Imaginary part of the interband dielectric constant and energy of Ag-Cu series (a)
Cu-rich and (b) Ag-rich [39] .................................................................................................... 17
Figure 2.6 Relationship between colour and composition in the Au-Ag-Cu system[62] ........ 19
Figure 2.7 Comparison of coloured rings from different precious metal alloys [61] ............. 20
Figure 2.8 Reflectivity curves of gold and its alloys (a) gold-silver alloys and (b) goldpalladium alloys [63] ............................................................................................................... 22
Figure 2.9 Crystal structures of coloured binary intermetallic compounds (a) Caesium
chloride structure and (b) Calcium fluoride (Courtesy CrystalMaker Software Ltd, UK) ...... 23
Figure 2.10 Reflectivity curves of coloured gold intermetallic compounds: AuAl2 (curve 1),
AuIn2 (curve 2) and AuGa2 (curve 3) [63]............................................................................... 24
Figure 2.11 CIE a* and b* colour coordinates of alloys along the 18 carat pseudo-binary, and
position of phase fields [70] ..................................................................................................... 29
Figure 2.12 Classification of nanoporous metals[76] .............................................................. 31
Figure 2.13 Nanoporous gold by dealloying Au-Ag (a) Model for dealloying [7], (b) SEM
micrograph of nanoporous gold by dealloying Au-Ag in nitric acid [89] and (c) Simulated
porous structure of nanogold which made from Au0.35Ag0.65 precursor [90] .......................... 32
Figure 2.14 Nanoporous silver generated from Ag-Al precursors (a) D–Al rich region and (b)
D–Al and Ag2Al region [116] .................................................................................................. 34
Figure 2.15 Nanoporous platinum produced by co-sputtered PtxSi1-x amorphous film for
different initial compositions (a) Pt0.10Si0.90 as deposited (b) isotropic open-cell foam (c)
Pt0.34Si0.66 as deposited (d) anisotropic columnar-type foam (e) Pt0.33 Si0.67 as deposited and (f)
anisotropic Voronoi [133] ........................................................................................................ 35
Figure 2.16 Nanoporous palladium by dealloying in various precursors (a) Pd-Co [146] (b)
Pd-Ni [139] and (c) Pd-Cu [142] ............................................................................................. 36
xii
Figure 3.1 A schematic diagram of magnetron sputtering mechanism .................................. 39
Figure 3.2 Relationship between sputter yield and atomic number of elements for argon ion
energy at 400 eV [155] ............................................................................................................ 40
Figure 3.3 A co-deposition technique during sputtering process, photograph taken during the
present project .......................................................................................................................... 40
Figure 3.4 Tube furnace for post-deposition annealing treatment ........................................... 41
Figure 3.5 Schematic of an X-ray diffractometer in the Bragg-Brentano configuration [156]43
Figure 3.6 Schematic diagram of XRD (a) conventional T-2T geometry and (b) grazing angle
geometry [160] ......................................................................................................................... 44
Figure 3.7 (a) High temperature furnace and (b) platinum heater bar with cavity [165] ....... 45
Figure 3.8 Information in powder diffraction pattern [167] .................................................... 45
Figure 3.9 A field emission scanning electron microscope (Zeiss Supra 55VP)..................... 47
Figure 3.10 Cross section preparation for TEM (a) schematic of stack sample prepared using
the sandwich technique and (b) Interface of thin film after polishing with diamond of 1Pm
(the arrow indicate the location of the glue line) [175] ........................................................... 49
Figure 3.11 An optical model of a thin film sample [177] ...................................................... 51
Figure 3.12 Diagram of the process for ellipsometry data analysis [178] ............................... 52
Figure 4.1 Phase diagram of Al-Au system by Okamoto, H.(1991) [182] .............................. 54
Figure 4.2 Phase diagram of Al-Pt system by McAlister, A.J. and Kahan, D.J.(1986) [182] . 55
Figure 4.3 Purple gold by investment casting (Courtesy JARAD Project by Srinakharinwirot
University, Bangkok Fashion City under the Ministry of Industry of Thailand, Thailand) .... 56
Figure 4.4 AuAl2- carat purple gold (top row) and AuIn2 - blue gold (bottom row)
(Courtesy Co. Reischauer GmbH, Idar Oberstein, Germany) [6]............................................ 56
Figure 4.5 Bi-metal casting (a) 950 Pd casting with injected wax for the 2nd bi-metal casting
process step and (b) Bi-metal castings of 14k blue gold (left) and 18k purple gold (right) with
950 Pd (Courtesy Vendorafa-Lombardi Srl, Valenza,Italy) [6] .............................................. 57
Figure 4.6 Purple gold by powder metallurgy process (Courtesy Lee Hwa Jewellery,
Singapore) [186] ...................................................................................................................... 57
Figure 4.7 Purple glory gemstone-like AuAl2 casting in setting on ring (courtesy M.B Cortie)
[180] ......................................................................................................................................... 57
Figure 4.8 Platigems and Platigem jewellery (Courtesy Mintek, South Africa) [187]............ 58
xiii
Figure 4.9 AuAl2-coated items made by depositing onto sterling silver costume jewellery by
the present author [181] ........................................................................................................... 58
Figure 4.10 Reflectivity of gold intermetallic compounds from experiment (solid curve) and
calculation (dashed curved) [191]............................................................................................ 60
Figure 4.11 Dielectric function of gold intermetallic compounds; H1 (solid curve) and H2
(dashed curved) [191] .............................................................................................................. 60
Figure 4.12 Dielectric function of ordered intermetallic compounds [192] ............................ 61
Figure 4.13 Reflectivity of ordered intermetallic compounds comparing with experimental
reflectivity of PtAl2 thin film [192] ......................................................................................... 61
Figure 4.14 Comparison CIE Lab colour coordinates of AuAl2, Au0.5Pt0.5Al2,
Au0.25Pt0.75Al2 and PtAl2 [192]................................................................................................. 61
Figure 4.15 CIE L*a*b* colour gamut of Au-Ni-Au tri layer in reflection [203] ................... 62
Figure 4.16 Deposition rate of aluminium, gold and platinum as function of current ............ 64
Figure 4.17 X-ray patterns of Pt-Al compound films by co-sputtering using
varying current level of aluminium, PtAl2 are formed by using current at 0.395 A (yellow
pattern) ..................................................................................................................................... 66
Figure 4.18 Morphology of Pt-Al compound films (a) PtAl400 (b) PtAl395 (c) PtAl360
and (d) PtAl335 .......................................................................................................................... 67
Figure 4.19 Comparison of reflectance spectra of PtAl2 in bulk and film ............................... 68
Figure 4.20 Comparison of morphology of surfaces of PtAl2 in (a) bulk and (b) film ........... 68
Figure 4.21 Comparison of X-ray patterns of PtAl2 in bulk and film...................................... 69
Figure 4.22 X-Ray patterns of Au-Al compounds ................................................................... 70
Figure 4.23 Morphologies of Au-Al films produced by using different power levels on the
gold target (a) 16 W (sample AuAl040) in low magnification, (b) 16 W (sample AuAl040) in
high magnification, (c) 26 W (sample AuAl060) in low magnification, (d) 26 W (sample
AuAl060) in high magnification and (e) 21 W (sample AuAl050) ............................................. 71
Figure 4.24 Reflectance spectra of Au-Al compounds ............................................................ 72
Figure 4.25 Comparison of X-Ray patterns of AuAl2, deposited at different temperature ..... 73
Figure 4.26 Microstructure of AuAl2 films, deposited at different temperatures
(a) below 400 qC and (b) at 400 qC ......................................................................................... 73
Figure 4.27 Comparison of reflectance spectra of AuAl2 in bulk and film ............................. 74
Figure 4.28 Morphologies of AuAl2 bulk sample by (a) SEM and (b) LM............................. 74
xiv
Figure 4.29 Morphologies of AuAl2 thin film (a) plan view and (b) cross-section ................. 75
Figure 4.30 X-ray patterns of AuAl2 in bulk and thin film samples ........................................ 75
Figure 4.31 Diagram illustrating the process for fabricating coloured intermetallic compounds
by controlling the chemical composition and thickness of (a) PtAl2 film and (b) AuAl2 film 76
Figure 4.32 Cross-sections of the thin films of the binary intermetallic compounds after
annealing at 400 qC (a) AuAl2 and (b) PtAl2 ........................................................................... 76
Figure 4.33 X-ray patterns of colour intermetallic compounds in different thicknesses of film
(a) PtAl2 films and (b) AuAl2 films ......................................................................................... 77
Figure 4.34 Reflectance (R) and transmittance (T) spectra of PtAl2 films (Exp) with model
fitted (Model Fit) to different thicknesses of film (a) 100 nm and (b) 40 nm ......................... 78
Figure 4.35 Reflectance and transmittance spectra of AuAl2 films with model fitted to
different thicknesses (a) 100 nm and (b) 40 nm ...................................................................... 79
Figure 4.36 Dielectric functions of coloured intermetallic compounds by reflectance and
transmission data (a) PtAl2 and (b) AuAl2 ............................................................................... 80
Figure 4.37 Colour of simulated thin films in CIE L*a*b* space, with the colour of each film
rendered into the surface of a spherical data point. Both reflectance (yellow) and
transmittance (grey) modes are shown for the different thicknesses (a) front view (b) top view
and (c) perspective view .......................................................................................................... 82
rendered into the surface of a spherical data point. Both reflectance (purple) and
transmittance (yellow-green) mode in different thickness of film are shown (a) front view (b)
top view and (c) perspective view ........................................................................................... 83
Figure 4.39 Dielectric functions of coloured intermetallic compounds found by analysis of
ellipsometric data (a) PtAl2 and (b) AuAl2 .............................................................................. 84
Figure 4.40 The two kinds of bi-layer films produced (a) AuAl2/PtAl2 and (b) PtAl2/AuAl2 85
Figure 4.41 The reflectance spectra of bi-layers of AuAl2/PtAl2 ............................................ 85
Figure 4.42 Cross-section of bi-layers films of PtAl2/AuAl2 before annealing (a) In lens mode
and (b) backscatter mode ......................................................................................................... 86
Figure 4.43 Cross-section of PtAl2/AuAl2 film after annealing under vacuum at 400 °C
(a) for 24 hours and (b) for 48 hours........................................................................................ 86
Figure 4.44 Calculated reflectance (a) and colour (b) of 200 nm PtAl2 film that has been overcoated with indicated thickness of AuAl2 ................................................................................ 87
xv
Figure 4.45 Calculated reflectance (a) and colour (b) of 200 nm AuAl2 film that has been
over-coated with indicated thickness of PtAl2 ......................................................................... 87
Figure 4.46 Schematic illustration of the arrangements of the four-layer films of Al-Au-Pt (a)
Au on the top and (b) Pt on the top .......................................................................................... 88
Figure 4.47 Cross-sections of four-layer films of Al-Au-Pt with Au on the top, as deposited at
25 °C (a) In lens and (b) RBSD ............................................................................................... 89
Figure 4.48 Cross-sections of four-layer films of Al-Au-Pt with Au on the top, after annealing
at 400 °C (a) SEM:In lens mode (b) SEM:RBSD mode and (c) TEM .................................... 90
Figure 4.49 Cross-sections of four-layer films of Al-Au-Pt with Pt on the top, after annealing
at 400 °C (a) SEM and (b) TEM .............................................................................................. 90
Figure 4.50 The reflectance spectra from the front side of a four-layered film of Al-Au-Pt (Au
on the top) as deposited at 25 °C before and after annealing. Data for a pure gold film of
30 nm thickness is shown for comparison ............................................................................... 91
Figure 4.51 The reflectance spectra from the back side of the above four-layered film, before
and after annealing ................................................................................................................... 92
Figure 4.52 The reflectance spectra from the front side of a four-layered film of Al-Au-Pt (Pt
on the top) as depositing at 25 °C before and after annealing. Data for a pure platinum thin
film of 30 nm thickness is shown for comparison ................................................................... 92
and after annealing ................................................................................................................... 93
Figure 4.54 X-ray patterns of four-layer films of AlAuPt comparing the structure before and
after annealing at 400 °C (a) pure gold layer on the top and (b) pure platinum layer on the top
(both were deposited at 25 qC), with patterns for AuAl2 and PtAl2films shown for comparison
.................................................................................................................................................. 94
Figure 4.55 The arrangement of the six-layered films (a) Al-Au-Pt and (b) Al-Au ................ 95
Figure 4.56 Cross-section views of the six-layer film of Al-Au-Pt after annealing at 400 °C
(a) SEM-In lens (b) SEM-RBSD and (c) TEM ....................................................................... 96
Figure 4.57 The reflectance spectra of the top of the six-layer film of Al-Au-Pt (Pt on the top)
after annealing .......................................................................................................................... 96
Figure 4.58 The X-ray pattern of the six-layer film of Al-Au-Pt after annealing, with patterns
for PtAl2 and AuAl2 films shown for comparison ................................................................... 97
Figure 4.59 Cross-sections of six-layer films of Al-Au (a) before annealing-In lens, (b) before
annealing – RBSD, (c) after annealing at 400 °C – In lens and (d) after annealing at 400 °C –
RBSD ....................................................................................................................................... 97
xvi
Figure 4.60 The X-ray pattern of the six-layer film of Al-Au before and after annealing, with
patterns for AuAl2 films shown for comparison ...................................................................... 98
Figure 4.61 The different arrangements of eight-layer films of Al-Au-Pt (a) 50 nm each layer,
Au on the top and (b) 50 nm each layer, Pt on the top ............................................................ 98
Figure 4.62 Cross-sectional views of eight-layered films of Al-Au-Pt (a) before annealing-In
lens, (b) before annealing – RBSD, (c) after annealing at 400 °C – In lens and (d) after
annealing at 400 °C – RBSD ................................................................................................... 99
Figure 4.63 The X-ray pattern of eight multi-layers films of Al-Au-Pt (Pt on the top) after
annealing, comparing with PtAl2 film .................................................................................... 100
Figure 4.64 The reflectance spectrum of the surface of eight multi-layers films of Al-Au-Pt
(Pt on the top) after annealing, comparing with a single PtAl2 film ...................................... 100
Figure 4.65 The morphologies of eight multi-layer films of Al-Au-Pt which Au on the top
after annealing at 400 °C (a) cross-sectional area and (b) surface area ................................. 101
Figure 4.66 The X-ray pattern of the eight-layered films of Al-Au-Pt (Au on the top) after
annealing. A pattern for a simple AuAl2 film is shown for comparison................................ 101
Figure 4.67 The reflectance spectrum of the surface of the eight-layer film of Al-Au-Pt (Au
on the top) after annealing, in comparison to that of a simple, single-layer AuAl2 film ....... 102
Figure 4.68 The arrangement of eight-layered films of Al-Au-Pt with each layer being 25 nm
thick (Au on the top) .............................................................................................................. 102
Figure 4.69 The X-ray patterns of the eight-layered sample produced with half the deposition
time of the standard eight-layered sample of Al-Au-Pt (Au on the top), both before and after
annealing, compared with that of a simple, single-layer AuAl2 film..................................... 103
Figure 4.70 The cross-sectional view of the eight-layer films of Al-Au-Pt in which layer
thickness was halved, (a) before annealing and (b) after annealing at 400 °C for 30 minute104
Figure 4.71 Average integrated peak areas of PtAl2 and/or AuAl2 over the (111), (200), (220)
and (311) peaks as a function of temperature ........................................................................ 105
Figure 4.72 Peak area of four layers stack formed by depositing pure metals at (111) of PtAl2
and AuAl2............................................................................................................................... 105
Figure 4.73 The design of stacks consisting of co-deposited precious metals and aluminium
(a) precious metals on the bottom (Al/(Au,Pt) ), (b) precious metals on the top ( (Au,Pt)/Al)
and (c) co-depositing precious metals on the top but with half the thickness ....................... 106
Figure 4.74 A comparison of X-ray patterns of thin films of (Au,Pt) solid solution to those
pure Au and pure Pt ............................................................................................................... 107
xvii
Figure 4.75 X-ray patterns of different arrangements of stacks made of a layer of
co-deposited Au and Pt, and Al, before and after annealing ................................................. 108
Figure 4.76 X-ray patterns of an Al/(Au,Pt) sample, followed by annealing at various
temperatures ........................................................................................................................... 108
Figure 4.77 Comparison of X-ray patterns of an Al/(Au,Pt) sample, followed by annealing at
400 qC and 500 qC ................................................................................................................. 109
Figure 4.78 Comparison of the X-ray diffraction patterns obtained after annealing the stacks
with 20, 60 and 120 nm of (Au,Pt) at 400 qC ........................................................................ 110
Figure 4.79 X-ray patterns of a mixed AuAl2/PtAl2 sample formed by co-depositing Au and
Pt onto Al. The fitted pattern was obtained by Rietveld refinement on a PtAl2 structure ..... 111
Figure 4.80 Cross-section of the Al/(Au,Pt) sample with 20 nm of co-deposited (Au,Pt) with
EDS elemental scan and mapping.......................................................................................... 112
Figure 4.81 High resolution TEM images of the Al/(Au,Pt) sample with 20 nm of codeposited (Au,Pt) (a) top layer and (b) bottom layer ............................................................. 113
Figure 4.82 The reflectance spectra of the 65 at.% Al-23 at.% Pt-12 at.% Au sample on its
front and back sides, compared with the front side of the sample before annealing and that of
a pure PtAl2 film .................................................................................................................... 114
Figure 4.83 The reflectance spectra of 56 at.% Al-22 at.% Pt-22 at.% Au on its front and back
sides, compared with the front side of the sample before annealing and a pure AuAl2 film . 114
Figure 4.84 X-ray diffraction patterns of samples made by co-depositing Au and Pt on top of
an Al layer (a) 65 at.% Al - 23 at.% Pt - 12 at.% Au and (b) 56 at.% Al - 22 at.% Pt - 22 at.%
Au. Data for before and after annealing, and for pure PtAl2 and pure AuAl2 is shown ........ 115
Figure 4.85 Lattice parameter and peak area of Al/(Au,Pt) sample as a function of
temperature (a) Lattice parameter of (Au,Pt)Al2 in Al/(Au,Pt) sample and (b) peak area of
(Au,Pt)Al2 phase (111) and Pt2Al3 (002) ............................................................................... 117
Figure 5.1 Various techniques for nanoporous platinum fabrication [207] ........................... 120
Figure 5.2 Pt-Al phase diagram [237].................................................................................... 121
Figure 5.3 The different lattice types of the intermetallic compounds in the
Pt-Al binary system [229] ...................................................................................................... 122
Figure 5.4 Effect of Al content on structure of sponges produced from precursors with ȤAl >
0.80 (a) X-ray diffraction pattern of increasing amount of Al, (b) X-ray diffraction patterns of
de-alloyed Pt sponges (c) and (d) SEM micrograph of isotropic foamy Pt sponge from
precursor with ȤAl = 0.88 and 0.85 respectively (e) TEM micrograph of Pt sponge from
precursor with ȤAl = 0.88 [239] .............................................................................................. 124
xviii
Figure 5.5 Simulation of the de-alloying of the sponges by using Monte Carlo model as a
function of ȤAl (a) Morphologies of sponges in various aluminium content (b) Ratio of surface
atoms to total atoms of sponges (ȣ) and ȤAl remaining in sponge. (c) Average mean and
Gaussian curvatures of sponges. (d) Effect of Lennard-Jones temperature on the de-alloying
of a starting alloy with ȤAl = 0.80. This work was performed by my co-authors [239] ......... 125
Figure 5.6 Three distinct morphologies of nanoporous platinum as correlated with cosputtering parameter, initial alloy composition and thickness [133] ..................................... 126
Figure 5.7 Effect of de-alloying system on ligament size of nanoporous platinum from
different alloy systems (a) ligament sizes of different noble metal-aluminium with dealloying with 5% HCl and 20% NaOH and (b) ligament sizes of platinum-gold-copper alloys
with varying noble metal content ........................................................................................... 126
Figure 5.8 Nanoporous platinum produced from Pt0.20Cu0.80, then de-alloyed in 93%H2SO4
and coarsened at different temperatures (a) 250 qC (b) 300 qC (c) 400 qC and (d) 500 qC
[246] ....................................................................................................................................... 127
Figure 5.9 De-alloying process on Pt-Al precursor (a) Bubble of H2 on Pt-Al precursor
immersing in alkali solution and (b) model of aluminium removing from Pt-Al precursor
[239] ....................................................................................................................................... 128
Figure 5.10 Flowchart showing preparation of Pt-Al precursors and the subsequent
nanoporous platinum .............................................................................................................. 129
Figure 5.11 Pt-Al precursor film deposited at 400 qC with ȤAl | 0.50 (a) XRD patterns
comparing with other phases from calculated and database and (b) SEM micrograph after dealloying showing that a nanoporous sponge did not form ..................................................... 130
comparing with Pt2Al3 from database (b) SEM micrograph before de-alloying (c) SEM
micrograph after de-alloying.................................................................................................. 131
Figure 5.13 Pt-Al precursor film deposited at 400 qC with ȤAl = 0.67 (a) XRD patterns
comparing with Pt2Al3 from database and (b) reflectance spectra ........................................ 132
Figure 5.14 ‘Mud-cracked’ sponges produced by de-alloying sample with ȤAl = 0.67 (a) a
porous and cracked film (b) cross-sectional view and (c) curled up porous and cracked film at
low magnification .................................................................................................................. 132
Figure 5.15 TEM micrographs of de-alloyed samples with ȤAl = 0.67 .................................. 133
Figure 5.16 Pt-Al precursor film with 0.67< ȤAl < 0.80 (a) XRD patterns of precursors with
ȤAl = 0.78 (deposited at 400 qC) and precursors with ȤAl = 0.75 (deposited at room
temperature then crystallized by heating ~400 qC), comparing with Pt8Pt21 from database and
reported İ phase (b) crystallization of İ phase at ~360 qC on heating up precursor with ȤAl =
0.75 (c) morphology of Pt-Al precursor with ȤAl = 0.78 as deposited (d) SEM micrograph of
xix
isotropic fibrous sponges in plain view and (e) SEM micrograph of isotropic fibrous sponges
in cross-sectional view ........................................................................................................... 134
Figure 5.17 Pt-Al precursor film deposited at 400 qC with ȤAl = 0.82 (a) (a) XRD patterns
comparing with Al-rich phases from database and İ phase (b) and (c) SEM micrograph of
isotropic fibrous sponges ....................................................................................................... 135
Figure 5.18 Pt-Al precursor film deposited at 400 qC with ȤAl > 0.90 (a) XRD pattern
comparing with PtAl6 and Al from database (b) SEM micrograph as deposited (c) SEM
micrograph after de-alloying (d) TEM de-alloying and (e) High resolution TEM after dealloying .................................................................................................................................. 136
Figure 5.19 XRD patterns of the precursors were deposited at room temperature
with ȤAl = 0.67 ...................................................................................................................... 137
Figure 5.20 Morphology of partially de-alloyed sponges produced from the precursors were
deposited at room temperature with ȤAl = 0.67 (a) plan view and (b) cross-sectional view .. 138
Figure 5.21 Morphology of sponge produced from a precursor with ȤAl | 0.75 that had been
deposited at room temperature ............................................................................................... 138
Figure 5.22 Morphology of Pt-Al precursors, deposited at room temperature with ȤAl > 0.80
(a) as deposited (b) ȤAl | 0.83 after de-alloying (c) ȤAl | 0.88 after de-alloying (d) ȤAl | 0.96
after de-alloying and (e) curled up porous Pt sponge and shown cross-sectional view ........ 139
Figure 5.23 TEM mapping on Pt-Al precursors, deposited at room temperature
with ȤAl = 0.92 ........................................................................................................................ 140
Figure 5.24 TEM-EDS analysis through the cross-sectional area of Pt-Al precursors,
deposited at room temperature with ȤAl = 0.92. The presence of Cu is due to redeposited
materials during PIPS ............................................................................................................ 140
Figure 5.25 Distribution of pore sizes from Pt-Al precursor with different mole fraction of Al
(a) ȤAl = 0.83 (b) ȤAl = 0.88 and (c) ȤAl = 0.96 ....................................................................... 141
Figure 5.26 TEM micrograph of sponge formed from precursor with ȤAl § 0.88 (a) a
continuous network of Pt surrounding the void and (b) lattice fringe image at high resolution
................................................................................................................................................ 142
Figure 5.27 Comparison of X-ray diffraction pattern between samples with ȤAl > 0.80, which
were deposited at room temperature and above 400 °C and the sample with ȤAl = 0.82, which
was deposited at elevated temperature ................................................................................... 143
Figure 5.28 Morphologies of samples with ȤAl >0.80, which were deposited at room
temperature, then annealed at various temperatures followed by de-alloying process in alkali
solution (a) as deposited at room temperature (b) annealed at 100 °C (c) annealed at 200 °C
(d) annealed at 300 °C (e) annealed at 400 °C and (f) annealed at 500°C............................. 144
xx
Figure 5.29 Morphologies of samples with ȤAl =0.83, which were deposited at room
temperature with various deposition times (a) 5 minutes (b) 10 minutes and (c) 30 minutes
................................................................................................................................................ 145
Figure 5.30 Pinhole sponge produced from precursors that deposited at room temperature
with high deposition rate of Pt in various Al contents (a) ȤAl | 0.62 (b) ȤAl | 0.67 and (c) ȤAl |
0.69 and (d) preferential dissolution along grain boundaries ................................................ 146
Figure 5.31 Foamy sponge produced from precursors that deposited at room temperature with
high deposition rate of Pt in various Al contents (a) ȤAl | 0.71 (b) ȤAl | 0.74 (c) ȤAl | 0.77 and
(d) view of interior of sponge through walls of crack ........................................................... 147
Figure 5.32 Morphologies of samples that deposited at room temperature with current level of
Pt at 0.005 A (a) partly foamy sponges and (b) fragile sponges ............................................ 148
Pt at 0.025 A (a) foamy sponges and (b) transparency foamy sponges film ......................... 148
Figure 5.34 Morphologies of samples that deposited at room temperature with different
current level of Pt (a) 0.050 A and (b) 0.075 A ..................................................................... 148
Figure 5.35 Foamy sponges produced from precursors that deposited at room temperature
with ȤAl = 0.96, then de-alloying by different solutions (a) 0.2M NaOH and (b) 0.2M Na2CO3
................................................................................................................................................ 149
Figure 5.36 Foamy sponges from precursors that deposited at room temperature with ȤAl =
0.92, then de-alloying by using Na2CO3 with different de-alloying times (a) 1 minute (b) 3
minutes (c) 5 minutes (d) 10 minutes and (e) 15 minutes...................................................... 150
xxi
List of Tables
Table 1.1 Structure and lattice constant of precious metals [1-3].............................................. 3
Table 1.2 Selected properties of precious metals [1-3].............................................................. 4
Table 2.1 Comparison between alloys and intermetallic compounds........................................ 7
Table 2.2 Definition of caratage in gold content [1] .................................................................. 9
Table 2.3 Varied content of alloying elements of carat gold alloys for jewellery [12] ............. 9
Table 2.4 Selected properties and applications of carat gold alloys in jewellery [12] ............ 10
Table 2.5 Onset of interband transition of selected coloured intermetallic compounds [41, 42]
.................................................................................................................................................. 17
Table 2.6 Coloured carat gold alloys based on the Au-Ag-Cu system [66] ............................ 21
Table 2.7 Coloured binary intermetallic compounds with CsCl and CaF2 structure [68] ....... 24
Table 2.8 Ternary coloured intermetallic compounds based on a precious metal [68] ........... 25
Table 2.9 Quaternary coloured intermetallic compounds based on a precious metal [68] ...... 27
Table 2.10 Classification of nanoporous materials and their properties [74] .......................... 30
Table 3.1 Sputter yields (atoms/ion) as a function of argon ion energy of selected metals
[154] ......................................................................................................................................... 40
Table 4.1 Pt-Al films fabricated by co-sputtering using varying currents on the aluminium
target. (The current on the platinum target was fixed at 0.125 A or the power was fixed at
~ 55-57 W) ............................................................................................................................... 66
Table 4.2 Au-Al films fabricated by co-sputtering using varying current level of gold. (Power
of Al was 204 W or current ~0.443-0.452 A) .......................................................................... 70
Table 4.3 Quantitative chemical analysis by EDS of Au-Al compounds ................................ 70
Table 4.4 Conditions, colour and XRD results of Au-Al films ............................................... 72
Table 4.5 A comparison of the CIE XYZ and CIE L*a*b* colour coordinates of thin film and
bulk samples of PtAl2 and AuAl2 ............................................................................................ 81
Table 4.6 Deposition conditions for the four-layer films of Al-Au-Pt .................................... 89
Table 4.7 Deposition conditions for the six-layered films of Al-Au-Pt .................................. 95
Table 4.8 Deposition conditions of eight-layer films of Al-Au-Pt. Each layer is 50 nm thick99
xxii
Table 4.9 Deposition conditions of eight-layered film of Al-Au-Pt in which each layer is 25
nm thick ................................................................................................................................. 103
Table 4.10 Chemical composition of a stack of Al/(Au,Pt) with various thicknesses of
precious metals, measured after annealing at 400 °C for 60 minutes .................................... 110
Table 5.1 List of nanoporous Pt produced by other research works ...................................... 152
Table 5.2 List of nanoporous Pt produced by this research project ....................................... 155
xxiii
Chapter 1
Introduction
1
The term 'precious metal' describes a noble-metal that is valuable and rare. There are
eight precious metals: gold (Au), silver (Ag), platinum (Pt), iridium (Ir), palladium (Pd),
rhodium (Rh), ruthenium (Ru) and osmium (Os). These eight precious metals occur close
together in periods 5 and 6 (groups VIII and Ib) of the periodic table (Figure 1.1). Gold and
silver are the best known of these but the platinum group metals or PGMs are also considered
to be precious metals. The precious metals have unique properties such as high lustre,
resistance to oxidation, high electrical conductivity and, in some cases, attractive strength at
high temperature.
Alkali metals
Lanthanide
Other nonmetals
Alkaline earth metals
Actinide
Halogen
Transition metals
Metalloid
Noble gas
Precious metals
Unknown properties
Post -transition metals
Figure 1.1 Periodic table of elements
All physical and chemical properties depend on atomic and crystal structure. Except
for osmium and ruthenium (which are close-packed hexagonal, HCP) the precious metals are
face-centred cubic, FCC. The two different crystal structures are shown in Figure 1.2. In
Table 1.1 the atomic numbers, atomic weights, crystal structures and lattice constants of the
eight precious metals are summarized. It can be seen that most of the precious metal family
share similar properties although Ag and Au stand out for their low electrical resistivity and
2
high thermal conductivity. Physical, thermal and mechanical properties of the eight precious
metals are shown in Table 1.2. The precious metals also have excellent resistance to chemical
and environment attack and hence are often known as the ‘noble’ metals.
Table 1.1 Structure and lattice constant of precious metals [1-3]
Element
Atomic
number
Ruthenium (Ru)
44
Atomic
weight
Crystal structure
101.07
HCP
Lattice constant (Å)
a=2.7056
c=4.2816
Rhodium (Rh)
45
102.91
FCC
3.8044
Palladium (Pd)
46
106.40
FCC
3.8902
Silver (Ag)
47
107.87
FCC
4.0862
Osmium (Os)
76
190.20
HCP
a=2.7340
c=4.3194
Iridium (Ir)
77
192.20
FCC
3.8392
Platinum (Pt)
78
195.09
FCC
3.9231
Gold (Au)
79
196.97
FCC
4.0786
(a)
(b)
Figure 1.2 Crystal structure of precious metals (a) face-centered cubic (FCC) and (b) closepacked hexagonal (HCP) [4]
3
Table 1.2 Selected properties of precious metals [1-3]
Properties
Ru
Rh
Pd
Ag
Os
Ir
Pt
Au
Density at 20 °C
(g/cm3)
12.45
12.41
12.02
10.49
22.61
22.65
21.45
19.32
Melting point (qC)
2310
1963
1552
961
3045
2447
1769
1064
Boiling point (qC)
3900
3700
2900
2210
5020
4500
3800
2808
Electrical resistivity at
0 °C ȝȍ · cm
6.80
4.33
9.93
1.59
8.12
4.71
9.85
2.06
Thermal conductivity
at 0-100 °C (W/m K)
105
150
76
425.0
87
148
73
315.5
Linear coefficient of
thermal expansion
(10-6/°C)
9.1
8.3
11.1
19.68
6.1
6.8
9.1
14.16
Tensile strength
(MPa)-As worked
wire
496
13791586
324414
290
-
20702480
207241
207-221
Hardness (HV)Annealed wire
200350
120140
37-44
25-30
300670
200240
37-42
25-27
Gold and silver have of course been used for thousands of years, but use of the PGMs
is more recent and dates from the 19th Century. Platinum has become an important substance
in many industrial applications. However, use of pure metals causes some limitation in
properties (in particular the tensile strength and hardness of the pure elements is rather low)
and the precious metals are therefore commonly used in commercial and industrial
applications as alloys or compounds. Alloying between the precious metals and with other
elements has been developed to gain alternative properties for use in both decorative and
industrial applications. These mixtures may take the form of conventional alloys,
intermetallic compounds, or blends of the two. Historically, the initial applications of
precious metals and their alloys or compounds have been in jewellery manufacture and
dentistry. Technological development to make the use of precious metals and their alloys or
compounds more efficient in those industries continues. However, the precious metals have
more recently been applied in many other applications, for example electronics, catalysis,
fuel cells, environmental remediation, the automobile industry, aerospace, optical devices,
plating and coating, medical and pharmaceutical products. Precious metal alloys and
compounds therefore continue to be an interesting subject.
4
There are several interesting groups of precious metal alloys and intermetallic
compounds and they are usually classified according to their majority component, e.g. goldbased, platinum-based etc., or according to the field of application, dental alloys, jewellery
alloys, etc. Precious metal alloys and compounds have been investigated in many different
aspects, many of which are summarized in the literature review in Chapter 2.
In the present research work the precious metal alloys and compounds have been
fabricated in the form of thin films by direct-current magnetron sputtering. The chemical
composition, structures and morphologies of those resultant thin films have been
characterized by various techniques which are explained in Chapter 3.
The main aims of this project have been to develop new applications of the precious
metals. In respect of the jewellery industry, which is of great economic importance in my
home country of Thailand, I have tried to find new and interesting technologies and alloys.
My main hypothesis in this part of the project has been that the distinctly different colours of
the various precious metal aluminides can be combined to yield new colour effects for
jewellery. In particular, I hypothesized that the compounds PtAl2 (brassy yellow) and AuAl2
(metallic purple) can be alloyed to yield a range of intermediate colours. It is generally stated
[5, 6] that these compounds would be immiscible but I proposed that a series of metastable
solid solutions could be formed by means of magnetron sputtering. Chapter 4 is dedicated to
this part of my project, the coloured intermetallic compounds of AuAl2 and PtAl2. Due to
their distinct metallic colours, they are especially interesting for jewellery and decorative
design purposes. AuAl2 can also be used in spectrally-selective coatings while PtAl2 is able to
be used as corrosion or oxidation resistant coatings, particularly at high temperatures.
Precious metals alloys have uses beyond the jewellery industry however. The Pt-Al
and Au-Al alloys and intermetallic compounds can be de-alloyed to form nanoporous
sponges of Pt and Au respectively. These have potential applications in the chemical industry
as catalysts and sensors [7]. In the second part of my project I have tried to understand how
the microstructure of precursor alloys can control the morphology of subsequent sponges.
My hypothesis has been that preparation of sponges from metastable precursors will produce
a different result than producing them from well-crystallized precursors. Chapter 5 deals with
the second theme, which is particularly related to platinum-aluminium intermetallics in
general and the nanoporous platinum sponges that can be formed by de-alloying them in
alkali solution.
Each of these chapters contains more specific literature reviews and experiments. The
overall conclusion and future works are presented in Chapter 6.
5
Chapter 2
Literature review
6
There are many interesting precious metal alloys and compounds and many practical
applications for them. The present research is focused on two themes: (i) coloured precious
metal alloys and intermetallic compounds and (ii) nanoporous precious metal sponges. The
work is motivated overall by the hypothesis that a better understanding of the binary and
ternary phase relationships within precious metal systems will enable the design and
fabrication of materials with specific and improved properties. In this Chapter I provide a
summary of what is already known on these topics.
2.1 General: precious metal alloys and intermetallic compounds
2.1.1 Definition of precious metal alloys and intermetallic compounds
The precious metals can combine with one or more other elements to produce either
alloys or intermetallic compounds. The term “alloy” is generally reserved for a material
basically comprised of a complete or partial solid solution of two or more elements, possibly
also containing a minor volume fraction of ceramic or intermetallic compounds. The crystal
structure and properties of the alloy are closely related to those of the base elements, possibly
somewhat modified by any minor phase or phases present. In contrast, the term “intermetallic
compound” is used to designate a combination of two or more elements in definite
proportions that form a new phase, with its own distinctive crystal structure and properties[8].
The bonding of intermetallic compounds is generally considered to be between metallic and
covalent in nature[9]. Some pairs of intermetallic compounds are mutually soluble and can
form a continuous range of isostructural compositions, in which case the term “intermetallic
alloy” is sometimes used. A comparison between definition of alloys and intermetallic
compounds are shown in Table 2.1.
Table 2.1 Comparison between alloys and intermetallic compounds
Alloys
Intermetallic compounds
Complete or partial solid solution
Form a new phase
Crystal structure and properties are closely
related to base elements
Distinctive crystal structure and properties
7
2.1.2 Applications of precious metals and their alloys and intermetallic
compounds
Precious metals and their alloys and compounds have a long history of uses in a wide
range of applications. Although these have advanced far beyond the original use as a means
to show off wealth, beauty and achievement, their use in jewellery remains economically
very important.
Jewellery
The major proportion of gold and silver consumption is for jewellery applications.
Gold, silver and platinum (and their alloys) are the main precious metals which have been
used in this application. The content of precious metals is a key consideration. The Ag-Cu
system is the most commonly used basis for silver alloys. 'Sterling silver' or '925Ag', which
contains 92.5 wt.% Ag and 7.5 wt.% Cu, is the most typical silver alloy used in jewellery
manufacturing. '835Ag' and '800Ag' are also used [10, 11]. The permissible content of gold is
varied more than for the other precious metals and depends on regional market preferences or
product niche. The term caratage refers to the proportion of pure gold, expressed on a scale of
zero to 24, with 24 being 100% Au. (In North America the spelling 'karat' and 'karatage' may
be used with 'carat' in those regions being reserved as a unit of mass for gemstones).
Fineness (gold content expressed in parts out of thousand) or weight percentage (of Au) is
also used, especially for bulk materials. The most popular common caratage levels are shown
in Table 2.2. The best-known gold alloys are based on the ternary alloy system of Au-Ag-Cu
and the quaternary alloy system of Au-Ag-Cu-Zn. The compositions of gold alloys are varied
for jewellery according to the colour, mechanical properties and processability required and
some of them are shown in Table 2.3. The physical properties and applications of gold alloys
are summarized in Table 2.4. Nickel and palladium have been used as alloying additions to
make so-called white gold alloys [12, 13]. The 990Au-Ti alloy has been developed to provide
acceptable mechanical properties at a very high caratage [14, 15].
Platinum alloys in jewellery usually have 90 wt.% of Pt or higher. The '950Pt' and
'900Pt' alloys are common levels of fineness of platinum which have been accepted in
jewellery manufacture. The alloying elements which commonly used in platinum are
palladium, ruthenium, cobalt, copper and iridium. For instance, 95%Pt-5%Cu and 95%Pt5%Ru are used as general purpose wrought alloys while 95%Pt-5%Co and 95%Pt-5%Pd are
suitable for casting in jewellery application [1, 16, 17].
8
Table 2.2 Definition of caratage in gold content [1]
Carat
Fineness
Percentage of pure gold
24
1000
100
23.76
990
99
22
916.6
91.66
21
875
87.5
20
833.3
83.3
18
750
75
15
625
62.5
14
585
58.5
12
500
50
10
416
41.6
9
375
37.5
8
333.3
33.3
Table 2.3 Varied content of alloying elements of carat gold alloys for jewellery [12]
Carat
Gold content (wt.%)
Content of alloy components (wt.%)
22
91.66
Ag 0-6, Cu 2-8.3
18
75
Ag 0-20, Cu 5-25
14
58.5
Ag 0-41.5, Cu 0-33.5, Zn 0-10
9
37.5
Ag 0-56, Cu 4-60, Zn 0-15
8
33.3
Ag 0-58.5, Cu 4-60, Zn 0-15
9
Table 2.4 Selected properties and applications of carat gold alloys in jewellery [12]
Carat
18
14
9
8
Melting
range
(qC)
Density
(g/cm3)
Hardness
(HV)
red
880-900
15.1
160
X
750/90
reddish
870-890
15.3
150
X
750/125
yellow
850-890
15.4
140
X
750/150
pale yellow
890-920
15.5
130
X
750/200
pale yellow
900-970
15.8
90
585/40
reddish
900-920
13.0
130
X
X
X
585/45
(+Zn)
yellow
835-860
13.0
100
X
X
X
585/100
reddish
yellow
820-870
13.3
140
X
585/200
yellow
830-850
13.6
170
X
585/260
pale yellow
830-850
13.7
160
X
585/300
pale yellow
820-890
13.9
140
X
X
X
X
X
X
585/340
green yellow
860-940
14.0
120
X
X
X
X
X
X
585/415
green yellow
10151020
14.3
50
375/90
reddish
870-930
11.2
120
X
375/60
yellow
870-950
11.0
95
X
375/150
pale yellow
800-860
10.5
100
X
X
X
X
333/90
reddish
890-930
10.9
110
X
X
X
X
333/75
yellow
850-910
10.9
105
X
333/120
pale yellow
800-860
10.5
100
X
Fineness
Au/Ag,
(rest Cu)
colour
750/40
Applications*
1
2
3
4
5
6
7
X
8
9
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
*Applications: 1Rings and brooches; 2 Deep pulling work; 3 Pressing work; 4 Enamelling; 5 Casting; 6 Chains
and lattice work; 7 Blanks for minting; 8 Wedding bands; 9 Pipes and seamless
10
Dentistry
The precious metal alloys used in dentistry range from Ag-Pd-based ('low noble
alloys' to Au-Pt based ('high noble alloys'). These are used as restorative materials as well as
in applications such as prosthetic appliances including inlays, crowns, and bridges. Alloying
additions are used to enhance or control properties such as mechanical strength, castability,
chemical stability, biocompatibility and colour. As in jewellery, the Au-Ag-Cu system is the
basic coloured gold alloy used in dentistry. Au-Ag-Cu-PGM alloys have been developed to
enhance tarnish and corrosion resistance of gold-based alloys in the oral environment.
Platinum and palladium are the PGMs which are also usually contained in dental alloys. Low
carat gold alloys for dental applications might not contain copper, with typically palladium
and indium or tin (Au-Pd-In or Au-Pd-Sn) being used instead [18]. However, palladium may
cause a reaction in patients who are allergic or hypersensitive, particularly in Europe. Pd –
free Au-Pt based high noble dental alloys have therefore been investigated. Ag-Au-Pt-Cu has
also been studied for dental applications [19]. Furthermore, dental gold alloys containing
platinum but no silver or copper have also been developed. Au-Pt-X (X= In, Sn, Fe, Zn) and
Au-Pt-In-Y (Y=Sn, Fe, Zn) are examples of such dental alloys [20, 21].
Medical uses
Precious metals are excellent candidates for certain medical applications. Similarly to
dental application, biocompatibility, corrosion resistance and durability are the key for use in
medical implants of various types. However precious metals or their compounds have also
been used in biomedical fields such as therapeutics and diagnostics. For example, silver and
its compounds have bactericidal properties and can be used as an antibacterial agent in wound
care and medical devices in order to prevent the bacterial infections [22, 23]. Gold
compounds have been used in drugs for rheumatoid arthritis, anticancer and antimicrobial
activity [24, 25]. A platinum compound (cisplatin) is well-known and widely used for its anticancer effect. Platinum group metals, particularly platinum and platinum alloys, have also
been used in a wide range of medical devices, components and implants. (The Cochlear
bionic ear, for example, uses Pt electrodes). Pt-Ir alloy is used for electrode material in
medical appliances such as pacemakers, defibrillators and electrophysiology catheters. The
tip of the guidewire in balloon angioplasty is usually made of Pt-W while Pt-Co can be used
for small tubular devices or stents. Other metals in platinum group metals in wire form are
used as microcoils for neurovascular devices. Rhodium foil is used as a filter inside X-ray
mammography equipment [26, 27].
Electronics and electrical technology
Precious metals usually have good electrical and thermal conductivity properties with
silver being the best electrical and thermal conductor of all metals. The electrical conductivity
of silver is 6.30 x 107 S/m while gold is 4.10 x 107 S/m at 20 qC. Thermal conductivity of
silver and gold is approximately 428 and 318 W/m K respectively [3, 28]. Although gold is
11
not quite as good as silver, it has better corrosion resistance and remains an excellent
conductor of heat and electricity. The platinum group metals also have relatively good
electrical conductivity and durability. Precious metals and their alloys have been used in
electronic components (connectors, contacts, printed circuit boards or PCB), electrical
switches (switches, contacts, relays and circuit breakers) and electrical devices (e.g. multilayer ceramic capacitors which are crucial to functioning in cars, mobile phones, computers,
televisions and aircraft). The precious metals in electronic applications can be used in various
forms including bulk materials, bimetals, and surface coatings (electroplated, sputtered,
evaporated, pastes, lacquers and solder connections). AgPd alloys or even pure Pd are used as
conductive material in multi-layer ceramic capacitors. AgCu-base alloys are used for vacuum
brazing in electronics or as 'silver solder'. Other elements (Zn, Sn, Cd, Pd, Mn and P) are
added to the base alloy to yield compositions like AgCuZn, AgCuZn, AgCuZnCd and
AgCuP. Pure silver and its alloys including AgCu, AgNi, AgW, AgMo and composite
materials such as Ag-CdO, Ag-SnO2, Ag-ZnO, Ag-graphite, and Ag-WC are also used for
electrical contacts [10, 29, 30]. Gold thin and thick films are commonly used as conductors in
electronics. Gold wire ("bonding wire") is also used to interconnect microprocessor chips to
their 'packaging frame'. Gold and its alloys are also used in different applications and
conditions. AuNi, AuCo and AuAg alloys are used as contact materials for low voltage
switches while binary or ternary alloys of Au with Ag, Pd or Cu are used at higher voltages.
AuPt and AuAgPt alloys are also used for electrical contacts under heavily corrosive
conditions. AuCr2 is used as resistor material in corrosive environments and has a stable
temperature coefficient of resistance (TCR) between -20 and +40 qC [11, 13, 31].
Platinum group metals and their alloys have also demonstrated excellent properties
for use in some electronic applications. For example, PtCo alloy has good magnetic
properties and is a candidate for magneto-optic data storage and hard drives. PtRh alloys
serve as high temperature thermocouples and windings for high temperature due to their high
melting points and stability [17, 26, 32].
Catalysts
Precious metal catalysts based on palladium, platinum, gold, silver, rhodium and
ruthenium have been used in various industries due to their high activity, selectivity and
stability. Silver is an excellent oxidation catalyst while gold and platinum group metals are
used in a variety of reactions for applications including pollution control, chemical
processing and fuel cells. Catalysts may be used in the form of nanoparticles of precious
metals or their alloys and compounds, or as nanostructured films of precious metals or dealloyed sponges of precious metals (often with other transition metals). AuPd catalyst is now
used commercially in vinyl acetate production. Pt-Al2O3, PtIr and PtRe play a role in the
petrochemical industry while PtRh is mainly used as an automobile catalyst. PtCoCr, PtCoFe,
PtRu or PtNi alloys are cathode catalysts in fuel cells [11, 31, 33, 34].
12
2.2 Optical properties of materials
The optical properties of materials can be considered to describe how they respond
when exposed to electromagnetic radiation. In this section, the literature review is focused on
the optical properties of metallic materials in general. The optical and electronic properties of
precious metal alloys and compounds have been investigated in the past for both scientific
and practical reasons. There are two main groups of coloured precious metals: alloys and
intermetallic compounds. More specific details are reviewed in topic 2.3 (specific precious
metal alloys) and 2.4 (specific precious metal intermetallic compounds) respectively. Both
groups are attractive candidates for use in decorative arts, ornaments and jewellery. A more
specific review of optical properties of the coloured CaF2 structure of precious metal
intermetallic compounds, particularly PtAl2 and AuAl2, is provided in Chapter 4. Here we
consider the following general subtopics in turn: colour, CIE-L*-a*-b* colour coordinate
system, dielectric function and plasmon resonances in precious metal nanoparticles.
2.2.1 The colour of metallic materials
Gold and copper are the only two metallic elements that are not silver-coloured. A
range of interesting colours arise when they are combined with one another and/or with third
or fourth elements. The colour of metal alloys or compounds is a function of their reflection
and absorption properties which in turn are based on their electronic or crystal structure. For
this reason, the formation of intermetallic compounds between other elements, e.g. Pt and Al,
can cause the new phase to have a strong colour too, even though the constituent elements
were not coloured. The optical properties of materials are therefore related to the interactions
between the electromagnetic radiation and atoms, ions, and/or electrons. Thus, colour is
generated by interaction of light (which is an electromagnetic wave or photon) and matter in
the frequency range of visible spectrum (approximately from 390 nm to 700 nm). The
interaction of photons with the electronic structure of metal leads to optical phenomena such
as absorption and reflection which are explained in terms of electron transitions from one
energy state to another within the electron band structure of the material. Metals consist of
partially filled high-energy conduction bands. When photons in the range of visible light are
directed on the metal surface, the energy that they carry excites the electrons into unoccupied
energy states above the Fermi level (the maximum energy level of the occupied states) as
shown in Figure 2.1a. Metals appear opaque to the visible light because the absorption
normally takes place in very thin outer surface layer which is ~ 0.1 m. Thus only metallic
films which are thinner than this can transmit the light. Most of the absorbed radiation is
reemitted from the metallic surface in the form of visible light of the same wavelength as the
incident light. Reemission takes place by decay electron transitions in the reverse direction of
the transition considered before and it is shown in Figure 2.1b. The amount of energy
absorbed by metals depends on the electronic structure of each particular metal and the
perceived colour of metal is determined by the wavelength distribution of the reflected light.
Most metals are silvery in appearance due to the fact that these metals can strongly reflect
over the entire range of the visible spectrum. In other words, the wavelength composition of
reemitted photons of the reflected beam is approximately the same as the incident beam.
13
Copper appears red-orange while gold appears yellow since some energy associated with
photons having short wavelengths is not reemitted as visible light, therefore the resulting
emerging light does not contain those wavelengths and its colour changes accordingly. For
these two metals, the part of the incident radiation absorbed is short wavelengths such as
green and blue. These metals scatter the incident light predominantly in the yellow-orange,
orange and red wavelengths [35].
(a)
(b)
Figure 2.1 Schematic representation of the mechanism of photon absorption (a) and emission
(b) for metallic materials [35]
2.2.2 The CIE-L*-a*-b* colour coordinate system
The colour of materials is associated with the physics of electromagnetic radiation in
the visible range and the perception of colour by the human eye and brain. Sir Isaac Newton
developed the first circular diagram of colour (the colour wheel) which is based on red,
yellow and blue in 1666. Since then the colour wheel has been further developed, and the
current one includes hue, saturation and brightness to provide more details about colour. An
accurate colour measurement is required to define colour appearance or colour difference. In
this project the CIE LAB system is used to specify colour. The Commission Internationale de
l’Éclairage (CIE) is the international authority on light, illumination, colour and colour spaces
which was established in 1913. In the beginning, CIE XYZ colour space was used to
represent possible colours but the values of the XYZ coordinate system do not match the
sensitivity of the human eye very well. Consequently, CIE LAB or CIE L*a*b* was defined
in 1976. In this scale colour 'distances' are correlated with the sensitivity of the average
human eye. The CIE LAB model is shown in Figure 2.2. The three coordinates of CIE LAB
are the brightness or lightness of the colour (L*) and the chromaticity indices a* and b*. The
brightness increases from the bottom to the top (L*=0 indicate black and L*=100 indicates
14
white).The a* axis spans the range from green to red (negative values indicate green while
positive values indicated red) while the b* axis indicates the range from blue to yellow
(negative values indicate blue and positive values indicate yellow). The different systems can
be transformed each other such as RGB to XYZ or XYZ to CIELAB by colour conversion
algorithms [36, 37].
Figure 2.2 CIE L*a*b* colour space [38]
2.2.3 Dielectric function
The dielectric function (also known as the dielectric permittivity) of a material is
related to the ability of that material to be polarized by an electric field. The dielectric
function is a complex function, and like any complex number is composed by a real (Re) and
an imaginary (Im) part. Therefore the dielectric constant İ is actually a complex function of
the frequency (or photon energy) and can be written as the sum of a real part and an
imaginary part: H (Z) H1 (Z) iH 2 (Z) , where H1and H2 are two real numbers, the real and the
imaginary part of H, respectively and i is the imaginary unit, i 2
1 .
The dielectric function of an individual phase is closely correlated with its optical
properties, and is linked to its electronic configuration. Of course, alloys and compounds may
be present in a material as a single phase or as a mixture of phases. Consequently, the
dielectric function of both pure phase and mixtures of phases should be considered. For
example, the Au-Ag-Cu ternary alloy system contains solid solutions (Au-Ag and Au-Cu)
and a eutectic (Ag-Cu), which represent single- and two-phase compositions respectively.
The variation of the optical behaviour of the solid solutions (taken in isolation) is monotonic
as the composition changes, as shown in Figure 2.3 which shows the imaginary part of the
15
interband dielectric constant as a function of photon energy of Au-Ag (with varied Ag
content in at.%) and Au-Cu (with varied Cu content in at.%). For a solid solution or single
phase, the continuous shift of the absorption edge with concentration over the entire range is
displayed. The absorption spectrum moves and modifies its shape gradually as the
composition changes. However, once the compositional bounds of the solid solution are
exceeded the material will separate into two phases. The interband dielectric constant is
shown in Figure 2.4 for two cases for the Ag-Cu eutectic: Cu-rich and Ag-rich. In these
cases, the optical characteristic of the two phase composition is determined by volume
fractions of the two phases [39, 40]. Table 2.5 shows the onset of interband transitions in
some of the coloured intermetallic compounds which have CaF2 structure [41, 42]. The
imaginary part of the optical constants of pure phases can be calculated by the ab initio
method, and then the real part obtained by means of the Kramers-Kronig relations.
(b)
(a)
81 at% Cu
70 at% Cu
40 at% Cu
25 at% Cu
12 at% Cu
21 at% Ag
41 at% Ag
62 at% Ag
94 at% Ag
Figure 2.3 Imaginary part of the interband dielectric constant as function of energy (a) Au-Ag
and (b) Au-Cu [39]
16
(b)
(a)
94 at% Cu
43 at% Cu
30 at% Cu
8 at% Cu
6 at% Cu
55 at% Cu
Figure 2.4 Imaginary part of the interband dielectric constant and energy of Ag-Cu series (a)
Cu-rich and (b) Ag-rich [39]
Table 2.5 Onset of interband transition of selected coloured intermetallic compounds [41, 42]
Onset of interband transition, in electron volts (eV)
AuAl2
2.2
AuGa2
2.0
AuIn2
1.7
PtAl2
2.8
PtGa2
2.5-4.5
2.2.4 Plasmon resonances in precious metal nanoparticles
Nanoparticles of noble metals, in particular Au and Ag, have been studied for their
optical properties because they can support a plasmon resonance in the visible and nearinfrared regions of the spectrum. Plasmon resonances are optical phenomena that can occur
when conduction electrons couple with photons to produce an oscillation of charge at the
surface of a metal. The position of the resonance can modified by varying the nanoparticle
size and shape [43]. Moreover, Au and Ag can be easily formed into complex nanostructures
by chemical, physical and lithographic methods [44] which facilitates these studies. Some
17
studies by Blaber et al. have compared the different elements [45, 46]. Numerous hybrid or
alloyed nanoparticle systems, containing for example, Cu, Pt or Pd in addition to the Au or
Ag, have also been investigated [47-52]. The plasmon resonances of alloyed and intermetallic
nanoparticles are interesting because the behaviour of such hybrids is different from that of
the bulk materials. For example, Au and Pt have poor miscibility in bulk materials [53] but
they can form a solid solution in nanoparticles smaller than about 5 nm [54, 55]. A similar
result holds for Au and Fe [56]. In the case of ordered intermetallic compounds, Cu3Au is
normally ordered up to 240 qC in bulk materials [53] but if the nanoparticle size is less than 5
nm, ordered Cu3Au would not be formed [57]. Hybrid nanoparticles of the coinage metals
have been investigated due to the strong plasmon resonance of the constituent elements, with
Au-Ag and Ag-Au core-shell particles having been investigated since early work in 1964 by
Morris and Collins [58, 59]. Multi-shell structures such as Au-Ag-Au-Ag can also be formed
and they have interesting optical properties. For example, there is a sequence of colour
changes in multi-shell Au/Ag colloids. These start from gold colloid which is red, but after
overcoating with Ag the colloid becomes yellow due to the plasmon resonance being blue
shifted. Then Au-Ag-Au, produced by adding a layer of Au, has a deep blue colour due to
red-shifting of the resonance. Lastly, if a layer of Ag is added to make Au-Ag-Au-Ag,
resonance blue-shifts again to give an orange colour [59]. Moreover, hybrids with platinum
group metals including Au-Pd, Au-Pt and Ag-PGMs have been studied as well as hybrids
with magnetic metals Au-Ni, Au-Fe, Au-Co and Ag-Ni.
2.3 Specific precious metal alloys
The ternary phase diagram of Au-Ag-Cu (the three 'coinage metals') is the most basic
and best-known gold alloy system. The colours of these three elements are totally different.
The reflectance curves are shown in Figure 2.5. Reflectance of silver is close to 100 % over
the visible range thus silver appears white while reflectance of copper and gold is large at
yellow-red wavelengths and smaller in the blue-green region. Due to the difference in
position of the reflection edges of these two metals, copper is reddish/orange and gold is
yellow [60]. Varied colours, such as yellow, white, “red” and “green”, can be obtained using
different compositions of Au, Ag, Cu, sometimes with minor alloying additions of other
elements. Addition of copper makes the alloy reddish while addition of silver gives the alloy
a greenish to whitish hue, as shown in Figure 2.6. Some coloured rings from different
precious metal alloys are shown in Figure 2.7 [61]. Consideration of caratage is also
important in consideration of coloured gold alloys used in jewellery. A range of variously
coloured gold alloys can be produced at each caratage level [32-34], although there is
obviously more flexibility at the lower carat levels. Alloys at the 9 carat level can be “white”,
different shades of yellow from pale to rich yellow, greenish, and even pink and “red”, 14
carat gold alloys can be pale green, yellow and red while 18 carat gold alloys can be greenyellow, or different shades of yellow, pink and red. In 22 carat gold alloys the colours can be
yellow, deep yellow and pink, however, as gold content increases above 22 carat (22/24, or
91.6%Au), the yellow colour of gold usually dominates. The colour of some carat gold alloys
based on the Au-Ag-Cu system is shown in Table 2.6. Addition of up to 15 wt.% Zn in the
18
Au-Ag-Cu system can change the colour from red to reddish yellow or dark yellow.
Cadmium addition of up to 4 wt.% in the Au-Ag-Cu system can be used to make greenish
alloys. A light green 18 carat gold has been produced with 75Au-23Cu-2Cd while 75Au15Ag-6Cu-4Cd results in a dark green alloy. The colours that occur in this system can be
explained by band gap theory [33-36].
Ag
Au
Cu
Figure 2.5 Reflectance spectra of copper, silver and gold
Figure 2.6 Relationship between colour and composition in the Au-Ag-Cu system[62]
19
“White” gold is also considered in the jewellery industry to be a “coloured” gold, and
has been developed to compete with platinum. The “bleaching” of the yellow gold colour
with small alloying additions is caused by a change in the electronic structure. Silver, nickel,
zinc and palladium are usually used as "bleaching" agent with the use of the different
bleaching agents resulting in subtly different white hues. The reflectance spectra of gold
combined with various bleachers are compared in Figure 2.8. Figure 2.8(a) shows that the
reflectivity edge of silver with 50 at.% gold is in the green region of the spectrum while the
reflectivity edge for higher silver contents in gold alloys has moved into the blue and violet
regions so that almost the entire visible spectrum is reflected. In the latter case the alloy takes
on an increasingly white colour. The decolouring effect of additions of palladium is different
from that of silver: the reflectivity in the yellow region is decreased when adding more
palladium, as shown in Figure 2.8(b) [63]. In commercial compositions more than one
bleaching alloying addition may be made in order to improve the white colour. However,
other aspects such as workability, oxidation and cost must also be considered [64]. For
example, in the Au-Cu-Ni-Zn system, nickel is the primary bleaching agent, and it has a low
cost and strong bleaching effect. Zinc is the secondary bleacher. Unfortunately, some
individuals develop an allergic skin reaction to nickel and its use in jewellery is restricted in
some countries. Au-Pd-Ag is another system which contains white gold alloys. In this case,
palladium is the primary bleacher and silver is the secondary one. Silver is used because it
brings excellent workability and low cost, even though it is only a moderate bleaching agent
and has limitations in respect of sulphur tarnishing. Silver is therefore only used to bleach
low carat alloys [64, 65].
Figure 2.7 Comparison of coloured rings from different precious metal alloys [61]
20
Table 2.6 Coloured carat gold alloys based on the Au-Ag-Cu system [66]
Carat
Gold wt.%
Silver wt.%
Copper wt.%
Colour
91.6
8.4
-
Yellow
91.6
5.5
2.8
Yellow
91.6
3.2
5.1
Deep yellow
91.6
-
8.4
Pink/rose
75.0
25.0
-
Green-yellow
75.0
16.0
9.0
Pale yellow, 2N
75.0
12.5
12.5
Yellow, 3N
75.0
9.0
16.0
Pink, 4N
75.0
4.5
20.5
Red, 5N
58.5
41.5
-
Pale green
58.5
30.0
11.5
Yellow
58.5
9.0
32.5
Red
37.5
62.5
-
White
37.5
55.0
7.5
Pale yellow
37.5
42.5
20.0
Yellow
37.5
31.25
31.25
Rich yellow
37.5
20.0
42.5
Pink
37.5
7.5
55.0
Red
22
18
14
9
21
Curve 1
Curve 2
Curve 3
Curve 4
Curve 5
Fine gold
Silver with 50 at% gold
Fine silver
Curve 1
Curve 2
Curve 3
Curve 4
Curve 5
Curve 6
Fine gold
Gold with 5 at% palladium
Fine palladium
(a)
(b)
Figure 2.8 Reflectivity curves of gold and its alloys (a) gold-silver alloys and (b) goldpalladium alloys [63]
2.4 Specific precious metal intermetallic compounds
2.4.1 Colour of pure phases
Some precious intermetallic compounds have attractive fundamental properties such
as high hardness, high wear resistance, good oxidation resistance or specialized electronic
properties [67]. A few of them display beautiful colours that are not found in the metallic
elements [67, 68]. The coloured intermetallic compounds require a highly symmetrical
crystalline structure in order to ensure that electron band structure allows sp-d hybridization.
The coloured binary intermetallic compounds which have B2 (cP2 or CsCl) and C1 (cF12 or
CaF2) crystal structure the best-known examples [67, 68]. The crystal structure of CsCl and
CaF2 are shown in Figure 2.9 (a) and (b) respectively. Table 2.7 shows some examples of
coloured binary intermetallic compounds with these crystal structures. PdIn is red and PdMg,
PdAl are yellow in B2 type, PtAl2 PtGa2 and PtIn2 are yellow, AuAl2 is purple and AuGa2
22
and AuIn2 are blue in the C1 type. The reflectance spectra of purple gold and blue gold are
shown in Figure 2.10.
(a)
(b)
Figure 2.9 Crystal structures of coloured binary intermetallic compounds (a) Caesium
chloride structure and (b) Calcium fluoride (Courtesy CrystalMaker Software Ltd, UK)
The colour of intermetallic compounds can be modified in some cases by adding a
third element. For example, in the B2 compounds, indium in PdIn can be substituted by 100%
of Al. The colour alters from red (PdIn) to yellow (PdAl). Another way to change colour of
B2 aluminides is to increase the Pd content of PdIn. Replacement of In by higher valence
elements such Sn and Sb in In is shifts the colour from red to yellow to silvery. On the other
hand, lower valence elements including Ag, Cu and Mg can be substituted for In up to 10%
and the red colour becomes more intense. The C1 compounds behave differently to the B2
type due the more covalent nature of their bonding. Therefore, ternary elements can generally
only be substituted into C1 compounds to a very limited extent. There are, however, some
ternary and quaternary coloured intermetallic compounds that are related to the C1
compounds. These are the so-called Zintl derivative compounds, filled zinc blende structure
or AB2-derived compounds [68]. There are various elements that can be used to build Zintl
derivative compounds. The number of valence electrons (Nval) is related to the colour of the
compounds. A value of Nval from 5 to 8 is likely to vary the colour from yellow to red, violet,
and blue. The fcc lattice provides octahedral and tetrahedral sites. Generally, the octahedral
sites are occupied by metal or metalloid elements of Group 13-15, such as Sb, Bi, Si, Ge, Sn,
Pb, Al, or Ga (in which the valence electron count is 3-5) while tetrahedral sites are occupied
by late transition metals or noble metals of Group 8-11 such as Co, Ni, Ru, Rh, Pd, Ir, Pt, Cu,
Ag, Au. The metals of Group 1 and 2 such as Li and Mg may occupy both of octahedral and
tetrahedral sites. Some ternary and quaternary coloured intermetallic compounds based on the
precious metals are listed in Table 2.8 and 2.9.
23
Table 2.7 Coloured binary intermetallic compounds with CsCl and CaF2 structure [68]
CsCl
CaF2
Compound
Colour
Compound
Colour
FeAl
Brown
CoSi2
Dark blue
CoAl
Yellow
NiSi2
Gray blue
CoGa
Yellow
PtAl2
Yellow
NiAl
Blue
PtGa2
Yellow
NiGa
Bluish
PtIn2
Yellow
PdIn
Red
AuAl2
Purple
PdMg
Yellow
AuGa2
Blue
PdAl
Yellow
AuIn2
Blue
Figure 2.10 Reflectivity curves of coloured gold intermetallic compounds: AuAl2 (curve 1),
AuIn2 (curve 2) and AuGa2 (curve 3) [63]
24
Table 2.8 Ternary coloured intermetallic compounds based on a precious metal [68]
Compound
Colour
Nval
Compound
Colour
Nval
Ag Li2 Al
yellow-pink
6
Rh Li2 Ga
light-yellow
5
Ag Li2 Ga
yellow
6
Rh Li2 In
silver-coloured
5
Ag Li2 In
gold-yellow
6
Rh Li Al2
yellow
7
Ag Li2 Sn
dark red-violet
7
Rh Li Ga2
light blue
7
Ag Li2 Pb
blue-violet
7
Rh Li In2
silver-coloured
7
Ag2 Li Sn
light blue
7
Au Li2 Ga
green-yellow
6
Ir Li2 Ga
silver-coloured
5
Au Li2 In
green-yellow
6
Ir Li2 In
silver-coloured
5
Au Mg Sn
red-violet
7
Ir Li2 Sn
silver-coloured
6
Au Li Sb
red-violet
7
Ir Li Al2
red-violet
7
Au Li2Sn
pink
7
Ir Li Ga2
light violet
7
Au Li2Pb
violet
7
Au Li0.3 Al2
blue
7.3
Au Li0.5 Ga2
blue
7.5
Au Li0.6 In2
gray
7.6
Au Mg Sb
gray
8
Au Li2 Sb
bluish
8
Ru Li Al2
yellow
7
Ru Li Ga2
gray
7
Ru Li In2
silver-coloured
7
25
Table 2.8 Ternary coloured intermetallic compounds based on a precious metal (cont.) [68]
Compound
Colour
Nval
Compound
Colour
Nval
Pd2 Li Ge
brown-yellow
5
Pt Li2 Al
bright-yellow
5
Pd2 Li Sn
brown-yellow
5
Pt Li2 Ga
bright-yellow
5
Pd2 Li Pb
brown-yellow
5
Pt Li2 In
brass-yellow
5
Pd Li2 Al
rose
5
Pt Li2 Sn
yellow
6
Pd Li2 Ga
brass-yellow
5
Pt Mg Sn
reddish-brown
6
Pd Li2 In
brown-yellow
5
Pt Li Al2
copper-red
7
Pd Li2 Ge
yellow
6
Pt Li Ga2
brown-pink
7
Pd Li2 Sn
yellow
6
Pt Li In2
pink
7
Pd Li2 Pb
brown-yellow
6
Pt Li2 Sb
brass-yellow
7
Pd Mg Sn
brown-yellow
6
Pt Mg Sb
violet
7
Pd Li Al2
violet
7
Pd Li Ga2
silver-coloured
7
Pd Li In2
silver-coloured
7
Pd Li2 Sb
brass-yellow
7
Pd Mg Sb
violet
7
Pd Mg1.5 Sb
light violet
8
Pd Mg2 Sb
blue-gray
9
26
Table 2.9 Quaternary coloured intermetallic compounds based on a precious metal [68]
Compound
Colour
Nval
Pd Li Mg Sn
red-violet
7
Pd Li Mg Sb
gray-blue
8
Pt Li0.5 Mg Sn
dark red
6.5
Pt Li0.5 Mg0.5 Sn
pink
6.5
Pt Li Mg Sn
copper-red
7
Pt Li Mg0.5 Sb
copper-red
7
Pt Li0.5 Mg Sb
dark blue-violet
7.5
Pt Li Mg Sb
red-violet
8
Ir Li Mg Sn
gray-blue
7
Au Li Mg Sn
gray
7
2.4.2 Alloying effects
The addition of alloying elements plays a role to enhance the properties of precious
metal alloys and compounds as well as to change their colour. As mentioned above, in the
well-known alloys based on Au-Ag-Cu, the typical alloying elements used for a colour effect
are copper (reddening), silver (greening) and zinc, nickel and palladium (whitening).
However, colour changes in other precious metal alloys and compound systems have also
been investigated. The Au-Cu-Al alloy known as Spangold is interesting because of its
surface texture. There are two different colours based on 18 carat gold alloy. The alloy 76%
Au-19%Cu-5%Al (wt.%) is yellow while the alloy of 76%Au-18%Cu-6%Al is pinkish [69].
The colour of Au-Cu-Al systems have been studied by measuring and plotting CIE a* and b*
colour coordinates of compositions along the 18 carat pseudo-binary as shown in Figure 2.11.
Starting from Au-Cu, the Į-phase alloys show a red-gold colour, which changes to yellow
when Al content increases. When the composition reaches the ȕ-phase, the yellow colour
shifts to a pink/apricot colour. At higher Al content, when the Ȗ-phase replaces the ȕ-phase,
the colour turns to silver. Finally, some purple AuAl2 can be formed in this system. However,
the end point of the 18 carat pseudo-binary is actually a two phase Al + AuAl2 mixture, in
which the colour will be lighter purple due to the bleaching effect of the Al. Another
composition, 61%Al-24%Cu-15%Au contains the phase AuAl2, CuAl and CuAl2. These
phases are respectively pink, tan and yellow as seen with light microscopy [70].
The optical properties and effect of indium additions on Au-Pt based dental alloys
were studied [20]. The samples were prepared by melting in a high-frequency induction
27
furnace, then cold rolled followed by a homogenizing heat treatment at high temperature.
There were two systems of sample alloys with different chemical composition. The first
system was binary Au-Pt (AP5: 95 at%Au-5 at%Pt and AP10: 90 at%Au-10 at%Pt). Another
system was ternary Au-Pt-In (AP10-In2: 88at%Au-10 at%Pt-2 at%In and AP10-In4: 88
at.%Au-10 at.%Pt-4 at.%In). The spectral reflectance for the incident CIE standard illuminant
D65 was collected as a function of the wavelength at 10 nm intervals from 360 to 740 nm.
Three coordinates L*, a* and b* colour space were determined. The reflectances of Au, Pt
and binary Au-Pt alloys were compared. The pronounced step near 520 nm (approximately
2.4 eV) in the spectral reflectance-wavelength curve for pure gold, which is responsible for
the rich yellow colour of gold, became less pronounced with increasing platinum content.
This is due to the decolouring (“bleaching”) effect of Pt on Au. In ternary Au-Pt-In, the
addition of a small amount of indium to Au-Pt alloy increased the reflectance in the longwavelength range and decreased the reflectance in the short-wavelength range of the visible
spectrum. The number of valence electrons per atom (e/a) of the ternary Au-Pt-In increased
with increasing indium content. This change caused an increase in both chromaticity indices
including a* and b* values, which gave a gold tinge to the alloy.
In later research, Au-Pt-based ternary alloys containing a small amount of In, Sn, Fe,
Zn and quaternary alloys containing 2 at% In and a small amount of Sn, Fe, Zn were
investigated, with a focus on colour variations by CIELAB, chroma and hue values[21]. The
addition of Sn, Fe, In slightly increased chromaticity index a* which giving a very light tint
of red but significantly increased chromaticity index b* which increased yellow colour. This
effect was strongest in Fe, and the addition of Fe only was obviously sufficient to increase
lightness. Fe was therefore the most effective addition to give a bright yellow colour. On the
other hand, the addition of Zn did not change colour coordinates in these alloys. The chroma
depended on chromaticity index b*, and the addition of Fe increased the chroma most
effectively, followed by Sn and In additions. The chromas in the quaternary alloys were
greater than those ternary alloys. The hue angle, h was not influenced by the alloying
addition. Another quaternary Ag-Au-Pt-Cu alloy system was also investigated [19]. The
samples of quaternary Ag-Au-Pt-Cu containing 10at% Pt and 10-35 at% Au were prepared in
a high frequency induction furnace under an argon atmosphere. The optical properties and
microstructures were analysed by spectrophotometric colorimetry, optical microscopy and
electron probe microanalysis. The Ag-Au-Pt-Cu alloys annealed at 850 °C were composed of
a major phase of the Ag-Au rich matrix and minor phase of the Pt-Cu rich small grains
embedded in the matrix. The Pt-Cu rich phase was almost colourless. The Ag-Au rich matrix
therefore controlled the colour of the quaternary alloys. Three-dimensional colour
coordinates including L* (lightness), a* (red-green) and b* (yellow-blue) in CIELAB varied
as Au/Ag atomic ratio in alloy. Increasing Au/Ag atomic ratio from 0.130 to 0.996, the L*
value decreased from 90.4 to 86.9 and b* value increased from 8.0 to 14.4 whereas the a*
value did not vary significantly. This gave a pale yellow colour. It also found that the chroma
(C*) is mainly caused by systematic increase in yellowness of the Ag-Au-Pt-Cu alloys with
increasing Au/Ag atomic ratio.
28
The colour of intermetallic compounds can be changed with temperature. The colour
of (Cu, Ag, Au)-(Zn,Cd) E-brasses in different temperature has been studied [71]. These
materials are based on the Hume Rothery E electron compound and its associated martensite.
They are yellow, red and pale-gold at liquid nitrogen condition but change to pink, purple and
violet at room temperature, then their colour shifts to red for common brass and grey for the
other alloys at 300 qC. The colour change is due to the effect of thermal vibration of the
crystal lattice on the energy levels of band electrons [68].
Figure 2.11 CIE a* and b* colour coordinates of alloys along the 18 carat pseudo-binary, and
position of phase fields [70]
2.5 Nanoporous precious metal sponges
Porous materials with small pore sizes and large effective surface areas are attractive
as advanced materials in multiple applications and many industries including
microelectronics, manufacturing, medicine, clean technology, and environment. Porous
metals in particular may find use in important applications such as catalysis, energy
conversion and storage systems, electrochemical sensors, surface-enhanced Raman scattering
(SERS) and plasmonics [72, 73]. This is because of their enhanced ability to interact with
atoms, ions and molecules on their surfaces. Pore size plays an important role in advanced
science and technology [74]. According to the International Union of Pure and Applied
Chemistry (IUPAC), porous materials are divided into three categories according to their
pore size: microporous materials (0.2-2 nm), mesoporous materials (2-50 nm) and
macroporous materials (50-1000 nm). However, the term “nanoporous” is also often used and
refers to porous materials with pores that are in the range of diameter between 1 and 100 nm
[74, 75]. Pores also have various shapes and morphology such as cylindrical, spherical,
hexagonal and slit types [74] . Porous materials can be produced from various materials
including organic materials, inorganic materials, organic-inorganic hybrid materials, metals,
metal oxides, carbon and polymer [73, 76]. Table 2.10 shows classification of nanoporous
materials and their properties in comparison. Porous metals, the subject of this review, may
29
be prepared by techniques such as galvanic replacement reaction, the combination of block
copolymer template with deposition, chemical reduction of metal ions, hydrothermal
synthesis method, powder metallurgy, filter casting, potential-controlled anodization,
electroplating, electrodeposition, surfactant emulsion template, wet-chemical strategy and
template-printing method. However, de-alloying is the most popular method for preparing
nanoporous metals because it is a simple and effective method for fabricating structures with
a three-dimensional (3D) bicontinuous nanostructure and open nanoporosity, large surface
areas and excellent physical, mechanical and chemical properties [76, 77].
De-alloying is defined as an alloy corrosion process in which a selective electrolytic
dissolution of one component from solid solution takes place. The less noble metal is
removed and the more stable element is retained in a porous structure. The crucial factors to
be considered when attempting to form nanoporous structures by de-alloying are whether the
precursor is a homogeneous single phase (desirable), there is sufficient galvanic series
difference of the alloy components (essential), that the composition is above the parting limit
(essential) and that rapid surface diffusion of the noble metal components can take place
essential) [78, 79]. There are various examples of nanoporous metals (NPMs) fabricated by
the de-alloying method. Nanoporous metals are divided into three main groups: three
dimensional nanoporous metals, low-dimensional nanoporous metals and nanoporous metalbased composites. A classification of these nanoporous metals is shown in Figure 2.12 [76,
80]. In this chapter we focus on nanoporous metals, particularly nanoporous precious metals
including nanoporous gold (np-Au), nanoporous silver (np-Ag), nanoporous platinum (np-Pt)
and nanoporous palladium (np-Pd).
Table 2.10 Classification of nanoporous materials and their properties [74]
Pore size
Polymer
Carbon
Glass
Meso-macro
Micro-meso
Meso-macro
Micro-meso
Meso-macro
High
0.3-0.6
Low
0.3-0.6
Medium
0.3-0.6
Low
0.1-0.7
Low
Surface
area/Porosity >0.6
Oxides
Metal
Permeability
Low-medium Low-medium High
Low-medium High
Strength
Medium
Low
Strong
Weakmedium
Strong
Thermal
stability
Low
High
Good
Mediumhigh
High
Chemical
stability
Low-medium High
High
Very high
High
Costs
Low
High
High
Medium
Medium
Life
Short
Long
Long
Long
Long
30
Nanoporous metals (NPMs)
Low-dimensional nanoporous metals
Three-dimensional nanoporous metals
Nanoporous metals
Nanoporous nanowires
(np-Au, np-Ag, np-Cu, np-Pt, np-Pd, np-Ni)
(np-Au, np-PtCo, np-PtNi)
Nanoporous alloys
-
np-Au-based alloys
(np-AuAg, np-AuPd)
np-Pt-based alloys
(np-PtAu, np-PtRu, np-PtNi)
Nanoporous nanotubes
(np-Au, np-AuAg)
Nanoporous nanoparticles
Nanoporous nanoshells
Nanoporous metal-based composites
Np-Au-based composites
-Metal-decorated np-Au composites
(Pt@np-Au, Ag@np-Au, Au@np-Au, Ni@np Au,
Sn@np-Au)
-Metal oxide-decorated np-Au composites
(TiO2@np-Au, Al2O3@np-Au, MnO2@np-Au)
-Polymer-decorated np-Au composites
(PPy@np-Au)
Np-Cu-based composites
(Au@np-Cu, Ag@np-Cu, Nanotubular
mesoporous PtCu and PdCu)
np-Pd-based alloys
(np-PdCu, np-PdNi)
Np-Ag-based composites
(Nanotubular mesoporous PtAg and PdAg)
Figure 2.12 Classification of nanoporous metals[76]
31
2.5.1 Nanoporous gold (np-Au)
Nanoporous gold has a long history as it was used in an ancient technique for gilding
Au-Cu items by the Indians of pre-Columbian Central America: copper was removed or
converted to a surface film of copper oxide and, after annealing, the spongy gold-rich surface
regions remaining after de-alloying became a shiny gold layer [81]. In the 1960s, Au-Cu was
the focus of a transmission electron microscopy (TEM) study into corrosion by Pickering and
Swann [82]. The Au-Cu binary alloy system is suitable for gaining a fundamental
understanding of the selective dissolution process due to solid solution miscibility of Au and
Cu over the entire composition range. It is known that the nature of the electrolyte is one of
the important factors for the de-alloying process. The interaction between alloy and
electrolyte influences the atomic dissolution and diffusion. De-alloying in the Au-Cu system
has been studied in several electrolytes including sodium chloride (NaCl) [83], sodium
sulfate (Na2SO4) and sulfuric acid (H2SO4) [84-86], and nitric acid (HNO3) [87, 88].
Similarly to the Au-Cu system, the Au-Ag system is also able to generate nanoporous gold.
Actually, the Au-Ag system is now probably the most well-known model system for
understanding de-alloying mechanisms [89]. In 1970s, de-alloying Au0.50Ag0.50 alloy in nitric
acid was studied by Forty [81]. Ag is first dissolved in the electrolyte and gold forms a
porous structure which shows a morphology of gold islands and silver channels as shown in
the model in Figure 2.13 (a). Later Erlebacher et al. showed that de-alloying Au0.26Ag0.74 in
nitric acid also forms a porous network of interconnected ligaments, shown in Figure 2.13 (b)
[89]. Modeling and simulation are employed to study the formation of the 3D nanoporosity as
shown in Figure 2.13 (c) The 3D image also shows the interconnections of the gold ligaments
and nanopore channels [89, 90].
(b)
(a)
(c)
Figure 2.13 Nanoporous gold by dealloying Au-Ag (a) Model for dealloying [7], (b) SEM
micrograph of nanoporous gold by dealloying Au-Ag in nitric acid [89] and (c) Simulated
porous structure of nanogold which made from Au0.35Ag0.65 precursor [90]
Another early study into the de-alloying of Ag from Au-Ag alloy was conducted by
Sieradzki and Newman [91]. There are many subsequent publications which deal with the
parameters for generating nanoporous gold by de-alloying Au-Ag precursor. These show that
it is possible to control the pore size and morphology, which are related to the surface area
and which, in turn, affect the chemical, physical and mechanical properties of nanoporous
32
metals. The main parameters for controlling nanoporosity are applied potential [92-94],
electrolyte pH and concentration [94-96], composition of noble metals [95, 97-100]
dealloying time and temperature [94, 101, 102] and annealing treatment [98, 103, 104].
Apart from Au-Cu and Au-Ag, np-Au may also be produced by de-alloying many
other Au-based alloys. For example, the Au-Zn system may be used to fabricate nanoporous
gold. The electrochemical alloying and de-alloying methods are general methods for
preparing nanoporous gold from Au-Zn [105-107]. The Au-Al system is one of the other
interesting candidates, and there have been several studies of nanoporous sponges made from
it. Cortie et al. produced mesoporous gold sponges by de-alloying an AuAl2 precursor in
alkaline solution. The AuAl2 precursor could be either a bulk materials or a thin film. The
bulk AuAl2 samples came in solid buttons which were produced by a vacuum-arc furnace
while AuAl2 thin films were produced by dc magnetron sputtering technique. The gold
sponges have a high specific surface area, with an average pore diameter of 2-20 nm [108,
109]. Zhang et al. reported that nanoporous gold ribbons can be fabricated from Al-Au alloys
through rapid solidification and de-alloying as an alternative simple method for nanoporous
gold. The morphology of nanoporous gold ribbons depends on the phase composition [110,
111]. Wang et al. investigated the formation of nanoporous gold by de-alloying AuAl2
precursor in various kinds of inorganic and organic acid solutions which can influence the dealloying process and morphology. It was found that the de-alloying kinetics in organic acids
are slower than in inorganic acids [112].
2.5.2 Nanoporous silver (np-Ag)
Nanoporous silver may become useful for some commercial applications because it is
cheaper than other precious metals. There may, for example, be possibilities in regard to
optical applications [113, 114]. The Ag-Al binary alloy is an efficient precursor for
fabricating nanoporous silver by de-alloying. Zhang et al reported that nanoporous silver
films were fabricated using a DC magnetron co-sputtering to deposit Ag and Al, followed by
chemical de-alloying. After de-alloying, a uniform 3-D open continuous porous network with
ligament and channel width between 10 nm-50 nm was formed [115]. Nanoporous silver has
also been produced by de-alloying of melt-spun Ag-Al alloys in acidic or alkaline solution
[116-119]. It was found that a nanoporous silver ribbon with open, bicontinuous ligamentchannel structure was generated. As with np-Au, the composition of the precursor alloys and
de-alloying solutions has some influence on the formation and morphology of the sponge.
Precursors in the Ag-Al system usually contain two phases: D–Al solid solution (which can
contain up to 25 at.% Ag at elevated temperatures) and Ag2Al intermetallic compound, with
40-100 at.%Al overall. The dissolution rate of D–Al was faster than Ag2Al at room
temperature. Consequently, nanoporous silver structure was mostly formed in the former D–
Al regions from their content of dissolved Ag. SEM images in figure 2.14 show
morphologies of the two phases after the de-alloying process. A higher de-alloying
temperature could leach more Al out of the Ag2Al [120].
33
(a)
(b)
Figure 2.14 Nanoporous silver generated from Ag-Al precursors (a) D–Al rich region and (b)
D–Al and Ag2Al region [116]
Solution concentration and de-alloying time are two important factors to control the
pore size of nanoporous silver. Qiu reported that a higher acid concentration or a shorter dealloying time resulted in smaller pores within nanoporous silver. A smaller pore gave a better
result for Surface-Enhanced Raman Scattering (SERS) [121].
The Ag-Cu system is also suitable for use as a precursor binary alloy for producing
nanoporous silver by de-alloying, even though, under equilibrium conditions at room
temperature, these two elements exhibit only negligible mutual solid solubility. There are
several methods for fabricating nanoporous silver from Ag-Cu alloy but a key first step is to
prepare a metastable solid solution of the two elements. This could be achieved by
mechanical alloying [122] or rapid solidification [123]. Thereafter the Cu can be selectively
removed by, for example, supercritical CO2 solvent [114] or electrochemical selective
dissolution [123]. The Ag-Zn binary alloy is another alternative system to produce
nanoporous silver film, in this case by two steps of electrochemical formation followed by
electrochemical de-alloying [106, 124]. Similar methods have been used to obtain
nanoporous silver with the Ag-Sn system [125]. Apart from binary metal systems, some
metal/salt composites such as Ag/CaCO3 have also been reported to be possible precursors
for fabricating nanoporous silver [126].
2.5.3 Nanoporous platinum (np-Pt)
Nanoporous platinum is another attractive nanoporous precious metal, particularly
because of its catalytic properties. Pickering et al investigated Pt-Co and Pt-Fe exposed to
HCl gas at elevated temperatures and found that microporous platinum was formed [127].
Pugh et al. successfully produced nanoporous platinum with a pore size of approximately 3.4
nm by electrochemical de-alloying of Pt-Cu foil in H2SO4 [128]. Pt0.25Cu0.75 was a suitable
precursor for the purpose of achieving this pore size. Nanoporous platinum with even higher
surface area can be fabricated by a two-step process at room temperature which is related to
the electrodeposition and electrochemical de-alloying from Pt-Cu alloy [129, 130]. Similarly,
34
in Pt-Zn alloy a two-step process can be used to produce a nanoporous platinum with high
surface area [131]. Thorp et al found that amorphous PtxSi1-x films can also be used as
precursor to produce nanoporous platinum by electrochemical de-alloying in hydrofluoric
acid (HF) solutions [132]. A pore size of 5-20 nm was produced. Antoniou et al investigated
the morphologies of nanoporous platinum formed from the Pt-Si system by varying the
deposition parameters including initial composition, thickness, and sputter bias conditions.
They found that there are three different morphologies: isotropic open cell foam, anisotropic
columnar and Voronoi-type foam [133]. Examples of these nanoporous platinum
morphologies are shown in Figure 2.15. The Pt-Al system is another interesting precursor for
generating nanoporous platinum by de-alloying. In this case a parting threshold at about
Pt0.40Al0.60 in NaOH was observed [134].
Figure 2.15 Nanoporous platinum produced by co-sputtered PtxSi1-x amorphous film for
different initial compositions (a) Pt0.10Si0.90 as deposited (b) isotropic open-cell foam (c)
Pt0.34Si0.66 as deposited (d) anisotropic columnar-type foam (e) Pt0.33 Si0.67 as deposited and (f)
anisotropic Voronoi [133]
2.5.4 Nanoporous palladium (np-Pd)
Nanoporous palladium can be formed by chemical de-alloying of binary alloys in
processes analogous to those discussed above for the other precious metals. There are two
factors to be considered for nanoporous palladium formation. Firstly, the standard electrode
potential of the sacrificial element must be lower than that of palladium in order to obtain
dissolution easily. Another factor is that the binary alloy should form a single solid solution
phase in precursor. Therefore, iron (Fe), cobalt (Co) and nickel (Ni) were selected for a study
by Hakamada et al. [135-137]. Bulk samples of these binary alloys were prepared by arc
melting, followed by electrochemical de-alloying in H2SO4 solution. It was found, for
example, that Pd0.20Co0.80 could be de-alloyed to form a nanoporous structure with pore and
35
ligament size of 15-20 nm (figure 2.16a). The thermal annealing could be used to increase the
ligament size up to 650 nm. The electrodeposition of Pd-Co alloys followed by
electrochemical de-alloying is another technique for fabricating nanoporous palladium [136,
138]. Chen et al. found that Pd0.20Ni0.80 alloys could generate nanoporous palladium (Figure
2.16b) too and that the residual Ni in the nanoporous structure could be tuned by control of
the electrochemical potentials during the de-alloying process [139]. Li and Balk prepared PdNi alloy films by magnetron co-sputtering then de-alloyed them in H2SO4. The alloy
compositions and de-alloying conditions were varied to obtain a uniform nanoporous
palladium film with a sponge-like and interconnected ligament structure. The ligament and
pore size in this work was approximately 10 nm [140] but some refinement of the ligament
and pore size can be achieved by adding surfactants during de-alloying process [141]. The
use of Pd-Cu alloys as precursor for nanoporous palladium has also been considered. There is
only a relatively small difference in the galvanic series between Pd and Cu, therefore, the
important factor is to keep a constant ratio between Cu leaching and Pd diffusion. It was
found that de-alloying in an ionic liquid was suitable for obtaining nanoporous palladium
(Figure 2.16 (c)) [142]. Finally, Pt-Al alloy precursors have also been studied, even though
Al-based alloys are usually formed as intermetallic phases which some report as preventing
the formation of nanoporous structure. However, nanoporous palladium with ultrafine
ligament (3–6 nm in size) was produced by de-alloying of an Al–Pd alloy in an alkaline
solution [143-145].
.
(a)
(b)
(c)
Figure 2.16 Nanoporous palladium by dealloying in various precursors (a) Pd-Co [146] (b)
Pd-Ni [139] and (c) Pd-Cu [142]
36
Chapter 3
General Experimental
37
3.1
Overview
Precious metal alloys and compounds can be produced by several methods. These
methods include conventional casting, vacuum arc melting or powder metallurgy for making
bulk samples, and chemical vapour deposition (CVD) and physical vapour deposition (PVD)
for making nanoscale films. Thin film technology is an attractive method to produce
nanoscale materials due to its low materials cost and reduced energy consumption compared
to the production of bulk materials. In addition, nanoscale materials are interesting because
their properties are sometimes different from bulk materials. Thin films have been widely
used in electronic devices, optical coating, instrument hard coating, sensors, solar cell,
magnetic materials, catalysts and decorative applications [147-154].
There are two main processes of thin film deposition: CVD and PVD [155]. There are
several CVD processes such as Atmospheric Pressure Chemical Vapour Deposition
(APCVD), Low Pressure Chemical Vapour Deposition (LPCVD), Metal-Organic Chemical
Vapour Deposition (MOCVD), Plasma Enhanced Chemical Vapour Deposition (PECVD),
Laser Chemical Vapour Deposition (LCVD), Photochemical Vapour Deposition (PCVD),
and Chemical Beam Epitaxy (CBE) [154, 155]. The PVD process options are divided into
two categories: evaporation and sputtering. Sputtering is a unique deposition process. This
type of deposition uses high energy gaseous atoms to eject atomic sized solid materials from
a source and transfer them to form a surface layer on a substrate. The technique of magnetron
sputtering is very useful for fabricating thin films of elements, alloys or compounds, and two
materials can be readily co-sputtered if needed. The thin films which are obtained by
sputtering have high purity, fully dense microstructure, uniformity and good adhesion.
Magnetron sputtering can be reproduced with a high degree of control.
Therefore, in the experiments described in the present PhD, precious metal alloys and
compounds have been mainly fabricated as thin films by PVD with the magnetron cosputtering technique. Sputtered films are generally poorly crystalline if deposited at room
temperature and, in most cases heat treatment has been applied here during deposition or after
deposition. These precious metal alloy and compound films were characterized by a variety
of techniques for chemical composition, crystal structure, morphology and optical properties.
There are two main groups of precious metals alloys and compounds in this doctoral project.
These are (a) the intermetallic compounds of AuAl2 and PtAl2 and (b) the many intermetallic
compounds in the Pt-Al system. The present chapter explains the general experimental
techniques which are common to all the chapters of this thesis. Specific details relevant to
individual alloy systems will be given in their own chapters.
38
3.2
Materials preparation
3.2.1 Magnetron sputtering
Magnetron sputtering is one of the methods of physical vapour deposition, and is used
worldwide for fabricating thin films. The process is conducted in a vacuum chamber which
contains the target (cathode) and substrate (anode). Magnetic fields are applied to the system
to trap and control the path of electrons. A low pressure of inert gas, typically argon, is
flowed into the chamber during the sputtering process. Positively charged gas ions (Ar+ ion)
are accelerated towards the target material and eject atomic sized particles from it. These
atomic-sized fragments then travel to the substrate and deposit there to form thin films. The
mechanism of magnetron sputtering is shown in Figure 3.1. In this project, the focus is alloys
and intermetallic compounds of precious metals including Pt-Al, Au-Al and Pt-Ag. A codepositing technique with direct current magnetron sputtering is suitable to produce and
investigate these alloys and compounds. Two targets were set up in the chamber at a distance
of 150 mm from the substrate holder. The substrate holder could be rotated to produce
homogenous thin films. The base pressure of system was better than 10-6 Torr and during
sputtering the pressure of flowing argon was maintained at 10-3 Torr. Separate DC power
supplies were connected to each target. Current or power level on each power supply could
be adjusted to produce desired stoichiometry and deposition rate. The ratio of atoms removed
from the surface of solid to the number of incident ions is known as sputter yield and is
different for each element. Sputter yields of selected metals are shown in Table 3.1. The
variation with elemental atomic number is compared in Figure 3.2. A quartz crystal
microbalance sensor and monitor were used for calibrating deposition rate and thickness of
each pure element before co-sputtering was undertaken. Figure 3.3 shows the set up for the
co-deposition technique inside the chamber during sputtering process.
Figure 3.1 A schematic diagram of magnetron sputtering mechanism
39
Table 3.1 Sputter yields (atoms/ion) as a function of argon ion energy of selected metals
[154]
Metals
Ag
Al
Au
Pt
Argon Ion Energy
500 eV
2.5
0.9
1.7
1.0
300 eV
1.7
0.6
1.1
0.7
1000 eV
3.5
1.5
2.5
1.6
Figure 3.2 Relationship between sputter yield and atomic number of elements for argon ion
energy at 400 eV [155]
Target
Target
Rotating substrate holder
Figure 3.3 A co-deposition technique during sputtering process, photograph taken during the
present project
40
Several substrates have been used in the present project, including microscope glass
slides, silicon wafers, tantalum foil and austenitic stainless steel foil. The microscope glass
slide were cleaned by sonication in detergent solution for 15-20 minutes, then rinsed with
water, then rinsed with distilled water and finally being dried under N2. The silicon wafer and
metallic substrates were cleaned by sonication in acetone, then rinsed with acetone or ethanol
and, finally, dried under N2.
3.2.2 Heat treatment
The formation of thin films is associated with nucleation and growth processes. There
are three main steps: (1) a target of the chosen material is used to generate a flux of
appropriate atomic or molecular species; (2) these species are transported to the substrate and
(3) a solid deposit is formed on the substrate. In many cases an annealing treatment is
required to crystallize the sample, remove defects, and grow the grains. Annealing may also
cause subsequent phase transformations and colour changes. I used two different methods of
annealing in the present project.
The first method used halogen lamps located within the chamber to heat the sample during
deposition. Thermocouples were used to monitor and control the temperature. Thin film
samples were then heated up to an appropriate temperature during the sputtering process.
Another method was to anneal the sample using a tube furnace. This method is a postdeposition process. After deposition, thin film samples were cut into small segments, covered
with a protective piece of microscope glass slide and wrapped with aluminium foil. Then the
wrapped samples were placed in ceramic boat crucible. The tube furnace was then purged
with a stream of N2 gas during annealing. Copper shavings were also placed in the tube
furnace in order to reduce oxidation in the samples. The tube furnace set up for postdeposition annealing treatment is shown in Figure 3.4.
Figure 3.4 Tube furnace for post-deposition annealing treatment
41
3.3
Materials characterization
Thin films of precious metal alloys and compounds were fabricated using several
deposition conditions. In many cases these samples also had various forms of post-deposition
processing. The microstructures and properties of these nanoscale films have been
investigated by various techniques including X-ray diffraction (XRD) - using both a
laboratory instrument and synchrotron radiation, scanning electron microscopy (SEM), and
transmission electron microscopy (TEM). Spectrophotometry, ellipsometry and computer
modelling were used for investigating optical properties.
3.3.1 X-ray diffraction
X-ray diffraction is a non-destructive analytical technique for the study of crystal
structures and atomic spacing. XRD is also commonly used for identification of crystalline
materials and analysis of the proportion of phases present. The interaction between X-rays
and electrons of atoms leads to wave interference and the phenomenon of diffraction which
can be expressed by Bragg’s law, nO 2d sin T , where n is an integer, O is the wavelength of
the incident x-ray beam, d is the spacing between the planes in atomic lattice and T is the
diffraction angle. Constructive interference or diffraction only occurs when Bragg’s law is
satisfied. A diffraction pattern is obtained by diffraction from different planes of atoms at a
specific angle. Both the angle of diffraction and the intensity of the diffracted peak are unique
for each material and can be identified like fingerprints. Diffraction patterns are usually
plotted as intensity against Bragg angle (2T). An unknown sample is primarily identified by
comparing with known standards in the power diffraction database which contains basic
information including chemical formula, name of substance, crystallographic data, data on
specimen and data on diffraction pattern [156, 157].The significant diffraction pattern
components are peak position, peak intensity and peak shape. These three components are
related to the crystal structure of material including unit cell parameters, atomic parameters
and crystallinity. Peak width (often taken as Full Width at Half Maximum (FWHM)) is also
important for estimating particle or grain size and residual strain. It can be obtained by fitting
a peak with Gaussian, Lorentzian, Gaussian-Lorentzian functions. Generally, a source of Xrays and a suitable detector are required for the power diffraction technique. The X-rays are
usually generated using one of two different sources. The first is laboratory X-ray
instrumentation or conventional X-ray source which is the most commonly used source of xrays. These devices use an X-ray sealed tube of a specific metal target to generate
electromagnetic radiation. The second source is synchrotron radiation which is generated by
high energy electrons which are confined in a storage ring. Those electrons accelerate in a
circular orbit and emit electromagnetic radiation [158, 159]. In this project, both laboratorybased instrumentation and synchrotron radiation were used for analyzing samples. The
Rietveld refinement technique has been used for analyzing X-ray powder diffraction data as
well as determining lattice parameters.
42
Laboratory X-ray instrumentation
The laboratory-based instrumentation or diffractometer consists of a source of
radiation, a monochromator to ensure a single wavelength, slits to adjust the shape and size of
the beam, a sample, a detector to record the diffraction pattern and a goniometer to adjust the
sample and the detector positions. The sample rotates at a constant angular velocity while
detector rotates at double angular velocity around the sample in order to detect Braggreflected waves. A schematic of a X-ray diffractometer is shown in Figure 3.5. A Siemens
D5000 X-ray diffractometer which used Cu KD radiation (where Ȝ is taken as 1.54184 Å) has
been used for this project (actually there are two closely situated wavelengths KD1 and KD2
but in most cases in the present project the peaks are broad enough to obscure this fact). The
grazing incidence mode has been applied for the thin film samples. The grazing incidence
technique uses a very small incident angle of X-ray beam on the specimen (typically 0.1q3.0q) in order to obtain as much signal as possible from the thin film and as little as possible
from the substrate of the film. The diffracted beam will be parallel focused onto slits in front
of a detector [160]. Figure 3.6 shows a comparison of the basic principle of grazing incidence
technique for thin film geometry and the conventional T-2T geometry. A scintillator with
photomultiplier was used as a detector. In this research, the angle of incidence was set at 0.5q
while the divergence slit was 0.2q. The diffractometer was operated at 40 kV and 30 mA. The
2T range was between 20q and 90q, and counts were accumulated for 3 seconds at each step.
Diffrac Plus software was used for data collection. Phase identification could also be
performed using the search and match features of this software in combination with the JCPDF database.
Figure 3.5 Schematic of an X-ray diffractometer in the Bragg-Brentano configuration [156]
43
Figure 3.6 Schematic diagram of XRD (a) conventional T-2T geometry and (b) grazing angle
geometry [160]
Synchrotron Radiation
Synchrotron radiation is generated by a machine that accelerates charged particles, in
general electrons, passing them through a succession of the bending magnets. The charged
particle beam travels in a circular orbit and emits tangentially propagating electromagnetic
radiation. The emitted radiation, which covers a wide energy range of spectrum, from the
infra-red to X-ray region, has high intensity and brightness compared with a conventional Xray source. The radiation may be selectively filtered to obtain any selected wavelength. It is
also highly collimated. The low beam divergence allows for improved resolution. Therefore,
it is possible to obtain very narrow and well-defined peaks with a FWHM of < 0.03q in 2T. In
contrast, a laboratory X-ray source may give broader peaks with a FWHM of 0.15q in 2T.
These advantages of synchrotron radiation can be applied to the powder diffraction technique
[161, 162]. The synchrotron experiments for this project were carried out at the Australian
Synchrotron on its powder diffraction beamline. The powder diffraction beamline is located
on a bending magnet source and has been designed to operate over the energy range 5-30 keV
[163]. In order to monitor phase transformations as a function of temperature and time, the
high-temperature diffraction stage was used for in-situ high temperature XRD studies from
25 qC to 1000 qC. Heating was achieved by a platinum bar through which a controlled current
was passed (Figure 3.7). Various ramp rates were used. A Mythen microstrip detector was
applied in these experiments and it provides fast data acquisition and good data quality. Thin
film samples of precious metal alloys and compounds were prepared on tantalum or stainless
steel foil for the synchrotron experiments. After the diffraction patterns were obtained, the
peaks due to the tantalum or stainless steel substrate were removed and then selected sample
peaks fitted with pseudo-Voigt functions using FitYK [164]. The diffraction patterns at
various temperatures were compared in order to observe any structural changes.
44
(b)
(a)
Figure 3.7 (a) High temperature furnace and (b) platinum heater bar with cavity [165]
Rietveld Refinement
The Rietveld refinement is a powerful technique which was created by Hugo Rietveld
for the diffraction analysis of crystalline materials. The Rietveld method involves using a
least squares procedure to refine a theoretical line profile until it matches the measured
profile in order to minimize the difference between the calculated and the experimental
powder diffraction pattern. The diffraction from overlapping peaks is accounted for in this
technique. It is an accurate way to determine the structure of materials. The Rietveld method
can be used with data collected from X-ray laboratory instrument or synchrotrons [166].
There is a lot of information in a powder pattern which may be revealed by using Rietveld
refinement as shown in Figure 3.8 [167].
Figure 3.8 Information in powder diffraction pattern [167]
45
3.3.2 Scanning Electron Microscopy (SEM)
Scanning electron microscopy is a powerful facility for analysing surfaces by using a
focused beam of electrons. The electrons interact with atoms in the sample and generate a
variety of signals such as secondary electrons (SE), backscattered electrons (BSE),
characteristic X-rays, Auger electrons and cathodoluminescence [168]. These signals can be
detected and provide information about the sample including microstructure, morphology,
grain orientation, and chemical composition. SEM produces high-resolution images with
reasonable depth-of-field. For this project, secondary electrons, backscattered electrons and
characteristic X-rays were used. Secondary electrons and backscattered electrons are
primarily used for imaging samples. Secondary electrons are the most common signal for
image formation and provide information on morphology and topography while
backscattered electrons provide either compositional or topographic information.
Backscattered electrons are valuable for showing contrast in composition in multiphase
samples. Backscattered electrons are reflected or back-scattered out of the specimen by
elastic scattering during interaction with specimen atoms. Regions of high average atomic
number generate a greater number of backscattered electrons than regions of lower atomic
number and are hence brighter in the image. Characteristic X-rays result from the electrons of
the electron beam interacting with the inner orbit electrons of the sample. The incoming
electron may eject an inner-shell electron thereby leaving a vacancy. An outer shell electron
then falls into the vacancy and the atom emits an X-ray having energy equal to the energy
difference between two shells. The energy or wavelength of the x-ray can be detected by
spectrometers. Energy dispersive spectrometers (EDS) are most commonly used in SEMs
[169].
In the present project I mainly used a Zeiss Supra 55VP, which is shown in figure 3.9.
This is a field emission scanning electron microscope (FESEM) which has high resolution
imaging. It was mainly used to obtain topographical, morphological and compositional
information from samples in this project. The microscope was operated on high vacuum
mode with an acceleration voltage at 5-20 kV. A higher acceleration voltage and the In-lens
secondary electron detector were used for the high resolution imaging. The backscattered
electron detector was used to obtain lower resolution images showing atomic number
contrast. The Oxford EDS system was used for elemental characterization. In this case a
lower acceleration voltage and a higher working distance were employed. Also used in this
project was a Zeiss Evo LS15, which is a thermionic tungsten electron gun SEM. This SEM
is also equipped with a Bruker EDS Quantax 400 with silicon drift detector (SDD) which
allows for high speed elemental analysis, however, the resolution available on images was
inferior to that on the 55VP.
There were several types of samples in this project, including bulk materials, thin
films and small particles, all on different supporting materials and substrates. Therefore,
samples have been prepared in a variety of ways for SEM. In all case they had to be
electrically connected to the sample holder using conductive tape or silver paint to prevent or
minimize sample charging. Aluminium stubs proved convenient to use as a sample holder in
many cases. These stubs can be modified for a particular type of specimen. For example, bulk
46
samples were cut and embedded in Bakelite by hot mounting. Then, these bulk samples were
attached to the aluminium stub with conductive carbon tape. Copper double-sided tape was
also sometimes used to connect the metallic part of the sample to the aluminium stub. Thin
film samples were cut or fractured into small pieces and then stuck down onto an aluminium
stub with conductive carbon tape. The standard aluminium stubs could also be used to hold a
sample at an angle of 90q in order to examine its cross-sectional area. Small particle samples
from chemical processing were transferred to 200 mesh TEM copper grids and placed on an
aluminium stub [170].
Figure 3.9 A field emission scanning electron microscope (Zeiss Supra 55VP)
3.3.3 Transmission Electron Microscopy (TEM)
Transmission electron microscopy is a powerful characterisation technique with high
spatial and analytical resolution. It operates by transmitting a tightly focussed electron beam
through a very thin specimen. Images, crystallographic information and accurate
compositional analysis are obtained from a TEM, which in general has a higher resolution
than a SEM. The high energy (100 to 400 kV) electron beam of a TEM is generated by an
electron source such as tungsten (W) or lanthanum hexaboride (LaB6). As in a SEM, the
electron beam is controlled by electromagnetic lenses and apertures. An objective lens creates
the diffraction image in the back focal plane and the image of the specimen in the image
plane. Intermediate lenses are used to magnify the image or the diffraction pattern on the
screen. Bright-field (BF) and dark-field mode (DF) imaging modes are commonly used on a
TEM. An objective aperture that is positioned at specific locations in the back focal plane is
used to control these two different imaging modes. When the aperture is positioned to pass
47
only the transmitted or un-diffracted electrons, the resulting image is referred to as a brightfield (BF) image. The aperture absorbs electrons that are scattered. In this case the contrast in
the sample is roughly proportional to the absorption of the transmitted beam, so empty
regions appear bright and dense regions appear dark. In contrast, when the aperture is
positioned to pass only specific diffracted electrons, the image is referred to as dark-field
(DF) image. In this case the diffracting regions of the specimen appear bright while other
regions are dark [171, 172]. Scanning transmission electron microscopy (STEM) mode works
on the same principle as a SEM. Secondary electrons, backscattered electrons and X-rays are
produced similarly to the SEM but a STEM has a higher signal level and better spatial
resolution. The transmitted or diffracted beams are detected with various detectors on the
column axis. Transmitted electrons are collected on the axis to create the bright field. An
annular dark-field detector can be used in the STEM mode to obtain high-angle annular darkfield (HAADF) or Z-contrast imaging. Furthermore, X-rays generated from electron
excitations in the specimen can be also used for microanalysis and elemental mapping by
EDS [173].
Three different TEMs were used in this project. The Philips CM12 is a conventional
analytic TEM. It was operated with LaB6 source at various accelerating voltages from 20-120
kV. BF/DF imaging and diffraction mode can be obtained and displayed via a CCD camera.
The sample holder is double tilt holder with -60q to +60q. A maximum resolution is
approximately 0.5 nm. The other two TEMs were used with the assistance of Dr. Annette
Dowd. The JEOL 1400 is operated with a high brightness LaB6 electron source at an
accelerating voltage in the range 40 to 120 kV. It is equipped with a Gatan large-area camera.
STEM imaging with BF/DF imaging can be obtained and high-definition diffraction patterns
can be acquired. A JEOL EDS system is used for X-ray microanalysis and elemental
profiling and mapping. The point resolution is 0.38 nm and lattice resolution is 0.2 nm. The
JEOL 2200 FS is a high resolution TEM with a 200 kV field emission gun and a built-in
Omega filter to produce energy-filtered imaging and chemical analyses of specimens. TEM
images with BF/DF imaging and diffraction patterns can be acquired on its Gatan Ultrascan
CCD camera. STEM mode can be performed BF and HAADF imaging. An advanced EDS
analysis permits point, line scan and area mapping with X-ray signals. The point and lattice
resolution are 0.23 nm and 0.1 nm respectively.
TEM specimens must be very thin in order to be transparent to electrons, certainly not
more than 100 nm for the present types of materials. So sample preparation is a crucial
process for TEM. There were two different types of specimen prepared in this project: thin
films and fine particles/sponges. For thin film specimens, a cross sectional area was often
prepared in order to study the internal structure. The 'sandwich' technique was applied to
prepare the cross-sections. Thin film samples, which had been deposited on a Si wafer, were
prepared and cut into two pieces by a diamond scribe. The dimensions were approximately 3
mm x 1 mm. Two Si wafer pieces were also prepared with the same dimensions as the
sample. The two coated surfaces were then placed face-to-face and the two additional pieces
of Si wafer attached to the backside of each piece to provide more structural support. The
four pieces were then glued as stack-like sandwiches by using epoxy resin. Figure 3.10 shows
48
the schematic of a stack sandwich sample and interface of the thin film. The glued sandwich
was placed into an oven at 120qC for 30 minutes. After that, the sandwich was attached to a
polishing stub using 'crystal bond' adhesive. This is used for temporary bonding and can be
dissolved later using acetone. The stub is mounted on to tripod polisher and polishing begun
with abrasive paper (grit size 1200), followed by diamond lapping film from 30 Pm, 9 Pm, 3
Pm and 1 Pm respectively. Once one side is smooth, shiny and scratch-free, the sample can
be removed and cleaned with acetone. The next step was to prepare the other side of sample
for polishing. A piece of Si wafer was placed on the polishing stub with cyanoacrylate glue as
a base for the specimen. The stub with Si wafer base is mounted back on to a tripod polisher.
The legs of the tripod are adjusted to position the glass plate parallel to the Si wafer surface,
and then the surface is polished with 30 Pm diamond lapping film. After that the sample was
attached to the polished side facing down on Si wafer using cyanoacrylate glue and was left
overnight. The process of polishing began with the abrasive paper for aligning the sample.
The tripod polisher was adjusted to generate a thin wedge, where a wedge angle of about 0.5
was introduced. Next, the sample is polished again with diamond lapping film from 30 Pm, 9
Pm, 3 Pm and 1 Pm. When it is sufficiently thin, the sample is transferred to a slot copper
grid which was compatible with the TEM sample holder. Finally, the sample was gently
thinned to electron transparency using a 691Gatan precision ion polishing system (PIPS). An
argon ion beam bombards the thin sample and sputters away material until the sample is thin
enough for electron transparency [174]. Several variables can be controlled including voltage,
angle of incidence and sample rotation. The ion gun is operated at approximately 3.5 keV.
The angle of incidence is set at 8q either above or below the sample. The sample rotation
speed was 2-3 rpm. Preparation of the fine particle samples involved far less processing than
the thin films. The fine particles were first suspended in liquid (ethanol or water) then
transferred to a holey carbon-coated copper 200 mesh grid for TEM observation.
(c)
(b)
(a)
(b)
(a)
Figure 3.10 Cross section preparation for TEM (a) schematic of stack sample prepared using
the sandwich technique and (b) Interface of thin film after polishing with diamond of 1Pm
(the arrow indicate the location of the glue line) [175]
49
3.3.4 Determination of optical properties
Optical properties of metals are determined by the way light interacts with their
surfaces as well as by the role of human perception to see the colour or lustre. In this project,
thin films in several nanoscale thicknesses have been fabricated. Their optical properties are
interesting to study, particularly those of the coloured intermetallic compounds. The colours
of these intermetallic compounds are exceptional compared to the usual silver or silver-grey
of most metals. Reflection and transmission as a function of wavelength and angle of incident
radiation is measured using a spectrophotometer or an ellipsometer. Ellipsometry analysis
software (WVASE32) was employed to model and analyze the reflectance, transmission and
the ellipsometric data. These data were used to extract the dielectric function (n and k) of
each material. The OpenFilters software was also used in this research project in order to
indicate colour of these thin films in the Commission Internationale de l’Éclairage (CIE)
system and to predict the properties of various designs of stack. In this section, the optical
measurement tools and techniques used in my project are described.
Spectrophotometry
Spectrophotometry is the quantitative measurement of the reflection or transmission
as function of wavelength when the light interacts with materials. Measurements are
conducted using a spectrophotometer. The spectrophotometer consists of three elements: (a) a
light source for providing illumination of the appropriate wavelengths, (b) a monochromator
for selecting the precise wavelength of interest and (c) a detector for measuring the amount of
light reflected or transmitted by the sample [176]. A Perkin Elmer Lambda 950
spectrophotometer with a universal reflectance accessory (URA) was used in this project.
This spectrophotometer has a double beam and a double monochromator and can take
measurements from the near-ultraviolet (300 nm) through to the near-infrared (2500 nm).
Tungsten-halogen or deuterium lamps are used as light sources.
Ellipsometry
Ellipsometry is an optical measurement technique that measures the change in
polarized light after light is reflected from or transmitted through a sample. Ellipsometry
measures the amplitude ratio ( < ) and phase difference (') between light waves known as pand s-polarized light waves. In the present project the ellipsometer was primarily used to
determine thickness and optical constants of various thin film samples. The optical constants
are obtained from the data by constructing an optical model using the WVASE software. Two
ellipsometry parameters ( < , ') are defined from the ratio of the amplitude reflection
coefficients for p- and s-polarization or defined by U tan < exp i' .The optical model for
thin film sample as shown in Figure 3.11, U is expressed by equation 3.1.
tan < exp i'
U ( N 0 , N1 , N 2 , d , T 0 )
50
(Equation 3.1)
Figure 3.11 An optical model of a thin film sample [177]
If the thickness (d) of the thin film is known or can be estimated, then the complex refractive
index of thin film can be expressed by equation 3.2. In this condition n1 and k1 can be
obtained directly from the two measured ellipsometry parameters.
N1
n1 ik1
(Equation 3.2)
The optical properties can alternatively be represented as the complex dielectric function (H)
where
H H1 iH 2
H1
n 2 k 2 and H 2
(Equation 3.3)
2nk
(Equation 3.4)
(H1 is the real part and H2 the imaginary part of complex dielectric constant [177]).
The ellipsometer is a tool for collecting ellipsometry data. The ellipsometer contains
the basic components which are light source, polarization generator, polarization analyzer
and detector. A Variable Angle Spectroscopic Ellipsometer (VASE) was used to measure thin
film samples with the assistance of Dr Angus Gentle. The ellipsometry data was collected
from 300 to 3300 nm. The angle of incidence range was 15q-90q.
Data analysis software of optical properties
(a) WVASE32
WASE32 by J.A. Woollam Co. is an ellipsometric analysis package which was used
to model and analyse the ellipsometric data, and reflection and transmission data for this
project with the assistance of Dr Angus Gentle. After measuring the thin film samples with
either the spectrophotometer or the ellipsometer, the reflectance, transmission and
ellipsometry parameters were obtained. The optical constants can be obtained by data
analysis. The process of data analysis is shown in Figure 3.12. After obtaining experimental
data, a model was constructed to describe the layer of thin film sample. The optical constants
51
were obtained by fitting the ellipsometric data or reflection and transmission data with a
Lorentz oscillator model which retained Kramers-Kronig consistency.
Figure 3.12 Diagram of the process for ellipsometry data analysis [178]
(b) OpenFilters
OpenFilters is an open-source package. It was used to create multilayer models of
thin films and to calculate the expected optical properties including reflection, transmission,
absorption and colour. The dielectric functions used in OpenFilters were either those obtained
in this project by measurement, or in some cases, data from the literature. After a thin film
stack had been designed, its colour was calculated in XYZ, xyY, L*a*b* and L*u*v* colour
spaces using the appropriated angle, illuminant and observer defined by the CIE system
[179].
52
Chapter 4
The AuAl2-PtAl2 system
53
4.1 Background
The colours of precious metal alloys and compounds were discussed in Section 2.2.
Apart from being of decorative value, these materials are exploited in engineering and
industrial applications for their specialised optical properties. As mentioned, coloured
intermetallic compounds with CaF2 crystal structure are also well-known especially AuAl2,
which is known as "amethyst" or "purple gold" or "purple glory" [180] and the bright yellow
compound PtAl2, which is sometimes known as "platigem". In this chapter I focus on these
two coloured intermetallic compounds and, in particular, show how they can be combined to
produce a range of new metallic colours.
4.1.1 Review of the fabrication and applications of AuAl2 and PtAl2
AuAl2 and PtAl2 are coloured intermetallic compounds with the CaF2 crystal
structure. There is some commercial interest in them particularly for jewellery and decorative
applications [5, 6, 64, 180, 181]. Their bright metallic colours are attractive: AuAl2 is deep
purple while PtAl2 is golden yellow. Both of these compounds can only be formed in a very
narrow range of stoichiometry, centred on 33 at.% of Au or Pt as shown in Figure 4.1 and
4.2. For this reason, it is challenging to achieve their unique colours.
Figure 4.1 Phase diagram of Al-Au system by Okamoto, H.(1991) [182]
54
Figure 4.2 Phase diagram of Al-Pt system by McAlister, A.J. and Kahan, D.J.(1986) [182]
In addition, intermetallic compounds are notoriously brittle. Various processing
techniques have been attempted so far to produce coloured intermetallic compounds and
overcome the brittleness. Investment or lost wax casting (which is the conventional casting
technique for jewellery items) has been used for producing purple and blue gold, Figure 4.3
and 4.4, [5, 6] but brittleness is a problem. Microalloying additions such as Pd in AuAl2 have
been investigated. The results showed brittleness decreasing due to the formation of a ductile
second phase [6]. Bi-metal casting techniques have been adapted to produce bi- or multicoloured jewellery. Examples of a 18 carat purple gold and a 14 carat blue gold with Pd made
by bi-metal casting are shown in Figure 4.5 [6, 183]. AuAl2 has been produced by powder
metallurgical techniques as claimed in a Japanese patent [184, 185] and has also been
successfully commercially manufactured as hallmarked 18 carat purple gold in Singapore as
shown in Figure 4.6 [186-188]. Mintek in South Africa used vacuum arc melting to produce
small buttons of either AuAl2 or PtAl2 that can be faceted as precious stones. Samples of
these purple and yellow gemstone-like castings are shown in Figure 4.7 and 4.8. These items
are suitable for setting in jewellery such as earrings, brooches and pendants only, due to the
relatively low abrasion and tarnish resistance of these coloured intermetallic compounds [64,
180, 187]. Another technique is physical vapour deposition (PVD), which can also be used to
fabricate 18 carat purple gold as a coating or nanoscale film. Some jewellery items which
have been coated with 18 carat purple gold are shown in Figure 4.9 [181].
The coloured intermetallic compounds have also attracted technological interest in
particular on account of their optical properties. They have reflection coefficients that are
strongly dependent on wavelength in the visible spectrum. Therefore, coloured intermetallic
55
compounds including AuAl2 and PtAl2 have potential application as spectrally-selective
coatings [189, 190]. Spectrally-selective coatings can, for example, be applied on windows
for energy conservation. AuAl2 films which were prepared by co-deposition direct current
magnetron sputtering have been investigated as a spectrally selective coating for windows,
but were found to be uncompetitive with gold films [181]. It has also been predicted that
nanostructures of AuAl2 and PtAl2 should manifest a localized surface plasmon resonance
[181, 191, 192].
Figure 4.3 Purple gold by investment casting (Courtesy JARAD Project by Srinakharinwirot
University, Bangkok Fashion City under the Ministry of Industry of Thailand, Thailand)
Figure 4.4 AuAl2- carat purple gold (top row) and AuIn2 - blue gold (bottom row)
(Courtesy Co. Reischauer GmbH, Idar Oberstein, Germany) [6]
56
(b)
(a)
Figure 4.5 Bi-metal casting (a) 950 Pd casting with injected wax for the 2nd bi-metal casting
process step and (b) Bi-metal castings of 14k blue gold (left) and 18k purple gold (right) with
950 Pd (Courtesy Vendorafa-Lombardi Srl, Valenza,Italy) [6]
Figure 4.6 Purple gold by powder metallurgy process (Courtesy Lee Hwa Jewellery,
Singapore) [186]
Figure 4.7 Purple glory gemstone-like AuAl2 casting in setting on ring (courtesy M.B Cortie)
[180]
57
Figure 4.8 Platigems and Platigem jewellery (Courtesy Mintek, South Africa) [187]
Figure 4.9 AuAl2-coated items made by depositing onto sterling silver costume jewellery by
the present author [181]
58
Other technological applications are also being investigated or are already in use.
AuAl2 has potential applications as a superconductor or resistor [193, 194] while PtAl2 can be
used as a corrosion or oxidation resistant coating, particularly for high-temperature turbine
blades [67]. Furthermore, aluminium-containing precious metal compounds, including AuAl2
and PtAl2, are suitable precursors for preparing mesoporous metals. It is possible to use these
sponges as catalysts, electrodes or sensors [108, 109, 195-197].
4.1.2 Review of the optical properties of AuAl2 and PtAl2
Dielectric functions
The dielectric functions of coloured precious intermetallic compounds, especially
those with the CaF2 structure, have been previously investigated with particular interest in
AuAl2 [41, 181, 191, 192, 198-202] and PtAl2 [192, 199, 200]. The imaginary part of the
optical constants of the pure phase can also be estimated by Density Functional Theory
calculations, and then the real part can be obtained by means of the Kramers-Kronig
relations, and the colour explained in terms of an interband transition. In a typical
experimental study, AuAl2 was prepared by melting pure metals in sealed evacuated quartz
tubes and zone refining. It was reported that interband transitions appeared at 2.2 eV [41]. In
another study of AuAl2, this time made by arc melting, the transition was at 2.125 eV.
Similar compounds such as PtAl2 can be made by substituting platinum for gold. In a typical
study, bulk samples were also prepared by arc-melting. The onset of the interband transition
of PtAl2 was at 2.834 eV [199].
As described in the literature review the dielectric properties of materials can be
described by their n and k, or H1 and H2. As part of this PhD project I participated in a study
conducted by Dr Vicki Keast of University of Newcastle to determine the optical properties
of AuAl2 and PtAl2 by a combination of measurement and modelling by density functional
theory. The experimental and calculated reflectivity of gold intermetallic compounds are
compared in Figure 4.10. It was found that the reflectivity spectra for AuAl2, AuGa2 and
AuIn2 are quite similar but the dielectric function of AuAl2 is significantly different, as
shown in Figure 4.11. The H1 of AuAl2 crosses zero at 2 eV. It has a bulk plasma frequency
that is in the optical region and provides the purple colour [191]. Furthermore, optical
properties of AuAl2, Au0.5Pt0.5Al2, Au0.25Pt0.75Al2 and PtAl2 were also investigated by
calculation. It was found that the main peak in H2 increases in energy as the gold content
decreases as shown in Figure 4.12. The calculated reflectivity spectra of these compounds are
compared to experimental data in Figure 4.13. The CIELAB colour coordinates were
obtained by calculation and are compared for these four compounds (Figure 4.14) [192].
59
Figure 4.10 Reflectivity of gold intermetallic compounds from experiment (solid curve) and
calculation (dashed curved) [191]
Figure 4.11 Dielectric function of gold intermetallic compounds; H1 (solid curve) and H2
(dashed curved) [191]
60
Figure 4.12 Dielectric function of ordered intermetallic compounds [192]
Figure 4.13 Reflectivity of ordered intermetallic compounds comparing with experimental
reflectivity of PtAl2 thin film [192]
Figure 4.14 Comparison CIE Lab colour coordinates of AuAl2, Au0.5Pt0.5Al2,
Au0.25Pt0.75Al2 and PtAl2 [192]
61
As a side-project, I also collaborated with colleagues on verifying an idea for tuning
the colour of Au-Ni mixtures and alloys. In this project we fabricated nanoscale thin film
stacks of Au-Ni. Here my role was to assist with the fabrication and measurements, and to
supply specific samples used in the analysis. We have found that the optical properties of the
alternating Au/Ni films are controlled by varying the number and thickness of the layers of
stacks fabricated from the constituent elements. The colour gamut of tri-layer of Au-Ni-Au is
shown Figure 4.15 [203]. If the layers are thin enough to permit some transmission of light,
then the colours will mix and new effects occur. I also applied this strategy to thin films
stacks of AuAl2 and PtAl2 (see later).
Figure 4.15 CIE L*a*b* colour gamut of Au-Ni-Au tri layer in reflection [203]
Plasmon resonances in AuAl2 and PtAl2
As mentioned previously, many intermetallic compounds with the CaF2 crystal
structure are strongly coloured. These properties will also have an effect on any plasmon
resonances in the nanoparticles. It has also been predicted that nanostructures of AuAl2 and
PtAl2 should manifest a localized surface plasmon resonance. In AuAl2, which has a purple
colour, it has been predicted that nanoparticles would have two broad plasmon resonances
centred on about 2-2.5 eV [181] while PtAl2, which is yellow, is associated with a bulk
plasmon at about 3 eV [192].
Alloying effects
Unfortunately, the coloured intermetallic compounds with CaF2 crystal structure, in
particular blue and purple gold, are brittle and have a low corrosion resistance. This must be
considered when using these materials for any technological purpose. Further alloying
additions have been proposed as a way to solve these drawbacks of blue and purple gold. The
influence of alloying additions on these properties as well as colour effects was investigated.
Blue (AuGa2 and AuIn2) and purple gold (AuAl2) share the same crystal structure (CaF2).
62
The brittleness in these intermetallic compounds can be reduced by ternary alloying additions
of, for example, palladium, copper or silver. One problem is that in most cases the desirable
colour fades when the third element is included. Platinum is an interesting element which
could potentially be added to the coloured gold compounds. This is because platinum forms
coloured intermetallic compounds with the same crystal structure and stoichiometry PtX2
(X=Al, Ga, In) as the blue and purple gold. Platinum can therefore potentially exchange for
gold in the same atom ratio of (Au,Pt)X2. This has been reported to occur in the compounds
with indium. With increasing platinum content the colour changes from blue (AuIn2) to
apricot (PtIn2) [5]. Furthermore, the brittleness decreases, possibly because precipitation of
the PtIn2 results in grain refinement of blue gold matrix. A colour change effect by
substitutional alloying addition must also be considered. For example, micro-alloying
additions increase the fracture resistance without affecting the colour of 14 carat blue gold
and 18 carat purple gold. Overall, the material performance is improved when micro-alloying
additions and third element additions like palladium are used in these coloured intermetallic
compounds [6]. However, the question of how to improve corrosion resistance of blue and
purple gold by alloying additions is still open. An attempt has been made to modify the
colour of yellow platinum (PtAl2) with additions of between 5 and 25 wt.% Cu [189]. In that
case all samples were prepared by vacuum-arc melting under an argon atmosphere. The
optical properties were determined by the reflection spectrophotometer in the wavelength
range 450-900 nm. CIELAB colour coordinates a* and b* were calculated from the measured
data in order to describe the colour of the intermetallic compound. It was found that the effect
of increasing the copper content was to change the colour of the compound from the brassyellow of PtAl2 through orange to copper-pink. The colour change was said to be related to
an increase in the lattice parameter suggesting that solid solution alloying had occurred.
4.2 Objective of this chapter
Gold and platinum have the same face-centred cubic (FCC) structure, have similar
physical properties, electronegativity and atomic diameter, and form the XAl2 compound with
the same CaF2 or cF12 (Pearson symbol) crystal structure. It might be expected that the two
compounds would exhibit unlimited mutual solubility. However, no Au-Pt-Al ternary section
appears to be available in the literature and so the nature of the microstructures along the
isopleth connecting these two compounds was unknown. AuAl2 and PtAl2 are fully metallic
and have attractive colours. This raises the interesting issues of (i) whether AuAl2 and PtAl2
are mutually soluble or, if not, (ii) whether mixed microstructures of these two compounds
could be used to tune the colour between these two coloured end-points.
4.3 Experimental details specific to this chapter
Thin films of AuAl2 and PtAl2 were prepared by direct current (DC) magnetron
sputtering in high vacuum. The films were deposited onto glass, silicon, stainless steel or
tantalum foil substrates. The glass slides were cleaned with detergent solution for 15-20
63
minutes then rinsed with distilled water and dried under N2. The silicon and metallic
substrates (tantalum or austenitic stainless steel foil) were cleaned with acetone then rinsed
with ethanol and dried under N2. The gold, platinum and aluminium sputtering targets were
of 99.99% purity. The composition was controlled by setting the appropriate current or power
level. The calibration curves for pure aluminium, gold and platinum are shown in Figure
4.16. The three main tasks of this part of the project were to prepare single layer, bi-layer and
multi-layer films of these coloured intermetallic compounds.
Figure 4.16 Deposition rate of aluminium, gold and platinum as function of current
a) Single layer films: Films of PtAl2 or AuAl2 were prepared by co-sputtering the elements
onto a substrate held at 400 qC. The current or power level on the elemental sputtering targets
were varied and controlled to obtain the composition of the compounds.
b) Bi-layer films: These coloured intermetallic compounds films were prepared by cosputtering the elements at 400 qC. There were two variations, PtAl2/AuAl2 and AuAl2/PtAl2.
The substances are listed in the order in which they were deposited. The samples were then
annealed in the vacuum chamber at 400 qC for 24 hours or 48 hours.
c) Multi-layer films: These films were prepared by depositing as a stack in different
sequences and using different techniques, all at room temperature. There are two distinct
types of stack: a stack formed by depositing individual pure metals in sequence and a stack
formed by co-depositing the precious metals and then the Al. After deposition, these films
were then annealed at 400qC for 30 minutes to allow interdiffusion and reaction. Several
different sequences of these films were fabricated.
64
For comparison purposes bulk samples of PtAl2 and AuAl2 were produced by melting
pure gold or platinum with aluminium in a vacuum arc furnace. These bulk samples were
prepared by Mintek in South Africa. The composition and morphology of the samples were
analysed by light microscopy (LM), X-ray diffraction (XRD) in grazing incidence mode,
scanning electron microscopy (SEM) with an energy dispersive spectrometry (EDS) and
transmission electron microscopy (TEM). An in situ diffraction experiment in a synchrotron
was also carried out in order to investigate the conversion of multi-layers of the elements into
intermetallic compounds. In this case the samples were heated from room temperature to
1250 qC at 50 °C/min and cooled afterwards at the same rate.
The optical transmittance and reflectance of the films were obtained using a PerkinElmer Lambda 950 UV/VIS spectrophotometer in the spectral range of 300 to 2500 nm. The
optical constants (n and k, or equivalently H1 and H2) were also extracted from data measured
at different angles by using a Variable Angle Spectroscopic Ellipsometer (VASE).The
WVASE32 program from J.A. Woollam Co.,US, was applied to calculate the dielectric
functions from the measured optical properties. The computer program OpenFilters [179] was
used to simulate the colours of various designs of film using the measured dielectric
functions.
4.4 Results and discussion
4.4.1 Single layer films of coloured intermetallic compounds
The coloured intermetallic compounds PtAl2 and AuAl2 have a very narrow range of
stability. Successful synthesis required careful adjustment of the current or power level
during sputtering. Nevertheless, pure phase PtAl2 and AuAl2 films were fabricated
successfully.
Platinum aluminide fabrication
The fabrication of PtAl2 is quite difficult because it is formed in a very narrow range
of composition. Also, other phases in the Pt-Al system such as Pt8Al21 and Pt2Al3 can be
formed in the composition range near to PtAl2. Consequently, during sputtering, the current
applied to the platinum target was fixed at 0.125 A, but that of the aluminium target was
varied between 0.200 and 0.400 A. The compounds obtained were closely correlated with the
current used. When aluminium content (ie. current) increased, the phases formed changed
from Pt2Al Æ Pt5Al3 Æ PtAl Æ Pt2Al3 Æ PtAl2 Æ Pt8Al21 as shown in Table 4.1 and the Xray patterns in Figure 4.17. The morphologies of the various Pt-Al coatings are compared in
Figure 4.18.
65
Table 4.1 Pt-Al films fabricated by co-sputtering using varying currents on the aluminium
target. (The current on the platinum target was fixed at 0.125 A or the power was fixed at
~ 55-57 W)
PtAl400
PtAl395
Current
of Al
(A)
0.400
0.395
Power
of Al
(W)
160
160
PtAl384
0.384
155
PtAl372
0.372
150
PtAl360
PtAl335
PtAl300
PtAl265
PtAl250
PtAl200
0.360
0.335
0.300
0.265
0.250
0.200
145
135
108
100
100
68
Sample
Colour
Possible compounds from XRD patterns
Silvery
Yellow
Yellow some
parts
Yellow some
parts
Yellow some
parts
Silvery
Silvery
Silvery
Silvery
Silvery
amorphous
PtAl2
PtAl2, unidentified peak
PtAl2, Pt2Al3
Pt2Al3, PtAl2
Pt2Al3, unidentified peak
Pt2Al3, unidentified peak
Pt2Al, Pt5Al3, PtAl, unidentified peak
Pt2Al, Pt5Al3, unidentified peak
Pt2Al, unidentified peak
Figure 4.17 X-ray patterns of Pt-Al compound films by co-sputtering using
varying current level of aluminium, PtAl2 are formed by using current at 0.395 A (yellow
pattern)
66
(a)
(b)
100 nm
100 nm
(d)
(c)
100 nm
100 nm
Figure 4.18 Morphology of Pt-Al compound films (a) PtAl400 (b) PtAl395 (c) PtAl360
and (d) PtAl335
Despite these difficulties, PtAl2 can be reproducibly produced by magnetron cosputtering by careful control of the factors of the process, such as the current or power
applied to each target, the deposition temperature and the deposition time. Success is readily
confirmed with the naked eye because PtAl2 is yellow. The reflectance spectrum of PtAl2 was
also measured and the film showed increasing reflectance with increasing wavelength. A
comparison of bulk PtAl2 (produced by arc melting) with thin-film PtAl2 which was
fabricated by magnetron sputtering showed a small shift in the absorption of bulk PtAl2 but
the reflectance spectra still had much the same shape as shown in Figure 4.19. The surface
morphologies of these two different samples of PtAl2 were similar with regard to their fine
structure as shown in Figure 4.20. This is important as the perceived or measured colour of a
material can be influenced by the morphology of its surface.
67
Figure 4.19 Comparison of reflectance spectra of PtAl2 in bulk and film
(a)
(b)
10 Pm
100 nm
Figure 4.20 Comparison of morphology of surfaces of PtAl2 in (a) bulk and (b) film
Furthermore, both bulk and thin film PtAl2 were characterized by XRD. The X-ray
patterns are shown in Figure 4.21. The (200) peak of the PtAl2 is missing from the patterns
collected from the thin-films which may be due to crystallographic texture in the sample. The
chemical compositions were analyzed by EDS, with the bulk sample analyzing at Pt 34 at.%,
Al 66 at.% while the thin-film sample was Pt 33 at.%, Al 67at.%. The lattice parameter of the
PtAl2 film and bulk samples were obtained by Rietveld refinement and found to be 0.5941
and 0.5923 nm respectively, at room temperature. These values are close to database values
of 0.5920 to 0.5930 nm recorded in JCPDF (pattern card number 00-003-1006 and 03-0652983).
68
Figure 4.21 Comparison of X-ray patterns of PtAl2 in bulk and film
Gold aluminide fabrication
Similarly to PtAl2, the film of AuAl2 was fabricated by dc magnetron co-sputtering.
Based on our experience in fabricating PtAl2 and due to the fact that the compounds have the
same chemical stoichiometry and crystal structure, the electrical power on the aluminium
target was fixed at 204 W. The power applied to the gold target was then adjusted over a
range 16-26 W in order to find the appropriate deposition rate for producing AuAl2. Varying
the power level of the gold target affects the fraction of gold in the film and the film's colour
as shown in Table 4.2 and 4.3. XRD results and the X-ray pattern in Figure 4.22 indicated
that the appropriate power level of gold and aluminium can produce pure AuAl2 phase
(sample AuAl050). It was also possible to fabricate AuAl2 with a lower content of gold
(sample AuAl040) in this arrangement. However, this scenario resulted in excess aluminium
being present in the sample. In another instance of this arrangement, it was shown that it was
possible to fabricate intermetallic compound of AuAl2 using a higher gold content (sample
AuAl060), but then the sample contained some AuAl and sometimes even Au4Al. Figure 4.23
outlines the morphologies of these films which are presented to explain the different phases
in each sample.
69
Table 4.2 Au-Al films fabricated by co-sputtering using varying current level of gold. (Power
of Al was 204 W or current ~0.443-0.452 A)
Samples
AuAl060
AuAl050
AuAl040
Power
of Au
(W)
26
21
16
Current
of Au
(A)
0.062
0.052
0.041
Colour
Possible compounds from XRD patterns
Purple (dull)
Reddish Purple
Pale purple
AuAl2, AuAl, Au4Al
AuAl2
AuAl2, Al
Table 4.3 Quantitative chemical analysis by EDS of Au-Al compounds
AuAl060
AuAl050
AuAl040
Au (at %)
40
38
28
Al (at %)
60
62
72
Figure 4.22 X-Ray patterns of Au-Al compounds
70
(a)
(b)
100 nm
1 Pm
(c)
(d)
200 nm
1 Pm
(e)
200 nm
Figure 4.23 Morphologies of Au-Al films produced by using different power levels on the
gold target (a) 16 W (sample AuAl040) in low magnification, (b) 16 W (sample AuAl040) in
high magnification, (c) 26 W (sample AuAl060) in low magnification, (d) 26 W (sample
AuAl060) in high magnification and (e) 21 W (sample AuAl050)
71
All of the above samples are purple to the naked eye but they do have slightly
different hues. The reflectance spectra are compared in Figure 4.24. All of the spectra show a
dip in reflectance at ~ 550 nm (green) but the dip is more pronounced in the spectrum of
sample AuAl050 (the pure AuAl2). Thus this film is a more intense reddish purple. AuAl040 is
composed of AuAl2 and Al whereas AuAl060 has AuAl2 as the matrix phase with small
amounts of the other phases such as AuAl and Au4Al in the film. The addition of these other
phases (which are believed to be silvery-grey in colour like most intermetallic compounds)
affected the colour of these samples. Therefore, the spectra of the impure samples showed a
reduced reflectance compared to that of the sample of pure AuAl2 phase.
Figure 4.24 Reflectance spectra of Au-Al compounds
Table 4.4 Conditions, colour and XRD results of Au-Al films
Sample
AuAl2_S1
AuAl2_S2
Power Deposition Assumed Temperature
colour
of Au time (min) thickness
(°C)
(watts)
(nm)
21
10
300
< 400
Pale purple
21
10
300
400
Reddish
purple
XRD
AuAl2
AuAl2
The effect of the various deposition parameters was investigated. The power or
current level is the major factor that must be controlled for fabricating the appropriate
72
compound and their effect is similar to the results described previously for PtAl2. The
temperature of the substrate during deposition affects the crystalline structure and the colour
of AuAl2. If the deposition temperature was less than 400 °C, then AuAl2 was still formed but
it was less crystalline than deposits formed at 400 °C. Figure 4.25 compares these films. The
quantitative chemical composition of these films is approximately 37 at.% Au. SEM images
of their microstructures are shown in Figure 4.26.
Figure 4.25 Comparison of X-Ray patterns of AuAl2, deposited at different temperature
(a)
(b)
100 nm
100 nm
Figure 4.26 Microstructure of AuAl2 films, deposited at different temperatures
(a) below 400 qC and (b) at 400 qC
On balance, I found that AuAl2 can be formed more easily than PtAl2. According to the
relevant phase diagrams, there are fewer other phases with compositions near that of AuAl2,
73
in contrast to the situation for PtAl2. The results showed that AuAl2 could be formed even
when using different power levels, deposition times and deposition temperatures, with some
AuAl2 formed in each case even though the purple hue of films varied a bit. In addition, the
reflectance curves of AuAl2 which was produced by arc melting (bulk) and AuAl2 which was
fabricated by magnetron sputtering (film) are of similar form although the film has a stronger
colour because it has a slightly higher reflection in the violet and red regions, and less
reflection in green, as shown in Figure 4.27. However, the morphologies of the two different
forms of AuAl2 sample are different, shown in Figure 4.28 and 4.29. A dendritic structure can
be observed in the AuAl2 bulk sample with some segregation of Al, which had most likely
occurred during solidification. In contrast, the morphology of AuAl2 film is dense and
homogeneous.
Figure 4.27 Comparison of reflectance spectra of AuAl2 in bulk and film
(a)
(b)
10 Pm
50 Pm
Figure 4.28 Morphologies of AuAl2 bulk sample by (a) SEM and (b) LM
74
(a)
(b)
100 nm
100 nm
Figure 4.29 Morphologies of AuAl2 thin film (a) plan view and (b) cross-section
Additionally, both bulk and thin film AuAl2 were characterized by XRD. The X-ray
patterns are shown in Figure 4.30. The chemical compositions were analysed by EDS, Au 33
at.% and Al 67 at.% in bulk as well as Au 34 at.% and Al 66 at.% in film. The lattice
parameter of the AuAl2 film and bulk samples were obtained by Rietveld refinement and
found to be 0.5993 and 0.5999 nm respectively, at room temperature. These values are in
agreement with the 0.5995 to 0.5999 nm recorded in Pearson’s Handbook of Crystallographic
Data for Intermetallic Phases [204] and JCPDF (pattern card number 00-017-0877 and 03065-2984).
Figure 4.30 X-ray patterns of AuAl2 in bulk and thin film samples
75
Optical properties of single layers of coloured intermetallic compounds
Determination of optical properties by reflection and transmission data
Although single layers of the binary intermetallic compounds could be fabricated by
co-sputtering, as described above, it was found that it was difficult to simultaneously control
both the thickness of the films and their composition. Therefore, another technique was also
applied. In this case pure films of each element were sequentially deposited. Aluminium was
the first layer, followed by platinum or gold on top. The second layer was made in half the
deposition time compared to the first. Thereafter the sample was annealed at 400 qC for 30
minutes to react the elements and form the intermetallic compound. A diagram illustrating
this process is shown in Figure 4.31. After annealing these films at 400 qC in the vacuum
chamber, the cross-sectional area of these binary intermetallic compounds was investigated
and is shown in Figure 4.32. The thickness of these films is about 100 nm.
Pt
Al
Anneal
PtAl2
at 400 qC
(a)
Au
Al
(b)
Anneal
AuAl2
at 400 qC
Figure 4.31 Diagram illustrating the process for fabricating coloured intermetallic compounds
by controlling the chemical composition and thickness of (a) PtAl2 film and (b) AuAl2 film
(a)
(b)
100 nm
100 nm
Figure 4.32 Cross-sections of the thin films of the binary intermetallic compounds after
annealing at 400 qC (a) AuAl2 and (b) PtAl2
76
Two different thicknesses (40 and 100 nm) of these films were fabricated and then
analysed by XRD, Figure 4.33. The X-ray patterns of the PtAl2 films are similar even in the
different thicknesses. However, while AuAl2 film which is 100 nm thick shows a crystalline
structure and strong peaks at (111), (220) and (311), the structure of the thinner AuAl2 film is
quite different. However, the colour of these films showed the expected purple for AuAl2 and
yellow for PtAl2 at either thickness. These films of both compounds were measured for
optical reflectance and transmittance with a UV/VIS spectrophotometer and the resultant data
were comparing with the fitted model. The data for PtAl2 films are shown in Figure 4.34
while that for AuAl2 films are shown in Figure 4.35.
(a)
(b)
Figure 4.33 X-ray patterns of colour intermetallic compounds in different thicknesses of film
(a) PtAl2 films and (b) AuAl2 films
77
(a)
10
0
nm
(b)
(a)
Figure 4.34 Reflectance (R) and transmittance (T) spectra of PtAl2 films (Exp) with model
fitted (Model Fit) to different thicknesses of film (a) 100 nm and (b) 40 nm
78
(a)
(b)
Figure 4.35 Reflectance and transmittance spectra of AuAl2 films with model fitted to
different thicknesses (a) 100 nm and (b) 40 nm
79
The complex refractive indices, n and k, (and equivalently İ1 and İ2) of these coloured
intermetallic compound films were obtained by using the WVASE32 software. The
dielectric constants of these coloured intermetallic compound films are compared with the
optical properties data from the bulk samples (which had been prepared by arc-melting in an
argon atmosphere [199]) in Figure 4.36. The shapes of the dielectric functions are broadly
similar for PtAl2 except for the fact that the H1 for the thin film crosses the X-axis whereas
that for the bulk samples apparently does not. This crossing would be associated with a
localized plasmon resonance. However, there is a bigger discrepancy in the case of AuAl2,
possibly due to a difference in microstructure between the two samples. The İ1 peaks present
at ~ 3.25 eV in PtAl2 and ~2.75 eV in AuAl2.
(a)
(b)
Figure 4.36 Dielectric functions of coloured intermetallic compounds by reflectance and
transmission data (a) PtAl2 and (b) AuAl2
80
Open Filters was used to simulate the colour of these films using both the CIE XYZ
and CIE LAB systems. The values of these colours are compared between the bulk and film
samples in reflection and transmission mode and are shown in Table 4.5. The colour of
nanoscale AuAl2 and PtAl2 films can be varied both in reflectance and transmittance mode by
controlling the thickness. Simulated CIE LAB colour parameters for a wide range of
hypothetical films were also obtained by using OpenFilters. The chromaticity indices a* and
b* were plotted to show the different colours expected when the film thickness changed for
PtAl2 and AuAl2, shown in Figure 4.37 and 4.38 respectively. The a* parameter indicates the
amount of red or green colour (negative values indicate green while positive values indicate
red). The b* parameter indicates the amount of yellow and blue (negative values indicate blue
while positive values indicate yellow). The reflectance of PtAl2 shows bright yellow colour
on thicker film (100nm) and darker on thinner films (5nm). The transmittance of PtAl2 for all
thicknesses displays low values of chromaticity indices and close to and grey colour (at zero).
The reflectance of AuAl2 film shows more reddish purple on thicker film (100 nm) while the
transmittance of AuAl2 film is yellowish green and became brighter in thinner film (5 nm).
Table 4.5 A comparison of the CIE XYZ and CIE L*a*b* colour coordinates of thin film and
bulk samples of PtAl2 and AuAl2
PtAl2
X
Y
Z
L*
a*
b*
Bulk : Reflection
53.965
56.068
53.054
79.653
1.736
7.537
Bulk :Transmission
0.079
0.090
0.083
0.813
-0.265
0.218
Film : Reflection
53.579
55.884
38.214
79.548
1.200
23.661
Film :Transmission
0.451
0.502
1.002
4.532
-1.045
-6.514
X
Y
Z
L*
a*
b*
Bulk : Reflection
43.829
41.767
51.169
70.710
12.542
-5.995
Bulk :Transmission
0.993
1.062
0.138
9.496
-0.589
14.393
Film : Reflection
39.630
34.348
45.266
65.238
23.377
-9.204
Film :Transmission
1.381
2.265
0.449
16.82
-19.463
22.575
AuAl2
81
(a)
(b)
0 nm
100 nm
T
0 nm
R
100 nm
(c)
rendered into the surface of a spherical data point. Both reflectance (yellow) and
transmittance (grey) modes are shown for the different thicknesses (a) front view (b) top view
and (c) perspective view
82
(b)
(a)
0 nm
T
R
100 nm
0 nm
100 nm
(c)
rendered into the surface of a spherical data point. Both reflectance (purple) and
transmittance (yellow-green) mode in different thickness of film are shown (a) front view (b)
top view and (c) perspective view
83
Determination optical properties by ellipsometry
Thin films of either AuAl2 or PtAl2 (which had been produced by the usual codeposition technique) were measured by ellipsometer. The thickness of these films was
approximately 300 nm, which is thick enough to make them opaque. The angles of
measurement were set at 65q,70q and 75q. The optical constants (n and k) of these coloured
intermetallic compounds were then obtained from each of the different three angles. The
average of n and k was then used to calculate the H1 and H2 for the films. The dielectric
constants of the two coloured intermetallic compounds are in Figure 4.39. The shapes of the
dielectric functions are similar to the previous results that had been obtained from the
reflectance and transmission data (Figure 4.36 above). The İ1 of PtAl2 crossed zero at about 3
eV while it is about 2.25 eV for AuAl2.
(a)
(b)
Figure 4.39 Dielectric functions of coloured intermetallic compounds found by analysis of
ellipsometric data (a) PtAl2 and (b) AuAl2
84
4.4.2 Bi-layers of coloured intermetallic compounds
Once suitable deposition parameters were found for each compound separately, the
parameters were reapplied to produce the bi-layer samples. There are two different
arrangements, namely PtAl2/AuAl2 and AuAl2/PtAl2, as shown in Figure 4.40. The
reflectance of the bi-layer films has been measured on both sides, i.e. the front (top) and the
back (through the glass slide).
The bi-layer films of AuAl2/PtAl2 are yellow on the front side and reddish purple on
the back side as shown in Figure 4.41. Chemical composition of each layer was measured by
SEM/EDS. The top layer is comprised of approximately 34 at.% Pt while the bottom layer is
comprised of 33 at.% Au. Bi-layer films in which AuAl2 was on the top were also fabricated
and analysed using the same methods. SEM images of the cross-sections of typical bi-layer
films are shown in Figure 4.42.
PtAl2
AuAl2
AuAl2
PtAl2
(b)
(a)
Figure 4.40 The two kinds of bi-layer films produced (a) AuAl2/PtAl2 and (b) PtAl2/AuAl2
Figure 4.41 The reflectance spectra of bi-layers of AuAl2/PtAl2
85
(b)
(a)
100 nm
100 nm
Figure 4.42 Cross-section of bi-layers films of PtAl2/AuAl2 before annealing (a) In lens mode
and (b) backscatter mode
Both coloured intermetallic compounds share the same CaF2 structure or cF12
(Pearson symbol) crystal structure. They have similar physical properties. Furthermore,
platinum and gold have same face-centred cubic (FCC) structure, similar electronegativity
and atomic diameter. Therefore, it was initially considered that the two compounds might
exhibit unlimited mutual solubility. Post-deposition heat treatment was applied to the above
bi-layer samples to see whether the two phases would interdiffuse. The annealing temperature
was at 400 qC for 24 hours or 48 hours under vacuum. Contrary to our initial expectation, the
two compounds seem to be nearly mutually insoluble. The cross-sections of these films after
annealing were observed and shown in Figure 4.43.
(a)
(b)
100 nm
100 nm
Figure 4.43 Cross-section of PtAl2/AuAl2 film after annealing under vacuum at 400 °C
(a) for 24 hours and (b) for 48 hours
86
However, even though the two phases are insoluble, it is still the case that nanoscale
duplex microstructures comprised of them would have an effective dielectric function that
can be tuned between that of the components. Optical properties of such films were simulated
using OpenFilters. Calculated reflectance spectra of 200 nm PtAl2 film that has been overcoated with various thickness of AuAl2 are shown in Figure 4.44(a). Simulated CIE LAB
colour parameters of these bi-layer films were obtained. Thicknesses varying from 0-100 nm
of AuAl2 were applied and shown in Figure 4.44(b). Similarly, reflectance spectra of 200 nm
AuAl2 film that has been over-coated with various thickness of PtAl2 and the simulated CIE
LAB colour parameters are shown in Figure 4.45 (a) and (b), respectively.
(b)
(a)
(b)
0 nm
100 nm
Figure 4.44 Calculated reflectance (a) and colour (b) of 200 nm PtAl2 film that has been overcoated with indicated thickness of AuAl2
(a)
10
0
100 nm
(b)
0 nm
Figure 4.45 Calculated reflectance (a) and colour (b) of 200 nm AuAl2 film that has been
over-coated with indicated thickness of PtAl2
87
4.4.3 Multi-layer films of coloured intermetallic compounds
Multi-layer films were prepared by depositing a sequence of the elements as a stack at
25qC and then annealing the samples. Two different deposition schemes were applied, with
either a stack formed of pure elements being deposited in sequence, e.g. Al/Pt/Al/Au or a
stack of co-deposited elements, e.g. Al/Pt-Au, being made. This was done in order to
examine (i) if AuAl2 and PtAl2 are mutually soluble and (ii) how the multi-layer structure
affected the colour.
A stack formed by depositing pure metals
The first type of film was a stack comprised of layers of pure metals, each deposited
as the pure element at 25qC. A number of these stacks, with 4, 6 or 8 layers, were made.
After deposition, the stacks were annealed at 400qC for 30 minutes in order to react the
elements by solid state diffusion.
Four layer stacks
There were two different arrangements of these films. The first arrangement started
with a first layer of aluminium, followed by platinum, aluminium and gold. The other type
started the first layer with aluminium but followed with gold, aluminium and platinum. Both
types of these multi-layers films are shown in Figure 4.46. The deposition conditions for
fabricating these films are presented in Table 4.6. The cross-sections of these multi-layered
films, as deposited at room temperature, were analysed and shown in Figure 4.47 using both
the in lens and backscatter modes. Four layers are shown with the in lens mode. The contrast
between the layers is also shown in backscatter mode (the higher atomic number element
shows brighter on the image). Therefore, the brighter layers are gold or platinum while the
darker layers are aluminium.
Pt
Al
Au
Al
Au
Al
Pt
Al
Substrate
Substrate
(b)
(a)
Figure 4.46 Schematic illustration of the arrangements of the four-layer films of Al-Au-Pt (a)
Au on the top and (b) Pt on the top
88
Table 4.6 Deposition conditions for the four-layer films of Al-Au-Pt
Layer
order
1
2
3
4
Metal targets
Current (A)
Deposition time
Al
Pt or Au
Al
Au or Pt
0.4
0.1
0.4
0.1
5 minutes
2 minutes
5 minutes
2 minutes
After that, these films were annealed at 400qC for 30 minutes. The result was that the
four distinct layers merged to become only two layers, as seen in SEM images taken in both
in lens and backscatter mode in Figure 4.48 and Figure 4.49. TEM images also showed the
two separate layers quite clearly.
(a)
(b)
100 nm
100 nm
Figure 4.47 Cross-sections of four-layer films of Al-Au-Pt with Au on the top, as deposited at
25 °C (a) In lens and (b) RBSD
89
(a)
(b)
100 nm
100 nm
(c)
50 nm
Figure 4.48 Cross-sections of four-layer films of Al-Au-Pt with Au on the top, after annealing
at 400 °C (a) SEM:In lens mode (b) SEM:RBSD mode and (c) TEM
(b)
(a)
100 nm
100 nm
Figure 4.49 Cross-sections of four-layer films of Al-Au-Pt with Pt on the top, after annealing
at 400 °C (a) SEM and (b) TEM
90
The colour of these films changed substantially after annealing at 400 °C. To measure
this change a glass slide was used as the substrate. The reflectance spectra of both sides of
the samples were then measured by spectrophotometer in the visible region before and after
annealing. The four-layer films which had gold on the top were greenish yellow before
annealing. Their colour was noticeably different to that of a 30 nm pure gold film that was
used as a comparator. After annealing, the film on the top turned reddish purple. A
comparison of the spectra of the four-layer films which had gold on the top is shown in
Figure 4.50 before annealing and after annealing. Data for the thin film of gold is shown for
comparison. According to their cross-sectional views, the four-layer films have been
converted to two layers during annealing. Therefore, the back side of these films was also
measured, through the glass slide substrate. Figure 4.51 shows the different of reflectance
spectra between before and after annealing.
The other type of four-layer film had platinum on the top. These were silvery before
annealing, with the colour no different to that of a 30 nm film of pure platinum used as a
comparator. After annealing the upper layer turned yellow, as shown in Figure 4.52. In
contrast, the back side of these films turned from silvery to purple after annealing, as shown
in Figure 4.53. The X-ray patterns of these films are compared after deposition at 25 qC
(before annealing) and after annealing at 400 qC. The results indicated that AuAl2 or PtAl2
was formed on the top of the sample, depending on which element was uppermost in the
stack, Figure 4.54, and that they did not interdiffuse.
Figure 4.50 The reflectance spectra from the front side of a four-layered film of Al-Au-Pt (Au
on the top) as deposited at 25 °C before and after annealing. Data for a pure gold film of
30 nm thickness is shown for comparison
91
and after annealing
Figure 4.52 The reflectance spectra from the front side of a four-layered film of Al-Au-Pt (Pt
on the top) as depositing at 25 °C before and after annealing. Data for a pure platinum thin
film of 30 nm thickness is shown for comparison
92
and after annealing
(a)
50
nm
93
(b)
Figure 4.54 X-ray patterns of four-layer films of AlAuPt comparing the structure before and
after annealing at 400 °C (a) pure gold layer on the top and (b) pure platinum layer on the top
(both were deposited at 25 qC), with patterns for AuAl2 and PtAl2films shown for comparison
Six layer stacks
Six-layer sequences were prepared at 25 qC in the same way as for the four-layer
sequences described above. The first layer of these films was aluminium, which was followed
by platinum, aluminium, gold, aluminium and platinum, as shown in Figure 4.55(a). The
deposition conditions including current and deposition time for each layer are presented in
Table 4.7. Another arrangement of aluminium and gold was prepared using the same
deposition conditions for the purpose of comparison, shown in Figure 4.55 (b). Each layer of
this film is approximately 50 nm thick. After deposition these six-layer films were annealed
at 400qC for 30 minutes.
These Al-Au-Pt stacks reacted to form three layers during annealing, as shown in
SEM and TEM images, Figure 4.56. The top surface of the first of these films turned yellow
after annealing as shown in the reflectance spectrum in Figure 4.57. The X-ray pattern of this
six -layered film after annealing is shown in Figure 4.58, with the pattern for PtAl 2 shown for
comparison. Clearly, both PtAl2 and AuAl2 formed, but there were also some other phases
generated during annealing.
94
Pt
Al
Au
Al
Pt
Al
Au
Al
Au
Al
Au
Al
Substrate
Substrate
(a)
(b)
Figure 4.55 The arrangement of the six-layered films (a) Al-Au-Pt and (b) Al-Au
Table 4.7 Deposition conditions for the six-layered films of Al-Au-Pt
Layer
order
1
2
3
4
5
6
Metal target
Current (A)
Deposition time
Al
Pt
Al
Au
Al
Pt
0.4
0.1
0.4
0.1
0.4
0.1
5 minutes
2 minutes
5 minutes
2 minutes
5 minutes
2 minutes
Cross-sections of the six-layered film of Al-Au are shown for the as-deposited form in
Figure 4.59 (a) and (b). After annealing at 400 qC for 30 minutes, these films reacted to
become nearly a single layer, as seen in SEM images, Figure 4.59 (c) and (d). This film was
purple in colour by naked eye observation. The X-ray pattern of this Al-Au six-layered film
also indicated that AuAl2 was formed after annealing as shown in Figure 4.60.
(a)
(b)
100 nm
100 nm
95
(c)
100 nm
Figure 4.56 Cross-section views of the six-layer film of Al-Au-Pt after annealing at 400 °C
(a) SEM-In lens (b) SEM-RBSD and (c) TEM
Figure 4.57 The reflectance spectra of the top of the six-layer film of Al-Au-Pt (Pt on the top)
after annealing
96
Figure 4.58 The X-ray pattern of the six-layer film of Al-Au-Pt after annealing, with patterns
for PtAl2 and AuAl2 films shown for comparison
(a)
(b)
100 nm
100 nm
(c)
(d)
200 nm
200 nm
Figure 4.59 Cross-sections of six-layer films of Al-Au (a) before annealing-In lens, (b) before
annealing – RBSD, (c) after annealing at 400 °C – In lens and (d) after annealing at 400 °C –
RBSD
97
Figure 4.60 The X-ray pattern of the six-layer film of Al-Au before and after annealing, with
patterns for AuAl2 films shown for comparison
Eight-layer stacks
Eight-layered stacks were fabricated in a similar manner to that described above. The
two different arrangements are shown in Figure 4.61 (a) and (b). Each layer of pure metal is
approximately 50 nm thick. The deposition conditions of these films are given in Table 4.8.
Pt
Al
Au
Al
Pt
Al
Au
Al
Au
Al
Pt
Al
Au
Al
Pt
Al
Substrate
Substrate
(b)
(a)
Figure 4.61 The different arrangements of eight-layer films of Al-Au-Pt (a) 50 nm each layer,
Au on the top and (b) 50 nm each layer, Pt on the top
98
Table 4.8 Deposition conditions of eight-layer films of Al-Au-Pt. Each layer is 50 nm thick
Layer
order
1
2
3
4
5
6
7
8
Metal target
Current (A)
Deposition time
Al
Au or Pt
Al
Pt or Au
Al
Au or Pt
Al
Pt or Au
0.4
0.1
0.4
0.1
0.4
0.1
0.4
0.1
5 minutes
2 minutes
5 minutes
2 minutes
5 minutes
2 minutes
5 minutes
2 minutes
(b)
(a)
100 nm
100 nm
(c)
(d)
100 nm
100 nm
Figure 4.62 Cross-sectional views of eight-layered films of Al-Au-Pt (a) before annealing-In
lens, (b) before annealing – RBSD, (c) after annealing at 400 °C – In lens and (d) after
annealing at 400 °C – RBSD
The cross-section of the eight-layer film which had platinum on the top is shown
before and after annealing in Figure 4.62. The chemical inhomogeneity of the eight layers is
99
much reduced by annealing to the extent that the sample seems to consist for the most part of
only four layers. However, the X-ray result of these films, Figure 4.63, indicated that it was
the PtAl phase, not PtAl2, that formed after annealing. In support of this, the reflectance
spectrum of this films, Figure 4.64, did not indicate a yellow colour.
Figure 4.63 The X-ray pattern of eight multi-layers films of Al-Au-Pt (Pt on the top) after
annealing, comparing with PtAl2 film
Figure 4.64 The reflectance spectrum of the surface of eight multi-layers films of Al-Au-Pt
(Pt on the top) after annealing, comparing with a single PtAl2 film
100
The other type of eight-layered films, with gold on the top, also reacted to four layers
after annealing under the same conditions, Figure 4.65 (a). The flat surface area of these films
is homogenous, shown in Figure 4.65 (b). Their X-ray pattern indicated that AuAl2 was
formed after annealing process, as shown in Figure 4.66. The flat surface also turned purple
after annealing, as shown as reflectance spectrum in Figure 4.67.
(a)
(b)
100 nm
100 nm
Figure 4.65 The morphologies of eight multi-layer films of Al-Au-Pt which Au on the top
after annealing at 400 °C (a) cross-sectional area and (b) surface area
Figure 4.66 The X-ray pattern of the eight-layered films of Al-Au-Pt (Au on the top) after
annealing. A pattern for a simple AuAl2 film is shown for comparison
101
Figure 4.67 The reflectance spectrum of the surface of the eight-layer film of Al-Au-Pt (Au
on the top) after annealing, in comparison to that of a simple, single-layer AuAl2 film
The effect of the thickness of the individual layers was also investigated. An eightlayer film was prepared by using half the deposition times of the previous eight-layer film
with the gold on top. The thickness of each layer was approximately 25 nm. The stack and
deposition conditions of these films are shown in Figure 4.68 and Table 4.9, respectively. The
X-ray patterns of these films are shown for the condition before annealing and after annealing
at 400 qC. The X-ray results indicated that AuAl2 was formed, Figure 4.69. Similarly to the
thicker eight-layer films, four separate layers developed after annealing at 400 qC for 30
minutes. The cross-sectional views of these multi-layered films before and after annealing are
shown in Figure 4.70.
Au
Al
Al
Pt
Al
Au
Al
Al
Al
Al
Pt
Substrate
Figure 4.68 The arrangement of eight-layered films of Al-Au-Pt with each layer being 25 nm
thick (Au on the top)
102
Table 4.9 Deposition conditions of eight-layered film of Al-Au-Pt in which each layer is 25
nm thick
Layer
order
1
2
3
4
5
6
7
8
Metal target
Current (A)
Deposition time
Al
Pt
Al
Au
Al
Pt
Al
Au
0.4
0.1
0.4
0.1
0.4
0.1
0.4
0.1
2 minutes 30 seconds
1 minutes
1 minutes
1 minutes
1 minutes
Figure 4.69 The X-ray patterns of the eight-layered sample produced with half the deposition
time of the standard eight-layered sample of Al-Au-Pt (Au on the top), both before and after
annealing, compared with that of a simple, single-layer AuAl2 film
103
(a)
(b)
100 nm
100 nm
Figure 4.70 The cross-sectional view of the eight-layer films of Al-Au-Pt in which layer
thickness was halved, (a) before annealing and (b) after annealing at 400 °C for 30 minute
Formation of the intermetallic compounds during post-deposition annealing could be
followed by in situ XRD in a synchrotron on four layer stack formed by depositing pure
metals. Both of the two different arrangements (Pt and Au on the top) were investigated. In
Figure 4.71 the average of the area of (111), (200), (220) and (311) peaks of PtAl 2 and AuAl2
are shown for samples heated up from 50 q C up to 1250qC with ramp rate at 100 qC/min and
cooling down at the same rate. Under this condition, crystallization of intermetallic
compounds started at about 180 qC and was substantially complete at about 600 qC. Further
heating reduced the proportion of these intermetallic compound phases, possibly due to
oxidation or reaction of the thin film with its substrate. When the data have been considered
for evidence for the (111), (200), (220) and (311) peaks of PtAl2 and AuAl2, it was apparent
that both phases were present simultaneously during heating up between about 200 qC and
700 qC. It assumed that PtAl2 and AuAl2 did not interdiffuse, hence the two sets of peaks. In
Figure 4.72, the areas under the (111) peaks of PtAl2 and AuAl2 are shown. Furthermore,
these data supported that the view that AuAl2 formed at a lower heat-treatment temperature
than PtAl2.
104
Figure 4.71 Average integrated peak areas of PtAl2 and/or AuAl2 over the (111), (200), (220)
and (311) peaks as a function of temperature
Figure 4.72 Peak area of four layers stack formed by depositing pure metals at (111) of PtAl2
and AuAl2
105
A stack formed by co-depositing precious metals
The second strategy was to form a stack of co-deposited Au and Pt, interleaved with a
layer of pure Al. The co-deposited layer was designed to be 50 at.% Au and 50 at.% Pt at
various thicknesses ranging from 20 nm to 120 nm. Deposition was carried out at room
temperature. After that the films were annealed in a tube furnace at 400qC for 60 minutes.
Different sequences of these layers were investigated. The designs of these films are shown
in Figure 4.73 (a) and (b). The effect of the thickness of co-deposited precious metals was
also investigated. This was done by reducing the deposition time, Figure 4.73 (c).
Al
50 at.% Au + 50 at.% Pt
50 at.% Au + 50 at.% Pt
Al
Substrate
Substrate
(b)
(a)
50 at.% Au + 50 at.% Pt
Al
Substrate
(c)
Figure 4.73 The design of stacks consisting of co-deposited precious metals and aluminium
(a) precious metals on the bottom (Al/(Au,Pt) ), (b) precious metals on the top ( (Au,Pt)/Al)
and (c) co-depositing precious metals on the top but with half the thickness
The composition of Au-Pt in the co-deposited films was verified by EDS and XRD.
The composition in the 120 nm thick film was 54 at.% Au and 46 at.% Pt. When compared to
those of pure Au and pure Pt, the X-ray pattern of this film, Figure 4.74, indicates that a solid
solution of the two elements had formed with a lattice parameter of 0.4006 nm which lies
between pure Au films (0.4077 nm for our Au films, compared to a literature value of 0.4079
nm for the bulk) and pure Pt films (0.3935 nm for our Pt films, compared to a literature value
of 0.3924 nm for the bulk). Therefore, from Vegard’s rule, a lattice parameter of 0.4006 nm
would be expected for a solid solution. Under equilibrium conditions Au and Pt do not form a
solid solution at room temperature, however metastable solid solutions in this system can
certainly be found, as for example after quenching alloy samples from elevated temperatures.
In the present case, the freshly deposited material has not had any time to attain equilibrium
and so is in the form of a metastable fcc solid solution. After the co-deposited Au-Pt was
successfully produced, a single layer of pure Al film was deposited following the procedure
which was described previously.
106
Pt
Au
AuPt
Pt
Au
Au
AuPt
Pt
Au
AuPt
Pt
Au
AuPt
Pt
Figure 4.74 A comparison of X-ray patterns of thin films of (Au,Pt) solid solution to those
pure Au and pure Pt
As mentioned, there are two different arrangements of a stack of co-deposited Au and
Pt, interleaved with the layer of pure Al. The X-ray patterns of these films, as-deposited and
after annealing at 400qC for 60 minutes, are compared in Figure 4.75. It seems that
(Au,Pt)/Al did not show differences between as-deposited and annealed conditions, while
new phases were formed in the Al/(Au,Pt) sample after annealing at 400 qC. This is probably
because the Al layer was protected from oxidation by a top layer of noble metal. The X-ray
patterns of the Al/(Au,Pt) sample after annealing at various temperature are compared with
as-deposited samples in Figure 4.76. These X-ray patterns showed that a thin film of (Au,Pt)
solid solution was formed when the elements were co-deposited at 25 qC, and this solid
solution was stable up to a heat treatment of 300 qC. At higher temperatures (400 qC and 500
qC) a phase transformation started. The X-ray patterns of the Al/(Au,Pt) sample that had
been annealed at 400 qC and 500 qC were analysed and are compared in Figure 4.77. At an
annealing temperature of 400 qC, the Al, Au and Pt each existed separately, ie. the (Au,Pt)
solid solution had separated into individual regions of Au and Pt but unidentified new phases
were also formed when the sample was heated at 500 qC. These new peaks did not match any
compounds in the database.
107
Figure 4.75 X-ray patterns of different arrangements of stacks made of a layer of
co-deposited Au and Pt, and Al, before and after annealing
Figure 4.76 X-ray patterns of an Al/(Au,Pt) sample, followed by annealing at various
temperatures
108
Figure 4.77 Comparison of X-ray patterns of an Al/(Au,Pt) sample, followed by annealing at
400 qC and 500 qC
Three different thicknesses (120, 60 and 20 nm) of co-deposited Au-Pt on the top
layer were analysed by XRD after annealing at 400 qC, Figure 4.78. These samples were also
analysed by EDS, Table 4.10. The X-ray patterns showed that different compounds had been
formed. As mentioned previously, for the Al/(Au,Pt) film with 120 nm of co-deposited AuPt, the Al, Au and Pt were separated after annealing at 400 qC. In contrast, the Al/(Au,Pt)
film with 60 nm of co-deposited Au-Pt mainly formed PtAl compounds after annealing. The
un-identified peak marked as ‘*’ is assumed to be pure gold. The composition of Al/(Au,Pt)
with 60 nm of co-deposited Au-Pt sample was 45 at.% Al, 44 at.% Pt and 11 at.% Au.
Interestingly, Al/(Au,Pt) with 20 nm of co-deposited Au-Pt formed a phase with cF12
structure. EDS analysis of this sample indicated that its composition was 65 at.% Al, 23 at.%
Pt and 12 at.% Au. In Figure 4.79, Rietveld analysis of this sample yielded a lattice parameter
of a= 0.5952 nm, which close to a value of 0.5958 nm that could expected form applying
Vegard’s Law to a mixture of AuAl2 and PtAl2 in the composition ratio of the film.
109
*
Figure 4.78 Comparison of the X-ray diffraction patterns obtained after annealing the stacks
with 20, 60 and 120 nm of (Au,Pt) at 400 qC
Table 4.10 Chemical composition of a stack of Al/(Au,Pt) with various thicknesses of
precious metals, measured after annealing at 400 °C for 60 minutes
Thickness of Au-Pt layer as deposited
Composition (at.%)
120 nm
60 nm
20 nm
Pt
47
44
23
Au
26
11
12
Al
27
45
65
110
Figure 4.79 X-ray patterns of a mixed AuAl2/PtAl2 sample formed by co-depositing Au and
Pt onto Al. The fitted pattern was obtained by Rietveld refinement on a PtAl2 structure
A cross-sectional microstructure of Al/(Au,Pt) with 20 nm of co-deposited (Au,Pt)
was analysed by TEM including EDS elemental scan and mapping, Figure 4.80. It is clear
that in this case an Au-Pt-Al solid solution was formed. The top layer of the surface contains
Pt, Au and Al. It is polycrystalline and lattice fringe image is shown in Figure 4.81 (a). The
deeper part has a slightly higher concentration of Au while the surface is Pt rich. The bottom
part of sample contains an Al-rich layer which is amorphous, Figure 4.81(b). There appears
to be some mass transfer of the film to the Si boundary.
111
Figure 4.80 Cross-section of the Al/(Au,Pt) sample with 20 nm of co-deposited (Au,Pt) with
EDS elemental scan and mapping
112
(b)
(a)
0.298 nm
{200}
Figure 4.81 High resolution TEM images of the Al/(Au,Pt) sample with 20 nm of codeposited (Au,Pt) (a) top layer and (b) bottom layer
An Al/(Au,Pt) sample with 20 nm of co-deposited (Au,Pt) (65 at.% Al- 23 at.% Pt-12
at.% Au overall) was deposited onto a glass slide so that the optical properties of both sides
of the film could be observed. After annealing, this sample was yellow on both sides by
naked eye observation. In Figure 4.82 I compare the reflectance spectra of both sides of this
sample after annealing with that of the front side before it had been annealed and with that of
a pure PtAl2 film which was prepared by the co-sputtering technique described previously.
Although the reflectivity is overall a bit lower than for the pure PtAl2 film, it is clear that
annealing has produced a XAl2-type compound right through the film.
The amount of gold was increased in another sample (also with 20 nm thickness of
(Au,Pt). In this case the overall analysis was 56 at.% Al-22 at.% Pt-22 at.% Au). After
annealing the back side of sample was purple while the top was silvery. The reflectance
spectra of this sample are shown in Figure 4.83. The reflectance of the sample before
annealing and that of pure AuAl2 film was prepared by co-sputtering are shown for
comparison.
The X-ray diffraction patterns of both samples before and after annealing are
compared with each other and with pure PtAl2 and pure AuAl2 in Figure 4.84. From this data
it seems that the 65 at.% Al- 23 at.% Pt-12 at.% Au sample formed (Au,Pt)Al2 after
annealing. However, the increased amount of Au and decreased amount of Al in sample 56
at.% Al-22 at.% Pt-22 at.% Au has led to the formation of one or more phases in addition to
the (Au,Pt)Al2.
113
Figure 4.82 The reflectance spectra of the 65 at.% Al-23 at.% Pt-12 at.% Au sample on its
front and back sides, compared with the front side of the sample before annealing and that of
a pure PtAl2 film
Figure 4.83 The reflectance spectra of 56 at.% Al-22 at.% Pt-22 at.% Au on its front and back
sides, compared with the front side of the sample before annealing and a pure AuAl2 film
114
(a)
(b)
Figure 4.84 X-ray diffraction patterns of samples made by co-depositing Au and Pt on top of
an Al layer (a) 65 at.% Al - 23 at.% Pt - 12 at.% Au and (b) 56 at.% Al - 22 at.% Pt - 22 at.%
Au. Data for before and after annealing, and for pure PtAl2 and pure AuAl2 is shown
115
An Al/(Au,Pt) sample consisting of ~200 nm of Al followed by a co-deposition of a
combination of 54 at.% Au and 46 at.% Pt with ~100 nm thickness was fabricated on
austenitic stainless steel foil. The formation of the intermetallic compounds during postdeposition treatment was examined in this sample using X-ray diffraction in a synchrotron
beamline. This sample was heated from 30 °C up to 1000° C with ramp rate at 3 °C/min. The
peaks of Pt and Al elements are visible up to a temperature of 310 ° C. The lattice parameter
of Au is similar to that of Al and was obscured by the peak of the relatively thick Al
underlayer. At 312 °C the peaks of a cF12 phase appear with a lattice parameter of 0.599 nm.
In Figure 4.85 (a), the high temperature lattice parameters of AuAl2 (purple line) and PtAl2
(yellow line) have been provided by extrapolating from room temperature using an estimate
of 13x10-6 K-1 for the coefficient of thermal expansion. The (Au,Pt)Al2 phase has a lattice
parameter intermediate between that of AuAl2 and PtAl2. The value is very close to what
might have been expected by applying Vegard's rule to a mixed solid solution of (Au,Pt)Al2.
When the temperature reaches above 350 °C, Pt2Al3 phase appeared while (Au,Pt)Al2 phase
is decreased as shown in Figure 4.85(b). However, the lattice parameter of the (Au,Pt)Al2
phase was increased to the value expected for AuAl2. Clearly, the Pt content of the original
(Au,Pt)Al2 phase has transferred to the newly formed Pt2Al3 phase, leaving the remaining
(Au,Pt)Al2 gold-rich. Raising temperature up at about 380°C, (Au,Pt)Al2 completely
disappeared and AuAl and other PtxAly compounds were possibly formed. Only an isothermal
experiment could prove that the mixed (Au,Pt)Al2 phase is actually metastable, but this seems
the most probable situation. The sequence of reactions as this sample approaches equilibrium
is probably:
(Au,Pt)+Al o (Au,Pt)Al2 o AuAl2 + Pt2Al3 o AuAl +PtxAly + Pt2Al3
(a)
116
(b)
Figure 4.85 Lattice parameter and peak area of Al/(Au,Pt) sample as a function of
temperature (a) Lattice parameter of (Au,Pt)Al2 in Al/(Au,Pt) sample and (b) peak area of
(Au,Pt)Al2 phase (111) and Pt2Al3 (002)
4.5 Conclusion
Coloured intermetallic compounds are attractive particularly in jewellery and
decorative applications. Two well-known coloured intermetallic compounds are purple gold
(AuAl2) and golden platinum (PtAl2). These two coloured intermetallic compounds share the
CaF2 or cF12crystal structure. Since the two compounds share the same structure and have
similar lattice parameters they might be expected to show unlimited mutual solubility by the
Hume-Rothery rules. The Pt-Au-Al ternary phase diagram appears to have not yet been
constructed so I investigated the situation myself. There are two interesting issues to be
highlighted: (i) whether AuAl2 and PtAl2 are mutually soluble, or not, and (ii) if not, then
whether nanoscale duplex microstructures comprised of these two compounds could be used
to tune the colour between that of the components.
The very narrow range of stoichiometry is a challenge for the fabrication of samples
of these unique coloured intermetallic compounds. Nevertheless, with care, PVD with the cosputtering technique has been applied to produce them. Several types of layers and
arrangements of these coloured intermetallic thin films were designed. Pure single layers of
AuAl2 and PtAl2 were successfully fabricated and compared with data from bulk samples
made externally by vacuum arc melting. The colour of these thin films can be tuned by
controlling their thickness. Bi-layers and multilayers of coloured intermetallic compounds
have been produced in order to explore two interesting issues mentioned previously.
Considering the first issue, are AuAl2 and PtAl2 mutually soluble? Examination of the
bi-layers of AuAl2/PtAl2 and of stacks formed by depositing pure metals showed clearly that,
if the two coloured intermetallic compounds are formed separately, then they will not
subsequently interdiffuse. This shows that the equilibrium situation is one of mutual
immiscibility, as suggested by Klotz [5]. However, metastable solid solutions of either Au
117
and Pt, or AuAl2 and PtAl2, may be prepared by magnetron co-deposition at room
temperature. The present work has also served to highlight that quite precise control of the
deposition and heat-treatment parameters is required to produce the desired phases, in terms
of film composition as well as time and temperature of the post-temperature heat-treatment.
The second issue was what the possible range of colours would be if tuned between
that of the two coloured intermetallic compounds. It is certainly possible to produce
metastable solid solution of AuAl2-PtAl2with a corresponding range of intermediate colours.
However, it should also be recognized that very similar colours can be presented to an
external observer by either mutual solid solutions of the (Au,Pt)Al2 type or by phase mixtures
of AuAl2 and PtAl2. The mixing effect in this ternary system is similar to that occurring in the
binary Au-Ni system in which a range of equivalent colour effects can be achieved by true
alloying or by a two-phase mixture of the elements [203]. Since a (Au,Pt)Al2 solid solution is
actually metastable and may decompose during heat-treatments, it is suggested here that
multi-layers of pure AuAl2 and PtAl2 are the preferred option. These would be stable under
equilibrium conditions and immiscible. Either way, however, a range of attractive
intermediate colour effects can be achieved using these two intermetallic compounds.
118
Chapter 5
Nanoporous platinum sponges
119
5.1 Background
Nanoporous platinum is an attractive material due to its unique structure and
properties. In particular, it has high specific catalytic activity, high surface area, stability and
biocompatibility [205-207]. Therefore, nanoporous platinum has potential applications in
catalysis, electrodes, sensors, actuators and fuel cells [73, 207-211]. As mentioned in Chapter
2, there are various methods to produce nanoporous platinum but de-alloying is probably the
most suitable and effective method. Generally, there are two steps for fabricating nanoporous
metallic sponges by this route: alloying and de-alloying. For those two steps, there are
alternative techniques for the nanoporous platinum sponges fabrication as shown in Figure
5.1[207]. The first step is the alloying process. Three main techniques can be applied
including melting techniques, bi-layer fabrication techniques and co-deposition techniques.
Most commonly, Pt-based binary alloys are produced as precursors including Pt-Cu [128,
129, 212-214], Pt-Ag [215], Pt-Zn [131, 216], Pt-Ni [217-219], Pt-Si [132, 133, 220], Pt-Co
[127, 221] and Pt-Al [134, 143, 222, 223]. The Pt-based ternary alloys can also be fabricated
into nanoporous platinum sponges. Examples include Pt-Au-Ag [224], Pt-Au-Cu [225], PtAu-Al [226] and Pt-Ni-Si [227] or platinum composites. After fabricating the precursors,
annealing for homogenization should be carried out. Then the second step for producing
nanoporous platinum sponges is de-alloying. Free corrosion and corrosion under
electrochemical potential control are options for de-alloying step.
Figure 5.1 Various techniques for nanoporous platinum fabrication [207]
In the material science view the structure of materials affects their properties and
hence their applications. Therefore, many researchers have paid attention to controlling the
structure of nanoporous platinum sponges. It has been found that various morphology types
can be obtained by varying parameters during either the alloying or the de-alloying processes.
Here I will concentrate on the sponges produced by de-alloying the Pt-Al films described
previously in Chapter 4. We found that Pt-Al system is a very interesting system. There are
many different phases that can be formed by controlling the composition deposited during co-
120
sputtering. Other parameters applied during the alloying and de-alloying processes were also
studied.
The Pt-Al binary alloy system was selected for use as the precursor for producing
nanoporous platinum by de-alloying. The Pt-Al phase diagram is quite complex with
intermetallic compounds including Pt5Al21, Pt8Al21, PtAl2, Pt2Al3, PtAl, E, Pt5Al3, Pt2Al and
Pt3Al as shown in Figure 5.2 [228]. These compounds have different crystal structures such
as cubic, trigonal, tetragonal, hexagonal and orthorhombic, Figure 5.3 [229]. Some
metastable phases such as PtAl6, PtAl5, PtAl4, Pt6Al21 and PtAl3 have also been reported [228,
230-234]. For example the tetragonal PtAl5 or H phase was found in as-quenched samples
[234]. The trigonal PtAl4, or more probably cubic Pt5Al21 as previously reported [229, 230,
235, 236], formed by peritectic reaction at approximately 800 qC from PtAl3 while Pt6Al21
phase transforms into Pt5Al21 during heat treatment at 200 qC for several hours [230].
Another metastable phase is orthorhombic PtAl6 which can be preserved close to the melting
point of the alloy [231]. Besides thermal history, the Pt concentration is also a key factor
controlling the formation of PtAl6, PtAl5, PtAl2 and Pt2Al3 intermetallic phases [232].
Figure 5.2 Pt-Al phase diagram [237]
121
Figure 5.3 The different lattice types of the intermetallic compounds in the
Pt-Al binary system [229]
The precursor is the most important factor that controls the morphology of the
nanoporous platinum formed by de-alloying. Generally, the nanoporous metallic materials
have a bi-continuous, vermicular morphology with metal ligaments and open channels [80,
81]. These structures are formed by a phase separation process at the solid and electrolyte
interface, with aspects of the process being analogous to those of a spinodal decomposition
[89, 238]. However, there is also another sponge morphology reported. This is formed by
interconnected spherical cavities [109, 133, 196]. Therefore, I have suggested that there are
two main types of nanoporous metal sponge: isotropic fibrous sponge and isotropic foam
sponge [239]. There is also the possibility of forming an anisotropic fibrous sponge with
metal ligaments aligned in the growth direction [133, 227]. Other morphologies such as like
the 'cracked mud' type of sponge were formed by shrinkage and fracture of a de-alloyed
matrix [240, 241].
There is a critical range of precursor compositions over which de-alloying processes
can form a sponge. The minimum content of sacrificial metal required is defined as the dealloying threshold. The parting limit refers to the concentration of a noble metal in an alloy at
the de-alloying threshold [76]. Alloys with more than that content of noble metal will not dealloy readily or at all. Simmonds et al. studied the de-alloying in 4 M NaOH solution of
various compositions of Pt-Al alloys produced by using dc co-sputtering deposition. It was
found that at Pt0.40Al0.60, aluminium dissolution occurred while at Pt0.80Al0.20 there was no
122
evidence of aluminium dissolution occurring [134]. Cortie et al. reported that mesoporous
platinum with pores size in range 2 to 20 nm can be produced from PtAlx (x § 2) precursor.
PtAlx was prepared by co-depositing the elements using high vacuum dc magnetron
sputtering, then the film was de-alloyed in 0.5 M NaOH solution [109]. Nahm et al. studied
the effect of Al content on nanoporous platinum thin film produced from a Pt-Al precursor.
PtxAl1-x (x § 0.27, 0.52, and 0.76) were prepared by rf magnetron co-sputtering method. The
nanoporous platinum structure was formed by electrochemical dissolution of Al in 0.5 M
H2SO4. It was found that Pt0.52Al0.48 showed both enhanced catalytic activity and mechanical
stability after 500 cycles[223]. In general, these previous studies have showed that the mole
fraction of active metal, FA must be more than the parting limit, which is approximately 0.5
[242-244]. A value of FA in the range of about 0.65 to 0.88 is most suitable for formation of
nanoscale sponges for PtAlx [109, 143, 245].
In addition, however, the different atomic ratios of noble and active metals may result
in the formation of different intermetallic phases in the precursor. This may also have an
influence on the morphology of sponges. Recently, we published the effect of precursor
stoichiometry on the morphology of nanoporous platinum sponges[239]. In this publication
we examined whether this control of microstructure can be used to influence the morphology
of the nanoporous platinum film formed after de-alloying. For this work I fabricated PtAlx
thin film precursors with mole fraction of Al between 0.30 and 0.90 at various deposition
temperatures and my experimental work on these samples forms the bulk of the Results
section of the present Chapter. It was found that the precursors that had been deposited at
elevated temperatures and with mole fraction of Al between 0.65 and 0.90 produced the
classic isotropic fibrous sponges. However isotropic foamy sponges can be produced from
precursors that were deposited at room temperature. We concluded that the formation of
fibrous sponge requires an equilibrated precursor while foamy morphologies will be formed
if the precursor is metastable. The effect of increasing Al content (ȤAl > 0.80) on the structure
of sponges is shown in Figure 5.4 [239]. The crystallinity of the Pt sponges decreased when
ȤAl was increased towards 0.88. Furthermore, it has found using in situ X-ray diffraction in a
synchrotron experiment that the relative proportion of İ and PtAl4 depends on the thermal
history.
123
Figure 5.4 Effect of Al content on structure of sponges produced from precursors with ȤAl >
0.80 (a) X-ray diffraction pattern of increasing amount of Al, (b) X-ray diffraction patterns of
de-alloyed Pt sponges (c) and (d) SEM micrograph of isotropic foamy Pt sponge from
precursor with ȤAl = 0.88 and 0.85 respectively (e) TEM micrograph of Pt sponge from
precursor with ȤAl = 0.88 [239]
In addition, my co-authors in reference [49] performed a simulation of the de-alloying
of the sponges using a computer program based on the Metropolis Monte Carlo algorithm.
This was used to investigate the range of possible sponge morphologies. The average mean,
and Gaussian, curvatures of surface regions of the simulated sponges were computed by
fitting biquadratic surface patches. As stoichiometry changed in the model system, the
average mean and Gaussian curvature of the sponges also systematically changed as shown in
Figure 5.5 [49].
124
Figure 5.5 Simulation of the de-alloying of the sponges by using Monte Carlo model as a
function of ȤAl (a) Morphologies of sponges in various aluminium content (b) Ratio of surface
atoms to total atoms of sponges (ȣ) and ȤAl remaining in sponge. (c) Average mean and
Gaussian curvatures of sponges. (d) Effect of Lennard-Jones temperature on the de-alloying
of a starting alloy with ȤAl = 0.80. This work was performed by my co-authors [239]
Apart from the initial alloy composition, other factors including deposition
parameters, de-alloying solution and the de-alloying temperatures and times are also able to
control the microstructure of nanoporous metals, especially the characteristic size of
ligaments and channels [80, 207]. Antoniou et al. synthesized nanoporous platinum through
electrochemical de-alloying in HF solution from co-sputtered PtxSi1-x amorphous films. It was
found that the three different morphologies of nanoporous platinum could be obtained by
varying the deposition parameters including composition and sputter bias conditions. These
distinct morphologies are shown in Figure 5.6 as correlated with alloy composition, thickness
and sputter bias conditions [133]. Kloke et al. reported the effect of de-alloying solution on
ligament size as shown in Figure 5.7. The ligament sizes of different alloy systems and
different (alkaline and acidic solutions) de-alloying solutions were compared [207]. Abburi et
al. fabricated nanoporous platinum by chemical de-alloying of co-sputtered Pt-Cu films that
had been coarsened at various temperatures. It was shown that the coarsening temperature
had an influence on the pore size of nanoporous platinum. Pt0.20Cu0.80 de-alloyed in
93%H2SO4 and coarsened at 250, 300,400, and 500 qC yielded pore sizes of <5, 5-10, 25 and
35 nm respectively as shown in Figure 5.8. It was also found that the optimum de-alloying
time was 15 minutes on 150 nm thick film of Pt0.20Cu0.80 [246].
125
Figure 5.6 Three distinct morphologies of nanoporous platinum as correlated with cosputtering parameter, initial alloy composition and thickness [133]
Figure 5.7 Effect of de-alloying system on ligament size of nanoporous platinum from
different alloy systems (a) ligament sizes of different noble metal-aluminium with dealloying with 5% HCl and 20% NaOH and (b) ligament sizes of platinum-gold-copper alloys
with varying noble metal content
126
Figure 5.8 Nanoporous platinum produced from Pt0.20Cu0.80, then de-alloyed in 93%H2SO4
and coarsened at different temperatures (a) 250 qC (b) 300 qC (c) 400 qC and (d) 500 qC
[246]
5.2 Objectives of this chapter
The Pt-Al binary alloys are an interesting option for use as precursors for the
production of nanoporous platinum by de-alloying. Furthermore, the morphologies of
nanoporous platinum play important roles in their applications. In this chapter I investigate
how nanoporous Pt can be fabricated from Pt-Al precursors through chemical de-alloying.
My objective is to find out how parameters such as precursor composition and structure, and
de-alloying condition control the morphologies of nanoporous platinum. Finally, I try to
explain these effects.
5.3 Experimental detail specific to this chapter
Pt-Al precursors were prepared by co-depositing the elements under high vacuum
using DC magnetron sputtering on inert substrates. In these experiments, the substrates were
glass slides, silicon wafers or austenitic stainless steel foil, depending on what
characterization techniques were to be applied subsequently. The surfaces of the substrates
were cleaned using detergent or acetone in a sonicator then rinsed with distilled water and/or
acetone and dried in N2. The base pressure of the chamber was ‫׽‬10í6 Torr and a flow of Ar
was at ‫ ׽‬2mTorr during deposition. The amount of each element deposited was controlled by
monitoring deposition rates on a quartz crystal microbalance and by independently varying
the current or power level on each target. A series of Pt-Al samples, with aluminium contents
127
varying from 50 at.% to ‫׽‬98 at.% were prepared. The deposition temperature was at room
temperature or 400 qC. Some samples, which had been co-deposited at room temperature,
were subsequently annealed at temperatures from 100 to 500 qC in a tube furnace with N2
flow. De-alloying of selected samples was carried out by immersing those samples in 0.2 M
NaOH or 0.2 M Na2CO3 solution. During the de-alloying process, aluminium is removed and
bubbles of H2 formed, as illustrated in Figure 5.9. It was assumed that process was complete
once bubbling stopped. Pt-Al precursors and de-alloying samples were prepared in various
series in order to investigate the effect of processing parameters on morphology of
nanoporous platinum. A flowchart of various parameters during sputtering (Al composition
control, deposition rate, deposition time, deposition temperature, and annealing temperature)
and de-alloying process (de-alloying solution and de-alloying time) for this experiment is
displayed in Figure 5.10.
5 mm
(a)
(b)
Figure 5.9 De-alloying process on Pt-Al precursor (a) Bubble of H2 on Pt-Al precursor
immersing in alkali solution and (b) model of aluminium removing from Pt-Al precursor
[239]
Two sources of X-ray diffraction were used to characterize the samples. The first one
was a Siemens D5000 Diffractometer using Cu KĮ radiation (Ȝ = 0.154184 nm and the
incidence angle was set at 0.5°). The second source was on the powder diffraction beamline
at the Australian Synchrotron using a standard flat plate arrangement inside an Anton Paar
furnace with an argon atmosphere (Ȝ = 0.1261 nm, the incident angle ȍ of 2° and beam crosssection 4 mm × 0.2 mm). The morphology of the samples was obtained by imaging on a
Zeiss Supra 55VP SEM. Selected precursors and de-alloyed samples were also characterised
by TEM (Philips CM120 and, for precise EDS analysis and hi-resolution imaging, on a 200
kV JEOL 2200FS).
128
Preparation of Pt-Al precursors
Co-sputtering
deposition
Composition control
(varying Al content form 50 – 98 at.% )
Selected
samples
Deposition rate
Fast rate
Deposition time
Slow rate
5 min
10 min
Deposition temperature
Room
temp
30 min
Elevated
temp
Annealing
100 qC
200 qC
300 qC
400 qC
500 qC
Preparation of nanoporous Pt
De-alloying
De-alloying time
De-alloying solution
NaOH
Na2CO3
1 min
3 min
5 min
10 min
15 min
Figure 5.10 Flowchart showing preparation of Pt-Al precursors and the subsequent
nanoporous platinum
129
5.4 Results and discussion
Nanoporous platinum sponges were prepared under various conditions. As mentioned
previously in the Experimental section, preparation of Pt-Al precursor and chemical dealloying are the two distinct steps for producing nanoporous platinum. During these processes
many parameters have been varied including Al content, deposition rate, deposition time,
deposition temperature, annealing temperature, de-alloying solution and de-alloying time.
These parameters are related to the chemical composition, film thickness and microstructure
of the films and have a subsequent effect on the morphology of the nanoporous platinum. In
this section, I divided the effects into subtopics including effect of composition, effect of
temperature, effect of deposition rate, effect of deposition time and effect of de-alloying
parameters.
5.4.1 Effect of composition
Pt-Al precursors were prepared by varying current or power level on the Al sputter
target in order to obtain the samples with Al contents from 50 to 98 at.% (0.50 <ȤAl< 0.98).
In one set of experiments, the Pt-Al precursor films were deposited at elevated temperature (~
400qC). In this case the films became crystalline with various intermetallic phases forming.
These samples were used to investigate the correlation between the microstructure of asdeposited films and ratio of Al to Pt. According to the Pt-Al phase diagram, increasing Al
content controlled the formation of various phases. Precursors with ȤAl <0.60 were expected
to form PtAl and other Pt-rich phases including Pt5Al3 and Pt2Al. In fact, the X-ray
diffraction pattern of the sample with ȤAl | 0.50 showed a mixture of Pt2Al3 and either Pt5Al3
or Pt2Al, but, the PtAl phase expected from the equilibrium phase diagram did not occur, as
shown in Figure 5.11(a). The precursors in this range did not form sponge morphologies after
de-alloying process due to them containing more than the parting limit of Pt. The surface was
smooth and displayed an absence of obvious porosity as shown in Figure 5.11(b).
(a)
(b)
500 nm
comparing with other phases from calculated and database and (b) SEM micrograph after dealloying showing that a nanoporous sponge did not form
130
The precursors with 0.60 <ȤAl <0.67 should form 100% Pt2Al3 at ȤAl =0.60 and 100%
PtAl2 at ȤAl = 0.67. Samples with composition closer to ȤAl = 0.60 produced partially dealloyed sponges. The X-ray pattern of ȤAl | 0.60 indicated that Pt2Al3 was formed, shown in
Figure 5.12(a). The SEM images before and after de-alloying were compared in Figure
5.12(b and c). The precursors deposited at elevated temperature with ȤAl = 0.67 were
confirmed to form pure PtAl2 by both their cubic diffraction pattern and their obvious brassy
yellow colour [189]. The X-ray diffraction pattern and reflectance spectra of precursors as
deposited are shown in Figure 5.13(a) and (b) respectively. PtAl2 only forms over a very
narrow range of compositions and it is difficult to make it. It is also essential to either deposit
at an elevated temperature or to do a subsequent heat treatment to produce crystalline
samples. However, care had to be taken to avoid oxidation of Al during the deposition
process otherwise Pt2Al3 was formed instead of PtAl2. Samples with composition closer to ȤAl
=0.67 de-alloyed rapidly resulting in a porous and cracked film, with a characteristic
microstructure of elongated voids, channels and cracks, surrounded by higher density
material or ‘mud-cracked’ mesoporous sponges, shown in Figure 5.14. It is also evident that
shrinkage has taken place. The remaining Pt contained some internal spherical or vermicular
porosity to the solid as shown in TEM images (Figure 5.15).
(b)
100 nm
(c)
(a)
200 nm
comparing with Pt2Al3 from database (b) SEM micrograph before de-alloying (c) SEM
micrograph after de-alloying
131
(b)
100
Figure 5.13 Pt-Al precursor film deposited at 400 qC with ȤAl = 0.67 (a) XRD patterns
comparing with Pt2Al3 from database and (b) reflectance spectra
(a)
(b)
100 nm
200 nm
20 Pm
(c)
Figure 5.14 ‘Mud-cracked’ sponges produced by de-alloying sample with ȤAl = 0.67 (a) a
porous and cracked film (b) cross-sectional view and (c) curled up porous and cracked film at
low magnification
132
Figure 5.15 TEM micrographs of de-alloyed samples with ȤAl = 0.67
Next I consider precursors in the range 0.67 <ȤAl <0.80. According to the equilibrium
phase diagram, there are two mixed phase regions in this range. A mixture of PtAl2 and
Pt8Al21 should be formed in range of 0.67 <ȤAl <0.72, and then close to 100% Pt8Al21 at about
ȤAl = 0.73. Another mixed phase region of Pt8Al21 and PtAl4 should be formed in range 0.73
<ȤAl <0.80, followed by close to 100% PtAl4 at about ȤAl = 0.80. In this range, phase
formation is complicated due to the possible formation of the metastable intermetallic
compounds which were mentioned previously. The precursors with ȤAl = 0.78 formed Pt8Al21
as indicated by XRD patterns comparing with JCPDF database in Figure 5.16(a). The peak
marked ‘*’ in the samples deposited at elevated temperature could be due to the metastable İ
phase or (PtAl5) [234]. A sample with ȤAl = 0.75 deposited at room temperature and heated in
a synchrotron X-ray diffraction experiment at 3 °C·miní1, crystallized at about 360 °C,
Figure 5.16 (b). The peaks were formed in this sample were similar to those reported for İ
phase. The peaks marked ‘V’ in this sample are probably due to Pt8Al21 formation. Dealloying of precursors with 0.67 <ȤAl <0.80 that deposited at elevated temperature produced
isotropic fibrous sponges. The morphologies as deposited and de-alloyed are shown in Figure
5.16 (c), (d) and (e).
When the precursors contained ȤAl > 0.80, the microstructure should contain Al with
PtAl4[247]. The presence of Al was confirmed by XRD in samples with ȤAl = 0.82, as shown
in Figure 5.17(a). However, the most obvious phase in this sample was İ instead of PtAl 4.The
precursor with ȤAl = 0.82 still produced classic isotropic fibrous sponges, Figure 5.17 (b) and
(c). Finally, the precursors with ȤAl > 0.90 that deposited at elevated temperature were rapidly
de-alloyed. There was no sponge formed, but the Pt component formed a disordered and
fragile mass. The XRD pattern indicated that PtAl6 and pure Al were formed as shown in
Figure 5.18 (a) and the SEM micrograph of the as deposited material also showed the Al
crystals before de-alloying process in Figure 5.18 (b). SEM and TEM micrographs of
133
samples de-alloyed from the precursors ȤAl > 0.90 are shown in Figure 5.18 (c) – (e). SEM
images showed a low density and disordered mass of very friable Pt and TEM images
showed that the filaments of Pt are actually comprised of equiaxed Pt nanocrystals. The Al
content of de-alloyed samples was analysed by EDS on TEM. It was found that the ȤAl was of
the order of 0.10 to 0.15, in agreement with values in the literature [143, 248]. Evidently,
there is always a residual Al content that is encapsulated by Pt.
(a)
(c)
100 nm
(d)
Temperature (qqC)
500
400
100 nm
300
(e)
200
100
(b)
10
20
30
100 nm
2-Theta
q
Figure 5.16 Pt-Al precursor film with 0.67< ȤAl < 0.80 (a) XRD patterns of precursors with
ȤAl = 0.78 (deposited at 400 qC) and precursors with ȤAl = 0.75 (deposited at room
temperature then crystallized by heating ~400 qC), comparing with Pt8Pt21 from database and
reported İ phase (b) crystallization of İ phase at ~360 qC on heating up precursor with ȤAl =
0.75 (c) morphology of Pt-Al precursor with ȤAl = 0.78 as deposited (d) SEM micrograph of
isotropic fibrous sponges in plain view and (e) SEM micrograph of isotropic fibrous sponges
in cross-sectional view
134
100 nm
(b)
20 nm
(c)
(b)
100(a)
Figure 5.17 Pt-Al precursor film deposited at 400 qC with ȤAl = 0.82 (a) (a) XRD patterns
comparing with Al-rich phases from database and İ phase (b) and (c) SEM micrograph of
isotropic fibrous sponges
(e)
135
(a)
(b)
(c)
200 nm
200 nm
(d)
(e)
Figure 5.18 Pt-Al precursor film deposited at 400 qC with ȤAl > 0.90 (a) XRD pattern
comparing with PtAl6 and Al from database (b) SEM micrograph as deposited (c) SEM
micrograph after de-alloying (d) TEM de-alloying and (e) High resolution TEM after dealloying
136
5.4.2 Effect of temperature
In this project, the Pt-Al precursors were prepared using deposition either at elevated
temperature (~ 400 qC) or at room temperature. The precursor films deposited at elevated
temperature were previously discussed in the section on the effect of composition. In the
present section I compare what happens when similar compositions are deposited instead at
room temperature. The situation for precursor films that had been deposited at room
temperature but then annealed (prior to de-alloying) at various temperatures between 100 qC
and 500 qC is also discussed.
Effect of deposition temperature
The most noticeable difference was that, while the Pt-Al precursor films deposited at
elevated temperature were crystalline with a variety of phases present, those deposited at
room temperature were nearly amorphous. However in both cases, samples with ȤAl <0.60 dealloyed rather slowly, with a slow production of H2 bubbles and they did not form sponges. It
was previously mentioned that, in the case of the crystalline samples, there was a marked
change in sponge morphology at compositions close to ȤAl = 0.67. According to equilibrium
phase diagram, the crystalline Pt-Al precursors should have been 100% PtAl2 at ȤAl = 0.67
and, indeed, the brassy yellow PtAl2 phase would have been expected to have formed at
elevated temperature [109, 249]. However, samples with the 1Pt:2Al composition that had
been produced at room temperature were not crystalline as shown Figure 5.19. Furthermore,
de-alloying of these samples produced a different kind of sponge from the porous and
cracked film made from crystalline samples of this composition. Instead, the precursors with
ȤAl = 0.67 that had been deposited at room temperature produced partially de-alloyed
‘pinhole’ sponges as shown in Figure 5.20 (a). A cross-sectional view down one of the
cavities in the film is shown in Figure 5.20 (b).
Figure 5.19 XRD patterns of the precursors were deposited at room temperature
with ȤAl = 0.67
137
(b)
(a)
100 nm
100 nm
Figure 5.20 Morphology of partially de-alloyed sponges produced from the precursors were
deposited at room temperature with ȤAl = 0.67 (a) plan view and (b) cross-sectional view
Precursors which were deposited at room temperature in the range 0.67 < ȤAl < 0.80
developed foamy sponges, with morphology varying systematically from partially de-alloyed
pinhole type (ȤAl = 0.67) to completely foamy (ȤAl = 0.80) when de-alloyed (Figure 5.21). In
contrast, fibrous sponges had been produced from the precursors which were deposited at
elevated temperature in the same ȤAl range.
100 nm
Figure 5.21 Morphology of sponge produced from a precursor with ȤAl | 0.75 that had been
deposited at room temperature
Foamy sponges are completely formed when ȤAl > 0.80 (up to ~ 0.95) from the
precursors which were deposited at room temperature, Figure 5.22. The Pt-Al precursor in the
as-deposited form had uniform and smooth appearance as shown in Figure 5.22 (a). Some
parts of the foamy sponges are curled up during de-alloying process. Therefore, the entire
foamy sponge is shown as well as cross-sectional view in Figure 5.22 (e). To further
investigate the microstructure of such films, a Pt-Al precursor film (which had been deposited
at room temperature with ȤAl = 0.92) was prepared for TEM cross-sectional analysis by the
138
sandwich technique. Elemental mapping in the TEM of this sample showed that the film was
homogenous in composition. Evidently, it had formed a solid solution during deposition at
room temperature (Figure 5.23 and Figure 5.24).
(a)
(b)
100 nm
100 nm
(d)
(c)
100 nm
100 nm
(e)
100 nm
Figure 5.22 Morphology of Pt-Al precursors, deposited at room temperature with ȤAl > 0.80
(a) as deposited (b) ȤAl | 0.83 after de-alloying (c) ȤAl | 0.88 after de-alloying (d) ȤAl | 0.96
after de-alloying and (e) curled up porous Pt sponge and shown cross-sectional view
139
Figure 5.23 TEM mapping on Pt-Al precursors, deposited at room temperature
with ȤAl = 0.92
100 nm
Figure 5.24 TEM-EDS analysis through the cross-sectional area of Pt-Al precursors,
deposited at room temperature with ȤAl = 0.92. The presence of Cu is due to redeposited
materials during PIPS
Similarly to the precursors deposited at elevated temperature, the Al content of
samples deposited at room temperature also had an influence on the pore size of the sponges.
When Al content was increased, the pore sizes of the sponges also became larger. The pore
size distribution of nanoporous Pt that had been produced from Pt-Al precursors with
different mole fractions of Al are compared in Figure 5.25. The average pore size in
nanoporous Pt that had been produced from Pt-Al precursors with ȤAl = 0.83 was 14.4 ± 1.8
nm, that of Pt-Al precursors with ȤAl = 0.88 was 18.0 ± 4.9 nm nd that of Pt-Al precursors
with ȤAl = 0.96 was 35.6 ±4.9 nm. Furthermore, TEM micrographs of sponge from precursors
140
with ȤAl | 0.88, Figure 5.26, show that Pt is in the form of a continuous and low density
filament network that surrounds the voids.
(d
10
0
(c)
Figure 5.25 Distribution of pore sizes from Pt-Al precursor with different mole fraction of Al
(a) ȤAl = 0.83 (b) ȤAl = 0.88 and (c) ȤAl = 0.96
141
(a)
(b)
Figure 5.26 TEM micrograph of sponge formed from precursor with ȤAl § 0.88 (a) a
continuous network of Pt surrounding the void and (b) lattice fringe image at high resolution
Effect of annealing
The Pt-Al precursor films that had been deposited at room temperature produced
isotropic foamy sponges when de-alloyed. This type of sponge is noticeably different in
appearance to the isotropic fibrous sponges produced from precursors deposited at elevated
temperature. In this section, I investigate the effect of post-deposition (but pre-de-alloying)
annealing temperatures from 100 qC to 500 qC applied to selected samples produced by
deposition at room temperature. These were the same compositions that produced foamy
sponges when de-alloyed directly from the as-deposited (at room temperature) condition. It
was found that the samples with ȤAl > 0.80 which were deposited at room temperature and
then heated above 400 °C crystallized to form the same phases (H phase and Al) as the
samples with ȤAl > 0.80 deposited at elevated temperature. The XRD patterns of samples with
ȤAl > 0.80, which were deposited at room temperature and then heated 400 °C and 500 °C
were compared with the sample with ȤAl = 0.82 which was deposited at elevated temperature
as shown in Figure 5.27. The morphologies of the sponges stayed foamy as the annealing
temperature was raised, but it was noticeable that the pore size became quite bimodal, Figure
5.28. Probably Al-rich regions were formed during the crystallization which was nearly
completely dissolved away during the de-alloying process leaving the large voids observed.
142
Figure 5.27 Comparison of X-ray diffraction pattern between samples with ȤAl > 0.80, which
were deposited at room temperature and above 400 °C and the sample with ȤAl = 0.82, which
was deposited at elevated temperature
(a)
(b)
20 nm
20 nm
(d)
(c)
20 nm
20 nm
143
(e)
(f)
20 nm
20 nm
Figure 5.28 Morphologies of samples with ȤAl >0.80, which were deposited at room
temperature, then annealed at various temperatures followed by de-alloying process in alkali
solution (a) as deposited at room temperature (b) annealed at 100 °C (c) annealed at 200 °C
(d) annealed at 300 °C (e) annealed at 400 °C and (f) annealed at 500°C
5.4.3 Effect of deposition time
The duration of deposition ('deposition time') has a direct and obvious effect on the
thickness of the film produced. In this section, the effect of the thickness of precursor thin
film on their morphology of the sponge produced by de-alloying is studied. Here the different
thicknesses of films were produced by varying the deposition time only. (In principle,
thickness can also be varied by changing the current or power applied to the sputter targets
but this might exert other changes on the microstructure.) A precursor composition (ȤAl =
0.83) that was known to produce the foamy sponge was selected. The deposition rates of Pt
and Al were fixed but deposition times of 5, 10 and 30 minutes were applied during the cosputtering Pt and Al. The foamy sponges produced from the three different thicknesses are
compared in Figure 5.29. They are similar although the thicker precursor films seem to
produce slightly denser foamy sponges.
(a)
(b)
20 nm
20 nm
144
(c)
20 nm
Figure 5.29 Morphologies of samples with ȤAl =0.83, which were deposited at room
temperature with various deposition times (a) 5 minutes (b) 10 minutes and (c) 30 minutes
5.4.4 Effect of deposition rate
In this section, the deposition rates of the two elements were controlled by adjusting
power or current on each target to obtain the appropriate composition. Previously, the current
level of Pt target was fixed and only the current levels of aluminium were varied. In this
section I also varied the rate of deposition of Pt (by controlling the current on the Pt target) to
see if that had any effect on the morphology of the foamy sponges. A series of Pt-Al
precursors was deposited at room temperature with twice the current (fast rate, 0.050 A) for
comparing to those samples that had been deposited at room temperature formerly (slow rate,
0.025 A). The deposition rate of Pt did not affect the precursors with ȤAl < 0.60 since they
hardly de-alloyed anyway and these samples will not be discussed further. Samples with
compositions closer to ȤAl = 0.60 started producing partially de-alloyed pinhole sponges and
developed increasingly more open sponges until the composition approached ȤAl = 0.70, as
shown in Figure 5.30 (a), (b) and (c). These samples also show that the de-alloying process is
not uniform over the surface and that there appears to be preferential attack and penetration
into the alloy along grain boundaries in Figure 5.30 (d).
The precursors which were deposited at room temperature with a high deposition rate
of Pt, start to produce true foamy sponges when ȤAl > 0.70. The surface of such a precursor in
the as-deposited state is smooth, dense and featureless. After de-alloying, foamy sponges
were formed, with the sponge morphology becoming more pronounced when Al contents
were increased. SEM micrographs show the development of foamy sponges from ȤAl | 0.71,
0.74 and 0.77 in Figure 5.31 (a), (b) and (c) respectively. The interior of this latter foamy
sponge is revealed within the crack in Figure 5.31 (d).
145
(a)
(b)
100 nm
(c)
100 nm
(d)
100 nm
200 nm
Figure 5.30 Pinhole sponge produced from precursors that deposited at room temperature
with high deposition rate of Pt in various Al contents (a) ȤAl | 0.62 (b) ȤAl | 0.67 and (c) ȤAl |
0.69 and (d) preferential dissolution along grain boundaries
(a)
(b)
20 nm
146
20 nm
(d)
(c)
20 nm
20 nm
Figure 5.31 Foamy sponge produced from precursors that deposited at room temperature with
high deposition rate of Pt in various Al contents (a) ȤAl | 0.71 (b) ȤAl | 0.74 (c) ȤAl | 0.77 and
(d) view of interior of sponge through walls of crack
Since foamy sponges had been formed, the current level of Al was fixed in that range.
Then the current level of Pt was varied as 0.005 A, 0.025 A, 0.050 A and 0.075 A (all these
samples were produced by deposition at room temperature). The sample which used the
lowest deposition rate or current level of Pt (0.005 A) was rapidly de-alloyed and was
analysed by SEM to have had ȤAl | 0.98 before de-alloying. Interruption of the de-alloying
followed by examination of the surfaces by SEM showed that the partially de-alloyed surface
certainly did have a foamy morphology, Figure 5.32 (a), and completely de-alloyed samples
produced a very fragile sponge, Figure 5.32(b). The samples which were deposited with
current level of Pt = 0.025 A ( ȤAl | 0.92) produced foamy sponges. The usual foamy sponges
are shown in Figure 5.33 (a) and at low magnification this precursor thin film are transparent
in Figure 5.33 (b). When a current level of Pt= 0.050 A was used for preparing precursors
(ȤAl | 0.83), a foamy sponge was formed and the pore size was smaller, Figure 5.34 (a), than
those of the precursors made using the 0.025 A current, as shown in Figure 5.33(a). Finally,
the current level of Pt was increased to 0.075A which produced a precursor with ȤAl | 0.73.
After de-alloying, pinhole sponges were formed in Figure 5.33 (b).
147
(a)
(b)
100 nm
20 nm
Pt at 0.005 A (a) partly foamy sponges and (b) fragile sponges
(a)
(b)
200 nm
20 nm
Pt at 0.025 A (a) foamy sponges and (b) transparency foamy sponges film
(a)
(b)
20 nm
20 nm
Figure 5.34 Morphologies of samples that deposited at room temperature with different
current level of Pt (a) 0.050 A and (b) 0.075 A
148
5.4.5 Effect of de-alloying parameters
Previously, I examined the effect of varying process parameters during the cosputtering deposition. In this section, I focus on the effect of varying the de-alloying
parameters. De-alloying solution and de-alloying time were the two obvious parameters to
investigate. Selected precursors that produced foamy sponges were immersed in two different
de-alloying solutions. Alkaline solutions are usually used for the fabrication of nanoporous
metals by de-alloying Al-based alloy precursors. Here I used 0.2 M sodium hydroxide
(NaOH) and 0.2 M sodium carbonate (Na2CO3) as de-alloying solutions. NaOH solution is
strongly basic while Na2CO3 solutions are weakly basic. Using Na2CO3 for de-alloying Pt-Al
precursor took a longer time than for NaOH because the dissolution rate of Al slower in
Na2CO3 than in NaOH [250]. However, the morphologies of foamy sponges produced by dealloying using either NaOH or Na2CO3 are similar as shown in Figure 5.35. For the slower
dissolution rate of Al in Na2CO3 it became possible to consider the optimal de-alloying time.
The dissolution rate is very high in the beginning of immersion, followed by a period of
reduced activity, caused perhaps by the formation of a protective layer of Pt or H2 bubbles.
After that, the protective layer was slowly removed. A series of foamy sponges from
precursors that had been deposited at room temperature with ȤAl = 0.92, then de-alloyed by
using Na2CO3 with different de-alloying times (1, 3, 5, 10 and 15 minutes) are shown in
Figure 5.36. In general, however, all the sponges have a similar appearance.
(a)
(b)
100 nm
100 nm
Figure 5.35 Foamy sponges produced from precursors that deposited at room temperature
with ȤAl = 0.96, then de-alloying by different solutions (a) 0.2M NaOH and (b) 0.2M Na2CO3
149
(a)
(b)
100 nm
100 nm
(c)
(d)
100 nm
100 nm
(e)
100 nm
Figure 5.36 Foamy sponges from precursors that deposited at room temperature with ȤAl =
0.92, then de-alloying by using Na2CO3 with different de-alloying times (a) 1 minute (b) 3
minutes (c) 5 minutes (d) 10 minutes and (e) 15 minutes
5.4.6 Comparison between my nanoporous Pt sponges and those in the literature
Nanoporous Pt produced by de-alloying of binary or ternary alloys normally shows a
bicontinuous ligament-channel structure with a length scale of few nanometers. The fine
structure is probably due to the low surface diffusivity of Pt (<10-19 cm2/s) [77]. Nanoporous
Pt has been produced by a variety of techniques before and these influence the resulting
morphology. In this section, the nanoporous Pt reported in the literature are summarized and
compared to the nanoporous Pt sponges produced here by de-alloying Pt-Al precursors.
150
Pugh et al. produced nanoporous Pt from Pt-Cu precursors [128, 212]. The
bicontinuous structure of porosity was formed by Pt-Cu precursors which had been prepared
by arc melting then cold rolled to a foil, and finally annealed. After that, an electrochemical
potential was applied in an electrolyte for obtaining nanoporous Pt. The morphologies
obtained were found to depend on the method of de-alloying. For example, a Pt0.25 Cu0.75
precursor was immersed in 1 M H2SO4, at 1.2V with a saturated calomel electrode (SCE) for
2.4 hours and produced nanoporous Pt with pore diameter about 3.4 nm. Of course the
sponge could be coarsened afterwards by annealing at several hundred degrees Celsius.
Abburi et al. also fabricated nanoporous Pt from Pt-Cu precursors [246], but the precursors
were prepared by co-sputtering. In this case the de-alloying was done by immersing these
precursors in 93% H2SO4. As before, a bicontinuous structure of nanoporous Pt was formed
with pore size in the range 5-35 nm and this could be tuned by a coarsening heat treatment.
Nahm et al. synthesized nanoporous Pt from Pt0.52Al0.48 precursor by co-depositing
using RF magnetron sputtering technique at 400qC. De-alloying was performed in 0.5 M
H2SO4 solution with an applied electrochemical potential. A fibrous nanoporous Pt structure
was produced. Zhang et al. also produced nanoporous Pt from Pt-Al precursors. The
Pt0.12Al0.88 precursor was prepared by melt spinning as the ribbons, and then chemical dealloying in 20wt%NaOH solution at 90 qC was applied. After de-alloying, a bicontinuous
ligament-channel structure was revealed.
Antoniou et al. fabricated nanoporous Pt through electrochemical de-alloying in
aqueous HF from Pt-Si that had been co-sputtered at room temperature [133]. Three different
nanoporous Pt morphologies were found by varying deposition parameters. Pt0.10Si0.90
produced isotropic open-cell foam while anisotropic columnar and Voronoi type foam are
obtained from Pt0.34Si0.66 and Pt0.33Si0.67 precursors, respectively. The ligament diameter and
grain size were about 5 nm for all structures.
In my own research work, the nanoporous Pt sponges produced from Pt-Al precursors
showed a range of morphologies ranging from bicontinuous isotropic fibrous sponges, to
foamy sponges and even pinhole sponges. Morphologies and processing parameters of
nanoporous Pt from other research works are listed and compared this project to in Table 5.1
and 5.2.
151
Table 5.1 List of nanoporous Pt produced by other research works
Composition
Precursor preparation
De-alloying process
Morphology
Reference
Pt0.25 Cu0.75
Arc melting/cold rolled /annealing
De-alloying potential control in 1 M
H2SO4 at 1.2 V SCE for 2.4 hours
Pugh 2003
[128]
Pt0.25 Cu0.75
H2SO4 at 1.2 V SCE for 2.4 hours/ heat
treatment at 500 °C for 60 minutes
Pugh 2003
[128]
Pt0.20Cu0.80
HClO4 at 1.24 V NHE for 20 hours/ heat
treatment at 800 °C for 15 minutes
Pugh 2005
[212]
Pt0.20Cu0.80
H2SO4 at 0.692 V with NHE for 24 hours /
heat treatment at 500 °C for 30 minute
Pugh 2005
[212]
152
Table 5.1 List of nanoporous Pt produced by other research works (cont.)
Composition
Pt0.20 Cu0.80
Co-sputtering technique
De-alloying process
Morphology
Free corrosion in 93% H2SO4/ heat
treatment at 400 qC
Reference
Abburi 2012
[246]
(d)
Pt0.33 Al0.67
Co-depositing using dc magnetron
sputtering technique at 400 qC
Free corrosion in 0.5 M NaOH
Cortie 2006
[109]
Pt0.52Al0.48
Co-depositing using rf magnetron
De-alloying potential control in 0.5 M
H2SO4
Nahm 2009
[223]
Pt0.12Al0.88
Melt spinning
Free corrosion in 20wt%NaOH solution at
90 qC
Zhang 2009
[143]
100
153
Table 5.1 List of nanoporous Pt produced by other research works (cont.)
Composition
De-alloying process
Morphology
Reference
Pt0.10Si0.90
Co-sputtering technique at room
temperature
De-alloying potential control in HF
Antoniou
2009[133]
Pt0.34Si0.66
temperature
Antoniou
2009[133]
Pt0.33Si0.67
temperature
Antoniou
2009[133]
154
Table 5.2 List of nanoporous Pt produced by this research project
Composition
Pt0.33 Al0.67
De-alloying process
Morphology
Reference
This work
200 nm
Pt0.22 Al0.78
This work
100 nm
Pt0.18Al0.82
This work
100 nm
Pt0.10Al0.90
This work
100 nm
155
Table 5.2 List of nanoporous Pt produced by this research project (cont.)
Composition
Pt0.12 Al0.88
De-alloying process
Morphology
Free corrosion in 0.2 M NaOH or Na2CO3
sputtering technique at room
temperature (current level =0.025 A
for Pt)
Reference
This work
100 nm
Pt0.04 Al0.96
for Pt)
This work
20 nm
Pt0.31Al0.69
for Pt)
This work
200 nm
Pt0.23Al0.77
for Pt)
This work
100 nm
156
5.5 Conclusion
Nanoporous platinum has received considerable attention due to its various
applications, real or potential, in catalysis, biomedical stimulation electrodes, sensors,
mechanical actuators and fuel cells. There are several methods by which it can be prepared
and in each of these there are a range of process parameters that can be adjusted. In this
project, nanoporous platinum was fabricated by first making Pt-Al precursors by codeposition using dc magnetron sputtering under high vacuum and the de-alloying them using
alkaline solutions. The properties of nanoporous platinum are very likely to be closely related
to their microstructure. Therefore I have examined the ways in which process parameters
might control the characteristics of nanoporous platinum. It was discovered stoichiometry of
the precursor is the primary controlling factor, followed by the temperature at which it had
been deposited. The stoichiometry controls which phases form which in turn has an effect on
the sponge morphology. The effect of the Al content can be summarized into discrete
intervals:
(i) Precursors with ȤAl < 0.60 did not form sponges either after deposition at elevated
or room temperature,
(ii) Precursors with ȤAl = 0.67 and deposited at elevated temperature produced 'mudcracked' mesoporous sponges,
(iii) Precursors with 0.67< ȤAl < 0.90 at produced at elevated temperature produced
classic isotropic fibrous sponges. In this range, the complex phases including Al-rich phases
such as Pt8Al21 and meta-stable phase (H phase or PtAl5) were formed.
(iv) Precursors with ȤAl > 0.90 produced a disordered and fragile mass instead of a
mechanical robust sponge. Also, these precursors contained a mixture between PtAl6 and
pure Al.
(v) Precursors that had been deposited at room temperature produced different
morphologies of Pt sponges to those that had been deposited at elevated temperature.
(vi) In the room temperature case, the sponge morphology changed from pinhole to
unusual isotropic foamy sponges in the range 0.67 < ȤAl <0.96.
(vii) Precursors which had been deposited at room temperature are amorphous and
become crystalline after annealing at 400-500 qC. De-alloying of such heat-treated samples
still produced foamy sponges but some oxide formation occurred on the surface during heat
treatment process.
(viii) The deposition rate of noble metal, Pt in this case, has influence on the
characteristics of nanoporous platinum sponges. At high deposition rate of Pt, the precursors
with 0.60 < ȤAl < 0.70 produce pinhole sponge type. When the precursors with ȤAl > 0.70
isotropic foamy sponges started to form.
157
Chapter 6
Conclusions and future work
158
Precious metal alloys and their intermetallic compounds have some unique
technological properties. They find applications in fields as varied as dentistry, medicine,
electronics and electrical technology, and catalysis. In addition, the gold- and platinum-based
materials are highly valued for use in jewellery and decorative applications due to the
combination of their corrosion resistance, appearance, ductility and rarity. Although there are
many interesting precious metal alloys and compounds, in this present research project I
focused on the gold - platinum - aluminium system. The overall motivation of this present
research work is first to better understand the binary and ternary phase relationships within
this ternary system and second to investigate how methods of fabrication might be optimized
to enhance their properties.
The results can be divided into two parts:
In the first part I investigated coloured intermetallic compounds of AuAl2 and PtAl2,
mainly for jewellery applications. There may, however, be some other applications for
coatings of these materials as well, for example spectrally-selective coatings for AuAl2 and
corrosion or oxidation resistant coatings for PtAl2.
The second part of the project was on platinum-aluminium intermetallic compounds
for making nanoporous platinum sponges, and on the sponges themselves. These sponges
have great potential in catalyst and sensor applications.
I used physical vapour deposition in both parts of the project to make most of my
samples. Specifically, I used direct current magnetron co-sputtering in a vacuum system to
fabricate thin films of the alloys and intermetallic compounds of this project. Part of this
project involved a systematic investigation of the deposition conditions and post-deposition
heat treatments. The films were characterized in terms of their microstructures and properties
by various techniques including X-ray diffraction (using both a laboratory instrument and a
synchrotron), scanning electron microscopy, and transmission electron microscopy with
energy dispersive spectroscopy. The optical properties of these intermetallic compounds were
observed by spectrophotometry and ellipsometry. Computer modelling was also used for
investigating their optical properties. In this present chapter I present a summary of the
overall conclusions of my project. The most promising aspects for future work are also
suggested.
6.1 The AuAl2-PtAl2 system
The coloured intermetallic compounds AuAl2 and PtAl2 have the CaF2 (also
designated as cF12) crystal structure. They are of practical and scientific interest because of
their unique colours (purple and yellow respectively) which are completely different from the
colours of their components.
Gold and platinum have the same fcc structure and have similar electronegativity and
atomic diameter so might be expected to exhibit unlimited mutual solubility. However, their
binary phase diagram shows that they are, in fact, nearly mutually insoluble, at least under
159
equilibrium conditions. There is no ternary phase diagram of Au-Pt-Al available in the
literature and therefore the relationship between AuAl2 and PtAl2 was unknown. For these
reasons, two interesting issues were raised (i) whether AuAl2 and PtAl2 are mutually soluble,
or not, and (ii) whether solid solutions or mixed microstructures of these two compounds
could be used to tune the colour between purple and yellow end-points.
The first task of this part of the investigation was to develop means to fabricate pure
AuAl2 and PtAl2 thin films by co-sputtering techniques. Due to the very narrow ranges of
stability in their binary phase diagrams this required tight control of the appropriate
composition. The aluminium content should be about 66-67 at.% in order to form these
coloured intermetallic compounds. Their compositions were controlled by setting the
appropriate current or power level during sputtering. Pure AuAl2 and PtAl2 thin films were
successfully fabricated. These coloured intermetallic compound thin films were then
compared with bulk samples of PtAl2 and AuAl2 that had been produced by melting pure
gold or platinum with aluminium in a vacuum arc furnace by an outside organization.
The compositions and lattice parameters of the AuAl2 films and bulk samples were
very similar. The reflectance curves of AuAl2 of both thin films and bulk samples were
similar but the film sample had a stronger colour because it had a slightly higher reflection in
the violet and red regions, and less reflection in the green. However, the microstructures of
the two different forms of AuAl2 sample were completely different. The AuAl2 bulk sample
showed a dendritic structure with some segregation of aluminium while the AuAl2 thin film
was dense and homogeneous.
The chemical compositions and lattice parameters of the thin film and bulk PtAl2
samples were also reasonably similar. The reflectance curves of PtAl2 for both thin films and
bulk samples were similar too except that the film sample showed a small shift in the
absorption relative to bulk PtAl2. The microstructures of these two samples were similar,
being relatively homogenous with a very fine structure.
The measured optical properties of these coloured intermetallic compounds were used
in simulations in order to show the range of colours that could be obtained in these films by
varying their thicknesses.
The second task of this part of the investigation was to design and fabricate multilayer
structures in order to explore the interesting issues mentioned above. Bi-layer and multi-layer
films of coloured intermetallic compounds were prepared in various arrangements. Bi-layer
films of PtAl2/AuAl2 and AuAl2/PtAl2 seem to be nearly mutually insoluble after postdeposition heat treatment. Multi-layer films of Au, Pt and Al with a stack formed by
depositing pure metals were also fabricated. It was found in this case that separate domains of
AuAl2 and PtAl2 were formed after post-deposition heat treatment. From this result, it is clear
that, when two coloured intermetallic compounds are formed separately, they do not
interdiffuse later. A stack formed by co-depositing Au and Pt followed by Al was another
strategy that was followed for this investigation. As mentioned, Au and Pt are immiscible
under equilibrium conditions at temperatures below 1260 °C, nevertheless a metastable
160
mutual solid solution of these two compounds may be prepared by magnetron sputtering. The
co-deposited film of 54 at.%Au and 46 at.%Pt film formed a solid solution with a lattice
parameter of 0.4006 nm which lies between that of the pure Au films (0.4077 nm) and the
pure Pt films (0.3935 nm). After post-deposition heat-treatment this metastable film reacted
with the layer of Al to form a metastable solid solution of (Au,Pt)Al2. This solid solution had
optical properties intermediate between those of AuAl2 and PtAl2.
An alternate strategy for tuning the colour was to form very thin film stacks of pure
AuAl2 and pure PtAl2. Provided that the layers are so thin that light can penetrate through the
top layer, then some mixing of the colours is possible. Similar hues can be obtained by this
means as with the metastable (Au,Pt)Al2 solid solutions. Overall, the present work has shown
that the composition and colour of the coatings can be controlled by deposition and heattreatment parameters, and that the desired phases (AuAl2, PtAl2 or (Au,Pt)Al2) can be
produced as required.
This work has shown that the use of thin films of the coloured intermetallic
compounds might be an alternative approach for the jewellery industry. The use of thin films
solves the problem of the bulk intermetallic compounds being very brittle, and it solves a
potential caratage problem in the case of the PtAl2 : this is because PtAl2 (78.3 wt.% Pt) has
too low a Pt content to satisfy the usual industry standards of 90 or 95wt.% Pt. However, if
PtAl2 is applied together with conventional Pt alloys the composite item will still be
hallmarkable as Pt. Thus there is no limitation for item design in this case. This is less of a
problem for AuAl2 because that contains 75.8 wt.% Au, which already satisfies the popular
18 carat (75 wt.% Au) standard. Nevertheless, even 22 carat (91.7 wt.% Au) items can benefit
from the inclusion of thin films of AuAl2 by this means.
Finally, this work has shown that a range of interesting colour effects can be obtained
by two different strategies: metastable films of (Au,Pt)Al2 and stacks of very thin films of
AuAl2 and PtAl2. The incorporation of other coloured intermetallic compounds could be
considered in any future work on this subject.
6.2 Nanoporous platinum sponges
A variety of metals such as gold, nickel, copper etc. can be prepared in meso- or
nanoporous forms. Nanoporous platinum is one of the most interesting of the nanoporous
metals because of its catalytic properties and hence numerous applications. The binary alloys
and intermetallic compounds of the Pt-Al system are attractive candidates to use as
precursors for fabricating nanoporous platinum by the de-alloying method. In the present
work Pt-Al precursors were prepared by co-deposition using dc magnetron sputtering and this
was followed by de-alloying in alkaline solutions. The aim of this part of the research was to
discover how parameters (during either precursor preparation or de-alloying) control the
morphologies of the resulting nanoporous platinum. In particular I sought to discover what
the factor was that changed the sponge morphology from the usual fibrous type reported by
most other investigators to the foamy type usually produced in our research group.
161
X-ray diffraction, either with a laboratory instrument or the Australian Synchrotron,
was used to characterize the precursors. Scanning electron microscope and transmission
electron microscope with elemental analysis were also used to examine selected precursors
and de-alloyed samples.
It was found that stoichiometry of the precursor and the deposition temperature played
an important role in controlling the nanoporous platinum sponge morphology. The Pt-Al
stoichiometry and the temperature controlled which phases formed and hence sponge
morphology after de-alloying. If FAl is the mole fraction of Al then the situation in respect of
stoichiometry and deposition temperature can be summarized as follows:
Precursors with ȤAl < 0.60 did not form sponges either after deposition at elevated
temperature or at room temperature. Their content of Al is below the ‘parting limit’.
Precursors with ȤAl = 0.67 and deposited at elevated temperature produced 'mud-cracked'
mesoporous sponges while precursors with 0.67< ȤAl < 0.90 produced at elevated temperature
produced classic isotropic fibrous sponges. In this range, Pt8Al21 and meta-stable phase (H
phase or PtAl5), which are complex phases with Al-rich phases, were formed. When
precursors with ȤAl > 0.90 were dealloyed then a disordered and fragile mass of Pt sponge was
obtained. These precursors contained a mixture between PtAl6 and pure Al. The deposition
temperature is another important parameter effecting morphologies of platinum sponges. It
was discovered that precursors that had been deposited at room temperature produced very
different morphologies to those that had been deposited at elevated temperature. In the
precursors that had been deposited at room temperature, the sponge morphology changed
from pinhole to unusual isotropic foamy sponges in the range 0.67 < ȤAl <0.96. The
precursors which had been deposited at room temperature are amorphous and after heat
treatment at more than 400 qC they became crystalline. After de-alloying these heat treated
and crystalline samples, foamy sponges are still produced but some oxide formations
appeared on the surface.
Finally, the deposition rate of noble metal also has an influence on the morphology of
sponges. At high deposition rates of Pt, the precursors with 0.60 < ȤAl < 0.70 produced
pinhole sponge types while isotropic foamy sponges started to form at the precursors with ȤAl
> 0.70.
No actual testing of the catalytic power of these sponges was done in the present
project. This would be a good thing to follow up with in future work. The present work has
shown that fabrication of a Pt-Al precursor followed by de-alloying might be an alternative
option for producing nanoporous platinum for catalytic, optical and electrochemical
applications. This process may be less complex than currently available methods and it
offers the prospect of obtaining various morphologies by controlling processing parameters.
Other properties of these sponges including mechanical, optical, sensing, actuation, electrical,
thermal and magnetic properties should be considered in future investigations. It is also
obvious that this method could be applied to produce sponges from other binary or ternary
alloy systems.
162
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Properties and applications of metastable precious

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