5.5 Nanostructured MoS 2 and WS 2 for the Solar Production of

Nanostructured MoS2 and WS2 for the Solar Production of Hydrogen
Investigators
Thomas F. Jaramillo, Assistant Professor, Chemical Engineering, Stanford University;
Zhebo Chen, Graduate Student, Chemical Engineering, Stanford University;
Jakob Kibsgaard, Post-doctoral Researcher, Chemical Engineering, Stanford University.
Abstract
We have successfully synthesized MoS2 nanoparticles of various sizes using a reverse
micelle encapsulation method, and studied their opto-electronic properties for the
synthesis of fuels by photoelectrochemistry (PEC). The nanoparticles exhibit a size
dependent blueshift in both the direct and indirect band gaps relative to each other and to
bulk films. In pursuing these studies, we developed a low temperature sulfidization
process using a custom H2S/H2 gas mixture that enables Mo to be sulfidized at
temperatures as low as 150ºC. The sulfidized nanoparticles are fully stable when
exposed to air and do not spontaneously re-oxidize. The development of nanowire
morphologies have yielded the production of anodic alumina oxide templates with pores
below the size necessary for quantum confinement. We have also performed initial
synthesis of a nanoscaled bi-continuous double-gyroid template. Both of these
templating methods require the optimization of molybdenum and tungsten
electrodeposition, which we have achieved. Opto-electronic studies of the supported
nanoparticles show significant bandgap enlargement from 1.2 eV to approximately 2 eV,
as we had originally targeted for PEC applications. This exploratory work has shown that
nanostructured MoS2 is indeed a promising material for the synthesis of fuels from
sunlight.
Introduction
The fact that the sun provides us with nearly 10,000 times more energy than we
consume strongly compels us to find a cost-effective way to harness solar power.
Photoelectrochemical (PEC) hydrogen production is one promising approach (see Figure
1), first demonstrated with TiO2 in 1972 [1]. Since then, research in this area has aimed to
improve device efficiency in several ways, including semiconductor doping/alloying [26], tandem absorbers [7], and dye-sensitization [8-10]. Despite these efforts, there exists
no material system that can simultaneously satisfy all the materials criteria required for
cost-effective PEC hydrogen production; new materials with new properties are needed.
This proposal aims to transform research in this area by engineering inexpensive
semiconductors such as MoS2 and WS2 at the nano-scale in order to tailor their surface
and bulk properties for efficient PEC water-splitting. The fundamental, cross-cutting
knowledge gained regarding the chemical, catalytic, optical, and electronic phenomena
Figure 1. PEC H2 production. Courtesy: John Turner, NREL.
within these nano-scale dichalcogenide semiconductors will contribute broadly to a
number of different scientific fields such as photovoltaics and heterogeneous catalysis,
facilitating a broad mix of technological innovations.
Hydrogen has emerged as a contender among clean-burning fuels. While significant
challenges remain in its storage and use, its high specific energy content and clean
combustion make it an attractive option. Hydrogen already plays an enormous role in our
chemical and fuel industries with an annual production of 30 million tons (15 Teramoles) [11]. Unfortunately, most commercial H2 is produced by steam reforming [2] – an
energy-intensive, fossil fuel based process with substantial greenhouse gas emissions.
Whether H2 produced by “green” methods such as solar PEC water-splitting is best used
as a clean fuel, as a reagent for greener synthesis of foods/chemicals, or for synthesis of
higher-value liquid fuels is a question that economics will ultimately answer. Storing an
intermittent energy source such as the sun as hydrogen is inherently flexible and will help
reduce greenhouse gas emissions on a terrestrial scale.
Background
The PI, Jaramillo, recently took part in a techno-economic study of hydrogen
production by photocatalysis and photoelectrochemistry, sponsored by the DOE. As
described in the published report, if PEC R&D targets can be met in the laboratory
regarding efficiency, stability, and material cost, a large-scale solar-to-hydrogen plant can
be constructed with H2 costs at the plant gate as low as $1.60/kg H2. Note that 1 kg H2 is
equivalent in energy content to 1 gallon of gasoline. This outcome further inspires R&D
efforts in this field [12]. During the course of this 1-year project, no major breakthroughs
in device efficiency have been reported by our group or others. The world-record remains
at 12.4%, the GaInP2/GaAs device reported by NREL in reference [7]. Nevertheless,
incremental advancements have been made on other devices by the conventional
approaches described above: doping/alloying, tandem cells, and dye sensitization.
Results
Our investigation of nanostructured transition metal dichalcogenides for solar
production of hydrogen has resulted in the synthesis of molybdenum and tungsten
nanoparticles in various, controlled sizes, the production of anodic alumina oxide
templates with pores in the range necessary for quantum confinement, optimization of
molybdenum and tungsten electrodeposition and sulfidization, as well as the development
of a low-temperature sulfidization procedure as well as a synthetic route for a doublegyroid mesoporous templating agent.
Synthesis of well-defined nanoparticles was the first crucial step in this project. We
developed of a low-temperature sulfidization procedure that was an important step in the
synthetic route as we were able to mitigate nanoparticle sintering. Using a custom
mixture of H2S and H2 gas, we have optimized the sulfidization temperature for
molybdenum and tungsten at 150ºC and 400ºC, respectively. The low temperature
required for the sulfidization of molybdenum in particular minimizes sintering effects
that would cause the nanoparticles to coalesce into larger structures.
We have successfully synthesized molybdenum and tungsten nanoparticles in the 525 nm range using a reverse micelle encapsulation method. Four different molecular
weight poly(styrene-block-2-vinylpyridine) polymers were used in the synthesis. The
resulting nanoparticles exhibited a trend in size consistent with the molecular weight of
the vinylpyridine block, as confirmed by atomic force microscopy (AFM) and scanning
electron microscopy (SEM). The particle sizes could be tuned between 5-25 nm.
Figure 1. AFM images of MoS2 nanoparticles on Si(100). The molecular weight ratios
of styrene:vinylpyridine for the block copolymer used in the synthesis were (a)
27700/4300, (b) 42500/7800, (c) 81000/21000, (d) 172000/42000. Images are 100µm x
100 µm.
Mo(IV) 3d5/2
Mo(IV) 3d3/2
27700/4300
Mo(IV) 3d5/2
Mo(IV) 3d3/2
32500/7800
(b)
Counts (a.u.)
Counts (a.u.)
(a)
S 2s
238
236
234
232
230
228
226
224
222
220
218
S 2s
238
236
234
232
Binding Energy (eV)
230
228
226
224
222
220
218
Binding Energy (eV)
Mo(IV) 3d5/2
Mo(IV) 3d5/2
81000/21000
172000/42000
Mo(IV) 3d3/2
Mo(IV) 3d3/2
238
236
234
232
230
228
226
Binding Energy (eV)
224
222
220
(d)
Counts (a.u.)
Counts (a.u.)
(c)
S 2s
218
S 2s
238
236
234
232
230
228
226
224
222
220
218
Binding Energy (eV)
Figure 2. XPS of the Mo(IV) 3d5/2 and 3d3/2 and S (2s) peaks of MoS2 nanoparticles
prepared using (a) 27700/4300, (b) 42500/7800, (c) 81000/21000, and (d) 172000/42000
molecular weight polymers.
Following oxygen plasma treatment to remove the polymer from the nanoparticle, xray photoelectron spectroscopy (XPS) confirmed fully oxidized (6+ valent) states for
both molybdenum and tungsten. Molybdenum nanoparticles were then sulfidized in our
reactor as described above. XPS confirmed complete MoS2 composition with no oxide
component, Figure 2. This was a critical result as the nanoparticulate MoS2 was shown
to be stable in air and water.
Ultraviolet-visible (UV-Vis) spectroscopy confirmed a size-dependent blue-shift in
the absorption onset of nanoparticulate MoS2 versus bulk films, Figure 3. This showed
that quantum confinement can in fact be achieved with MoS2, up to ~ 2 eV which was the
bandgap target of the investigation.
(a)
27700/4200
81000/21000
172000/42000
Bulk film
(b)
(αhν)
2
Absorbance (a.u.)
27700/4200
81000/21000
172000/42000
Bulk film
400
600
800
1000
1200
1400
1.0
1.5
2.0
2.5
3.0
3.5
E (hν) / eV
Wavelength (nm)
(c)
(αhν)
1/2
27700/4200
81000/21000
172000/42000
Bulk film
1.0
1.5
2.0
2.5
3.0
3.5
E (hν) / eV
Figure 3. (a) UV-Vis absorption data of MoS2 film and nanoparticles supported on
fluorine-doped tin oxide. A blueshift in the absorption onset is observed with decreasing
size. Tauc plots indicate an increase in both the (b) direct and (c) indirect band gaps.
The bulk of the effort in this project was focused on proving the concept/principle
with MoS2 nanoparticles; however significant progress was made in developing methods
for the other structures – namely nanowires and a double-gyroid mesoporous structure.
Templating methods were employed to produce the latter two morphologies. In general,
the synthesis procedure is to (1) synthesize a porous template – the negative of the
desired structure, (2) electrodeposit Mo/MoO3 into the template, and (3) remove the
template by etching. Below we describe our efforts to produce these structures.
A general procedure for the electrodeposition of molybdenum and tungsten has been
optimized in our laboratory, Figure 4. The results from this work are directly applicable
to the development of nanostructures from the anodic alumina and double-gyroid
templating methods we investigated for this project. This procedure could also
potentially be extended to future studies of ultrathin films that may exhibit onedimensional quantum confinement. XPS confirmed MoS2 and WS2 for these films after a
H2/H2S furnace treatment to 150 ºC and 400 ºC, respectively.
700
(a)
Thickness (nm)
600
500
400
300
200
100
0
0
1000
2000
3000
4000
Length (µm)
(b)
700
Thickness (nm)
600
500
400
300
200
100
0
0
1000
2000
Length (µm)
3000
4000
Figure 3. Profilometry of (a) Mo and (b) W films prepared by electrodeposition.
Anodized aluminum oxide templates have been produced with pores on the order of
13 nm, Figure 5, which is below MoS2’s Bohr exciton radius of 15 nm. Future work will
be needed to refine anodization parameters in order to improve the monodispersity and
size control of the pores. This will ultimately be followed by development of nanowires
using the electrodeposition method that we have optimized.
(a)
(b)
Figure 5. SEM of anodized alumina oxide templates, exhibiting pore sizes of ~13 nm.
The double-gyroid nanostructure required the construction of a custom dip coater
with humidification control, which we have successfully developed. Humidity can be
controlled over a range of 20-70% (optimal double-gyroid formation occurs between 4060%). We have produced SiO2 double-gyroid template films in our laboratory, and
subsequently filled them with Pt by electrodeposition. Etching the SiO2 template in HF
produces a thin-film of mesoporous, double-gyroid Pt, Figure 6. Our initial attempts to
replicate this structure with molybdenum have not been as successful, however we are
continuing our efforts in this area.
Figure 6. SEM and TEM images of a double-gyroid Pt mesoporous structure synthesized
by electrodeposition into an SiO2 template, followed by template etching in HF.
Conclusions
The work conducted in this exploratory project has answered two important
questions: (1) Quantum confinement can be achieved in supported nanoparticles of MoS2,
and (2) the bandgap of MoS2 can be tuned to ~ 2 eV which is the ideal target for PEC
applications. Significant progress was also made in developing new routes to other
morphologies of nanostructured MoS2 – namely nanowires and a double-gyroid
mesoporous structure. Our results have confirmed that this approach is promising indeed
for manipulating semiconductor band structure for the synthesis of fuels from sunlight – a
technology that could lead to substantial reductions in emissions of greenhouse gases by
means of producing clean, solar-derived fuels.
Publications
1. 217th National Meeting of the Electrochemical Society, Vancouver, B.C.
“Nanostructured MoS2 for Solar Hydrogen Production”, Z. Chen and T.F.
Jaramillo, April 2010.
2. 239th National Meeting of the American Chemical Society, San Francisco, CA.
“Nanostructured MoS2 for Solar Hydrogen Production”, Z. Chen and T.F.
Jaramillo, March 2010.
3. University of California at Santa Cruz (invited). “Nano-scaled materials for the
synthesis of fuels from sunlight.” T.F. Jaramillo, February 2010.
4. University of California at Berkeley (invited). “Nano-scaled materials for the
synthesis of fuels from sunlight.” T.F. Jaramillo, December 2009.
5. 2009 American Institute of Chemical Engineers Annual Meeting, Nashville, TN.
“Nanostructured MoS2 for the Photoelectrochemical (PEC) Production of
Hydrogen.” T.F. Jaramillo, November 2009.
6. University of Texas at Austin (invited). “Nano-scaled materials for the synthesis
of fuels from sunlight.” T.F. Jaramillo, October 2009.
7. 2009 Global Climate Energy Project Research Symposium, Stanford, CA.
“Nanostructured MoS2 and WS2 for the Solar Production of Hydrogen.” T.F.
Jaramillo, October 2009.
8. University of California, Berkeley, Nanosciences and Nanoengineering Institute
(BNNI), Nanoscale Science and Engineering (NSE) Seminar, Berkeley, CA.
“Designing nano-scaled, non-precious metal catalysts for hydrogen evolution”
T.F. Jaramillo, March 2009.
9. Chevron Corporation, Richmond, CA, March 2009. “Solar Fuels by
Photoelectrochemistry (PEC)” T.F. Jaramillo, March 2009.
References
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78, 581-592.
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Review of Materials Research 2005, 35, 1-27.
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S.D.; McFarland, E.W.; J. Comb. Chem., 2005, 7, 264.
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Chemical Intermediates 2003, 29, 233-256.
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Photoelectrochemical (PEC) Hydrogen Production”, DOE Report (2009) Contract
# GS-10F-009J.
Contacts
Thomas F. Jaramillo: [email protected]
Zhebo Chen: [email protected]
Jakob Kibsgaard: [email protected]