Surface-Sensitive Spectro-electrochemistry

Article
pubs.acs.org/JPCC
Surface-Sensitive Spectro-electrochemistry Using Ultrafast 2D ATR IR
Spectroscopy
Davide Lotti, Peter Hamm, and Jan Philip Kraack*
Department of Chemistry, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
S Supporting Information
*
ABSTRACT: A new method is presented for the combination of spectroelectrochemistry and femtosecond 2D IR spectroscopy. The key concept is
based on ultrathin (∼nm) conductive layers of noble metals and indium−tin
oxide (ITO) as working electrodes on a single-reflection attenuated total
reflectance (ATR) element in conjunction with ultrafast, multidimensional ATR
spectroscopy. The ATR geometry offers prominent benefits in ultrafast spectroelectrochemistry, that is, surface sensitivity for studying electrochemical
processes directly at the solvent−electrode interface as well as the application
of strongly IR-absorbing solvents such as water due to a very short effective path
length of the evanescent wave at the interface. We present a balanced
comparison between usable electrode materials regarding their performance in
the ultrafast ATR setup. The electrochemical performance is demonstrated by
vibrational Stark-shift spectroscopy of carbon monoxide (CO) adsorbed to
platinum-coated, ultrathin ITO electrodes. We furthermore measure vibrational
relaxation and spectral diffusion of the stretching mode from surface-bound CO dependent on the applied potential to the
working electrode and find a negligible impact of the electrode potential on ultrafast CO dynamics.
■
py.15−18 Such dynamics are largely unexplored to date for
interfacial systems and, in particular, near electrodes. It is
therefore highly desirable to make this information available
also from electrode−electrolyte interfaces.
In this paper, we report on a new spectroscopic method for
the resolution of ultrafast, multidimensional signals from
immobilized molecules at electrodes in solution. Our method
is based on attenuated total reflectance (ATR) spectroelectrochemistry13,19,20 in combination with the recently
developed ultrafast 2D ATR IR spectroscopy,21−23 which yields
interface-sensitive, multidimensional signals with sub-picosecond temporal resolution. Recent studies have shown that
2D ATR IR spectroscopy allows the investigation of even
organic monolayers equipped with low-absorbing (ε < 200 M−1
cm−1) local vibrational probes at submonolayer coverages.21,22
Here, we compare different electrode materials regarding their
applicability in time-resolved ATR IR spectroscopy and resolve
ultrafast dynamics from carbon monoxide (CO) adsorbed on
the electrode surfaces. We evaluate strengths and weaknesses of
electrode materials and show that a variation of the
electrochemical potential applied to the electrode shifts the
vibrational frequency of the immobilized CO with a rate of ∼24
cm−1 V−1 on the basis of the vibrational Stark-shift effect.24,25
We monitor ultrafast vibrational relaxation as well as spectral
diffusion of the CO molecules dependent of the electro-
INTRODUCTION
The dynamics of molecules at solid−liquid interfaces, which
can be electrified on demand, are of significant importance in
different fields of chemistry and physics, but are difficult to
access experimentally.1−7 Particularly, ultrafast vibrational
spectroscopy offers a high potential regarding identification
and characterization of transient species at electrode−electrolyte interfaces. These signals bear information on the structural
dynamics of, and interaction between, molecules and their
environment. The availability of ultrafast IR signals of samples
at electrode interfaces is valuable in many ways, and timeresolved spectroscopy for such samples gained considerable
attention in recent years.3,7−12 Spectro-electrochemistry in
conjunction with time-resolved spectroscopy is an intense field
of research, for instance, concerning redox-related biochemical
processes,13 electrocatalytic reactions14 or solar cells.4 In these
disciplines, very often questions emerge that relate to
vibrational, orientational, and energy-transfer dynamics of
molecules in direct contact with or in the vicinity of
electrode−solvent interfaces.
Ultrafast IR spectroscopy allows one to resolve vibrational
dynamics with sub-picosecond temporal resolution.15−18 It is
well-known that two-dimensional infrared (2D IR) spectroscopy15−18 allows obtaining such detailed information in
advantageous ways. In analogy to 2D NMR spectroscopy, a
correlation between two frequency axes in time-resolved signals
allows for the resolution of, for example, vibrational coupling,
chemical exchange, energy transfer, vibrational relaxation, or
spectral diffusion of chemical bonds in 2D IR spectrosco© 2016 American Chemical Society
Received: January 13, 2016
Published: January 14, 2016
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Deposition is performed with a sputtering current of 60 mA,
which results in a deposition rate of about 0.1 nm s−1. The
average thickness was measured using a quartz microbalance.
The sputtering process yields a structured surface of ITO
nanoparticles (NPs, Figure 2a), as determined by scanning
chemical potential and resolve that both quantities are only
very mildly sensitive to the applied potential.
■
MATERIALS AND METHODS
Spectro-Electrochemical Cell. Figure 1 shows a schematic
drawing of the home-built electrochemical ATR cell for
Figure 1. Sketch of the electrochemical ATR cell used in the ultrafast
2D ATR IR measurements. Carbon monoxide (CO) is adsorbed on
ultrathin platinum (Pt)-coated indium−tin oxide (ITO) electrodes.
Two collinear pump pulses focused on the reflecting plane of a singlereflection ATR element excite the adsorbed molecules, and a delayed,
weak probe pulse interrogates the induced dynamics. The electrochemical cell consists of a working electrode (WE), a counter
electrode (CE), and a reference electrode (RE). For clarity, the
electrochemical circuit involving the potentiostat is not shown, and the
prism and cell elements are not drawn to scale. “I” denotes the inlet of
the flow cell, whereas the outlet is located on the opposite side (not
visible).
ultrafast experiments. The assembly consists of a right-angle,
single-reflection CaF2 prism (top surface 10 × 14 mm,
Thorlabs), the reflecting plane of which is sputter-coated with
an ultrathin layer of either platinum (Pt) or indium−tin oxide
(ITO). These layers are used as the working electrode (vide
infra). The sputtered layer is incorporated in an electrochemical
circuit containing a potentiostat (IviumStat Inc.) via a thin
adhesive aluminum foil (Electron Microscopy Science Inc.)
sandwiched between the cell and the sputter-coated prism
surface. Additionally, the potentiostat is connected to counter
and reference electrodes, which are assembled inside a homebuilt polyether−ether ketone (PEEK) sample flow cell. The
counter electrode consists of a Pt-coated Ti wire (E-DAQ Inc.)
and is centered in the chamber of the flow cell. The reference
electrode (Ag/AgCl, Biosense Inc.) is located at the front side
of the PEEK cell.
The sample cell assembly is mounted on top of the reflecting
plane of the sputter-coated ATR prism using an O-ring
(Teflon) to allow solvent flow above the working electrode.
The O-ring has an inner diameter of 7 mm, which determines
the area of the electrochemically active surface. The sample
chamber of the flow cell is centered above the reflecting plane
of the ATR prism and has an inner diameter of about 5 mm and
a volume of about 0.5 cm3. Inlets and outlets of the sample cell
(I/O) are located on opposite sides of the flow cell and are
connected to Teflon tubes (outlet not shown in Figure 1).
Conductive layers as working electrodes on the ATR prism
are obtained by use of either Pt or ITO as an electrode material.
The ultrathin layers are sputter-coated on the reflecting plane of
the CaF2 ATR prism using Ar+ ion sputtering. In the case of
ITO a working distance of 50 mm, a base pressure of 8 × 10−5
mbar, and a working pressure of 9 × 10−2 mbar are used in a
Safematic CCU 100 sputter coater (ZMB, Zürich, Switzerland).
Figure 2. Scanning electron microscopy (SEM) images of sputtercoated, ultrathin electrode films on CaF2 substrates used as electrodes
in the spectro-electrochemistry experiments: (a) SEM image of a 5 nm
(average thickness) bare ITO film; (b) SEM image of a 5 nm (average
thickness) ITO film co-sputtered with a 0.1 nm ad-layer of Pt; (c)
SEM image of a 2.5 nm (average thickness) Pt layer. The scale bar is
50 nm in all images. Images were recorded with an in-lens secondary
electron detector at an acceleration voltage of 5 kV. The image pixel
size is 352.9 pm.
electron microscopy analysis (SEM Auriga 40, Carl Zeiss,
Oberkochen, Germany). For an average thickness of 5 nm ITO
of CaF2, the NPs (light gray) form a heterogeneous layer of
NPs, the diameters of which are all below 10 nm. These NPs
are largely interconnected, and only few gaps are formed (dark
gray), the size of which is estimated from the SEM images to be
<5 nm. In this form, the sputtered ITO layers have a typical
resistance of about 0.5−10 kΩ measured with two tips of a
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compared to water (n = 1.31) makes it necessary to use a very
large value for the angle of incidence (here >85°) to satisfy total
internal reflection conditions, as described previously.23
manual multimeter separated by about 1 cm, which is
sufficiently low for a quick equilibration of the electrochemical
potential on the surface.
Pt layers are sputter-coated either on top of the ITO
electrode layers (in this case with an average thickness of 0.1
nm) or on bare CaF2 prisms (average thickness of 2.5 nm)
using a sputtering current of 6 mA, which resulted in a
deposition rate of about 0.02 nm s−1. Otherwise, the deposition
parameters were the same for ITO (vide supra). In the case of
co-sputtered Pt/ITO layers, the 0.1 nm thin Pt layer serves only
as the adsorption site for the carbon monoxide (CO), whereas
in the case of the pure Pt layers (2.5 nm), the entire layer
functions as an electrode as well as adsorption site of the CO.
We note that the additional deposition of 0.1 nm Pt on top of
the ITO layers does not influence the general conductivity of
the substrate. The 0.1 nm co-sputtered Pt layers do not
qualitatively change the overall structure of the ITO electrode
layer (Figure 2b). The electrode surface consists again of
interconnected NPs (2−10 nm) with only a very few gaps (<5
nm). We state that for the very thin Pt ad-layers (0.1 nm) on
ITO it is not possible to discriminate between the ITO and Pt
NPs in the SEM image.
The deposition of Pt with an average thickness of 2.5 nm
results in an approximately continuous layer of Pt NPs, which
form large and interconnected island-type structures with sizes
exceeding 100 nm (Figure 2c). Here, narrow corrugations (<5
nm) are formed between the Pt islands. The large Pt islands
consist of aggregates formed by much smaller Pt NPs (<5 nm).
The sputtered Pt electrode layers exhibit a similar resistance
(0.5−10 kΩ) as compared to the ITO layers.
With regard to liquid sample preparation, CO-saturated
aqueous solutions were prepared by bubbling CO (4.8 grade,
Pangas) for 15 min through a double-distilled water solution
containing 0.1 M NaClO4 (puriss. p.a., Sigma-Aldrich) as
electrolyte. The prepared CO solutions were flown across the
electrode for a few minutes until adsorption was complete.23
Ultrafast Setup. For 2D ATR IR measurements, the
experimental setup is the same as described previously.21−23 In
brief, ∼90 fs mid-IR pulses (∼250 cm−1 bandwidth) centered
around 2000 cm−1 are generated from an OPA,26 which is
pumped by a 5 kHz regenerative amplifier at 800 nm. The
output energy is about 1.5 μJ per pulse. The major portion is
directed to a Mach−Zehnder interferometer to obtain coherent
pump pulse pairs for 2D IR spectroscopy. About 5% of the
OPA output is split off with a BaF2 wedge and used as probe
and reference beams for balanced signal detection on a 2 × 32
pixel MCT array detector mounted to a spectrograph. Pump,
probe, and reference beams all exhibit s-polarization with
respect to the reflecting prism surface and are focused on the
backside of the reflecting plane of the ATR crystal by use of offaxis parabolic mirrors. 2D spectra are generated by scanning the
coherence time (τ, Figure 1) between the two pump pulses
with a fixed population time (T, Figure 1) between the second
pump pulse and the probe pulse. A succeeding Fourier
transformation of the oscillating signal part27 yields the
absorptive 2D IR spectrum.
CaF2 is used as an ATR material due to its broad
transparency in the mid-IR region (down to ∼1100 cm−1). In
addition, more widely applied ATR crystals such as germanium,
silicon, or zinc selenide possess significantly lower bandgaps of
electronic excitation, which make these materials susceptible to
multiphoton absorption of the intense IR pulses. The
comparatively low refractive index (n = 1.41) of CaF2 as
■
RESULTS
We start with comparing two different materials for application
as ultrathin (∼nm) conductive layers in ultrafast ATR IR
spectroscopy, that is, sputter-coated metal layers (Pt, average
thickness = 2.5 nm) and ITO layers (average thickness = 5
nm). To choose a suitable electrode material for ultrafast
spectroscopy, a few key points need to be considered. Sputtercoated metal layers of at least 2 nm thicknesses generally
approach the percolation threshold (vide supra), and the
resulting conductivity28,29 allows them to be usable as
electrodes. This is guaranteed for both materials employed
here, as demonstrated in the Supporting Information (Figure SI
1) by recording cyclic voltammograms (CVs) of the sputtercoated samples in the presence of 0.1 M NaClO4 electrolyte
solution. As such, both materials are usable as electrodes.
With regard to mechanical stability of the conductive layers,
however, we find empirically during the processes of sample
preparation, handling, sample-cell assembling, and cleaning that
ultrathin Pt layers are significantly more susceptible to
mechanical stress compared to the ITO layers. This is of
particular importance in, for example, establishing the contact
between the conductive tape and the metal layer (Figure 1). As
such, ITO can be considered as an “easier to handle” material
regarding spectro-electrochemistry.
Another aspect is that the materials need to transmit the IR
evanescent wave in ATR spectroscopy. Specifically regarding
ultrafast spectroscopy, it is desirable to minimize the intrinsic
absorbance of the electrode layer to have available the
maximum light intensity for excitation and probing of the
adsorbate signals. For the two materials employed here, we find
that ITO exhibits the lower values of IR absorbance compared
to Pt samples (Figure SI 2). This hence makes ITO the more
suitable material because more photons are available for the
excitation of the adsorbate by the evanescent wave.
To demonstrate the applicability of the ultrathin layers in
ATR IR spectroscopy of adsorbates, Figure 3 shows in situ
measured21,23 ATR spectra of CO adsorbed to a sputter-coated
Pt layer (average thickness = 2.5 nm, black) on a bare CaF2
prism along with CO on a sputter-coated Pt layer (average
thickness = 0.1 nm, orange) on a 5 nm ITO layer on a CaF2
prism. Both materials result in a single band in the range of
Figure 3. In-situ measured ATR IR absorption spectrum of CO on Pt/
ITO (0.1 nm/5 nm) electrodes (orange) versus ATR IR absorption
spectrum of CO on 2.5 nm Pt (black). The 2.5 nm Pt spectrum
appears inverted as an antiabsorptive signal (negative ATR
absorbance) due to line shape effects stemming from the metal layer.
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Figure 4. Pump−probe signals for 2.5 nm Pt (left column, panels a, c, e) and ITO/Pt electrodes (right column, panels b, d, f): (a, b) Signals in the
presence of the neat electrolyte solution; (c, d) signals in the presence of CO-saturated electrolyte solution; (e, f) difference signals ((c) minus (a)
and (d) minus (b)) showing only the CO signal contributions. The color-coded intensity scale is depicted in mOD.
2000−2100 cm−1, which corresponds to linear bound CO on
Pt.23 We note that the different CO band positions are most
likely the result of a dependence of the CO band position on
metal particle sizes as observed previously.30 An absorptive
band is observed for Pt/ITO layers, whereas anti-absorption31−33 is determined for the 2.5 nm Pt layer. The antiabsorption signals are the result of line shape distortion effects
generally observed for adsorbates on metal surfaces near the
percolation threshold.31,33−37 We note that the thickness of the
Pt layer as used here has been purposely tuned to result in this
anti-absorption signal to avoid band shapes with dispersive
contributions.31,33−37
Next, we evaluate the applicability of the two different
electrode materials regarding the implementation in ultrafast IR
spectroscopy. Because the conductive layers intrinsically absorb
IR light (Figure SI 2), a background signal is observed in timeresolved spectroscopy from the electrode materials alone (in
the absence of adsorbed CO) as shown below. To minimize
this effect, it is thus advantageous to keep the conductive layers
as thin as possible. Figure 4 shows ATR pump−probe signals
for different measurements of a 2.5 nm Pt film on CaF2 (left
column) and of 0.1 nm Pt on 5 nm ITO (right column).
Signals are shown for bare conductive layers incubated with the
electrolyte solution (a, b) and after CO adsorption (c, d), along
with the difference signals between CO adsorption and the
electrolyte-incubated electrode layers (e, f). For 2.5 nm Pt
layers (a), a spectrally broad and positive (red) signal is
observed, which evolves slowly on the experimental time scale
of 40 ps. This signal stems from direct Pt excitation of
conduction electrons at the Fermi edge and subsequent thermal
energy redistribution (heating) of the substrate. This signal is
changed in the presence of adsorbed CO (c) to higher/lower
signal magnitudes in spectral regions of GSB/SE (∼2070
cm−1)/ESA (∼2025 cm−1), which correspond to the CO signal
contributions. Note that the CO signals are inverted with
respect to the conventional representation of differential
absorbance (GSB/SE ≡ decrease absorbance, ESA ≡ increase
in absorbance) in pump−probe spectroscopy due to the antiabsorptive line shape of the CO band (Figure 3, black line).
The difference between the signals in (a) and (c) reveals the
signal contributions of only the adsorbed CO (e), which clearly
shows GSB/SE signals (red) as well as ESA signals (blue),
decaying on a 10 ps time scale.
In the case of ITO/Pt layers the bare electrode signal (b) is
negative (reduced absorption, blue), and the dominant signal
intensity ceases during the initial 500 fs after excitation. The
pure CO signals (GSB/SE ∼ 2060 cm−1 and ESA ∼ 2020 cm−1
(f)) are again obtained after subtraction of the bare Pt/ITO
background signals from the CO on ITO/Pt data (d). Note
that here the GSB/SE and ESA signals are negative/positive
differential absorption as expected from the positive absorption
signals of the CO ATR band (Figure 3). The CO signals decay
on a time scale of about 10 ps, as seen also for CO on Pt (e).
The temporal evolution of the CO-related signals is discussed
below.
Overall, both conductive layers are applicable in ultrafast
spectroscopy to reveal the sought adsorbate signals. However,
due to the much shorter-lived background signal of the ITO as
well as the better mechanical stability of ITO, we will focus on
the application of ITO/Pt layers in ultrafast measurements
throughout the rest of the paper.
Vibrational Stark-Shift Spectroscopy. We now turn to
the application of the ultrathin conductive layers in spectroelectrochemistry. The linearly bound CO on Pt NPs sensitively
responds to electrochemical potentials of the electrode.38,39
Incorporation of the conductive layers as working electrodes in
an electrochemical circuit therefore allows for vibrational Starkshift spectroscopy5,24,38,40−43 as demonstrated below. The CO
stretch frequency can be varied in a potential range from −1 V
to about +0.5 V. In this potential range CO is stable on small Pt
NPs, whereas complete CO oxidative desorption is observed
for potentials larger than +0.5 V.38,39 Cyclo-voltammetry (CV)
is used here to demonstrate the electrochemical characteristics
of the sputter-coated Pt and ITO layers, covered with Pt NPs
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observed together with ESA signals (red). As the potential is
varied from negative (−1.0 V (a)) to positive (+0.4 V (c))
values, the central frequency of the CO band changes between
2037 and 2070 cm−1. Recording the CO band maxima
projected on the probe axis at a series of different potentials
(Figure 6d) reveals that the central frequency (open circles) is
shifted upon potential variation in a linear manner. A linear fit
to the data yields a slope of ∼24 cm−1 V−1, which is in good
agreement with previous results (25−30 cm−1 V −1).8,38,39
The origin of this spectral shift in known as a vibrational
Stark-effect,5,24,38,40−43 which is based on alterations of the
vibrational properties and chemical binding properties of
adsorbates in electric fields near electrodes. We note that due
to the heterogeneous structure of the sputter-coated electrode
layers in our experiment, there will be a spatial variation of the
local electric field around the NPs, which is difficult to quantify.
Due to the applied s-polarization of excitation and probing
beams in our experiments, however, the dominant signal stems
from molecules adsorbed at the sides of the NPs. In this
respect, the close agreement between the observed Stark-tuning
rate in this work and previous8,38,39 results indicates that the
spatial variation of the local electric field is not significantly
different.
For all potentials the GSB/SE signals in the 2D ATR IR
spectra are strongly elongated along the diagonal, indicating a
large degree of inhomogeneity in the vibrational line shape of
the immobilized CO, as discussed previously.23 The center-line
slope (CLS) method46,47 is used here to quantify inhomogeneity of the GSB/SE signals. We state that linear fits to the CLS
for the different 2D spectra at 1 ps delay show very similar
values (∼0.75−0.8), irrespective of the potential. This indicates
that the distribution of oscillators contributing to the broad
absorption band does not significantly depend on the applied
potential. A detailed analysis of the temporal evolution of the
CLS values from 2D ATR IR signals will be discussed further
below.
Ultrafast CO−Dynamics in Dependence of Potential.
We additionally studied ultrafast vibrational relaxation of the
adsorbed CO molecules on the Pt/ITO dependent on the
applied potential. Figure 7 shows cuts along the temporal axis
of background-subtracted ATR IR pump−probe data at the
position of the maximum GSB/SE signal for CO at the two
potentials of +0.4 V (open circles, ∼2070 cm−1) and −1.0 V
(solid triangles, ∼2037 cm−1), which are the two potentials of
highest charge employed in this study. Experimental data are
and adsorbed CO. Starting at 0 V toward positive potentials
with a scan rate of 50 mV s−1, the CV of the 2.5 nm Pt sample
(Figure 5, black) shows an intense oxidation peak at about 0.45
Figure 5. Cyclic voltammograms of sputter-coated conductive Pt (2.5
nm, black) and ITO/Pt (5 nm/0.1 nm, orange) layers incubated with
CO-saturated aqueous NaClO4 (0.1 M) solutions recorded at a
temperature of 294 K and a scan rate of 50 mV s−1 at pH 7.
V versus Ag/AgCl, which corresponds to the characteristic
oxidative desorption of CO on Pt.44,45 In contrast to that, a CV
on an ITO electrode covered with 0.1 nm Pt (Figure 5, orange)
shows a much broader oxidative desorption wave with lower
intensity, which peaks at about 0.4 V. This reduced wave simply
corresponds to a lower number of adsorption sites for the CO
due to a much lower average Pt thickness.
To demonstrate the vibrational Stark-shift spectroscopy,
Figure 6 shows 2D ATR IR spectra of CO on ITO/Pt for a
population delay of 1 ps and for three potentials, those being
−1.0 V (a), 0.0 V (b), and +0.4 V (c), all versus Ag/AgCl.
These signals have been obtained by subtraction of two
separately measured spectra of the electrolyte solution on the
ITO electrodes with and without CO.23 GSB/SE (blue) are
Figure 6. Normalized 2D ATR IR signals of CO absorbed on ultrathin
ITO/Pt electrodes for different potentials and a population time T = 1
ps: (a) −1.0 V; (b) 0 V; (c) +0.4 V; (d) spectral positions of the GSB/
SE (blue) CO band maxima projected on the probe axis against the
applied potential (open circles, experimental data; solid red line, linear
fit with slope Δ = 24 cm−1 V−1).
Figure 7. Vibrational relaxation of CO GSB/SE signals on ITO/Pt
electrodes dependent on the applied potential. Solid triangles
correspond to a potential of −1.0 V, whereas open circles represents
data from a potential of +0.4 V. Solid lines visualize single-exponential
fits to the experimental data. The vibrational relaxation time constants
are only mildly dependent on the applied electrode potential.
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represented by symbols, whereas single-exponential fits are
represented by solid lines. We find vibrational relaxation time
constants of 2.5 ± 0.1 ps (−1.0 V) and 3.1 ± 0.1 ps (+0.4 V).
Both values are slightly faster, but in overall good agreement
with previously reported (3−4 ps)23 vibrational relaxation of
CO on ultrathin metal layers. This demonstrates that the CO
vibrational lifetime is only very mildly dependent on the applied
electric field at the surface.
Furthermore, ultrafast spectral diffusion of CO on ITO/Pt
electrodes was investigated by recording 2D ATR IR spectra for
a series of population delays during the vibrational relaxation
dependent on the applied potential. Being able to record
spectral diffusion dynamics at electrode−electrolyte interfaces
allows, for instance, obtaining information on how and on
which time scales the adsorbate interacts with the electrolyte
molecules. Figure 8 shows 2D ATR IR spectra for population
Figure 9. Spectral diffusion dynamics extracted from CLS values for a
series of 2D ATR IR GSB/SE signals of CO adsorbed in ITO/Pt
electrodes at +0.4 V (open circles) and −1 V (solid triangles). Solid
lines correspond to exponential fit further described in the text.
data. On the basis of the limited temporal window for the
investigation we cannot decide whether the offset is a truly
static contribution or if it represents dynamics on a time scale
that is much longer than our observation window.21 The fits
return τSD/Δ2 values of 8.6 ± 0.9 ps/0.26 (+ 0.4 V, open
circles) and 9.7 ± 0.6 ps/0.21 (−1.0 V, solid triangles). The
similarity between these time constants and offsets indicates
that the dynamics of spectral diffusion are very similar for the
two potentials.
■
DISCUSSION
Our results demonstrate the potential of 2D ATR IR
spectroscopy regarding the resolution of vibrational dynamics
dependent on an electrochemical potential of adsorbates on
electrode surfaces. The main information of this study is
represented by the vibrational relaxation along with spectral
diffusion of CO adsorbed to an electrode surface (Figures 4, 7,
and 9) under the influence of the vibrational Stark-effect.
Traditionally, the effect of variations in vibrational frequency of
adsorbates with an applied external electric field is explained by
either changes in the electron density in (anti)bonding
orbitals,5,24,41,48 that is, strengthening and weakening of a
chemical bond, or alterations of the relative energetic positions
of vibrational levels within an anharmonic potential.42,43 The
latter effect stems from a larger average bond- length (and thus
a higher dipole moment) for excited vibrational levels, which
hence experience different energetic (de)stabilization in an
external electric field relative to the vibrational ground
state.42,43 A combination of vibrational Stark-shift and ultrafast
spectroscopy, as developed here, therefore allows experimentalists to gain valuable insight into intra- and intermolecular
dynamics under the impact of purposely applied electric fields.
Vibrational relaxation typically occurs on the picosecond
time scale and provides insight into the interaction between an
adsorbate and its environment. In the case studied here, the
environment of adsorbed CO consists of the electrode surface
as well as the electrolyte solution. Aside from variations in bond
strengths and (de)stabilization of vibrational levels (vide supra),
the interfacial electric field can also affect the alignment of the
adsorbate molecules with respect to the surface8,25 or alter the
structure and dynamics of the solvent (electrolyte) environment. 11,25,49,50 All of these points can influence the
redistribution of the adsorbates’ excess vibrational energy
after excitation. The results presented here for vibrational
relaxation of CO on sputter-coated, polycrystalline Pt layers are
in a good agreement with the nearly potential-invariant
Figure 8. 2D ATR IR spectra of CO on ITO/Pt electrodes at different
potentials of (a, b) +0.4 V and (c, d) −1 V, both for population times
of 0.5 ps (a, c) and 7 ps (b, d). White lines correspond to CLS of the
GSB/SE signals (blue), which is used to quantify the degree of
inhomogeneity of the sample. Red signals represent ESA contributions. Note that the 2D spectra are normalized to maximum GSB/SE
signal magnitudes to facilitate comparison.
times of 0.5 and 7 ps for CO adsorbed on ITO/Pt, again for the
two most separated potentials of +0.4 V (a, b) and −1.0 V (c,
d). For both potentials, the GSB/SE signals are initially (a, c)
strongly elongated along the diagonal line with similar CLS
values of about 0.8−0.85. Temporal evolution of the GSB/SE
signals reduces the CLS values to about 0.6, again very similar
for both potentials (b, d).
Plotting CLS values dependent on a series of population
times T allows gaining insight into the temporal dynamics
underlying spectral diffusion (Figure 9). For both potentials the
CLS curves start at values about 0.8−0.85 and evolve to values
around 0.4 during 10 ps of relaxation. Note that the temporal
window for investigations is limited by the vibrational lifetime
of about 3 ps (Figure 7), which prevents observation of the
complete decay of the CLS curves. The temporal evolution of
both curves suggests an exponential decay including an offset
(CLS(T) = Δ1 exp(−T/τSD) + Δ2), which we use to fit the
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the electrolyte solution. The determined spectral diffusion time
constants are rather slow as compared to, for example,
hydrogen-bonded carbonyl functional groups in amides.56,57
This sluggish spectral diffusion is likely to stem from the
confined nature of the water at the solid−liquid interface.58−60
In this context, investigations of the structure of interfacial
water in direct contact with charged oxide surfaces have
revealed significant structural variations of water molecules
dependent on surface charge.11,49,50 For instance, water is
known to form spatially extended hydrogen-bond networks,
specifically at negative surface charge when the first layer of
water molecules is strongly hydrogen-bonded to, for example,
the surface oxide. Here, our results obtain only indirect
information about the water dynamics11,50 via spectral diffusion
of the surface-bound CO molecules. The observed near
invariance of spectral diffusion with the electrode potential
may indicate that a simple electrostatic influence of the applied
electric field on reorientation and hydrogen bonding of water
molecules in close proximity to the surface is too weak to
influence the intermolecular dynamics. It is thus more likely
that the near-surface water molecules need to establish a certain
chemical interaction, such as hydrogen bonding between the
negatively charged surface oxides and the water,11,50 to
significantly influence spectral diffusion dynamics. Such
chemical interactions are not very likely for the charged Pt
NPs employed here.
General Context of 2D ATR IR Spectro-electrochemistry. Most studies concerning ultrafast vibrational
spectroscopy at electrified or charged interfaces employ
ultrafast SFG spectroscopy, which is based on a combination
of mid-IR and visible beams.3,7−9,12,49,50,54,61 Recent studies
also successfully demonstrated 2D IR spectroscopy combined
with electrochemistry in external reflection conditions62 using a
metal mirror as an electrode, however, without signals directly
from the interface.63,64 When working in such a configuration,
the excitation light travels through the (generally) aqueous
electrolyte solution twice, which demands liquid layers of only a
few micrometers in thickness, due to the spectrally broad and
strong IR absorbance of water.63,64 This is a little less of an
issue in external reflection SFG spectroscopy, because the signal
light is frequency-converted to the visible spectral range at the
interface before the second path through the liquid phase. In
ATR spectroscopy, in contrast, challenges concerning sample
thicknesses are circumvented due to the very low penetration
depth of the evanescent wave at the interface. The highest
intensity of the evanescent field is present directly at the solid−
liquid interface, giving rise to the well-known surface sensitivity
of ATR spectroscopy. In addition to molecules immobilized at
the surface, ATR spectroscopy is also expected to be able to
resolve dynamics from molecules within the Helmholtz double
layer.65 As a rule of thumb, the evanescent fields produced at
the interface exhibit penetration depths of about one
wavelength of the applied light (here ca. 5 μm), thin enough
so that water absorption is not yet a severe problem.66
Moreover, the penetration depth can easily be tuned to both
smaller and larger values by varying the angle of incidence66
and the polarization67 of the IR light. A penetration depth of a
few micrometers is sufficiently large to also investigate
electrochemically active samples in bulk solution. We note
that a polarization dependence of 2D ATR IR signals of
adsorbates has been presented recently68 and reveals that both
p- and s-polarizations are exploitable to reveal information
about signals from the interface.
vibrational lifetime of CO on single-crystal Pt electrodes
observed with sum-frequency generation (SFG) spectroscopy.51,52 It can therefore be concluded that the vibrational
lifetime is in a general manner only very mildly affected by
changes in the sigma-donating, π*-accepting electronic bonding
mechanism of CO to the Pt NPs41 as well as charge-induced
structural changes of the complex hydrogen-bond network of
water molecules near the charged surface.11,49,50
It is furthermore well-known, that the vibrational lifetime of
adsorbed CO depends strongly on the electronic structure of
the surface. For instance, the vibrational lifetime is different by
multiple orders of magnitude for metals (e.g., Pt, Cu) and
insulators (NaCl) as surfaces for immobilization.53 This result
has been interpreted in that there exists strong coupling
between the vibrational states and the continuum electronic
states of the metal to which the excess vibrational energy is
transferred after excitation.53,54 The results presented here thus
moreover indicate that this electron−nuclear coupling is not
strongly affected by (i) changes in the electron density of the
metal NPs upon change of the potential at the interface and (ii)
the Stark-shifted ground and excited vibrational levels of the
adsorbate and (iii) possible image dipole contributions as
invoked previously54 for adsorbates at metal surfaces.
Next we turn to the spectral diffusion dynamics, which
originate from the temporal fluctuations of individual oscillator
frequencies under the envelope of the CO absorption band. In
general, spectral diffusion also occurs on time scales of a few
hundreds of femtoseconds up to tens or hundreds of
picoseconds with usual broadening mechanisms stemming
from, for example, solvent−sample interaction, intramolecular
fluctuations of the samples’ structure, or static inhomogeneity.15,47,55 Because the diatomic molecules employed here lack
the possibility of intramolecular structural fluctuations, the
main contributions to spectral diffusion are likely to stem from
solvent−adsorbate interaction and molecular alignment on the
surface as well as static inhomogeneity. In general, we observe
that the GSB/SE signal elongation represented by the CLS
values are fairly high at early delays (>0.8 at <1 ps, Figure 9).
This is thus similar to that observed previously23 in 2D ATR IR
spectroscopy, but our CLS values are higher than observed, for
instance, by 2D SFG spectroscopy.7 We note, however, that
these differences are most likely due to different sample
preparation techniques for 2D ATR and 2D SFG spectroscopy.
The static contributions (Δ2) in the CLS dynamics (Figure
9) probably originate from the specific structure of the NPs on
the electrode surface (Figure 2). In this respect we note that
similar static offsets in spectral diffusion of CO on noble metal
alloys have been observed previously23 as well and have been
interpreted in terms of the spatially varying NP structure in the
alloy. It is therefore likely that the Δ2 values observed here are
of the same origin. By the observation of similar elongation of
the GSB/SE peaks along the diagonal line at a series of
potentials (Figure 6) along with very similar static offsets in
spectral diffusion dynamics (Figure 9), we can therefore
conclude that no significant potential-dependent alignment or
ordering of adsorbed molecules with respect to the surface
occurs which would change the static offsets in the CLS curves
or interaction between the water molecules and the adsorbate
at the interface.
The largest part of spectral diffusion stems from the dynamic
contribution (Δ1, 8−10 ps, Figure 9), which originates from,
for example, the hydrogen-bonding interaction with the
adsorbed CO and reorientation of the water molecules23 in
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■
CONCLUSION
We have presented a new method for measuring time-resolved,
multidimensional IR spectra from immobilized molecules at
electrode−electrolyte interfaces. Experimental benefits and
pitfalls of different electrode materials have been compared
and emphasize the advantages of ultrathin (∼nm) conductive
layers. Using a combination of electrochemical ATR spectroscopy with ultrathin ITO electrodes and the recently developed
2D ATR IR spectroscopy, multidimensional vibrational Starkshift spectra of CO adsorbed to platinum nanoparticles have
been acquired. In a potential window from −1.0 to +0.5 V the
CO band was Stark-shifted at a rate of ∼24 cm−1 V −1.
Vibrational relaxation as well as spectral diffusion of
immobilized CO has been investigated in detail and shows
negligible dependence on the applied electrode potential. From
the presented results we expect that ultrafast 2D ATR IR
spectro-electrochemistry will find broad application in various
research fields which involve purposely charged interfaces as
well as redox- and electron-transfer dynamics across solid−
liquid interfaces.
It is well-known that upon binding to metal NPs, the
oscillator strength of the CO molecule increases due to a
chemical enhancement effect.36,69 This chemical enhancement
effect thus contributes here as well, together with recently
characterized electromagnetic enhancement effects in 2D ATR
IR spectroscopy for sputter-coated metal layers.68 Such
enhancement effects significantly help the investigation of
ultrafast vibrational dynamics at interfaces, where the number
of contributing sample molecules is only low.
A first future extension of the current work will involve the
transfer of redox-active samples from bulk solution to organic
monolayers21,22 to allow investigations of electrocatalytic
reactions70 or potential-dependent orientational dynamics.71
In addition to this, a second major extension can be envisioned
to involve time-resolved investigations concerning dynamics
due to fast potential jumps at the interface,72 pH jumps,73 or
temperature jump experiments.74
■
with regard to preparation of CO solutions, is gratefully
acknowledged.
■
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.6b00395.
Stability of working electrodes and infrared spectra of
electrode materials (PDF)
■
REFERENCES
AUTHOR INFORMATION
Corresponding Author
*(J.P.K.) E-mail: [email protected]. Phone: +41 44
63 544 77. Fax: +41 44 63 568 38.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The work has been supported by the Swiss National Science
Foundation (Grant CRSII2_136205/1) and by the University
Research Priority Program (URPP) for solar light to chemical
energy conversion (LightChEC). Experimental support by the
Center of Microscopy and Imaging at the University of Zürich
(ZMB) with regard to the preparation and characterization of
ITO layers, as well as by Henrik Braband (University of Zürich)
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