Surface-Sensitive Spectro-electrochemistry Using Ultrafast 2D ATR IR

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Surface-Sensitive Spectro-Electrochemistry Using Ultrafast 2D ATR IR Spectroscopy Davide Lotti, Peter Hamm, and Jan Philip Kraack J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00395 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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The Journal of Physical Chemistry

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

Corresponding Author *[email protected] Phone: +41 44 63 544 77 Fax: +41 44 63 568 38

TOC Graphic

Keywords: Spectro-Electrochemistry,

Stark-Shift

Spectroscopy,

Attenuated

Total

Reflectance

Spectroscopy, 2D ATR IR spectroscopy, Carbon monoxide, Nanoparticles, Surface Spectroscopy 1 ACS Paragon Plus Environment


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ABSTRACT A new method is presented for the combination of spectro-electrochemistry 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, multi-dimensional ATR spectroscopy. The ATR geometry offers prominent benefits in ultrafast spectro-electrochemistry, i.e. 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 useable electrode materials regarding their performance in the ultrafast ATR setup. The electro-chemical performance is demonstrated by vibrational Stark-shift spectroscopy of carbon monoxide (CO) adsorbed to platinumcoated, ultrathin ITO electrodes. We furthermore measure vibrational relaxation and spectral diffusion of the stretching mode from surface-bound CO in dependence of the applied potential to the working electrode and find a negligible impact of the electrode potential on ultrafast CO dynamics.

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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. Making available ultrafast IR signals of samples at electrode interfaces is valuable in many ways and time-resolved 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 bio-chemical processes13, electro-catalytic reactions14 or solar cells4. In these disciplines, very often questions emerge which relate to vibrational, orientational and energy-transfer dynamics of molecules in direct contact with, or in the vicinity of electrodesolvent 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, e.g. vibrational coupling, chemical exchange, energy transfer, vibrational relaxation or spectral diffusion of chemical bonds in 2D IR spectroscopy.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 3 ACS Paragon Plus Environment


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based on Attenuated Total Reflectance (ATR) spectro-electrochemistry13,19,20 in combination with the recently developed ultrafast 2D ATR IR spectroscopy21–23 which yields interfacesensitive, multi-dimensional 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 electro-chemical 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 effect24,25. We monitor ultrafast vibrational relaxation as well as spectral diffusion of the CO molecules in dependence of the electro-chemical potential and resolve that both quantities are only very mildly sensitive to the applied potential.

Materials and Methods The Spectro-Electrochemical Cell Figure 1 shows a schematic drawing of the home-built electrochemical ATR cell for ultrafast experiments. The assembly consists of a right-angle, single reflection CaF2 prism (top surface 10x14 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. 4 ACS Paragon Plus Environment

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Additionally, the potentiostat is connected to counter- and reference electrodes, which are assembled inside a home-built poly-ether-ether-keton (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.

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 IndiumTin-Oxide (ITO) electrodes. Two collinear pump pulses are focused on the reflecting plane of a single reflection ATR element, excite the adsorbed molecules and a delayed, weak probe pulse interrogates the induced dynamics. The electro-chemical cell consists of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE). For clarity, the electro-chemical 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, while the outlet is located on the opposite side (not visible).

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 electro-chemically active surface. The sample chamber of the flow-cell is centered above the reflecting plane of the 5 ACS Paragon Plus Environment


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ATR prism and has an inner diameter of about 5 mm and a volume of about 0.5 cm3. In- 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 Fig. 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 case of ITO a working distance of 50 mm, a base-pressure of 8x10-5 mbar and a working pressure of 9x10-2 mbar are used in a Safematic CCU 100 sputter coater (ZMB, Zürich). 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, Fig. 2 (a)), as determined by scanning electron microscopy analysis (SEM Auriga 40, Carl Zeiss, Oberkochen, Germany). For an average thickness of 5 nm ITO of CaF2, the NPs (light grey) form a heterogeneous layer of NPs, the diameters of which are all below 10 nm. These NPs are largely inter-connected and only few gaps are formed (dark grey), the size of which is estimated from the SEM images to be less than 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 manual multi-meter separated by about 1 cm, which is sufficiently low for a quick equilibration of the electro-chemical 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 case of cosputtered Pt/ITO layers the 0.1 nm thin Pt layer serves only as the adsorption sites for the carbon monoxide (CO) while in 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 6 ACS Paragon Plus Environment

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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 qualitatively not change the overall structure of the ITO electrode layer (Fig. 2 (b)). The electrode surface consists again of inter-connected NPs (2 – 10 nm) with only 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.

Figure 2. Scanning Electron Microscopy (SEM) images of sputter-coated, 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 7 ACS Paragon Plus Environment


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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.

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 (Fig. 2 (c)). 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. Regarding liquid sample preparation, CO-saturated aqueous solutions were prepared by bubbling CO (4.8 grade, Pangas) for 15 minutes 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

The 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 2x32 pixel MCT array detector mounted to a spectrograph. 8 ACS Paragon Plus Environment

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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 off-axis parabolic mirrors. 2D spectra are generated by scanning the coherence time (τ, Fig. 1) between the two pump pulses with a fixed population time (T, Fig. 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 multi-photon absorption of the intense IR pulses. The comparatively low refractive index (n = 1.41) of CaF2 as compared to water (n = 1.31) makes it necessary to use a very large value for the angle of incidence (here > 85°) in order to satisfy total internal reflection conditions, as described previously23.

Results We start with comparing two different materials for the application as ultrathin (~nm) conductive layers in ultrafast ATR IR spectroscopy, i.e. sputter-coated metal layers (Pt, average thickness 2.5 nm) and ITO layers (average thickness 5 nm). In order to choose a suitable electrode material for ultrafast spectroscopy a few key-points need to be considered. Sputter-coated metal layers of at least 2 nm thicknesses generally approach the percolation threshold (vide supra) and the resulting conductivity28,29 allows them to be useable as electrodes. This is guaranteed for both materials employed here, as demonstrated in the supporting information (Fig. SI 1) by recording cyclic voltammograms (CVs) of the sputter-



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coated samples in the presence of 0.1 M NaClO4 electrolyte solution. As such both materials are useable as electrodes. Regarding 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 for, e.g. establishing the contact between the conductive tape and the metal layer (Fig. 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 in order 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 (Fig. SI 2). This hence makes ITO the more suitable material since more photons are available for the excitation of the adsorbate by the evanescent wave. In order to demonstrate the applicability of the ultrathin layers in ATR IR spectroscopy of adsorbates, Fig. 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 sputtercoated Pt layer (average thickness 0.1 nm, orange) on 5 nm ITO layer on a CaF2 prism. Both materials result in a single band in the range of 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 previously30. An absorptive band is observed for Pt/ITO layers, while anti-absorption31–33 is determined for the 2.5 nm Pt layer. The anti-absorption signals are the result of lineshape distortion effects generally observed for adsorbates on metal surfaces near the percolation threshold.31,33–37 We 10 ACS Paragon Plus Environment

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note that the thickness of the Pt layer as used here has been purposely tuned to result in this anti-absorption signals in order to avoid band-shapes with dispersive contributions.31,33–37 0.04

ATR abs. / OD

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0.02

Pt/ITO Pt

0.00 -0.02 -0.04

2000

2050

ω / cm

2100 -1

Figure 3. In-situ measured ATR IR absorption spectrum of CO on Pt/ITO (0.1 nm/5 nm) electrodes (orange) vs. ATR IR absorption spectrum of CO on 2.5 nm Pt (black). The 2.5 nm Pt spectrum appears inverted as an anti-absorptive signal (negative ATR absorbance) due to lineshape effects stemming from the metal layer.

Next, we evaluate the applicability of the two different electrode materials regarding the implementation in ultrafast IR spectroscopy. Since the conductive layers intrinsically absorb IR light (Fig. SI 2), a background signal is observed in time-resolved spectroscopy from the electrode materials alone (in the absence of adsorbed CO) as shown below. In order to minimize this effect, it is thus advantageous to keep the conductive layers as thin as possible. Fig. 4 shows ATR pump-probe signals for different measurements of a 2.5 nm Pt film on CaF2 (left column) and 0.1 nm Pt on 5 nm ITO (right column). Signals are shown for bare conductive layers incubated with the electrolyte solution ((a) and (b)), after CO adsorption ((c) and (d)) along with the difference signals between CO-adsorption and the electrolyteincubated electrode layers ((e) and (f)). For 2.5 nm Pt layers, (a), a spectrally broad and 11 ACS Paragon Plus Environment


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positive (red) signal is observed which evolves slowly on the experimental timescale 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 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 anti-absorptive lineshape of the CO band (Fig. 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 timescale.

Figure 4. Pump-Probe signals for 2.5 nm Pt (left column, (a), (c), (e)) and ITO/Pt electrodes (right column, (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


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((c) minus (a)), and ((d) minus (b)) showing only the CO signal contributions. The colorcoded intensity scale is depicted in mOD.

In 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 (Fig. 3). The CO signals decay on a timescale of about 10 ps, as seen also for CO on Pt, (e). The temporal evolution of the COrelated signals is discussed below.

Overall, both conductive layers are applicable in ultrafast spectroscopy in order 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 manuscript.

Vibrational Stark-Shift Spectroscopy We now turn to the application of the ultrathin conductive layers in spectro-electrochemistry. 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 Stark-shift spectroscopy5,24,38,40–43 as demonstrated below. The CO stretch frequency can be varied in a potential range of - 1 V to about + 0.5 V. In this potential range CO is stable on small Pt NPs, while complete CO 13 ACS Paragon Plus Environment


oxidative desorption is observed for potentials larger than + 0.5 V.38,39 Cyclo-voltammetry (CV) is used here to demonstrate the electro-chemical characteristics of the sputter-coated Pt and ITO-layers, covered with Pt NPs and adsorbed CO. Starting at 0 V towards positive potentials with a scan-rate of 50 mV s-1, the CV of the 2.5 nm Pt sample (Fig. 5, black) shows an intense oxidation peak at about 0.45 V vs. Ag/AgCl which corresponds to the characteristic oxidative desorption of CO on Pt.44,45 Contrasting to that, a CV on an ITO electrode covered with 0.1 nm Pt (Fig. 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. 40

20

I / µA

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Pt/ITO Pt

0

-20 -1

0

1

V vs. Ag/AgCl

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.

In order to demonstrate the vibrational Stark-shift spectroscopy, Fig. 6 shows 2D ATR IR spectra of CO on ITO/Pt for a population delay of 1 ps and for three potentials, i.e. – 1.0 V (a), 0.0 V (b), and + 0.4 V (c), all vs. Ag/AgCl. These signals have been obtained by subtraction of two separately measured spectra of the electrolyte solution on the ITO 14 ACS Paragon Plus Environment

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electrodes with an without CO.23 GSB/SE (blue) are 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 – 2070 cm-1. Recording the CO band maxima projected on the probe axis at a series of different potentials (Fig. 6 (d)) 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.

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 are experimental data, the solid red line is a linear fit with a slope Δ = 24 cm-1 V-1.



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The origin of this spectral shift in known as a vibrational Stark-effect5,24,38,40–43 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 sputtercoated 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 lineshape of the immobilized CO, as discussed previously23. 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 in dependence of 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 16 ACS Paragon Plus Environment

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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 represented by symbols while 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.

0

norm. ∆OD / mOD

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+0.4V -1.0V

-1 1

10

T / ps

Figure 7. Vibrational relaxation of CO GSB/SE signals on ITO/Pt electrodes in dependence of the applied potential. Solid triangles correspond to a potential of – 1.0 V while 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.

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 in dependence of the applied potential. Being able to record spectral diffusion 17 ACS Paragon Plus Environment


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dynamics at electrode-electrolyte interfaces allows for instance obtaining information on how, and on which timescales, the adsorbate interacts with the electrolyte molecules. Fig. 8 shows 2D ATR IR spectra for population times of 0.5 ps 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) and (d)).

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 in order to facilitate a comparison. 18 ACS Paragon Plus Environment

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Plotting CLS values in dependence of a series of the population times T allows gaining insight into the temporal dynamics underlying spectral diffusion (Fig. 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 (Fig. 7), which prevents the observation the complete decay of the CLS curves. The temporal evolution of both curves suggests an exponential decay including an offset (CLS(T) = ∆1 exp(−T/τSD) + ∆2) which we use to fit the data. Based on 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 timescale that is much longer than our observation window.21 The fits return τSD/∆2 values of 8.6 ± 0.9ps / 0.26 (+ 0.4 V, open circles) and 9.7 ± 0.6ps / 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. 1.0 0.8

CLS

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0.6 0.4

+0.4V -1.0V

0.2 0.0

2

4

6

8

10

T / ps

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) – 1 V (solid triangles). Solid lines correspond to exponential fit further described in the text.



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Discussion Our results demonstrate the potential of 2D ATR IR spectroscopy regarding the resolution of vibrational dynamics in dependence of an electro-chemical 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 (Figs. 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 orbitals5,24,41,48, i.e. strengthening and weakening of a chemical bond, or alterations of the relative energetic positions of vibrational levels within an anharmonic potential42,43. The latter effect stems from a larger average bondlength (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 inter-molecular dynamics under the impact of purposely applied electric fields.

Vibrational relaxation typically occurs on the picosecond timescale 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) environment11,25,49,50. 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, poly-crystalline Pt layers are in a good agreement with the nearly potential-invariant vibrational lifetime of CO on single crystal Pt 20 ACS Paragon Plus Environment

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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 exist 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 changing 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, also spectral diffusion occurs on timescales of a few hundreds of femtoseconds up to tens or hundreds of picoseconds with usual broadening mechanisms stemming from, e.g. solvent-sample interaction, intra-molecular fluctuations of the samples` structure, or static inhomogeneity.15,47,55 Since the diatomic molecules employed here lack the possibility of intra-molecular structural fluctuations, the main contributions to spectral diffusion are likely to stem from solvent-adsorbate interaction, 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 < 1ps, Fig. 21 ACS Paragon Plus Environment


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9). This is thus similar as observed previously23 in 2D ATR IR spectroscopy, but our CLS values are higher than observed for instance by 2D SFG spectroscopy7. 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 (∆2) in the CLS dynamics (Fig. 9) probably originate from the specific structure of the NPs on the electrode surface (Fig. 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 ∆2 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 (Fig. 6) along with very similar static offsets in spectral diffusion dynamics (Fig. 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 (∆1, 8 – 10 ps, Fig. 9) which originates from, e.g. the hydrogen-bonding interaction with the adsorbed CO and reorientation of the water molecules23 in the electrolyte solution. The determined spectral diffusion time constants are rather slow as compared to, e.g. hydrogen-bonded carbonyl functional groups in amides56,57. 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 in dependence of 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 22 ACS Paragon Plus Environment

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to, e.g. the surface oxide. Here, our results only obtain 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 re-orientation and hydrogen-bonding of water molecules in close proximity to the surface is too weak to influence the inter-molecular 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 water11,50, in order to significantly influence spectral diffusion dynamics. Such chemical interaction are not very likely for charged Pt NPs as employed here.

General Context of 2D ATR IR Spectro-Electrochemistry Most studies concerning ultrafast vibrational spectroscopy at electrified or charged interfaces employ ultrafast Sum-Frequency-Generation (SFG) spectroscopy which is based on a combination of mid-IR and visible beams.3,7–9,12,49,50,54,61 Recent reports also successfully demonstrated 2D IR spectroscopy combined with electro-chemistry obtained exclusively in the mid-IR spectral region in external reflection conditions62 using a metal mirror as electrode63,64; however, without signals directly from the interface. 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 microns 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, since 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 well23 ACS Paragon Plus Environment


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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 micrometer is sufficiently large to also investigate electro-chemically 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-polarization are exploitable to reveal information about signals from the interface.

Conclusion We have presented a new method for measuring time-resolved, multi-dimensional 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 electro-chemical ATR spectroscopy with ultrathin ITO-electrodes and the recently developed 2D ATR IR spectroscopy, multi-dimensional vibrational Stark-shift spectra of carbon monoxide (CO) adsorbed to Platinum nanoparticles have been acquired. In a potential window of – 1.0 V 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


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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 effect36,69. 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 concerning the investigation of ultrafast vibrational dynamics at interfaces, where 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 in order to allow investigations of electrocatalytic reactions70 or potential-dependent orientational dynamics71. 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 interface72, pH-jumps73 or temperature jump experiments74.

Acknowledgements 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) with regard to preparation of CO solutions, is gratefully acknowledged.

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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 IndiumTin-Oxide (ITO) electrodes. Two collinear pump pulses are focused on the reflecting plane of a single reflection ATR element, excite the adsorbed molecules and a delayed, weak probe pulse interrogates the induced dynamics. The electro-chemical cell consists of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE). For clarity, the electro-chemical 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, while the outlet is located on the opposite side (not visible).

ACS Paragon Plus Environment


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Figure 2. Scanning Electron Microscopy (SEM) images of sputter-coated, 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.


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0.04

ATR abs. / OD

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0.02

Pt/ITO Pt

0.00 -0.02 -0.04

2000

2050

ω / cm

2100 -1

Figure 3. In-situ measured ATR IR absorption spectrum of CO on Pt/ITO (0.1 nm/5 nm) electrodes (orange) vs. ATR IR absorption spectrum of CO on 2.5 nm Pt (black). The 2.5 nm Pt spectrum appears inverted as an anti-absorptive signal (negative ATR absorbance) due to lineshape effects stemming from the metal layer.



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Figure 4. Pump-Probe signals for 2.5 nm Pt (left column, (a), (c), (e)) and ITO/Pt electrodes (right column, (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 colorcoded intensity scale is depicted in mOD.


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40

20

I / µA

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Pt/ITO Pt

0

-20 -1

0

1

V vs. Ag/AgCl

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.



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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 are experimental data, the solid red line is a linear fit with a slope Δ = 24 cm-1 V-1.


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0

norm. ∆OD / mOD

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+0.4V -1.0V

-1 1

10

T / ps

Figure 7. Vibrational relaxation of CO GSB/SE signals on ITO/Pt electrodes in dependence of the applied potential. Solid triangles correspond to a potential of – 1.0 V while 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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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 in order to facilitate a comparison.


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1.0 0.8

CLS

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0.6 0.4

+0.4V -1.0V

0.2 0.0

2

4

6

8

10

T / ps

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) – 1 V (solid triangles). Solid lines correspond to exponential fit further described in the text.


Surface-Sensitive Spectro-electrochemistry Using Ultrafast 2D ATR IR

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