Pt(111) Electrode Interface Probed by

C , 2015, 119 (46), pp 26056–26063. DOI: 10.1021/acs.jpcc.5b10086. Publication Date (Web): October 22, 2015. Copyright © 2015 American Chemical Soc...
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Electronic Structure of the CO/Pt(111) Electrode Interface Probed by Potential-Dependent IR/Visible Double Resonance Sum Frequency Generation Spectroscopy Shuo Yang,†,‡ Hidenori Noguchi,*,†,‡,§,∥ and Kohei Uosaki*,†,‡,§ †

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan § International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan ∥ PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: To clarify the origin of the anomalous increase of the intensity of the sum frequency generation (SFG) peak corresponding to the stretching of preadsorbed CO on a Pt(111) electrode in H2SO4 solution prior to anodic oxidation CO, potential-dependent SFG measurements were carried out by using eight different visible-light energies (527∼635 nm). Anomalous SFG intensity peaks were observed at all visible light except at 527 nm, and the potential of the SFG peak shifted with the visible-light energy by 1 V/eV, showing that the origin of the anomalous increase of SFG intensity is not due to the potential-dependent geometric structure change of adsorbed CO suggested before but due to a resonance of visible and/or SF light with electronic transition from the Fermi level of Pt(111) to the 5σa antibonding state of adsorbed CO. The present work showed that SFG spectroscopy can probe not only the molecular structure but also the electronic structure of the electrochemical interface, which is difficult to be determined by other methods.

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INTRODUCTION To understand the mechanism and improve the efficiency of processes at solid surfaces both in the gas phase, such as heterogeneous catalytic reactions,1,2 and in the liquid phase, such as electrochemical reactions,3,4 it is essential to have in situ, real time information on the geometric structure of the solid surface, electronic structure of the solid/gas or solid/ liquid interface, and molecular structures of reactant, product, intermediate, and solvent at the interface. While many techniques to obtain such information are available for solid/gas interfaces,5 most of them are not applicable to solid/liquid interfaces because of the presence of liquid, mostly water, which interacts strongly with a probe such as an electron. As results of concentrated efforts in the last 30 years, many useful methods applicable to the solid/liquid interface have been developed.6 For example, geometric structures can be determined by scanning probe microscopy7−9 and surface X-ray scattering,10,11 and molecular structure can be determined by infrared reflection absorption spectroscopy (IRAS), 1 2−1 4 surface-enhanced Raman spectroscopy (SERS),15−17 and X-ray absorption fine structure (XAFS).18,19 Techniques to probe the electronic structure at solid/liquid interfaces in situ are, however, still limited. Second harmonic generation (SHG) spectroscopy is one solution to probe the surface electronic structure; however, the spectroscopic SHG © 2015 American Chemical Society

study is rather limited, and the interpretation of the results is not straightforward.20−23 X-ray absorption near edge structure (XANES) can prove the oxidation state of elements24−26 and electronic states at the electrode/electrolyte solution interface.27 We recently developed an in situ electrochemical X-ray photoelectron spectroscopy (XPS) system combined with a specially designed electrochemical cell with a thin (ca. 20 nm) Si window, which separates the solution from vacuum, so that the electronic structure of the electrochemical interface can be determined in situ.28 The latter two methods are useful but of limited use because the synchrotron X-ray source is required. Thus, techniques, which can be applied to laboratory based experiments, should be developed. The adsorbed CO/Pt single-crystal interface is one of the most studied systems in ultrahigh vacuum (UHV),29−31 gas phase,32−34 and electrochemical35−37 conditions since CO is one of the simplest adsorbed molecules and is an intermediate and/or a poison of important interfacial processes such as water−gas shift reaction38,39 and methanol fuel cells.40,41 When a molecule is chemisorbed on a metal surface, interfacial electronic structure will be significantly modified by the Received: October 15, 2015 Revised: October 22, 2015 Published: October 22, 2015 26056

DOI: 10.1021/acs.jpcc.5b10086 J. Phys. Chem. C 2015, 119, 26056−26063

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION SFG Measurements. A broadband femtosecond laser system employed for SFG measurement is illustrated in Figure 1. A seed pulse (790 nm, 90 fs, 5 nJ/pulse, 82 MHz) from the

hybridization of electronic states of the metal and molecular orbitals, and as a result new energy states are formed at the interface.42,43 In the case of chemisorbed CO on Pt, the adsorbed CO is considered to interact with a Pt surface through s electron donation from the CO 5σ orbital of CO to the vacant 6sp band of Pt and π electron back-donation from the occupied 5d band of Pt into the vacant 2π* of CO.44−46 The interaction between Pt and CO creates CO-derived antibonding states, 2π*a and 5σa, above the Fermi level of Pt and bonding states, 2π*b and 5σb, below the Fermi level.47,48 A large number of experiments in UHV environment49−53 and theoretical calculations42,43,54 have been carried out to clarify these hybridization states. Dose et al. reported that the unoccupied 5σa antibonding state of adsorbed CO at Pt(110) is located at around 2.0 eV above the Fermi level by using inverse photoemission spectroscopy.51 The positions of unoccupied and occupied states of the atop CO band on Pt(111), 2π*a, and 2π*b, respectively, were determined as 4.3 eV above and 0.3 eV below the Fermi level by two-photon photoemission and photoemission measurements52 and photoemission measurement,49 respectively, in UHV environment, but electron spectroscopy cannot be applied to electrochemical interfaces. Although SHG studies for the CO/Pt(111) system in electrolyte solutions were also carried out,55−58 application to probe the electronic structure is limited.59 Bae et al. applied XANES to the CO/Pt system in an acid electrolyte and suggested the transitions from the Pt 2p(3/2) orbital to unoccupied states above the Fermi level were localized predominantly on CO, but the resolution was not high enough to specify the CO states.27 Recently, Somorjai and his colleagues reported that IR/visible sum frequency generation (SFG) spectroscopy can elucidate the electronic structure of the CO/Pt(111) interface in UHV environment by showing that the intensity SFG peak corresponding to CO stretching is enhanced by the resonance of visible light with the electronic transition between the bonding 2π*b and 5σa states at the interface, i.e., IR/visible double resonance (DR)-SFG.53 SFG has already been applied to the CO/Pt interface in an electrochemical environment60−63 but to study molecular structure as another addition of vibrational spectroscopy such as IRAS12−14 and SERS.15−17 If SFG enhancement takes place at the CO/Pt(111) interface in an electrolyte solution, the potential, at which the enhancement takes place, may vary with energy of the visible light since Fermi level and, therefore, the energy difference between the unoccupied antibonding state of the adsorbed CO and highest occupied energy level of electrons in Pt can be tuned by electrode potential. In this work, broadband SFG measurements of the CO adlayer on the Pt(111) electrode surface were carried out in a CO-free 0.5 M H2SO4 solution as a function of potential using visible light of eight different incident energies. The intensity of the SFG peak corresponding to CO stretching vibration increased prior to anodic CO oxidation, and the potential of the SFG peak linearly depended on the incident visible light energy with a slope of 1 V/eV. This result shows that the increase of SFG intensity is due to a surface electronic resonance, in which the energy of visible and/or SF light becomes equal to the energy of interfacial electronic transition from the Fermi level of Pt(111), which is tuned by the electrode potential, to the 5σa antibonding state of adsorbed CO.

Figure 1. Block diagram of the broadband femtosecond laser system.

Ti:sapphire oscillator (Tsunami 3960-L2S, Spectra-Physics), which was pumped by a Nd:YVO4 laser (Millennia-Vs, Spectra Physics), was amplified in a regenerative amplifier (Titan 4812RGA/4823S/C, Quantronix) pumped with a Nd:YLF laser (Darwin 527-40-M, Quantronix), yielding a fundamental pulse centered at 790 nm of 2.5 mJ energy, 120 fs duration, and 1 kHz repetition rate. 40% of fundamental output was used to generate a broadband frequency tunable femtosecond IR probe from 3 to 10 μm (typically, 5 μJ/pulse, 200 cm−1@5 μm) in the optical parametric generation/optical parametric amplification/ difference frequency generation (OPG/OPA/DFG) system (TOPAS 8034, light conversion). A residual of fundamental output was introduced into a second harmonic bandwidth compressor (SHBC) and another OPG/OPA system to generate a narrowband frequency tunable picosecond visible pulse (e.g., 4 ps, 99.95%, Taiyo Nippon Sanso) was bubbled into the electrolyte solution for 10 min at 0 mV, followed by purging with Ar gas for another 40 min to remove CO in solution. For electrochemical SFG measurements, the electrode was pressed firmly against the sapphire window to form a thin electrolyte layer of ca. 1−2 μm between the electrode and the window to avoid the strong IR absorption by water.64 Attenuation of IR light before reaching the electrode surface by passing through ca. 1−2 μm thick water with the angle at 60°, which is calculated using the Lambert−Beer equation of exp(−αL) with the absorption coefficient of water (α) as 400 cm−1 at 4.8 μm,65 is 0.88−0.94. A series of SFG spectra were acquired at the Pt(111) electrode with preadsorbed CO in the 0.5 M H2SO4 solution with an accumulation time of 50 s for each spectrum using visible light of eight different energies, while potential was scanned positively from −150 to 900 mV with a scan rate of 1 mV/s.66,67

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RESULTS Electrochemical Characteristics. Figure 3(a) shows CV of the Pt(111) electrode in a CO-free Ar-saturated 0.5 M

Figure 2. Schematic diagram of the spectroelectrochemical cell.

sapphire plate which was sandwiched by an acid-proof O-ring to hold the electrolyte into the cell and also utilized as the optical window for SFG measurements. The glassware and other parts of the cell were cleaned in a freshly prepared piranha solution (3:1 concentrated H2SO4:H2O2) to remove organic contaminants, followed by thorough rinsing with MilliQ water (TOC < 10 ppb, electric resistivity >18 MΩ·cm) and with the electrolyte solution prior to use. Ag/AgCl (sat. NaCl) and a coiled Pt wire were used as a reference electrode (RE) and a counter electrode (CE), respectively. A commercial Pt(111) single-crystal disk (MaTeck: thickness = 5 mm, ϕ = 10 mm) was used as a working electrode (WE), which was hooked to the end of an airtight movable glass piston via a Pt wire, which was welded onto the unpolished side of the Pt(111) disk. A 0.5 M H2SO4 aqueous solution prepared by H2SO4 (analytical grade, Wako Pure Chemical) and Milli-Q water was used as an electrolyte solution. The electrode potential was controlled by a potentiostat/galvanostat (HABF-501A, Hokuto Denko), and both potential and current were recorded by a data logger (GL900, Graphtec). The Pt(111) disk was annealed in a gas-oxygen flame and slowly cooled in Ar gas flow followed by a quenching in Milli-Q water. It was then transferred to a spectroelectrochemical cell with the surface covered with Milli-Q water and immersed in the electrolyte solution, which had been deaerated in advance by passing Ar gas (>99.99%) through the solution for 30 min while keeping the potential at 0 mV. Cyclic voltammograms (CVs) were obtained in a hanging meniscus configuration first in Ar-saturated 0.5 M H2SO4 solution to confirm the cleanliness

Figure 3. CVs of the Pt(111) electrode in CO-free Ar-saturated 0.5 M H2SO4 solution (a) without preadsorbed CO between −100 and +450 mV with a sweep rate of 20 mV/s and (b) with preadsorbed CO between −150 and +900 mV with a sweep rate of 1 mV/s for the 1st (black line) and the 2nd (red line) cycles.

H2SO4 solution obtained by scanning the potential between −100 and +450 mV with a sweep rate of 20 mV/s. The features of the CV of the Pt(111) electrode without preadsorbed CO such as characteristic flat hydrogen adsorption/desorption waves between the −100 and 100 mV region and the sharp spikes at ca. 250 mV associated with adsorption/desorption of sulfate or bisulfate anions are in good agreement with those previously reported for the clean and well-ordered Pt(111) electrode by other groups.68−70 Figure 3(b) shows the CV of the Pt(111) electrode with preadsorbed CO in the CO-free Ar-saturated 0.5 M H2SO4 solution obtained by sweeping the potential positively from −150 to 900 mV and negatively to −150 mV (first cycle: black) and then positively to 900 mV and negatively to −150 mV (second cycle: red) with a sweep rate of 1 mV/s. In the first positive going scan, the hydrogen desorption peak was completely suppressed by adsorbed CO; the anodic current started to flow when the potential became more positive than 26058

DOI: 10.1021/acs.jpcc.5b10086 J. Phys. Chem. C 2015, 119, 26056−26063

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where χ(2) eff,NR is the nonresonant contribution for the surface nonlinear susceptibility; ω is the IR frequency; and Aq, ωq, Γq, and φ correspond to the amplitude, resonant frequency, damping factor, and phase difference between the resonant and nonresonant terms of the qth vibrational mode, respectively. Figure 5 shows the typical results of potential dependencies of (a) peak wavenumber, (b) full width at half-maximum

ca. 400 mV; and a sharp peak due to the stripping of adsorbed CO was observed at 500 mV.67,71 The current became nearly zero at around 550 mV. The charge for CO oxidation obtained by integrating the area of the CO stripping peak was ca. 460 μC/cm2, which is in good agreement with the previously reported value for the oxidation of the CO monolayer,67 confirming that the CO monolayer was formed on the Pt(111) electrode. In the negative going scan of the first cycle, the current−potential relation was similar to that of the bare Pt(111) electrode, implying that the CO monolayer was fully removed by anodic oxidation and a clean Pt(111) surface was recovered. The recovery of the clean Pt surface was further proved by the CV of the second cycle, which is exactly the same as that of the clean Pt(111) electrode without the trace of the CO oxidation peak. Potential-Dependent SFG Spectra Obtained by Using Visible Light of Various Energies. Figure 4 shows typical

Figure 5. Potential dependencies of (a) peak wavenumber, (b) fwhm, and (c) amplitude of the SFG peak derived from the fitting of the SFG spectra obtained by using visible light of 560 nm (Figure 4(c)).

(fwhm) of the peak, and (c) amplitude, derived from the fitting of the SFG spectra obtained by using visible light of 560 nm (Figure 4(c)). Since each SFG spectrum was obtained by 50 s accumulation during 1 mV/s potential scan, results are on average of 50 mV and are presented at the midpotential of accumulated potential range, e.g., −125 mV for the data accumulated between −150 and −100 mV. Figure 5(a) shows that the peak position shifted to higher wavenumber by ca. 28 cm−1/V, which is a reasonable value for Stark tuning rate,75 as potential became more positive up to ca. 375 mV and then to lower wavenumber. Such a downward shift can be explained by the reduction of the dipole−dipole interaction among CO molecules as a result of the decrease of CO coverage due to anodic oxidation, which started at around 400 mV (Figure 3(b)).76,77 The peak width was almost constant but slightly increased just prior to CO oxidation (Figure 5(b)) as already reported for CO stripping at the Pt(111) electrode in acidic solution by IRAS72,78 and SFG67 measurements, suggesting a constant coverage of the atop CO until the CO oxidation started. Slight broadening of the width when the CO oxidation started supports the disordered distribution of CO molecular interaction upon oxidation of adsorbed CO. In addition, the increase of the SFG amplitude prior to CO oxidation was observed as shown in Figure 4(c) in agreement with the previous reports.62,67 During the positive going scan, the amplitude was almost constant up to 25 mV and then increased by 70% toward 225 mV, started to decrease, and finally disappeared completely at around 525 mV, as a result of

Figure 4. Typical potential-dependent SFG spectra of the Pt(111) electrode with preadsorbed CO in the 0.5 M CO-free Ar-saturated H2SO4 solution obtained by using visible light of (a) 635, (b) 605, (c) 560, and (d) 527 nm. Solid lines are the best fits according to eq 1.

potential-dependent SFG spectra obtained by using visible light of (a) 635, (b) 605, (c) 560, and (d) 527 nm during potential scan between −150∼−100, −50−0, 50−100, 200−250, 300− 350, 350−400, and 500−550 mV. More SFG spectra obtained by using visible light of four other energies (620, 590, 575, and 550 nm) can be found in the Supporting Information. All SFG spectra show a single peak at around 2070 cm−1, corresponding to CO stretching of CO adsorbed on the atop site of the Pt(111) surface.35,67,72 For more detailed analysis, the spectra were fitted to the following equation73,74 ISFG(ω) ∝

(2) χeff,NR

+

∑ q

A⃡ eff, q ω − ωq − i Γq

2

exp(iφ) (1) 26059

DOI: 10.1021/acs.jpcc.5b10086 J. Phys. Chem. C 2015, 119, 26056−26063

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intensities of visible and IR lasers. When visible light of 635 nm was used, the intensity increased immediately after the potential scan was started from −150 mV, reached a maximum at −25 mV, decreased, reached a constant value at 75 mV, started to decrease again at 375 mV, and finally disappeared completely at around 525 mV, as a result of electrochemical oxidation of adsorbed CO. In contrast, when visible light of 550 nm was used, the intensity stayed constant until the potential became as positive as 175 mV, reached a maximum at ca. 225 mV, and decreased gradually to become completely 0 at around 475 mV. When the visible light of 527 nm was used, the intensity stayed constant until the potential became as positive as 375 mV and then decreased without the increase. These results show that the potential dependencies of SFG characteristics of the CO/ Pt(111) electrode are strongly affected by the energy of visible light. This is more clearly seen in the bottom panel of Figure 6(a), which shows a contour map of the SFG amplitude as functions of the potential and wavelength of visible light obtained by using visible light of 635 nm (1.95 eV), 620 nm (2.0 eV), 605 nm (2.05 eV), 590 nm (2.10 eV), 575 nm (2.15 eV), 560 nm (2.21 eV), 550 nm (2.25 eV), and 527 nm (2.35 eV). It is clear that the peak potential became more positive as the wavelength of visible light became shorter, except for the measurement using visible light of 527 nm. Figure 6(b) shows the relation between SFG peak potential with the energy of both incident visible light and generated SF light. A linear dependence between the visible/SF light energy and electrode potential was observed with the slope of 1 V/eV. The reason why the SFG peak was not observed when visible light of 527 nm was used can be explained by this relation. The SFG peak must be observed at 375 mV if visible light of 527 nm was used, but the amount of adsorbed CO started to decrease already at this potential, which is in the preoxidation region of CO and is close to the CO main oxidation region; therefore, the SFG peak was not observed.

electrochemical oxidation of adsorbed CO, as proved by CV (Figure 3(b)). The increase of the SFG amplitude was observed in all the cases except when visible light of 527 nm was used. The top panel of Figure 6(a) shows the potential dependencies of

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DISCUSSION The anomalous increase of the SFG intensity prior to CO oxidation was already reported by two groups. Somorjai and his colleagues showed the increase of SFG in CO-free solution by using visible light of 532 nm and suggested the changes in the orientation and hyperpolarizability of adsorbed CO to be the possible reasons for the increase of SFG intensity.67 Wieckowski and his colleagues reported the increase of SFG intensity in CO-saturated and partially saturated 0.1 M H2SO4 solutions but not in CO-free solution by using visible light of 804 nm and proposed that the SFG intensity increase was caused by the potential-induced phase transition of the CO adlayer from (2 × 2) to (√19 × √19),62 which has been also accounted for by the increase of SHG intensity at CO/Pt(111) electrode prior to the anodic oxidation of adsorbed CO in COsaturated solution.56,77,79,80 Neither the changes in the orientation and hyperpolarizability nor the phase transition of adsorbed CO can explain the present results of the visible light energy-dependent SFG peak potential as these are not affected by visible light energy. Although the molecule hyperpolarizability of CO depends on visible light energy and a change in the hyperpolarizability might cause the increase of SFG intensity,67 the potential, at which the SFG peak is observed, should not be affected by visible light energy. It is also unlikely that the orientation of adsorbed CO is affected by visible light energy. The increase of SFG and SHG intensities due to the potential-induced phase transition of the CO adlayer from (2 ×

Figure 6. (a) Normalized SFG amplitude as a function of potential obtained by using visible light of 635 nm (1.95 eV), 575 nm (2.15 eV), 550 nm (2.25 eV), and 527 nm (2.35 eV) (top panel) and a contour map of the SFG amplitude as functions of the potential and wavelength of visible light obtained by using visible light of 635 nm (1.95 eV), 620 nm (2.0 eV), 605 nm (2.05 eV), 590 nm (2.10 eV), 575 nm (2.15 eV), 560 nm (2.21 eV), 550 nm (2.25 eV), and 527 nm (2.35 eV) (bottom panel). (b) Peak potential as functions of the input visible (bottom axis) and output SFG energy (top axis).

normalized SFG amplitude obtained by using visible light of 635 nm (1.95 eV), 575 nm (2.15 eV), 550 nm (2.25 eV), and 527 nm (2.35 eV). The results obtained by using visible light of 560 nm (2.21 eV) can be found in Figure 5(c). Here the SFG amplitude was normalized by making that before the increase as 1, since the absolute values of SFG amplitude obtained by using visible light of different energies cannot be compared directly as they are dependent on the experimental conditions including 26060

DOI: 10.1021/acs.jpcc.5b10086 J. Phys. Chem. C 2015, 119, 26056−26063

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The Journal of Physical Chemistry C 2) to (√19 × √19) can be possible in fully or partially COsaturated solution but not in CO-free solution where extra CO required for the transformation cannot be provided from the solution, although the surface structure of the CO adlayer in CO-free solution is not well-defined. Furthermore, the phase transition potential should not affect the visible light energy. SFG peaks were observed at potentials more negative than the phase transformation potential (ca. 300 mV vs Ag/AgCl).56,81 Since the anomalous increase in CO-free solution has been observed only for SFG not for IR, Raman, or SHG, one may speculate that there is a SFG specific reason. In this respect, DR-SFG, which was reported by Somorjai and his colleagues for CO/Pt(111) in UHV,53 may be accounted for by the potential-dependent behavior of the SFG intensity, although they did not mention their previous result of anomalous increase of SFG intensity in the electrochemical environment66 in their DR-SFG paper.53 DR-SFG, in which SFG intensity is enhanced not only by the vibrational resonance but also by electronic resonance when the energy of input visible light or output SF light becomes equal to the transition energy of two electronic states at the interface, was first experimentally demonstrated by Raschke et al.82 Somorjai and his colleagues suggested that the electronic SFG enhancement at CO/Pt(111) in UHV is caused by the resonance of visible/SF light with the electronic transition from 2πb* to 5σa. In the present study, SFG measurements were carried out with a visible light of a given energy as a function of potential. If one of the electronic states is varied with potential, the energy difference between the two states is also varied with potential, and therefore, the SFG enhancement takes place at a specific potential where the energy difference between the two states becomes equal to the energy of visible/SF light. As a result, an SFG peak should be observed in the SFG intensity−potential relation as experimentally observed. Taking into account the results of Somorjai and his colleagues for CO/Pt(111) in UHV, we propose that SFG enhancement at the CO/Pt(111) electrode in solution is caused by the resonance of visible/SF light with the electronic transition from the Fermi level of Pt, which is varied linearly with potential, to the 5σa state of adsorbed CO as schematically shown in Figure 7. When the energy of visible light is increased, the Fermi level at which the resonance takes place becomes lower, and as a result the peak potential is expected to become more positive. A linear dependence between the energy of the visible/SF light and the potential of the SFG peak with the slope of 1 V/eV observed experimentally in this study supports

this model. Similar transition from the Pt 2p(3/2) orbital to unoccupied states at the CO/Pt system in an acid electrolyte has been suggested by Bae et al. using XANES27 as mentioned before. One must note that although the SFG enhancement is caused by the resonance of either the input visible photon (1.95−2.35 eV) or the corresponding output SF photon (2.2− 2.6 eV) with the electronic transition between two electronic states at the surface it is not possible to separate these two contributions since the energy difference between the visible and SF light is ca. 0.25 eV and the resolution of the present SFG setup is not high enough to distinguish this small energy difference.

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CONCLUSIONS In conclusion, it is demonstrated that electrochemical SFG spectroscopy can probe not only molecular structure but also the electronic state at the CO/Pt(111) interface in solution. SFG measurements by using eight different visible light energies showed that the SFG peak corresponding to stretching of preadsorbed CO on the Pt(111) electrode in H2SO4 solution increased anomalously prior to anodic oxidation of CO, and the potential of the anomalous peak shifted negatively when the visible light energy decreased with the linear relation by 1 V/ eV, showing that the origin of the anomalous increase of SFG intensity is due to the resonance of visible and/or SF light with electronic transition from the Fermi level of Pt(111) to the 5σa antibonding state of adsorbed CO.

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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.5b10086. Figure of potential-dependent SFG spectra obtained by using visible light of various energies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +81-29-860-4841. E-mail: [email protected]. *Tel.: +81-29-860-4301. Fax: +81-29-851-3362. E-mail: uosaki. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was partially supported by World Premier International Research Center Initiative on Materials Nanoarchitectonics (WPI-MANA) and the Development of Environmental Technology using Nanotechnology from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. H.N. acknowledges the Japan Science and Technology Agency, PRESTO, for financial support.

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REFERENCES

(1) Ertl, G. Reactions at Solid Surfaces, 2nd ed.; John Wiley & Sons: Hoboken, N.J., 2009. (2) Somorjai, G. A.; Li, Y. Introduction to Surface Chemistry and Catalysis, 2nd ed.; John Wiley & Sons: Hoboken, N.J., 2010. (3) Lipkowski, J.; Ross, P. N. Electrocatalysis; Wiley-VCH: New York, N.Y., 1998. (4) Wieckowski, A. Interfacial Electrochemistry: Theory, Experiment, and Applications; Marcel Dekker: New York, N.Y., 1999.

Figure 7. Model of electronic structure of the CO/Pt(111) electrode interface, showing the potential-induced Fermi level shift and electronic coupling of visible energy to the electronic transition between the Fermi level and 5σa state. 26061

DOI: 10.1021/acs.jpcc.5b10086 J. Phys. Chem. C 2015, 119, 26056−26063

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The Journal of Physical Chemistry C (5) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1986. (6) Uosaki, K. In Situ Real-Time Monitoring of Geometric, Electronic, and Molecular Structures at Solid/Liquid Interfaces. Jpn. J. Appl. Phys. 2015, 54, 030102. (7) Uosaki, K.; Koinuma, M. In-Situ and Real Time Monitoring of the InSe Surface by Atomic Force Microscopy with Atomic Resolution During Electrochemical Reactions. J. Electroanal. Chem. 1993, 357, 301−306. (8) Naohara, H.; Ye, S.; Uosaki, K. Electrochemical Layer-by-Layer Growth of Palladium on an Au(111) Electrode Surface: Evidence for Important Role of Adsorbed Pd Complex. J. Phys. Chem. B 1998, 102, 4366−4373. (9) Takakusagi, S.; Kitamura, K.; Uosaki, K. In Situ Real-Time Monitoring of Electrochemical Ag Deposition on a Reconstructed Au(111) Surface Studied by Scanning Tunneling Microscopy. J. Phys. Chem. C 2008, 112, 3073−3077. (10) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaacs, H. S. In Situ XRay-Diffraction and -Reflectivity Studies of the Au(111)/Electrolyte Interface: Reconstruction and Anion Adsorption. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 10321−10338. (11) Kondo, T.; Morita, J.; Hanaoka, K.; Takakusagi, S.; Tamura, K.; Takahasi, M.; Mizuki, J.; Uosaki, K. Structure of Au(111) and Au(100) Single-Crystal Electrode Surfaces at Various Potentials in Sulfuric Acid Solution Determined by in Situ Surface X-Ray Scattering. J. Phys. Chem. C 2007, 111, 13197−13204. (12) Bewick, A.; Kunimatsu, K.; Pons, B. S.; Russell, J. W. Electrochemically Modulated Infrared Spectroscopy (EMIRS): Experimental Details. J. Electroanal. Chem. Interfacial Electrochem. 1984, 160, 47−61. (13) Gale, R. J. Spectroelectrochemistry: Theory and Practice; Plenum Press: New York, N.Y., 1988. (14) Iwasita, T.; Nart, F. C. In Situ Infrared Spectroscopy at Electrochemical Interfaces. Prog. Surf. Sci. 1997, 55, 271−340. (15) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine Adsorbed at a Silver Electrode. Chem. Phys. Lett. 1974, 26, 163−166. (16) Koglin, E.; Schwuger, M. J. In Situ Analysis of Adsorbed Molecules by Microsurface-Enhanced Raman Scattering. Faraday Discuss. 1992, 94, 213−220. (17) Zhang, B.; Li, J.-F.; Zhong, Q.-L.; Ren, B.; Tian, Z.-Q.; Zou, S.Z. Electrochemical and Surfaced-Enhanced Raman Spectroscopic Investigation of CO and SCN− Adsorbed on Aucore−Ptshell Nanoparticles Supported on GC Electrodes. Langmuir 2005, 21, 7449− 7455. (18) Dewald, H. D. Use of EXAFS to Probe Electrode−Solution Interfaces. Electroanalysis 1991, 3, 145−155. (19) McBreen, J.; Scherson, D. A. Spectroscopic Tools for the Analysis of Electrochemical Systems: Proceedings of the International Symposium; The Electrochemical Society: Pennington, N.J., 2002. (20) Guyot-Sionnest, P.; Tadjeddine, A.; Liebsch, A. Electronic Distribution and Nonlinear Optical Response at the Metal-Electrolyte Interface. Phys. Rev. Lett. 1990, 64, 1678−1681. (21) Corn, R. M.; Higgins, D. A. Optical Second Harmonic Generation as a Probe of Surface Chemistry. Chem. Rev. 1994, 94, 107−125. (22) Yang, D.; Bizzotto, D.; Lipkowski, J.; Pettinger, B.; Mirwald, S. Electrochemical and Second Harmonic Generation Studies of 2,2′Bipyridine Adsorption at the Au(111) Electrode Surface. J. Phys. Chem. 1994, 98, 7083−7089. (23) Awatani, T.; Yagi, I.; Noguchi, H.; Uosaki, K. Second Harmonic Generation Study on Electrochemical Deposition of Palladium on a Polycrystalline Gold Electrode. J. Electroanal. Chem. 2002, 524−525, 184−193. (24) Tryk, D. A.; Bae, I. T.; Scherson, D.; Antonio, M. R.; Jordan, G. W.; Huston, E. L. In Situ La, Ce, and Nd L-Edge X-Ray Absorption Fine Structure Study of an Intermetallic Metal Hydride Electrode in an Operating Alkaline Battery. J. Electrochem. Soc. 1995, 142, L76−L78.

(25) Masuda, T.; Fukumitsu, H.; Fugane, K.; Togasaki, H.; Matsumura, D.; Tamura, K.; Nishihata, Y.; Yoshikawa, H.; Kobayashi, K.; Mori, T.; et al. Role of Cerium Oxide in the Enhancement of Activity for the Oxygen Reduction Reaction at Pt− CeOx Nanocomposite Electrocatalyst - An in Situ Electrochemical XRay Absorption Fine Structure Study. J. Phys. Chem. C 2012, 116, 10098−10102. (26) Masuda, T.; Fukumitsu, H.; Takakusagi, S.; Chun, W.-J.; Kondo, T.; Asakura, K.; Uosaki, K. Molecular Catalysts Confined on and Within Molecular Layers Formed on a Si(111) Surface with Direct SiC Bonds. Adv. Mater. 2012, 24, 268−272. (27) Bae, I. T.; Scherson, D. A. In Situ Core-Electron Spectroscopy of Carbon Monoxide Adsorbed on High-Area Platinum in an Acid Electrolyte. J. Phys. Chem. 1996, 100, 19215−19217. (28) Masuda, T.; Yoshikawa, H.; Noguchi, H.; Kawasaki, T.; Kobata, M.; Kobayashi, K.; Uosaki, K. In Situ X-Ray Photoelectron Spectroscopy for Electrochemical Reactions in Ordinary Solvents. Appl. Phys. Lett. 2013, 103, 111605. (29) Morgan, A. E.; Somorjai, G. A. Low Energy Electron Diffraction Studies of Gas Adsorption on the Platinum (100) Single Crystal Surface. Surf. Sci. 1968, 12, 405−425. (30) Ertl, G. Reactions at Well-Defined Surfaces. Surf. Sci. 1994, 299−300, 742−754. (31) Somorjai, G. A.; Li, Y. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: Hoboken, N.J., 2010. (32) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; Jon Wiley & Sons: New York, N.Y., 1996. (33) Jensen, J. A.; Rider, K. B.; Salmeron, M.; Somorjai, G. A. High Pressure Adsorbate Structures Studied by Scanning Tunneling Microscopy: CO on Pt(111) in Equilibrium with the Gas Phase. Phys. Rev. Lett. 1998, 80, 1228−1231. (34) Starr, D. E.; Liu, Z.; Hävecker, M.; Knop-Gericke, A.; Bluhm, H. Investigation of Solid/Vapor Interfaces Using Ambient Pressure X-Ray Photoelectron Spectroscopy. Chem. Soc. Rev. 2013, 42, 5833−5857. (35) Leung, L.-W. H.; Wieckowski, A.; Weaver, M. J. In Situ Infrared Spectroscopy of Well-Defined Single-Crystal Electrodes: Adsorption and Electrooxidation of CO on Pt(111). J. Phys. Chem. 1988, 92, 6985−6990. (36) Marković, N. M.; Ross, P. N., Jr. Surface Science Studies of Model Fuel Cell Electrocatalysts. Surf. Sci. Rep. 2002, 45, 117−229. (37) Feliu, J. M.; Herrero, E. Surface Electrochemistry and Reactivity. Contrib. Sci. 2010, 6, 161−172. (38) Phatak, A. A.; Koryabkina, N.; Rai, S.; Ratts, J. L.; Ruettinger, W.; Farrauto, R. J.; Blau, G. E.; Delgass, W. N.; Ribeiro, F. H. Kinetics of the Water−Gas Shift Reaction on Pt Catalysts Supported on Alumina and Ceria. Catal. Today 2007, 123, 224−234. (39) Grabow, L. C.; Gokhale, A. A.; Evans, S. T.; Dumesic, J. A.; Mavrikakis, M. Mechanism of the Water Gas Shift Reaction on Pt: First Principles, Experiments, and Microkinetic Modeling. J. Phys. Chem. C 2008, 112, 4608−4617. (40) Chang, S.-C.; Weaver, M. J. In Situ Infrared Spectroscopy of CO Adsorbed at Ordered Pt(100)-Aqueous Interfaces: Double-Layer Effects upon the Adsorbate Binding Geometry. J. Phys. Chem. 1990, 94, 5095−5102. (41) Iwasita, T. Fuel Cells: Spectroscopic Studies in the Electrocatalysis of Alcohol Oxidation. J. Braz. Chem. Soc. 2002, 13, 401−409. (42) Bagus, P. S.; Nelin, C. J.; Bauschlicher, C. W. Bonding of CO to Metal Surfaces: A New Interpretation. Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 28, 5423−5438. (43) Föhlisch, A.; Nyberg, M.; Hasselström, J.; Karis, O.; Pettersson, L. G. M.; Nilsson, A. How Carbon Monoxide Adsorbs in Different Sites. Phys. Rev. Lett. 2000, 85, 3309−3312. (44) Blyholder, G. Molecular Orbital View of Chemisorbed Carbon Monoxide. J. Phys. Chem. 1964, 68, 2772−2777. (45) Sung, S.-S.; Hoffmann, R. How Carbon Monoxide Bonds to Metal Surfaces. J. Am. Chem. Soc. 1985, 107, 578−584. (46) Ishi, S.; Ohno, Y.; Viswanathan, B. An Overview on the Electronic and Vibrational Properties of Adsorbed CO. Surf. Sci. 1985, 161, 349−372. 26062

DOI: 10.1021/acs.jpcc.5b10086 J. Phys. Chem. C 2015, 119, 26056−26063

Article

The Journal of Physical Chemistry C (47) Avouris, P. On the Nature and Spectroscopy of the Affinity Levels of Adsorbates on Metals. Phys. Scr. 1987, 35, 47. (48) Matsumoto, Y.; Watanabe, K. Coherent Vibrations of Adsorbates Induced by Femtosecond Laser Excitation. Chem. Rev. 2006, 106, 4234−4260. (49) Shirley, D. A.; Stöhr, J.; Wehner, P. S.; Williams, R. S.; Apai, G. Photoemission from Noble Metals and Adsorbates Using Synchrotron Radiation. Phys. Scr. 1977, 16, 398. (50) Dose, V.; Rogozik, J.; Bradshaw, A. M.; Prince, K. C. Inverse Photoemission from CO Co-Adsorbed with K on Pt(111). Surf. Sci. 1987, 179, 90−100. (51) Rangelov, G.; Memmel, N.; Bertel, E.; Dose, V. Inverse Photoemission Study of Carbon Monoxide Bonding to Transition Metals. Surf. Sci. 1991, 251−252, 965−970. (52) Anazawa, T.; Kinoshita, I.; Matsumoto, Y. Two-Photon Photoemission Study of CO/Pt(111). J. Electron Spectrosc. Relat. Phenom. 1998, 88−91, 585−590. (53) Chou, K. C.; Westerberg, S.; Shen, Y. R.; Ross, P. N.; Somorjai, G. A. Probing the Charge-Transfer State of CO on Pt(111) by TwoDimensional Infrared-Visible Sum Frequency Generation Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 153413. (54) Grönbeck, H.; Andreoni, W. Theoretical Study of (CO)n Chemisorption on Pt and Pt3: Structural, Electronic and Vibrational Properties. Chem. Phys. Lett. 1997, 269, 385−390. (55) Lynch, M. L.; Barner, B. J.; Corn, R. M. In Situ Second Harmonic Generation Studies of Chemisorption at Well-Ordered Pt(111) Electrodes in Perchloric Acid Solutions. J. Electroanal. Chem. Interfacial Electrochem. 1991, 300, 447−465. (56) Akemann, W.; Friedrich, K. A.; Linke, U.; Stimming, U. The Catalytic Oxidation of Carbon Monoxide at the Platinum/Electrolyte Interface Investigated by Optical Second Harmonic Generation (SHG): Comparison of Pt(111) and Pt(997) Electrode Surfaces. Surf. Sci. 1998, 402−404, 571−575. (57) Pozniak, B.; Mo, Y.; Scherson, D. A. The Electrochemical Oxidation of Carbon Monoxide Adsorbed on Pt(111) in Aqueous Electrolytes as Monitored by in Situ Potential Step-Second Harmonic Generation. Faraday Discuss. 2002, 121, 313−322. (58) Fromondi, I.; Scherson, D. Surface Dynamics at Well-Defined Single Crystal Microfacetted Pt(111) Electrodes: In Situ Optical Studies. Faraday Discuss. 2008, 140, 59−68. (59) Yagi, I.; Nakabayashi, S.; Uosaki, K. Excitation Wavelength Dependent Three-Wave Mixing at a CO-Covered Platinum Electrode. J. Phys. Chem. B 1997, 101, 7414−7421. (60) Dederichs, F.; Friedrich, K. A.; Daum, W. Sum-Frequency Vibrational Spectroscopy of CO Adsorption on Pt(111) and Pt(110) Electrode Surfaces in Perchloric Acid Solution: Effects of Thin-Layer Electrolytes in Spectroelectrochemistry. J. Phys. Chem. B 2000, 104, 6626−6632. (61) Noguchi, H.; Okada, T.; Uosaki, K. Photoinduced Surface Dynamics of CO Adsorbed on a Platinum Electrode. J. Phys. Chem. B 2006, 110, 15055−15058. (62) Lagutchev, A.; Lu, G. Q.; Takeshita, T.; Dlott, D. D.; Wieckowski, A. Vibrational Sum Frequency Generation Studies of the (2 × 2)→(√19×√19) Phase Transition of CO on Pt(111) Electrodes. J. Chem. Phys. 2006, 125, 154705. (63) Xiong, W.; Laaser, J. E.; Mehlenbacher, R. D.; Zanni, M. T. Adding a Dimension to the Infrared Spectra of Interfaces Using Heterodyne Detected 2D Sum-Frequency Generation (HD 2D SFG) Spectroscopy. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 20902−20907. (64) Seki, H.; Kunimatsu, K.; Golden, W. G. A Thin-Layer Electrochemical Cell for Infrared Spectroscopic Measurements of the Electrode/Electrolyte Interface. Appl. Spectrosc. 1985, 39, 437−443. (65) Wieliczka, D. M.; Weng, S.; Querry, M. R. Wedge Shaped Cell for Highly Absorbent Liquids: Infrared Optical Constants of Water. Appl. Opt. 1989, 28, 1714−1719. (66) Lu, G. Q.; Lagutchev, A.; Dlott, D. D.; Wieckowski, A. Quantitative Vibrational Sum-Frequency Generation Spectroscopy of Thin Layer Electrochemistry: CO on a Pt Electrode. Surf. Sci. 2005, 585, 3−16.

(67) Chou, K. C.; Markovic, N. M.; Kim, J.; Ross, P. N.; Somorjai, G. A. An in Situ Time-Dependent Study of CO Oxidation on Pt(111) in Aqueous Solution by Voltammetry and Sum Frequency Generation. J. Phys. Chem. B 2003, 107, 1840−1844. (68) Clavilier, J. The Role of Anion on the Electrochemical Behaviour of a {111} Platinum Surface; an Unusual Splitting of the Voltammogram in the Hydrogen Region. J. Electroanal. Chem. Interfacial Electrochem. 1979, 107, 211−216. (69) Scherson, D. A.; Kolb, D. M. Voltammetric Curves for Au(111) in Acid Media: A Comparison with Pt(111) Surfaces. J. Electroanal. Chem. Interfacial Electrochem. 1984, 176, 353−357. (70) Jaaf-Golze, K. A.; Kolb, D. M.; Scherson, D. On the Voltammetry Curves of Pt (111) in Aqueous Solutions. J. Electroanal. Chem. 1986, 200, 353−362. (71) Baldelli, S.; Markovic, N.; Ross, P.; Shen, Y.-R.; Somorjai, G. Sum Frequency Generation of CO on (111) and Polycrystalline Platinum Electrode Surfaces: Evidence for SFG Invisible Surface CO. J. Phys. Chem. B 1999, 103, 8920−8925. (72) Kitamura, F.; Takeda, M.; Takahashi, M.; Ito, M. CO Adsorption on Pt(111) and Pt(100) Single Crystal Surfaces in Aqueous Solutions Studied by Infrared Reflection-Absorption Spectroscopy. Chem. Phys. Lett. 1987, 142, 318−322. (73) Shen, Y. R. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation. Nature 1989, 337, 519−525. (74) Ito, M.; Noguchi, H.; Ikeda, K.; Uosaki, K. Substrate Dependent Structure of Adsorbed Aryl Isocyanides Studied by Sum Frequency Generation (SFG) Spectroscopy. Phys. Chem. Chem. Phys. 2010, 12, 3156−3163. (75) Lambert, D. K. Stark Effect of Adsorbate Vibrations. Solid State Commun. 1984, 51, 297−300. (76) Persson, B. N. J.; Ryberg, R. Vibrational Interaction between Molecules Adsorbed on a Metal Surface: The Dipole-Dipole Interaction. Phys. Rev. B: Condens. Matter Mater. Phys. 1981, 24, 6954−6970. (77) Akemann, W.; Friedrich, K. A.; Stimming, U. PotentialDependence of CO Adlayer Structures on Pt(111) Electrodes in Acid Solution: Evidence for a Site Selective Charge Transfer. J. Chem. Phys. 2000, 113, 6864−6874. (78) Furuya, N.; Motto, S.; Kunimatsu, K. Vibrational Spectroscopy on Platinum Single-Crystal Electrodes: Part I. In-Situ Infrared Spectroscopic Studies of the Adsorption and Oxidation of CO on Pt(111) in Sulphuric Acid. J. Electroanal. Chem. Interfacial Electrochem. 1988, 239, 347−360. (79) Pozniak, B.; Mo, Y.; Stefan, I. C.; Mantey, K.; Hartmann, M.; Scherson, D. A. In Situ Time-Resolved Second-Harmonic Generation from Pt(111) Microfacetted Single-Crystal Platinum Microspheres. J. Phys. Chem. B 2001, 105, 7874−7877. (80) Fromondi, I.; Zhu, H.; Feng, Z.; Scherson, D. Dynamics of Oxidation of Well-Defined Adsorbed CO Phases on Pt(111) in Aqueous Acidic Electrolytes: Simultaneous in Situ Second Harmonic Generation and Differential Reflectance Spectroscopy. J. Phys. Chem. C 2014, 118, 27901−27910. (81) Rodes, A.; Gómez, R.; Feliu, J. M.; Weaver, M. J. Sensitivity of Compressed Carbon Monoxide Adlayers on Platinum(111) Electrodes to Long-Range Substrate Structure: Influence of Monoatomic Steps. Langmuir 2000, 16, 811−816. (82) Raschke, M. B.; Hayashi, M.; Lin, S. H.; Shen, Y. R. DoublyResonant Sum-Frequency Generation Spectroscopy for Surface Studies. Chem. Phys. Lett. 2002, 359, 367−372.

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