Insights into the Electrooxidation Mechanism of Formic Acid on Pt

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Insights into the Electrooxidation Mechanism of Formic Acid on Pt Layers on Au Examined by Electrochemical SERS Hwakyeung Jeong and Jongwon Kim* Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 28644, Korea S Supporting Information *

ABSTRACT: The electrooxidation of formic acid (FA) on Pt has received great attention because of its fundamental significance as a model reaction and its technical importance in fuel cells. Pt layers modified on Au surfaces were recently reported to exhibit enhanced electrocatalytic activity for FA oxidation; however, the mechanistic details have not been clearly elucidated. In this work, the mechanism of FA electrooxidation on Pt layers on Au surfaces was investigated via in situ electrochemical surface-enhanced Raman scattering (SERS). SERS-active DAR@Pt(n) substrates were prepared using the self-terminating electrodeposition of Pt on dendritic Au rod (DAR) surfaces, wherein the amount and coverage of Pt were precisely controlled by applying a different number of potential steps (n) during the electrodeposition process. The electrocatalytic activity of FA was highly dependent on the Pt coverage and thickness on DAR@Pt(n), which was investigated by electrochemical SERS. The amount of CO produced by the dehydration of FA, the potential-dependent SERS intensity variation, and the Stark slopes were examined on different DAR@ Pt(n) surfaces. DAR@Pt(1) surfaces with island-type Pt layers on Au exhibited typical electrooxidation behavior that has been proposed to proceed through direct reaction pathways; however, adsorbed CO produced by dehydration was observed, indicating that the indirect electrooxidation of FA operates even on this surface. As the Pt coverage on DAR@Pt(n) increased, a greater amount of CO was produced by FA dehydration, and the adsorbed CO persisted longer in the early stage of FA electrooxidation. The direct electrooxidation of FA was mostly prohibited by the adsorbed CO initially produced by the dehydration of FA on DAR@Pt(15) with monolayer-level Pt layers. The present work provides insight into the mechanistic interpretation of FA electrooxidation on Pt−Au systems.

1. INTRODUCTION The electrochemical oxidation of formic acid (FA) on Pt surfaces has received great attention because of its prospective utility in low-temperature fuel cells and because it is a model electrocatalytic reaction of small organic molecules.1−3 Various Pt nanoparticles were prepared for application as electrocatalysts in FA electrooxidation, and the dependence of catalytic activity on their size and shape was examined.4−7 Two parallel mechanisms are widely suggested for the electrooxidation of FA: direct and indirect pathways.1−3,8,9 In the direct pathway, FA is electrooxidized to CO2 through reactive intermediates (eq 1).

It was recently reported that Pt layers decorated on Au surfaces exhibited enhanced electrocatalytic activity for FA electrooxidation compared with pure Pt surfaces.13−18 Ptdecorated Au nanoparticles13 and dendrimer-encapsulated Au@Pt nanoparticles16 were shown to exhibit better electrocatalytic activity for FA oxidation than unmodified Pt catalyst systems. Ding and co-workers modified nanoporous Au surfaces with ultrathin Pt layers and examined the variation of the catalytic activity of the Pt−Au systems for FA oxidation depending on the amount of Pt layers.15 Lee and co-workers investigated the atomic-level utilization of electrochemically deposited Pt on octahedral Au nanoparticles for the electrooxidation of FA.18 These previous works demonstrate that the amount and coverage of Pt layers modified on Au surfaces affect the electrooxidation behavior of FA, which is assumed to originate from the variation in the reaction mechanism on different Pt−Au catalysts. Despite the recent investigations on the electrooxidation of FA on Au−Pt systems, the detailed

HCOOH → HCOOads + H+ + e− → CO2 + 2H+ + 2e− (1)

The indirect pathway involves adsorbed CO as a poisoning reaction intermediate (eq 2). HCOOH → COads + H 2O → CO2 + 2H+ + 2e−

(2)

The electrooxidation of FA on Pt surfaces proceeds via the indirect reaction pathway, whereas direct pathways occur on Pd surfaces.10−12 © XXXX American Chemical Society

Received: August 25, 2016 Revised: October 6, 2016

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evaporation (Au/Si, KMAC, Korea) were used as substrates for electrodeposition. The Au/Si substrate was confined in a Viton O-ring with an inner diameter of 2.9 mm and was used as a working electrode after it was cleaned for 1 min in a piranha solution. Bare Pt surfaces were prepared from flat massive Pt electrodes, which were mechanically polished with alumina powder from a large particle size down to the smallest size (ca. 0.05 μm). Scanning electron microscopy (SEM) analysis was performed using an ULTRA PLUS field emission microscope (Carl Zeiss). X-ray photoelectron spectroscopy (XPS) was performed using an Ulvac-PHI (PHI Quantera-II). SERS spectra were obtained using a homemade Ramboss microRaman spectrometer equipped with an integral microscope (Olympus BX 51). A 1200 grooves/mm grating dispersed the radiation onto a thermoelectrically cooled CCD (Andor Technology). The excitation source was 632.8 nm radiation from an air-cooled He/Ne laser with radiation power of 10 mW. The spectral resolution was estimated as 3 cm−1. 2.2. Formation of DAR@Pt(n) Substrates. The DAR surfaces were fabricated using electrodeposition from solutions containing 36 mM Au(I) ions complexed with sulfite anions, as reported in our previous work.12,30,31 A constant deposition potential of −0.83 V with a total deposition charge of 0.03 C (charge density of 0.45 C cm−2, deposition time of ∼100 s) was applied for the formation of well-defined DAR structures (a typical SEM image of DAR is shown in Figure 1, inset).

mechanistic aspect of FA electrooxidation has not been clearly elucidated. Vibrational spectroscopic techniques coupled with electrochemical measurements are useful to address the mechanistic aspect of electrochemical reactions. In particular, electrochemical surface-enhanced Raman spectroscopy (SERS) has been effectively utilized to investigate electrochemical reactions.19−23 Two prerequisites should be met for the mechanistic investigation of Pt−Au catalytic systems for FA electrooxidation using electrochemical SERS techniques. First, modification protocols are required to precisely control Pt layers on Au surfaces. Second, the Pt−Au catalytic electrodes should be highly SERS-active and stable in electrochemical environments. Conventional flat-type Pt−Au electrode surfaces cannot be used for electrochemical SERS investigations because SERSactive substrates require roughened or nanostructured surface morphology. Underpotential deposition methods followed by galvanic replacement reactions or electrochemical deposition methods have been traditionally used for the modification of Au surfaces with Pt layers.24,25 However, precise control of the Pt layer thickness and coverage is limited in these methods. Moffat and co-workers recently developed a self-terminating electrodeposition process for the precisely controlled growth of Pt layers on Au surfaces.26,27 This method was applied to investigate the electrocatalytic activity of electrochemical reactions as a function of the Pt layer thickness and coverage on Au surfaces.28,29 However, the Au substrates used in these works were flat Au surfaces, which are not SERS active. We have reported that dendritic Au rod (DAR) structures, prepared by a simple one-step electrodeposition method, are highly SERS active and stable in electrochemical environments.30,31 We also demonstrated that DAR can be used as SERS-active cores in the borrowed SERS systems to prepare SERS-active Pt or Pd surfaces in electrochemical investigations.12 In the present work, we applied the self-terminating electrodeposition process of Pt onto DAR structures to prepare SERS-active Au@Pt substrates. The resulting Pt-modified Au surfaces serve as efficient electrochemical SERS substrates to examine the electrooxidation of FA. The coverage and thickness of the Pt layers on the DAR surfaces were precisely controlled, and their effects on the electrocatalytic behavior of FA electrooxidation were examined. In situ electrochemical SERS measurements were performed during the electrooxidation of FA, and the dependence of the electrocatalytic behavior of FA electrooxidation on the coverage and thickness of Pt layers on DAR surfaces was investigated from a mechanistic point of view.

2. EXPERIMENTAL SECTION 2.1. Reagents and Instruments. All solutions were prepared using purified water (Milli-Q, 18.2 MΩ·cm). A noncyanide, sulfite-based commercialized electrolyte (Techni Gold 25ES) was purchased from Technic Inc. (Cranston, RI). K2PtCl4 was purchased from Alfa Aesar, and all other chemicals were purchased from Aldrich. A CHI 660C (CH Instruments) was used for all electrochemical measurements. Three-electrode systems housed in a homemade Teflon cell were used for electrodeposition (Supporting Information for the specific cell geometry). Pt wires and Ag/AgCl electrodes were used as the counter and reference electrodes, respectively. All potentials were reported relative to the Ag/AgCl reference electrode (3 M NaCl). Au films deposited on silicon wafers by thermal

Figure 1. (a) CVs of the DAR@Pt(n) surfaces in 0.1 M HClO4. Scan rate = 50 mV/s. Inset: SEM image of DAR nanostructures (scale bar = 200 nm). (b) XPS spectra of the Au 4f and Pt 4f region of the DAR@ Pt(n) substrates.

Ultrathin Pt layers were formed on the DAR surfaces using selfterminating electrochemical deposition techniques that were recently developed by Moffat and co-workers.26−28 The electrodeposition of Pt layers was performed from solutions containing 3 mM K2PtCl4 and 0.5 M NaCl (pH = 4) by stepping the potential between −0.75 V for 10 s and 0.4 V for 30 s. The potential stepping process was repeated for the desired number of times (n) to control the amount of Pt B

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The Journal of Physical Chemistry C deposited on the DAR surfaces, which was denoted as DAR@ Pt(n). After the electrodeposition of Pt, the DAR@Pt(n) substrates were subjected to voltammetric cycles in solutions containing 0.1 M FA and 0.1 M H2SO4 until stable voltammetric behavior toward FA electrooxidation was achieved.28

3. RESULTS AND DISCUSSION 3.1. Characterization of the DAR@Pt(n) Substrates. Figure 1a shows the cyclic voltammograms (CVs) of DAR and DAR@Pt(n) electrodes in 0.1 M HClO4. The DAR surfaces exhibited redox waves corresponding to Au oxide formation/ dissolution, whereas the DAR@Pt(n) surfaces exhibited redox waves corresponding to hydrogen adsorption/desorption and Pt oxide formation/dissolution. As the number of potential stepping processes (n) in the formation of DAR@Pt(n) increased, the redox peaks for Pt oxide and hydrogen adsorption increased and the Au oxide peaks decreased. The electrochemical surface areas (ESAs) were estimated based on the charge consumed during Au oxide dissolution (Au surfaces) and hydrogen adsorption (Pt surfaces),32 and the ESA ratios between Au and Pt were plotted as a function of n (Figure S1). The surface coverage of Pt on DAR@Pt(1) of approximately 30% indicated that submonolayers of Pt were deposited. The Pt coverage gradually increased with n and reached ∼100% on DAR@Pt(15) surfaces. The shape of the hydrogen adsorption peaks of the DAR@Pt(15) surfaces was well-defined, as found on bulk Pt surfaces (Figure S2). Thus, the Pt layers on DAR@ Pt(15) can be considered as a monolayer level on the Au surfaces. DAR@Pt(30) exhibited similar voltammetric features as DAR@Pt(15), with slightly increased ESAs. XPS measurements were performed to estimate the amount of Pt deposited on DAR@Pt(n), as shown in Figure 1b. As n increased, the Pt 4f peaks increased and the Au 4f peaks decreased. In particular, the Pt 4f peak intensity linearly increased with n, indicating that the amount of electrochemically deposited Pt is proportional to the number of potential stepping processes. The binding energies of the Pt 4f peaks on DAR@Pt(n ≥ 5) of 71.4 eV corresponded to that of metallic Pt, and a slightly smaller binding energy (71.1 eV) was obtained on DAR@Pt(1). This result indicated that the electronic structure of Pt on DAR@Pt(1) was slightly modified compared to the other DAR@Pt(n ≥ 5) substrates.18 We confirmed that the deposition of Pt on DAR substrates did not result in significant morphological changes by comparing the SEM images of DAR@Pt(n) (Figure S3) and DAR (Figure 1, inset). We next examined the SERS properties of DAR@Pt(n) substrates using CO as a probing molecule. Figure 2a shows SERS spectra of CO adsorbed on DAR@Pt(n) surfaces in COsaturated solutions. The low wavenumber region corresponds to M−CO bonds, and the high wavenumber region corresponds to CO vibrations adsorbed on the surfaces. The detailed assignment of the five vibrational bands is presented in Table S1 based on previous results in the literature.12,24,33,34 On the DAR surfaces, no band was observed in the low wavenumber region (Figure S4); thus, bands 1 and 2 observed on the DAR@Pt(n) surfaces correspond to Pt−CO stretching modes. The characteristic CO band on Au surfaces (band 5) was observed on DAR@Pt(1) surfaces, and the intensity of band 5 decreased on DAR@Pt(5) and DAR@Pt(10). This result indicates that the exposed Au surface area decreased with the successive deposition of Pt on the DAR surface. Band 5

Figure 2. (a) SERS spectra of CO adsorbed on DAR@Pt(n) surfaces in CO-saturated 0.1 M H2SO4 solution at −0.1 V. (b) Raman shift of band 2 (Pt−CO) and band 4 (CO) for CO adsorbed on DAR@ Pt(n) surfaces.

disappeared on DAR@Pt(15), indicating that the Au surfaces were completely covered by Pt layers, which is in agreement with the electrochemical results shown in Figure 1a. The SERS spectrum of DAR@Pt(30) surfaces was similar to that of DAR@Pt(15); however, the SERS intensity of DAR@Pt(30) decreased because of the diminished contribution of long-range SERS enhancement by Au cores.12,35,36 Figure 2b shows the variation in the Raman shift of bands 2 and 4 as a function of n for the DAR@Pt(n) surfaces. The vibrational frequencies of both bands were blue-shifted with increasing n, which was reported in a previous investigation.24 As the Pt coverage on DAR@Pt(n) increased, the interaction between the Pt layers and adsorbed CO became stronger, resulting in the blue-shift of the Raman band 2. For band 4, the blue-shift can be ascribed to the increase in the dynamic dipole−dipole coupling effects among the adsorbed CO due to their increased surface coverage at greater Pt domain sizes.37 On the basis of the electrochemical, XPS, and SERS results shown above, we suggest a schematic model of DAR@Pt(n) in terms of Pt modification modes on DAR surfaces in Scheme 1. In the case of DAR@Pt(1), island-type Pt layers cover the DAR surfaces. The amount of Pt increases on DAR@Pt(5) and DAR@Pt(10), and the size and coverage of the Pt domains increase. However, the Au surfaces are not completely covered by Pt layers at this stage. Fully covered Pt layers on the DAR surfaces are achieved on DAR@Pt(15), as confirmed by the C

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Pt(1) surfaces. However, several questions may be raised regarding the detailed electrooxidation mechanism of FA. Are adsorbed CO molecules not produced by dehydration of FA on DAR@Pt(1) surfaces? If any adsorbed CO molecules are produced on DAR@Pt(1) or DAR@Pt(5) surfaces, how much amount of CO are foamed on each surface? To address these issues regarding electrooxidation mechanism of FA on DAR@ Pt(n) surfaces, we performed electrochemical SERS investigations. 3.3. Electrochemical SERS of FAO at DAR@Pt(n) Substrates. We first examined the formation of adsorbed CO by dehydration of FA (HCOOH → COads + H2O) before the electrochemical oxidation of FA. The solid lines in Figure 4a show the SERS spectra of CO adsorbed on different DAR@ Pt(n) surfaces at open circuit potential obtained from FA-

electrochemical and SERS measurements. The amount of Pt layers further increases on DAR@Pt(30), and the properties of Pt become close to bulk Pt. On the basis of this model, we examined the electrochemical FA oxidation activity on different DAR@Pt(n) in conjunction with in situ electrochemical SERS measurements. 3.2. FA Electrooxidation on DAR@Pt(n). Figure 3 shows the forward linear scan voltammograms (LSVs) observed on

Figure 3. LSVs obtained in 0.1 M FA + 0.1 M H2SO4 on DAR@Pt(n) substrates. Scan rate = 10 mV/s.

DAR@Pt(n) surfaces in solutions containing 0.1 M FA and H2SO4. One oxidation peak (I) was observed at 0.3 V on the DAR@Pt(1) surfaces. On the DAR@Pt(5) surfaces, the current density of peak I decreased, and an additional oxidation peak (II) was observed at 0.6 V. The current density of peak I significantly decreased, and peak II became dominant on the DAR@Pt(10) surfaces. DAR@Pt(15) and DAR@Pt(30) exhibited similar electrooxidation waves predominantly through peak II. These electrooxidation behavior of FA is analogous to that observed on bulk Pt surfaces (Figure S5). It is widely accepted that the electrooxidation of FA on bulk Pt surfaces follows an indirect reaction pathway (eq 2). CO molecules produced by the dehydration of FA are adsorbed on Pt surfaces, which are further oxidized to CO2 (peak II). Therefore, it can be assumed that the DAR@Pt(15) and DAR@Pt(30) surfaces follow an indirect pathway during FA electrooxidation. On the other hand, the DAR@Pt(1) surfaces exhibit different FA electrooxidation behavior from that observed on DAR@ Pt(15) and DAR@Pt(30). Electrocatalytic FA oxidation behavior similar to that found on the DAR@Pt(1) surface has been reported for other Pt-modified Au surfaces when the Pt loading is at submonolayer levels.13,15,16,18,28 In these previous investigations, the enhanced electrocatalytic activity was interpreted based on the direct reaction pathway of FA (eq 1). In other words, FA is directly oxidized to CO2 through reactive intermediates by dehydrogenation. These mechanistic interpretations are reasonable and could be applied to DAR@

Figure 4. (a) SERS spectra of CO adsorbed on DAR@Pt(n) surfaces in 0.1 M FA + 0.1 M H2SO4 solution at open circuit potential (solid line) and CO-saturated 0.1 M H2SO4 solution at −0.1 V (dashed line; shown in Figure 2a). (b) Integrated SERS intensity ratio of the Pt− CO (atop) band for CO adsorbed on DAR@Pt(n) surfaces. D

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Figure 5. Electrochemical SERS spectra recorded on DAR@Pt(n) surfaces in 0.1 M FA + 0.1 M H2SO4 as a function of electrode potential (positive scan).

the electrooxidation of FA on different DAR@Pt(n) surfaces. As the electrode potential moved into positive regions, the SERS intensities of band 1 and 2 decrease. The trend and rate of intensity decrease were different on different DAR@Pt(n) surfaces, which is further analyzed below. Figure 6 shows the potential-dependent SERS intensity of band 2 (Pt−COads, atop) during the FA electrooxidation

containing solutions. Pt−CO bands were observed for all DAR@Pt(n) surfaces, indicating that the dehydration of FA occurs on all of these surfaces, although the amount of CO produced is different. Pt−COads bands were observed even on the DAR@Pt(1) surfaces, which indicates that the indirect pathway of FA electrooxidation occurs on these surfaces. To obtain quantitative information about the amount of adsorbed CO on different DAR@Pt(n) surfaces, the SERS activity of the respective DAR@Pt(n) substrates should be considered. We referred to the SERS spectra of each DAR@Pt(n) surface obtained in CO-saturated solution (Figure 2a) to estimate their intrinsic SERS activity (shown as dotted lines in Figure 4a for comparison). The SERS intensity of the Pt−COads (atop) band measured in FA-containing solution was divided by that measured in CO-saturated solution for each DAR@Pt(n) surface. Figure 4b provides quantitative information about the COads produced by FA dehydration at each DAR@Pt(n) surface. On the DAR@Pt(1) surface, the SERS intensity of COads produced by FA dehydration is 28% of that obtained in CO-saturated solution. The intensity ratio gradually increases with n to reach 82% on DAR@Pt(30) surfaces. These results indicate that the dehydration of FA becomes more facile as the Pt coverage on the DAR surfaces increases. The Raman shifts of band 2 from the COads produced by FA dehydration are greater than those observed from CO-saturated solution on the respective DAR@ Pt(n) surface. This result confirms that the amount of COads produced by FA dehydration is smaller than that obtained from CO-saturated solution because the Raman frequency is redshifted as the amount of COads increases.37 We next performed in situ SERS measurements during the electrooxidation of FA on DAR@Pt(n) surfaces, and the potential-dependent SERS spectra are shown in Figure 5. Bands 1 and 2 correspond to Pt−COads produced by FA dehydration, and band 6 corresponds to the surface oxide of Pt. The differences in SERS intensities between DAR@Pt(n) surfaces originate from the differences in intrinsic SERS activity and the amount of CO produced by FA dehydration between DAR@ Pt(n) surfaces; thus, the direct comparison is somewhat limited. However, the SERS intensity variation as a function of electrode potential provides mechanistic information regarding

Figure 6. Potential dependence of the integrated SERS intensity for the Pt−CO (atop) bands observed in Figure 5 on (a) DAR@Pt(1, 5, 10) and (b) DAR@Pt(15, 30) substrates.

measured from Figure 5. On DAR@Pt(1) surfaces, the SERS intensity uniformly decreased from −0.2 V and disappeared at 0.4 V (Figure 6a). In contrast, the intensity of band 2 decreased gradually at first and then steeply at approximately 0.4 V on the DAR@Pt(10) surfaces. The DAR@Pt(5) surfaces exhibited intensity variation behavior between that of the DAR@Pt(1) and DAR@Pt(10) surfaces. On DAR@Pt(15) surfaces, the SERS intensity remained nearly constant up to 0.2 V and then E

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The Journal of Physical Chemistry C steeply decreased (Figure 6b). A similar trend was observed on DAR@Pt(30), with reduced SERS intensity due to the smaller intrinsic SERS activity. These results indicate that the COads produced by FA dehydration persists longer in the early stage of FA electrooxidation as the Pt coverage on DAR increases. We also examined the variation of the SERS intensity of band 1 (Pt−COads, bridge) during FA electrooxidation on the DAR@ Pt(n) surfaces (Figure S6). The overall trends of the SERS intensity of band 1 were similar to those of band 2, confirming that COads remained at more positive potential regions as the Pt coverage increased. For the DAR@Pt(1) surface, the SERS intensity of the bridge COads band decreased more quickly than that of the atop COads band. The different FA electrooxidation behavior observed in Figure 3 can be interpreted based on the SERS measurements presented above. According to previous reports, the FA electrooxidation behavior observed on DAR@Pt(1) surface could be supposed to proceed by a direct pathway (eq 1). The SERS results presented in this work revealed that the indirect pathway involving COads as a reaction intermediate (eq 2) also occurred on the DAR@Pt(1) surface. However, the amount of COads produced by FA dehydration was relatively small, and the Pt−CO band quickly decreased as the electrode potential moved to positive regions. Electrooxidation of FA began at 0.0 V, and the peak potential was observed at 0.3 V, at which potential the intensity of Pt−COads decreased significantly. On DAR@Pt(5) surfaces, the initial amount of COads increased and the intensity of Pt−COads slowly decreased with a positive potential scan. Therefore, the direct pathway oxidation current (peak I) decreased, and the indirect oxidation current (peak II) appeared. On DAR@Pt(10) surfaces, the electrooxidation of FA by the direct oxidation pathway was greatly diminished because a significant amount of COads remained up to 0.4 V. Therefore, the electrooxidation of FA proceeded dominantly through indirect pathways. The Pt layers on the DAR surface reached monolayer levels on the DAR@Pt(15) surfaces, on which the initial amount COads by FA dehydration became significantly high. The initial COads was mostly retained up to 0.2 V, which prohibited the direct oxidation of FA in the positive potential regions. The SERS intensity of Pt−COads steeply decreased from 0.3 V, and the indirect electrooxidation of FA began at 0.5 V. COads was completely removed at 0.6 V, where the oxidation currents were maximized. The SERS behavior of COads on DAR@ Pt(30) was the same as observed on DAR@Pt(15), resulting in similar electrooxidation activity. This result indicates that Pt layers above the monolayer level on Au surfaces do not affect the FA electrooxidation activity. During the in situ electrochemical SERS measurements, potential-dependent variation in the Raman shift provides information about the absorption characteristics of metal surfaces. The Raman shift of the Pt−COads bands represents the adsorption strength between CO and Pt surfaces, and the potential-dependent change in the Raman shift can be explained by the Stark effect.38 The Raman shifts of band 2 on DAR@Pt(n) surfaces were plotted as a function of the electrode potential (Figure 7a). As the electrode potential moved to positive regions, the Raman shifts linearly decreased. This behavior is a typical Stark effect, indicating that the adsorption strength between CO and Pt weakened with positive potential excursion. On DAR@Pt(n) surfaces, the Stark slopes (dν/dE) was dependent on n, as shown in Figure 7b. In previous

Figure 7. (a) Potential dependence of the Raman shift for Pt−CO (atop) bands observed in Figure 5 on DAR@Pt(n) substrates. (b) Stark slopes of the DAR@Pt(n) surfaces as a function of the number of EC-ALD.

investigations, the Stark slopes for Pt−COads bands were approximately −10 cm−1/V.34,39,40 In this work, a Stark slope of −25 cm−1/V was measured on DAR@Pt(1), which is significantly greater than those reported in previous research. The Pt surfaces used in the previous research were massive Pt surfaces or completely covered Pt layers on Au surfaces. Stark slopes close to the previously reported values were obtained on DAR@Pt(15) and DAR@Pt(30), wherein the Pt layers completely cover the Au surfaces. In contrast, the Pt layers on DAR@Pt(1) are island-type submonolayers, which may result in a significantly greater Stark slopes. Therefore, the adsorption strength between CO and Pt on DAR@Pt(1) is weakened faster than other DAR@Pt(n) surfaces, which contributes to the high FA electrooxidation activity on DAR@Pt(1) through direct reaction pathways. The magnitude of the Stark slopes on DAR@Pt(n) surfaces decreased with n, indicating that the Pt−CO adsorption strength is retained stronger with positive potential scans as the coverage of the Pt layers on Au increased. Thus, the direct electrooxidation of FA became unfavorable as the Pt coverage increased. The Stark effect of band 4 (atop CO stretching) on the DAR@Pt(n) surfaces was also examined (Figure S7). The Stark slope on the DAR@Pt(1) surface was 48 cm−1/V, which is significantly greater than those reported on bulk Pt surfaces (∼30 cm−1/V) in previous studies.39,40 As the Pt coverage on the DAR@Pt(n) surfaces increased with n, the Stark slope decreased and reached 28 cm−1/V on DAR@Pt(30) surfaces. These results are consistent with those observed for band 2, confirming again that COads persist stronger with positive potential scan as the Pt coverage on the DAR increases. In addition to the amount of initially formed CO and its potentialdependent intensity variation, the Stark slopes for Pt−CO bands measured on DAR@Pt(n) surfaces provide insight into the dependence of the electrooxidation mechanism of FA on the Pt coverage on the Au surface. CO formation by FA F

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dehydration is suppressed on the DAR@Pt(1) surfaces, and the adsorption strength between CO and Pt is quickly weakened during the positive potential excursion. These combined effects unanimously explain the unique FA electrocatalytic activity on DAR@Pt(1) surfaces.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08611. Additional SEM images, electrochemical, and SERS data (PDF)

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REFERENCES

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4. CONCLUSIONS We investigated the mechanism of FA electrooxidation on Pt layers modified on Au surfaces using in situ electrochemical SERS measurements. SERS-active Au@Pt substrates, DAR@ Pt(n), were prepared using self-terminating electrodeposition of Pt on DAR surfaces. The amount and coverage of Pt on DAR@ Pt(n) were precisely controlled by applying a different number of potential steps (n) during the electrodeposition processes. The DAR@Pt(n) surfaces exhibited different FA electrooxidation behavior depending on n, which was investigated by electrochemical SERS measurements. DAR@Pt(1) surfaces with island-type Pt layers on Au exhibited typical electrooxidation behavior that have been supposed to proceed through direct reaction pathways. Even on this surface, COads was produced by dehydration, indicating that the indirect electrooxidation of FA occurs. However, the amount of COads produced by FA dehydration is relatively small compared to other DAR@Pt(n) surfaces. The potential-dependent SERS intensity and Stark slopes revealed that the adsorption strength between CO and Pt was quickly weakened during the positive potential excursion, which enabled the direct electrooxidation of FA on DAR@Pt(1). As the Pt coverage on DAR@Pt(n) increased, a greater amount of CO was produced by FA dehydration, and the COads persisted longer in the early stage of FA electrooxidation. On DAR@Pt(15), with monolayer Pt layers on Au, the direct electrooxidation of FA was mostly prohibited by the COads initially produced by the dehydration of FA. DAR@Pt(30) exhibited similar SERS behavior to DAR@Pt(15), indicating that Pt layers above the monolayer level on Au surfaces do not affect the electrooxidation activity of FA. The results demonstrated in the present work provide insight into the mechanistic interpretation of electrooxidation of FA on Pt−Au systems. Further application of the DAR@ Pt(n) system to other electrochemical reactions will be the focus of future research.

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AUTHOR INFORMATION

Corresponding Author

*Phone +82 43 261 2284; fax +82 43 267 2279; e-mail [email protected] (J.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2014R1A2A1A11050622 and NRF2016M3D1A1021145). G

DOI: 10.1021/acs.jpcc.6b08611 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.6b08611 J. Phys. Chem. C XXXX, XXX, XXX−XXX