Au(111)-Supported Pt Monolayer as the Most Active Electrocatalyst

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The Au(111) - Supported Pt Monolayer as the Most Active Electrocatalyst Toward Hydrogen Oxidation and Evolution Reactions in Sulfuric Acid Weicheng Liao, and ShuehLin Yau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05259 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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The Journal of Physical Chemistry 1

The Au(111) - Supported Pt Monolayer as the Most Active Electrocatalyst Toward Hydrogen Oxidation and Evolution Reactions in Sulfuric Acid

Weicheng Liao and Shuehlin Yau* Department of Chemistry, National Central University, Jhongli, Taiwan 320

Submitted to J. Phys. Chem. C

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The Journal of Physical Chemistry

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ABSTRACT: As a sub-nanometer thick platinum (Pt) film can have catalytic properties different from those of the Pt bulk, the research on the preparation and characterization of a Pt monolayer is fundamental intriguing and may lead to cost-effective fuel cells. We devise an electroless deposition method to fabricate a Pt monolayer and use scanning tunneling microscopy (STM) to characterize its atomic structures. This method involves the use of carbon monoxide (CO) molecules as the reducing agent for PtCl62- complexes, yielding a CO-capped Pt film on an Au(111) substrate. The deposition of the Pt film stops at one atom thick. In order to expose the Pt film, the CO adlayer is stripped off by pulsing the potential to 0.96 V (vs. hydrogen reversible electrode) for 3 s in H2-saturated 0.1 M H2SO4. Atomic resolution STM imaging shows that the Pt adatoms arrange in two hexagonal arrays with different atomic corrugation patterns and a notable difference (5.5 %) in the Pt-Pt distance. The Pt film with a larger interatomic spacing of 0.287 nm is 2× more active than that of Pt(111), and may be the most active catalyst toward hydrogen evolution and oxidation reactions (HER and HOR) reported thus far.


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1. Introduction: Platinum (Pt) is renowned for its electroactivities toward a number of electrochemical reactions, such as hydrogen evolution and oxidation,1 oxidation of methanol,2 and oxygen reduction reaction.3-4 These redox reactions are essential to the operation of fuel cells. However, the high cost of Pt metal is one of the main impediments to the commercialization of Pt-based fuel cells. Therefore, reducing the Pt loading in the electrocatalyst or finding inexpensive alternatives are the chief strategies to bypass this obstacle. Besides the chemical composition of an electrocatalyst, the atomic arrangement of the surface of an electrocatalyst also influences its activity.5 Fabrication of nanoscale metallic entities and study of their catalytic activities have been one of the frontiers in the modern aspects of nanotechnology. The reactivity of nanomaterials has attracted tremendous attention in the last two decades since the discovery of gold nanoparticles as a catalyst toward CO oxidation. The cost consideration is met by reducing the loading of Pt metal in the catalyst, which should not be compromised by lower activity. As a versatile catalyst in the application of fuel cells, Pt nanoparticles have been extensively examined to reveal the activity toward a wide range of reactions. Stimming et al. report a current density of 1000 mA/cm2 for HER at a single Pt nanoisland ~2 nm wide.6 More recently,



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Tymoczko et al. fabricate a near surface alloy (NSA) by placing a monolayer of Cu atoms under the uppermost layer of Pt atoms on the Pt(111) electrode, which is more active than the bulk Pt(111) toward hydrogen evolution and oxidation reactions (HER and HOR).7 Nano-scale Pt entities, ranging from 1D monolayers to 3D particles, have been deposited on metal substrates of Au, Ru, etc,8-9 on semiconductors of silicon10 and TiO211, on carbon materials (graphite,12-13 carbon nanotubes,14-15), and on polymers (polyaniline,16 and Nafion17). These Pt-modified electrodes can have unique electrocatalytic activities. However, it is fair to state that the exact interfacial structures at most Pt entities remain elusive. Scanning tunneling microscopy (STM) has been used to show the preferential nucleation of Pt deposit at steps and the following growth into 3D cauliflower-type clusters on Au(111) and Au(100) electrodes under potential control.18 But an epitaxial electrodeposition of Pt on Au(111) is inferred from the X-ray diffraction experiments.19-20 Meanwhile, a Pt film is produced when Pt2+ replaces a predeposited Cu monolayer on a support.21-22 When electrodepositing Pt from a CO-saturated PtCl62- solution, the resultant Pt film is capped by CO molecules, which restricts its thickness to only one Pt atom.23 Recently, we found that carbon monoxide can serve as a reductant for PtCl62-, which yields a CO-capped Pt monolayer on Au(111) without potential control.24 We substantiate the study of an ordered Pt monolayer on Au(111)


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by showing that the CO adlayer can be removed to expose the Pt deposit without affecting the ordered state of the Pt monolayer. This supported Pt monolayer has Pt adatoms arranged in atomic structures different from those of bulk Pt(111) electrode, which enables tailoring unique electrocatalytic properties.25 In particular, the prepared Pt film exhibits higher electrocatalytic activities toward hydrogen evolution and oxidation reactions (HER and HOR) than the Pt(111) electrode. 2. Experimental section 2.1 Electrode Preparation. The Au(111) electrode (area = 0.0432 cm2) was a bead made with the end of a gold wire, and its preparation and pretreatment followed the conventional method. The ordered Au(111) electrode was coated with a thin Pt film by being dipped in a Pt dosing solution made of Na2PtCl6(25 µM) and Millipore water saturated with carbon monoxide gas, as previously reported24. After being removed from the solution, the Au(111) electrode was rinsed with CO-saturated Millipore water and subsequently with H2-saturated Millipore water, and then quickly transferred into an electrochemical or STM cell. The use of 0.1M H2SO4 saturated with hydrogen gas as the electrolyte to prevent the Pt film from oxidation, and thus losing potential control during transfer in this experiment, was different from the process reported in the previous work.24 The first rinsing was used to remove the extra PtCl62- on the as-prepared electrode, and then the CO was removed by the following



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rinsing. Both the CV and STM results were recorded with a reversible hydrogen electrode (RHE). 2.2 Electrochemistry. CV was performed with a three-electrode electrochemical cell, including an RHE reference electrode and Pt counter electrode at room temperature. The potentiostat was a CHI 627E, used with the hanging meniscus configuration. The cell for HOR/HER experiment contained 0.1 M sulfuric acid saturated with hydrogen and was blanketed with hydrogen (the hydrogen pressure in the cell was 1 atm). The 0.1 M H2SO4 was prepared by diluting ultrapure concentrated sulfuric acid purchased from Merck (Darmstadt, DFG) with Millipore triple-distilled water. The STM was a Nanoscope E (Veeco, Santa Barbara, CA) and the scanner was an A-head with a maximal scan size of 500 ×500 nm2. It was calibrated against the atomic structure of the Au(111). The tip was a tungsten tip etched by AC in 1 M KOH. After thorough rinsing with Millipore water, it was insulated by applying an Apeazon wax coating. All STM images were acquired with the constant-current mode and unfiltered. 3. RESULT AND DISCUSSION 3.1 STM Imaging 3.1.1 The CO-capped Pt Deposit on Au(111). Figure 1 is a topographic STM scan obtained with the as-prepared, CO-capped Pt/Au(111) electrode at 0.15 V in 0.1 M


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H2SO4. Well-defined terraces punctuated by monatomic steps (∆z = 0.24 nm) are readily identified. The patch-like islands, not seen with a bare Au(111), are ascribed to the Pt deposit. The pristine reconstructed Au(111) surface is lifted to the (1 × 1) phase, producing segregated gold islands imaged as bilayer islands occupying 4% of the surface area (Figure 1a).26 The physical height of a monoatomic Pt island should be 0.24 nm, which exceeds 0.10 nm measured from Figure 1a. This discrepancy is explained by the different electron tunneling barriers at the Pt adlayer and Au substrate. Also, CO molecules adsorbed on the Pt deposit, but not on the Au substrate, can contribute to this difference. The CO adlayer on the Pt deposit is revealed by high-resolution STM images shown in Figure 1b-e. In order to reveal the corrugation profile of this sample, the cross-section profile along the scan line marked in the inset is shown in Figure 1f. A few Pt islands are particularly bright, which are raised 0.24 nm by gold aggregates. The Pt islands seen in Figure 1a are 5 ~ 30 nm wide with rugged and randomly oriented perimeters. The internal atomic structure on a typical Pt island is revealed by two close-up STM scans shown in Figures 1b and c, where prominent wavy corrugation patterns superimposed on an ordered structure are readily identified (Figure 1b). This feature is similar to the reconstructed Au(111) surface, where Au atoms in the uppermost plane are uniaxially compressed by 4% in the



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direction.26 Meanwhile, it is reported that Pt(111) also reconstructs in a similar fashion, where the uppermost Pt plane is compressed to yield an incommensurate structure.27 In addition, STM reveals a hollow “star” pattern, featuring six corrugated lines converging into a vacancy site located at the center of this image. This structure is reported in a theoretical study, proposing that Pt atoms can occupy (F) or (H) sites to give segregated domains separated by corrugated ridges (Figure 1c).28 This model is supported by a 0.9 nm mis-alignment between the molecular rows in any two domains on the opposite sites of the depression, as delineated by the three lines drawn in Figure 1c. Depending on the degree of compression, the reconstructed Pt(111) can form honeycomb, wavy triangle, bright star, and Moiré patterns.28

Figure 1. In situ STM topography (a) and high-resolution scans (b-e) acquired with


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the Au(111) electrode modified by a CO-capped Pt adlayer at 0.15 V in 0.1 M H2SO4. A few Pt islands are particularly bright (a), attributed to gold aggregates, as outlined by the model shown in panel (f). Panels (b) and (c) show superstructures of corrugated wavy lines and a depressed star pattern on an ordered CO adlattice. The six radiating lines in panel (c) define six domains, designated arbitrarily as F (fcc) and H (hcp) registries. The capping CO adlayer and the underneath Pt deposit sometimes are imaged simultaneously, as exemplified by the original (d) and Fourier-filtered (e) high-resolution STM scans. The feedback current and bias voltage are 1 nA and 60 mV. CO molecules line up in a direction rotated from the close-packed Pt atoms by 19°, as indicated in panel (e).

Shown in Figures 1d and e are original and Fourier-filtered molecular resolution STM images acquired at a Pt island. Different ordered patterns are seen on the two sides of these images. The one on the right is hexagonal with a nearest neighbor spacing of 0.287 ± 0.02 nm, which is ascribed to the Pt adlayer. This somewhat smaller atomic spacing than that of the Au(111) substrate results in the obvious corrugated patterns (∆z ~0.03 nm) seen in the STM image (Figure 1b and c). To its left, the array comprises features separated by 0.37 nm and one of the close-packed rows is rotated from that of the neighboring Pt domain by 19°, as outlined by the lines



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drawn in Figure 1e. These features yield a (√7 × √7)R19.1° structure with four spots per unit cell or a coverage of θ = 4/7 = 0.571. (The coverage in this study is defined by the ratio between the number of adsorbate and substrate atoms.) This structure is reported previously with CO molecules adsorbed on a Pt(111) electrode in CO-free perchloric acid.29 This STM result indicates that the Pt adlayer is capped by CO molecules, which is in line with the reported IR results. Electrodeposition of a Pt monolayer on Au(111) is also possible in the presence of solution CO.23 It is thought that Pt deposition is hindered by the capping CO adlayer, leading to a one-atom thick Pt film on Au(111).23 The spatial arrangement of CO molecules on Pt substrate is insensitive to the physical state, whether it is a film or a single crystal electrode. 3.1.2 The CO-free Pt adlayer on Au(111). Given the high surface energy of Pt metal, it seems to be difficult to prepare an ordered Pt deposit on the Au(111) electrode. In order to study the electracatalytic property of the supported Pt film, one needs to remove the CO capping layer on the Pt deposit. The easiest way to do this is to sweep the potential to 0.96 V in N2 – saturated 0.1 M H2SO4. (The CO stripping voltammogram is shown in the supporting information.) However, this potential sweeping method inevitably causes the oxidation of the Pt adlayer in N2-saturated solution, and irreversibly alters its structure. Consequently, a different method is devised to accomplish this, ie applying a potential pulse to 0.96 V for 3 s in


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H2-saturated 0.1 M H2SO4, followed by controlling the potential at 0.15 V. The sample is then characterized by STM. (The relevant electrochemical results are shown in Figure S2 in the supporting information.) The resultant electrode reveals patches of Pt islands 20-40 nm wide and 0.13 nm high on the Au(111) electrode (Figure 2a). Compared to the CO-capped Pt deposit (Figure 1a), Pt islands have well-defined triangular and hexagonal shapes with perimeters aligned in the close-packed atomic directions of the Au(111) substrate. The reason for this preferential alignment of step edges in the Au directions is the formation of (111) and (100) microfacets by Pt adatoms and the Au(111) substrate, leading to minimal dangling bonds or kink sites on the Pt deposit.30 The changes in Pt islands signal a substantial movement of Pt adatoms after the capping CO adlayer is removed.

Figure 2. In situ STM images acquired with a Pt – modified Au(111) electrode at 0.15 V in 0.1 M H2SO4. Panel (a) shows smooth CO-free Pt islands on wide Au(111) terraces. Pt islands reshape to yield the well-defined hexagons seen in panel (b). Panel



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(c) is an atomic resolution STM image collected over a Pt island. Pt atoms can reside at fcc (F) and hcp (H) sites of the Au(111) substrate, resulting in the skewing of atomic rows revealed by the line segment. These images were collected with 0.8 nA and 12 mV.

Zooming in onto a Pt island reveals a hexagonal array with corrugated atomic heights, as seen in Figure 2c. Within the accuracy of STM, Pt adatoms were aligned with the close-packed atomic rows of the Au(111) substrate and an in-plane atomic spacing of 0.287 nm, which is 0.51% smaller than the presumed 0.2885 nm of the underneath Au(111) – (1 × 1) structure. A line is drawn in Figure 2c to show that the Pt atoms are not aligned, suggesting that they occupy the fcc (F), hcp (H), and asymmetric sites of the Au(111) substrate. This occupation of multiple registries of Pt adatoms results in the dissimilar STM intensities of Pt adatoms seen in Figures 1c and 2b, which is in line with the stacking fault models proposed for the reconstructed Au(111) and Pt(111). The reconstructed Pt(111) also exhibits the same corrugation pattern.27-28 The structure seen in Figure 2b resembles the reconstructed Pt(111), and thus is referred as the R phase in the following discussion. These STM results indicate that the Pt adlayer can assume spatially ordered structure without CO admolecules. 3.1.3 The Moiré Structure of Pt Adlayer. The stability of the R phase against


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potential modulation is examined here. The distinct R phase is revealed by STM between 0 to 0.79 V in 0.1 M H2SO4. However, a protracted STM scanning at 0.8 V leads to noisy images, suggesting moving Pt adatoms and/or movement of ions on the electrode. However, setting the potential to 0.2 V yields clear imaging again. An STM snapshot taken afterward with 1 nA feedback current and 20 mV bias voltage is shown in Figure 3a, revealing drastic changes at the Pt deposit. The newly formed structure features a long-range modulation of intensity, which has been observed with a number of studies on heteroepitaxial deposition, such as Ni and Co deposited on Au(111) and Pt(111) electrodes.31-33 The atom-resolution STM scan shown in Figure 3b reveals a long range intensity modulation arising from dissimilar atomic corrugation heights. This Moiré pattern is referred as the M phase in the following. Compared with the mostly smooth morphology seen with the R phase (Figure 2a), Pt islands in the M phase are decorated with protruded clusters, and their perimeters are notably rugged or kinked (Figure 3a). These features appearing 0.25 nm higher than their supporting Pt islands are ascribed to monatomic clusters of Pt. This M phase and the associated defects stay unchanged when the potential is switched to 0 V, the onset potential for hydrogen evolution. Since the R-to-M phase transition occurs at E > 0.8 V, where oxidation of Pt begins, adsorption of anions or production of OH species on the Pt deposit can trigger relocation of Pt atoms under



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this condition. This M phase has more defect sites than the R phase. The long-range intensity modulations seen with the M phase (Figure 3a) are not all aligned in the same manner. They can be rotated from the axis of Au(111) by 0 ~ 20°, as illustrated by the two groups of arrows representing the directions of intensity undulation of two different Moiré patterns seen in Figure 3a. Shown in Figure 3b is a high-resolution STM scan of the M phase with a long-range intensity modulation of 4.4 nm running in 16°with the of the Au(111) substrate. The close-packed Pt adatoms are aligned in a direction 15° off from the intensity modulation. This M structure is explained by the ball model depicted in Figure 3c, consisting of a hexagonal Pt pattern superimposed on an Au(111) network. The atomic directions of the Pt adlayer and Au(111) are rotated by 1° from each other. The Pt lattice with an in-plane atomic spacing of 0.271 nm is contracted with respect to Au substrate (dnn = 0.289 nm) by 6.0%.

Figure 3. In situ STM images (a and b) showing the Moiré (M) structure at 0.1 V in 0.1 M H2SO4. Not all M patterns are aligned in the same way, as illustrated by the


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groups of arrows depicted in panel (a). Each mound of the M phase is surrounded by Pt adatoms residing at fcc (F) and hcp (H) sites of Au(111) substrate. The intensity modulation of this M phase is rotated from the Au axis by 15°. The imaging conditions are 1 nA and 20 mV. Panel (c) shows a tentative ball model, consisting of two hexagonal networks of the Au(111) substrate (black) and Pt adlayer (red). The in-plane atomic spacings are 0.289 and 0.271 nm for Au and Pt. The close-packed atomic direction is rotated from the Au axis by 1°.

The stability of this M phase vs. potential modulation is then studied by STM imaging in 0.1 M H2SO4. First, an STM image is acquired at 0 V, followed by switching the potential to 0.3 V, and another STM scan over the same area is acquired. These are shown in Figure 4a and b, respectively. The same surface morphology of these images indicates that the thermal drift, the common problem in STM imaging, was negligible in this experiment. The M phase is seen on the protruded Pt islands superimposed on the Au(111) substrate. The faint corrugated lines seen in the background indicate local the reconstructed Au(111). In order to compare the periodicities (P) of the M phases seen at 0 and 0.3 V, we obtain two cross-section profiles along the same row of the M patterns seen at the central island in these two images. The resultant profiles reveal two very different P values of 4.76 and 3.92 nm



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at 0 and 0.3 V, respectively. The in-plane atomic spacing among Pt adatoms thus clearly varies with the potential. In order to substantiate P as a function of potential, a series of STM images are obtained as the potential is changed positively from 0 to 0.8 V (not shown). The results are plotted in Figure 4d. The most notable change in the value of P is observed as the potential increases from 0 to 0.3 V, where it decreases notably from 4.76 to 3.92 nm, followed by a constant regime (P = 3.92 nm) from 0.3 to 0.8 V. Given the periodicity (P) of the Moiré pattern, one can apply the formula of dPt = (dAu·P)/(dAu + P) to calculate the in-plane atomic spacing of Pt adatoms, where dPt and dAu are the diameters of Pt and Au. As summarized in Table I, the Pt-Pt spacing decreases from 0.272 to 0.269 nm, representing a contraction of 1.1% over a 0.3 V interval (0~0.3 V). The 0.269 nm in-plane atomic spacing is 3.3% smaller than that of the Pt(111) surface, suggesting that the Pt adatoms reduce the surface energy by this manner.


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Figure 4. In situ STM images (a-b), revealing the M phase at 0 and 0.3 V in H2 – saturated 0.1 M H2SO4. Panel (c) shows the section profiles along the same line of undulation indicated in panels a and b. Panel (d) is a plot of the periodicity of the intensity modulation as a function of potential.

Since a variation in-plane atomic spacing of Pt is observed between 0 and 0.3 V, where under-potential deposition (UDP) of hydrogen occurs, the adsorbed H adatoms could contribute to this result. On the other hand, this variation of Pt-Pt spacing is not noted with the R phase between 0 and 0.3 V. The UPD of hydrogen is thus not the



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prime reason for this variation in the in-plane spacing of Pt adlayer. Meanwhile, the Pt-Pt spacing stays unchanged over the double-layer charging region between 0.3-0.7 V, where charges are accumulated on the electrode surface. Starting at 0.8 V, oxidation of the electrode begins, resulting in a rougher surface that makes it difficult to achieve clear imaging. Protracted potential holding at 0.1 V (for 3 hours) was not able to restore the well-ordered M structure, indicating an irreversible restructuring process.

Table 1. Summary of the periodicities (P) and in-plane Pt spacing (dPt) of the M Pt adlayer on Au(111) observed at various potentials.

3.2 Electrocatalysis of Pt/Au(111) toward HER and HOR. Given the two hexagonal structures of the R and M structures of Pt on Au(111), it is then curious to learn if they have different electrocatalytic properties. The most studied HER and HOR are chosen for the test. An Au(111) electrode is first modified with a sub-monolayer Pt adlayer organized in R phase. The Pt coverage is determined to be 0.46 from the STM image shown in the inset of Figure 5a. Its activities towards HER


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and HOR are examined by voltammetry in the rotating disk electrode configuration. The CV result recorded at 10 mV/s between 0.4 and -0.2 V at 900 rpms is shown in Figure 5b. Then the potential is held at 0.8 V for 10 min to convert the R-phase to the M phase, followed by examining HER and HOR activities. It is emphasized that the results shown in Figure 5 are obtained with the same Pt/Au(111) electrode, meaning that the R and M phases have the same Pt coverages. Also, as revealed by in situ STM imaging, these Pt structures are stable against potential cycling between -0.2 and 0.4 V, the potential range used to study HER and HOR. Figure 5(b) shows the CVs recorded with the Au(111) electrode decorated with the R () and M (----) Pt deposit in H2-saturated 0.1 M H2SO4 electrolyte. The kinetics of HOR at the R and M phases Pt /Au(111) and Pt(111) are compared in terms of the exchange current density and Tafel slope, which are extrapolated from the i-V relationship at the low overpotential domain (η < 10 mV). This information is summarized in Table II. Close examination of Figure 5b reveals that the R phase gives rise to the steepest increase of current at the equilibrium potential (0 V), indicating it is the most active electrocatalyst for both HER and HOR. Indeed, the exchange current density determined from the micropolarization curve for the 0.46 ML R phase is 1.16 mA/cm2, which is 1.78 and 1.84 times larger than those of the M phase and Pt(111). Also, the HER current observed at -0.06 V with this R phase is



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1.71 times higher than Pt(111) and comparable with the result (1.81 times) reported with the near surface alloy (NSA) electrode of Pt on Cu.7, 34 Assuming only the Pt deposit (not Au substrate) is responsible for this HER activity the Pt adatoms in the R phase are 3.9 times more active than those in the Pt(111) plane.

Figure 5. CVs obtained from R phase (solid line) and M phase (broken line) Pt films on Au(111) and Pt(111) (dotted line) in 0.1 M H2SO4 saturated with N2 (a) and H2 (b). The scan rate and rotating speed were 10 mV/s and 900 rpm. Current density was taken the Au(111) geometric area into account. The Pt coverage was 0.46 ML, as determined from the STM images shown in the inset of panel (b). Panel (c) shows the Tafel plots based on the CV results shown in panel (b). Panel (d) shows two section profiles along the lines marked in panel (b).


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The Tafel plots (E vs. log((idi)/(id-i)) of these three electrodes are depicted in Figure 5c. First, the Tafel slope for Pt(111) obtained in this study is 68 mV/dec, which is comparable with the reported.35 This validates the use of the hanging meniscus method to study the kinetics of HOR. The R and M phases result in Tafel slopes of 38 and 65 mV/dec in the potential range of 15 to 40 mV, indicating that these two Pt phases produce H2 via different mechanisms and R phase provides the more efficient pathway than those of the M phase and Pt(111).

Table 2. The hydrogen electrocatalytic activities at room temperature.

The exchange current density is obtained from the micropolarization curve (±10mV) shown in Figure 5(b). All the references were operated in the conditions of ~pH1 H2SO4 or HClO4.



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As the activity of Pt electrode can be obscured by the cleanliness of the electrode, the concentration of hydrogen, temperature etc, it is difficult to have a fair comparison of the results reported by different groups. Thus, the performance of Pt(111) under the same conditions is used as the reference. Among the samples studied, the one with a coverage of 0.36 gives the highest HER activity, whose jo is 1.18 mA/cm2 for HER and HOR at the equilibrium potential (0 V). This is 1.9 times higher than that (0.63 mA/cm2) for Pt(111), and is comparable to that of NSA of Cu and Pt, the best HER catalyst reported thus far.6-7 However, the Pt loading used in this study is the lower than NSA. The reason for the notable difference in reactivity of R, M phases, and Pt(111) are multifold. First, the binding energy of H to electrode is important, as revealed by the volcano plot of exchange current density and H-M binding energy.7 However, the effect of H adsorption on HER is complicated by the underpotential deposition (UPD) of H occurring between 0.3 and 0.05 V, observed with all electrodes. It is reported that HUPD can block surface active sites needed for HER.35 The coverage of HUPD at the R and M phases are evaluated by integrating the charges pass between 0.32 and 0.05 V in N2 saturated 0.1 M H2SO4. After corrected for the double-layer charging and taking account of the coverage (0.46 ML) of Pt deposit, the R and M phases respectively


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result in 118 and 150 μC/cm2, which are 74% and 93% of that found for Pt(111). This result suggests that there are more reactive sites for HER on the R phase than the M phase (by 19%) at the onset potential for HER (0.05 V). This extent of HUPD on these electrodes can also correlate with the binding strength of H adatoms, which increases in the sequence of R < M < Pt(111). These Pt monolayers are structurally similar to the recently reported NSA of Pt-Cu, which shows a HER activity two times higher than at Pt(111). The optimal binding energy of H to the NSA is proposed to be the key to this improvement. This view can hold for the better HER activities seen for the R phase, judged from the similarity in their HUPD feature in the i-V profiles. As the electronic structure of a supported metal monolayer can have unique in-plane distance between adatoms,5, 36-37 we seek evidence for this electronic effect of R and M phases. Figure 5d shows the cross-section profiles along the scan lines marked by white lines in the two STM images (the inset in Figure 5b). These two images are obtained using the same tip over the same electrode, when the Pt monolayer assumes R and M structures. M and R phases appear 0.22 and 0.13 nm higher than the gold substrate, which is explained by different local work functions at these Pt deposits. In other words, R and M phases have a notable difference in their electronic structures, which is related to the 5.5% difference in the Pt-Pt spacing. This view is consistent with that derived from a previous study on the Pd



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monolayers supported by a number of foreign metal substrates.5 It is proposed that the variation of Pd-Pd spacing within the adlayer results in a shift of the d-band center of the Pd film, which affects the binding strength of H adatoms and the subsequent HER. For example, compared with Pd(111), the Pd monolayer on Au(111) has a larger interatomic spacing, which leads stronger bonding between H and Pd. The subsequent coupling of two H atoms into H2 becomes rate-determining 5, 38 However, we find that the R phase with a larger Pt-Pt spacing than Pt(111) shows a better HER/HOR performance. Although surface defects on a Pt electrode can influence its electrocatalytic activity,39-40 this factor is not the determining factor in causing the different HER/HOR activities of the R and M phases, as the rougher M phase is not more active. We conclude that it is the atomic structures of these two Pt adlayers, which eventually decide their activities. On the other hand, to reconcile the result showing a maximal activity of HER/HOR of the R-phase at a coverage of ~0.36, we propose that the gold sites near the Pt deposit can contribute to these reactions through the dubbed “spill-over” effect.41 The effect of substrate on the activity of Pt adlayer can be important, as is demonstrated by a study on the oxygen reduction reaction.42 Unfortunately, this study does not show clear information on the atomic structure of the Pt film. By contrast, the present in situ STM study provides a real-space view of Pt adlayers in a reacting


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environment. The well-ordered Pt monolayers will be applied to study reactions other than HER/HOR and in situ STM would be helpful to reveal the structure of the Pt adlayer as chemical species are adsorbed under potential control. CONCLUSION This study shows an electroless deposition method to fabricate an one-atom-thick Pt film on an Au(111) electrode by using CO molecules to reduce PtCl62-. The as-prepared Pt film is capped by CO admolecules, arranging in the (√7 × √7)R19.1° structure, as seen on Pt(111). In order to expose the Pt monolayer, the capping CO adlayer is stripped off by applying a potential pulse to 0.96 V for 3 s in H2-saturated 0.1 M H2SO4. The ordered atomic structure of the Pt film is not disrupted by this process, which is revealed by atomic resolution STM imaging, showing a hexagonal array with an interatomic spacing of 0.287 nm. This designated R phase is transformed into the M phase at E > 0.8 V, where the Pt-Pt spacing decreases from 0.272 to 0.269 nm as the potential increases from 0 to 0.3 V. These strained Pt monolayers both catalyze HER and HOR in acid media in extents notably different from Pt(111). The R phase with a 3.3% larger interatomic spacing than Pt(111) is particularly active. If the comparison is made on atomic scale, the R phase with a coverage of 0.36 results in the highest activity toward HER and HOR among all results reported thus far.



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at . Stripping voltammogram of CO adlayer on Pt-modified Au(111) in H2-saturated 0.1 M H2SO4. Current transients due to the electro-oxidation of CO adlayer on Pt-Au(111) electrode in 0.1 M H2SO4 saturated with H2 and N2.

ACKNOWLEDGMENTS The authors are grateful for technical help they received from Prof. C. C. Su (Institute of Organic and Polymeric Materials, National Taipei University of Technology). This research was funded by Ministry of Science and Technology of ROC (MOST 106-2113-M-008-005).


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TOC graphic


Au(111)-Supported Pt Monolayer as the Most Active Electrocatalyst

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