As Probed by In-situ Scanning Tunneling Microscopy

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Electrified Interfaces of Pt(332) and Pt(997) in Acid Containing CO and KI: As Probed by In-situ Scanning Tunneling Microscopy Jie Wei, Weicheng Liao, Jing Lei, Shuehlin Yau, and Yanxia Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09901 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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The Journal of Physical Chemistry

Electrified Interfaces of Pt(332) and Pt(997) in Acid Containing CO and KI: As Probed by In-situ Scanning Tunneling Microscopy

Jie Weia, Wei-cheng Liaob, Jing Leia, Shuehlin Yau*,b and Yan-Xia Chen*,a

aHefei

National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, 230026， China

b

Department of Chemistry, National Central University, Jhongli, Taiwan 320

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The role of step defects on an electrode is a central issue in the modern study of electrocatalysis. Although scanning tunneling microscopy (STM) has been used to characterize the electrified interfaces of low Miller-indexed single crystal electrodes, there has been little progress in the STM study of stepped Pt electrodes. Herein the structures of Pt(332) and Pt(997), two vicinal surfaces to the (111) plane, are examined by highresolution STM under potential control in 0.5 M H2SO4 solution containing iodide or carbon monoxide. These electrodes are annealed by a hydrogen flame and quenched in Millipore water, giving rise to rough step edges with poorly-defined atomic structures at 0.1 V (vs. reversible hydrogen electrode) in 0.5 M H2SO4. But step ledges are sharpened and aligned in the direction after adsorption of CO on both electrodes. In contrast, seesaw-like step lines are produced by iodine adsorption. Therefore, the step structure and mobility of Pt atoms are markedly influenced by the adsorbate. Pt(997) and Pt(332) electrodes with (111) facets that are 8 and 5 Pt atoms wide, respectively, afford (7  7)R19.1º - I and (3  3)R30º - I structures at 0.1 V. In comparison, the (2  2) – 3CO structure, as seen on Pt(111), is found on both Pt electrodes. Also, STM results yield linearly-bonded and 3-fold bonded CO molecules at the peaks and troughs of steps on Pt(332) and Pt(997). The origins for the faster CO oxidation rate at steps than that at terraces are discussed.

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1.

INTRODUCTION Owing to its unique interaction with many chemical species, platinum is well-known

as a versatile catalyst for a number of reactions.1-4 A Pt single crystal bead electrode can be fabricated readily by melting the end of a Pt wire and orienting it to expose the desired crystallographic plane. An annealing-and-quenching procedure has been devised to enable preparation of well-defined Pt electrodes without the need for vacuum equipment. This approach is frequently used in the modern study of electrified interfaces, in particular, to correlate atomic structure with electrocatalytic activity.5-8 Stepped surfaces with welldefined step and kink defects are used to enable systematic investigation of the influence of surface defects on the reaction kinetics and mechanism at the electrified interface.9 Carbon monoxide (CO) is an intermediate commonly found at the anode of low temperature polymer electrolyte membrane fuel cells (PEMFCs), because small organic molecules (SOMs) are not completely oxidized during the reactions taking place in these cells. CO can also be generated in the reforming process of H2.10, 11 Pt metal can outperform any other metal as the anode material of PEMFCs. Despite the extensive investigations carried out in previous decades, the correlation between the activity and structure of Pt eletrocatalysts toward the oxidation of SOMs is still unclear. A number of issues, such as the mobility of adsorbed CO, as well as the adsorption and kinetic oxidation of CO at step surfaces are still under debate.12-21 Studies most relevant to this work are summarized below. Because of their unique 3



coordination environment, Pt atoms at step sites and on flat terraces can be electronically different,22-25 which results in higher binding strength of CO molecules adsorbed at on-top sites on the upper part of step sites, compared with that of CO on terrace sites.13, 15, 19, 23, 26-29

There are significant differences in the IR spectra of the CO adlayer on stepped Pt

surfaces with different terrace widths reveal that the presence of periodic steps greatly influences the CO adlayers structure.30 However, previous efforts to interpret the surface structure of adsorbed CO based on the IR spectra have led to contradictory conclusions. For example, in one report, both atop and multi-coordinated CO molecules (denoted as COL and COM, respectively) are observed at intermediate and saturated CO coverages on Pt(443), Pt(332) and Pt(322).13 In contrast, from a systematic infrared spectroscopic study on CO adsorption on Pt[n(111)  111] with various terrace widths, Hori et al found that only COL is detected if the (111) terrace is fewer than 8 atoms wide.31 The discrepancies reveal that the structures of such stepped surfaces are highly dependent on the conditions of the electrode preparation and may also change during the electrochemical measurements. Furthermore, due to the vibrational coupling between COs adsorbed on steps and terraces, and the subsequent intensity borrowing, it is difficult to quantify the coverage of CO molecules adsorbed on each site. The characterization of spatial structure of CO molecules adsorbed at stepped electrodes is made more difficult by the finite width of the flat terrace.13, 32, 33

Oxidation of CO molecules at a Pt electrode proceeds by reaction with oxygenated species via the Langmuir-Hinshelwood mechanism.34 This reaction is sensitive to the 4


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spatial structure of Pt electrode. It is well-established that step/kinks35-38 and Pt ad-islands39 with low coordination numbers initiate CO oxidation at low potentials, e.g., the stripping potential for a saturated CO adlayer on Pt(553), Pt(554), and Pt(111) shifts positively with decreasing step density.12, 40 For stepped Pt surfaces, some have suggested that the trough of a step is the active site for CO oxidation, and CO molecules adsorbed on the terraces diffuse to these sites to get oxidized.12, 16, 17, 19 Others claim that after initiation of CO oxidation at the trough of a step, the oxidation front propagates to terrace sites, and CO molecules adsorbed on the top of a step are finally oxidized.20, 21, 41 In order to get insight into the role of steps in CO oxidation, one needs to learn how the width of the terrace affects the adsorption of CO molecules. As reported by many studies in vacuum, the structure of stepped Pt electrode can be very different from the nominal one.42, 43 Scanning tunneling microscopy (STM) with sub-nanometer spatial resolution is particularly useful for probing the local structures on conductive substrates. It has been employed to examine stepped surfaces/gas interfaces under conditions close to catalytic reaction conditions.42,

44-46

It is found that reconstruction is one of the most important

aspects of electrocatalysis, e.g., stepped Pt surfaces restructure greatly upon dosage with strongly adsorbed species, such as carbon monoxide and oxygen. In situ acquisition of atomistic images of stepped Pt surfaces can eventually unveil such changes. Within the context of this study, STM results obtained with CO on Pt(111) are summarized. A Pt(111) - (2  2) - 3CO structure is formed at E < 0.55 V (vs. RHE).47-50 This transforms to (√19  √19)R23.4° - 13CO (θCO = 13/19) before oxidation commences at 0.8 V. However, a (√7 5



 √7)R19.1° - 4CO (θCO = 4/7) structure is found at E < 0.2 V in the absence of solution CO.47 The influence of CO adsorption on the step structure of Pt(111) electrodes is examined by in situ STM, revealing the preferential formation of (111) microfacets when the potential is cycled in acid solution saturated with CO.37 However, there is no atomic level information on the exact local structure/composition of the step sites so far. In this contribution, we succeeded in achieving molecular-resolution STM imaging of CO molecules adsorbed on Pt(332) and Pt(997), which illustrates the value of STM in studying the spatial structures of rougher and more active Pt surfaces. We observe significant changes in the step structures of these electrodes upon CO adsorption, and we find different spatial structures of CO adsorbed near steps. This adsorbate-induced restructuring event is further investigated by examining iodine adsorption on these stepped Pt surfaces. The reasons that the step structures of these Pt electrodes vary notably with the spatial structures of CO and iodine adsorbed, as well as the role of the step structure in CO oxidation are discussed.

2. EXPERIMENTAL SECTION 2.1. Methods. The STM used here was a Nanoscope IIIa (Veeco). The tip was made out of tungsten etched electrochemically by AC in 1 M KOH. After thorough rinsing with Millipore water and drying with acetone, it was insulated by coating with a thin film of Apiazon wax. This as-prepared W tip was most stable at 0.4 V, where the faradaic current was only a few pA and this was the tip potential set for all STM imaging experiments. 6


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The platinum electrodes used for STM and voltammetric measurements were made by melting the end of a Pt wire (φ = 0.5 mm) using a H2/O2 torch. The single-crystal bead electrode was oriented by the laser alignment method, followed by cutting and polishing to expose the Pt(332) and Pt(997) planes. These two (111) vicinal surfaces are nominally Pt[n(111)  (110)] with n = 6 and 9. The electrode was pre-treated by annealing with a hydrogen torch to red hot for 5 min, then it was quenched immediately in Millipore water bubbled with a hydrogen stream for at least 5 min. The sample was removed from the quenching tube, covered with a thin film of water, and transferred to the electrochemical or STM cell. The electrode was quickly brought under potential control, and its potential was scanned rapidly from the OCP to 0.1 V. In this study we examined iodine and CO adsorbed on these electrodes. The former is formed by adding KI solution into the STM cell. The final [KI] is ~1 mM. The latter is obtained by bubbling CO directly into the cell while it is under potential control. Cyclic voltammetry was performed in a 3-electrode cell equipped with a Pt wire as the counter electrode (CE) and a reversible hydrogen electrode (RHE) as the reference electrode. All potentials used here are quoted against a RHE scale. For measuring the CO oxidation current transient, a saturated CO adlayer was established by exposing the Pt electrode to CO-saturated 0.5 M H2SO4 for 5 min under potential control at 0.1 V. The excess CO dissolved in the solution was purged away by constantly bubbling the solution with N2 for more than 20 min. Then, the electrode potential was stepped to 0.7 V to strip off the CO adlayer, and the i-t curves were recorded simultaneously. 7



2.2. Materials. The supporting electrolyte was 0.5 M H2SO4, prepared by diluting concentrated H2SO4 obtained from Merck KGaA (96% Suprapure reagent) and ultra-pure water (18.2 M cm, from Milli Q water system). N2 and CO have purity of 99.995%. KI and CO were purchased from Merck KGaA and Qiaoyi Gas (Taiwan), respectively.

3. RESULTS AND DISCUSSION 3.1. Electrochemistry. Figure 1 shows the CVs recorded at 50 mV/s with the as-prepared Pt(332) and Pt(997) electrodes in 0.5 M H2SO4 solution. There is a prominent pair of peaks (fwhm = 10 mV) at 0.125 V (A1/C1) in both profiles, attributable to underpotential deposition (UPD) of hydrogen at the (110)-oriented step sites.21, 51 The amount of charge passed at the Pt(332) and Pt(997) electrodes is 45 and 32 μC/cm2 respectively, which is consistent with the reported and theoretical values calculated using the relationship (l/(n - 2/3)), where n is the number of terrace atoms.52-54 HUPD at the (111) facets is responsible for the current plateau between 0.35 and 0.05 V. A pair of broad and reversible features at 0.45 and 0.48 V (A2/C2) for Pt(997) and Pt(332), respectively, are associated with the adsorption and desorption of bisulfate anions. This contrasts markedly with the sharp spike seen at ~0.45 V on Pt(111), as the (bi)sulfate adlayer transforms from a disordered to an ordered state. The absence of a current spike on Pt(332) and Pt(997) suggests that adsorbed HSO4- anions on these surfaces are disordered, possibly because their (111) facets are just too narrow to afford an ordered (bi)sulfate structure. This confirms the lack of a disorder-to-order transition of the 8


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adlayers on these surfaces. In addition, when the step density on the electrode is higher, the A2/C2 current peak becomes smaller, and the potential becomes more positive. On Pt(110), the A2/C2 is totally absent.12 All of these facts suggest that the existence of (110) steps on the (111) terraces disturbs the (bi)sulfate adsorption. On the other hand, adsorbed (bi)sulfate causes passivation of these electrodes and blocks the activation of water until at least 0.6 V. These CV features are consistent with previous reports.55-64

Figure 1. Steady-state CVs obtained at 50 mV/s with Pt(332) and Pt(997) electrodes in 0.5 M H2SO4 solution. The inset shows the ball models of Pt(332) and Pt(997) surfaces in top view and side view.

Figure 2 shows the stripping profiles of CO molecules adsorbed on the Pt(332) and 9



Pt(997) electrodes in CO-free 0.5 M H2SO4. As the potential is swept positively at 50 mV/s to 0.9 V, a CO monolayer is first adsorbed at 0.1 V. Both profiles are featureless until 0.55 V for Pt(332) and 0.6 V for Pt(997), as highlighted in inset a. The current increases rapidly to yield sharp peaks at 0.82 V (A3) for Pt(332) and 0.87 V (A4) for Pt(997), resulting from the oxidation of adsorbed CO molecules to CO2. The charges associated with these CO stripping peaks at Pt(332) and Pt(997) are 320 and 360 C/cm2, which translate to CO coverages of 0.67 ML and 0.75 ML (or 1.06 and 1.13  1014 molecules/cm2) on Pt(332) and Pt(997) electrodes. This is in good agreement with a previous finding that on the stepped surface(Pt(hkl)), in which the CO saturation coverage is found to be slightly smaller compared with that of Pt(111) – (2  2) – CO structure.13, 23, 30, 40, 65 Inset b of Figure 2 shows chronoamperometric results obtained with the CO-coated Pt(997) and Pt(332) electrodes, whose potentials are switched abruptly from 0.1 to 0.7 V in 0.5 M H2SO4. Initially, induction periods of ca. 5 and 10 s are observed on Pt(332) and Pt(997), respectively. This is followed by oxidation of CO molecules adsorbed on the Pt(332) and (997) electrodes, as indicated by a rapid increase in the currents, to peak values of 93 and 81 A/cm2, respectively, followed by rapid decay. The similar shapes of these i-t profiles indicate that CO oxidation follows the same mechanism.12, 14, 66 The Pt(332) electrode exhibits faster CO oxidation kinetics, due to its higher step density, which is in line with reported results.12, 20

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Figure 2. The stripping CVs recorded at 50 mV s-1 with CO-saturated Pt(332) (solid line) and Pt(997) (dotted line) electrodes. The saturated CO adlayer was prepared by bubbling CO into the electrochemical cell at a fixed electrode potential of 0.1 V. Inset a shows magnified versions of these profiles in the potential region from 0.4 V to 0.9 V. Inset b shows i-t curves recorded with CO-covered Pt(332) and Pt(997) electrodes as the potential is stepped from 0.1 to 0.7 V. All electrolytes are CO-free 0.5 M H2SO4.

In order to have a detailed understanding of the role of steps in CO oxidation, atomic level information on the CO adlayer structure on stepped surfaces is essential. Therefore, we conducted in situ STM imaging of CO molecules adsorbed on Pt(332) and Pt(997) electrodes in H2SO4 with and without CO in the solution. 11



3.2. STM Imaging of Pt(332) and Pt(997) electrodes in sulfuric acid 3.2.1. Bare and CO-coated Pt(332) electrode. Figure 3a shows constant-current STM images collected over the as-prepared Pt(332) electrode at 0.1 V in 0.5 M H2SO4, revealing 150-nm-wide terraces with long protruded (z = 0.23 nm) stripes running vertically. Two high-resolution STM scans shown in Figures 3b and 3c reveal (111) stripes aligned on top of a terrace. On average, the (111) facet on the Pt(332) surface is ~1.3 nm wide, as measured between the peaks of two neighboring steps, while the step heights of the (111) facets vary from facet to facet, with a typical value near 0.2 nm. These dimensions roughly equal the ideal values (inset in Figure 1). But at the center of Figure 3c, there are facets as wide as 2 nm, which indicates local restructuring of this electrode. This finding resembles that seen with Pt(997) in vacuum,67 which is associated with the adsorption of impurities, such as O2, during the annealing and quenching processes in vacuum. Close examination of Figure 3c reveals that most step edges are rough and randomly decorated by nano-islands, which contrast markedly with the smooth step edges seen in vacuum.45 This is probably due to the annealing-and-quenching process used to pretreat such stepped Pt surfaces, which results in oxidation and a high density of defects, as reported with Pt(111), Pt(10,10,9) and Pt(11,10,10) surfaces.42, 68 The Pt(557) surface exposed to 1 torr O2 in vacuum also undergoes roughening.45 At 0.1 V the oxide layer on Pt(332) is already reduced and the electrode surface is occupied by underpotentially deposited hydrogen atoms, but the altered Pt(332) surface is 12


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not restored to the ideal state (Figure 3). The very narrow (111) facet on this Pt(332) surface makes it difficult to achieve atomic-resolution STM images on this substrate. According to cross-section analysis in Figure 3d, the (111) facet is atomically smooth. Although the morphology of the Pt(332) sample is largely expected, a high density of kinks on steps is observed. These local defects are clearly revealed by real-space STM imaging (Figure 3). The present results reveal that the surface morphology of the as-prepared Pt(332) electrode in an electrochemical environment is far from the ideal structure usually envisioned, possibly because of restructuring during the course of the sample annealing and quenching processes. The structures of stepped electrodes with strongly adsorbed-species, such as iodine and CO, will be discussed next. Unlike hydrogen, strongly-adsorbed species greatly modify the step structures of Pt electrodes.

Figure 3. In situ constant-current STM images collected over the as-prepared Pt(332) 13



electrode at 0.1 V in 0.5 M H2SO4. The arrows marked in panels a - c point in the directions. (111) facets in the marked area (panel c) are wider than 1.3 nm. Panel d shows the corrugation profile along the broken line marked at the upper end of panel b. The bias voltage and tunneling current are 300 mV and 1 nA.

The structure of Pt(332) is investigated by STM imaging at 0.1 V in CO-saturated 0.5 M H2SO4. Terraces spanning hundreds of nanometers and 2~3 steps are formed close to one another at the upper and lower ends of Figure 4a. These features are likely due to polishing damage.69 Two steps in the upper section (S1) of Figure 4a are straight and aligned in the direction, while three steps in the lower end (S2) are relatively rougher. (These two groups (S1 and S2) are rotated by 30º from each other.) A high-resolution STM scan (Figure 4c) on the terrace site reveals distinct (111) facets. In strong contrast to the poorly-defined step morphology seen in Figure 3c, the steps are mostly straight, with random breaks and protruded sections, as shown in Figure 4b and 3c. On average, the (111) facet is 1.25 nm wide and punctuated by steps 0.17 nm height, which are roughly consistent with expected values for the Pt(332) surface (inset in Figure 1). Comparing to the images presented in Figure 3, we see that the bare Pt(332) electrode is altered by the adsorption of CO molecules, yielding a much better-defined morphology. This change suggests vast relocations of Pt atoms by the adsorbed CO molecules.

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Figure 4. In situ STM images acquired with a Pt(332) electrode at 0.1 V in CO-saturated 0.5 M H2SO4 (a-c). Groups of two and three elongated step lines (S1 and S2) are shown in the upper and lower ends of panel a. All marked arrows point to the directions.

Three representative high-resolution STM images of the CO-saturated Pt(332) electrode surface are displayed in Figure 5. There are distinct striped (111) facets whose widths vary between 0.7 and 2.5 nm (Figures 5a and b). All (111) facets shown in Figure 5a are decorated with ordered CO adlayers. Close examination of Figure 5b reveals three typical (111) facets denoted by  , and , which are 1.3, 1.8, and 2.3 nm wide (5, 8 and 11 Pt atoms, respectively) on the (111) facets. The most populated one is the  domain, whose width of 1.3 nm is close to the ideal value of Pt(332) terraces. Occasionally, a much narrower facet is also noted, marked by , whose 0.7 nm width is too small to accommodate a whole (2  2) unit cell. It is the least common, and possibly least stable, surface structure. Regardless of the width of the (111) facet, the step edge is always straight and aligned in the direction. Close examination of Figure 5b reveals that the Pt atoms at all edges are terminated with CO molecules spaced by 0.56 nm, or 2 times the Pt-Pt distance. CO molecules adsorbed on the edges exhibit roughly the same corrugation heights, as revealed 15



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by the profile (Figure 5c) taken along the step line, which is marked by the broken line shown in Figure 5b. This implies that they are adsorbed in the same configuration, linearly bonded to Pt atoms. Evidently, adsorbed CO molecules cause restructuring of the kinked step edges into straight (110) edge segments, and modification of the width of the (111) facet and step structures of the Pt(332) electrode. This effect is similar to that on the COmodified steps on Pt(111) electrode.37 In contrast to the smoothing effect induced by continuous potential cycling in CO-saturated solution, dosing with CO molecules at fixed potential enables migration of Pt atoms at step edges and transformation of crooked steps (Figure 3c) into (110)-oriented ones on Pt(332) (Figure 4c and 5a,b). This may be due to much narrower terraces on these surfaces, which leads to stronger adsorbate-adsorbate and adsorbate-edge interactions in the confined spaces.23,

70

These results obtained in

electrochemical environments contrast markedly with the roughening effect of adsorbed CO on Pt(557) in vacuum at high CO pressure ( > 1 torr).42 Despite the short-range ordering (< 25 Å) of the CO adlayer, high-resolution STM imaging succeeds in revealing details of the surface structure, as illustrated by the highresolution scan over the  domain (Figure 5d). A corresponding model of this STM image is shown in Figure 5e. Two neighboring step edges and a trough are marked by two solid lines and a dotted line. A (2  2) – 3CO structure is identified, whose unit cell is denoted by the marked rhombus containing four bright corners and 2 dimmer spots. These features are attributed to atop and 3-fold CO admolecules, respectively, as seen with Pt(111).47 Besides such well-known adlayer structures, protrusions with intensity similar to the 3-fold 16


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CO molecules are noted in the area above the (2  2) unit cell. In particular, the row of weaker spots denoted by the dotted line is ascribed to CO molecules adsorbed at the trough of a step. They can coordinate to 3-fold sites on the microfacet at the step, in agreement with previous IR studies for CO adsorption on such surfaces with low CO coverage (only steps are occupied).13 These features are represented by yellow circles in the model (Figure 5e), which differs significantly from the (2  2) structure. The CO coverage calculated from this STM image is 0.7 ML, which is slightly higher than that determined from cyclic voltammetry (0.64 ML). The difference is likely due to the existence of (111) facets with various widths on the Pt(332) surface and to the existence of CO in the solution during STM measurements. The coverage of CO at the step is estimated to be 0.68 ML on Pt(332). This value is roughly consistent with the 0.7 ML observed for (110) step sites of Pt(hkl), which is much higher than that for (100) step sites (ca. 0.4 ML).15 Based on the coverage, it is suggested that most CO molecules on (110) step sites are in the COL configuration, and those on (100) step sites are in the bridgebonded configuration. The preference of CO for hollow sites in the step trough of Pt(332) is probably induced by the Smoluchowski effect,18,

71, 72

according to which there is a

redistribution of surface charge at steps, with a lower d-electronic density at the upper part of the steps and accumulation of charge density at the bottom of the steps. At lower potentials (electrode with higher charge density), CO prefers hollow sites over top sites,47, 73-75

because hollow site CO coordination is more stabilized at larger negative electrode

potentials by the greater dπ- 2π* back-bonding occurring under these conditions. 17



Figure 5. In situ molecular resolution STM images (a and b) recorded with the Pt(332) electrode at 0.1 V in CO-saturated 0.5 M H2SO4. The marked arrow in panel a indicates the direction of . (111) facets marked in   and  are 1.3, 1.8, and 2.3 nm wide. All ordered structures are (2  2) – CO, denoted by the marked rhombus in panel b (a zoomin of panel a). Panel c shows the corrugation profile along the broken line marked at the lower end of panel b. Panel d shows the high-resolution STM scan over the  domain. A rectangular molecular arrangement is seen at the trough. The red and yellow circles in the ball model (panel e) denote CO molecules adsorbed in the peak and trough of a step. The typical STM imaging conditions are 300 mV in bias voltage and 1 nA feedback current.

3.2.2. CO-coated Pt(997). Figure 6a-c shows the STM results obtained with the as18


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prepared Pt(997) electrode at 0.1 V in CO-saturated 0.5 M H2SO4. From Figure 6a, we see that most steps that are 0.23 nm high are poorly aligned and rough, except the ones aligned in the direction. These are seen more clearly in Figure 6b. The (111) facets are clearly observed in the high-resolution STM scan shown in Figure 6c. They are 1.8~2.5 nm wide, separated by 0.1-0.2 nm height steps, as compared with the ideal values 2 nm width and 0.22 nm height for the Pt(997) surface (inset in Fig 1a). Similar to step structures of Pt(332) described above, CO adsorption also results in straight step lines aligned in the direction. A high density of kinked sites is also noted.

Figure 6. In situ STM images acquired with a Pt(997) electrode at 0.1 V in CO-saturated 0.1 M H2SO4 (a-c). The arrows marked in panel a-c point to the direction.

A high-resolution STM scan shown in Figure 7a, b reveals the spatial structure of CO molecules adsorbed on (111) facets. The ordered CO array is hexagonal with an intermolecular spacing of 0.6  0.3 nm, which is two times the Pt-Pt distance of 0.278 nm, indicating a (2  2) - CO structure on the (111) terraces. This is also apparent in the profile (Figure 7d) taken along the step line marked by a broken line (d) shown in Figure 7b. Only 19



the four corners of the unit cell are imaged by the STM; CO molecules adsorbed at 3-fold hollow sites are not seen in this experiment. This would give a CO coverage of nearly 0.75, which is consistent with the CV results (Fig. 2). It should be emphasized that, due to the confined width in the step trough, the structure of CO in this region is a little different from that on a (111) terrace. CO molecules adsorbed on the edges exhibit roughly the same corrugation heights, as revealed by the profile (Figure 7e) taken along the step line marked by the broken line(e) shown in Figure 7b. This analysis implies that they are also adsorbed in the same configurations, linearly bonded to Pt atoms. A ball model corresponding to the (2  2) structure is depicted in Figure 7c with only the corner CO molecules shown for simplicity. In the step trough, CO molecules are also adsorbed in multifold sites just like those on Pt(332). Comparing Figure 7b, c with Figure 5d, e, it is clear that the domain size of the (2  2) structure is three times larger on Pt(997) than on Pt(332). While the domain sizes of the (2x2) structures on both electrodes are much smaller than that on Pt(111), due to the disruption of flat (111) terraces by steps and kinks.

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Figure 7. In situ STM images acquired with a Pt(997) electrode at 0.1 V in CO-saturated 0.5 M H2SO4 (a-b). A ball model is shown in (c) to account for the structure seen on the (111) facet. Blue and red circles represent Pt atoms and CO molecules, respectively. Yellow circles represent CO molecules that are proposed to reside at the troughs of steps. Panel d and e show the corrugation profiles along the broken lines (d and e respectively) marked at panel b.

The STM results obtained with stepped Pt surfaces with and without CO adsorption are summarized as follows: i) STM imaging of Pt(332) at 0.1 V in 0.5 M H2SO4 reveals that the (110) steps are severely kinked. Adsorption of CO molecules at 0.1 V re-orients steps on Pt(332) and Pt(997) to the (110) direction. ii) The local structure of the CO adlayer near steps differs greatly from that of (2  2) on the terrace. CO molecules are adsorbed at atop and 3-fold hollow sites on the upper and lower edges of a step. CO molecules are adsorbed more strongly at multifold sites at the step trough than on terraces. iii) CO 21



adsorption straightens steps on Pt(997) and Pt(332), but kinks at random breaks of step lines and protruded sections (see in Fig. 4c, 5a, 5b, 6c) are abundant. The latter can be important catalytic sites.39 The different CO bonding configuration and the significantly smaller saturated coverage of COad at the step trough of the stepped Pt surface compared with that on terraces reveals that the step trough has a special electronic structure, and the binding of CO to such sites is much weaker than the binding to terrace sites. This is probably the origin for the higher CO oxidation activity of stepped Pt surface comparing with that of Pt(111).9, 37-39

3.2.3. Iodine-coated Pt(332) and Pt(997) electrodes. Iodine adsorbs strongly on a Pt surface and passivates it. For this reason, iodine on Pt has been used as a model system in the STM study of Pt electrodes. Three iodide adlattices have been identified on Pt(111): symmetric and asymmetric a (3  3) with θ = 0.44, a (√7  √7)R19o – I with θ = 0.43, and a (√3  √3)R30º - I with θ = 0.33 at potentials where hydrogen evolves.76-78 In order to substantiate the effect of adsorbate on the structure of the stepped Pt surface in electrochemical environment, we examined Pt(332) and Pt(997) electrodes modified with iodine. The obtained results are compared with those observed following CO modification of the electrode surface. STM images of Pt(997) and Pt(332) electrodes in 0.5 M H2SO4 + 1 mM KI at 0.1 V are given in Figure 8. Iodide ion undergoes spontaneous, oxidative adsorption on these electrodes, as reported with the Pt(111) electrode.76, 78-80 Striped (111) facets defined by 22


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seesawed steps are noted on both electrodes, which contrasts markedly with the linear steps seen with CO modification (Figure 5a and 6c). This contrast is shown to stem from the different spatial structures of these adsorbates and the fact that Pt atoms located at step edges are mobile under the influence of adsorbates at room temperature. On average, the (111) facets on Pt(332) and Pt(997) are 1.3 and 2 nm wide, respectively, but wider and narrower facets are also seen. Although iodine adatoms form several ordered lattices, including (7  7)R19.1º (denoted as 7 hereafter), as well as symmetric and asymmetric (3  3) structures76, 78-80 on Pt(111), it is unknown how iodine adatoms would arrange when the width of the (111) facet shrinks to a dimension comparable to the unit cell of an ordered iodine structure. Compared with Pt(111), which has a terrace width of a few hundred nanometers, these stepped surfaces have flat (111) facets of 1 to 2 nm, and it takes more effort to obtain highquality STM atomic images on these rougher surfaces. An extraordinarily wide (111) facet (~4.4 nm) is found on Pt(997), where two ordered hexagonal arrays are identified (Fig. 8b). The close-packed rows of spots in these two domains are rotated from each other by 38º, as indicated in Figure 8b. The nearest neighbor spacings are 0.73 nm in these two domains. These arrays are rotational domains of the (7  7)R19.1º - I structure. Although only the corner iodine adatoms occupying atop sites are imaged under the present condition, it is likely that this is the same as that formed on Pt(111). Two I adatoms residing at 3-fold hollow sites inside the unit cell are not imaged. This 7 adlattice is also seen on narrower (111) facets (Fig. 8c). It is the major structure, along with some minor disordered arrays 23



found on narrow facets. The lower intensity corresponding to iodine adatoms located at the trough of steps stems from their lower physical height. In Figure 8d-e, we show atomic-resolution STM images acquired with the Pt(332) electrode at 0.1 V in 0.5 M H2SO4 + 1 mM KI. On the (111) facet, ordered iodine arrays are imaged, where strings of 3 or more iodine atoms are formed along the direction or 30º rotated from the axis. The average distance between two neighboring iodine adatoms is 0.48 nm, indicating a (3  3)R30º - I structure. This contrasts with the 7 structure seen on the Pt(997) electrode (Figure 10c) and the (3  3) structure seen with Pt(10,10,9),68 which has a (111) facet that is 20 Pt atoms wide. It seems that the ~1.3 nm wide (111) facet on the Pt(332) surface could not maintain a stable 7 structure, and the iodine adatoms were forced to adopt an ordered structure with a smaller unit cell. Apparently, this 3 structure is not the favored iodine adlattice on the much wider (111) plane under the present condition. Close examination of step edges in Figures 8c and e reveals that all spots are corners of the 7 and 3 iodine structures. This is possible only under the condition that Pt atoms at step edges are shuffled to sites where an ordered iodine structure needs them. In other words, the integrity of the iodine structure determines the structure of the step. A ball model accounting for the 3 structure is shown in Figure 8f, where iodine adatoms are placed on the 3-fold hollow sites of (111) facet.

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Figure 8. In situ STM images acquired with Pt(997) (a-c) and Pt(332) (d,e) electrodes at 0.1 V in 0.5 M H2SO4 + 1 mM KI. Step edges of both electrodes are seesaw-like. Panel b shows one (111) facet supporting two ordered domains with the close-packed atomic rows rotated from each other by 38º. Panels c and e show (7  7)R19.1º and (3  3)R30º I structures. Panel f is a ball model of the (3  3)R30º - I structure with Pt atoms and I adatoms are colored in blue and red. Similar to CO adsorption, the strongly adsorbed iodine reconfigures the steps on Pt(997) and Pt(332). In contrast with the new spatial structure of CO found at these Pt electrodes, iodine is adsorbed on the (3  3)R30º structure, as found on Pt(111). These results reveal that both the nature of the adsorbate and the width of (111) facet affect the spatial structure of adsorbate and the steps structure.

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4. CONCLUSION The interfaces of stepped Pt(997) and Pt(332) electrodes in 0.5 M H2SO4 have been examined by voltammetry and in situ STM. Our results confirm that these electrodes with very different step densities result in different amounts of H atoms deposited (HUPD) at 0.125 V, and different potentials where adsorbed CO molecules are stripped off. Highresolution STM imaging reveals poorly-defined atomic structures at step sites on these two electrodes covered with hydrogen atoms. The widths of the (111) facets on Pt(332) and Pt(997) electrodes vary notably, possibly resulting from restructuring in the course of the annealing process. The atomic structures at steps are altered notably by the adsorption of iodine and CO, which are known to interact strongly with Pt electrodes. The rough step edge on these stepped Pt electrodes at 0.1 V in 0.5 M H2SO4 became straight and aligned in the direction in the presence of CO. Evidently, the adsorption of CO molecules at steps on Pt(997) and Pt(332) surfaces induces migration of Pt atoms at steps. CO molecules adsorbed on the edge arrange themselves in an orderly fashion, and their structure repeats at distance intervals equal to two times the in-plane Pt-Pt distance on both electrodes. CO molecules sitting on peak and trough of step are adsorbed in linearly and 3-fold bonded configurations. A high density of kinks is observed on these CO-covered electrodes. More importantly, in situ STM reveals that CO molecules are more loosely adsorbed at troughs than at peaks of steps. This can facilitate generation of OH species and their subsequent reaction with CO admolecules at positive potentials. Finally, in contrast with the straight 26


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steps seen with CO-coated Pt electrodes, steps become clearly zigzagged upon iodine adsorption. Therefore, it is fair to state that in situ STM imaging is indispensable in characterizing local surface features such as steps, which can act as the active sites in electrocatalysis.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] [email protected] Notes The authors declare no competing financial interest

ACKNOWLEDGEMENTS This research was funded by the Ministry of Science and Technology of ROC (MOST 1072113-M-008-005), National Natural Science Foundation of China (no. 21473175) and 973 Program from the Ministry of Science and Technology of China (no. 2015CB932301).

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As Probed by In-situ Scanning Tunneling Microscopy

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