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Surface X-ray Scattering of Stepped Surfaces of Platinum in an Electrochemical Environment: Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111) Nagahiro Hoshi,*,† Masashi Nakamura,† Osami Sakata,‡ Akira Nakahara,† Keita Naito,† and Hiroyuki Ogata† †
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522 Japan ‡ Research and Utilization Division, Japan Synchrotron Radiation Research Institute/SPring-8, Kouto, Sayo, Sayo-gun, Hyogo 679-5198, Japan ABSTRACT: Real surface structures of the high-index planes of Pt with three atomic rows of terraces (Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)) have been determined in 0.1 M HClO4 at 0.1 and 0.5 V(RHE) with the use of surface X-ray scattering (SXS). The surfaces with two atomic rows of terraces, Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111) = 2(111)(100), are reconstructed to a (1 2) structure according to previous studies. However, the surfaces with three atomic rows of terraces have pseudo-(1 1) structures. The interlayer spacing between the first and the second layers, d12, is expanded 13% on Pt(331) compared to that of the bulk, whereas it is contracted 37% on Pt(511). The surface structures do not depend on the applied potential on either surface.
1. INTRODUCTION The catalytic activity and the selectivity of electrochemical reactions strongly depend on the surface structures of the electrodes.1-4 Well-defined single-crystal electrodes play a key role in the determination of the structure of the active sites of reactions. Real surface structures have been determined in ultrahigh vacuum (UHV) on the low-index planes of Pt5-7 as well as the high-index planes8 using low-energy electron diffraction (LEED). In aqueous electrolytes, the structure of the lowindex planes of Pt has been determined with the use of scanning tunneling microscopy (STM)9-15 and surface X-ray scattering (SXS).16-18 Pt(111) and Pt(100) have an unreconstructed (1 1) structure, whereas Pt(110) has a (1 1) or (1 2) structure depending on the cooling conditions after annealing.19,20 The interlayer spacing between the first and second layers, d12, is the same as that of the bulk in the double-layer region on Pt(111) and Pt(100), whereas it is expanded in the adsorbed hydrogen region. On Pt(110), the topmost rows of Pt atoms are expanded into the electrolyte in the double-layer region, and additional expansion occurs because of the adsorption of hydrogen atoms.19 The expansion increases as Pt(111) (2%) < Pt(100) (2.5%) < Pt(110) (10%) in 0.5 M H2SO4. In UHV, d12 is contracted on Pt(110) because of the attractive interaction between the first and second layers due to the localization of electrons in the r 2011 American Chemical Society
second layer.21-24 The adsorbed hydrogen atoms tend to decrease the localization of electrons, causing the expansion in the adsorbed hydrogen region.19 On the high-index planes of Pt, there have been few reports on the real structure in aqueous electrolytes. Pt(311) = 2(100)(111) is reconstructed to the (1 2) structure in 0.1 M HClO4.25 This result is the same as that obtained by LEED and field microscopy in UHV.26-28 The value of d12 is contracted compared with the interlayer spacing of the bulk. The in-plane structure and the interlayer spacings do not depend on the applied potentials on Pt(311), which contradicts the results on the low-index planes of Pt. Pt(310) = 3(100)-(110) has the unreconstructed (1 1) structure, and no relaxation is found within the experimental error.29 Adsorbed CO induces the expansion of d12 on the low-index planes of Pt.30-34 However, adsorbed CO does not affect the value of d12 and the in-plane structure of Pt(310).29 For the elucidation of the real surface structures of the surfaces with three atomic rows of terraces in aqueous electrolytes, we have recorded the SXS on Pt(331) = 3(111)-(111) and Received: January 17, 2011 Revised: February 9, 2011 Published: March 07, 2011 4236
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Figure 1. Hard-sphere models of Pt(331) and Pt(511).
Pt(511) = 3(100)-(111) in 0.1 M HClO4. Figure 1 shows the hard-sphere models of the bulk-terminated structures of the surfaces examined.
2. EXPERIMENTAL SECTION A single-crystal bead of Pt with a cross-sectional area of 0.20-0.25 cm2 was prepared with the method reported previously.35-37 The crystal was oriented and polished with diamond slurries down to 0.3 μm. The polished surface was annealed in a H2/O2 flame at about 1300 °C to remove the distortion caused by the mechanical polishing and cooled to room temperature in an Ar-H2 atmosphere with a volume ratio of 9:1. Electrolytic solutions were prepared using ultrapure water treated with Milli-Q plus low TOC (Millipore) and suprapure-grade chemicals (Merck). The purity of Ar was higher than 99.9999%. All of the potentials were referred to the RHE. SXS was measured in a 0.1 M HClO4 solution in which no anion is strongly adsorbed on Pt surfaces. SXS measurements were performed at BL13XU of SPring-8 for surface and interface structural determinations. The electrochemical cell for SXS was made of PTFE, into which counter and reference electrodes were inserted. The top of the cell was covered with a polypropylene window onto which the electrode surface is pushed during the SXS measurement. The cell was set on a multiaxis diffractometer in the hatch 1 of BL13XU.31 The X-ray energy was 20 keV. Integrated intensities were measured by rocking scans around the axis of the surface normal. The diffracted beam was measured using the symmetric ω = 0 mode.38 The electrode potential was set to 0.1 and 0.5 V. The following rods were measured in 0.1 M HClO4: Pt(331) = 3(111)-(111) (H, K) = (0, 1) (1, 1) (-1, 0) (2, 2) (1, 3) (0, 2) Pt(511) = 3(100)-(111) (H, K) = (1, 2) (1, 1) (-1, 1) (-1, 0) (0, -1) (3, -2) Specular rods (0 0 L), which are sensitive to the variation of the interlayer spacing, were not measured in this work. However, structural refinements using several independent nonspecular rods also give the same information on the interlayer spacing as specular rods. Many papers have determined interlayer spacings without specular rod measurements.39-45 The intensities reported herein are corrected for Lorentz and polarization factors.38 ROD software was used for the structure refinements.46 A monoclinic coordinate system was used for Pt(331) and Pt(511) crystals in which the reciprocal wave vector was Q = Ha* þ Kb* þ Lc*, where L is along the surface normal direction. On Pt(331), a* = 10.3901 nm-1, b* = 23.2329 nm-1, c* = 3.6734 nm-1, a = 0.6204 nm, b = 0.2775 nm, and c = 1.710 nm (R = 77.079°, β = 90°, and γ = 90°). The unit cell was composed of 19 layers of Pt. A total of
Figure 2. Voltammograms of Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111) electrodes in HClO4 saturated with Ar. Scanning rate = 0.050 V s-1. 211 (0.5 V) and 219 (0.1 V) reflections for the CTR measurement were used for the structural analysis. On Pt(511), a* = 8.7159 nm-1, b* = 23.0602 nm-1, c* = 3.0815 nm-1, a = 0.7341 nm, b = 0.2775 nm, c = 2.0389 nm (R = 100.89°, β = 90°, γ = 90°). The unit cell was composed of 27 layers of Pt. A total of 297 (0.5 V) and 280 (0.1 V) reflections for the CTR measurement were used for the structural analysis. The surface structure was optimized by changing nine structural parameters between the first and third layers, as well as scale and roughness factors. The Debye-Waller and roughness factors were 0.2 and 0, respectively.
3. RESULTS AND DISCUSSION Voltammograms of Pt single-crystal electrodes give peaks characteristic of their orientations in the adsorbed hydrogen 4237
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Figure 3. Representative crystal truncation rods (CTRs) on the Pt(331) = 3(111)-(111) electrode in 0.1 M HClO4 saturated with Ar. Blue and red squares show the data at 0.1 and 0.5 V, respectively. Solid and dotted lines represent the calculated intensity based on the pseudo-(1 1) and (1 2) models, respectively.
Table 1. Nine Structural Parameters of the Optimized Structure of Pt(331) = 3(111)-(111)a
Table 2. Interlayer Spacing between the mth and nth Layers, dmn, on Pt(331) = 3(111)-(111)
a/Å
b/Å
c/Å
d12/Å
d23/Å
d34/Å
dbulk/Å
first layer second layer
-0.13 ( 0.03 -0.19 ( 0.03
0.14 ( 0.02 0.01 ( 0.01
0.17 ( 0.02 0.05 ( 0.02
1.02 ( 0.04
0.94 ( 0.03
0.91 ( 0.01
0.90
third layer
-0.04 ( 0.01
0.02 ( 0.01
0.01 ( 0.01
a
The parameters are given by the deviation from the bulk-terminated positions.
region in 0.5 M H2SO4. The voltammograms of the prepared electrodes agreed with those reported previously;36,47-51 we judged that the surfaces are well-defined. SXS was measured in 0.1 M HClO4 in which no anion is strongly adsorbed on the Pt electrodes. Figure 2 shows voltammograms of Pt(331) and Pt(511) in 0.1 M HClO4. The potentials at which SXS was measured are shown by the arrows. 3.1. Pt(331) = 3(111)-(111) Electrode. Figure 3 shows the representative crystal truncation rods (CTRs) of the Pt(331) electrode. The CTRs at 0.5 V are on the same line as those at 0.1 V, indicating that the structure at 0.5 V is the same as that at 0.1 V. These results differ from those on the low-index planes of Pt where the interlayer spacings are expanded in the adsorbed hydrogen region compared to that in the double-layer region. No potential dependence of the interlayer spacing is found on the high-index planes reported previously: Pt(311) = 2(100)-(111)25 and Pt(310) = 3(100)-(110).29
The CTRs are reproduced well using the pseudo-(1 1) model, as shown by the solid lines in Figure 3. The (1 2) model cannot reproduce the data. The same results were obtained in the other CTRs. These results support the fact that the Pt(331) surface has the pseudo-(1 1) structure. Table 1 shows the structural parameters of the optimized model of Pt(331). The in-plane positions of the atoms are shifted. Atoms are lifted between the first and the third layers, but the displacement along the c axis decreases with the depth of the layer. Therefore, the value of d12 is expanded 13% compared with the interlayer spacing of the bulk, as shown in Table 2. Such a large expansion is not found on the low-index and high-index planes of Pt in the double-layer region. The optimized model of Pt(331) is shown in Figure 4. 3.2. Pt(511) = 3(100)-(111) Electrode. Figure 5 shows the representative CTRs of the Pt(511) = 3(100)-(111) surface at 0.1 and 0.5 V in 0.1 M HClO4. The CTRs at both potentials are on the same lines. The surface structure of Pt(511) is independent of the applied potential, which also applies to the other highindex planes. We tried to fit the data on the basis of the (1 1) and (1 2) models. Pt(511) has two types of (1 2) structures as shown in 4238
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Figure 4. Optimized model of Pt(331) = 3(111)-(111).
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Table 3 summarizes the deviation of the structural parameters from the bulk positions on Pt(511). The deviation of the in-plane
Figure 6. Hard-sphere models of the unreconstructed and the reconstructed Pt(511) = 3(100)-(111) surfaces.
Figure 5. Crystal truncation rods (CTRs) of the Pt(511) = 3(100)-(111) surface in 0.1 M HClO4 saturated with Ar. Blue and red squares show the experimental data at 0.1 and 0.5 V (RHE), respectively. Green, solid, and broken black lines present the CTRs calculated on the basis of (1 1), (1 2)-1, and (1 2)-2 models. The models are shown in Figure 6. 4239
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Table 3. Nine Structural Parameters of the Optimized Structure of Pt(511) = 3(100)-(111)a a/Å
b/Å
c/Å
first layer
0.03 ( 0.01
0.05 ( 0.02
-0.12 ( 0.03
second layer third layer
0.12 ( 0.03 0.04 ( 0.02
-0.01 ( 0.01 0.04 ( 0.01
0.15 ( 0.04 0.14 ( 0.03
a
The parameters are given by the deviation from the bulk-terminated positions.
Table 4. Interlayer Spacing between the mth and nth Layers, dmn, on Pt(511) = 3(100)-(111) d12/Å
d23/Å
d34/Å
dbulk/Å
0.49 ( 0.07
0.77 ( 0.07
0.90 ( 0.03
0.76
Figure 8. Hard-sphere models of the unreconstructed and reconstructed structures of the surfaces with two atomic rows of terraces.
Figure 7. Optimized model of Pt(511) = 3(100)-(111).
positions is small in the first layer. In the vertical direction, however, the atoms in the first layer move downward, and those in the second and third layers are lifted. Thus, the value of d12 is remarkably contracted 35% on Pt(511), as shown in Table 4. Such a large contraction has not been reported previously. Figure 4 illustrates the direction of the deviation of atomic positions on Pt(511). The atoms in the third layer are lifted 0.15 Å (Table 3), whereas those below the fourth layer remain at the bulk positions. The local relaxation down to the third layer causes the large expansion of d34 (18%) in Table 4. 3.3. Motive Force of the Reconstruction. Figure 8 shows the hard-sphere models of the (1 1) and the (1 2) structures of the surfaces with two atomic rows of terraces in the bulk-terminated structure: Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111). The (111) terrace width increases from 2 to 3 after the reconstruction. The surface energy of the (111) structure is low because of the close-packed structure; the surface energies of Pt(110) and Pt(311) are stabilized by the increase in the (111) terrace width. The (1 1) and (1 2) structures of Pt(511) = 3(100)-(111) and Pt(331) = 3(111)-(111), which have three atomic rows of terraces, are shown in Figure 9. On Pt(511), the (100) terrace width increases after the reconstruction; however, the reconstruction
does not affect the width of the (111) structure in this step. The reconstruction increases the (111) terrace width of the lower layers on Pt(331) with the consumption of the (111) terrace of the upper layers. The total terrace width does not change on Pt(331). The discussion above supports the fact that the increase in the (111) terrace width induces the reconstruction to the (1 2) structure on surfaces with two atomic rows of terraces: Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111). 3.4. Possible Factors for the Relaxation. Previous studies on the stepped surfaces reported that the value of d12 is contracted compared with the interlayer spacing of the bulk because of the attractive force resulting from the electron transfer from the topmost atoms to the bottom of the steps.21-24 According to our SXS results, d12 on Pt(511) is contracted 35%, whereas that on Pt(331) is expanded 13%. Adsorbates such as hydrogen atoms and CO cause the relaxation on the low-index planes of Pt; we tried to reproduce the relaxation on the high-index planes by the incorporation of the adsorbed water molecule in the model, which is the only adsorbate at 0.5 V (RHE). We used a Vienna ab initio simulation package (VASP) to reproduce the relaxations.52-54 Adsorbed water molecules on the on-top site of the step atoms induce the contraction of d12 on Pt(511); water adsorption on the step may be the cause of the remarkable contraction on Pt(511). The adsorbed water at the terrace reduces the contraction on Pt(331) compared to that in the model without adsorbed water. However, we cannot reproduce the expansion of d12 on Pt(331) by the incorporation of adsorbed water. Other factors such as the applied potential might cause the remarkable expansion of Pt(331). A full-scale theoretical calculation including the applied potential will be necessary to elucidate the cause of the expansion of d12 on Pt(331). 4240
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Figure 9. Hard-sphere models of the unreconstructed and reconstructed structures of the surfaces with three atomic rows of terraces.
4. CONCLUSIONS The real surface structures of Pt(331) = 3(111)-(111) and Pt(511) =3(100)-(111) have been determined using surface X-ray scattering (SXS) at 0.1 and 0.5 V(RHE) in 0.1 M HClO4. Small shifts of the in-plane positions are found compared with the bulk-terminated positions on both electrodes; Pt(331) and Pt(511) have pseudo-(1 1) structures. The interlayer spacing between the first and second layers, d12, is expanded 13% on Pt(331) whereas that on Pt(511) is contracted 35%. ’ AUTHOR INFORMATION Corresponding Author
*Phone and Fax: 81-43-290-3384. E-mail: hoshi@faculty. chiba-u.jp.
’ ACKNOWLEDGMENT SXD measurements were supported by the Japan Synchrotron Radiation Research Institute (JASRI) under proposal numbers 2007A1258 and 2007B1227. This work was supported by Grants-in-Aid for Scientific Research 20550007 and partially by a grant from the New Energy and Industrial Technology Development Organization. ’ REFERENCES (1) Adzic, R. In Modern Aspects of Electrochemistry; White, R. E., Bockris, J. O’M., Conway, B. E., Eds; Plenum Press: New York, 1990; Vol. 21, Chapter 5. (2) Lamy, C.; Leger, J. M. J. Chim. Phys. 1991, 88, 1649–1671. (3) Markovic, N. M.; Ross, P. N., Jr. Surf. Sci. Rep. 2002, 45, 117–229. (4) Ye, S.; Kondo, T.; Hoshi, N.; Inukai, J.; Yoshimoto, S.; Osawa, M.; Itaya, K. Electrochemistry 2009, 77, 2–20. (5) Hagstrom, S.; Lyon, H. B.; Somorjai, G. A. Phys. Rev. Lett. 1965, 15, 491–493. (6) Sandy, A. R.; Mochrie, S. G. J.; Zehner, D. M.; Grubel, G.; Huang, K. G.; Gibbs, D. Phys. Rev. Lett. 1992, 68, 2192–2195. (7) Zhang, X. -G.; Van Hove, M. A.; Somorjai, G. A.; Rous, P. J.; Tobin, D.; Gonis, A.; MacLaren, J. M.; Heinz, K.; Michl, M.; Lindner, H.; Muller, K.; Ehsasi, M.; Block, J. H. Phys. Rev. Lett. 1991, 67, 1298–1301.
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