Langmuir 2007, 23, 10879-10882
10879
In Situ Surface X-ray Scattering of Stepped Surface of Platinum: Pt(311) Akira Nakahara,† Masashi Nakamura,† Kazushi Sumitani,‡ Osami Sakata,‡ and Nagahiro Hoshi*,† Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba UniVersity 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522 Japan, and Materials Science DiVision, Japan Synchrotron Radiation Research Institute/SPring-8, Kouto, Sayo, Sayo-gun, Hyogo 679-5198, Japan ReceiVed May 29, 2007. In Final Form: September 5, 2007 Surface structure of a stepped surface of Pt, Pt(311) ()2(100) - (111)), has been determined under potential control in 0.1 M HClO4 with the use of in situ surface X-ray scattering (SXS). The crystal truncation rods (CTRs) are reproduced well with the (1 × 2) missing-row model. Relaxation of surface layers, which is observed on the low-index planes of Pt, is not found on Pt(311) in the “adsorbed hydrogen region”. CTRs at 0.10 (RHE) have the same feature as those at 0.50 V, showing that the surface layers of Pt(311) have no potential dependence. Scanning tunneling microscopy (STM) also supports the (1 × 2) structure of Pt(311) in 0.1 M HClO4.
Introduction Pt is a major electrocatalyst for fuel cells because of its high catalytic activity and chemical stability. Activity of an electrochemical reaction strongly depends on surface structure,1-3 as is the case of a solid-gas interface.4 A stepped surface of Pt plays a key role for the estimation of the structure giving high catalytic activity, because its terrace and step structures can be modified systematically. It is important to determine the real surface structure of a stepped surface of Pt in a solid-liquid interface for the development of electrocatalysts with high activity. Surface structures of Pt have been widely studied in ultrahigh vacuum (UHV).5-7 A Pt(111) surface gives an unreconstructed (1 × 1) structure at room temperature, whereas the interlayer spacing between the first and second layers (d12) is expanded about 2.5%.8 The Pt(100) surface also has (1 × 1) structure at room temperature after the flame-annealing treatment, although the (1 × 1) structure transforms irreversibly to a quasi-hexagonal structure above 390 K.9 The expansion of interlayer spacing ∆d12 on Pt(100)-(1 × 1) is within 0.25% of the bulk spacing.10 Pt(110) exhibits a reconstructed (1 × 2) missing row structure at room temperature.11 The value of ∆d12 is contracted 18.4% compared to that of the bulk.12,13 * Corresponding author. E-mail:
[email protected]. † Chiba University. ‡ Japan Synchrotron Radiation Research Institute. (1) Adzic, R. 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) Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Rep. 1991, 14, 1-107. (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. (8) Van Hove, M. A. Physics of CoVered Solid Surfaces, Landolt-Boernstein; Springer-Verlag: Germany, 1999. (9) Norton, P. R.; Davies, J. A.; Creber, D. K.; Sitter, C. W.; Jackman, T. E. Surf. Sci. 1981, 108, 205-224. (10) Davies, J. A.; Jackman, T. E.; Jackson, D. P.; Norton, P. R. Surf. Sci. 1981, 109, 20-28 (11) Thiel, P. A.; Esrup, P. J.; Hubbard, A. T. The Handbook of Surface Imaging and Visualization; CRC Press: Boca Raton, 1995. (12) Sowa, E. C.; Van Hove, M. A.; Adams, D. L. Surf. Sci. 1988, 199, 174182.
In solid-liquid interfaces, many papers have reported real surface structures of the low-index planes of Pt with the use of scanning tunneling microscopy (STM)14-17 and surface X-ray scattering (SXS).18-20 Pt(111) and Pt(100) have a (1 × 1) structure.14,15,19 The surface structure of Pt(110) depends on the cooling condition after the flame-annealing treatment.21,22 The values of ∆d12 expand on all the low-index planes of Pt in the “adsorbed hydrogen region”. Surface relaxation of Pt(110) is more remarkable than of Pt(111) and Pt(100): the value of ∆d12 of Pt(110) shows the expansion of about 10% at 0.05 V(RHE), which is three times as large as those of Pt(111) and Pt(100). The interlayer spacing ∆d12 tends to expand in the liquid phase, which is opposite to the case in UHV.10 In the case of high-index planes, field-ion microscopy and low-energy electron scattering (LEED) show (1 × 2) periodicity of the Pt(311) ()2(100) - (111)) surface in UHV.23-25 Calculation based on density functional theory (DFT) predicts the stability of Pt(311)-(1 × 2) compared to Pt(311)-(1 × 1).26 The calculated value of ∆d12 shows significant contraction of -19% ()0.23 Å) on Pt(311)-(1 × 2). All the previous studies on Pt stepped surfaces have been done ex situ. No real surface structure has been determined in solid-liquid interfaces other than STM study on gold electrodes.27 We report the real structure of the Pt(311) ()2(100) - (111)) (13) Vlieg, E.; Robinson, I. K.; Kern, K. Surf. Sci. 1990, 233, 248-254. (14) Sashikata, K.; Furuya, N.; Itaya, K. J. Vac. Sci. Technol., B 1991, 9, 457-464. (15) Tanaka, S.; Yau, S.-L.; Itaya, K. J. Electroanal. Chem. 1995, 396, 125130. (16) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648-1660. (17) Itaya, K. Prog. Surf. Sci. 1998, 58, 121-248. (18) Tidswell, I. M.; Markovic´, N. M.; Ross, P. N. Phys. ReV. Lett. 1993, 71, 1601-1604. (19) Tidswell, I. M.; Markovic´, N. M.; Ross, P. N. J. Electroanal. Chem. 1994, 376, 119-126. (20) Lucas, C.; Markovic´, N. M.; Ross, P. N. Surf. Sci. 1996, 340, L949L954. (21) Lucas, C.; Markovic´, N. M.; Ross, P. N. Phys. ReV. Lett. 1996, 77, 49224925. (22) Markovic´, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. Surf. Sci. 1997, 384, L805-L814. (23) Gaussmann, A.; Kruse, N. Surf. Sci. 1992, 266, 46-50. (24) Yamanaka, T.; Inoue, Y.; Matsushima, T. Chem. Phys. Lett. 1997, 264, 180-185. (25) Kose, R.; King, D. A. Chem. Phys. Lett. 1999, 313, 1-6. (26) Jenkins, S. J.; Petersen, M. A.; King, D. A. Surf. Sci. 2001, 494, 159-165. (27) Gao, X.; Hamelin, A.; Weaver, M. J. Surf. Sci. Lett. 1992, 274, L588L592.
10.1021/la701566w CCC: $37.00 © 2007 American Chemical Society Published on Web 09/29/2007
10880 Langmuir, Vol. 23, No. 22, 2007
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surface in an electrolytic solution under potential control with the use of in situ SXS and STM. Experimental Section A single crystal bead of Pt with a cross-sectional area between 0.20 and 0.25 cm2 was prepared with the method reported previously.28-30 The crystal was oriented and polished with diamond slurries down to 0.3 µm. The polished surface was annealed in an H2/O2 flame at about 1300 °C to remove distortions caused by the mechanical polishing and cooled down to room temperature in an atmosphere of Ar-H2 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 the potentials were referred to RHE. SXS was measured in 0.1 M HClO4 solution in which no anion is strongly adsorbed on Pt surfaces. SXS measurements were performed at BL13XU for surface and interface structural determination in SPring-8. The electrochemical cell for the surface X-ray scattering was set on multi-axis diffractometer in hatch 1 of BL13XU.31 The energy of the X-rays 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. The electrode potential was set at 0.1 and 0.5 V. The following rods were measured in 0.1 M HClO4: 0.5 V: (H, K) ) (1, 0) (2, -1) (1, -1) (-1, -1) (0, -1) (-1.5, 0) 0.1 V: (H, K) ) (1, 0) (2, -1) (1, -1) (-1, -1) (0, -1) The intensities reported herein are corrected for Lorentz and polarization factors.32 ROD software was used for the structure refinements.33 A monoclinic coordinate system was used for the Pt(311) crystal in which the reciprocal wave vector was Q ) Ha* + Kb* + Lc*, where a* ) 0.1366 nm-1, b* ) 0.2365 nm-1, c* ) 0.04827 nm-1, a ) 0.4806 nm, b ) 0.2775 nm, c ) 1.3016 nm (R ) 90°, β ) 90°, γ ) 106.78°), and L is along the surface normal direction. The unit cell was composed of 11 layers of Pt. A total of 146 (0.5 V) and 135 (0.1 V) reflections for the CTR measurement were used for the structural analysis. The surface structure was optimized by changing 9 structural parameters between the first and the third layers, scale factors, and roughness factor. The optimization was estimated using χ2 that is defined by the following formula: χ2 )
1 N-p
∑ hk
(
)
exp 2 2 |Fcalc hk | - |Fhk |
σhk
2
where N and p are the number of structure factors and free parameters in the model, respectively, and σhk is a statistical error. The statistical error does not contain systematic error due to variation of the surface quality and diffractometer misalignments, because the Pt(311) surface has p1 symmetry. The STM image was measured at 0.5 V in 0.1 M HClO4 using Nanoscope E (Digital Instrument). The tungsten tip was electrochemically polished in 4 M KOH solution using a dc power source. STM images were obtained in “height mode” (i.e., at constant current). The Pt(311) electrode was flame-annealed and transferred to the STM cell containing 0.1 M HClO4.
Results and Discussion Voltammograms of Pt electrodes give peaks corresponding to the adsorption and desorption of hydrogen atoms between 0.05 (28) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205-209. (29) Furuya, N.; Koide, S. Surf. Sci. 1989, 220, 18-28. (30) Hoshi, N.; Tanizaki, M.; Koga, O.; Hori, Y. Chem. Phys. Lett. 2001, 336, 13-18. (31) Sakata, O.; Furukawa, Y.; Goto, S.; Mochizuki, T.; Uruga, T.; Takeshita, K.; Ohashi, H.; Ohata, T.; Matsushita, T.; Takahashi, S.; Tajiri, H.; Ishikawa, T.; Nakamura, M.; Ito, M.; Sumitani, K.; Takahashi, T.; Shimura, T.; Saito, A.; Takahashi, M. Surf. ReV. Lett. 2003, 10, 543-547. (32) Altman, M. S.; Estrup, P. J.; Robinson, I. K. Phys. ReV. B 1988, 38, 5211-5214. (33) Vlieg, E. J. Appl. Crystallogr. 2000, 33, 401-405.
Figure 1. Voltammogram of the Pt(311) ()2(100) - (111)) surface in (a) 0.5 M H2SO4 and (b) 0.1 M HClO4. Scanning rate: 0.050 V s-1.
and 0.40 V. The potential and charge of the peaks strongly depend on the terrace and step structure of the surfaces.34 Figure 1a shows a voltammogram of the Pt(311) electrode in 0.5 M H2SO4 solution. The voltammogram was identical to that reported previously;29,35-38 the prepared electrode is confirmed to be welldefined. Figure 1b presents the voltammogram in 0.1 M HClO4. SXS measurements were done in the adsorbed hydrogen (0.1 V) and the double layer region (0.5 V). Coverage of adsorbed hydrogen is 0.85 at 0.10 V, whereas neither hydrogen nor anion is adsorbed at 0.5 V. Figure 2 shows representative CTRs of Pt(311) at 0.5 V. Calculated curves based on a (1 × 2) model give the lowest χ2. Refinement based on (1 × 1) and (1 × 3) structures cannot be fit to the data. If the surface is reconstructed to the (1 × 2) structure, scattering intensity will be observed at fractional order rod. Reflections from superstructure are found at (-1.5, 0, L) as shown in Figure 3. These results support the determination that the Pt(311) surface is reconstructed to a (1 × 2) structure in the liquid phase, as is the case of UHV. Calculated curves are slightly deviated from the experimental data in Figure 2. The STM image found a partially disordered structure on Pt(311) in 0.1 M HClO4. The deviation may be due to the partially disordered structures. It is probable, however, that the majority of the Pt(311) surface has a reconstructed (1 × 2) structure, because the calculated curves based on (1 × 1) and (1 × 3) models are significantly deviated from the data. The (1 × 2) structure was optimized by the least-squares method for determination of the detailed atomic coordinates. Lattice constants are initially set as follows: a ) 0.9612 nm, b ) 0.2775 nm, and c ) 1.3016 nm. Figure 4b shows the optimized structure: the areas of (111) facets are enlarged, and (100) facets tend to reconstruct to the psuedo-hex structure. The (1 × 2) missing-row reconstruction is lifted on clean (110) surfaces of Ir, Pt, and Au. The motive force of the reconstruction is attributed to the decrease of the surface energy due to the formation of (111) microfacets.26 The enlarged (111) facets of Pt(311)-(1 × (34) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245-253. (35) Motoo, S.; Furuya, N. Ber. Bunsen Ges. 1986, 91, 457-461. (36) Feliu, J. M.; Rodes, A.; Orts, J. M.; Clavilier, J. Polish J. Phys. Chem. 1994, 68, 1575-1595. (37) Markovic´, N. M.; Marinkovic, N. S.; Adzic, R. R. J. Electroanal. Chem. 1988, 241, 309-328. (38) Hoshi, N.; Hori, Y. Electrochim. Acta. 2000, 45, 4263-4270.
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Langmuir, Vol. 23, No. 22, 2007 10881 Table 1. Interlayer Spacingsa of Pt(311)-(1 × 2) at 0.1 and 0.5 V in 0.1 M HClO4 d12(111) /Å
d12(100) /Å
d23(111) /Å
d23(100) /Å
dbulk /Å
1.16
1.22
1.23
1.17
1.18
a
The values of dmn(111) and dmn(100) show interlayer spacing between m and n layers on (111) and (100) facets, respectively.
Figure 2. Representative CTRs of Pt(311) ()2(100) - (111)) at 0.5 V in 0.1 M HClO4. Triangles: integrated intensity of diffracted X-ray beam. Blue, red, and green lines show calculated intensities based on Pt(311)-(1 × 2), ideal Pt(311)-(1 × 1), and Pt(311)-(1 × 3) models, respectively.
Figure 3. Fractional rod of Pt(311) at 0.5 V in 0.1 M HClO4.
Figure 4. Schematic top and side views of Pt(311) ()2(100) (111)) surface: (a) unreconstructed (1 × 1) structure; (b) reconstructed (1 × 2) structure.
2) may also lower the surface energy. The Au(311) surface is not, however, reconstructed to a (1 × 2) structure in aqueous solution.27 The activation energy for the surface reconstruction on Au(311) may be higher than that on Pt(311). CTRs in the “adsorbed hydrogen region” (0.1 V) were similar to those at 0.5 V, showing that the surface layers have no potential dependence on Pt(311). According to the studies of Markovic´ et al.,39 the values of d12 are enlarged on the low-index planes of Pt in the “adsorbed hydrogen region”. These results suggest that the Pt(311)-(1 × 2) surface is more rigid than the lowindex planes. (39) Markovic´, N. M.; Ross, P. N., Jr.; Wieckowski, A. Interfacial Electrochemistry - Theory, Experiment, and Applications; Marcel Dekker: New York, 1999; p 821.
Figure 5. In situ STM image of the Pt(311) ()2(100) - (111)) surface at 0.5 V in 0.1 M HClO4. Scanning area: 5.00 × 5.00 nm2. Bias voltage: 78.13 mV. Tunneling current: 19.26 nA.
In Figure 4, all the atoms are lifted 0.03 Å in the first layer. The atoms in the second layer are also lifted 0.05 Å on the (111) facet, whereas the others are displaced inwardly 0.01 Å on the (100) facet. No displacement occurs in the third layer. Table 1 shows the interlayer spacing of Pt(311)-(1 × 2). The values of d12 depend on the structure of facets: d12 contracts 0.02 Å on the (111) facet compared with the bulk layer spacing, whereas it expands 0.04 Å on the (100) facet. The value of d23 expands 0.05 Å on the (111) facet, but it contracts 0.01 Å on the (100) facet. Although anisotropy is found in the relaxation of Pt(311)(1 × 2), ∆d12 and ∆d23 are within 3% of the bulk spacing. The relaxation is remarkably smaller than that obtained by the DFT calculation by Jenkins et al.,26 where d12 contracts about 0.2 Å (∆d12 nearly 20%). The contraction of the surface in UHV is attributed to the electrostatic attractive force between the first and second layers resulting from the electron transfer from the topmost atoms to the bottom of the missing row.40 The electrostatic interaction between the layers may be weakened by the adsorption of water molecules in 0.1 M HClO4. We tried to refine the structure using the model that includes adsorbed water. However, the calculated CTRs are almost identical to those without adsorbed water. The contribution of adsorbed water to a CTR is less intense than that of Pt in the range of Figure 2, because the electron densities of oxygen and hydrogen are 1 order of magnitude smaller than that of Pt. Figure 5 shows an in situ STM image of the Pt(311) surface at 0.5 V in 0.1 M HClO4. The distances between the neighboring spots (arrow b in Figure 5) are about 0.3 nm, which almost agrees with the interatomic distance of Pt atoms (0.28 nm). The distances between the neighboring step lines (arrow a in Figure 5) are about 1.1 nm, which coincides with the value of Pt(311)(1 × 2) (0.96 nm). Bright and dark spots show (100) and (111) facets, respectively. These features match the Pt(311)-(1 × 2) surface very well, supporting the result of the SXS strongly. (40) Ho, K. M.; Bohnen, K. P. Phys. ReV. Lett. 1987, 59, 1833-1836.
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Conclusions Surface X-ray scattering shows that the Pt(311) surface has a (1 × 2) reconstructed structure at 0.1 and 0.5 V in 0.1 M HClO4. Surface relaxation, which is observed on the low-index planes of Pt, is not found on Pt(311) in the “adsorbed hydrogen region”. STM measurement also supports the (1 × 2) reconstructed structure in 0.1 M HClO4.
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Acknowledgment. This work was supported by a grant from the New Energy and Industrial Technology Development Organization, and by Japan Synchrotron Radiation Research Institute (JASRI) under Proposal No. 2005B0097 and 2006A1629. LA701566W
