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Identification and Quantification of Oxygen Species Adsorbed on Pt(111) Single-Crystal and Polycrystalline Pt Electrodes by Photoelectron Spectroscopy Mitsuru Wakisaka,† Hirokazu Suzuki,‡ Satoshi Mitsui,‡ Hiroyuki Uchida,§ and Masahiro Watanabe*,§ Fuel Cell Nanomaterials Center, Interdisciplinary Graduate School of Medicine and Engineering, and Clean Energy Research Center, UniVersity of Yamanashi, Takeda 4, Kofu 400-8510, Japan ReceiVed September 17, 2008. ReVised Manuscript ReceiVed NoVember 24, 2008 We have positively identified oxygen species on Pt(111) single-crystal and polycrystalline Pt electrodes in N2purged 0.1 M HF solution by X-ray photoelectron spectroscopy combined with an electrochemical cell. Four oxygen species (Oad, OHad, and two types of water molecules) were distinguished. The binding energies of each species were nearly constant over the whole potential region and independent of the single- or polycrystalline electrodes. The coverages, however, varied considerably and were dependent on the electrode potential. We have for the first time demonstrated clear differences in the surface oxidation processes for Pt(111) and polycrystalline Pt electrodes.
Introduction The development of highly active cathode catalysts for the oxygen reduction reaction (ORR) is one of the most important subjects to be investigated to achieve high efficiency in polymer electrolyte fuel cells (PEFCs). We were the first to find an enhancement of ORR at the Pt skin layer formed on Pt alloys with nonprecious transition metals such as Fe, Co, and Ni.1-3 The layer formation results from a dissolution of the second metal and a subsequent rearrangement of the surface layers with well-defined Pt(111)-(1 × 1) terraces,4 contrary to general predictions of the structure formation (e.g., such as a Pt skeleton).5 The skin formation was confirmed by electrochemical scanning tunneling microscopy and quartz crystal microbalance studies by us,4,6 suggesting the enhancement by an effect of the electronic structure of the layer modified by the underlying alloys.1-7 The ORR on such Pt skin surfaces is enhanced without changing either the Tafel slope (ca. 120 mV/decade) or the activation energy (ca. 40 kJ/mol) in a potential region for the practical operation of PEFCs, as found by the use of the channel-flow double-electrode method.8 In that work, we suggested that an increased pre-exponential factor Z in the Arrhenius equation should be responsible for the enhancement for the rate-determining step (rds), presumably the first electron-transfer step in the reaction scheme. Very recently, we have for the first time clarified and quantified the oxygen species adsorbed on the Pt skin/PtFe alloy * Corresponding author. E-mail:
[email protected]. † Fuel Cell Nanomaterials Center. ‡ Interdisciplinary Graduate School of Medicine and Engineering. § Clean Energy Research Center. (1) Toda, T.; Igarashi, H.; Watanabe, M. J. Electrochem. Soc. 1998, 145, 4185. (2) Toda, T.; Igarashi, H.; Watanabe, M. J. Electroanal. Chem. 1999, 460, 258. (3) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750. (4) Wan, L. J.; Moriyama, T.; Ito, M.; Uchida, H.; Watanabe, M. Chem. Commun. 2002, 58. (5) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241. (6) Uchida, H.; Ozuka, H.; Watanabe, M. Electrochim. Acta 2002, 47, 3629. (7) Stemenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2002, 106, 11970. (8) Wakabayashi, N.; Takeichi, M.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 5836.
and poly-Pt surfaces in 0.1 M HF electrolyte solution saturated with N2 or O2 by the use of an XPS system combined with an electrochemical cell and found increased Oad coverage on the Pt skin surface. We took the Oad, correlated to the large Z value, as the limiting reactant in the rds and proposed an enhanced ORR mechanism.9 From the point of view of the development of the surface science aspects and finding clues to the design of high-performance cathode catalysts, such an approach, as described above on well-defined catalyst surfaces, is essential. Recently, in situ X-ray absorption techniques have been used to examine the oxygen species on Pt-based electrodes.10,11 However, it is difficult to distinguish the various types of possible adsorbed species (e.g., oxygen atoms (Oad), hydroxyl groups (OHad), and water molecules (H2Oad)) because of the insufficient resolution. In contrast, X-ray photoelectron spectroscopy (XPS) has distinct advantages in surface sensitivity and energy resolution, with the ability to detect small binding-energy (BE) shifts, as reported by us.9,12 However, the structural characteristics of the adsorbed oxygen species, such as the adsorption site and configuration, have remained unclear because Pt skin/Pt alloy surfaces and polycrystalline Pt surfaces with relatively large roughness factors have consisted of a variety of crystal facets, steps, or kinks. The purpose of this research is to identify and quantify the various oxygen species at a well-defined Pt(111) single-crystal electrode in N2-purged solution, comparing them to those existing on polycrystalline Pt, and to support the previous results reported in the literature.9
Experimental Section A single-crystal Pt(111) with a diameter of 5 mm was made by the crystallization of a molten ball formed at the end of pure Pt wire in a hydrogen flame and mechanical polishing with successively (9) Wakisaka, M.; Suzuki, H.; Mitsui, M.; Uchida, H.; Watanabe, M. J. Phys. Chem. C 2008, 112, 2750. (10) Murthi, V. S.; Urian, R. C.; Mukerjee, S. J. Phys. Chem. B 2004, 108, 11011. (11) Tada, M.; Murata, S.; Asaoka, T.; Hiroshima, K.; Okumura, K.; Tamida, H.; Utga, T.; Nakanishi, H.; Mastumoto, S.; Inada, Y.; Nomura, M.; Iwasawa, Y. Angew. Chem., Int. Ed. 2007, 46, 4310. (12) Wakisaka, M.; Mitsui, S.; Hirose, Y.; Kawashima, K.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2006, 110, 23489.
10.1021/la803050r CCC: $40.75 2009 American Chemical Society Published on Web 01/16/2009
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Figure 1. O 1s spectra for a Pt(111) single-crystal electrode emersed from a 0.1 M HF solution saturated with N2.
finer grade alumina pastes down to 0.05 µm.13 Before the XPS measurement, the Pt(111) surface was freshly prepared by annealing in a hydrogen flame and subsequently quenching with pure water.14 A polycrystalline Pt surface was prepared on an Au disk by dc sputtering a Pt target.8,9,12 All of the XPS measurements were carried out in a closed ultrahigh vacuum system combined with an electrochemical cell made of Neoflon (polychlorotrifluoroethylene), placed in a separate chamber under an atmosphere of ultra-highpurity N2 (99.9999%, Sumitomo Seika, Japan) or O2 (99.999%, Sumitomo Seika, Japan) to characterize the electrode surfaces without exposing them to air.9,12 The employed XPS apparatus has been described in detail elsewhere.12 To obtain O 1s spectra without any interference, we have chosen a 0.1 M HF electrolyte solution because HF contains neither oxygen nor a specifically adsorbing anion. Each test electrode was polarized at a given potential for 5 min in an N2-purged solution of 0.1 M HF. Then, the electrode was emersed from the solution under potential control and promptly transferred to a UHV chamber by a combination of pumps. The electrode surface was spontaneously cooled to below 220 K because of the evaporation of an electrolyte droplet remaining on the electrode surface during the transfer to UHV (freeze evacuation). The XPS measurement was carried out within 6 min after electrode emersion. A reversible hydrogen electrode (RHE) was used to control the electrode potential, which was placed outside the chamber and connected to the electrochemical cell with a Teflon tube.
Results and Discussion Figure 1 shows a series of O 1s spectra for a Pt(111) singlecrystal electrode immersed in a 0.1 M HF solution saturated with N2 at electrode potentials E from 0.4 to 1.1 V versus the reversible hydrogen electrode (RHE). The intensities and shapes of the O 1s spectra were found to change with the potential. The changes can be attributed to the changes in coverage and composition of oxygen-containing species adsorbed on the Pt electrode accompanying surface oxidation. We then deconvoluted the O 1s spectra into several asymmetric Lorentzian-Gaussian peaks with a linear background. Curve fitting was performed for all spectra with the full width at half-maximum and tail scale fixed as constants while allowing the peak energies and areas to vary.9 Typical results for the O 1s deconvolution are presented in the Supporting Information (section 1). On the basis of the deconvolutions, four oxygen species were distinguished with discrete BEs for potentials of E < 1.1 V. Figure 2c shows the BEs of the doconvoluted oxygen species on Pt(111) as a function of the electrode potential. Surprisingly, the same four oxygen species are distinguished at both Pt(111) (13) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (14) Clavilier, J.; Achi, K. E.; Rodes, A. J. Electroanal. Chem. 1989, 272, 253.
Figure 2. Comparisons of the properties at Pt(111) single-crystal and poly-Pt electrodes in 0.1 M HF solution purged with N2: voltammograms (50 mV s-1) at (a) Pt(111) and (b) poly-Pt. Deconvoluted BEs for different oxygen species at (c) Pt(111) and (d) poly-Pt, with the coverage changes of each oxygen species as a function of electrode potential at (e) Pt(111) and (f) poly-Pt. Symbols: (O) Oad, (2) OHad, (0) H2Oad,1, and (9) H2Oad,2.
single-crystal and poly-Pt, the same as those at the Pt skin on the Pt-Fe alloy.9 The near (same) identity of the BEs on both surfaces (Figure 2d) suggests that the chemical state of each oxygen species might be independent of the crystal face exposed. The BEs of the four oxygen species were found to be nearly constant at 529.6, 530.5, 531.1, and 532.6 eV, with variations of (0.2 eV. The species at 529.6 and 530.5 eV can be assigned to OHad and Oad, in agreement with the assignments of these species on Pt(111) formed in UHV.15 The species at 532.6 eV can be assigned to H2Oad, also in good agreement with the value found for Pt(111) in UHV.16 The remaining species exhibited a BE of 531.1 eV, close to that for OHad. However, because this species was observed on poly-Pt, even at E ) 0.4 V, where no surface oxidation had occurred (Figure 2b and Supporting Information, section 1), we assign this species to a second type of H2Oad, with a chemical state being different from that for the one observed at 532.6 eV. For poly-Pt, the validity of this assignment to H2Oad had been intensively discussed in the previous XPS study, where we assigned the species at 531.1 eV to H2Oad adsorbed at Pt sites distinct from the atomically flat Pt(111) facet and/or a different configuration of H2Oad.9 In the present study, however, the BEs of the respective oxygen species were found to be independent of the type of electrode (i.e., single-crystalline or polycrystalline). Therefore, the two types of H2Oad were not ascribed to different crystal facets. Also, the second type of H2Oad was not attributed to a solvated water molecule interacting with F- or HF2- because survey spectra for both Pt(111) and polycrystalline Pt revealed no F signal at any electrode potential. It has been well-established that water molecules form an icelike bilayer structure on the Pt(111) surface at low temperature in UHV.16 Recently, Ogasawara et al. have analyzed this bilayer structure by using XPS and evaluated the distance between the two water layers from the O 1s BE split on the basis of final-state hole-screening theory.17 The theory explains the lower O 1s BE (15) Kinne, M.; Fuhrmann, T.; Zhu, J. F.; Trankenschuh, B.; Denecke, R.; Steinruck, H. P. Langmuir 2004, 20, 1819. (16) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1. (17) Ogasawara, H.; Brena, B.; Nordlund, D.; Nyberg, M.; Pelmenschikov, A.; Pettersson, L. G. M.; Nilsson, A. Phys. ReV. Lett. 2002, 89, 276102.
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Figure 3. Structural models for (a) the bilayer on Pt(111)15 and (b) the mixed OHad/H2Oad,1 layer, denoted as Pt(111)-(3 × 3)-3(OH + H2O).21
of the first-layer water molecule and predicts a short distance between the molecule and the Pt surface due to the image charge. Thus, the H2Oad observed herein at 531.1 eV can be assigned to the first-layer water, which is bound directly to the Pt surface through an O lone pair, whereas the feature at 532.6 eV can be assigned to the second-layer water, which interacts with the first layer via hydrogen bonding. The proposed bilayer model is illustrated in Figure 3a. On the basis of this model, the theoretical coverage of H2Oad is 0.67 at Pt(111). Hereafter, the two water species at 531.1 and 532.6 eV are denoted by H2Oad,1 and H2Oad,2, respectively. The BE of H2Oad,1 formed in HF solution was significantly lower than that reported for H2Oad formed in UHV, indicating a shorter bond to Pt, probably as a result of an electric field effect (i.e., the lowered Fermi level of Pt). Toney et al. have reported such an effect on the bond distance between water and the Ag(111) electrode surface in their X-ray scattering study. They observed that the average Ag-O distance decreased by 1 Å at potentials above that of zero charge (PZC) in NaF solution.18 A theoretical study has reported that a change of 0.96 Å in the Pt-water distance yields a shift of 1.9 eV in BE.17 Therefore, the low BE for H2Oad,1, 531.1 eV, is quite reasonable for E > PZC (ca. 0.3 V vs RHE19). The intensity of H2Oad,2 was fairly small over the whole potential region, as compared to that of H2Oad,1. This might be due to the desorption of H2Oad,2 that is weakly bound to the first layer during the electrode transfer to UHV. In contrast, the strongly bound H2Oad,1 remained on the electrode surface during the XPS measurement probably because of the freeze-evacuation process. Figure 2e,f shows the fractional coverages (atomic ratio of each oxygen species to Pt) of the oxygen species (Oad, OHad, and H2Oad,1) as a function of electrode potential at Pt(111) and polyPt, respectively. The coverages were calculated on the basis of the deconvoluted O 1s photoelectron intensities normalized by that for Pt 4f.9 On Pt(111), OHad appeared at E > 0.6 V, accompanying the surface oxidation current in the “butterfly” region (Figure 2a).14,20 Very recently, Rai et al. simulated the formation of OHad in the butterfly region by density functional theory.21 However, to our knowledge, the present work is the first direct experimental evidence that the anodic current in this region is due to OHad formation as the first step of Pt surface oxidation via the following scheme:
H2Oad f OHad+ H++ e-
(1)
At E ) 0.80 V, the coverage θ of OHad was found to reach 0.36 (86 µC cm-2), in good agreement with the 92 µC cm-2 (18) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Surf. Sci. 1995, 335, 326. (19) Weaver, M. J. Langmuir 1998, 14, 3932. (20) Wagner, F. T.; Ross, P. N. J. Electroanal. Chem. 1988, 250, 301. (21) Rai, V.; Aryanpour, M.; Pitsch, H. J. Phys. Chem. C 2008, 112, 9760.
charge density measured electrochemically from 0.50 to 0.80 V (excluding the sharp spike; see below). It should be noted that θ[H2Oad,1] also increased in the butterfly region. This increase can be explained on the basis of hydrogen bond formation between H2Oad and the OHad appearing in this region. It has been reported in a thermal desorption study that the water molecule was more strongly adsorbed on Pt surfaces covered with OHad.22 If the species at 531.1 eV were assigned to OHad rather than H2Oad,1, then the electron/surface Pt site (e/Pt) value based on the coverage must deviate from that independently measured from the voltammogram. Our assignments provide the best agreement between the e/Pt values estimated from the photoelectron intensities and those from the voltammograms over the whole potential range studied (Supporting Information, section 2). Remarkably, as θ[OHad] and θ[H2Oad,1] became nearly equal at E ) 0.80 V, the sum of these coverages reached a maximum of 0.68, coinciding within experimental error with the reported coverage of bilayer water molecules.16,17 This coincidence suggests that the honeycomb framework of the bilayer remained during the initial surface oxidation process (0.50-0.80 V). A structural model for the mixed OHad/H2Oad,1 layer at 0.80 V is illustrated in Figure 3b. This structure, with θ[OHad] ) θ[H2Oad,1], should be the most stable among the possible mixed OHad/H2Oad layers because the hydrogen-bonding network (HBN) is complete, as reported in the literature.23 The decrease in θ[H2Oad,1] at E > 0.80 V suggests that HBN was disturbed and H2Oad,1 became unstable. It should be noted that the sharp spike in the voltammogram around 0.80 V, similar to that observed in sulfuric acid solution,20 might reflect a structural change of this adlayer relating to the HBN. Recently, Berna´ et al. suggested in a discussion of their CV data that the sharp spike around 0.8 V in perchloric acid solution could be ascribed to the formation of OH originating from a water molecule interacting with the perchlorate anion.24 In the present research, however, such solvation water interacting with F- or HF2- was never observed. At E ) 0.90 V, θ[Oad] commenced to increase, whereas θ[OHad] decreased. This suggests that Oad originated from the oxidation of OHad via the following scheme:
OHad f Oad+ H++ e-
(2)
At E > 1.00 V, both θ[Oad] and θ[OHad] were found to increase, accompanying the large anodic peak (Figure 2a). In contrast to Pt(111), θ[Oad] and θ[OHad] on poly-Pt commenced to increase in almost the same manner at E ) 0.8 V. It should be pointed out that Oad forms on Pt(111) at almost the same potential as on polycrystalline Pt, whereas OHad appears on Pt(111) at lower potential than on polycrystalline Pt, suggesting that the formation of OHad is very sensitive to crystal face structures such as terraces, steps, kinks, and grain size whereas that of Oad is less dependent on such structure. It has been proposed by Conway et al. on the basis of voltammetric measurements that the formation of OHad is followed by a place exchange between OHad (or Oad) and the Pt atom within the surface lattice at polycrystalline Pt in H2SO4 solution.25 You et al. have also proposed that the place exchange of adsorbed oxygen species occurs on Pt(111) at E > 1.025 V in HClO4 solution on the basis of in situ surface X-ray scattering.26 However, in the present research, we did not obtain clear evidence for the (22) Fischer, G. B.; Gland, J. L. Surf. Sci. 1980, 94, 446. (23) Clay, C.; Haq, S.; Hodgson, A. Phys. ReV. Lett. 2004, 92, 046102. (24) Berna´, A.; Climent, V.; Feliu, J. M. Electrochem. Commun. 2007, 9, 2789. (25) Conway, B. E. Prog. Surf. Sci. 1995, 49, 331. (26) You, H.; Zurawski, D. J.; Nagy, Z.; Yonco, R. M. J. Chem. Phys. 1994, 100, 4699.
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place exchange at E < 1.1 V on either Pt(111) or polycrystalline Pt electrodes in HF solution. At E g 1.2 V, the place exchange and/or a bulk oxidation of Pt took place in HF solution, as indicated by the appearance of a bulk PtO signal in the Pt 4f region (Supporting Information, section 3). The difference in the oxidation process (i.e., presence vs absence of place exchange in the high-potential region) might be ascribed to a difference in the supporting electrolyte anion.
Conclusions In the present research, we have positively identified the oxygen species on Pt(111) single-crystal in comparison with those on poly-Pt electrodes in an N2-purged 0.1 M HF solution by XPS. Four types of oxygen species were distinguished, with BEs at 529.6, 530.5, 531.1, and 532.6 eV; the first two were assigned to Oad and OHad, and the latter two were assigned to the bilayer water molecules, H2Oad,1 and H2Oad,2. The BEs of the respective oxygen species were found to be nearly constant over the whole potential region and independent of the Pt electrode types. In contrast, the quantitative coverage of each species varied considerably and was found to depend on the electrode potential and the crystal faces as well as the electronic property modified by Pt alloying.9 Furthermore, we have for the first time demonstrated the clear difference in the surface oxidation process
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for Pt(111) and polycrystalline Pt electrodes. It is essential in understanding the enhancement of ORR at the Pt skin layer on Pt alloys with nonprecious transition metals to establish the oxygenated species on the Pt surface with well-defined surface properties (crystal structure, roughness factor, alloy composition, electronic state, etc). The present identification and quantification of such species adsorbed on the Pt(111) single-crystal in comparison with polycrystalline Pt electrodes by XPS allow us to see the quantitative correlation between the catalytic activity and adsorbates existing on various types of catalyst surfaces, which strongly contributes to our understanding of the enhancement of ORR at the Pt skin layer and other new catalysts and gives important clues to the design of new, higher-activity electrocatalysts. Acknowledgment. This work was partially supported by the fund for the HiPer-FC Project of NEDO and a Grant-in-Aid (no. 20750007) for Scientific Research from the Ministry of Education, Science, Culture, Sports, and Technology of Japan. Supporting Information Available: Deconvolution of O 1s spectra, validity of oxygen species assignment, and Pt 4f spectra for bulk oxidation of the polycrystalline Pt. This material is available free of charge via the Internet at http://pubs.acs.org. LA803050R
