ARTICLE pubs.acs.org/JPCC
Outermost Surface Structures and Oxygen Reduction Reaction Activities of Co/Pt(111) Bimetallic Systems Fabricated Using Molecular Beam Epitaxy Toshimasa Wadayama,* Hirosato Yoshida, Koichiro Ogawa, Naoto Todoroki, Yoshinori Yamada, Kanji Miyamoto, Yuki Iijima, and Tatsuya Sugawara Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
Kazuki Arihara, Seiho Sugawara, and Kazuhiko Shinohara Advanced Materials Laboratory, Nissan Research Center, Nissan Motor Co. Ltd., 1, Natsushima-cho, Yokosuka, 237-8523, Japan ABSTRACT: We studied oxygen reduction reaction (ORR) activities for outermost surfaces of 0.3 nm thick Co deposited on Pt(111) (Co0.3 nm/Pt(111)) bimetallic systems fabricated using molecular beam epitaxy at various Co deposition temperatures. Results show that Co0.3 nm/Pt(111) fabricated at temperatures lower than 393 K displays extra low-energy electron diffraction (LEED) spots outside the integer ones, indicating incoherent epitaxial growth of Co. A new IR band that is attributed to linearly bonded carbon monoxide (CO) on the Pt site influenced by neighboring Co atoms emerges at 2052 cm 1 for 333 K fabricated Co0.3 nm/Pt(111), in addition to the CO Pt and CO Co bands. With increasing fabrication temperature, the new band shifts to higher frequencies and reaches 2082 cm 1 for 773 K fabricated Co0.3 nm/Pt(111), which has a diffuse (11) LEED pattern. We evaluated the dependence of the deposition temperature on the lattice parameters of the Co0.3 nm/ Pt(111) and ascribed the band at 2082 cm 1 to adsorbed CO on a Pt-enriched topmost surface having 6-fold symmetry. Although the incoherent epitaxial Co layer was unstable in 0.1 M HClO4 aqueous solution, the Pt-enriched topmost surface is rather stable and the ORR activity is 10 times higher than that for clean Pt(111). The activities for Pt0.3 nm,0.6 nm/Co0.3 nm/Pt(111) artificial sandwich (superlattice) surfaces were also evaluated. The obtained results indicate that the Co atoms located at the second atomic layer strongly modify the electrocatalysis of the topmost surface.
1. INTRODUCTION Atomic-level designs of specific surface structures are of great importance for innovative industrial processes. Fabrication of well-defined bimetallic surfaces by vacuum deposition of metals onto different single-crystal metal substrates has been studied intensively to create new surface materials, particularly for magnetic devices and heterogeneous catalysts.1 7 In particular, bimetallic systems of platinum and magnetic elements, such as iron, nickel, and cobalt, are of interest. Precise control of the atomic arrangements for such systems is indispensable for the development of novel magnetic information-storage devices.8 10 Pt-based alloys are also of interest for their role as highly efficient, stable, and inexpensive catalysts, and the design of the alloy outermost surface is the key to achieving high activity and stability for practical use as electrode catalysts in fuel cells.11 15 Indeed, alloying Pt with Fe, Ni, Co, Pd, Ru, etc. improves not only the oxygen reduction reaction (ORR) activity at the cathode electrode but also the carbon monoxide (CO) tolerance of the anode electrode surface.11 13 Furthermore, electrochemically fabricated core shell structures in which non-Pt nanosized r 2011 American Chemical Society
particles are covered by a monolayer of Pt show higher ORR activities than that for the corresponding Pt particles.16 18 The published results indicate that electronic as well as chemical properties of the alloy surfaces determine catalytic activities.8 18 Therefore, from a catalytic perspective, molecular behavior on well-defined bimetallic surfaces is expected to provide clues for elucidating the role of the outermost surface structure of the Ptbased alloy catalyst for the electrocatalysis mechanism. However, discussion of the mechanism is complicated because of uncertainties related to the outermost surface structures. Therefore, to clarify the effect of the outermost surface structures of the alloy catalysts, Stamenkovic et al. evaluated ORR activities for the (111), (100), and (110) single-crystal planes of the Pt3Ni intermetallic compound. Their pioneering work underscores the superior ORR activity for the (111) surface, 10 times higher than that for the corresponding clean Pt(111).19,20 Furthermore, Received: April 25, 2011 Revised: August 19, 2011 Published: August 26, 2011 18589
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Figure 1. LEED patterns of Co0.3 nm/Pt(111) surfaces fabricated at various substrate temperatures. Insets: magnified images around original spots. The incident electron energy was 75 eV. All patterns were collected at 323 K.
we have studied electrochemical properties for the Ni0.3 nm/ Pt(111) bimetallic surfaces fabricated in an ultra-high-vacuum (UHV) condition using molecular beam epitaxy (MBE). The ORR activities depend on the topmost surface structures and the Pt-enriched surface having 6-fold symmetry, giving 8 times more activity than the clean Pt(111).21 Regarding the Pt Co bimetallic surfaces, Co/Pt(111) has been studied mainly for its magnetic properties.22 24 Surface structural investigations for the Co/Pt(111) have been conducted using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and synchrotron X-ray diffraction;25 in these reports, the authors discussed the correlation between the Co and Pt atomic composition near the surface and the annealing temperature in UHV. Scanning tunneling microscopic (STM) studies of Co/Pt(111) show that surface alloying of the Pt substrate with deposited Co proceeds by Pt incorporation into the deposited Co layer through surface defects.26,27 Regarding surface chemical properties, IR reflection absorption spectroscopy (IRRAS) is a powerful tool to study the adsorption of simple molecules onto well-defined metal and alloy surfaces.28 33 Indeed, Al-Shamery and co-workers studied adsorption and desorption behaviors of CO on Co0.9 nm/Pt(111) surfaces annealed at various temperatures using IRRAS and temperature-programmed desorption (TPD).34,35 They demonstrated the ligand effect for the Pt Co bimetallic surfaces, that is, the electronic influence of Co on the Pt surface site for CO adsorption. We reported a similar influence of the deposited Co to the substrate Pt atoms for Co/Pt(111) and Co/Pt(100)-hex bimetallic systems.36,37 Recently, electrochemical properties for Pt Co thin films fabricated in UHV by codeposition of Pt and Co have been investigated by Soriaga and co-workers38 by using low-energy ion scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), LEED, and TPD. They discussed the degradation of the Pt3Co thin film through corrosion of surface Co atoms. In addition to the above-mentioned UHV approaches, Wakisaka et al.39 reported fabrication of Pt100 xCox(111) (x = 9 35) single crystals by the hydrogen oxygen flame annealed method: a series of cyclic voltammetric (CV) curves for the samples depend seriously upon the Co compositions (x). This study specifically examines the outermost surface structures and their ORR activities of 0.3 nm thick Co deposited Pt(111) (Co0.3 nm/Pt(111)) bimetallic surfaces fabricated by vacuum deposition of Co onto a clean Pt(111) under UHV
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Figure 2. IRRAS spectra of 1.0 L CO-exposed Co0.3 surfaces fabricated at 333 843 K substrate temperatures.
nm/Pt(111)
conditions at substrate temperatures of 333 843 K. We conducted LEED and IRRAS measurements of adsorbed CO on the Co0.3 nm/Pt(111) bimetallic surfaces. The results demonstrate that the deposited Co grows incoherent epitaxially on Pt(111) at 333 K, although the outermost Pt-enriched layer on the underlying Co atoms is generated by the Co deposition on the substrate at 773 K. The UHV-prepared samples are transferred to the electrochemical system without being exposed to air, and the electrochemical properties of the samples are evaluated using a rotating disk electrode (RDE) method in a perchloric acid solution. The results reveal that the Pt-enriched layer is 10 times more active for ORR than clean Pt(111).
2. EXPERIMENTAL SETUP The experimental equipment used in this study has been described elsewhere.36,37 In brief, the UHV system is equipped with several components, including IRRAS, LEED, a quadruple mass spectrometer (Q-mass), and an electron beam evaporator. The Pt(111) (99.999%), Ar (>99.9999%), and N2 (>99.9995%) were used. The electrolyte solution was prepared from perchloric acid (HClO4, suprapur, Merck and Co., Inc.) and Milli-Q water. After conducting an electrochemical surface cleaning procedure of the type described by Markovic et al.,40 we recorded cyclic voltammetric (CV) curves for the samples in Ar-purged 0.1 M HClO4 without disk rotation. Current densities were estimated with respect to the geometric surface area of the Pt(111) substrate exposed to the solution. We then conducted linear sweep voltammetry (LSV) measurements in O 2 -saturated 0.1 M HClO4. The ORR activities of the samples were evaluated from the kinetic controlled current density (ik) at 0.9 V using the Koutecky Levich equation.40 42
3. RESULTS AND DISCUSSION 3.1. LEED Patterns for Co 0.3 nm /Pt(111) Fabricated at Various Substrate Temperatures. Figure 1 shows LEED
patterns for Co 0.3 nm /Pt(111) fabricated at substrate temperatures within 333 843 K. A 0.3 nm thickness of Co is expected to correspond to approximately 1 monolayer (ML) because the atomic radius of Co is 0.125 nm. 23 The Co deposition on the clean Pt(111) substrate surface was investigated previously using LEED, AES, and photoemission spectroscopies.25 Recently, Varga and co-workers reported STM observations of Co thin film growth on Pt(111). 26,27 The results demonstrate that Co deposition is sensitive to both Co thickness and substrate temperature. As is clearly evident in the figure, a 0.3 nm thick Co deposition at a substrate temperature of 333 K induces LEED spots outside the integer ones that stem from the Pt(111) substrate. Small hexagonal satellites of this type have been reported in the LEED patterns of incoherent epitaxially grown bimetallic Co/Pt(111)22 24 and in large lattice-mismatched bimetallic systems, such as Ni/Pt(111)43 and Pt/Ni(111). 44 The results presented in Figure 1 indicate that Co grows epitaxially on Pt(111) at the Co deposition temperature of 333 K under these experimental conditions. As can be clearly seen in Figure 1, with increasing Co deposition temperature, the LEED subspots become blurred. The surfaces fabricated at temperatures greater than 693 K show a diffused 11 pattern. These results suggest that higher deposition temperatures cause surface structural changes. The in-plane lattice spacing for the Co0.3 nm/Pt(111) surfaces can be estimated from the distances between the LEED spots at a given incident electron beam energy. Figure 1 shows that the distances decrease concomitantly with increasing deposition temperature, which suggests that, as the deposition temperature increases, the in-plane lattice distances approach distances of the Pt(111) substrate because the atomic radii of Co and Pt are, respectively, 0.125 and 0.135 nm.23 This result might reflect incorporation of deposited Co into the Pt substrate, that is, surface alloying induced by thermal energy of the substrate during deposition. 3.2. IRRAS Spectra of Adsorbed CO on Co0.3 nm/Pt(111) Surfaces. The C O stretch frequencies of adsorbed CO depend strongly on the structure and composition of the topmost part of the alloy surface. Therefore, we measured the IRRAS spectra of the CO adsorbed on Co0.3 nm/Pt(111) that had been fabricated
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Figure 3. IRRAS spectra of 1.0 L exposed CO on Co0.3 nm/Pt(111) fabricated at 773 K (top) and 0.3 nm thick Pt (middle) and 0.6 nm thick Pt (bottom) on Co0.3 nm/Pt(111) fabricated at 333 K. Dashed line: peak frequency for linear-bonded CO on Co0.3 nm/Pt(111) fabricated at 773 K. Inset images: side-view models for the corresponding 773 K Co0.3 nm/Pt(111) and Pt0.3 nm,0.6 nm/Co0.3 nm/Pt(111).
at elevated substrate temperatures. Figure 2 shows trends of Co0.3 nm/Pt(111) fabricated at various temperatures. For clean Pt(111) (the trace nearest the bottom), a strong band at 2092 cm 1 dominates the spectrum of the 1.0 L CO exposed surface and is accompanied by a weak band at 1855 cm 1. The 2092 and 1855 cm 1 bands are assigned, respectively, to the ontop and bridge sites of the CO adsorbed on Pt(111).45,46 In contrast, for the Co0.3 nm/Pt(111) fabricated at 333 K, a band at 2007 cm 1 dominates the spectrum along with bands at 2091 and 2052 cm 1. Given the atomic radius of Co (0.125 nm), the incoming Co is expected to cover the substrate Pt(111) surface with a deposition thickness of 0.3 nm. The absorption intensity for the 2007 cm 1 band depended upon the deposited Co thicknesses (not shown here). Beitel et al.47 and Lahtinen et al.48 reported the CO adsorption on clean Co(0001) by IRRAS, in which the linear-bonded CO bands are located at around 2000 2050 cm 1. Therefore, the bands at 2007 and 2089 cm 1 can be assigned, respectively, to the CO adsorbed on the deposited Co and on uncovered substrate Pt sites. Close inspection of the figure shows that a new band appeared at 2052 cm 1 (indicated by an arrow) for the surface fabricated at 333 K. The new band shifts to a higher frequency as the Co deposition temperature increases and reaches 2082 cm 1 for the surface fabricated at 773 K. For Co0.3 nm/Pt(111) fabricated at 473 K, the CO Co band disappears. For Co0.3 nm/Pt(111) fabricated at 693 K, a strong band appears at 2076 cm 1 with a shoulder at 2090 cm 1. A marked absorption at 2090 cm 1 dominates the spectrum of the Co0.3 nm/Pt(111) fabricated at 843 K, which is almost identical to the spectrum for the clean Pt(111). Therefore, the band at around 2082 cm 1 might be attributed to the specific surface structure generated by thermal activation processes. Baudoing-Savois et al.25 investigated surface structures of Cox/Pt(111) fabricated at various temperatures and found that the negative surface segregation energy for the Pt Co bimetallic system49 appears to induce surface segregation of the substrate Pt atoms, thereby generating a Pt-enriched topmost surface. Therefore, at elevated substrate temperatures, there is both an incorporation of incoming Co into the substrate Pt lattice and surface segregation of the substrate Pt atoms, thereby forming a specific outermost surface structure on which adsorbed 18591
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Figure 4. Lattice parameter ratios for the Co0.3 nm/Pt(111) to the clean Pt(111) as evaluated from the distance between LEED spots (upper panel) and the C O stretch frequencies (lower panel) versus the Co deposition temperature. The corresponding values for clean Pt(111) (dashed lines) are also shown.
CO gives rise to new bands. Jerdev and Koel50 showed that, at temperatures less than 800 K, the diffusion of incoming Fe atoms into Fe/Pt(111) is limited to the near-surface region. Similarly, the concentration of Pt and Ni in PtxNi1 x(111) surfaces oscillates at around the bulk value.51 Generation of a pure Pt layer comprising the topmost surface atoms by UHV annealing of the Pt Ni and Pt Fe bimetallic surfaces has also been demonstrated.19,20 Consequently, in this study, the Co atoms deposited near the surface are expected to modify the surface Pt atoms electronically as well as chemically, leading to frequency shifts of the adsorbed CO bands. To investigate the outermost surface structure of the Co0.3 nm/ Pt(111) fabricated at 773 K in detail, we fabricated Pt/Co/ Pt(111) “sandwich” (superlattice) structures with 0.3 nm thick and 0.6 nm thick Pt depositions on the 333 K Co0.3 nm/Pt(111). Considering the atomic radius of Pt (0.135 nm), the Pt thicknesses of 0.3 and 0.6 nm almost correspond, respectively, to 1 and 2 monolayers (MLs) of Pt. The IRRAS spectra for adsorbed CO of the resulting sandwich surfaces recorded after 1.0 L CO exposure are depicted in Figure 3. The C O stretch frequency of adsorbed CO on the Co0.3 nm/Pt(111) (2082 cm 1) fabricated at 773 K is located in the same frequency range (2077 2084 cm 1) as that of the sandwich surfaces. Therefore, we can conclude that the 2082 cm 1 band for the 773 K Co0.3 nm/ Pt(111) results from CO adsorption on an almost pure Pt layer comprising the topmost surface, generated by surface segregation of the substrate Pt atoms. From these results, we can deduce that the Pt-enriched topmost layer is generated during Co deposition at 773 K and that the CO adsorbed onto the Pt-enriched surface creates the new band at 2082 cm 1. As shown in Figure 2, the single absorption band at 2082 cm 1 dominates the IRRAS spectrum of Co0.3 nm/Pt(111), fabricated at 773 K. In contrast, clean Pt(111) displays linearly bonded (2092 cm 1) and bridge-bonded CO (1885 cm 1) bands. The disappearance of the bridge-bonded CO Pt band from the surfaces might also be related to modification of the electronic structure on the outermost part of the surface. Watanabe and coworkers reported CO adsorption on Pt alloy electrode surfaces in
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electrochemical systems;11 their FTIR ATR investigations revealed that the alloying of Pt causes less-intense bridge adsorption of CO on the surfaces. They inferred that weaker backdonation of d electrons at the alloy electrode engenders a lessintense bridge-bonded CO band. In addition, the HR EELS investigation by Atli et al.52 showed that the on-top CO is the prevalent adsorbed species for Pt80Fe20 (111). At any rate, the 6-fold symmetry of the topmost Pt surface on the underlying Co atoms reveals specific surface properties that differ greatly from those of clean Pt(111). By contrast, the C O stretch frequencies for the CO adsorbed on the Co0.3 nm/Pt(111) fabricated at 843 K are almost identical to those of clean Pt(111). Therefore, most of the deposited Co should diffuse into the bulk Pt substrate, leaving an almost pure Pt(111) surface. 3.3. Lattice Parameters and C O Stretch Frequencies for Co0.3 nm/Pt(111) Surfaces. In-plane atomic distances can be estimated from the separations in the LEED diffraction spots for a given incident electron beam energy. Ratios of the spot separations for Co0.3 nm/Pt(111) fabricated at various deposition temperatures (a*(alloy)) to the spot separations for clean Pt(111) (a*(Pt(111)) as a function of Co deposition temperatures are shown in the upper panel of Figure 4. For comparison, the dependence of C O stretch frequencies on the deposition temperature for CO adsorbed on the corresponding surfaces is shown in the lower panel. As shown in Figure 4, the in-plane lattice distances for Co0.3 nm/Pt(111) are close to those for Pt(111) and are almost identical to those for clean Pt(111) above 773 K. In other words, one cannot identify the topmost surface structures based on a visual inspection of the LEED patterns. In contrast, the C O stretch frequencies of CO molecules located at the on-top sites on the surface Pt sites gradually approach the frequency of the CO adsorbed on clean Pt(111). For example, at a deposition temperature of 773 K, the in-plane distances for Co0.3 nm/ Pt(111) are almost identical to those for clean Pt(111), whereas the C O stretch frequency is 2082 cm 1 or about 10 cm 1 red shifted from the value for clean Pt(111). Because of the negative surface segregation energy,49 a higher deposition temperature not only activates Co incorporation into the Pt surface but also leads to surface segregation of the substrate Pt atoms, resulting in modified Pt compositions near the bimetallic surfaces. The underlying Co atoms should affect the outermost Pt-enriched layer electronically as well as chemically, inducing specific surface properties that are much different from those of clean Pt(111). Such perturbations influence the vibrational properties of the adsorbates. Pioneering work by Rodriguez and Goodman5,6 revealed that the C O stretch vibration is sensitive to the polarizability of the pseudomorphic admetal layers on singlecrystal substrates. Furthermore, red shifts in linearly bonded C O stretch frequencies are reported for CO adsorbed on the Pt sites of Pt Co bimetallic surfaces.34 37 According to the Pt Co phase diagram,53 Pt Co alloys form disordered solid solutions with face-centered cubic (fcc) lattice structures at sufficiently high temperatures (1800 1100 K). Below 1100 1000 K, two ordered structures are present: Pt3Co, in which ordered Co atoms can occupy any of the four equivalent sites (L12), and PtCo, in which Pt and Co atoms occupy alternating close-packed planes (L10). On the Co-rich side, a Pt Co alloy phase with a hexagonal closest-packing (hcp) structure is present. In contrast, as described earlier, for Co0.3 nm/Pt(111), the substrate Pt atoms tend to segregate to the top part of the surface, even at temperatures lower than 473 K, 18592
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Figure 6. CV curve for Co0.3 nm/Pt(111) fabricated at 773 K (red solid line, upper panel). The curves for the clean Pt(111) (dashed dotted line, upper) and Pt0.3 nm,0.6 nm/Co0.3 nm/Pt(111) “sandwich” surfaces (navy dotted and solid lines, lower panel) are also presented for reference (sweep rate = 50 mV/s). Inset images: side-view models for the corresponding surfaces. Figure 5. CV (a) and LSV (b) curves for samples of clean Pt(111) and Co0.3 nm/Pt(111) fabricated at 333 K (epitaxial Co layer). Sweep rates of CV and LSV measurements are, respectively, 50 and 10 mV/s.
suggesting that segregation proceeds not through atomic exchange between Pt and Co (substitutional diffusion) but through diffusion paths, for example, defects in the deposited Co layer. Incoherent epitaxy of the deposited Co might be responsible for such low-temperature Pt segregation. Indeed, STM studies of Co growth on Pt(111) show that misfit dislocations in the Co layer contribute to the formation of the outermost Pt layer. 26,27 Furthermore, the bulk phase transition of Co from hcp to fcc at 700 K seems to correlate with such segregation. In fact, Baudoing-Savois et al. demonstrated that, in the case of Cox/ Pt(111) (x > 6 MLs) fabricated at room temperature, the 6 ML thick Co layer is hcp and allows little Pt incorporation, although annealing in UHV above 670 K induces an easy incorporation of Pt to form a homogeneous Pt Co fcc alloy.25 For the Co0.3 nm/ Pt(111), in this study, such phase transitions of Co and Pt Co alloys might also be related to low-temperature Pt surface segregation. 3.4. Electrochemical Properties for Epitaxial Co Layer. A CV curve for the sample of Co0.3 nm/Pt(111) fabricated at 333 K is shown in Figure 5a (solid line). The curve for the Pt(111) (dashed dotted line) is characterized by symmetrical features located at 0.05 0.35 V (hydrogen-related peaks) and at 0.8 V (hydroxyl-related peaks, forming a so-called “butterfly”).40 42,54,55 In contrast, the CV curve for the 333 K fabricated Co0.3 nm/ Pt(111) (Co incoherent epitaxial layer) recorded immediately after the sample transfer from UHV to the electrochemical system exhibits that a significant change in hydrogen-related features, accompanied by complete disappearance of the butterfly peaks. The Pt skin for the Pt3Ni(111),19,20 and furthermore, for our MBE-prepared Ni/Pt(111) bimetallic system,21 shows similar changes in the hydrogen-related features. Therefore, change in hydrogen-related features might stem from electronic influence of the deposited Co atoms on the surface Pt atoms, although siteblocking of the substrate Pt atoms by the Co epitaxial layer cannot be ruled out. The LSV measurement under O2 saturation of the 0.1 M HClO4 solution was conducted after the CV measurement; the result is presented in Figure 5b. The curve for the clean Pt(111)
(dashed dotted line ) is also depicted for reference. At around 0.4 0.6 V, the current densities for the 333 K fabricated Co0.3 nm/Pt(111) (solid line) are much lower than the limiting current density of the clean Pt(111) (ca. 5.7 mA/cm2). Moreover, in the mixed kinetic diffusion control region (0.8 1.0 V), the onset potential shifts negatively. These results indicate that the electrochemical properties of the 333 K fabricated Co0.3 nm/ Pt(111) differ greatly from those for the clean Pt(111). The deposited Co atoms present at the topmost surface are expected to dissolve into the 0.1 M HClO4. Furthermore, they are expected to be oxidized during the potential sweeps in the O2saturated solution, that is, during LSV measurements. Actually, the CV curve recorded in Ar-purged 0.1 M HClO4 after the LSV measurement (Figure 5a, dashed line) shows hydrogen adsorption desorption signals at the defect sites with (110) symmetry54 (ca. 0.1 V, arrow), and also hydroxyl-group-related features at around 0.6 1.0 V. The result suggests that the bimetallic surface after the LSV measurements is not the epitaxial Co layer but that it is polycrystalline platinum. Whichever happens to be true, the CV results presented in Figure 5a indicate that the epitaxial Co layer is unstable in the 0.1 M HClO4 solution. Therefore, we focus hereinafter on the ORR activity for the Pt-enriched topmost surface fabricated by the Co deposition at the substrate temperature of 773 K. 3.5. Oxygen Reduction Reaction Activity for Pt-Enriched Layer. Figure 6 summarizes CV curves for the Co0.3 nm/Pt(111) (red solid line) fabricated at 773 K and the artificially prepared Pt/Co/Pt(111) “sandwich” surfaces with 0.3 nm thick (ca. 1 ML thick, navy dotted) and 0.6 nm thick (2 MLs thick, navy solid) Pt depositions on the 333 K Co0.3 nm/Pt(111). The curve for clean Pt(111) (dashed dotted) is also depicted for reference. As can be clearly seen from the figure, the Pt-enriched topmost surface fabricated at 773 K and the Pt/Co/Pt(111) sandwich surfaces reveal a significant change in hydrogen-related peaks accompanied by disappearance of the butterfly peaks. The Pt skin of Pt3Ni(111) exhibited similar changes in its hydrogen-related and hydroxyl-related features.20 Furthermore, our electrochemical study of MBE-fabricated Ni/Pt(111) bimetallic systems showed a similar CV curve for the Pt-enriched topmost surface.21 If the deposited Co atoms are present at the outermost part of the surface, then the Co atoms are expected to dissolve into 0.1 M 18593
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Figure 7. (a) LSV curves for clean Pt(111), 773 K Co0.3 nm/Pt(111), and Pt0.3 nm,0.6 nm/Co0.3 nm/Pt(111), recorded at 1600 rpm (sweep rate = 10 mV/s). (b) Koutecky Levich plots for the clean Pt(111) (top) and 773 K Co0.3 nm/Pt(111) (bottom) surfaces.
HClO4,38 causing surface defects. Hydrogen adsorption desorption at the defect sites with (110) symmetry would induce signals at around 0.13 V.54 Actually, our previous results for CV measurements of the Ni0.3 nm/Pt(111) model catalysts21 show that dissolution of the Ni atoms remaining at the topmost surface causes the hydrogen-related redox features at 0.13 V. Furthermore, as presented in Figure 5a, the redox features appeared on the CV curve of the epitaxial Co layer recorded after the LSV measurements. On the contrary, the defect-induced signals are very weak on the CV curve of the Pt-enriched layer, indicating that the Co atoms are almost absent at the outermost part of the surface of the 773 K Co0.3 nm/Pt(111), which has the same surface symmetry as the clean Pt(111), and that the surface structure remained atomically flat during CV measurements. The CV curve for the Pt0.3 nm/Co0.3 nm/Pt(111) (navy dotted) shows the redox feature at 0.13 V (indicated by an arrow) (Figure 6). As shown in section 3.1, the LEED pattern for the 0.3 nm thick Co deposition at 333 K indicates that Co grows incoherent epitaxially on the Pt(111). Therefore, the 0.3 nm thick Pt deposition at 333 K might be insufficient to cover the Co epitaxial layer, probably because of surface roughness caused by incoherence of the deposited Co(111) domains and the substrate Pt(111) lattice. Indeed, as shown in Figure 3, the full width at half-maximum of the adsorbed CO band (15 cm 1) for the Pt0.3 nm/Co0.3 nm/Pt(111) surface is much wider than that for the 773 K Co0.3 nm/Pt(111) (7 cm 1), suggesting that the topmost surface is inhomogeneous. Thus, the deposited Co atoms might remain at the topmost surface and dissolve into the electrolyte during the CV measurements, resulting in the surface defect sites. In contrast, as for the Pt0.6 nm/Co0.3 nm/Pt(111) (navy solid), the 0.6 nm thick Pt is sufficient and, thereby, the hydrogen-related redox feature is absent on the CV curve (navy solid). One might notice that the hydrogen-related charge (Q updH) for the Pt0.6 nm/ Co0.3 nm/Pt(111) surface is slightly higher than that for the 773 K Co0.3 nm/Pt(111). The result might indicate a less-effective electronic influence of the Co epitaxial layers on the topmost Pt layer. Wakisaka et al.39 reported a series of CV curves for the Pt100 xCox(111) (x = 9 35) single-crystal surfaces. They first demonstrated that, with increasing Co composition, Q updH deceased monotonically, accompanied by a positive shift of the
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onset potentials for the hydroxyl-related features. The trend corresponds well to the theoretical results reported by Roques and Anderson.56 Taking into account the results by Wakisaka et al.39 and, furthermore, the definite near-surface structures of the Pt/Co/Pt(111) sandwich, the concentration of the Co atoms located just below the Pt-enriched topmost surface of the 773 K Co0.3 nm/Pt(111) might be higher than the Pt65Co35(111) sample prepared by the hydrogen oxygen flame method.39 LEISS,19,38,57,58 and/or STM,59 would provide us essential information for the alloy composition at the topmost surface, although no attempts has been made in this study. At any rate, the CV results shown in Figure 6 indicate that the deposited Co atoms located below the Pt-enriched topmost surface strongly modify the electrochemical properties of the outermost Pt layer. We measured LSV curves for the 773 K Co0.3 nm/Pt(111), Pt0.6 nm/Co0.3 nm/Pt(111), and Pt0.3 nm/Co0.3 nm/Pt(111) surfaces in oxygen-saturated 0.1 M HClO4; the results are shown in Figure 7a along with results for the clean Pt(111). All the Ptenriched surfaces show positive shifts in half-wave potential; the shift for the 773 K Co0.3 nm/Pt(111) is 75 mV relative to the clean Pt(111). We evaluated specific ORR activities of the catalysts by kinetic current densities (ik) using the Koutecky Levich equation.40 42 We present the inverse current densities as a function of inverse of the square root of rotation rates for the 773 K Co0.3 nm/Pt(111) and clean Pt(111) in Figure 7b. Linear plots of the samples shown in Figure 7b clearly indicate firstorder kinetics for ORR.60 On the basis of the ik values at 0.9 V estimated from the intercepts of the corresponding straight lines, the Pt-enriched topmost surface generated by the deposition of Co onto the Pt(111) at 773 K was an approximately 10 times higher ORR activity. The Pt/Co/Pt(111) sandwich surfaces showed relatively low ORR enhancements: estimated enhancement factors for the Pt0.3 nm/Co0.3 nm/Pt(111) and Pt0.6 nm/Co0.3 nm/Pt(111) are 4 and 6, respectively. As shown in Figure 6, the CV curve for the former sandwich surface revealed the hydrogen-related redox feature, suggesting dissolution of the surface Co atoms. A lesser amount of the Co atoms might result in relatively low ORR enhancement (factor of 4). In other words, complete covering of the underlying Co atoms by the topmost Pt layer is a key for achieving the highest ORR activity. In contrast, as for the latter sandwich surface, the relatively low enhancement (factor of 6) is probably caused by a less-effective electronic contribution (“ligand effect”) of the Co atoms located at the third layer. The results obtained for the sandwich surfaces clearly show that the ORR activity for the Pt-enriched topmost surface is determined by the underlying alloying element (Co) located at the subsurface region. We have reported the electrochemical properties of a Ptenriched surface of MBE-fabricated Ni/Pt(111) in which the ORR activity of the surface was 8 times higher than that for the clean Pt(111).21 In contrast, as shown in this study, the 773 K Co0.3 nm/Pt(111) revealed a 10 times higher ORR activity: the activity for the Co/Pt(111) is ca. 10% higher than that for the Ni/ Pt(111). A similar difference in ORR activity has been reported for Pt3M alloys,19,57,58 in which the activity for the Pt3Co is greater than that for the Pt3Ni. To date, enhancements in ORR activity by alloying of Pt with transition metals have been discussed theoretically:56,61 64 the “d band model” proposed by Nørskov and co-workers61 successively explains the trend in the activity enhancement. Furthermore, the lattice strain of the topmost Pt layer might correlate with the ORR activity 18594
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The Journal of Physical Chemistry C enhancement. Hyman and Medlin64 showed that compressive strain of the Pt/Pt3Co destabilizes the surface intermediates, such as O, OH, OOH, O2, and H2O. Roques and Anderson56 investigated molecular behaviors on the topmost Pt surface as a function of Co concentration at the second layer. They showed that, the higher the Co concentration, the weaker the adsorption energies of H2O and OH on the topmost surface. The abovementioned theoretical results clearly demonstrate that interface structures between the topmost Pt layer and the underlying alloying elements (Co,Ni) determine ORR activity enhancements. Our electrochemical results for the artificial Pt/Co/ Pt(111) sandwich surfaces (Figures 6 and 7) also indicate that the electrochemical properties of the Pt-enriched surface are quite sensitive to the Co concentration at the second and/or third layers. The difference in the surface electronic property and/or strain might cause the different ORR activities of the Co/ Pt(111) and Ni/Pt(111). Surface scientific investigations of alloy surfaces before (as-prepared) and after electrochemical measurements38,65 would give us a clue for the comprehensive understanding not only of the ORR enhancement but also of the durability of the Pt-based alloy catalysts.
4. SUMMARY We investigated the outermost surface structures of Co0.3 nm/ Pt(111) fabricated at various substrate temperatures using LEED and IRRAS. The 0.3 nm thick Co deposition at 333 K on the Pt(111) showed incoherent epitaxial growth of Co. The in-plane lattice distances increased concomitantly with increasing Co deposition temperature. The distances for the surfaces fabricated at 773 K were almost identical to those of clean Pt(111). For surfaces fabricated at temperatures lower than 473 K, the IRRAS bands attributable to adsorbed CO on the surface Pt sites influenced by neighboring Co atoms are positioned at around 2050 2060 cm 1; for surfaces fabricated at 773 K, the band is located at 2082 cm 1. For surfaces fabricated at 843 K, the linearly bonded C O stretch frequencies are almost identical to those for clean Pt(111), at around 2090 cm 1. These results suggest that the deposition of 0.3 nm thick Co onto a clean Pt(111) substrate at 773 K generates the Pt-enriched topmost surface, an almost pure Pt atomic layer having 6-fold symmetry. The epitaxial Co layer on the Pt(111) was unstable in the electrochemical environment. In contrast, the Pt-enriched layer is rather stable in the oxygen-saturated 0.1 M HClO4 solution. Its ORR activity was 10 times higher than the activity of Pt(111), clearly indicating that underlying Co atoms enhance the ORR that proceeds on the Pt-enriched topmost surface. Consequently, the atomic design of near-surface structures and compositions of Pt-based alloys produced by thermal processing are essential for developing highly active electrode catalysts for polymer electrolyte fuel cells. ’ AUTHOR INFORMATION Corresponding Author
*Phone: +81-22-217-7319. Fax: +81-22-217-7319. E-mail:
[email protected].
’ ACKNOWLEDGMENT This study was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
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T.W. expresses his thanks to the Ministry of Education, Culture, Sports, Science and Technology of Japan for Grant-in-Aid for Scientific Research (B) (22360298).
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