Structural and Electrochemical Characterization of Metastable PtAu

Jan 15, 2010 - Structural and Electrochemical Characterization of Metastable PtAu Bulk and Surface Alloys Prepared by Crossed-Beam Pulsed Laser Deposi...
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Structural and Electrochemical Characterization of Metastable PtAu Bulk and Surface Alloys Prepared by Crossed-Beam Pulsed Laser Deposition Eric Irissou,† Francois Laplante,‡ Sebastien Garbarino, Mohammed Chaker, and Daniel Guay* INRS-EMT, 1650 BouleVard Lionel-Boulet, C.P. 1020, Varennes, Que´bec J3X 1S2, Canada ReceiVed: September 3, 2009; ReVised Manuscript ReceiVed: December 21, 2009

Crossed-beam pulsed laser deposition in a moderate He background gas pressure was used to prepare PtAu thin films. The Pt bulk content was determined by neutron activation analysis, whereas X-ray diffraction and X-ray photoelectron spectroscopy were used to assess the bulk and the surface structure of the films, respectively. It is shown that metastable PtAu alloys with a unique fcc structure are formed over the whole composition range. The surface composition of the films closely follows the bulk content, and X-ray photoelectron spectroscopy reveals that the surface of the films is also made of a PtAu alloy. These films are stable under ambient conditions. The electrochemical properties of these films were determined by cyclic voltammetry in H2SO4 electrolyte, and their reactivity toward the electrooxidation of CO and the electroreduction of O2 was assessed. The CO stripping peak potential value increases with the Au content, indicating an increased binding energy in comparison with polycrystalline Pt. Similarly, there is a cathodic shift of the Pt oxide reduction peak for the Au-rich alloy that indicates stronger Pt-O binding energies as compared with Pt-rich alloy electrodes. At the surface, the presence of Au in close proximity to Pt atoms induces a shift of the d-band center of the Pt atoms that translates into stronger bonds with CO- and O-containing species at the surface of the samples. As far as we can tell, the surface composition and structure of the deposits are not modified following the electrochemical measurements. Introduction Over the last years, the platinum-gold system has received increasing attention due to its superior catalytic properties for a number of reactions. For example, the Pt-Au systems have been reported to be more active than pure Pt for the oxidation of ethylene glycol,1 methanol,2–7 formic acid,2,8–11 nitrite,12 and the reduction of O2.3,13–18 The addition of Au has also been shown to increase the stability of Pt-based electrocatalysts6,16,19 and reduced the poisoning of the electrocatalyst surface.1,3 The Pt-Au system has a positive heat of formation,20 and the equilibrium phase diagram displays a large immiscibility gap below 1260 °C (critical temperature).21 As a result, the synthesis of PtAu alloys is challenging and it is expected that the procedure used to prepare mixed Pt-Au catalysts will have a strong influence on the composition and structure of the resulting material. For example, a recent study showed that catalysts prepared by impregnation from Pt and Au precursors are not different from those obtained from monometallic Pt, suggesting that the presence of Au did not affect the catalytic performance of Pt. This was attributed to phase segregation of Pt and Au due to their small miscibility gap.22 In some cases, the fact that alloying between Pt and Au atoms is not taking place, nevertheless, results in a performance increase.16,23 A performance increase has also been observed for Pt@Au core-shell nanoparticles that are made of a Au core nanoparticle on which a thin layer of Pt is deposited.24,25 Yet, alloying of Pt and Au is expected to drastically modify the electronic structure * To whom correspondence should be addressed. E-mail: [email protected]. † Permanent address: National Research Council Canada - Industrial Materials Institute, 75 Boulevard de Mortagne, Boucherville (QC) Canada J4B 6Y4. ‡ Permanent address: Centre de recherche et de de´veloppement Arvida, 1955 Boulevard Mellon, C.P. 1250, Saguenay (QC) Canada G7S 4K8.

of the resulting material, thereby allowing an electrocatalytic enhancement. Recently, it has been shown that the reactivity of a given metal atom in different surroundings is determined by the location of the center of the d band relative to the Fermi level.26–28 Shifts in the surface d states can be obtained by alloying,27,29,30 by forming overlayers,27,31 by strain,28 or by changing the coordination number of the surface metal atoms.32,33 Only a few reports have appeared in the literature giving clear indication that bulk alloying of Pt and Au has been achieved. In most of these cases, the synthesis of PtAu alloys has been reported for a single composition close to 1:1,2,8–10,12,13 whereas the synthesis of PtAu alloys with different bimetallic compositions (other than 1:1) have been achieved in a very limited number of reports. For example, Luo et al. and Mott et al. have shown by X-ray diffraction that carbon-supported PtAu alloy nanoparticles of several bimetallic compositions could be prepared by a two-phase synthesis protocol,34,35 whereas Brown et al. have used co-sputtering to achieve homogeneous mixing of Pt and Au atoms.17 Although the alloy character of the core of PtAu nanoparticles and films can readily be established through X-ray diffraction analysis, it is more challenging to establish that Pt and Au are alloyed at the surface of the nanoparticles and films. This is important since the surface free energies of Pt and Au differ from each other and that surface segregation is expected. In that respect, Mott et al. gave the first evidence through infrared spectroscopic data of CO adsorption that both the core and the surface of the platinum-gold nanoparticles exhibit bimetallic alloy properties.35 During the past decade, pulsed laser deposition (PLD) has established itself as a versatile method for the synthesis of nanocrystalline and cluster-assembled films when deposition is performed in a moderate pressure.36 Indeed, we showed recently

10.1021/jp908524u  2010 American Chemical Society Published on Web 01/15/2010

PtAu Bulk and Surface Alloys Prepared by CBPLD that gold nanostructured thin films37,38 and Pt nanoparticles39 could be prepared by PLD in an inert background atmosphere at room temperature and that control over the diameter of the nanoparticles could be readily achieved, allowing the investigation of the effect of nanoparticle size on their electrochemical stability.40 More recently, we showed that mixed Pt-Sn41 electrocatalysts could be prepared by dual beam pulsed laser deposition and that the resulting material exhibits distinctly different properties than pure Pt. Also, we have investigated the influence of an inert background gas on crossed-beam pulsed laser deposition using two dissimilar targets (Pt-Au) and have determined the optimal deposition conditions.42 In this study, we will investigate the structural and physicochemical properties of mixed Pt-Au films prepared by crossedbeam pulsed laser deposition (CBPLD). Using X-ray diffraction analysis and X-ray photoelectron spectroscopy, we will show that bulk and surface Pt-Au alloys can be prepared over the whole composition range. The electrocatalytic properties of these films will be investigated, and the electrooxidation of CO and reduction of O2 as compared to pure Pt will be studied. The reasons affecting the electrocatalytic behavior of such alloys will be discussed. Experimental Section PtAu thin films were prepared by crossed-beam pulsed laser deposition (CBPLD) in a moderate He background gas pressure. The setup used to perform the CBPLD experiments was designed according to Tselev et al.,43 and a detailed description can be found elsewhere.42 In brief, two laser beams (KrF @ 248 nm; 17 ns pulse width; repetition rate, 100 Hz; fluences, 1.5-8.0 J cm-2) are synchronously focused onto two pure (>99.99%) solid targets (Pt and Au) whose normal to the surface makes a 90° angle to each other. The two resulting ablation plumes propagate into a vacuum chamber (base pressure ) 5 × 10-5 Torr) filled with helium gas (He pressure ) 1.6-3.2 Torr), crossing each other at 3-5 cm in front of the substrate. At this point, a resulting plume traveling in a direction making a 45° angle with both initial plumes propagate to the substrate. A diaphragm is inserted at the interaction zone of the two initial plumes to avoid direct deposition of materials on the substrate. The Pt and Au contents of the film were varied by either (i) varying independently the fluence on each target or (ii) slightly rotating one target with respect to the other so that the normal to both targets does not intersect at a right angle (asymmetrical CBPLD43). All experiments were performed at room temperature, and the thicknesses of all films were in the 75-150 nm range. The substrates consisted of 6 × 10 mm glass plates previously cleaned with sulfochromic and hydrofluoric acid. They were then thoroughly rinsed with distilled water and dried with a stream of purified nitrogen. The crystalline structure of the mixed PtAu films was characterized by X-ray diffraction (XRD) using a Bruker AXS D8 advance diffractometer with Cu KR radiation. All measurements were performed at a grazing incidence angle of 5°. Quantitative analyses were carried out by the Rietveld method44 using GSAS and EXPGUI softwares.45,46 Neutron activation analyses (NAA) were performed to measure the bulk Pt and Au concentrations. The surface morphology was investigated by scanning tunneling microscopy (STM) using a Nanoscope III microscope from Digital Instruments. All measurements were performed at room temperature and in ambient air conditions. The surface composition and electronic structure were investigated by means of X-ray photoelectron spectroscopy (XPS) performed using a VG Escalab 220i-XL equipped with an Al

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Figure 1. XRD patterns of PtAu films prepared by crossed-beam pulsed laser deposition. The Pt content is (A) 100, (B) 86, (C) 76, (D) 67, (E) 62, (F) 49, (G) 38, (H) 33, (I) 27, (J) 19, and (K) 0 atom %.

KR monochromatic source. All samples were analyzed with a spot size of 250 × 1000 µm at five different locations. The relative Pt (and Au) surface content was calculated from the following equation [Pt]surf ) [Pt]/([Pt] + [Au]) × 100, where [Pt] and [Au] were established by integrating the surface area under the Pt and Au 4f core-level peaks, respectively. The C 1s core-level peak at 284.6 eV was used as a reference. For electrochemical characterization, the PtAu thin films were deposited by CBPLD on glassy carbon (GC) electrodes that were then used as working electrodes in a standard three compartment cell. Prior to each electrode characterization, the glassware and the electrochemical cell were cleaned according to a wellestablished method.47 The auxiliary and the reference electrodes were a platinum gauze and reversible hydrogen electrode (RHE), respectively; all mentioned potential values are, therefore, referred to the RHE scale. The electrochemical testing of the electrodes was divided in three different steps. First, cyclic voltammetry (CV) curves were initially recorded at room temperature in deareated (Argon N5.0, Praxair) 0.5 M ultrapure sulfuric acid (A300-212, Fisher Scientific) until a stable CV was obtained. Carbon monoxide (N2.5, Praxair) adsorption was then performed by bubbling purified CO (oxygen trap, CRS) for 3 min with the electrode potential held at 0.3 V. Residual CO was removed by bubbling Ar for 15 min, and two successive CVs were recorded. Finally, voltammogramms were recorded in O2-saturated (N4.3, Praxair) 0.5 M H2SO4 solution by linear sweeping of the electrode potential from 1.45 to 0.05 V. Results and Discussion Characterization of the As-Deposited Materials. Figure 1 shows the X-ray diffraction (XRD) patterns of mixed Pt-Au films. The bulk Pt content of each film, as determined by neutron activation analysis, is given in the caption. Pure metal films ([Pt] ) 0 and 100 atom %) were obtained using a conventional PLD configuration (single target).38 Each XRD pattern exhibits a unique series of five diffraction peaks that can be indexed to a simple face-centered cubic (fcc) phase. As seen in Figure 1, there is a gradual shift of the various peak positions toward the larger 2θ values as the Pt content increases from 0 to 100 atom %. There is also a discernible peak broadening as the bulk Pt content increases from 0 to 100 atom %, suggesting that the crystallite sizes are reduced as the Pt content is increased.

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Figure 2. Variation of the lattice parameter a as a function of the Pt bulk content.

According to the Pt-Au phase diagram, Pt and Au are immiscible below 1260 °C (critical temperature)21 and the material is made of two distinct fcc structures (Pt-rich and Aurich fcc phases). The immiscibility gap between Pt and Au is large and reaches ∼85 atom % at 400 °C. For Pt-Au films prepared at temperature below the critical temperature, the existence of an immiscibility gap should yield to an XRD pattern exhibiting two sets of diffraction peaks, each set belonging to a distinct fcc phase. This is obviously not the case here, and Pt-Au films made of a unique fcc structure are formed over the whole composition range from [Pt]bulk ) 0-100 atom %. Independent measurements (not described here) have allowed us to estimate that the substrate temperature increases from 25 to ∼70 °C during the deposition of a 150 nm thick PtAu film. This estimation is consistent with the results of Kusumori et al., where the occurrence of a phase transition in SiC was used to estimate the temperature rise occurring at the surface of a substrate during PLD.48 The maximum temperature reached by the substrate surface is far below the critical temperature above which Pt and Au are totally miscible. Therefore, the formation of metastable PtAu alloys must be related to the deposition conditions (far from equilibrium) reached during CBPLD. The fcc lattice parameter, a, of a few selected Pt-Au films was extracted from a Rietveld analysis of their XRD histograms. The bulk Pt atomic concentration, [Pt]bulk, of these films was assessed by neutron activation analysis, and a plot of the lattice parameter a versus [Pt]bulk is displayed in Figure 2. There is a linear variation of the lattice parameter a with respect to [Pt]bulk, indicating that Vegard’s law holds true for Pt-Au films prepared by CBPLD. This is a strong indication that a substitutional PtAu alloy can be formed over the whole composition range. Once rearranged, the equation of the linear curve describing the variation of a with respect to [Pt]bulk can be used to estimate the bulk Pt content from the lattice parameter a of the fcc phase ([Pt]bulk (atom %) ) -601 × a (Å) + 2451). The metastable character of the deposits is nicely confirmed by an examination of the XRD pattern of the film after thermal treatment. Figure 3 shows a comparison between the XRD pattern of an as-deposited [Pt] ) 49 atom % film and the same film after it was annealed at 450 °C under argon atmosphere for 72 h. The positions of the diffraction peaks of pure Au and Pt are indicated at the bottom. After annealing, two sets of diffraction peaks are clearly observed in the XRD pattern, indicating that the crystalline structure of the film has evolved from a single to two distinct fcc phases, confirming the metastable character of the as-deposited film. One series of peaks

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Figure 3. XRD patterns of a PtAu film ([Pt] ) 49 atom %): (A) as deposited and (B) annealed at 450 °C for 72 h. The peak positions of pure Au and pure Pt are indicated.

Figure 4. Variation of the mean crystallite size as a function of the Pt bulk content.

originates from a compound with a lattice parameter a ) 4.043 Å, whereas the second series of peaks corresponds to a compound with a ) 3.927 Å. On the basis of the relationship established previously that relates the lattice parameter a and the Pt bulk concentration, it is estimated that one of the phases corresponds to Au(Pt) with a Pt content of ∼21 atom %, whereas the other one corresponds to Pt(Au) with a Au content of ∼9 atom %. Both of these values are slightly larger than expected on the basis of the Pt-Au binary phase diagram, indicating that equilibrium conditions might not be reached after a 72 h heating period. Nevertheless, these results nicely demonstrate that the synthesis of metastable PtAu films has been achieved. The XRD patterns of PtAu films kept at room temperature during a period of 12 months are identical to those of the native films, indicating that the metastable alloy is stable at room temperature. Variation of the XRD peak broadening with composition can be accounted for on the basis of a modification of the crystallite size and the introduction of strain in the films. For the Pt-Au system deposited by PLD, strain-induced broadening of the diffraction peaks was less than 0.04% and does not vary with the composition of the film. This is thought to reflect the fact that the a lattice parameters of pure Pt and Au do not differ by more than 4% from each other. In contrast, as depicted in Figure 4, the variation of the crystallite size with [Pt]bulk, as determined from the Rietveld analysis, is nonlinear with respect to the Pt bulk content, unlike what was previously observed in the case of the lattice parameter. The crystallite size decreases from ∼27 to ∼7 nm as [Pt]bulk increases from 0 to ∼35 atom %. For [Pt]bulk

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Figure 7. XPS core electron binding energy difference, ∆BE, defined as Au 4f7/2 - Pt 4f7/2, the bulk Pt content.

Figure 5. Scanning tunnelling microscopy image of a Pt-Au film with [Pt]bulk ) 50 atom %.

Figure 6. Variation of the Pt surface concentration ([Pt]sur) with respect to the Pt bulk concentration ([Pt]bulk).

> 35 atom %, the crystallize size stays fairly constant at ∼7 nm. The crystallite sizes deduced from Rietveld analysis are consistent with measurements performed by scanning tunneling microscopy (STM) that was used to image the surface of the film. An STM image of a film with [Pt]bulk ) 50 atom % is shown in Figure 5. Spherically shaped particles are observed at the surface of the sample. The mean size (standard deviation) of these structures is 6.63 nm (0.79 nm). This value coincides well with the crystallite size obtained from the Rietveld analysis, strongly suggesting that the bulk of the film is made of nanoparticles similar to those observed at its surface. X-ray photoelectron spectroscopy was used to estimate the Pt surface content ([Pt]surf), and Figure 6 shows a plot of [Pt]surf against [Pt]bulk. The [Pt]surf error bars were estimated from the standard deviation of five different measurements realized on each deposit. In each case, the error bars are small (less than 2 atom %), indicating that the deposits have a uniform composition. As seen in Figure 6, [Pt]surf varies linearly with [Pt]bulk, indicating that there is no segregation at the surface of the

deposit within the first 2-3 nm thick layer probed by XPS. Furthermore, depth profiling of [Pt]surf established through ion etching of a [Pt]bulk ) 60 atom % deposit (not shown) reveals that the Pt surface content varies by less than 1 atom % throughout the whole thickness of the film, showing that the composition of the film is very homogeneous up to the outermost layer of the film. The Pt and Au 4f core-level XPS spectra of PtAu films were recorded. The curves (not shown) display the characteristic 4f7/2 and 4f5/2 core-level peaks of Pt (∼71 and ∼74 eV, respectively) and Au (∼84 and ∼ 87 eV, respectively). On the basis of the literature,49 these two doublets are assigned to Pt and Au in their metallic states. A closer inspection of these curves reveals that there is a small, but significant, shift in the position of the 4f7/2 and 4f5/2 core-level peaks between these samples. To eliminate errors in the absolute values of the binding energies, the binding energy difference between the Au and Pt 4f7/2 corelevel peaks, ∆BE, of the samples has been computed with respect to the bulk Pt content of the film. As seen in Figure 7, ∆BE varies from ∼13.4 eV for the Au-rich sample to ∼12.7 eV for the Pt-rich sample. The Au 4f7/2 peak position shifts from 83.8 to 83.6 eV (lower binding energy) as [Pt]bulk increases from 9% to 96%. In contrast, the Pt 4f7/2 core-level peak shifts from 70.3 to 70.9 eV (higher binding energy) as [Pt]bulk increases from low to high values. The binding energy difference between the commonly accepted values of Au and Pt 4f7/2 core-level peaks of bulk Au and Pt (83.8 and 70.9 eV, respectively) is 12.9 eV.49 It has been reported that the X-ray photoemission spectra of Pt and Au surface atoms are displaced to lower binding energy compared with Pt and Au bulk atoms. Using synchrotron radiation with an excitation energy that minimizes the mean free path of the photoelectron and increases the surface sensitivity, Ho¨rnstro¨m et al. have shown that the 4f core-level binding energy of Pt and Au surface atoms are shifted by ∼0.3 eV toward the lower binding energy compared with Pt and Au bulk atoms.50 This is due to the fact that surface atoms have different coordination numbers and do not experience the same potential as bulk atoms. However, laboratory-based X-ray photoemission spectrometers do not offer the surface sensitivity that would allow resolving surface from bulk core-level peaks. At best, the surface core-level peak will cause a small broadening of the bulk core-level peak with only a small effect on the position of the peak.51 Moreover, as mentioned earlier, surface core-level peaks are shifted to lower binding energy and both Au and Pt 4f7/2 core-level peaks would be expected to shift in

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the same direction. This is not the case here because Pt and Au core-level peaks shift in opposite directions with respect to their bulk counterparts. It is well-known that the core-level peak shifts toward higher binding energy and becomes progressively broader as the cluster size is decreased.52 This has been nicely confirmed recently in a study of isolated Pt and Au nanoparticles prepared by pulsed laser deposition, where it was shown that the Pt and Au 4f corelevel peaks are shifted by ∼+0.60 and +0.15 eV toward larger binding energy compared to bulk Pt and Au as the diameter of the nanoparticle reaches 1.8 and 2.4 nm, respectively.40,53 The shift in core-level binding energies and the changes of the peak width with the diameter of the Pt nanoparticles agree well with experimental results reported elsewhere for Pt and other noble metal nanoparticles deposited on a carbon support.54–66 However, there is a disagreement over the assignment of these characteristics to initial- or final-state effects. Thus, for example, Mason,54 Vijayakrishnan et al.,59 Lopez-Salido et al.,56 and, more recently, Zhang et al.55 concluded that initial-state effects, such as change in the atomic coordination number, metal-insulator transition, or charge transfer effects, dominate the binding energy shift for small nanoparticles deposited on poorly conducting surfaces. These conclusions are not supported by other studies where the spectral changes are related to final-state screening due to relaxation effects.60,61,64–66 Independent of the mechanisms responsible for this effect, decreasing the size of nanoparticle shifts the core-level peaks to higher binding energy, and again, both Pt and Au 4f core-level peaks would be expected to shift in the same direction if a variation of the particle size was responsible for this effect. Moreover, as shown in ref 40, the shift of the core-level peaks becomes negligible as the diameter of the Pt nanoparticle exceeds ∼6 nm, which corresponds to the smallest crystallite size observed here. On the basis of this, shifts of the Pt and Au 4f core-level peaks due to a difference in particle size are not expected. Measurements of Au and Pt core-electron binding-energy shifts occurring when these elements are made the minor constituents in alloys of the other element have appeared in the literature. For example, a -0.30 eV shift was reported for Au alloyed with Pt (Au0.1Pt0.9) compared to bulk Au samples.67 Au and Pt are totally miscible at that composition. A negative shift of the core-level binding energy of Au dissolved in Pt was also reported by Ho¨rnstro¨m et al. for Au0.15Pt0.85 and Au0.02Pt0.98.50 Similar conclusions were reached when Au overlayers were deposited on a Pt substrate. For example, a -0.25 eV shift of the Au core-level peaks to lower binding energy is noticed when the thickness of Au deposited on Pt(100) is reduced from 4 ML (bulk gold) to 1 ML.68 Similarly, Berg et al. have noticed a -0.39 eV difference between the Au 4f core-level peaks of gold as the thickness of the Au overlayer is reduced from 14 to 1 Å.69 This was explained by the difference in the electronegativity of both elements, Au being the most electronegative metallic element on the Pauling scale. The -0.2 eV shift in the binding energy of the Au 4f core-level peaks we have observed as the concentration of Pt is increased from 0 (pure gold) to 100 atom % (pure Pt) is consistent with these previous findings. It gives a strong indication that a PtAu surface alloy is formed. To the best of our knowledge, a Pt 4 f core-level X-ray photoemission spectroscopy of a Pt overlayer grown on a Au substrate has not appeared in the literature. Instead, ab initio methods have predicted a +0.43 eV shift of the Pt d band for a pseudomorphic overlayer of Pt on Au(111).27 Consistently, a higher desorption temperature of CO adsorbed on a monolayer of Pt deposited on Au(111) was recorded.31 Evidence of the

Irissou et al. PtAu surface exhibiting bimetallic alloy properties has also been presented for PtAu nanoparticles (∼2 nm diameter) using FTIR spectroscopy measurements of adsorbed CO.35 Recently, Xiao et al. have simultaneously observed an upward (downward) shift of the Pt (Au) 4f core-level peaks in platinum-gold alloy nanoparticles deposited on multiwalled carbon nanotubes.12 These results are consistent with those of the present study and are indicative that formation of a true PtAu surface alloy formation does indeed occur. The surface composition is generally different from that of the bulk. In the Pt-Au system, such a difference between the bulk and surface composition is expected because the surface free energy of gold (1.41 J cm-2) is lower than that of platinum (2.34 J cm-2).70 Estimates of the surface segregation energy fall in the range of 50-100 kJ mol-1.71–73 A few studies have shown that this is indeed the case. For example, using Auger electron spectroscopy, Schwarz et al. have shown that the equilibrium surface concentration of Pt-5% Au is essentially a monolayer of Au at 600 °C.71 Similarly, Tsong et al. have shown using SIMS measurements that the Au concentration in the top (first) layer of dilute gold alloys of platinum (4.1%) is as high as 99% at 600 °C and that the Au concentration decreases monotonically into the bulk with a characteristic depth of about three atomic layers.74 At room temperature, Pedersen et al. have shown that deposition of Pt at a coverage from 0.02 ML up to 2.5 ML on Au(111) initially leads to substitutional alloying of 3% Pt into the top Au layer, followed by growth of islands with a mixed composition of Pt and Au.31 Temperature-programmed desorption (TPD) measurements have shown that CO readily adsorbs on the freshly prepared surface. However, after heat treatment to 600 K, exposure to CO does not result in any CO being adsorbed on the surface.31 This is indicative that Pt diffused away from the surface as CO does not adsorb on the pure gold surface. As mentioned earlier, there is an energetic advantage in Au capping the Pt structures and it was hypothesized that, at high temperature, the surface of the sample rearranges itself to minimize its energy. Thus, at room temperature, reorganization of the electrode surface would be kinetically hindered and the PtAu alloyed surface could exist even if the surface free energy of Pt and Au differs by ∼1.0 J cm-2. Characterization of the Materials after Electrochemistry. XRD and XPS analyses of a few selected PtAu films were conducted after they were characterized for their electrocatalytic activities for CO oxidation and O2 reduction (see below). In all cases, the XRD pattern after electrochemistry is identical to that of the as-deposited film, indicating that dealloying is not occurring and that the metastable character of the Pt-Au films is preserved (see Figure S1 of the Supporting Information). The same assertion holds true in the case of the XPS data that show no change in the Pt surface content and no modification of the binding energy difference between the Au and Pt 4f core-level peak after electrochemistry (see Figure S2 of the Supporting Information).This indicates that the surface content of the films is not modified by the electrochemical procedure and that dealloying is not occurring at the surface of the films. Electrocatalytic Activity. In Figure 8, stable cyclic voltammograms (CVs) for representative PtAu alloys in Ar-purged 0.5 M H2SO4 are shown. The Au content (atom %) varied from 1% in Figure 8B to 95% in Figure 8F. For the sake of comparison, the CV curves of pure polycrystalline platinum and pure polycrystalline gold are also presented in Figure 8A. All CVs display the well-known voltammetric features in sulfuric acid solution for (i) the pure Pt/hydrogen sorption region for E < 0.40 V and Pt oxide formation/removal for E > 0.75 V and

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Figure 8. Cyclic voltammetric profiles (15 mV s-1) between 50 and 1400 mV (dashed line) and between 50 and 1800 mV (solid and dotted lines) for (A) polycrystalline Pt (dashed and solid lines) and polycrystalline Au (dotted line). The bulk PtAu alloy compositions are (B) Pt99Au1, (C) Pt86Au14, (D) Pt47Au53, (E) Pt20Au80, and (F) Pt5Au95. All measurements were realized in Ar-saturated 0.5 M H2SO4.

Figure 9. Voltammetric profiles (15 mV s-1) following adsorption of a CO monolayer for (A) polycrystalline Pt, (B) Pt86Au14, (C) Pt47Au53, (D) Pt20Au80, and (E) Pt5Au95 in Ar-saturated 0.5 M H2SO4.

for (ii) pure Au/double layer charging for E < 1.15 V and Au oxide formation/removal at E > 1.15 V. Hence, CVs for PtAu alloys tend to appear as a linear combination of the CVs for both pure metals.17 During the reverse sweeps, fingerprints of both metals are observed as well-defined cathodic peaks, that is, around ca. 1.16 and 0.76 V for gold and platinum oxide reduction peaks, respectively. It is interesting to note that, at PtAu alloy electrodes, both single metal oxide reduction peaks are observed, which is in good agreement with previous voltammetric studies of PtAu alloys.1,75 By integrating the charges involved in both metal oxide reduction waves,76 the electrochemical determination of the Pt surface content leads to similar values ((4%) as those extracted from XPS surface analysis. A closer inspection of the CVs in Figure 8 reveals that the peak potential value for gold oxide reduction remained constant at ca. 1.16 V for the whole range of alloy composition, whereas a significant potential shift is observed for the platinum oxide reduction peak, from E ) 0.79 V for Pt99Au1 to E ) 0.74 V for Pt20Au80. Therefore, potential peaks of the two reduction waves appear at slightly less anodic potentials on PtAu alloys than on the respective pure metals, for example, 1.18 V for polycrystalline Au and 0.81 V for polycrystalline Pt. The slightly larger oxide reduction hindrance as compared with the pure metals was already shown to reflect the reactivity of PtAu surface alloys.75,77 The cathodic shift of the Pt oxide reduction peak for the Au-rich alloy reveals the stronger Pt-O binding energies as compared with Pt-rich alloy electrodes. This might be related to the fact that the lattice parameter of PtAu alloys increases with the Au content. Oxygen-type adsorbates, such as O and OH species, were also recently found to be stabilized at the

surface of expanded Pt as compared to bulk Pt.78 It must be mentioned that an increase in the oxophilicity of platinum was also recently correlated to a size effect, and a 130 mV cathodic shift of the Pt oxide reduction peak was observed as the particle size is decreased from 30 to 1 nm.79 However, in this study, such a size effect can be ruled out as the particle size remained constant at ca. 7 nm for PtAu alloys with [Pt]surf > 20 atom % (see Figure 4). Two consecutive CVs for PtAu alloy/glassy carbon electrodes in 0.5 M H2SO4 for the oxidation of a monolayer of adsorbed CO (solid line) were recorded, and for clarity, only the resulting positive sweeps are presented in Figure 9. In every case, the presence of CO at the alloy surface (solid line) induced a suppression of the hydrogen desorption peaks at the platinum surface atoms, from ca. 0.05 to 0.40 V. At such low sweep rates (15 mV s-1), the whole CO monolayer is oxidized during the first sweep, as illustrated by the recovery of the typical voltammetric features in the second positive sweep (dashed line), which are strikingly similar to previous results in Ar-saturated electrolyte (Figure 8). For the Pt-rich alloy electrodes, a well-defined CO stripping anodic peak is obtained, which correlates with the conventional behavior on the polycrystalline (pc) Pt electrode depicted in Figure 9A. However, the CO peak potential value increases slightly as the Au content of the alloy is increased. The CO stripping peak potential value is 0.79, 0.82, 0.84, and 0.86 V for Pt100, Pt86Au14, Pt47Au53, and Pt20Au80 electrodes, respectively. This shift of the CO stripping peak potential results from an increased binding energy in comparison with pc Pt due to a lattice constant increase of the fcc phase as the Au content of the alloy is augmented. A similar conclusion was reached by

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Irissou et al. Conclusion We have shown that metastable PtAu films can be prepared over the whole composition range by CBPLD. The bulk and the surface of these films are made of PtAu alloys. The presence of both Pt and Au atoms at the surface of the films translates into stronger bonds with CO- and O-containing species. This yields to an increased value (more positive) of the CO stripping peak potential and a decreased value (more negative) of the Pt oxide reduction peak. Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Research Chair program of Canada.

-1

Figure 10. Voltammetric profiles (15 mV s ) for representative PtAu alloys in O2-saturated 0.5 M H2SO4.

Mott et al. who were investigating the adsorption of gaseous CO at SiO2-supported PtAu nanoparticles by FT-IR.35 Recent DFT calculations showed that the d-band center of Pt atoms increased with Au concentration in the PtAu alloys.31 Such increase in the d-band center results in a stronger adsorption of CO and OH species at the surface of Pt atoms.80 Higher CO oxidation potential can be attributed to the higher Pt-O and Pt-CO binding energies in the case of Au-rich PtAu alloys, which is consistent with the cathodic shift observed previously in the cathodic reduction peak of the Pt oxide (Figure 8). It is also worth noting that, on the Au-rich alloy electrodes, an additional CO oxidation peak is present at ca. 1.10 V. The CO oxidation charge involved in the 0.95-1.35 V potential range is about ca. 2, 43, and 80% of the total CO stripping charge at Pt86Au14, Pt47Au53, and Pt20Au80 surfaces, respectively. It seems that, for PtAu alloys with intermediate composition, CO oxidation occurred at both metal sites, as evidenced by the two different oxidation waves occurring at ca. 0.85 V (Pt surface sites) and ca. 1.10 V (Au surface sites). In the case of Pt5Au95 in Figure 9E, the charge involved in the oxidation process at 1.10 V is dramatically decreased compared with the other alloys. This is attributed to the low amount of CO initially adsorbed at the platinum surface atoms and to the fact that the mean particle size at the surface of the Pt5Au95 alloy is ca. 25 nm (see Figure 4) larger than the limiting value below which CO adsorption is occurring even on pure gold particles.81 Figure 10 displays linear sweep voltammetry for PtAu alloy electrodes in O2-saturated 0.5 M H2SO4. The gold oxide reduction peak is apparent for the Au-rich alloy surfaces at ca. 1.20 V, while the hydrogen adsorption peaks remained present for the Pt-rich alloys at potentials below 0.40 V. The main reduction peak at ca. 0.75 V corresponds to both molecular oxygen and platinum oxide reduction processes. A gradual positive shift in the ORR peak potentials is observed from 0.37 V (Pt5Au95) to 0.77 V (Pt86Au14), which indicates that the kinetics of O2 reduction increased with increasing the Pt content. Such behavior corroborates quite well with numerous ORR studies at PtAu alloy surfaces.82,83 As previously described in Figure 8, the platinum oxides formed at Au-rich alloys are more difficult to reduce as compared with Pt-rich PtAu alloy surfaces. The increase in oxophilicity favors the formation of OH species that are known to be responsible for blocking the O2 adsorption sites;84 such enhanced oxophilicity resulted in a reduced ORR activity at the PtAu surface alloys, that is, a gradual peak potential shift toward lower potential values as the Au content of the alloy is increased.

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