ARTICLE pubs.acs.org/JPCC
MetalSupport Interactions between Nanosized Pt and Metal Oxides (WO3 and TiO2) Studied Using X-ray Photoelectron Spectroscopy Adam Lewera,*,† Laure Timperman,‡ Agata Roguska,§,|| and Nicolas Alonso-Vante*,‡ †
Department of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland Laboratory of Electrocatalysis, LACCO-UMR-CNRS 6503, University of Poitiers, 4, rue Michel Brunet - B27 BP 633, F-86022 Poitiers, France § Institute of Physical Chemistry, PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland
)
‡
ABSTRACT: Platinum nanoparticles have been selectively deposited on composites of titanium oxide-carbon and tungsten oxide-carbon. Selectivity of the deposition made it possible to investigate changes in electronic properties of both platinum and oxide support, induced by the so-called strong metalsupport interactions (SMSI). X-ray photoelectron spectroscopy (XPS) was used, and changes in binding energy of Pt 4f, Ti 2p, and W 4f core-levels and Pt 4f peak asymmetry were determined. These parameters allowed us to state the changes in local electron density, when Pt is deposited on oxide support. In all cases the binding energy of the Pt 4f signal for platinum deposited on an oxide support was significantly lower in comparison to samples where Pt was solely supported onto carbon. The increase in Pt 4f XPS signal asymmetry was observed. This suggests an increased electron density on Pt. No electron donor could be identified from the analysis of the oxide supports. To explain the observed data, at least two effects must be considered: (i) alloy formation between Pt and the oxide support and (ii) partial charge transfer from substrate to Pt, which can be correlated to previously observed increased activity toward oxygen reduction reaction.
’ INTRODUCTION The use of composites containing platinum-group metals and TiO2 or WO3 has been a subject of many studies.17 In particular, the inertness of those oxides makes those materials very attractive for applications in relatively strong oxidative conditions, for example at the cathode of a working fuel cell.4,7 Additionally, as compared to systems without metal oxides, those systems often exhibits superior electrocatalytical activity, for instance in terms of enhanced tolerance for methanol,3,811 ethanol,12 and carbon monoxide1315 or increased activity toward oxygen reduction reaction (ORR).11,1622 The enhancement of the catalytic activity, due to the presence of metaloxide support, is often called strong metalsupport interactions (SMSI) and significant effort has been devoted to understand this phenomenon,1,13,19,2426 due to its importance to many fields of science, in particular to material science and electrocatalysis. SMSI has also been observed for photocatalytic systems.23 It is worth to note that SMSI has been reported for many combinations of different oxides and different noble metals. In particular, Pt/TiO 2 exhibits increased activity toward ORR2,3,14,19,20,2729 and hydrogen oxidation2 and also increased CO tolerance.13,14 In the case of Pt/WO3 the enhancement has been observed for ORR2,18,29,30 and hydrogen2 and methanol oxidation.8 A composite systems, containing noble metal, deposited on oxides also shows increased activity in r 2011 American Chemical Society
electrocatalytic reactions; the enhancement has also been shown for PtRu/WO3, which is more active toward methanol electrooxidation:10 for RuSe/WO3 toward ORR11,16,17,21,22 and for PtSn/WO3 toward ethanol oxidation.12 Different ways of preparation were described for those noble metaloxide systems, like electro-deposition,28,31 vacuum deposition techniques,32 or thermal evaporation onto Nafion membranes.27 Although the interactions between metal and oxide substrate is best investigated if the catalyst is selectively deposited onto it, and no oxide bulk phase is present, SMSI is usually explained in terms of partial charge transfer1,16,24,25 or substrate-induced change in the lattice parameter of the metal deposited.19,20 To address the possible change in electronic properties of the components, X-ray photoelectron spectroscopy (XPS)33,34 has been utilized. It is especially useful to study the possible charge transfer, which is often considered to be responsible for the increased electrocatalytical activity due to SMSI.1,24,25,33 Particularly the change of the electronic properties of the metals when deposited on TiO2 has been investigated, and attributed to overlapping of d orbitals (occupied) from deposited metal, and the unoccupied d orbitals of the support.1 Further research done by Goepel et al.,35 shows that the surface defects Received: July 18, 2011 Revised: August 31, 2011 Published: September 05, 2011 20153
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play a significant role, introducing electronic states in the empty gap of TiO2, being able to act as electron donors, and specific adsorption sites for hydrogen and carbon monoxide. Herein we report on the changes of electronic properties of nanosized Pt, selectively photodeposited on the oxide (TiO2 or WO3)carbon composite: Pt/TiO2/C and Pt/WO3/C. As such, a significant amount of oxide substrate is in contact with Pt and practically no “bulk” oxide exists, which allowed us to comment also on the changes of the electronic structure of the oxide substrate. Both systems exhibit increased activity toward ORR20 which is correlated to down-shift the binding energy (BE) of Pt 4f electrons. Such down-shift suggests an increased electron density on Pt, which can explain the increased activity toward ORR.
’ EXPERIMENTAL SECTION Sample Preparation. Pt was photodeposited on composites of TiO2-carbon or WO3-carbon, as described elsewhere.19,20 The preparation of TiO2/C was inspired by the solgel method to synthesize TiO2 anatase. The carbon support (Vulcan XC-72 from Cabot Corporation, Boston, Massachusetts, USA; specific surface area: 250 m2/g; particle size: 2050 nm) has been ultrasonically dispersed into isopropanol for 2 h. Titanium isopropoxide, precursor of TiO2, was then added, followed by water in order to hydrolyze titanium isopropoxide to TiO2. This process was performed at 0 °C, and the suspension was maintained under stirring for 24 h. For WO3/C, the carbon was stirred with water under ultrasonic bath for 2 h. The chemical precursor of WO3, sodium metatungstate, was added to the suspension. Diluted HCl water solution was then added dropwise into the suspension, and the reaction was maintained for 6 h under stirring. Oxidecarbon composites were separated by filtration, washed with pure water, and dried under vacuum. For WO3/C, the powder was also heat treated at 450 °C under nitrogen atmosphere for 5 h. The oxide was embedded in the carbon matrix forming nanoparticles of ca. 5 nm unevenly distributed. The junction between the oxide nanoparticles and carbon revealed an ohmic contact formation. Pt has been photodeposited, as previously described.19 In brief, a TiO2/C or WO3/C suspension in deaerated water was prepared in a glass reactor having an optical quartz window. Isopropanol and Pt2+ complex (H2PtCl6 3 6H2O) were added, and the suspension was subsequently stirred for 3 h under illumination via a UV lamp (Xe lamp, 159 W). Visible and infrared wavelengths were cutoff via a filter (hot mirror UV, Silica). The electronhole pairs photogenerated via UV irradiation on TiO2 (anatase energy gap 3.2 eV) or WO3 (energy gap 2.6 eV) of the composites allowed for the reduction of Pt2+ to platinum Pt0.36 This process allowed for the deposition of metallic nanoparticles (average size of 2.0 ( 0.9 nm) selectively onto TiO2 or WO3.20,29 We have discussed the morphology of the investigated materials in more detail elsewhere.20,29 Finally the samples analyzed were 8 wt % Pt on 5 wt % TiO2carbon (Pt/TiO2/C) and 5 wt % Pt on 5 wt % WO3carbon (Pt/WO3/C). Additionally, 8 wt % Pt/C synthesized via carbonyl method37 and commercial 10 wt % Pt/C samples as well as composites of TiO2/C and WO3/C were used as references. XPS Measurements. X-ray photoelectron spectroscopy measurements were performed with Microlab 350 spectrometer using Al Kα non-monochromated radiation (1486.6 eV; 300 W) as the exciting source. The pressure during analysis was 5.0 107 Pa. The binding energy of the target elements
Figure 1. XP spectra of the Pt 4f region registered for Pt/C, Pt/TiO2/C, and Pt/WO3/C samples. A significant down-shift (270 or 290 meV) of Pt 4f BE can be observed, when Pt is deposited on oxidecarbon composites.
(Pt 4f, W 4f, Ti 2p, O 1s, and C 1s) was determined at a pass energy of 50 eV, with a resolution of 0.83 eV, using the binding energy of carbon (C 1s: 284.44 eV)38 as the internal standard. XP Spectra Fit. XP spectra were fitted using CASA XPS software. GaussianLorentzian (GL) line shape with Shirley background or DoniachSunjic (DS) line shape with linear background were used for electron’s BE and asymmetry analysis, respectively. Two different line shapes and background types have been used, due to the fact that (i) Shirley background effectively removes signal asymmetry and (ii) DS line shape (as opposed to GL profile) can fit asymmetric signals, but it is not well suited to determine the BE peak and full width at half-maximum (fwhm).34 XPS spectra of Pt 4f, W 4f, Ti 2p, O 1s, and C 1s regions have been registered for all investigated samples.
’ RESULTS AND DISCUSSION Pt 4f BE down-Shift As a Result of Local Charge Density Change Due to Interactions with Oxide Substrates. Regis-
tered Pt 4f spectra were fitted with one “f” doublet, using GL line shape. The peak area of 4f7/2 and 4f5/2 components has been fixed at a 4:3 ratio, and a theoretical spinorbit splitting of 3.33 eV has been used.39 BE of Pt 4f7/2 component has been determined from the fit. It becomes evident, that two different models must be considered, one for Pt deposited on TiO2/C or WO3/C, and the second for Pt deposited on C. In the case of Pt deposited on WO3/C the registered spectrum can be fitted with Pt 4f7/2 component having BE equal to 70.94 eV, whereas for Pt/TiO2/C this signal is located at 70.90 eV. Pt 4f signal for Pt/C has been observed at BE higher by ca. 270 or 290 meV (71.19 eV), as compared to Pt deposited on WO3/C or TiO2/C, respectively. Difference in Pt 4f BE suggests a local increase of the electron density on Pt, when Pt is deposited on any of the two oxides investigated. The slight decrease in Pt 4f BE has already been reported for Pt deposited on oxides of titanium, which confirms our findings,25 and the increased electron density in similar systems is considered as the source of SMSI’s 20154
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Figure 2. XP spectra of the O 1s region registered for 8% Pt/C sample, fitted with single, broad GL line shape, BE equal to 531.85 eV, for all present oxygen-containing species.
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Figure 4. XP spectra of the O 1s region registered for Pt/TiO2/C sample. An additional signal at BE = 529.58 eV, not present either in the case of Pt/C, or in the case of TiO2/C has been detected. This signal is most probably correlated to PtTiO2 alloy formation.
Table 1. O 1s Signal Deconvolution for Pt/TiO2/C and Reference Samples with the Relative Oxygen Species Amounts (at. %) component’s BE sample
529.58 eV
530.55 eV
531.85 eV
18%
82%
26%
63%
Pt/C
100%
TiO2/C Pt/TiO2/C
11%
Figure 3. XP spectra of the O 1s region registered for TiO2/C sample, fitted with two GL line shapes: first to reflect all species present on the Pt/C (at 531.85 eV, exactly as in Figure 2), and the second signal at lower BE (530.55 eV), to reflect oxygen from TiO2.
enhancement of catalytic activity,1,2426 which is also consistent with our findings. Due to the lowest fwhm, Pt 4f signal parameters (fwhm, G-L ratio etc.) obtained for Pt deposited on WO3/C, were used to fit all other Pt 4f spectra. In case of Pt deposited on TiO2, the Pt 4f signal is virtually identical to that of Pt deposited on WO3, this latter being slightly broader (see Figure 1). In the case of both Pt/C samples a significant shoulder on the high BE side can be observed, most probably due to a more oxidized Pt surface, i.e. presence of Pt oxides, which can be detected at 72.33 eV38,39 and accounts for ca. 30% of the registered signal. To maintain the clarity of presentation and due to the relative uncertainty due to BE change, the spectrum fit is not shown. Reduction of Pt surface oxides in the case of Pt deposited on TiO2 or WO3 is easy to understand in terms of increased electron density on Pt. It is worth noting, that reduced Pt surface has been found as a prerequisite for effective ORR,40 and in our case it correlates with the presence of oxide support with enhancement of activity of those materials toward ORR.20
Figure 5. XP spectra of the O 1s region registered for the Pt/WO3/C sample, fitted with three components, at 531.85 eV for background, oxygen-containing species (as in Figure 2), at 530.55 eV for WO3, and at 533.00 eV for tungsten bronzes. No signal for the PtWO3 alloy (which should be located at ca. 529.58 eV, as in Pt/TiO2/C, Figure 4) can be detected.
PtTiO2 Alloy Formation As Evidenced from O 1s Peak Analysis. In the case of Pt/TiO2/C samples, O 1s XP spectra
have been deconvoluted using three components at 529.58, 530.55, and 531.85 eV; see Figures 24. Due to the complexity 20155
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Figure 6. XP spectra of the O 1s region registered for WO3/C sample, virtually identical to the same region registered for Pt/WO3/C (Figure 5).
Figure 8. XP spectra of Ti 2p region registered for TiO2/C sample, fitted with one p-type GL doublet. Ti 2p5/2 BE equal to 459.13 eV.
Table 2. O 1s Signal Deconvolution for Pt/WO3/C and Reference Samples with the Relative Oxygen Species Amounts (at. %) component’s BE sample
530.50 eV
531.85 eV
WO3/C
11%
60%
29%
Pt/WO3/C
11%
58%
31%
Pt/C
533.00 eV
100%
Figure 9. XP spectra of Ti 2p region registered for TiO2/C and Pt/ TiO2/C. The BE scale was changed to enhance the difference in BE and fwhm. The Ti 2p signal in the case of Pt presence is shifted down and broadens compared to that in the case of Pt-free sample. This suggest an additional component at lower BE side.
Figure 7. XP spectra of Ti 2p region registered for Pt/TiO2/C sample, fitted with one p-type GL doublet. Ti 2p5/2 BE typical for TiO2 (458.99 eV), with a slight down-shift due to the Pt presence.
of the oxygen XPS spectra, the following procedure has been used: Pt/C sample has been used as a base, and fitted with one component (BE = 531.85 eV) for all oxygen containing species. We did not separate it in distinct species (i.e., Pt oxides, surface OH groups etc.) as this spectrum contains one featureless signal, and any deconvolution would be vague (see Figure 2). O 1s signal in case of this (Pt/C) sample is relatively broad (fwhm ca.
3.5 eV), due to presence of oxygen-containing species on carbon and platinum, including Pt oxides. The parameters of the peak, i. e., BE, fwhm, asymmetry, etc., have been fixed, and used for fitting other spectra. Only the peak area was adjusted, when other O 1s spectra were deconvoluted. This approach let us identify additional components, or BE shifts, present in the TiO2/C and Pt/TiO2/C spectra. In the case of the TiO2/C sample, an additional signal at 530.55 eV has been identified, with fwhm equal to 1.37 eV, which can obviously be attributed to oxygen in TiO2. Parameters of that signal have been used in identical way, as of Pt/C, to fit the Pt/TiO2/C spectra. It is interesting to note, that Pt/TiO2/C cannot be fitted, using only signals present for Pt/C and TiO2/C, and the third component at 529.58 eV has to be used to fit the experimental data. The origin of that signal is still unclear; from its BE it should be correlated to metal (Ti or Pt) oxide, but it is not present either on Pt/C or on TiO2/C sample. We can tentatively attribute it to oxygen bonded to Ti alloyed with 20156
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Figure 10. XP spectra of W 4f region registered for Pt/WO3/C sample, fitted with one f-type GL doublet, W 4f7/2 BE is equal to 34.53 eV, typical value for WO3.
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Figure 12. XP spectra of the W 4f region registered for Pt/WO3/C and WO3/C samples. In both cases the registered spectra are virtually identical, and this is most probably due to different response of W 4f signal to lattice strain.
Competition between Charge Transfer and StructureInduced Electronic Structure Modifications. Ti 2p signal has
Figure 11. XP spectra of the W 4f region registered for WO3/C sample. W 4f7/2 BE is equal to 34.53 eV, a typical value for WO3.
Pt; as such Ti atoms have different electronic properties (see below). Relative amounts of different signals are presented in Table 1. With respect to Pt/WO3/C and WO3/C samples a similar analysis was used. Spectra have been deconvoluted using three O 1s components: at 530.50, 531.85, and 533.00 eV (see Figures 2, 5, and 6). The signal at 530.50 eV can be attributed, most probably, to oxygen bonded to hexavalent tungsten in WO3 (similarly to TiO2, as above). The signal at 531.85 eV is related to O-containing species present on carbon surface (see above), and the third component, at 533.00 eV can be related to nonstoichometric tungsten oxides (“tungsten bronzes”), where tungsten has an oxidation state less than six. Both WO3-containing samples have very similar O 1s spectra, contrary to TiO2-containing samples. The relative amount of different oxygen species are summarized in Table 2. From the O 1s signals in case of Pt deposited on WO3/C, no conclusion can be drawn about the possibility of alloy formation between platinum and tungsten (WO3).
been investigated for Pt/TiO2/C sample and compared to reference sample: TiO2/C (see Figures 79). Change in BE of Ti 2p core level, when TiO2 acts as a support for Pt nanoparticles, is observed. Namely, in TiO2/C Ti 2p signals BE is equal to 459.13 eV, as compared to 458.99 eV in the case of the Pt-containing sample. These values are within the range reported for TiO2.39 BE of the Ti 2p signal for Pt/TiO2/C is in agreement with data published for Pt deposited on bulk TiO2,25 where similar downshift was observed. Besides for the Ti 2p3/2 signal, the fwhm changes from 1.678 eV for TiO2/C to 1.788 eV for Pt/TiO2/C. It is again worth to note that, due to the preparation technique, where a small amount of TiO2 is deposited on C and Pt is located only on TiO2 sites, the recorded changes must be due to PtTiO2 interactions. The observed 0.1 eV downshift of the Ti 2p emission peak and the 0.1 eV increase in fwhm suggest that a new electronic state of Ti, at the lower BE side, is present. It is in the form of a shoulder on the low BE side for Pt/TiO2/C in Figure 9. In general, such a state can be due to either different lattice parameters (PtTiO2) or to a lower than 4+ oxidation state of Ti. When directions of BE changes for Pt 4f and Ti 2p signals are compared, it is clear that Ti cannot be an electron donor. Another explanation could be based on the presence of the lattice strain. It is known, that alloying Ti with Pt leads to a lattice strain20 producing BE shifts.41 This phenomenon presents a different mechanism: that of the charge transfer one. Both do not have to be in opposite directions; indeed it has been observed for Pt/Ru alloys that Pt and Ru peaks shift in the same direction when alloy is formed,41 similarly as observed here. But the charge transfer cannot be completely excluded, as indicated by Pt 4f signal width and asymmetry changes, when Pt is deposited on TiO2 sites. Therefore, both phenomena suggest that there is an increased electron density on Pt, as discussed above (peak width) and below (peak asymmetry). Hence, the most probable explanation must be based on the assumption that the observed BE shifts are a superposition of both: charge transfer and lattice strain. The shifts of Pt 4f and Ti 2p XPS signals toward lower BE when Pt is deposited on TiO2 has been previously observed by 20157
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Table 3. Details of Pt 4f peak deconvolution and asymmetry analysis sample name
Pt 4f7/2 BE
α (eq 1)
A (eq 2)
8 wt % Pt 5 wt %TiO2/C
70.90
0.28
0.28
8 wt % Pt 5 wt % WO3/C 8 wt % Pt/C homemade
70.94 71.19
0.25 0.25
0.23 0.24
10 wt % Pt/C ETEK
71.19
0.25
0.24
A ¼ 1
Chen et al.,25 and it has been suggested that the SMSI’s enhancement of catalytic activity is correlated to the degree to which the support can act as a donor of electrons.1,25 It has been also proposed that charge transfer is due to formation of intermetallic compounds1,24 or comes from subsurface TiO225 acting as an electron donor, which cannot be observed using XPS, due to the surface sensitivity of the technique.25 This is not the case herein, as the samples investigated are characterized by oxide nanoparticles on the surface of carbon, where Pt is selectively photodeposited. Such arrangement assures that any charge transfer from oxide should be clearly seen on XPS spectra of Ti, as the inelastic mean free path of photoelectrons (ca. 15 nm in those conditions34) must be higher than the thickness of oxide nanoparticles. Moreover, Chen et al.25 claim that for different Ti oxides there is no direct correlation between the hydrogen uptake (SMSI) and Pt 4f or Ti 2p BE. Their observations are consistent with ours, when we take into account the interplay between alloy formation and charge transfer to explain the observed BE shifts. The analysis of the W 4f peak of WO3/C or Pt/WO3/C samples also favors the alloy formation argument, as the partial source of changes of the electronic properties of Pt and WO3 support. It is based on the examination that the observed W 4f signals are virtually identical, regardless if sample contains Pt or not, see Figures 1012 (BE equal to 34.53 eV). Since there is a difference in Pt 4f BE, when Pt is deposited on WO3 (see Figure 1), the lack of change of W 4f BE must be due to a canceling effect between the BE change due to charge transfer and alloy formation, or due to different response of W 4f to lattice strain,41 which has been observed for those samples.20 Change in the Electron’s Density of States Close to the Fermi Edge, As Evidenced from the Pt 4f Signal Asymmetry Change. To further analyze the electronic properties of the investigated materials, Pt 4f peaks’ asymmetry analysis has been performed (Table 3). To determine the asymmetry, Pt 4f XPS spectra (Figure 1) were fitted with DoniachSunjic profile42 Y ðEÞ ¼
role on the investigated samples, to state on the changes in electrocatalytic activity toward oxygen reduction reaction, due to metalsupport interactions. 20 Experimental asymmetry index has been also determined. It can be expressed as follows:
πα 1 E E0 þ ð1 αÞ tan cos 2 γ ððE E0 Þ2 þ γ2 Þð1 αÞ=2 Γð1 αÞ
ð1Þ where Y(E) is the registered intensity of XPS peak, E0 is binding energy, Γ is the gamma function, γ is the lifetime broadening, and α is the asymmetry parameter. Constant background has been subtracted before the fit. This approach is well suited for analysis of the asymmetry of XPS signals, which is correlated to the density of states at the Fermi level.34,42 In general, any change in material’s density of states close to the Fermi level is directly correlated to changes in adsorption strength on its surface, and as a result in the modification of the electrocatalytic activity.4345 Thus, it is imperative to check if this phenomenon plays a significant
fwhmL fwhmR
ð2Þ
where fwhmL is the peak width at the half-maximum, measured only for higher BE side of the peak only, and fwhmR is the peak width at the half-maximum, measured only for lower BE side of the peak. It measures the peak departure from the symmetric form, and is completely independent of the line shape used for deconvolution. Thus, the peak with an ideal symmetry would have A = 0, and the more asymmetric peak, the higher A, similar as α, but independent of the assumed line shape. In all cases, α parameter from eq 1 was close to 0.25, whereas the result for Pt/TiO2/C again shows slightly higher asymmetry (0.28) than the results for Pt/C. A and α have been determined within a precision of (0.01, which confirms that there is a small change in the asymmetry in the case of Pt deposited on TiO2/C and no significant change in case of Pt deposited on WO3/C. The utmost care has been exercised to address the influence of Pt 4f signal resulting from formation of Pt oxides or Pt3Ti alloy (located at higher BE than the signals for elemental Pt) on the observed asymmetry. Both those phenomena can increase the asymmetry to some degree, but the α parameter is based on the fit to theoretical line shape, which include the “tail”, i.e., the far left portion of the spectrum, most influenced by inelastically scattered (by valence band) photoelectrons, where no signal from Pt oxides or Pt3Ti alloy is located. Validity of the obtained data is assured by two facts: (i) a very good agreement has been found between the α and the A parameter and (ii) the lowest asymmetry has been observed for Pt/C samples, where Pt oxides are present. The observed change in asymmetry is small, most probably due to high initial asymmetry of Pt 4f signals, as a result of high electron density of states close to the Fermi edge. The reason why Pt 4f signal asymmetry does not change in the case of Pt deposited on WO3/C is still unclear.
’ CONCLUSIONS Summing up the observed changes of the electronic properties of nanosized Pt, selectively photodeposited on the oxide sites of TiO2carbon, or WO3carbon, Pt/TiO2/C and Pt/WO3/C, suggest that the enhancement toward ORR, observed when Pt is deposited either on TiO2 or WO3,20 and the observed BE shifts of core-level electrons are most probably due to changes in the electronic properties of Pt induced by at least two factors: (i) the charge transfer from the oxide to Pt atoms and (ii) change in the lattice parameter due to alloy formation. The charge transfer alone cannot explain the observed BE shifts as those were observed toward the same direction in case of Pt/TiO2/C (namely lower BE values) for Pt 4f and Ti 2p levels. It is also the case for Pt/WO3/C sample, where observed BE shifts were toward lower BE for Pt 4f and showed no change for W 4f levels. To explain the observed BE shifts in that system the change of Pt 4f, Ti 2p, and W 4f BE, with opposite sign and different magnitude, due to the alloy formation, must also play a significant role, as the observed BE shifts are a result of superposition of those two effects. 20158
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The Journal of Physical Chemistry C The observed BE shift of Pt 4f level suggests that there is an increased electron density on Pt atoms, which can explain the enhanced electrocatalytic activity of Pt/TiO2/C and Pt/WO3/C toward ORR. To further investigate the electron density change we analyzed the Pt 4f signal asymmetry, and those results shows small increase in asymmetry in the case of Pt deposited on TiO2/ C and no change (within the experimental error) in case of Pt deposited on WO3/C. The difference is subtle, due to the initial high asymmetry of those signals, but also confirms our thesis. Increased electron density on Pt has also consequences in terms of oxidation state of Pt. Namely, when Pt is deposited on oxidecarbon substrate, then Pt surface contains significantly less surface oxides, which can be in part responsible for the observed increase in electrocatalytic activity. Those observation are in good agreement with the literature data,1,24,25 but a new explanation based on the interplay between lattice strain and charge transfer has been provided.
’ AUTHOR INFORMATION Corresponding Author
*Phone/Fax: +48 22 822 0211 ext. 524. E-mail:alewera@chem. uw.edu.pl (A.L.). Phone/Fax: +33549453625; 0033549453580. E-mail:
[email protected] (N.A.-V.).
’ ACKNOWLEDGMENT This work has been partially supported by Ministry of Science and Higher Education (Poland) under Project N N204 527739 and Ministry of Science and Higher Education (Poland)/PHC (France) under “Polonium” Project: 7832/R09/R10 and PHC 20100SH. One of us, L.T., was supported by a fellowship of the “Ministere de l’Enseignement Superieur et Recherche”. ’ REFERENCES (1) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, 100, 170. (2) Shim, J.; Lee, C. R.; Lee, H. K.; Lee, J. S.; Cairns, E. J. J. Power Sources 2001, 102, 172. (3) Xiong, L.; Manthiram, A. Electrochim. Acta 2004, 49, 4163. (4) Chhina, H.; Campbell, S.; Kesler, O. J. Electrochem. Soc. 2007, 154, B533. (5) Kulesza, P. J.; Faulkner, L. R. J. Am. Chem. Soc. 1988, 110, 4905. (6) Kulesza, P. J.; Faulkner, L. R. J. Electroanal. Chem. 1988, 248, 305. (7) Kulesza, P. J.; Faulkner, L. R. J. Electrochem. Soc. 1989, 136, 707. (8) Shukla, A. K.; Ravikumar, M. K.; Arico, A. S.; Candiano, G.; Antonucci, V.; Giordano, N.; Hamnett, A. J. Appl. Electrochem. 1995, 25, 528. (9) Kulesza, P. J.; Faulkner, L. R. J. Electroanal. Chem. 1989, 259, 81. (10) Barczuk, P. J.; Tsuchiya, H.; Macak, J. M.; Schmuki, P.; Szymanska, D.; Makowski, O.; Miecznikowski, K.; Kulesza, P. J. Electrochem. Solid-State Lett. 2006, 9, E13. (11) Kolary-Zurowska, A.; Zieleniak, A.; Miecznikowski, K.; Baranowska, B.; Lewera, A.; Fiechter, S.; Bogdanoff, P.; Dorbandt, I.; Marassi, R.; Kulesza, P. J. J. Solid State Electrochem. 2007, 11, 915. (12) Miecznikowski, K.; Kulesza, P. J. J. Power Sources 2011, 196, 2595. (13) Yoo, S. J.; Jeon, T.-Y.; Lee, K.-S.; Park, K.-W.; Sung, Y.-E. Chem. Commun. 2010, 46, 794. (14) Sun, X. L.; Saha, M. S.; Banis, M. N.; Zhang, Y.; Li, R. Y.; Cai, M.; Wagner, F. T. J. Power Sources 2009, 192, 330. (15) Neophytides, S. G.; Zafeiratos, S. H.; Jaksic, M. M. J. Electrochem. Soc. 2003, 150, E512.
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(16) Lewera, A.; Miecznikowski, K.; Hunger, R.; Kolary-Zurowska, A.; Wieckowski, A.; Kulesza, P. J. Electrochim. Acta 2010, 55, 7603. (17) Kulesza, P. J.; Miecznikowski, K.; Baranowska, B.; Skunik, M.; Kolary-Zurowska, A.; Lewera, A.; Karnicka, K.; Chojak, M.; Rutkowska, I.; Fiechter, S.; Bogdanoff, P.; Dorbandt, I.; Zehl, G.; Hiesgen, R.; Dirk, E.; Nagabhushana, K. S.; Boennemann, H. J. Appl. Electrochem. 2007, 37, 1439. (18) Savadogo, O.; Beck, P. J. Electrochem. Soc. 1996, 143, 3842. (19) Vogel, W.; Timperman, L.; Alonso-Vante, N. Appl. Catal., A 2010, 377, 167. (20) Timperman, L.; Lewera, A.; Vogel, W.; Alonso-Vante, N. Electrochem. Commun. 2010, 12, 1772. (21) Kulesza, P. J.; Miecznikowski, K.; Baranowska, B.; Skunik, M.; Fiechter, S.; Bogdanoff, P.; Dorbandt, I. Electrochem. Commun. 2006, 8, 904. (22) Miecznikowski, K.; Kulesza, P. J.; Fiechter, S. Appl. Surf. Sci. 2011, 257, 8215. (23) Yixuan, C.; Zhaobin, W.; Yanxin, C.; Huaxin, L.; Zupei, H.; Huiqing, L.; Yonglei, D.; Chuying, Y.; Wenzhao, L. J. Mol. Catal. 1993, 21, 275. (24) Horsley, J. A. J. Am. Chem. Soc. 1979, 101, 2870. (25) Chen, B. H.; White, J. M. J. Phys. Chem. 1982, 86, 3534. (26) Cui, G.; Shen, P. K.; Meng, H.; Zhao, J.; Wu, G. J. Power Sources 2011, 196, 6125. (27) Hanarp, R.; Gustavsson, M.; Ekstrom, H.; Eurenius, L.; Lindbergh, G.; Olsson, E.; Kasemo, B. J. Power Sources 2007, 163, 671. (28) Mentus, S. V. Electrochim. Acta 2005, 50, 3609. (29) Timperman, L.; Alonso-Vante, N. Electrocatal. 2011, 2, 181. (30) Kulesza, P. J.; Grzybowska, B.; Malik, M. A.; Galkowski, M. T. J. Electrochem. Soc. 1997, 144, 1911. (31) Kokkinidis, G.; Stoychev, D.; Lazarov, V.; Papoutsis, A.; Milchev, A. J. Electroanal. Chem. 2001, 511, 20. (32) Tammeveski, K.; Arulepp, M.; Tenno, T.; Ferrater, C.; Claret, J. Electrochim. Acta 1997, 42, 2961. (33) Corcoran, C. J.; Tavassol, H.; Rigsby, M. A.; Bagus, P. S.; Wieckowski, A. J. Power Sources 2010, 195, 7856. (34) Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Grant, J. T., Eds.; IM Publications and Surface Spectra Limited: Chichester, U.K., 2003. (35) Goepel, W.; Rocker, G.; Feierabend, R. Phys. Rev. B 1983, 28, 3427. (36) Sugimura, H.; Uchida, T.; Kitamura, N.; Masuhara, H. Chem. Lett. 1993, 22, 379. (37) Alonso-Vante, N. Fuel Cells 2006, 6, 182. (38) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble Jr, J. R. NIST Standard Reference Database 20, Version 3.4 (Web Version). (39) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy. A Reference Book of Standard Spectra for Identification and Interpretation of XPS Spectra; Physical Electronic, Inc.: Eden Praire, MN, 1995. (40) Davis, D. G., Jr. Talanta 1960, 3, 335. (41) Lewera, A.; Zhou, W. P.; Hunger, R.; Jaegermann, W.; Wieckowski, A.; Yockel, S.; Bagus, P. S. Chem. Phys. Lett. 2007, 447, 39. (42) Doniach, S.; Sunjic, M. J. Phys. C: Solid State Phys. 1970, 3, 285. (43) Kibler, L. A.; El-Aziz, A. M.; Hoyer, R.; Kolb, D. M. Angew. Chem., Int. Ed. 2005, 44, 2080. (44) Mavrikakis, M.; Hammer, B.; Norskov, J. K. Phys. Rev. Lett. 1998, 81, 2819. (45) Norskov, J. K.; Bligaard, T.; Logadottir, A.; Bahn, S.; Hansen, L. B.; Bollinger, M.; Bengaard, H.; Hammer, B.; Sljivancanin, Z.; Mavrikakis, M.; Xu, Y.; Dahl, S.; Jacobsen, C. J. H. J. Catal. 2002, 209, 275.
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