Facile Fabrication of Pt, Pd and Pt-Pd Alloy Films on Si with Tunable Infrared InternalReflection Absorption and Synergetic Electrocatalysis

Chao Wang,† Bin Peng,† Hai-Nan Xie,‡ Han-Xuan Zhang,† Fei-Fei Shi,† and Wen-Bin Cai*,†

Shanghai Key Laboratory for Molecular Catalysis and InnoVatiVe Materials and Department of Chemistry,Fudan UniVersity, Shanghai 200433, China, and The College of Chemistry & Chemistry Engineering,Fuzhou UniVersity, Fuzhou 350108, China

ReceiVed: April 15, 2009; ReVised Manuscript ReceiVed: June 18, 2009

A new facile fabrication of Pt, Pd and their alloy films on Si, intended mainly for in situ ATR surface infraredspectroscopy and electrocatalysis study, has been achieved through an alternate electroless deposition approachfrom very simple acidic baths with hydrazine dihydrochloride as the reducing agent. Compositional analysesreveal that Pt and Pd elemental ratios in the alloy films are close to those in the plating baths, and higher inskin layers compared to those of bulk films. Electrochemical and IR spectroscopic characterizations suggestthe Pt-Pd alloy films may exhibit synergetic effects in surface electrochemistry toward oxidation of COadlayer and formic acid. Of particular interest is the reproducible control of internal reflection absorptionresponses through adjusting Pt and Pd compositions in the films. More enhanced surface IR absorption withnormal band shape and direction was observed for surface species at Pt and Pd electrodes. For the as-depositedPt-Pd alloys, in addition to less enhanced IR absorption intensity, bipolar and totally inverted bands mayappear for different alloy films.

1. Introduction

Pt, Pd and Pt-Pd alloys are excellent materials for electro-catalytic oxidation of small organic molecules1-3 and reductionof oxygen molecules4 relevant to the proton exchange membranefuel cells, as well as for a number of non-electrochemicalcatalytic reactions, involving hydrogenation.5 With the nearlysame atomic size, Pt and Pd can form a solid solution with thefcc structure over the entire atomic concentration range. It canbe expected that this atomically mixed Pd-Pt alloy may be quitedifferent from simply palladized Pt in terms of surface (elec-tro)chemistry. In fact, the palladized Pt(111) electrodes showedsurface adsorption corresponding independently to Pd patchesand Pt patches, and Pd-alloyed Pt(111) electrodes exhibited apropensity for synergetic effect of two elements.6,7 A Pt-Pdbimetallic alloy with an appropriate composition reportedlypossesses a higher (electro)catalysis and selectivity toward somespecific important reactions, like the selective partial hydrogena-tion of 1,3-cyclooctadiene to cyclo-octene.8 Hence, a facilefabrication of Pt-Pd alloys with controllable compositions isof significance for (electro)catalytic applications.

Exploring the adsorption and reaction on Pt, Pd and Pt-Pdalloy surfaces is essential for understanding a specific reactioncourse and thus helpful for the design of an efficient (electro)-catalyst. To this end, surface-enhanced infrared absorptionspectroscopy (SEIRAS) with attenuated total reflection (ATR)configuration is a powerful tool due to its simple surfaceselection rule and high surface sensitivity in probing adsorptionand reaction at (but not limited to) electrode/electrolyte inter-faces.9 Successful implementation of this technique in surface(electro)chemistry requires the appropriate fabrication of ananoparticle metal film on an ATR IR prism.10,11

Electroless deposition of Pt and Pd films on the reflectingplane of a Si prism from alkaline plating baths with hydrazineas the reducing agent has been reported by Osawa’s group.12,13

Nevertheless, either a patented recipe with unknown additivesfor plating Pt12 or a quite different and complicated one forplating Pd13 had to be adopted. In addition to the complicationin the baths, contacts with alkaline solutions may lead to theaccumulative corrosion of Si prisms. As for the Pt-Pd alloyfilms, no prior reports on their chemical codeposition on Si aswell as on their application to ATR-SEIRAS could be found inthe literature, despite one regarding the palladized Pt filmelectrode on Si.7

Meanwhile, surface infrared absorption properties, includ-ing the band intensity, direction and shape for adsorbates,were found to depend on the film nanostructures, largelybased on the external reflection14,15 and transmission16,17

absorption measurements. However, tuning ATR infraredabsorption through deposition of metal films has not beenattempted. Given the advantages of ATR-SEIRAS in probinginterfacial adsorption and reaction, the above issue shouldbe addressed for a better understanding of SEIRA effect anda more extensive application of this technique.

In the present work, we aim to simplify the wet chemistryfabrication of Pt, Pd and Pt-Pd alloy films on Si and to exploretheir initial applications to ATR-SEIRAS and electrocatalysis.A novel electroless deposition from very simple acidic baths ispresented herein to prepare the desired Pt, Pd and their alloyfilm electrodes on Si. These films are characterized structurally,electrochemically and spectroscopically. Interestingly, the as-prepared alloy films exhibit tunable surface enhanced IRabsorption and synergetic surface electrochemistry. The “syn-ergy” used in the present paper is a general concept, coveringbimetallic coeffects leading to unique electrochemical andoptical responses which are not a simple addition of themonometallic responses.

* To whom correspondence should be addressed. E-mail: wbcai@fudan.edu.cn. Phone: +86-21-55664050. Fax: +86-21-65641740.

† Fudan University.‡ Fuzhou University.

J. Phys. Chem. C 2009, 113, 13841–13846 13841

10.1021/jp9034562 CCC: $40.75 2009 American Chemical SocietyPublished on Web 07/10/2009

2. Experimental Section

2.1. Fabrication of Films. The reflecting plane of a hemi-cylindrical Si prism was polished with alumina powders withsuccessively decreasing sizes down to 0.05 µm, ultrasonicatedin acetone and ultrapure Milli-Q water sequentially and repeat-edly for removing surface impurities, and then terminated withhydrogen according to ref 13. The Si reflecting plane wasactivated with Pd seeds by contacting it with 1 mM PdCl2

containing 0.5% HF for 2.5 min at 30 °C before chemicaldeposition with desired films from simple acidic baths withhydrazine dihydrochloride as the reducing agent.

2.2. FE-SEM, ICP-AES and XPS. Surface morphology andmicrostructure of the Pt, Pd and Pt-Pd bimetallic films werecharacterized with a field emission scanning electron microscope(FE-SEM, Hitachi S-4800); inductively coupled plasma (ICP)atomic emission spectroscopy (AES, Hitachi P-4010) was usedto determine the amount of metal species from the depositedfilm dissolved in aqua-regia, for the estimation of the massequivalent thickness of the films as well as the average elementalpercentages in the Pt-Pd alloys. XPS spectra of Pt-Pd alloyfilms were recorded on a Perkin-Elmer PHI-5000C ESCA, andanalyzed using the PHI-MATLAB software provided by PHICorporation to determine the Pt and Pd atomic percentages inthe skin layers of Pt-Pd alloy films.

2.3. Electrochemistry and ATR-FTIR Spectroscopy. Theas-deposited films served as the working electrodes with a Ptmesh and reversible hydrogen electrode (RHE) as the counterand reference electrodes, respectively. All electrolyte solutionswere prepared with GR grade reagents and ultrapure water(Milli-Q). Before experiments, the electrolytes were Ar-deaer-ated for 60 min.

In situ ATR-FTIR measurements at the incident angle of 70°and a spectral resolution of 4 cm-1 were performed on VarianExcalibur 3100 FT-IR spectrometer in conjunction with aCHI660B electrochemistry workstation. Single-beam spectrawere sampled in CO-saturated 0.5 M HClO4 at 0.1 V, and thecorresponding reference spectra were taken at 1.0 V where theCO adlayer could be completely removed. All the spectra areshown in the absorbance units defined as -log(I/I0), where Iand I0 represent the sample and reference spectra, respectively.

3. Results and Discussion

3.1. Electroless Deposition of Pt, Pd and Pt-Pd AlloyFilms. Table 1 gives the recipes and conditions selected forthe electroless deposition of Pt, Pd and Pt-Pd alloys withdifferent compositions, as well as the Pt/Pd mole ratiosdetermined ICP and XPS assays. The electroless depositionmainly follows three different mechanisms: autocatalytic deposi-tion, substrate catalyzed deposition, and galvanic displacement.The seeding of initial discontinuous Pd layer on Si proceeds

via the galvanic displacement, roughly according to the fol-lowing reaction:

The subsequent electroless deposition of Pt, Pd or Pt-Pdalloys occurs via the substrate (Pd seeds) catalyzed deposition,which can approximately be represented by the reactions asfollows:

3.2. Film Characterizations. Shown in Figure 1 are selectedSEM images for Pt, Pt-Pd[B], Pt-Pd[D] and Pd films chemicallydeposited on Si. It can be seen that Pt, Pt-Pd[B] and Pt-Pd[D]

films have a similar morphology with a more densely intercon-nected and finer nanoparticle underlayer on top of which aresparsely scattered larger nanoparticles. The nanoparticles appearto be in elliptical shapes with sizes from 40 to 80 nm, witheach larger nanoparticle surface being somewhat rough in finernanostructured scales. Specifically, the films grew more homo-geneous and smooth in the order from Pt, Pt-Pd[B] to Pt-Pd[D].Quite different from all the other films and also from thatobtained from the basic bath,13 the Pd film here exhibits a

TABLE 1: Bath Compositions, Plating Conditions, and ICP and XPS Assays for Pt, Pd and Pt-Pd Alloy Films

Pt/Pd

H2PtCl6 (mM) PdCl2 (mM) HCl (M) NH2NH2 ·2HCl (M) temp (°C) time (min) ICPa XPS

Pt 2 0 0.33 0.035 65 8 100/0 100/0Pt-Pd[A] 1 1 0.38 0.035 55 5 48/52 62/38Pt-Pd[B] 0.66 1.33 0.38 0.035 55 5 37/63 48/52Pt-Pd[C] 0.5 1.5 0.38 0.035 55 5Pt-Pd[D] 0.4 1.6 0.38 0.035 55 5 22/78 29/71Pt-Pd[E] 0.22 1.78 0.38 0.035 55 5 15/85 16/84Pt-Pd[F] 0.15 1.85 0.38 0.035 55 5 9/91 9/91Pd 0 2 0.42 0.035 50 10 0/100 0/100

a With calibration of the contribution from initial Pd seeds.

Figure 1. SEM images of (a) Pt, (b) Pt-Pd[B], (c) Pt-Pd[D], and (d)Pd films coated on Si by electroless deposition.

Si + 6F-+2PdCl2 f SiF62-+ 2Pd + 4Cl- (I)

PtCl62-+ N2H4 ·2HCl f Pt + 6H++ 8Cl-+ N2v

(II-a)

2PdCl42-+ N2H4 ·2HCl f 2Pd + 6H++ 10Cl-+ N2v

(II-b)

13842 J. Phys. Chem. C, Vol. 113, No. 31, 2009 Wang et al.

structure consisting of closely linked and much finer nanopar-ticles of irregular shapes.

From the ICP results, the mass equivalent thicknesses of thePt, Pt-Pd[A], Pt-Pd[B], Pt-Pd[D], Pt-Pd[E], Pt-Pd[F] and Pdfilms were 53, 51, 53, 50, 57, 48, and 45 nm, respectively. Aftercalibrating the elemental contribution from initial Pd seeds, theatomic ratio of Pt and Pd in a bulk film was listed in Table 1:48/52 for Pt-Pd[A], 37/63 for Pt-Pd[B], 22/78 for Pt-Pd[D], 15/85 for Pt-Pd[E] and 9/91 for Pt-Pd[F]. These values are to someextent close to the atomic ratios of Pt and Pd in the correspond-ing plating baths, i.e. 50/50, 33/67, 20/80, 11/88 and 8/92respectively, indicative of the effective codeposition of thesetwo metals from the simple acid plating baths with a properreducing agent.

It has been known that the surface stoichiometry of a binaryalloy can vary considerably compared to that of the bulk. Hence,XPS analysis was performed to examine the elemental composi-tion in the top layers of Pt-Pd alloy films. Except the C 1speak, all the other peaks can be assigned to either Pt or Pdelements, as indicated in Figure 2. From the intensities of thePt 4f7 peak and the Pd 3d5 peak, the Pt and Pd elemental ratiosin the skin layers were evaluated to be ca. 62/38, 48/52, 29/71and 16/84, 9/91 for Pt-Pd[A], Pt-Pd[B] Pt-Pd[D] Pt-Pd[E] andPt-Pd[F] films, respectively, which are larger than the corre-sponding bulk values, especially when the Pd/Pt ratios in thebulk films are not very high. In contrast, for the Pt-Pd alloysprepared through flame annealing or subjected to high-temper-ature treatment in UHV, the surfaces are enriched with Pd.8

Although Pd-segregation in the skin layers is favorable for thePt-Pd solid solution at equilibrium,18,19 a Pt rich surface wasalso found in Pt-Pd alloy colloidal dispersion by using EXAFSmeasurement.20 The somewhat abnormal Pt-rich surfaces forchemically deposited Pt-Pd alloys may be ascribed to the muchlower formation temperatures which resulted in a metastablealloy phase with slow Pt and Pd diffusion in the lattices.Thenew and “easy to go” approach provided here for the chemicaldeposition of Pt, Pd and Pt-Pd alloy films on Si with a wide-range of controllable compositions is very useful for potentialapplications in consideration of the (electro)catalytic activitiesbeing in many cases dependent on the alloy compositions.

3.3. Electrochemical Characterizations. Figure 3a andFigure 3b show the cyclic voltammograms (CVs) for thechemically deposited Pt, Pt-Pd[B], Pt-Pd[D], Pt-Pd[E], Pt-Pd[F]

and Pd film electrodes in 0.5 M HClO4. They are typical ofthose for corresponding polycrystalline bulk electrodes. The CVs

for Pt-Pd[B], Pt-Pd[D], Pt-Pd[E], and Pt-Pd[F] electrodes arerather different from those for monometallic electrodes, that is,in the hydrogen adsorption/desorption range, only a couple ofbroader and larger peaks can be seen without clear distinctionbetween strong and weak hydrogen adsorption/desorption sites.In the higher potential region, the reduction of surface oxide orhydroxide layer on Pt-Pd[B], Pt-Pd[D] Pt-Pd[E] and Pt-Pd[F]

film electrodes occurred at a more positive potential than thaton Pt and Pd film electrodes. It has been proposed that thesurface oxide/hydroxide layer may block the oxygen reductionreaction (ORR);21 the present result suggests that the as-preparedPt-Pd alloys could be potentially used as electrocatalyticmaterials for ORR.

The CO stripping curves for the six electrodes, as shown inFigure 3c, indicate that the surface electrochemistry of thePt-Pd alloy films are not linear superposition of that of Pt andPd films. Only singular CO oxidation peaks were detected at0.78, 0.80, 0.83, 0.845, 0.85, and 0.90 V for Pt, Pt-Pd[B],Pt-Pd[D], Pt-Pd[E], Pt-Pd[F] and Pd film electrodes, respec-tively, suggestive of the synergetic effect of the two atomicallymixed elements on the alloy surfaces, which is in accordancewith the IR result (vide infra).

The roughness factor (RF) of the as-prepared Pt film wasestimated to be ca. 8.5 by assuming 210 µC cm-2 for thehydrogen adsorption charge at a smooth Pt electrode. RF couldalso be estimated from the CO striping charge (Figure 3c) byassuming a charge of 420 µC cm-2 for the oxidation of COmonolayer.22 The latter evaluation approach is especially usefulfor the Pd and Pt-Pd film electrodes to avoid the interferenceof hydrogen absorption with hydrogen adsorption at lowerpotentials. Based on the second approach, the resulting RFvalues for the Pt, Pt-Pd[B], Pt-Pd[D] Pt-Pd[E], Pt-Pd[F] andPd films are ca. 8.3, 6.8, 6.7, 6.5, 5.8 and 7.8, respectively,close to those reported previously for Pt and Pd films chemicallydeposited on Si from basic baths,12,13 and largely matched withthe SEM observation.

Figure 3d shows the voltammetric curves measured fordifferent film electrodes in 0.5 M HClO4 containing 0.1 MHCOOH with current densities normalized to surface roughnessfactors. It can be seen Pt-Pd alloys and Pd films exhibit higherelectrooxidation currents at lower potentials than the Pt film.The electrooxidation of formic acid proceeds mainly throughthe dehydrogenation pathway leading to CO2 and the indirectdehydration pathway leading to CO intermediate. The COadsorption at lower potentials hinders the first pathway on thePt surface. By contrast, the indirect pathway is nearly negligibleon Pd electrode. Therefore, it can be expected that Pt-Pd alloyelectrodes display electrocatalytic activity higher than the Ptelectrode. A superb synergetic effect toward electrocatalysis offormic acid was found for Pt-Pd alloys with sufficiently highPd/Pt molar ratios. Specifically, the Pt-Pd[F] electrode with itsPt/Pd surface atom ratio around 1:10 showed an even muchbetter activity than the Pd electrode, with negatively shifted onsetand peak oxidation potentials as well as an increased peakcurrent. Third body (or ensemble) and electronic (or ligand)effects may account for this result. The former effect is due tothe dilution of Pt atoms in Pt-Pd[F] alloy, which ensures eachsingle Pt atom surrounded by Pd atoms, hindering the dissocia-tion of formic acid on neighboring Pt atoms to form CO,23 andpromoting direct electrooxidation through the dehydrogenationpath at lower potentials. The latter effect is due to the suitabledownshift of the d band center of Pd as a result of slight Ptalloying,24 enhancing the formic acid electrooxidation.25 Bycontrast, the voltammogram for a monolayer-palladized Pt

Figure 2. XPS survey spectra of (a) Pt-Pd[A], (b) Pt-Pd[B], (c)Pt-Pd[D], and (d) Pt-Pd[F] films chemically deposited on Si substrates.Details for the electroless deposition of Pt-Pd[A], Pt-Pd[B], Pt-Pd[D]

and Pt-Pd[F] films and surface composition are listed in Table 1.

Pt, Pd and Pt-Pd Alloy Films on Si J. Phys. Chem. C, Vol. 113, No. 31, 2009 13843

electrode in formic acid-containing solution resembles more orless that for a Pd electrode,6 reflecting a different surfacedistribution of Pt and Pd atoms on the Pt-Pd alloy and thepalladized Pt surfaces. Considering that Pd is one-fifth the costof Pt currently, our current finding may trigger the furthersynthesis-by-design of low Pt-content and Pd-based efficientnanocatalysts for formic acid electrooxidation.

3.4. In Situ ATR-SEIRAS on Pt, Pd and Pt-Pd AlloyElectrodes. In situ SEIRA spectra are shown in Figure 4 forPt, Pd and five Pt-Pd bimetallic films in CO-saturated 0.5 MHClO4 at 0.1 V. The bands at 2065 to 2075 cm-1 correspondto linearly bonded CO (COL) and those at 1869-1956 cm-1 tobridge-bonded CO (COB). The bands at 3650 and 1632 cm-1

are characteristic of ν(OH) and δ(OH) of interfacial free H2O,respectively.12 Obviously, the SEIRA bands of CO adsorbedon Pt and Pd film electrodes are normally directed. As usual,the COL band is predominant on Pt electrode whereas the COB

band is predominant on Pd electrode. The peak intensity is ca.0.040 Abs. for COL on Pt and is ca. 0.035 Abs. for COB on Pd,comparable to that obtained by using unpolarized IR irradiationfor COL on Pt film (ca. 0.05 abs)26 or COB on Pd film (ca.0.045)13 chemically deposited from basic recipes. Neverthelessthe band intensity for COL on Pt film observed here issignificantly less than that on a pinhole-free Pt overlayerelectrodeposited on Au film on Si (usually 0.07-0.12 abs).11

Notably, the band intensity up to ca. 0.3 Abs. for COL on a Ptfilm made from the patented basic plating bath was measuredby using p-polarized IR irradiation,12 in contrast, a much lowerband intensity (ca. 0.05 Abs.) was obtained on the same typeof Pt film with unpolarized IR irradiation.26 Therefore, onlyATR-SEIRAS results with unpolarized IR irradiation were usedin the above for a reasonable comparison.

The increase of the Pd content in Pt-Pd[A], Pt-Pd[B],Pt-Pd[C], and Pt-Pd[D] films appeared to result in bipolar bandsand even totally inverted bands (see curves b to e in Figure 4).The band distortion and inversion was mostly reported in theexternal IR reflection-absorption spectroscopy (IRAS) on Ptgroup metal films electrodeposited onto bulk electrodes.14,15 Inthose measurements, normally directed bands came with lessIR enhancement, inverted bands with more IR enhancement.Different from the previous results, the band intensity obtainedhere gradually decreased accompanied by a change of the bandshape. The changes of band shape and intensity were qualita-tively explained by various effective medium theories,15,27,28

which approximately relate the band intensity to the effectivemedium layer thickness, and the band shape (direction) to thefilling factor (or volume fraction) of nanoparticulate metals in

Figure 3. Cyclic voltammograms of (a) Pt (black), Pt-Pd[B] (green), and Pt-Pd[D] (blue) and (b) Pt-Pd[E] (orange), Pt-Pd[F] (violet), and Pd (red)film electrodes in 0.5 M HClO4 at 50 mV s-1. (c) Anodic stripping voltammograms for the CO-predosed corresponding electrodes in 0.5 M HClO4

at 10 mV s-1, CO was predosed by keeping the electrodes in CO-saturated 0.5 M HClO4 at 0.2 V for 30 min followed by Ar sparging for 90 min.(d) Positive-going voltammograms for the corresponding electrodes in 0.5 M HClO4 + 0.1 M HCOOH at 50 mV s-1 after normalizing surfaceroughness factors as evaluated from Figure 3c.

Figure 4. SEIRA spectra for (a) Pt, (b) Pt-Pd[A], (c) Pt-Pd[B], (d)Pt-Pd[C], (e) Pt-Pd[D], (f) Pt-Pd[F] and (g) Pd film electrodes in CO-saturated 0.5 M HClO4 at 0.1 V. Reference spectra were taken at 1.0V. The * band at ca. 1225 cm-1 is due to a trace amount of SiOx.

13844 J. Phys. Chem. C, Vol. 113, No. 31, 2009 Wang et al.

the effective medium. In particular, by using the advancedeffective medium model for the calculation of adsorption-induced differential external infrared spectra of particulate metalsdeposited on a substrate, Pecharroman et al. could predict thevarious combinations of band intensity and band shape.28 Inother words, a normally directed band can be accompanied byeither a higher or a lower band intensity, and so can an invertedband, depending on the dielectric constant, the thickness andthe filling factor of a film, as well as on the substrate and theincidence angle. Anyway, the present work represents the firstreport on reproducibly tuning ATR-SEIRA response throughfacile film fabrication. Given the same substrate, incidence angleand very close thickness, presumably the difference in fillingfactors (reflected by SEM images) and/or optical constants forfilms of different compositions may account for the unique ATR-SEIRAS responses in Figure 4. Specifically, the band inversionwas found on the most compact particulate film (see Figure 1c).Extension of such advanced theory to the calculation of internalreflection mode is planned in our group for better understandingthe experimental results.

It is noted that the shape and direction of the bands at 1124,3650, and 1632 cm-1 changed concurrently with that of the COL

and COB bands. In consideration of high concentrations forClO4

- and H2O in the electrolyte side, the solution contributionto the ATR-FTIR bands is sometimes hard to exclude since thesebands do not shift or shift very little with potential. Our currentresults further confirm that these bands are due to surface speciesor at least interfacial species adjacent to the surface.

In Figure 4, interestingly, the COL band is much strongerthan the COB band on all our Pt-Pd alloy electrodes like thaton Pt electrodes, which is in stark contrast to CO adsorptionon palladized Pt electrodes where CO adsorption on two metalpatches could be distinguished easily.6,7 Electrochemical deposi-tion of Pt-Pd bimetallic films on glassy carbon electrode wasalso reported;29 nevertheless, the resulting discrete IR bands forCO adsorption on Pt and Pd sites suggested that the obtainedPt-Pd film might not be an atomically mixed practical alloy.Furthermore, in our case, the COL band red-shifted and the COB

band blue-shifted on Pt-Pd alloys as compared to the COadsorption on Pt, in resemblance to that reported previously onPt-Pd(111) bulk alloy electrodes.6 All these strongly suggestthat our codeposited Pt-Pd bimetallic films are to a large extentatomically mixed alloys. Such a frequency/site-occupancychange has been attributed to the vibrational coupling betweenCO adsorbed on a more or less random arrangement of Pt andPd sites. Again, ensemble effect and electronic effect mayexplain the unique adsorption configurations found for CO onPt-Pd alloy electrodes,6 as in the case of CO adsorption onNi-P alloy electrode.30 The former arises from the fact thatthe CO bridge-adsorption may require an ensemble of severalneighboring Pd atoms; obviously, the “dilution” of surface Pdatoms by Pt atoms decreases the probability of forming COB

on neighboring Pd sites. The latter arises from partial electrontransfer from electropositive Pd to electronegative Pt,31 theredistribution of surface electrons may modify the CO bindingto Pd-Pt surfaces. The above effects may also explain thesynergy in CO oxidation on Pt-Pd alloy electrodes, asdemonstrated by one singular peak in Figure 3c between thosefor Pt and Pd electrodes.

4. Conclusions

A facile fabrication of Pt, Pd and Pt-Pd alloy films on Sihas been developed by electroless deposition from simpleacidic plating baths with hydrazine dihydrochloride as the

reducing agent. These films can be extended for in situ ATR-SEIRAS and electrocatalysis application. SEM images revealnanoisland structures for the films depending on theircompositions. ICP-AES analysis indicates that the Pt/Pdatomic ratios in the films are similar to those in thecorresponding plating baths, suggesting an effective codepo-sition of these two metals. XPS measurement points out thatthe skin layers of Pt-Pd alloy films contain more Pt thanthe underlying bulk, in contrast to the typical Pd-segregationon a flame annealed Pt-Pd alloy surface. Voltammetric andspectroscopic measurements suggest that the as-depositedfilms exhibit electrochemical and spectral features typical ofPt, Pd and their alloys. In particular, surface electrochemistryand infrared absorption of the as-deposited films can bereproducibly tuned through adjusting the plating recipes, andthus the composition and the nanostructured morphology offilms. Superb synergetic electrocatalysis toward formic acidoxidation was found for Pd-Pt alloy films at a sufficientlyhigh Pd/Pt molar ratio. Higher enhanced surface IR absorptionwith normal band shape and direction was observed forsurface species at Pt and Pd electrodes. For Pt-Pd alloys, inaddition to less-enhanced IR absorption intensity, bipolar andtotally inverted bands can be found for certain nanostructuredfilms.

Acknowledgment. This work is supported by NSFC (No.20673027, 20833005, and 20873031) and STCSM (No.08JC1402000 and 08DZ2270500). FFS thanks NSFC (No.J0730419) for partial financial support.

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