lable at ScienceDirect

Materials Chemistry and Physics xxx (2014) 1e7

Contents lists avai

Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

An X-ray photoelectron spectroscopy study of platinum nanoparticleson yttria-stabilized zirconia ionic support: Insight into metal supportinteraction

Spyridon Ntais*, Rima J. Isaifan, Elena A. BaranovaDepartment of Chemical and Biological Engineering, Centre for Catalysis Research and Innovation (CCRI), University of Ottawa, 161 Louis-Pasteur, Ottawa,ON K1N 6N5, Canada

h i g h l i g h t s

* Corresponding author. Tel.: þ1 6135625800x4044E-mail address: sntais@uottawa.ca (S. Ntais).

http://dx.doi.org/10.1016/j.matchemphys.2014.08.0330254-0584/© 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: S. Ntais,zirconia ionic support: Insight into mej.matchemphys.2014.08.033

g r a p h i c a l a b s t r a c t

� XPS study of Pt nanoparticles ofaverage sizes between 1.9 and 6.7 nmdeposited on YSZ.

� Pt4f peak of Pt/YSZ composites shiftsto lower BEs compared to Pt foil.

� Strong metal support interactiontakes place through charge transferor metal-oxygen vacancies.

a r t i c l e i n f o

Article history:Received 22 May 2014Received in revised form1 August 2014Accepted 19 August 2014Available online xxx

Keywords:CeramicsNanostructuresOxidesX-ray photo-emission spectroscopy (XPS)Surface properties

a b s t r a c t

An insight into the electronic interaction between platinum nanoparticles and ionically conductiveyttria-stabilized zirconia (YSZ) support is reported. To this end, Pt nanoparticles with four differentcrystallite sizes ranging from 1.9 to 6.7 nm were interfaced with YSZ resulting in �1 wt% Pt on YSZ. Theobtained four Pt/YSZ systems were studied using X-ray photoelectron spectroscopy and the changes ofthe recorded spectra are discussed in details. Pt4f peak is detected in all cases at lower binding energiescompared to the position of the peak of a platinum foil. More specifically, the Pt4f peak in the case of thesmallest nanoparticles shifts 0.9 eV compared to the peak position of the Pt foil. Furthermore, smallchanges are detected on the Zr3d, Y3d and O1s spectra after Pt nanoparticle deposition. The XPS resultsare interpreted on the basis of charge transfer that occurs from the oxide support to the platinumnanoparticles, which may be responsible for the observed strong metal-support interaction (SMSI) be-tween Pt and ionically conducting ceramics reported earlier for various catalytic reactions.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Nanoparticles (NPs) exhibit unique properties finding new ap-plications in many scientific fields and improving the alreadyexisted ones [1]. For instance, in the field of catalysis, Pt-group

; fax: þ1 6135625172.

et al., An X-ray photoelectrotal support interaction, Ma

nanosized systems show enhanced catalytic activity and selec-tivity compared to traditional polycrystalline catalysts [1,2].Nanosized catalysts appear to have improved catalytic performancewith the use of considerably less amounts of active phase. Thestudies until now have shown that various factors are responsiblefor their enhanced catalytic activity. Among them is the catalystparticle size, oxidation state, chemical and physical environment,composition as well as geometrical factors [2]. Of special interest inheterogeneous catalysis is the phenomenon of metal-support

n spectroscopy study of platinum nanoparticles on yttria-stabilizedterials Chemistry and Physics (2014), http://dx.doi.org/10.1016/

S. Ntais et al. / Materials Chemistry and Physics xxx (2014) 1e72

interaction (MSI), where the significant enhancement in catalyticactivity of metal/oxide-support systems is observed.

In early studies, Schwab et al. [3,4] hypothesized that catalystsupport can strongly influence the catalytic activity and the acti-vation energies of metal clusters. Later, Tauster et al. used themetal-support interaction phenomenon to explain the catalyticperformance of noble metals, e.g., Ir, Ru, Rh, Pd, Os, Pt supported ondifferent oxides, e.g., TiO2, MgO, Sc2O3, Y2O3, HfO2, ZrO2, V2O3,Nb2O5, Ta2O5 [5,6]. Since then, numerous studies have beendevoted to understanding the MSI phenomenon using various ox-ide substrates and a number of metal catalysts (Pt, Au, Pd, Ru, Rhetc.) and several comprehensive reviews have been published onthis topic [7,8].

Depending on the reducibility of the support, MSI can be weak,medium or strong [9]. Supports such as alumina, carbon, graphite,magnesia or silica show weak MSI. When zeolites are used assupport, one can refer to medium MSI, whereas strong metalsupport interaction (SMSI) is reported for the reducible oxidesupports, e.g. TiO2 and CeO2.

SMSI is usually explained in terms of either partial chargetransfer [10,11] from the substrate to the metal but a substrate-induced change in the lattice parameter of the deposited metalclusters has been also proposed [12]. The electronic interaction atthe metal-support interface has been shown to be enhanced whendopants are added resulting in electron transfer from the support tothe metal crystallites [13].

X-ray photoelectron spectroscopy (XPS) has beenwidely used toelucidate themechanism ofMSI. Studies on the Pt/TiO2 systemhaveshown a negative BE shift of the Pt4f peak attributed to theappearance of a fractional negative charge on Pt particles [10,11,14].This charge transfer mechanism has been proposed also for goldparticles supported on Fe2O3 where Audþ is converted to Aud� bycharge-transfer involving oxygen species [15]. Studies of platinuminterfaced with TiO2 system after high-temperature treatment,attributed SMSI to the reduction of Ti atoms and the formation ofintermetallic compounds (Pt3Ti) [16]. A recent XPS study of the Pt/TiO2 system after reduction at high temperature showed that theMSI mechanism differs under reductive or oxidative environmentbut in general, it takes place through migration/diffusion of lowvalence TiO2 onto the surface of metallic Pt [17].

The metal-support interaction phenomenon has been studiedusing XPS for both traditional oxide supports like TiO2[10,11,14,16,17], Al2O3 [18] or CeO2 [19] and for mixed oxides[20,21]. Several studies [22] and references therein have shownthat for the ZrO2-, TiO2- and CeO2-based supports, the mechanismof the MSI effect is functionally similar to the electrochemicalpromotion of catalysis (EPOC) phenomenon [23].

Recently, ionically conductive ceramics or solid electrolytes haveattracted a great amount of attention as promising active supportsfor nano-structured highly dispersed catalysts [24]. Among varioussolid electrolytes, yttria-stabilized zirconia (YSZ) shows excellentchemical and mechanical stability. The introduction of yttria into

Table 1XRD characteristics of Pt colloidal nanoparticles, peak maximum (2qmax (�)) and full widtand Pt/YSZ catalysts.

Pt/YSZ sample CNaOH (M) 2qmax (�) (111) FWHM (�)(111)

Pt/YSZ-1 0.056 39.72 1.59Pt/YSZ-2 0.06 39.64 2.78Pt/YSZ-3 0.08 39.72 3.64Pt/YSZ-4 0.20 39.33 12.76

a From XRD of colloidal Pt [33].b From TEMs of colloidal Pt [33].c From ADF-STEM images of the supported Pt/YSZ samples (Fig. 2).

Please cite this article in press as: S. Ntais, et al., An X-ray photoelectrozirconia ionic support: Insight into metal support interaction, Maj.matchemphys.2014.08.033

ZrO2 lattice creates oxygen vacancies in YSZ, which compensatecharge and provide high O2� ion conductivity at elevated temper-atures [25].

TheMSI of highly dispersed nanoparticles on YSZ, also termed as“self-induced EPOC” was first demonstrated over Pt NPs (10 nmaverage size) deposited on YSZ for propane oxidation and morerecently over Pt NPs of 3 nm average size for CO and tolueneoxidation [26e28]. It was found that for these reactions, Pt/YSZ hashigher activity than Pt deposited on carbon and g-Al2O3. It wasproposed that the electronic effect induces spontaneous back-spillover of ionic species from the support to the gas-exposedcatalyst surface thus resulting in improved performance of NPssupported on ionic conductors.

Up to date, the XPS studies of Pt/YSZ systems were carried outusing several mm-thick Pt film-electrodes deposited on YSZ [29,30]whereas no XPS studies on interaction of highly dispersed Ptnanoparticles with YSZ exist. Due to increasingly growing appli-cations of YSZ in various heterogeneous catalytic reactions [24], it isimperative to gain a better insight into the electronic and back-spillover (e.g. Od�) phenomena that take place in Pt NPs/YSZ cata-lytic systems and which is believed to be responsible for theiroutstanding catalytic performance.

In the present work, a detailed XPS study of highly dispersed PtNPs supported on yttria-stabilized zirconia is presented. To thisend, Pt NPs having four average particle sizes were prepared anddeposited on yttria stabilized zirconia (YSZ), resulting in Pt loadingof �1 wt%. The obtained Pt/YSZ catalysts have enhanced catalyticactivity for ethylene and CO oxidation as reported elsewhere [31],whereas this work presents the surface properties of those metallicNPs interfaced with YSZ and attempts to provide an insight into themetal-support interaction in the Pt/YSZ system using X-rayPhotoelectron spectroscopy analysis.

2. Experimental

2.1. Synthesis of the supported Pt nanoparticles

The synthesis of Pt nanoparticles was carried out in ethyleneglycol (EG, anhydrous 99.8% Sigma Aldrich) as described elsewhere[32]. In short, 0.1227 g of PtCl4 (Alfa Aesar, 99.9% metals basis) werediluted in 50 mL of EG containing various concentrations of sodiumhydroxide (0.056, 0.06, 0.08 and 0.20 M). NaOH was used to adjustthe pH of the synthesis solution in order to control the Pt particlesize [32e34]. The crystallite size of colloids was confirmed by XRDbeing 7.4, 3.8, 2.8 and 1 nm and was close to the average particlesizes of unsupported Pt NPs found from TEM (Table 1) [31]. The as-prepared colloidal nanoparticle solutions were deposited on YSZsupport by mixing an appropriate amount of the colloidal nano-particles with 1.5 g YSZ (Tosoh, 13 m2 g�1). In order to achieve highdispersion of the Pt NPs on the YSZ, the mixture was stirred/soni-cated for 24 h at room temperature. The resulting Pt/YSZ com-posites were extensively washed with de-ionized water and

h at half maximum (FWHM) of (111) Pt peak. Average particle size of Pt colloids [33]

Crystallite size ofPt NPs (nm) ±5%a

Average size fromTEM (nm)b

Average size of supportedPt/YSZ (nm)c

7.4 6.3 ± 0.5 6.7 ± 1.03.8 2.8 ± 0.8 4.4 ± 0.32.8 1.9 ± 1.0 3.0 ± 0.81.0 1.5 ± 0.5 1.9 ± 0.4

n spectroscopy study of platinum nanoparticles on yttria-stabilizedterials Chemistry and Physics (2014), http://dx.doi.org/10.1016/

Fig. 1. Pt4f XPS peaks for the Pt/YSZ-4 (a), Pt/YSZ-3 (b), Pt/YSZ-2 (c) and Pt/YSZ-1 (d)composites and a sputtered Pt foil (e).

S. Ntais et al. / Materials Chemistry and Physics xxx (2014) 1e7 3

separated by centrifugation at 6000 rpm. The washing procedurewas repeated ten times and finally the Pt/YSZ systems were dried ina freeze dryer for 10 h. The Pt metal loading in all cases wasconfirmed by inductive coupled plasma measurements (ICP-OESVarian Vista-Pro CCD spectrometer, Varian 720 ICP-AES) and wasfound to be close to 1 wt%. The prepared Pt/YSZ composites arenamed as Pt/YSZ-1 for the sample prepared using the largest andPt/YSZ-4 using the smallest NPs (Table 1).

2.2. XPS characterization

X-ray photoelectron spectroscopy (XPS) measurements werecarried out using a KRATOS Axis Ultra DLD with a Hybrid lens modeat 140 W and pass energy 20 eV using a monochromatic AlKa. TheXPS core level spectra were analyzed with a fitting routine whichdecomposes each spectrum into individual mixed Gaussian-Lor-entzian peaks using a Shirley background subtraction. In order toovercome charging effects, an electron gunwas used. The energy ofthe electrons was carefully selected to have the optimum signal tonoise ratio while at the same time to eliminate charging effects. TheXPS spectra were corrected by referencing the C1s core level line to284.6 eV.

2.3. Annular dark-field (ADF) scanning transmission electronmicroscopy (STEM) characterization

The FEI Titan3 80e300 microscope equipped with a CEOS ab-erration corrector for the probe forming lens and a monochromaticfield-emission gunwas used to acquire ADF-STEM images of Pt/YSZsamples. The specimens were prepared by sonicating the as-prepared catalyst powders in ethanol. One drop of the solutionwas then placed onto a 200 mesh TEM copper grid coated with alacey carbon support film (Ted Pella) and dried in air. The brightfeatures observed in the ADF-STEM imageswere identified as the Ptcatalyst using spatially-resolved energy dispersive X-ray spectros-copy (EDX). The use of Image J software allowed for the determi-nation of Pt particles size distribution. The particle size of Pt/YSZsamples was obtained from ADF-STEM images of the Pt/YSZ sam-ples by counting at least 300 particles (Table 1).

3. Results and discussion

The crystallite size of the as prepared Pt colloids was calculatedfromXRD diffractograms and Table 1 summarizes the 2qmax, FWHMof fcc (111) peak and the crystallite size, as well as the averageparticle size obtained from TEMmicrographs of the four Pt colloidsas reported earlier [31,33]. The synthesis of the Pt nanoparticles isperformed via the reduction of the platinum precursor (PtCl4) saltby ethylene glycol. The reduction of PtCl4 is accompanied by theoxidation of ethylene glycol to either glucolate ion or glucolic acid.Bock et al. [34] have proposed that glycolate ion serves as a stabi-lizing agent for PtRu nanoparticles. As pH increases the amount ofglucolate ion is high and the size of the obtained Pt NPs is smaller.

As it can be seen from Table 1, the crystallite size and the averageparticle size of the unsupported particles decreases with increasingNaOH concentration or pH of synthesis solution [33]. When theparticles are deposited on YSZ, which has low specific surface area (as ¼ 13 m2 g�1), the average particle size slightly increases.

The Pt/YSZ catalysts were characterized using XPS and the Pt4fpeaks for all composites are shown in Fig. 1. The deconvolutionwasperformed using doublets with spin orbit splitting 3.35 eV andintensity ratio Pt4f7/2:Ptf5/2 ¼ 4/3, while a peak asymmetry wasused based on the work of Hufner et al. [35]. For comparison reasonand in order to define the peak asymmetry of the metallic state, a Ptfoil was also measured and is shown in Fig. 1e.

Please cite this article in press as: S. Ntais, et al., An X-ray photoelectrozirconia ionic support: Insight into metal support interaction, Maj.matchemphys.2014.08.033

The deconvolution of the Pt4f peaks (Fig.1) reveals the existenceof one asymmetrical peak which is attributed to Pt in the metallicstate in all cases. The Pt4f7/2 peak in the case of Pt foil is detected at71.1 eV in good agreement with the reported value for bulk metallicPt [36]. The Pt4f peak in the case of the Pt/YSZ composites shows agradual shift towards lower BE with decreasing particle sizecompared to the peak position in case of Pt foil. It is worth to notethat our group has reported recently an opposite shift and an in-crease of the FWHM of the Pt4f peaks in the case of the same Pt NPssupported on carbon black (Vulcan-XC 72) with decrease of theparticle size [33]. In the case of the Pt/YSZ-1 (Fig. 1d) the peak isfound at 70.6 eV (Pt4f7/2). The Pt4f7/2 peak for the smallest NPs is at70.2 eV (Pt/YSZ-4, Fig. 1a). The Pt4f7/2 exhibit a tendency to shift tolower BEs as the size of the NPs decrease implying a particle sizeeffect on the position of the core level peak. Similar shift towardslower BEs of the Pt4f peak were reported for several Pt/metal oxidecomposite systems [11,12,14,17] as well as for Au NPs supported ondifferent substrates [15,37,38].

Table 2 summarizes the Pt4f peak positions and the corre-sponding values of FWHMof all Pt/YSZ samples. As it can be seen theFWHM shows a small increase as the Pt particle size decrease but asmall discrepancy occurs in the case of the Pt/YSZ-2 that exhibits less

n spectroscopy study of platinum nanoparticles on yttria-stabilizedterials Chemistry and Physics (2014), http://dx.doi.org/10.1016/

Table 2Binding energy and FWHMof the Pt4f7/2 peaks for four Pt/YSZ composites and the Ptfoil.

Pt/YSZ sample Binding energy (eV) FWHM (eV)

Pt foil 71.1 0.9Pt/YSZ-1 70.6 1.9Pt/YSZ-2 70.5 1.7Pt/YSZ-3 70.3 1.9Pt/YSZ-4 70.2 2.1

S. Ntais et al. / Materials Chemistry and Physics xxx (2014) 1e74

wide peak than the other samples. The changes of the FWHM of thePt4f peaks (shown in Table 2) are only qualitatively discussedmainlydue to two reasons. The first one is that for the deconvolution of thePt4f peaks an attempt to use in all cases the same limits of thebackground line was not possible due to the relatively low signal tonoise ratio. Small changes in the limits of the background line causeschanges of the FWHM of the deconvoluted peak that in some casescan be even around 0.2 ± 0.05 eV while has no significant influenceon the peak position. Additionally, the FWHM and the relative po-sition of the Pt4f peak can be affected by the degree of Pt NPsdispersion and agglomeration, the parameters that determine thegeometrical characteristics of the particles. It has been reportedearlier for Au NPs, that structural arrangements of Au atoms in thesurface have an effect on the characteristics of the XPS peaks [38].

To this end, all Pt/YSZ samples were studied using ADF-STEM toobtain a clearer insight into morphological characteristics of theprepared Pt/YSZ systems. Fig. 2 shows the representative ADF-STEM images of the Pt/YSZ samples. The bright dots correspondto Pt NPs. As can be seen, there are areas where Pt NPs dispersion isgood and NPs are well distributed on YSZ support. (Fig. 2aec).However, ADF-STEM also revealed the presence of NPs agglomer-ation (Fig. 2d and h) and formation of larger aggregates (Fig. 2h)and these observations are independent on Pt NPs size. The pres-ence of agglomeration in Pt/YSZ samples could, in some degreeinfluence the position and FWHM of the Pt4f XPS peak, howeverthe agglomeration in the current samples cannot be avoided due tothe low specific surface area of YSZ.

The Zr3d, Y3d and O1s XPS spectra for all Pt/YSZ samples wererecorded as well. The deconvolution of the Zr3d and Y3d peaks wascarried out using doublet peaks with spin orbit splitting 2.4 and2 eV and intensity peak ratio 1.5 [36]. The deconvoluted spectra forboth Pt/YSZ-4 and YSZ powder alone are shown in Fig. 3. Similarresults were obtained for the rest of the Pt/YSZ samples and thepeak positions, the FWHM and their % relative intensities aresummarized in Table 3.

The Zr3d5/2 and Y3d5/2 peaks for YSZ powder are detected ataround 181.8 and at 156.8 eV, respectively and are characteristic ofyttrium and zirconium atoms in yttria-stabilized zirconia [39]. Theanalysis of the O1s spectra for the YSZ (Fig. 3c, lower spectra)revealed the existence of three components at about 529.7 (OI),531.3 (OII) and 532.6 eV (OIII). The OI is attributed to the latticeoxygen atoms in the mixed YSZ oxide [39] while the OII componentis attributed to chemisorbed oxygen Od� on the catalyst surface[39e41]. The component at the highest BE (OIII) is due to oxygenspecies related to atmospheric carbon contamination (CeObonding) [42] but it can also be attributed to OH species [43]. In theliterature though its presence has been reported in the case ofmixed oxides and attributed to chemisorbed O2� [44]. The relativeintensities of the three oxygen components were found to beOI:OII:OIII ¼ 75:20:5.

The deposition of Pt nanoparticles causes small changes of Y3d,Zr3d and O1s XPS spectra (Fig. 3, upper spectra). The Zr3d and Y3dspectra of Pt/YSZ show a rather small shift towards higher BEs and atheir FWHM compared to those of pure YSZ. This small broadening

Please cite this article in press as: S. Ntais, et al., An X-ray photoelectrozirconia ionic support: Insight into metal support interaction, Maj.matchemphys.2014.08.033

of the Y3d and Zr3d peaks can be attributed to a change in thechemical environment of Y and Zr atoms after the deposition of Ptnanoparticles. This hypothesis is further supported by the changesin the O1s spectra. The analysis of the O1s reveals the existence ofthree peaks but now their relative intensity has changed. Morespecifically, the ratio of their relative intensities isOI:OII:OIII ¼ 65:25:10, showing an increase of the abundance ofoxygen containing species to which the OII and OIII are attributed.The changes on the relative intensity of the oxygen components arerather small but they show a similar trend for all Pt/YSZ samples.The increase of the relative intensity of the OIII peak can be due toan increase of CO species adsorbed on platinum sites and/or theincrease of the OH groups present on the surface. To this point anddue to the relatively low loading of Pt it is not clear whether thesechanges Y3d, Zr3d and O1s core level peaks are Pt-induced or aredue to residuals from the experimental procedure.

Furthermore, the Y/Zr surface atomic ratio was calculated usingthe intensities and the sensitivity factors of Y3d and Zr3d [36]. Thecorresponding value for the YSZ powder was found to be 0.24 butthe Pt/YSZ systems exhibit a small decrease of the Y/Zr atomic ratioof around 8%. More specifically, the Y/Zr atomic ratio was found inall cases to be around 0.22.

The pronounced down shift of the Pt4f core level peak (~1 eV) tothe lower binding energies is a strong indication of an enhancedelectron density of Pt nanoparticles when they are interfaced withYSZ. Previously, Doniach et al. [45] reported that the density ofstates at the Fermi level influence the asymmetry of the XPS corelevel spectra. Recently, changes of Pt4f peaks asymmetry were usedto explain shifts of the Pt4f peak in the case of Pt/TiO2/C [46]. As itwas mentioned above, the Pt4f peaks were deconvoluted using thesame degree of asymmetry found for the Pt4f of a sputteredoxygen-free foil. Additionally, due to the relatively low signal tonoise ratio of the Pt4f spectra, originated from the low Pt loadingthe deconvolution can be performed successfully using peaks withdifferent degree of asymmetry. Therefore, it is not safe to associatethe peak asymmetry of Pt4f peaks to the electron density around Ptatoms at this point. Moreover, due to the low content of platinumthe Pt-related Auger peaks show low signal to noise ratio notallowing the calculation of Auger parameters that could provideadditional information.

The observed Pt4f peak shifts can be explained by assuming theformation of PteY and/or PteZr alloys. The formation of alloys isusually accompanied by a shift of the core level peaks of bothmetals mainly toward opposite direction as we have mentionedelsewhere [47]. However, in our case the Y3d and Zr3d do not showany significant shift. The formation of a YePt and/or ZrePt alloyrequires the scission of YeO and/or ZreO bonds, which seemsrather unlikely.

It is proposed that the observed shifts of the Pt4f peak and thesmall changes in the FWHM of Y3d and Zr3d are due to the strongmetal-support interaction, where electrons are transferred fromYSZ support to Pt nanoparticles. This hypothesis is based on the factthat when two solids with different work functions are broughtinto a contact, electrons from the substance with the lower workfunction (WF) will be transferred to the material with the higherWF until the development of an interface potential will opposefurther electron flow. Bulk metallic Pt has WF~5.3 eV while it in-creases up to 9 eV for a single atom [48]. Tsiplakides et al. [49] hasreported that the work function of YSZ (~8 mol% Y2O3 doped ZrO2)is 5.14 eV at PO2 ¼ 1 atm which is lower compared to WF of Pt NPs.Therefore upon contact, electrons will flow from YSZ towardsplatinum leading to increase in Pt electron density and thus causingthe decrease of the B.E. of the corresponding Pt4f peaks. Theobserved down shift of Pt4f peak can be also explained based on thenature of YSZ. YSZ contains highlymobile oxygen vacancies and it is

n spectroscopy study of platinum nanoparticles on yttria-stabilizedterials Chemistry and Physics (2014), http://dx.doi.org/10.1016/

Fig. 2. Representative ADF-STEM images of Pt/YSZ composites: Pt/YSZ-4 (a) and (b), Pt/YSZ-3 (c) and (d), Pt/YSZ-2 (e) and (f) and Pt/YSZ-1 (g) and (h).

S. Ntais et al. / Materials Chemistry and Physics xxx (2014) 1e7 5

Please cite this article in press as: S. Ntais, et al., An X-ray photoelectron spectroscopy study of platinum nanoparticles on yttria-stabilizedzirconia ionic support: Insight into metal support interaction, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.08.033

Table 3XPS peak position of Zr3d5/2, Y3d5/2 and O1s components, the corresponding FWHM and % relative intensity, as well as the Y/Zr atomic ratio.

YSZ blank Pt/YSZ-1 Pt/YSZ-2 Pt/YSZ-3 Pt/YSZ-4

Zr3d5/2 181.8(1.25 eV, 100%)

181.8(1.35 eV, 100%)

181.9(1.35 eV, 100%)

181.8(1.3 eV, 100%)

181.8(1.35 eV, 100%)

Y3d5/2 156.8(1.3 eV, 100%)

156.9(1.40 eV, 100%)

156.8(1.35 eV, 100%)

156.8(1.4 eV, 100%)

156.7(1.3 eV, 100%)

O1s 529.7(1.3 eV, 74%)531.3(1.5 eV, 20%)532.6(1.6 eV, 6%)

529.7(1.4 eV, 66%)531.3(1.6 eV, 23%)525.6(1.6 eV, 11%)

529.7(1.35 eV, 64%)531.3(1.6 eV, 25%)532.8(1.7 eV, 11%)

529.7(1.4 eV, 66%)531.3(1.7 eV, 24%)532.6(1.65 eV, 8%)

529.8(1.45 eV, 67%)531.4(1.6 eV, 25%)532.8(1.7 eV, 10%)

Y/Zr 0.24 0.22 0.21 0.21 0.22

Fig. 3. Zr3d (a), Y3d (b) and O1s (c) of the Pt/YSZ-4 (upper spectra) and of YSZ powder (lower spectra).

S. Ntais et al. / Materials Chemistry and Physics xxx (2014) 1e76

possible that Pt would be more strongly attracted to oxygen va-cancy sites, therefore electrons from the vacancy could be trans-ferred to platinum. This will lead to an increase in the electrondensity of Pt and consequently result in a negative shift of the Pt4f7/2 peak position. In several XPS studies of metal/oxide supportsystems the observed negative shifts of the metal core level spectrawere associated with an observed reduction of the oxide support,e.g., formation of Ti3þ and Ce3þ in the case of TiO2 and CeO2,respectively [16,19]. Our results show a small change of thechemical environment around the yttrium and zirconium atoms. Aswe mentioned above, the Y/Zr atomic ratio calculated using theintensities of the XPS peaks shows a small decrease after thedeposition of Pt NPs. This decrease of the Y/Zr ratio might indicatethat platinum prefers to lie on Y sites thus causing a more pro-nounced decrease of Y3d signal, however further studies arerequired to confirm this.

The negative shift in the position of the peaks compared to thePt foil is a strong indication of the change in electron density of thePt NPs. Though, due to the very low content of Pt (1 wt%) and thefact that agglomeration takes place the signal of the recorded peaksis relatively low (low signal to noise ratio). Thus, the peak asym-metries and FWHM of the deconvoluted peaks cannot be safelyused to further support the changes in the electron density of Pt.More studies are currently in progress to clarify this point.

4. Conclusions

In this study the MSI effect generated between highly dispersedPt NPs of 1.9e6.7nm average size) and YSZ is investigated by X-rayphotoelectron spectroscopy for the first time. Unlike earlier XPSstudies using thick-film Pt electrodes, we have placed specialemphasize on the exploration of interaction between Pt nano-particles and YSZ in Pt/YSZ catalytic system with low Pt loading(1wt%) typically used in practical applications. We found that the

Please cite this article in press as: S. Ntais, et al., An X-ray photoelectrozirconia ionic support: Insight into metal support interaction, Maj.matchemphys.2014.08.033

position of Pt4f peak shows a significant shift up to 0.9 eV towardslower BEs compared to the position of the Pt foil peak. This shift isaccompanied by the small changes in the width of Y3d and Zr3dXPS core level peaks. The results are interpreted on the basis of astrong metal support interaction of Pt nanoparticles and YSZ. Thisinteraction takes place through the charge transfer from the YSZsupport to Pt atoms and seems to be strongly size dependent.Furthermore, Pt might be attracted more strongly to oxygen va-cancy sites present in the support thus allowing the transfer ofelectrons from the vacancy to platinum. The observed MSI takingplace at the interface of Pt NPs and YSZ can explain the high cata-lytic activity of the Pt/YSZ systems reported earlier.

Acknowledgement

The authors would like to thank the Natural Science and Engi-neering Research Council (NSERC), Discovery grant for financialsupport and Dr. M. Couillard from National Research Council (NRC)Canada for ADF-STEM measurements. R.J. Isaifan acknowledgesOntario Graduate Scholarship (OGS).

References

[1] G. Cao, C.J. Brinker, Annual Review of Nano Research, vols. 1e3, World Sci-entific Publishing Company, 2006.

[2] V. Polshettiwar, T. Asefa, G. Hutchings, Nanocatalysis: Synthesis and Appli-cations, John Wiley & Sons, Inc., Hoboken, New Jersey, 2013.

[3] G.M. Schwab, J. Block, W. Muller, D. Schultze, Zur Natur der katalytischenVerst€arker-Wirkung, Naturwissenschaften 44 (1957) 582e584.

[4] G.M. Schwab, J. Block, D. Schultze, Kontaktkatalytische Verst€arkung durchdotierte Tr€ager, Angew. Chem. 71 (1958) 101e104.

[5] S.J. Tauster, S.C. Fung, R.L. Garten, Strong metal-support interactions. Group 8noble metals supported on titanium dioxide, J. Am. Chem. Soc. 100 (1978)170e175.

[6] S.J. Tauster, S.C. Fung, Strong metal-support interactions: occurrence amongthe binary oxides of groups IIAeVB, J. Catal. 55 (1978) 29e35.

n spectroscopy study of platinum nanoparticles on yttria-stabilizedterials Chemistry and Physics (2014), http://dx.doi.org/10.1016/

S. Ntais et al. / Materials Chemistry and Physics xxx (2014) 1e7 7

[7] S. Bernal, J.J. Calvino, M.A. Cauqui, J.M. Gatica, C. Larese, J.A. Perez Omil,J.M. Pintado, Some recent results on metal/support interaction effects in NM/CeO2 (NM: noble metal) catalysts, Catal. Today 50 (1999) 175e206.

[8] Q. Fu, T. Wagner, Interaction of nanostructured metal overlayers with oxidesurfaces, Surf. Sci. Rep. 62 (2007) 431e498.

[9] R.L. Augustine, Heterogeneous Catalysis for the Synthetic Chemist, CRC Press,New York, 1995.

[10] M. Koudelka, A. Monnier, J. Sanchez, J. Augustynski, Correlation between thesurface composition of Pt/TiO2 catalysts and their adsorption behaviour inaqueous solutions, J. Mol. Catal. 25 (1984) 295e305.

[11] J.C. Colmenares, A. Magdziarz, M.A. Aramendia, A. Marinas, J.M. Marinas,F.J. Urbano, J.A. Navio, Influence of the strong metal support interaction effect(SMSI) of Pt/TiO2 and Pd/TiO2 systems in the photocatalytic biohydrogenproduction from glucose solution, Catal. Commun. 16 (2011) 1e6.

[12] W. Vogel, L. Timperman, N. Alonso-Vante, Probing metal substrate interactionof Pt nanoparticles: structural XRD analysis and oxygen reduction reaction,Appl. Catal. A 377 (2010) 167e173.

[13] E.C. Akubuiro, X.E. Verykios, Effects of dopants on performance of metalcrystallites 2. Further characterization of doped supports and catalysts,J. Catal. 113 (1988) 106e119.

[14] M. Spichiger-Ulmann, A. Monnier, M. Koudelka, J. Augustynski, Strong metal-support interactions, in: R.T.K. Baker, S.J. Tauster, J.A. Dumesic (Eds.), ACSSymposium Series. Washington, 1986, pp. 212e227.

[15] A. Horvath, L. Toth, L. Guczi, Gold nanoparticles: effect of treatment onstructure and catalytic activity of Au/Fe2O3 catalyst prepared by co-precipi-tation, Catal. Lett. 67 (2000) 117e128.

[16] S. Tang, G. Xiong, H. Wang, The nature of the SMSI state of the PtTiO2 system,J. Catal. 111 (1988) 136e145.

[17] J. Zhang, M. Zhang, Z. Jin, J. Wang, Z. Zhang, Study of high-temperaturehydrogen reduced Pt0/TiO2 by X-ray photoelectron spectroscopy com-bined with argon ion sputteringdDiffusion-encapsulation effect in rela-tion to strong metalesupport interaction, Appl. Surf. Sci. 258 (2012)3991e3999.

[18] E.I. Altman, R.J. Gorte, A photoelectron spectroscopy study of Pt particlessupported on a-Al2O3{0001}, Surf. Sci. 216 (1989) 386e394.

[19] T. Jin, Y. Zhou, G.J. Mains, J.M. White, Infrared X-ray photoelectron spectros-copy study of CO and CO, on Pt/CeO2, J. Phys. Chem. 91 (1987) 5931e5937.

[20] J.C. Serrano-Ruiz, G.W. Huber, M.A. S�anchez-Castillo, J.A. Dumesic,F. Rodríguez-Reinoso, A. Sepúlveda-Escribano, Effect of Sn addition to Pt/CeO2eAl2O3 and Pt/Al2O3 catalysts: an XPS, 119Sn M€ossbauer and microcalo-rimetry study, J. Catal. 241 (2006) 378e388.

[21] A. Dauscher, L. Hilaire, F. Le Normand, W. Miller, G. Maire, A. Vasquez,Characterization by XPS and XAS of supported Pt/TiO2-CeO2 catalysts, Surf.Interf. Anal. 16 (1990) 341e346.

[22] J. Nicole, D. Tsiplakides, C. Pliangos, X.E. Verykios, C. Comninellis, C.G. Vayenas,Electrochemical promotion and metalesupport interactions, J Catal. 204(2001) 23e34.

[23] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, ElectrochemicalActivation of Catalysis: Promotion, Electrochemical Promotion and Metal-Support Interactions, Kluwer Academic/Plenum Publishers, New York, 2001.

[24] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-Consuegra,J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou,E.A. Baranova, Ionically conducting ceramics as active catalyst supports,Chem. Rev. 113 (2013) 8192e8260.

[25] J.P. Goff, W. Hayes, S. Hull, M.T. Hutchings, K.N. Clausen, Defect structure ofyttria-stabilized zirconia and its influence on the ionic conductivity atelevated temperatures, Phys. Rev. B 59 (1999) 14202e14219.

[26] P. Vernoux, M. Guth, X. Li, Ionically conducting ceramics as alternative catalystsupports, Electrochem. Solid-State Lett. 12 (2009) E9eE11.

[27] H.A.E. Dole, R.J. Isaifan, F.M. Sapountzi, L. Lizarraga, D. Aubert, A. Princivalle,P. Vernoux, E.A. Baranova, Low temperature toluene oxidation over Ptnanoparticles supported on yttria stabilized-zirconia, Catal. Lett. 143 (2013)996e1002.

Please cite this article in press as: S. Ntais, et al., An X-ray photoelectrozirconia ionic support: Insight into metal support interaction, Maj.matchemphys.2014.08.033

[28] R.J. Isaifan, H.A.E. Dole, E. Obeid, L. Lizarraga, P. Vernoux, E.A. Baranova, Metal-support interaction of Pt nanoparticles with ionically and non-ionicallyconductive supports for CO oxidation, Electrochem Solid State Lett. 15(2012) E1eE3.

[29] S. Ladas, S. Kennou, S. Bebelis, C.G. Vayenas, Origin of non-faradaic electro-chemical modification of catalytic activity, J. Phys. Chem. 97 (1993)8845e8848.

[30] B. Luerßen, J. Janek, R. Imbihl, Electrocatalysis on Pt/YSZ electrodes, Solid StateIonics 141/142 (2001) 701e707.

[31] R.J. Isaifan, S. Ntais, M. Couillard and E.A. Baranova, Size-dependent activity ofPt/yttria-stabilized zirconia catalyst for wireless electrooxidation of ethyleneand carbon monoxide in oxygen free environment, submitted to J. Catalysis.

[32] E.A. Baranova, C. Bock, D. Ilin, D. Wang, B. Macdougall, Infrared spectroscopyon size-controlled synthesized Pt-based nano-catalysts, Surf. Sci. 600 (2006)3502e3511.

[33] R.J. Isaifan, S. Ntais, E.A. Baranova, Particle size effect on catalytic activity ofcarbon-supported Pt nanoparticles for complete ethylene oxidation, Appl.Catal. A: General. 464-465 (2013) 87e94.

[34] C. Bock, C. Paquet, M. Couillard, G.A. Botton, B.R. MacDougall, Size-selectedsynthesis of PtRu nano-catalysts: reaction and size control mechanism, J. Am.Chem. Soc. 126 (2004) 8028e8037.

[35] S. Hufner, G.K. Wertheim, Core-line asymmetries in the x-ray-photoemissionspectra of metals, Phys. Rev. B 11 (1975) 678e683.

[36] D. Briggs, M.P. Seah, in: second ed.Practical Surface Analysis, vol. 1, Wiley,New York, 1996.

[37] S. Arrii, F. Morfin, A.J. Renouprez, J.L. Rousset, Oxidation of CO on gold sup-ported catalysts prepared by laser vaporization: direct evidence of supportcontribution, J. Am. Chem. Soc. 126 (2004) 1199e1205.

[38] J. Radnik, C. Mohr, P. Claus, On the origin of binding energy shifts of core levelsof supported gold nanoparticles and dependence of pretreatment and mate-rial synthesis, Phys. Chem. Chem. Phys. 5 (2003) 172e177.

[39] D. Majumdar, D. Chatterjee, X-ray photoelectron spectroscopic studies onyttria, zirconia, and yttria-stabilized zirconia, J. Appl. Phys. 70 (1991)988e992.

[40] S. Ardizzone, M.G. Cattania, P. Lugo, Interfacial electrostatic behaviour of ox-ides: correlations with structural and surface parameters of the phase, Elec-trochim. Acta 39 (1994) 1509e1517.

[41] H.B. Wang, C.R. Xia, G.Y. Meng, D.K. Peng, Deposition and characterization ofYSZ thin films by aerosol-assisted CVD, Mater. Lett. 44 (2000) 23e28.

[42] S. Ntais, V. Dracopoulos, A. Siokou, TiCl4(THF)2 impregnation a flat SiOx/Si(10 0) Polycryst. Au foil: Determ. Surf. species using XPS, J. Mol. Catal. A: Chem.220 (2004) 199e205.

[43] J.S. Bae, S.S. Park, B.S. Mun, S.H. Park, E.S. Hwang, J. Kim, et al., Surfacemodification of yttria-stabilized-zirconia thin films under various oxygenpartial pressures, Thin Solid Films 520 (2012) 5826e5831.

[44] Q. Xu, D.P. Huang, W. Chen, H. Wang, B.T. Wang, R.Z. Yuan, X-ray photo-electron spectroscopy investigation on chemical states of oxygen on surfacesof mixed electroniceionic conducting La0.6Sr0.4Co1�yFeyO3 ceramics, Appl.Surf. Sci. 228 (2004) 110e114.

[45] S. Doniach, M. Sunjic, Many-electron singularity X-ray photoemiss. X-ray linespectra metals, J. Phys. Chem. C: Solid State Phys. 3 (1970) 285e291.

[46] A. Lewera, L. Timperman, A. Roguska, N. Alonso-Vante, Metalesupport in-teractions between nanosized Pt and metal oxides (WO3 and TiO2) studiedusing X-ray photoelectron spectroscopy, J. Phys. Chem. C 115 (2011)20153e20159.

[47] A. Allagui, M. Oudah, X. Tuaev, S. Ntais, F. Almomani, E.A. Baranova, Ammoniaelectro-oxidation on mono-dispersed PtIr nanoparticles, Int. J. Hydrogen En-ergy 38 (2013) 2455e2463.

[48] E. Roduner, Size matters: why nanomaterials are different, Chem. Soc. Rev. 35(2006) 583e592.

[49] D. Tsiplakides, C.G. Vayenas, Electrode work function and absolute potentialscale in solid-state electrochemistry, J. Electrochem Soc. 148 (2001)E189eE202.

n spectroscopy study of platinum nanoparticles on yttria-stabilizedterials Chemistry and Physics (2014), http://dx.doi.org/10.1016/