Published on Web Date: May 07, 2010

r 2010 American Chemical Society 1660 DOI: 10.1021/jz100511s |J. Phys. Chem. Lett. 2010, 1, 1660–1665

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Probing Transformations of Relevance in Catalysis on aSingle Oxide Layer: Fe on TiOx/Pt(111)Luca Artiglia,† Emanuele Cavaliere,‡,§ Gian Andrea Rizzi,† Luca Gavioli,‡,§ andGaetano Granozzi*,†

†Department of Chemical Sciences, via Marzolo 1, University of Padova, Italy, ‡Department of Mathematics and Physics,Universit�a Cattolica del Sacro Cuore, via dei Musei 41, I-25121 Brescia, Italy, and §Istituto Officina dei Materiali - CNR,Laboratorio TASC, Area Science Park - Basovizza, Strada Statale 14, Km.163.5 I-34149 Trieste, Italy

ABSTRACT The evolution of a Fe/z0-TiOx/Pt(111) model catalyst in ultrahighvacuum, studied by advanced surface science tools, reveals various phenomenaoccurring in different temperature ranges. In the room temperature (RT) < T <460 K range, the deposited Fe atoms assemble as heterogeneous Fe and FeOx

nanoparticles (NPs), templated by the ordered array of defects (troughs andpicoholes) of the oxide film. At 460 < T< 810 K, FeOx NPs become predominant,and ametastable FeOx/TiOxmixed oxide is formed, assisted by the interdiffusion ofFe through the oxide into the Pt substrate, which triggers the z0-TiOx structuretransformation into a different TiOx phase with a hexagonal pattern. At 810< T<1000 K, a Fe-mediated extraction of Pt produces Pt(111) islands, surrounded andpartly encapsulated by the same z0-TiOx phase, demonstrating the intrinsic highstability of the ultrathin oxide film. This represents one of the first examples of aphase transformation in a ultrathin oxide layer induced by an external metaldeposit.

SECTION Surfaces, Interfaces, Catalysis

I ron and iron oxide (FeOx) nanoparticles (NPs) play animportant role in many chemical catalytic processes,1,2

such as selective oxidations or the formation and clea-vage of C-C bonds (e.g., Fischer-Tropsch process). In addi-tion, FeOx magnetic NPs have several applications in fieldssuch as biomedicine, environmental sciences, informationstorage, micromechanics, and microfluidics.3-6 It is now wellestablished that the oxide support (OS), where the catalyticNPs are usually dispersed, can play a relevant role on theoverall system catalytic activity.7 In fact, several phenomenadue to the NPs/OS interactions, either static (e.g., chargetransfer, and interface states) or dynamical (e.g., encapsula-tion, spillover and reverse spillover, NP coalescence), havebeen well documented.8 When the OS is reducible and somenoble-likemetalNPs (e.g., PdandPt) are involved, theobservedeffects have been attributed to a strong metal-support inter-action, which has been described in detail.9-11 For reactivetransition metals of interest in catalysis, e.g., Fe and Co, thebehavior is still a matter of debate, since the actual processesstrongly depend on the balance between the surface energiesand themetal work-function, and can lead both to oxidation orencapsulation of the metal NPs.11,12

A deep understanding of the relevant elementary steps inreal catalytic processes can be obtained using transitionmetal/OS model systems based on ultrathin (UT) oxidefilms:13 the effects due to morphology, defects, charge trans-fer, and interfacial states can be conveniently detailed. Inaddition, the UT films themselves are currently investigated

as potential catalysts (monolayer catalysts) with improvedperformances.13-15

Here we investigate a Fe/TiOx/Pt(111) model system, pre-pared by Fe deposition on a well-characterized UT TiOx filmepitaxially grown on Pt(111)16 (hereafter indicated as z0-TiOx).Adopting a series of advanced surface science tools,i.e., synchrotron (SR)-based high-resolution X-ray photoemis-sion (HR-XPS), scanning tunneling microscopy (STM) andthermal programmed desorption (TPD), we follow the physi-cal and chemical changes occurring after ultrahigh vacuum(UHV)annealing in a wide temperature range (from 300 Kupto 1000 K).

The preparation of the z0-TiOx film (x=1.25) can be fine-tuned to obtain a zigzag-like wetting polar Ti-O bilayer,already described in great detail,17,18 where the Ti ions areat the interface with the Pt substrate and the O ions form thetopmost layer. The zigzag-like structure consists of an orderedarray of rows (troughs), spaced by 1.44 nm and separatingcompact TiOx stripes, where Ti ions present a differentchemical and structural environment.18 The troughs arecharacterized by Ti vacancies exposing the bare substratesurface, defined as picoholes, that have been demonstrated tobe effective as nucleation sites for metal NPs growth.19-21

Received Date: April 21, 2010Accepted Date: May 5, 2010

r 2010 American Chemical Society 1661 DOI: 10.1021/jz100511s |J. Phys. Chem. Lett. 2010, 1, 1660–1665

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Our model catalyst was prepared by depositing 0.5 mono-layer (ML) of Fe on the clean z0-TiOx phase. This system wasthen analyzed at room temperature (RT) and after shortannealing (20) in UHVat temperatures up to 1000 K.

Figure 1 shows the HR-XPS Ti 2p and Fe 2p core level dataobtained on the Fe(0.5 ML)/z0-TiOx system collected with aphoton energy of 550 and 830 eV, respectively. The electronkinetic energy is on the order of 100 eV, ensuring a highsurface sensitivity to the HR-XPSmeasurements. At RT, the Ti2p spectrum (Figure 1) presents a predominant componentcenteredat a binding energy (BE) of456.5eV, to be associatedwith Ti at the interface with the substrate.17,22,23 The 0.4 eVenergy shift toward lower BEwith respect to the clean phase isdue to the charge transfer and band bending resulting fromthe Fe deposition,11 while the larger peak width is due to asecond minor component at about 458.5 eV, which can beonly appreciated after deconvolution of the peak profile. TheTi 2p component at higher BE has been associated withoxidized Ti ions sandwiched between two oxygen layers.17

At RT, the Fe 2p spectrum (Figure 1) shows a predominantmetallic component centered at about 707.5 eV, with acontribution fromoxidized states (both Fe2þ and Fe3þ) visibleafter deconvolution of the peak profile.

Annealing the system up to 460 K results only in minorvariation of the Ti 2p andFe2p spectra, i.e. a slightly increasedintensity of the higher BE Ti 2p component and of the oxide-related Fe 2p peaks. However, starting from this temperature,somedistinct changes in theXPSdata are observed (Figure1):

(a) The growth of awell-resolved component at higher BEin the Ti 2p spectra, centered at about 458.5 eV. Suchpeak reaches its maximum intensity at about 760 Kand starts to decrease again above 810 K, completelydisappearing at T>910 K. At this temperature, the Ti2p peak shape obtained at RT for the clean z0-TiOx film

is recovered, apart from the BE shift, that is stillobserved, probably due to the presence of residualFeOx at the surface. However, the overall integral Ti 2pintensity remains almost unchanged in the investi-gated temperature range.

(b) A progressive broadening of the Fe 2p spectrum, dueto the intensity increase of FeOx components in theregionwhere the corresponding shakeup satellites areexpected and there is a decrease of the overall integralFe 2p intensity, becoming relevantwhen the tempera-ture goes beyond 800 K.

Such data suggest that there are two parallel phenomena,i.e., a progressive oxidation of Fe, already present at RT, butclearly increasing from 460 K, and a diffusion of Fe into thesubstrate, relevant beyond 800 K, while the amount of Ti isalmost unchanged.Moreover, a quite peculiar phenomenon isobserved: in the 460-810K temperature range both Ti and Febecome apparently more oxidized. However, the processoccurs at a constant oxygen amount since neither an oxygenload is given during the thermal treatments (done in UHV),nor an oxygen loss is observed using TPD. The importantpoint to outline is the high stability of theTi-Obond, since thepreparation of the z0-TiOx UT films is routinely carried out byannealing inUHVat high temperature (970K). In our opinion,the only way to reconcile the experimental data is to considera partial migration of Fe into the bulk substrate, and aconcomitant restructuring of the z0-TiOx film, leading to theformation of amixed FeOx/TiOx oxide. In other words, kineticphenomena, such as the interdiffusion of metals into thesubstrate bulk,9,24 have to be considered to explain the ratherunexpected oxidation process inUHV, as already suggested ina previous detailed investigation of the evolution of the z0-TiOx

film at high temperature in the same conditions.25

This hypothesis finds support from the STM and TPD datadiscussed below. In Figure 2 we show the evolution of thesystem morphology by STM examined at RT for the systemprepared and treated exactly in the same conditions. At RT(Figure 2a) the Fe deposit forms NPs, predominantly alignedalong the directions of the troughs.21,26 Note, however, thatthe NPs are both nucleated on picoholes and on the TiOx

stripes, as a consequence of the lowFemobility onTiOxdue tothe Fe -O strong interaction and diffusion barriers.26 Above460 K, the STM data (Figure 2b) indicate that NPs stand in aslightly increased order, even if their density decreases byabout 20% with respect to RT. Moreover, a different hexago-nal arrangement of the UT film starts to appear amid the NPs(better observed at higher T, see Figure 2c,d). The importantpoint to outline is that such modification of the TiOx layer isdefinitely driven by the presence of Fe: in fact, the z0-TiOx

phase alone does not show any modification upon UHVannealing in a temperature range up to 970 K. A similartransformation from the rectangular z0-TiOx phase to a hex-agonal phase has been already described to occur at hightemperature in the case of the Au/z0-TiOx model system,where the NPs do not coalesce, but rearrange their shapeand positions together with the adjacent regions of theoxide.27 We recall that, at 460 K, we start to observe moreclearly the higher BE component in the Ti 2p region, centered

Figure 1. Ti 2p and Fe 2p HR-XPS spectra collected with a photonenergy of 550 and 830 eV, respectively. The spectra were acquiredafter 20 heat treatments at increasing temperatures.

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at about 458.5 eV (Figure 1a), and the oxidation of Febecomes relevant (Figure 1b). We can then speculate thatbecause of the high tendency to alloywith Pt,28 Fe atoms havea propensity to diffuse through the TiOx phase defects(troughs and picoholes), thus activating a restructuring pro-cess of the z0-TiOx bilayer itself. Moreover, the NPs shapechanges from irregular and rounded to triangular and flat,showing on top a modulation that could be in tune with theone observed for a FeO(111) surface.29,30 Considering the XPSspectrum at this temperature, we believe that the NP havetransformed into FeO-like islands. Also, the NPs are locatedboth at the center and at the vertices of the surroundinghexagons of the underlying UT film. The situation iseven more evident if the temperature is raised up to 800 K(Figure 2d).

Figure 2e shows the STM data taken after a thermaltreatment at 910 K. One can observe that the UT film patternhas mostly recovered the typical zigzag-like z0-TiOx arrange-ment, while only few spots with the hexagonal phase are stillpresent. Also the islands are modified, with an apparentheight (measured in the same tunneling conditions) reducedfrom 4 nm down to 1.2 nm, and a shape changing fromtriangular to hexagonal. At 1000 K (Figure 2f) the situation islargely modified: islands become larger with corrugationtypical of a metallic surface and an apparent height of 0.1 nm,some of them being partially covered by a phase showingthe typical z0-TiOxmotif. Moreover, the islands are complete-ly surrounded by the recovered zigzag-like pattern typicalof the clean z0-TiOx phase. Consistently, the HR-XPS dataafter the annealing at 910 K show that the higher BE Ti 2pcomponent at 458.5 eV is almost totally absent (Figure 1a),

recovering the Ti 2p signature peculiar to the z0-TiOx reducedbilayer. At the same time the overall intensity of the Fe 2pspectra decreases, and a shift of the peak toward higher BE isobserved as a consequence of an increase of the FeOx/Fe

0

components ratio. Our interpretation of the presented datais that a major Fe dissolution into the Pt substrate is tak-ing place above 910 K, which leaves only a minor residualamount of FeOx at the highest temperatures. The body of thisexperimental evidence suggests that the large islands ob-servedby STMat 1000Kmightwell be uncovered portions ofthe Pt substrate, probably partially alloyed with Fe.

Such a hypothesis finds a strong support from the results ofTPD experiments using CO as a probe (Figure 3). It is useful torecall that, while XPS gives chemical information for a finitedepth slab, TPD probes the exposed chemisorption sites withrespect to the probemolecule. In Figure 3 we report TPD datafor the Fe(0.5 ML)/z0-TiOx sample, collected after exposure to1.0 L of CO, prior to and after a thermal annealingmade in thesame conditions used for HR-XPS and STM.31 Referring toliterature data,32-35 the desorption peaks shown can berelated to CO desorption from Fe2þ (142 and 156 K), Fe3þ

(190 K), and Fe0 (broad peak centered around 340 K withmany components) exposed sites. The TPD data of the as-grown sample (i.e., at RT)show that bothmetallic Fe and FeOx

sites (both Fe2þ and Fe3þ with a large predominance of theformer) are exposed, in agreement with the HR-XPS datapresented in Figure 1. The most relevant result is that thebroad metallic Fe0 peak is almost vanished above 460 K,confirming the XPS indication that above this temperaturemost of the Fe NPs are oxidized. In the 460 < T < 800 Ktemperature range, TPD spectra show that the peak due to

Figure 2. STM constant current images of the 0.5 ML Fe/z0-TiOx system taken at RTand after thermal treatments at increasing tempera-tures for 20. Topography images have been shaded to enhance the substrate and on-top corrugation of the features. All images have beenacquired at positive sample biases in the 0.3-2.0 V range, with tunneling currents between 0.1 and 1 nA.

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Fe3þ almost vanishes, while sites due to Fe2þ are still presentup to the highest explored temperature. Such data are inagreement with the assignment of the protrusions observedin the STM data to FeO-like islands previously proposed(Figure 2c,d), where O ions are the topmost layer as aconsequence of a spillover process. It is well-known that COis not able to bind to this kind of oxygen terminated FeOfilms,34,35 but a few active sites at the edges of the FeO islandsremain exposed and can thus coordinate the small amount ofCO that is visible with TPD. Above 810 K, the number ofresidual Fe2þ sites is very low, whereas a new broad peakcentered at about 400 K becomes predominant. As alreadyoutlined, this peak corresponds to COdesorption fromPt(111),supporting the interpretation of the islands observed in theSTM image of Figure 2f as uncovered Pt(111).

In conclusion, thiswork provides newdetailed informationon the thermal evolutionofa Fe(0.5ML)/z0-TiOx/Pt(111)modelcatalyst prepared in UHV. Such a system has been used tomonitor subtle transformations that might also be in opera-tion in real catalysts and of relevance in the emerging field ofmonolayer catalysis. Our results suggest that the systemevolution in the explored temperature range is largely drivenby the facile interdiffusion of the Fe into the Pt substrate.According to our data, we can single out three temperatureranges where different phenomena occur:

(i) RT < T < 460 K: The Fe atoms adsorbed on the UTTiOx film at RT assemble as heterogeneous Fe and

FeOx NPs both on the stripes and on the troughs of theUT film. In this temperature range, a progressivediffusion of Fe into the z0-TiOx phase defects (troughsand picoholes) and the creation of a first direct contactwith the Pt substrate takes place. When Fe goes indirect contact with Pt through the picoholes, a prefer-ential path for interdiffusion into the substrate isopened, and a mechanism for modifying the existingUT TiOx bilayer is started.

(ii) 460 < T < 800 K: An oxidation of both Ti and Feoccurs, probably leading to the formation of a meta-stable FeOx/TiOx mixed oxide. Such an oxidation isassisted by an interdiffusion of Fe into the Pt substrate,with the consequent increase in the O/M(Ti and Fe)ratio. During such a process, the z0-TiOx phase reorga-nizes into an hexagonal-TiOx pattern and FeO islandsare prevalently pinned at the apex of the hexagonsformed by the restructured UT TiOx film. However,based on the current data, we cannot exclude that theobserved islands are formed by the mentioned me-tastable mixed oxide.

(iii) 800 < T < 1000 K: While a residual amount ofFeO islands is present, a Fe-mediated extraction ofPt, which creates Pt(111) nanoislands, progressivelytakes place. These islands are surrounded and par-tially covered by the z0-TiOx, which is the most stablephase in these high temperature conditions.

EXPERIMENTAL METHODS

The data were collected in three different UHV equipments(for STM, TPD, and HR-XPSmeasurements, respectively), work-ing with a base pressure of <5 � 10-10 mbar, by reproducingexactly the sameconditions for theFe/z0-TiOx/Pt(111) preparationand annealing. Further details regarding the procedures adoptedto prepare the investigated TiOx film are reported in ref 21.

The STM data were acquired in constant current mode atRT, using an Omicron multiscan system, with tip to samplebias ranging from 0.3 to 1.0 V and tunneling current rangingfrom 0.1 to 1.0 nA.

The HR-XPS data were obtained at the Beamline forAdvanced diCHroism (BACH, BL 8.2) at the Elettra Synchro-tron Light Source in Trieste, Italy.36 The photon/pass energiesused to collect the Ti 2p and Fe 2p core levels were, respec-tively, 550 eV/20 eV and 830 eV/50 eV, allowing a totalresolution of 0.229 and 0.374 eV. Measurements were takenat normal emission, and the energy calibration was made onthe basis of the corresponding Fermi edge.

The TPDdatawere obtained using aHydenQuadrupoleMassSpectrometerandmonitoring them/c=28corresponding toCO.

AUTHOR INFORMATION

Corresponding Author:*To whom correspondence should be addressed. E-mail: gaetano.granozzi@unipd.it. Phone: þ39-049-8275158.

ACKNOWLEDGMENT This work was supported by the Universityand Research (MIUR) through the program PRIN 2005 and 2006,

Figure 3. COTPD spectra of 0.5 ML Fe/z0-TiOx/Pt(111) after a 1.0 Lexposure to CO and 20 thermal treatments at increasing tempera-tures. In order to highlight the relative abundance of the differentcomponents, we renormalized the data with respect to the in-tensity of the peak seen at the lowest T. A comparison of theirabsolute intensities can be derived from the reported scale factors.

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and by the University of Padova, through Grant CPDA071781. Wethank Alessandro Fortunelli (Pisa) for clarifying discussions on thecontent of the manuscript. We thank the BACH Beamline staff(Elettra Synchrotron, Trieste) for their technical assistance duringthe measurements.

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(31) Preliminary experiments have demonstrated that such ex-posure is capable of saturating all the exposed chemisorptionsites. All the TPD spectra reported in Figure 3 have beenobtained by subtracting a background associated with theclean UT z0-TiOx films. The clean UT film almost completelywets the Pt(111) substrate (96%), as judged from the evalua-tion of the CO desorbed from the corresponding exposedsites at about 400 K.

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(35) It is important to point out that, in literature, different metalFe surfaces are reported to have three distinctive desorptionpeaks in the temperature range 300-370K (namedR, β, andγ). In our case, we only observe one broad feature, probablyarising from the highly heterogeneous nature of Fe NPs.

(36) BACH - Beamline for Advanced diCHroism, 10, 2009; http://www.tasc.infm.it/research/bach/scheda.php