Oxygen-Induced Thermal Faceting of Pd Nanosized Crystals - The

Jan 15, 2010 - Institute of Experimental Physics, University of Wrocław, Plac Maksa Borna 9, 50-204 Wrocław, Poland, and Universite Libre de Bruxelles...
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Oxygen-Induced Thermal Faceting of Pd Nanosized Crystals Robert Bryl,*,†,‡ Tomasz Olewicz,†,‡,§ Thierry Visart de Bocarme´,‡ and Norbert Kruse‡ Institute of Experimental Physics, UniVersity of Wrocław, Plac Maksa Borna 9, 50-204 Wrocław, Poland, and UniVersite Libre de Bruxelles, Chimie Physique des Materiaux, Campus de la Plaine CP 243, B-1050 Bruxelles, Belgium ReceiVed: October 7, 2009; ReVised Manuscript ReceiVed: December 22, 2009

The oxygen-induced faceting of [111] and [100] oriented Pd nanosized crystals (“tips”) was studied by field ion microscopy (FIM). Annealing at temperatures of 500 K in the presence of submonolayer amounts of oxygen caused major reconstruction to occur. Regions of the {100} plane broke up into small {100}, {112}, and {012} facets. In addition, only {111} and significantly enlarged {011} facets occurred at the surface of the reconstructed Pd crystal. The faceting behavior is in accordance with recently calculated equilibrium shapes of Pd crystals in the presence of small amounts of adsorbed oxygen. Introduction The interaction of oxygen with palladium surfaces has been the subject of considerable interest since the early times of surface science.1 This interest has been at least partly triggered by the important role of Pd metal and Pd oxides in catalysis.2,3 Applications range from hydrogenation and oxidation reactions of hydrocarbons to fine chemicals production with enantioselective control and many more.4 With respect to hydrocarbon oxidation, PdO was long considered to be the catalytically active “phase” in C-H bond breaking. More recently, nonstoichiometric surface oxide phases have been observed at the surface of “PdO” and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy.5,6 Such nonstoichiometric phases may be particularly active in catalysis and their formation is expected to be associated with considerable surface structural changes. Interestingly, considerable complexity can even be encountered in oxygen submonolayer adsorption on single crystal Pd surfaces as revealed by low-energy electron diffraction (LEED).7 Dissociative oxygen adsorption on Pd surfaces usually occurs via molecular precursor states. Measurements of the sticking coefficient as a function of the surface coverage have revealed the kinetics and mechanism of the dissociation process.8-13 Oxygen-induced surface reconstruction on a variety of single crystal surfaces, subsurface oxygen diffusion, and near-surface oxidation on Pd {111} and {001} planes have also been reported in considerable detail.14-17 A review of the behavior of Pd and other Pt group metals under oxidizing conditions was recently given by Seriani and Mittendorfer.18 While oxygen adsorption on extended Pd 2D surfaces has been studied quite intensely in the past, less information is available about the behavior of 3D nanosized particles. This has been recognized by a number of groups,19-22 and considerable efforts are presently being devoted to improve our knowledge about such model catalysts. Quite generally, a major difference of a 3D metal particle as compared to a 2D extended * To whom correspondence should be addressed. E-mail: rbryl@ ifd.uni.wroc.pl. † University of Wrocław. ‡ Universite Libre de Bruxelles. § Current address: Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801.

single crystal surface is the simultaneous presence of a variety of small planes at the surface of the particle. Similar to oxygeninduced reconstruction of oriented single crystal surfaces, morphological changes of 3D crystals may take place. Such shape transformation may be associated with faceting, i.e. the “dissolution” of certain planes and the growth of others, driven by the anisotropy of the surface free energy. Shape transformation and faceting of metal surfaces by adsorbed oxygen was reported for a variety of transition metals.23-27 To the best of our knowledge only little experimental research has as yet addressed the question for structural changes following adsorption of oxygen on palladium nanoparticles. Equilibrium shapes of Pd nanocrystals (10 nm) grown on MgO (001) single crystals and subsequently annealed at high temperatures in the presence of low pressures of O2 were studied by Graoui et al.28 In a theoretical investigation of the equilibrium shape of Pd crystals in the presence of oxygen Rogal et al.29 have calculated surface free energies for PdO {111}, {110} and {100} planes and presented a constrained Wulff construction of PdO crystals for oxygen-poor and oxygen-rich limits. Recently Mittendorfer et al.30 calculated surface free energies for several clean and oxygen-covered Pd planes. The authors predicted Pd crystals to develop a number of planes including {112} and {133} in the presence and absence of oxygen. To provide information on the morphology of 3D metal crystals in the absence of nonmetallic support material field ion microscopy (FIM) may be of valuable help. The method provides atomic resolution at the surface of a sharp tip, which can be regarded an excellent model of a single nanosized metal particle. Morphological reshaping and faceting due to adsorption or chemical reaction can be imaged in real space as demonstrated previously.27,31 The present paper will address the faceting of 3D Pd nanocrystals in the presence of oxygen submonolayer adsorption. It will be shown that reliable structural information can be gleaned by imaging small facets and edges between facets. The agreement of the results with theoretical predictions of the equilibrium shape of oxygen-covered Pd crystals turns out to be excellent. Experimental Procedures The experiments were performed in a stainless steel UHV system with a base pressure of ∼1 × 10-8 Pa. The experimental setup was described in detail elsewhere.32

10.1021/jp909592s  2010 American Chemical Society Published on Web 01/15/2010

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Figure 1. FI patterns of the clean, ion sputtered and field evaporated Pd tip, taken in Ne, at 65 K. (a) a (111) oriented sharp tip, field strength F ≈ 36 V/nm, R ≈14 nm, (b) a (111) oriented tip, blunted, field strength F ≈ 36 V/nm, R ≈ 27 nm, (c) a (001) oriented sharp tip, field strength F ≈ 33 V/nm, R ≈ 19 nm.

Palladium tips were produced from a 0.125 mm wire (purity 99.95%) by electrochemical etching in a 20% aqueous solution of KCN. The tips were subsequently cleaned and shaped in the microscope chamber by cycles of annealing at temperatures up to 700 K, field evaporation, and Ne ion sputtering at 100-120 K. These procedures allowed us to produce specimens with radii of curvature between 14 and 19 nm. However, further annealing during the experiments caused an increase of these radii to values between 25 and 28 nm as calculated by counting the number of atomic step along zone lines.33 Reshaping and reetching of blunted tips with ion sputtering and field evaporation cycles helped readjust the radius but often resulted in breakdown, too. All the field ion (FI) micrographs presented in this work were obtained by using Ne (Messer Griesheim, purity 99.9999%) as an imaging gas (pressure 1 × 10-3 Pa). Imaging was performed at 65 K. The tip was cooled using a liquid helium cryostat. Micrographs were taken with a CCD camera (OMA Vision, EG&G Princeton Appl. Res., 512 by 512 pixel, dynamic range ) 18 bit) from a microchannel plate. During exposure to O2, the tip was held at 65 K. Oxygen (Messer Griesheim, purity 99.998%) was introduced into the system through a variable leak valve. The following doses were used: 1 L (pox ) 1 × 10-6 Pa, texp ) 133 s), 2 L (pox ) 1 × 10-5 Pa, texp ) 25 s), 3 L (pox ) 1.6 × 10-5 Pa, texp ) 25 s), and 6 L (pox ) 1.6 × 10-5 Pa, texp ) 50 s). We estimate the accuracy of exposure to be around 20% mainly because of geometric considerations since the pressures were measured by an UHV ion gauge (corrected for species’ sensitivities) placed remote from the tip specimen. The experiments were performed as follows. First, the tip was cleaned by field evaporation and Ne ion sputtering and the control FI patterns of the cleaned tip were recorded. Next the tip was annealed at temperatures between 300 and 750 K in the absence of oxygen. After each annealing the tip was quenched to 65 K, the imaging gas (Ne) was introduced into the system and a series of ion micrographs were recorded for different electric field strengths up to values of ∼36 V/nm. Then, the tip was exposed to a certain dose of oxygen while keeping it at 65 K. The oxygen-covered tip was subsequently heated to a target temperature between 300 and 750 K. After 120 s at this temperature the tip was cooled down for imaging in Ne. During the oxygen adsorption and annealing experiments no electric field was applied. To image the oxygen-treated Pd tip an electric field of relatively low strength was applied and slowly increased to avoid any destruction of surface structures by the onset of field evaporation/desorption. Eventually, the same

oxygen-covered tip was heated field-free to another, higher temperature and the imaging process was repeated. At the end of the adsorption studies the tip was cleaned by field evaporation and Ne ion sputtering so as to start a new set of experiments. Before reconditioning the tip, the cryostat was switched off to allow the tip and its support to warm up to room temperature. This procedure was mandatory to prevent the growth of a thick oxygen layer onto the tip assembly, as it could seriously affect imaging and create additional errors in determining the O2 exposure. We were not able to remove oxygen from the tip by thermal desorption, as fast blunting of the tip was observed at 750 K. At this temperature oxygen has also been reported to diffuse into the Pd bulk.14 Therefore the tip was cleaned by ion sputtering and field evaporation. Results Field ion (FI) micrographs of Pd tips prepared by Ne+ sputtering and field evaporation are presented in Figure 1. The majority of our experiments were conducted with (111) oriented Pd tips (parts a and b of Figure 1). Pd (001) oriented tips were obtained occasionally and served for control experiments (Figure 1c). Sharp tips like those presented in Figure 1a (R ≈ 14 nm) and Figure 1c (R ≈ 19 nm) turned out to be relatively unstable and fast blunting occurred while annealing to g700 K. Tips with radii of curvature R ≈ 24-28 nm (Figure 1b) were more stable than those with smaller ones. Thus the majority of the data reported here were obtained with these larger-size specimens. The FI patterns in Figure 1a and 1c are well developed in the very apex part of the tip. The peripheral {001} and {111} facets yet remain largely dark in these figures. In Figure 1b the tip is neatly divided up into a considerable number of facets in between the central (111) pole and the peripheral (001) plane. Thus a flattening due to field evaporation in Ne has taken place.34 The final morphology of the tip usually depends on the specific preconditioning, i.e. thermal annealing vs field evaporation. Additional spots in Figure 1b are most probably due to Pd adatoms. Such disorder can be easily observed in field evaporation of metals with relatively low melting point, like Cu, Ag, Au. or, as in the present case, Pd. As will be presented below; the oxygen-induced restructuring of the Pd tip is largely independent of the local disorder. Oxygen-exposed Pd tips were annealed at temperatures of 350-750 K. Blank heating experiments in the absence of oxygen were also performed and helped distinguish oxygeninduced faceting from mere thermally induced reconstruction. The FI images of these blank experiments are presented in Figure 2, while patterns from a Pd tip exposed to 3 L oxygen

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Figure 2. FI patterns of a Pd(111) tip annealed at temperatures in the absence of oxygen: (a) not annealed, (b) T ) 400 K, (c) T ) 500 K, (d) T ) 600 K, (e) T ) 700 K, and (f) T ) 750 K All micrographs were taken in Ne, at 65 K, with a field strength F around 35-36 V/nm except of f where F is somewhat lower. No facet development is visible. The central (111) pole shrinks with increasing temperature.

Figure 3. FI patterns (taken in Ne at 65 K) of (a) a clean Pd (111) oriented tip, R ≈ 27 nm, field strength F ≈ 36 V/nm, applied voltage U ) 10.37 kV and the same tip exposed to 3 L of oxygen and subsequently annealed at the following temperatures (b) T ) 350 K, U ) 9.83 kV, (c) T ) 400 K, U ) 10.08 kV, (d) T ) 450 K, U ) 9.86 kV, (e) T ) 500 K, U ) 9.84 kV, (f) T ) 550 K, U ) 10.34 kV, (g) T ) 650 K, U ) 10.34 kV, (h) T ) 750 K, U ) 11.52 kV. Development of facets is clearly visible after annealing at 500 K and above.

are shown in Figure 3. It is apparent that the FI images for the clean and O2 exposed tip are quite similar for low annealing temperatures. For example, the images in Figures 2b and 3c, taken at 400 and 450 K, respectively, differ only in details (the zone line contrast is more pronounced in Figure 2b). While the images of the oxygen-free surface (parts c-f of Figure 2) hardly change at further rising temperatures (some coarsening is observed up from 500 K; however, the overall morphology does

not alter), considerable changes are observed in the presence of preadsorbed oxygen. Dark regions of variable size are formed and separated by single or multiple lines of bright spots. Obviously, a strong faceting has occurred in the presence of adsorbed oxygen (parts e-h of Figure 3): adjacent facets produce edges, which appear bright while individual spots are associated with Pd atoms. Inner-atomic arrangements of the facets remain invisible. This may be attributed to the locally

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Figure 4. (a) The pattern of the tip exposed to 3 L of oxygen and annealed at 650 K, the same as Figure 3g. (b) Schematic drawing of the faceted Pd tip surface, exhibiting exposed facets. (c) A ball model of the faceted Pd tip apex, depicting the tip presented in parts f-h of Figure 3. Note that in this model the {001} planes and their neighborhood are enlarged at the expense of {011} to show their structure.

smaller field strength (as compared to the edges) and to the presence of adsorbed oxygen atoms, which are likely located in hollow positions between Pd atoms so as to further “smoothen” the surface of the facets. The overall morphology of the Pd tip is no longer close-to-hemispheric as at the beginning of the experiments. Facet edges are fully developed after annealing at 550-600 K. Higher annealing temperatures of 650 K cause no changes in the FI pattern compared to those obtained at 550 K. After further annealing at 750 K the patterns yet appear broader and more diffuse. Less regular facet edges are formed under these conditions. The increase of the best image voltage by around 10% after annealing at 750 K is caused by an increase of the average tip radius. The FI patterns of the oxygen-covered tip are independent of the cooling procedure. Identical images are obtained no matter if the sample is quenched or cooled in increments of 50 K to the imaging temperature. It should be noted that faceting in the region of the (001) plane starts at temperatures of around 400-450 K, which is lower than that of all other planes. Early stages of faceting of the (001) region are visible in Figure 3d. A schematic drawing of the nanosized palladium crystal along with its facet structure is presented in Figure 4b. During the faceting process a significant enlargement of the {011} planes is observed. The medium-sized (111) facet spans {011} planes. {001} planes shrink and their vicinal areas break up to well developed {112} and {012} facets. A ball model of the tip apex is shown in Figure 4c. A more detailed account on how facets are identified is given in the Discussion part of the paper. Experiments with varying oxygen exposure at otherwise identical temperatures have also been performed. Accordingly, exposing the Pd tip to 1, 2, 3, and 6 L of oxygen produces very similar results. In Figure 5 we show the FI patterns taken after exposing the tip to specific oxygen doses and annealing it at 600-700 K. For each exposure facets and edges appear after annealing at 500 K. The oxygen-induced faceting as described above is (as expected) independent of the orientation of the Pd nanocrystal. Experiments with a [100] oriented specimen provide very similar results as those presented above. Exposure to 1 and 3 L of oxygen leads to the images of Figure 6. Direct comparison with Figure 3 shows that the facet edges are less well-defined though. Independent of the tip orientation and radius, {011} facets seem always to be enlarged. Discussion and Conclusions The Results part of this paper has reported on strong faceting of Pd nanosized crystals in the presence of adsorbed oxygen.

Figure 5. FI micrographs (taken in Ne, at 65 K) of a Pd (111) tip exposed to various oxygen doses followed by annealing (a) 6 L, T ) 600 K, U ) 9.64 kV, (b) 3 L, T ) 600 K, U ) 9.86 kV, (c) 2 L, T ) 700 K, U ) 10.36 kV, (d) 1 L, T ) 650 K, U ) 4.80 kV. In case of the tip exposed to 1 L of O2 its average radius of curvature (R ) 14-16 nm) was almost two times lower than in case of higher exposures (R ) 24-28 nm). The general faceting pattern does not depend on the oxygen dose.

Exposures of 1-6 L O2 turned out to be sufficient to provoke this reconstruction. To provide coverage information, we refer to measurements of temperature-dependent sticking probabilities on oriented single crystal surfaces. According to Zheng and Altman,16 an exposure of 1 L O2 to Pd (001) at 335 K leads to a coverage of Θ(001) ) 0.07 monolayer whereas 6 L produce Θ(001) ) 0.2. In case of Pd (011), Yagi et al.12 report Θ(011) ) 0.5 after 1 L and Θ(011) ) 0.6 after 3 L O2 exposure. We assume very similar values for the facets of our Pd tip and estimate a value of Θ(011) ) 0.7 monolayer for an exposure of 6 L, i.e., close-to-saturation coverage. For oxygen adsorption on Pd (111) the initial value of the sticking coefficient depends strongly on the temperature,8,35 and no reliable estimation can be made for the coverage-dependent sticking coefficients at low temperatures. Results presented by Matsushima8 at low temperature and by Zheng and Altman15 at higher temperature lead to Θ(111) ) 0.07- 0.08 monolayer for 1 L and to Θ(111) ) 0.11- 0.12

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Figure 6. FI patterns (taken in Ne, at 65 K) of Pd (001) tip: (a) clean tip (the same as Figure 1c), (b) exposed to 1 L of oxygen and then annealed at 650 K, U ) 4.31 kV, (c) exposed to 3 L of oxygen and then annealed at 650 K, U ) 5.50 kV. The average radius of curvature of the tip was estimated for around 19 nm. The (001) oriented Pd tip exhibits the same faceting pattern as the (111) oriented Pd tip.

for 6 L, respectively. This is far below the monolayer limit, and according to ref 15, a saturation coverage of 0.25 on Pd (111) is only obtained for much higher exposure, ∼30 L O2. Oxygen dosing has been performed with Pd nanocrystals of nearly hemispherical shape as obtained after cycles of annealing, field evaporation, and Ne+ sputtering. No significant morphological change of the crystal is noticed during annealing of a clean Pd specimen, i.e., thermally induced faceting is absent up to annealing temperatures of 750 K. The only visible thermal effect is shrinkage of the central (111) plane (or peripheral {111} planes in case of a sharp [001] oriented tip). Oxygen interaction with Pd single crystals at 65 K leads to molecular adsorption.8-11 Subsequent annealing to temperatures between 300 and 750 K causes dissociation (besides molecular desorption). At T >230 K only atomic oxygen is supposed to remain on the Pd crystal surface.8,10,11 Atomic oxygen desorbs (after recombination) from Pd at 750-1000 K, although on {001} desorption may start already at 600-650 K.8,10,11 Thus, the faceting that we observe in annealing experiments at 500 K and above is clearly induced by the presence of submonolayer atomic oxygen. Note also that the faceting of the (001) plane starts even at lower temperatures, around 450 K. The uncertainty in determining the oxygen coverages on the different planes of the Pd crystal makes it difficult to arrive at more quantitative conclusions about the surface free energies, F(hkl), of individual planes and the anisotropy in F(hkl) providing the driving force for faceting. Inner-facet surface structures remain invisible in FIM. This complicates the facet identification in terms of Miller indices. In case of low-index facets such as (001), (011), and (111), the identification can be easily accomplished on the basis of symmetry arguments, but this method fails for (001) vicinals not developed to specific planes. Also, facets cannot be identified unambiguously by comparison with maps of stereographic projection as this method only applies to spherical crystals. In case of small facets of the [110] zone recent theoretical calculations30 provide clues in predicting an oxidized Pd crystal to expose {112} rather than {113} planes under UHV conditions. A careful analysis of plane directions in the FI images arrives at the same conclusion although the coverages are far too low to cause oxidation to occur. An additional facet between (001) and (011) is identified as (012), although (013) cannot be excluded. This facet does not appear in the Pd crystal shape presented in ref 30 as the authors calculated surface free energies of facets belonging to the [110] zone. Mittendorfer et al.30 have shown theoretically that a clean Pd crystal in equilibrium exposes a number of planes: {001} and {111} are dominant,

while smaller {112} and {133} planes are also present. Note that the dominant role of {111} and {100} as calculated for the equilibrium shape of a clean Pd crystal supports our view of a further flattening of these planes when annealing Pd specimens during preconditioning. According to the calculations presented in ref 30, when increasing the amounts of adsorbed oxygen on a Pd crystal at otherwise low coverages, {011} planes enlarge at the expense of {133}, while {112}, {001}, and {111} planes are still present. However, the relative contribution of the latter two decreases and the crystal rounds off. This picture is in good agreement with our observations. Accordingly, we find that {133} planes in the final crystal shape are absent while {011} planes are dominant, with {111} and {001} being relatively small. Despite the good qualitative agreement of our experimental observations with the theoretical calculations some disparities with other work are worth mentioning. Experiments by Graoui et al.28 with MgO-supported Pd particles of 10-15 nm size have shown that the adsorption of oxygen causes formation of {011} microfacets and expansion of {001} facets at the expense of {111} planes. The authors do not report on any other facets resulting from the oxygen-induced Pd shape transformation. The difference with our experiments is not unexpected because of the much higher exposure to oxygen at pressures up to the mbar region. While the exposure in their experiment corresponds to several 106 L, it is of 6 L, at the most, in our case. As shown in ref 30, high oxygen pressures cause surface oxidation to take place and to make only {100} and {111} facets along with small amount of {011} to contribute to the final crystal shape. One should also keep in mind the influence of the MgO support in the high-pressure experiments of ref 28. The stress in the interface region between Pd particles and MgO support may affect the final shape of the Pd particles. A striking feature of our results is the prominent role of the {011} facets in the reconstruction of the Pd crystal. Neither the oxygen dose nor the annealing temperature seem to significantly influence this behavior. According to ref 30 the lowest values of free energies of the planes and thus the largest areal spread of facets in the final shape of the crystals are rather expected for {111} or {001} facets (depending on the oxygen exposure). In the Wulff construction the ratio of free energies of facets {hkl} and {h′k′l′} equal the ratio of distances of these facets from the center of the crystal, i.e., γ(hkl)/γ(h′k′l′) ) d(hkl)/d(h′k′l′). Thus in principle the values of γ(hkl)/γ(h′k′l′) could be evaluated on the basis of the known equilibrium crystal geometry. However, extracting the detailed tip geometry from the FIM pattern of the faceted tip is not a straightforward procedure. First, the magnification of the pattern is not uniform over the

Thermal Faceting of Pd Nanosized Crystals tip surface so that some facets appear larger than others. Second, ion optical effects in the presence of high imaging fields may come into play. The problem of image deformation in field ion microscopy of faceted crystals has been recently discussed by Niewieczerzał et al.36 Third, and possibly most important, the field evaporation end form of a tip deviates from the thermal end form and the local tip radii vary quite considerably. For these reasons we refrain from providing here a quantitative evaluation in terms of Wulff constructions. On the other hand, these considerations do not contest the importance of {011} facets in the reconstruction. Note that the surface free energies of these facets are likely to be reduced by an optimization of Pd-O bonds. We finally turn to a discussion of some more general aspects in field ion imaging of 3D nanosized tips. In accordance with the experimental28 and theoretical results30 quoted above, the morphology of our faceted tip comes close to that of the equilibrium shape. However, the emitter tip is a needle with a small cone angle (not exceeding 10°). The influence of the shank is minor when considering the facet growth in the very apex part of the tip. On the other hand, the angle between [111] and [011] directions is relatively high, 54°44′. Thus in the case of a [111] oriented tip the development of small facets around (001) may be well affected by planes of the shank (note in this context that the (1j,1j,2) facet visible in parts d-h of Figures 3 is perpendicular to the tip axis). As the curvature of the tip is highest at the apex the surface stress is higher there than elsewhere. This stress may cause material transport by diffusion from the shank to the apex. The macroscopic result of this process is known as blunting. According to the results presented in this paper, blunting is of minor importance at 550 K. All the facets are already well developed at this temperature. The observed faceting is therefore an intrinsic property of the oxygen-covered planes. These arguments are strongly supported by measurements with two different tip orientations, [001] and [111], where the reconstruction phenomena observed for (001) are essentially the same. Size-dependent morphological changes provide another point of discussion here. Clearly, dependent on the size of the tip, facets may vary in number and structure. From the viewpoint of particle sizes in heterogeneous catalysis, field emitter tips are relatively large (close-to-hemispherical) nanosized crystals. While tips with radii of curvature down to 5-8 nm can be prepared using conventional methods of electrochemical etching, the radius of the tip in Figure 6 is 19 nm. The angle between [001] and [012] directions in this figure is around 26° meaning that the distance between the respective plane centers is around 9 nm. As seen in the FI images, this distance is comparable to the dimensions of these facets. Taking a lattice constant of 0.385 nm for Pd the size of the (001) facet corresponds to 30-35 atoms edge-to-edge (using an interatomic distance of 0.272 nm). From a more general point of view, the faceting of the oxygen-covered Pd tip may be regarded as an example of the so-called type-B thermal evolution of the crystal,37,38 as the crystal reconstruction form is polyhedral with sharp edges between facets at nonzero temperatures. The question arises how a crystal with sharp edges between planes behaves when its size diminishes. In case of crystals with rounded edges the behavior is known: the size of the facets decreases and below critical crystal dimensions edges strongly round off so as to make facets disappear.37 To the best of our knowledge, in case of “type-B crystals” with sharp edges a facet disappearance has never been reported (and is neither seen here).

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2225 The results presented in this paper call for additional experiments with high oxygen exposures. These studies are under way and should help bridge the gap to “PdO” formation.28 FIM has previously demonstrated that valuable information can be obtained on the morphology of oxidized nanocrystals.27,39 Conclusion The present paper reports on FIM results of the oxygeninduced faceting of nanosized Pd crystals at overall submonolayer coverages. Amounts of 1-6 L oxygen at annealing temperatures of 500 K cause the reconstruction. Our results are the first experimental proof of the recent theoretical prediction that the polyhedral equilibrium shape of a Pd nanosized crystal in the presence of small amounts of oxygen exhibits {001}, {011}, {111}, {112}, and (likely) {012} facets. Among these, {011} facets dominate in size, while {001} are small as they break up into {112} and {012} planes. Blank annealing in the absence of surface oxygen fail to produce this faceting. The shape transformation is of the “type-B thermal evolution”. Acknowledgment. R.B. thanks the Program for Cultural Exchange between Governments of Poland and Region of Wallonia for financial support during his stay in Brussels. We thank Matthieu Moors (ULB) for technical help during the experiments. Financial support by F.R.F.C. Grant No. 2451408 of the Fond de la Recherche´ Scientifique (FNRS) of Belgium is gratefully acknowledged. Part of the work was also executed within A.R.C. No. 04/09-312. References and Notes (1) Ertl, G.; Rau, P. Surf. Sci. 1969, 15, 443. (2) Lyubovsky, M.; Pfefferle, L. D. Appl. Catal. A: Gen. 1998, 173, 107. (3) Lyubovsky, M.; Pfefferle, L.; Datye, A.; Bravo, J.; Nelson, T. J. Catal. 1999, 187, 275. (4) Ertl, G.; Kno¨zinger, H.; Schu¨th, F.; Witekamp, J. Handbook of Heterogeneous Catalysis; Wiley-VCH, 2008. (5) Zemlyanov, D.; Aszalos-Kiss, B.; Kleimenov, E.; Teschner, D.; Zafeiratos, S.; Ha¨vecker, M.; Knop-Gericke, A.; Schlo¨gl, R.; Gabasch, H.; Unterberger, W.; Hayek, K.; Klo¨tzer, B. Surf. Sci. 2006, 600, 983. (6) Gabasch, H.; Unterberger, W.; Hayek, K.; Klo¨tzer, B.; Kresse, G.; Klein, Ch.; Schmid, M.; Varga, P. Surf. Sci. 2006, 600, 205. (7) Todorova, M.; Lundgren, E.; Blum, V.; Mikkelsen, A.; Gray, S.; Gustafson, J.; Borg, M.; Rogal, J.; Reuter, K.; Andersen, J. N.; Scheffler, M. Surf. Sci. 2003, 541, 101. (8) Matsushima, T. Surf. Sci. 1985, 157, 297. (9) Imbihl, R.; Demuth, J. E. Surf. Sci. 1986, 173, 395. (10) Matsushima, T. Surf. Sci. 1989, 217, 155. (11) Ohta, N.; Ohno, Y.; Matsushima, T. Surf. Sci. Lett. 1992, 276, L1. (12) Yagi, K.; Sekiba, D.; Fukutani, H. Surf. Sci. 1999, 442, 307. (13) Junell, P.; Honkala, K.; Hirsimaki, M.; Valden, M.; Laasonen, K. Surf. Sci. 2003, 546, L797. (14) Westerstro¨m, R.; Gustafson, J.; Resta, A.; Mikkelsen, A.; Andersen, J. N.; Lundgren, E.; Seriani, N.; Mittendorfer, F.; Schmid, M.; Klikovits, J.; Varga, P.; Ackermann, M. D.; Frenken, J. W. M.; Kasper, N.; Stierle, A. Phys. ReV. B. 2007, 76, 155410. (15) Zheng, G.; Altman, E. I. Surf. Sci. 2000, 462, 151. (16) Zheng, G.; Altman, E. I. Surf. Sci. 2002, 504, 253. (17) Lundgren, E.; Kresse, G.; Klein, C.; Borg, M.; Andersen, J. N.; De Santis, M.; Gauthier, Y.; Konvicka, C.; Schmid, M.; Varga, P. Phys. ReV. Lett. 2002, 88, 246103. (18) Seriani, N.; Mittendorfer, F. J. Phys.: Condens. Matter 2008, 20, 184023. (19) Schalow, T.; Brandt, B.; Laurin, M.; Schauermann, S.; Guimond, S.; Kuhlenbeck, H.; Libuda, J.; Freund, H.-J. Surf. Sci. 2006, 600, 2528. (20) Bratlie, K. M.; Lee, H.; Komvoupoulos, K.; Yang, P. D.; Somorjai, G. A. Nano Lett. 2007, 7, 3097. (21) Tsung, C. K.; Kuhn, J. N.; Huang, W. Y.; Aliaga, C.; Hung, L. I.; Somorjai, G. A.; Yang, P. D. J. Am. Chem. Soc. 2009, 131, 5816. (22) Lee, I.; Delbecq, C.; Morales, R.; Albiter, M. A.; Zaera, F. Nat. Mater. 2009, 8, 132. (23) Song, K.-J.; Demmin, R. A.; Dong, C.-Z.; Garfunkel, E.; Madey, T. E. Surf. Sci. 1990, 227, L79.

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