Formation of Ruthenium-Tin Nanoparticles on Al2O3/Ni3Al(111) from an OrganometallicPrecursor†

Alexander Uhl,‡ Yu Lei,§ Homa Khosravian,§ Conrad Becker,|,⊥ Klaus Wandelt,|

Richard D. Adams,# Michael Trenary,*,‡ and Randall J. Meyer§

Department of Chemistry, UniVersity of Illinois at Chicago, 845 W Taylor St, Chicago, Illinois 60607,Department of Chemical Engineering, UniVersity of Illinois at Chicago, 810 S Clinton St, Chicago, Illinois 60607,Institut fur Physikalische und Theoretische Chemie, UniVersitat Bonn, D-53115 Bonn, Germany,CINaM - CNRS - UPR 3118, associated with UniVersite de la Mediterranee and UniVersite Paul Cezanne,Campus de Luminy - Case 913, 13288 Marseille Cedex 09, France, and Department of Chemistry andBiochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208

ReceiVed: April 17, 2010; ReVised Manuscript ReceiVed: July 20, 2010

A thin Al2O3/Ni3Al(111) film was prepared under ultrahigh vacuum conditions by surface oxidation of aNi3Al(111) single crystal. Using scanning tunneling microscopy, it was found that the film does not cover thesubstrate entirely, which allows two surfaces and their adsorption properties to be investigated in a singlestudy. The sample was subsequently exposed to the vapor of an organometallic compound, Ru3(CO)9-(µ-SnPh2)3. The interaction between the ligand sphere of the adsorbate and the relatively inert oxide filmfavors diffusion rather than static adsorption; however, some coverage is observed also on alumina. Uponheating, the images of the sample surface suggest that the bimetallic centers of the molecules lose their ligandsand nucleate as particles on the surface. On the oxide film, the particles grow three-dimensionally, whereasthey do not go beyond monolayer thickness on the unoxidized surface areas. Particles can be found on theoxide even after heating the sample as high as 925 K, despite a pronounced diffusion at room temperature ofthe precursor to the unoxidized surface patches.

1. Introduction

In the past, many model catalysts have been prepared andstudied under ultrahigh vacuum conditions using the so-calledsurface science approach. The related literature can by no meansbe reviewed completely here; however, good introductions tosome topics relevant to the field are given by Freund,1 Goodmanand co-workers,2 Campbell,3 and Henry.4 Typically these modelsmimic their industrial counterpart in the form of nanoparticlessupported on thin oxide films, which in turn are grown on ametal single crystal surface. The films may be prepared in twoprincipal ways: either simply by oxidation of the surface of thesingle crystal substrate, or by heteroepitaxial growth. Themimicked active species is subsequently deposited on the surfaceof the film by physical vapor deposition (PVD). If the activespecies is metallic, then the metal in question is evaporated invacuo; if it is a second oxide, the corresponding metal may beevaporated in vacuo and subsequently oxidized, or simplydeposited in an oxygen environment.

However, this procedure suffers from the so-called “pressuregap” and “materials gap”, i.e., while the model may becomposed of the same material as the industrial catalyst, thevastly different methods and preparation conditions will prob-ably lead to significant structural differences, thus limiting thecomparability of the results. The most notable difference in theuse of PVD in the traditional surface science approach is that

industrial catalysts are typically prepared from a precursormolecule, in which the active metal center is anchoring variousinorganic and also organic ligands. These precursors are exposedto their respective supports, and then treated such that theypartially or fully deligate, so as to bind the remainder to thesurface. In that light it seems logical to make an effort to bridgethe materials gap by exposing the support to a gaseousorganometallic compound. This is done by exchanging only thedeposition method for the active species and maintaining allother parameters, so that the surface science approach can stillbe applied, and the benefits of investigating a sample at theatomic level can still be exploited.

While this procedure obviously does narrow the materialsgap, it also inherently complicates matters in several aspects incomparison to PVD. However, the additional complicationscontain valuable information that cannot be extracted fromtraditional surface science experiments. The first question toaddress is whether and under which conditions the ligatedmetallic entities will actually lose their ligands. Next, it mustbe investigated whether the presence of the ligands will alterthe nucleation and potentially also the sintering behavior whenthe sample is heated. Ultimately it is important to know to whatextent residual ligand fragments on the surface will influencethe reactivity. For all of these reasons, it is instructive to maintainoxide films grown in ultrahigh vacuum as the support, in orderto facilitate comparison to previous studies.

The catalyst of choice in this work, supported bimetallicRu-Sn particles, was shown to be active in the hydrogenationof a carbon double bond.5 In general, hydrogenation catalystsare platinum group metals (PGMs). However, in the industrialhydrogenation of 1,5,9-cyclododecatriene to cyclododecene, acompound pivotal to many subsequent processes, metallic Pd

† Part of the “D. Wayne Goodman Festschrift”.* To whom correspondence should be addressed.‡ Department of Chemistry, University of Illinois at Chicago.§ Department of Chemical Engineering, University of Illinois at Chicago.| Universitat Bonn.⊥ CINaM - CNRS - UPR 3118.# University of South Carolina.

J. Phys. Chem. C 2010, 114, 17062–1706817062

10.1021/jp103462u 2010 American Chemical SocietyPublished on Web 08/16/2010

is used, which is somewhat unselective and yields considerableamounts of the fully hydrogenated cyclododecane.5 Yet it wasdemonstrated by Adams et al. that addition of tin to ruthenium,another PGM, to form bimetallic Ru-Sn particles, will signifi-cantly enhance the selectivity to end the hydrogenation cascadebefore the last step.6,7 Therefore, supported Ru-Sn particlesand their properties deserve a more detailed investigation.

Hermans et al. have shown that ruthenium-tin carbonylclusters may be used as precursors for the respective silica-supported particles, which are employed in the hydrogenationof 1,5,9-cyclododecatriene, 1,5-cyclooctadiene, and 2,5-nor-bonadiene.8 Namely, the anion [Ru6C(CO)16SnCl3]- was used;9

however, the presence of chlorine in the precursor has led todifficulties in reproducing the results.10,11 To overcome thishurdle, Adams et al. synthesized a range of new ruthenium-tincarbonyl complexes that are free of chlorine.6,12 Among themis Ru3(CO)9(µ-SnPh2)3,13 which also was used as the precursorhere. Previously Yang et al. exposed the surface of a monolayerSiO2 film grown on Mo(112) to a solution of this compoundunder ambient conditions.14 When this sample was studied withscanning tunneling microscopy (STM), triangular protrusionswere observed and interpreted as the fully deligated butinternally unchanged Ru3Sn3 core of the precursor, where Snoccupies the vertices of an equilateral triangle, and Ru is sittingat positions close to the edge centers.

The work presented here continues the ongoing efforts in ourlaboratory of bridging the materials gap. Recently Lei et al. havestudied Rh/Al2O3/Ni3Al(111), in which the metal particles wereobtained from Rh(acac)(CO)2 as the precursor.15 It was foundthat preparing the active species from an organometallicprecursor had the additional advantage of yielding comparativelysmall particles. The same alumina substrate was also utilizedin the present set of experiments, while the precursor is nowthe bimetallic Ru3(CO)9(µ-SnPh2)3, to prepare supported Ru-Snparticles. The use of STM permits observing the process ofparticle formation and agglomeration as a function of annealingtemperature.

2. Experimental Section

All experiments presented here were carried out at theUniversity of Illinois at Chicago in an ultrahigh vacuum chamber(base pressure below 2 × 10-10 mbar) equipped with low energyelectron diffraction (LEED), a directional gas doser, andfacilities for sample cleaning via Ar+ ion bombardment. Thechamber also houses a commercial scanning tunneling micro-scope (model VT-STM; Omicron NanoTechnology GmbH,Taunusstein, Germany). Data analysis of all STM imagespresented in this work was done using the WSxM 5.0 software.16

Sample heating is done by electron beam bombardment. Thetemperature was monitored by a pyrometer (model OS3708;Omega Engineering Inc., Stamford, CT) above 600 °C (873 K),while for lower temperatures the heating power was calibratedprior to the experiments with a type K thermocouple spot-weldeddirectly to the sample. The spot-weld junction was detachedafter the calibration, to enable the transfer of the sample platebetween the manipulator and the STM stage.

The sample is a Ni3Al(111) single crystal (MaTecK GmbH,Julich, Germany) spot-welded onto a Ta plate, which can betransferred between the STM stage and the manipulator. Asdescribed elsewhere,17,18 it is possible to prepare a thin oxidefilm on this crystal by surface oxidation, resulting in Al2O3/Ni3Al(111). The procedure is summarized briefly in the fol-lowing. The sample is cleaned by bombardment with Ar+ ionsfor 20 min (chamber backfilled with 4 × 10-6 mbar Ar) at 600

K, followed by annealing at 1150 K and then 1000 K for 8 mineach. This cycle was repeated once. Successful cleaning ofNi3Al(111) is confirmed by a hexagonal (2 × 2) LEED pattern.Subsequently, the oxide film is prepared by heating the sampleto 1000 K, at which point it is exposed to 4 × 10-8 mbar O2

for 20 min (i.e., about 50 L). This is followed by annealing thesample in Vacuo to 1050 K for 5-7 min. Again, the cycle isrepeated once, at which point a rather complex LEED patternis observed, which indicates the presence of the surface oxide.(The LEED patterns are shown in the paper by Lei et al.,15 andalso in publications of Becker and co-workers.18,19)

The organometallic compound, Ru3(CO)9(µ-SnPh2)3, wassynthezised according to the procedures described in Adamset al.13 Being a solid at room temperature, it has to be heated to55 °C (slowly, over the course of 20 min) to transfer a sufficientamount of it into the vapor phase, which is then exposed to thealumina film. During exposure, the gas doser is positioned asclosely as possible to the sample (a sample-doser distance of0.5 mm or less). A rise in the background pressure to about10-9 mbar was observed during the exposure. Exposure timesranged from 2 to 4 min. Those times were chosen empirically,such that some coverage could be detected in the STM images,while the surface does not appear overloaded.

Assuming the sticking coefficient of Ru3(CO)9(µ-SnPh2)3 isunity, it is possible to determine the exposure. From the STMimages, the coverage can be derived, and using the fact that a1 Langmuir (L) exposure will lead to a 1 monolayer (ML)coverage (i.e., 100% of the surface is covered), the coveragecan be converted into the corresponding exposure. As outlinedbelow, the “apparent” coverage in Figure 2b is the closest tothe “real” coverage on the surface. It is found to be 0.6 ML(see below). Therefore, the corresponding exposure must be 0.6L, which is the same for all data shown in Figures 2, 4, and 5.In Figures 3 and 6, however, the exposure time was half of thatof Figure 2 (while all other parameters were the same), andhence in those experiments the exposure was only 0.3 L.

Since the aim of this work is to study the deligation,nucleation, and sintering behavior of the precursor and thenanoparticles that arise from it, the sample was heated tosequentially higher temperatures, up to 925 K. STM experimentswere performed only after the sample had cooled down to roomtemperature from the respective anneal. From the temperaturecalibration results it was found that 90 min were required forthe sample to cool from 925 K to room temperature.

3. Results and Discussion

The structure of the thin Al2O3/Ni3Al(111) films has beenstudied mainly by the group of Wandelt,17-21 but also byothers.22-27 Depending on the gap voltage used, its surfaceappears as either the “network structure” (gap voltage ) 3.2V), or the “dot structure” (gap voltage ) 2.3 V). Degen et al.found that these structures are actually two different appearancesof the same atomic lattice.19 This is due to the fact that differentgap voltages allow for imaging different electronic states of thesample with different spatial distributions. Degen et al. alsoreported that the dot structure is really a superstructure withrespect to the network structure (cf. Figure 3 in ref 19).

Figure 1a shows an STM image of the sample after surfaceoxidation of Ni3Al(111); the dot structure is clearly recognizablein the bottom part. Note also the depressions that surround eachdot in a hexagonal arrangement. This substructure is highlighted(characteristic length: ∼0.3 nm), and it is observed again inFigure 1b and Figure 4e-h. However, the top part of Figure 1adoes not contain dots, nor does it reveal any other distinct and

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regular structure. The area of the dot structure is elevated byabout 0.15 nm with respect to the dot-free area. Note that nogap voltage has been reported in the literature to yield an entirelyfeatureless appearance of the Al2O3/Ni3Al(111) oxide film, asobserved in the top part of Figure 1a. However, in an earlyattempt to prepare a thin alumina film on this substrate onlyislands of the oxide could be produced.28 It is concluded thatthe dot-free area must represent unoxidized Ni3Al(111). Thisis substantiated by the fact that different appearances of the filmoccur upon variation of the gap voltage, while in Figure 1a twodistinctly different areas are observed at the same voltage of2.3 V. The lack of any regular structure in the dot-free area isindicative of a metal surface (i.e., Ni3Al(111)), which showsthat the surface oxidation was incomplete. The surfaces of theoxide thin film and of the bare substrate would be expected toexhibit different interactions with the organometallic compound.Indeed the results presented here consistently support theassignment of the dot-free domain to bare Ni3Al(111). Figure1b shows an STM image of the oxide film taken at 0.7 V. Astructure of the same periodicity as in Figure 1a is indicated.Note that what is seen as depressions in the latter appears nowas protrusions on the pristine oxide film.

Figure 2a,b shows STM images after dosing Ru3(CO)9-(µ-SnPh2)3 for 4 min using the conditions described in theExperimental Section. Although it is reasonable to assume thatthe vapor consists of parent molecules, there is no directevidence of this. The sample remained at room temperaturebetween the exposure to gaseous Ru3(CO)9(µ-SnPh2)3 and theacquisition of these images. In Figure 2a, the gap voltage was0.7 V, while it was 2.3 V in Figure 2b. Both depict the samearea on the sample, and they were collected directly one afterthe other. The images show some domains that appear highlypopulated by adsorbates, regardless of the tunneling voltage,while other areas appear less covered. This observation reflectsthe coexistence of the oxide film and unoxidized patches, asseen in Figure 1a. Naturally the metallic surface should bindadsorbates more strongly than the oxide. Thus, the morepopulated areas are identified as Ni3Al(111). Figure 2b is usedfor a calibration of the exposure of Ru3(CO)9(µ-SnPh2)3. Theabundance of the oxide and metal surface are found to be 60%and 40%, respectively, from an averaged estimate in the 200 ×

200 nm2 images obtained in this work (Figures 2, 3, and 5).The coverage in Figure 2b is an estimated 33% or 0.33 ML onthe oxide, while on the metal patches it is 100% or 1 ML. Theoverall coverage of Figure 2b is therefore 0.6 ML, and theexposure must have been 0.6 L.

In Figure 2a, the oxide and metal patches are indicated as“o” and “m”, respectively. The boundaries between the oxidepatches are step edges of 0.25 nm in height (i.e., monatomic).Some metallic patches in Figure 2 appear at higher elevationthan some oxide patches. The height of the oxide patch at the

Figure 1. (a) STM image (gap voltage ) 2.3 V, tunneling current ) 0.1 nA) showing the two areas observed after the preparation of Al2O3/Ni3Al(111). The lower part shows the dot structure of the aluminum oxide film, while in the upper part bare Ni3Al(111) is exposed. The whitehexagon (characteristic length: ∼0.3 nm) indicates six depressions surrounding a bright dot, a structural element also observed in Figure 4e-h. (b)STM image (gap voltage ) 0.7 V, tunneling current ) 0.1 nA) of another oxide patch. The unit cell is indicated by the black hexagon.

Figure 2. STM image taken after oxidizing Ni3Al(111) and exposingthe surface to 0.6 L of Ru3(CO)9(µ-SnPh2)3. The sample was at roomtemperature throughout this experiment. (a) gap voltage ) 0.7 V,tunneling current ) 0.1 nA, oxide (o) and metal (m) areas are labeled;(b) gap voltage ) 2.3 V, tunneling current ) 0.1 nA; (c) Structure ofthe precursor molecule.

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lower left in Figure 2a is about the same as the adjacent metalpatch in the center left of the same image, of which the latteris highly covered with adsorbed precursor material. The darkoxide patch in the upper left corner, however, is ∼0.5 nm lowerthan both. This seems to be a sequence in which oxide andmetal terraces alternate, where the metal is so highly coveredthat its apparent height is the same as the next higher oxideterrace.

The coverage of the oxide domains is apparently dependenton the gap voltage, as derived from Figure 2. At 2.3 V, thesame area seems to be covered significantly more (yet still lessthan Ni3Al(111); Figure 2b) than at 0.7 V (Figure 2a). Thisapparent voltage dependence may be explained by differenttip-sample distances in the respective experiments: given aconstant tunneling current (0.1 nA in the present case), the tipis positioned closer to the sample for smaller gap voltages. It isconceivable that when the tip is very close to the sample, itwill cause adsorbates to diffuse away before they can be imaged.For this reason it is useful to distinguish between the “realcoverage” (i.e., the actual number of adsorbates on the surface),and the “apparent coverage” (i.e., the number of adsorbates thatcan be detected by STM at a certain gap voltage). Thisinterpretation implies a rather weak bond between Ru3(CO)9-(µ-SnPh2)3 and the surface of the oxide film, which is consistentwith the observation that it is less covered than the patchesof Ni3Al(111) at either gap voltage (0.7 or 2.3 V). Since thesample had been kept at room temperature between exposureto the organometallic compound and image acquisition, theadsorbates are most likely still fully ligated, which wouldleave the interaction even weaker than it would be fordeligated Ru-Sn species.

Figure 3 shows an STM image after a shorter exposure timeof 2 min (i.e., 0.3 L), while all other experimental conditionsare identical to those of Figure 2. As a result, both the oxidizedand unoxidized areas are less covered in comparison to Figure2, and it is observed that there is a significantly higher density

of adsorbates at the borders between Ni3Al(111) and the oxidefilm. This can be explained by a spillover of preadsorbedRu3(CO)9(µ-SnPh2)3 from alumina to Ni3Al(111), driven by thefaster diffusion on the oxide film and by the stronger adsorptionon the clean Ni3Al(111) surface. It is remarkable, however, thatthe effect can be observed already at room temperature.

The sample shown in Figure 2 (0.6 L) was subsequentlyheated to increasing temperatures. STM images were obtainedfor each temperature from both domains, and are presented inFigure 4a-d and e-h for Ni3Al(111) and Al2O3/Ni3Al(111),respectively. As expected, the two surfaces show a very differentinteraction with the adsorbed Ru3(CO)9(µ-SnPh2)3. At roomtemperature (Figure 2), despite the high coverage on Ni3Al(111)there is no coherent adsorbate phase, but many individualaggregations are distinguished. This is not surprising becauseof the repulsive interaction between the ligand spheres of theorganometallic entities.

Figure 4a depicts the metallic surface after heating the sampleto 475 K, where the adsorbates have just started to agglomerate.The height is measured as 0.22 ( 0.03 nm, and the averagewidth is about 3.0 ( 0.4 nm. Most notably, however, the surfacespecies are no longer closely packed, as it was observed in thecorresponding areas of Figure 2. Figure 4e shows an image ofthe oxide surface obtained under otherwise identical conditions.Most of the protruding features have not changed by comparisonto Figure 2, notwithstanding the observation of some largerentities that seem to originate from agglomeration of the smallerones. Note that a site-specific nucleation on the oxide film isobserved in the present work, although not as clearly asdiscussed by Lei et al. (Figure 3b in ref 15) and Khosravianet al.,29 due to a lower coverage. Both publications discuss thenucleation of Rh on atomic sites of the alumina film used inthe present work. Thus, it is concluded that 475 K marks theonset of an agglomeration process on both domains. Lei et al.describe a similar behavior and attribute the small features tothe still ligated precursor molecule, and the large ones to alreadyaggregated metal centers. Note that Figure 4d also demonstratesthat up to 925 K very little, if any, diffusion of Ru or Sn intoNi3Al has taken place. Massalski reports that all four combina-tions of a metal from the precursor and a metal from the support(Al-Ru, Al-Sn, Ni-Ru, Ni-Sn) have at least one stable alloyphase,30 but Figure 4d does not support the idea that alloyinghas occurred. The reason for this remains unclear at this point.

A comparison of Figure 4f,e shows that at 625 K the numberof small features has decreased in favor of the larger ones.Following Lei et al.,15 it is concluded that the agglomerationhas made progress, which is to be expected in light of the 150K difference between the two images. Between Figure 4a,b,however, the most striking change is the formation of surfacecavities. They are about as large as the particles, which havenot changed much in size, but have diminished in number. Thisapparent overall loss of adsorbed material may be explainedby the formation of volatile species.31 The organometallicprecursor, Ru3(CO)9(µ-SnPh2)3, contains CO, which is well-known to form the volatile Ni(CO)4. Note that such cavitieswere observed also upon exposure of Ni3Al(111) to the vaporof another carbonyl-containing compound, Rh(acac)(CO)2 (notshown). At this point, however, additional alloying betweenRu-Sn and Ni3Al cannot be ruled out as the source of the cavityformation.

Particles with well-defined polygonal shapes are observed inFigure 4c,d. (Note that a part of an oxide domain can be seenin the top right corner of Figure 4d.) The polygonal shapes aretypical for metallic particles, which suggest that the particles

Figure 3. STM image (gap voltage ) 0.7 V, tunneling current ) 0.1nA) taken after oxidizing Ni3Al(111) and subsequently exposing thesurface to 0.3 L of Ru3(CO)9(µ-SnPh2)3. The sample was at roomtemperature throughout the experiment. An oxide patch is found inthe left part, while the right part consists of unoxidized Ni3Al(111).Note that the concentration of adsorbates is constant on most ofNi3Al(111), while it is significantly increased on the border to the oxide.

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consist of the agglomerated metallic centers of Ru3(CO)9-(µ-SnPh2)3. This implies an at least partial deligation of the metalcenters prior to the aggregation. Still, the particles exhibit thesame height after annealing to 675 and 925 K as observed inFigure 4a. This means that the particles on the metal have amonolayer height throughout the entire heating series, notwith-standing individual particles having additional cluster-likefeatures sitting on top of them. Such a Frank-van der Merwe(i.e., layer-by-layer) growth is always indicative of a relativelystrong interaction with the support. Assuming that the surfacespecies have lost most, if not all, of their ligands after heatingthe sample to 675 K, the observed particles should only consistof Ru-Sn; perhaps along with a fraction of the correspondingcarbides. Laterally, the metal-supported particles are 4.0 ( 1.0nm wide for 675 K, and for 925 K the largest diameter is foundto be 12 nm. This observation clearly confirms particle sinteringupon heating the sample.

The particles found on the oxide (Figure 4g,h), however,appear to be much taller for the same temperatures. Their heightwas measured to be up to 0.9 nm, which definitely surpassesthe monolayer limit. In contrast to the flat particles onNi3Al(111), these features appear rather hemispherical. Theirdiameters increase from 1.3 ( 0.1 nm (Figure 4g) to 2.9 ( 0.2nm (Figure 4h), reflecting agglomeration just as on Ni3Al(111).However, it occurs three-dimensionally, instead of wetting thesurface as found on the unoxidized patches.

The oxide thin film, as seen in Figure 1b, should be relativelyinert, due to its termination by a layer of oxygen atoms. Moreover,the appearance of the underlying oxide film structure in Figure4g,h reveals striking similarities with the dot structure of the film(bottom part of Figure 1a): the dots seem to have disappeared afterdosing and subsequent heating, while the hexagonal arrangementof six depressions surrounding the original position of the dots canstill be observed. The unit cell of this structure is highlighted inFigures 1a,b and 4g; in all cases, the characteristic length is

∼0.3 nm. This is another confirmation for the assignment of thissurface to the oxide film.

All images in Figure 4 were taken at a gap voltage of 0.7 V,just as in Figure 1b, while in Figure 1a it was 2.3 V. Yet theindicated hexagonal structure appears as depressions in Figures1a and 4e-h, and as protrusions in Figure 1b. Since Figure 1shows the pristine sample and Figure 4 shows the surface afterexposure to the precursor compound and subsequent annealing,it is concluded that the latter treatment caused a contrastinversion from Figure 1b to Figure 4e-h. This makes the imagesof the precursor-covered oxide surface in Figure 4 look likethe pristine oxide surface in Figure 1a, but without thecharacteristic dots, as stated in the previous paragraph. Thisphenomenon is partially attributed to the fact that the dots arenot imaged at 0.7 V. Nevertheless, STM images of the surfacesshown in Figure 4e-h, but obtained at 2.3 V, do not featureany dots either. Therefore, the absence of the dots on theprecursor-covered oxide along with the observed contrastinversion indicate that a substantial alteration of the electronicproperties of the oxide film must have taken place when it wasexposed to Ru3(CO)9(µ-Sn(Ph)2)3 and then heated. This maybe explained by an interaction of leftovers of the CO and Phligands with the surface; however, a definite assignment wouldrequire extensive additional work.

The incomplete oxidation of the Ni3Al(111) surface altersthe diffusion behavior artificially, since the unoxidized areasact as diffusion sinks. Therefore, the nucleation behavior in thepresent work differs significantly from earlier studies, wherePVD was used to deposit metals on alumina films. Baumer andFreund published a comprehensive review on this topic, whichnot only contains a comparison of different metals deposited,but also an instructive discussion of their respective nucleationbehavior.32 Namely, the authors address nucleation at pointdefects, while on the Al2O3/Ni3Al(111) film the work of Wiltneret al. suggests that specific regular surface sites act as nucleation

Figure 4. (a-d) STM images (gap voltage ) 0.7 V, tunneling current ) 0.1 nA) taken from Ni3Al(111) after exposure to 0.6 L of Ru3(CO)9-(µ-Sn(Ph)2)3 and subsequent heating to (a) 475 K, (b) 625 K, (c) 675K, and (d) 925 K (the top right corner shows a part of an oxide domain). (e-h)STM images (gap voltage ) 0.7 V, tunneling current ) 0.1 nA) taken from Al2O3/Ni3Al(111) after exposure to 0.6 L of Ru3(CO)9(µ-Sn(Ph)2)3 andsubsequent heating to (e) 475 K, (f) 625 K, (g) 675K, and (h) 925 K. The white hexagon in (g) indicates the unit cell of the six depressions aspreviously observed in Figure 1 (characteristic length: ∼0.3 nm).

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centers.33 Those sites were found to be correlated to the networkand dot structure for exposure to vanadium and copper,respectively. Likewise, Khosravian et al. found nucleation ofRh clusters from a Rh(acac)(CO)2 precursor on both types ofsites.29 Figure 4e-h do not reveal such ordered nucleation, butthis is attributed to the coverage being too low. It is expectedthat on an oxide surface that regular sites are less attractive fornucleation than defect sites. In light of these differences, it issomewhat surprising that particle formation was observed onthe oxide in the present study at all. It is concluded that theonset of deligation requires less thermal energy than diffusionof Ru3(CO)9(µ-SnPh2)3 from the oxide to the metal surfacepatches.

Figure 5a,b shows the surface after exposure to the precursorand a subsequent anneal to 725 K. The lower left parts of eitherimage show the metal surface, and the upper right parts showthe oxide surface. At 725 K, the apparent coverage of the oxidedoes not seem to be a function of the gap voltage anymore.The large-scale images in Figure 5a,b taken at 0.7 and 2.3 V,respectively, are practically identical and reveal a very lowcoverage on the oxide area. Indeed, it seems to be much lesscovered now than in Figure 2b, not to mention Figure 2a. Sucha loss can occur through two main channels: (i) diffusion to

the metal patches, or (ii) desorption of intact (i.e., still ligated)Ru3(CO)9(µ-SnPh2)3. It is quite difficult to infer whether heatingthe sample from room temperature to 725 K caused any spilloverin addition to the one observed at room temperature (cf. Figure3), because heating also leads to deligation, and hence coveragesderived from Figures 2 and 5 are not directly comparable toeach other. Hence, the extent of the contribution of eitherchannel remains unclear.

Figure 6 shows the STM image of a blank experiment, wherethe clean (i.e., unoxidized) Ni3Al(111) surface was exposed tothe vapor of Ru3(CO)9(µ-SnPh2)3 for 2 min (0.3 L) andsubsequently heated to 725 K. The results are identical to thosefound in Figure 5 (lower left part of either image), except fora smaller coverage in the blank experiment, due to a shorterexposure time. This proves the previous assignment of the dot-free surface to unoxidized Ni3Al(111).

Assuming that at room temperature the precursor moleculeis still intact, this would lead to a lesser coverage on the oxidepatches than dosing an equivalent amount of metal by meansof PVD, because the ligand sphere of an organometalliccompound will always counteract immediate nucleation and thusfacilitate diffusion to an area where the molecule binds morestrongly. The data presented in Figure 3 demonstrate the effect.This is one important difference between metal deposition usingan organometallic precursor versus by PVD.

4. Conclusions and Outlook

The fact that the surface oxidation of the Ni3Al(111) substrateturned out to be incomplete enables a comparative study of thenucleation of Ru-Sn particles on two different surfaces sideby side. The data clearly show the vastly different nucleationbehavior of one metal alloy, Ru-Sn, on an oxide (Al2O3) andon another metal alloy, Ni3Al, and they are entirely consistentwith predictions using simple surface energy arguments. Metal-on-metal growth occurs by a preferential monolayer (Franck-vander Merwe) mechanism; however, on oxide surfaces, which aregenerally much more inert, the metallic adatoms favor bondsamong themselves, thus forcing Ru-Sn into a three-dimensional(Volmer-Weber) growth mode.

The results show that by using an organometallic precursor,small uniform metal nanoparticles can be successfully deposited

Figure 5. STM image of Ni3Al(111) and Al2O3/Ni3Al(111) afterexposure to 0.6 L of Ru3(CO)9(µ-SnPh2)3 and subsequent heating to725 K: (a) gap voltage ) 0.7 V, tunneling current ) 0.1 nA, (b) gapvoltage ) 2.3 V, tunneling current ) 0.1 nA.

Figure 6. STM image (gap voltage ) 2.3 V, tunneling current ) 0.1nA) of unoxidized Ni3Al(111) after exposure to 0.3 L of Ru3(CO)9-(µ-SnPh2)3 and subsequent heating to 725 K.

Formation of Ru-Sn Nanoparticles on Al2O3/Ni3Al(111) J. Phys. Chem. C, Vol. 114, No. 40, 2010 17067

onto an oxide thin film. The high thermal stability of the oxide-supported particles is remarkable, given that thermal diffusionof the precursor from the film to patches of bare Ni3Al(111)seems to occur already at room temperature, and also that mostof the precursor material deposited on the oxide has left it uponannealing to 725 K. A particularly striking finding is apparentlythe substantial modification of the electronic structure of theoxide film after exposing it to Ru3(CO)9(µ-SnPh2)3 and asubsequent heating treatment. The (albeit tentative) assignmentto an interaction with leftovers of the ligand sphere after thermaldeligation reveals that our preparation indeed introduces a novellevel of complexity to the structure of the model catalyst.

This work paves the way for subsequent studies of the surfacechemistry associated with model oxide-supported metal catalystsconsisting of metal nanoparticles of a uniform size. In particular,a novel way for the fabrication of Ru-Sn nanoparticles wasfound. Such species are active in the hydrogenation of cyclicpolyenes, such as 1,5,9-cyclododecatriene. Since STM lackschemical sensitivity, further studies of this system with othertechniques will have to address the composition and structureof the particles. Among the questions to be answered are whetherthis method really leads to the formation of bimetallic particlesor rather to a segregation of the two metals, whether thepotentially bimetallic particles are fully alloyed or have acore-shell structure, and (in the latter case) which metal isenriched at the surface of the particles.

Acknowledgment. A.U. gratefully acknowledges financialsupport from Deutsche Forschungsgemeinschaft through apostdoctoral fellowship. A.U. and M.T. acknowledge supportby the National Science Foundation under Grant CHE-1012201.Y.L., H.K., and R.J.M. acknowledge support by the PetroleumResearch Fund Grant #46131-G5. R.D.A. acknowledges NSFsupport under Grant CHE-0743190.

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