Controlling the Size and Distribution of Pd-Au Nanoparticles on TiO2(110)

Patrick Han and D. Wayne Goodman*Department of Chemistry, Texas A & M UniVersity, College Station, Texas 77843-3255

ReceiVed: October 16, 2007; In Final Form: February 25, 2008

Variation in the extent of annealing of a TiO2(110) surface was used to control the distribution of depositedPd, Au, and Pd-Au clusters at room temperature. Programming with respect to the metal deposition andthermal sintering was used to optimize the size distribution and cluster density of Pd-Au mixed-metal clusters.The influence of varying the evaporation sequence on the cluster distribution is demonstrated, and the resultsexplained using known sintering mechanisms.

Introduction

Supported bimetallic nanoparticles play a central role in manycatalytic processes.1-3 Important examples are Pd-Au catalystswhose applications include CO removal in car exhaust,4

trichloroethene production,5 hydrogen peroxide synthesis,6 andvinyl acetate manufacturing.7 The synthesis of alloy nanopar-ticles typically use two recipes: (i) ex situ fabrication beforedeposition on the substrate4,8-10 or (ii) in situ epitaxial growthby vapor deposition.11-14 Approach i focuses on the monodis-persity of size, shape, and metal composition of the supportedclusters; however, little real-space information regarding thedynamics of the clusters is available. Approach ii uses localprobes to study epitaxially grown clusters; however, this methodoften results in metal clusters with a broad size distribution andill-defined composition. A notable example is described in ref14 where the authors use a nanostructured alumina film onNi2Al(111) to produce a monodispersed array of Pd-Auparticles with a diameter of∼4 nm.

In the present work, surface oxygen vacancies are inducedby changing the annealing conditions of a TiO2(110) substrate.This, in turn, controls the size distribution and density ofsubsequently nucleated Pd and Au clusters. The TiO2(110)substrate was also used to synthesize Pd-Au clusters by thermalsintering. By control of the substrate, the sequence of deposition,thermal annealing, and the initial distribution of the depositedmetal, well-dispersed Pd-Au clusters can be obtained with anaverage size of∼5 nm. The interplay of the synthesis conditionson the final cluster density and size distribution are discussedand explained using known sintering mechanisms.

Experimental Section

The scanning tunneling microscopy (STM) experiments wereperformed in ultrahigh vacuum at room temperature, using aRHK 300 STM. All images were scanned in the constant-currentmode using an electrochemically etched W tip. All bias voltagesare reported with reference to the sample. An orderedTiOx(110) surface was obtained through sputtering and annealcycles (1 keV Ar+ sputtering for 10 min, annealing at 850°C)on a TiO2(110) single crystal. Pd, Au, and Pd/Au alloy clusterswere grown by vapor deposition followed by annealing thesubstrate at 630°C. The sample temperature was measured

optically using an infrared pyrometer, and the cluster volumeswere estimated from the STM images.

Results and Discussion

Clean TiO2(110) Surface.Figure 1 shows two sets of STMimages of clean surfaces of TiO2(110) at various stages ofsample preparation. Following the initial 10 min of sputteringwith Ar+ and an anneal at 850°C for 3 min, bright spotsbelieved to be missing O atoms are evident (Figure 1A andinset). A defect-free surface showing the (1× 1) structure can

* To whom correspondence should be addressed. E-mail: goodman@mail.chem.tamu.edu.

Figure 1. (A) STM image of a clean TiO2(110) surface after 10 minof Ar+ sputtering (E ) 1 keV), followed by 3 min of annealing at 850°C (300 nm× 300 nm,Vs ) 2.62 V, I t ) 74 pA). Bright areas in theinset are believed to be missing oxygen atoms (100 nm× 100 nm,Vs

) 2.62 V, I t ) 96 pA). (B) STM image of the same sample afteradditional annealing for 3 min at 850°C (300 nm× 300 nm,Vs )2.44 V, I t ) 63 pA). The inset shows the 1× 1 surface structure atatomic resolution, as well as some added rows (30 nm× 30 nm,Vs )2.44 V, I t ) 58 pA).

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10.1021/jp7100607 CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 04/03/2008

be obtained by annealing once more (850°C for 3 min) withoutsputtering (Figure 1B and inset). The appearance of an added-row structure associated with the (1× 2) reconstruction alongwith straightening of the step edges by annealing is apparent inthe images of Figure 1.

It is well-known that Pd deposited on a clean, defect-freeTiO2(110)-(1 × 1) surface preferentially nucleates at stepedges.16 Au shows a similar trend at low deposition coverages(<0.2 ML).17 One approach for controlling the metal clusternucleation sites and ensuring a uniform cluster distribution isto induce surface defects on the terraces prior to depositing Au.Various methods used to vary the cluster size and distributioninclude controlled Ar+ sputtering of the surface,18,19 use of afully cross-linked TiO2(110)-(1 × 2) substrate,16,20,21 andutilization of mixed metal oxide surfaces.22 In this study thepreference of Au for surface oxygen vacancies is used to controlthe surface distribution of the metal clusters.23 Surface oxygenvacancies can be generated by varying the annealing conditionsof TiO2(110) as shown in Figure 1A. One important advantageof this method is the elimination of any surface contaminantsby Ar+ sputtering prior to annealing. This is confirmed by theordered TiO2(110)-(1× 1) surface evident after the annealingprocess (Figure 1B). In the following section, we demonstratethat a clean, defect-free TiO2(110) surface decorated with well-dispersed metal clusters can be obtained using a partiallyannealed substrate.

Behavior of Pd and Au Clusters.To demonstrate the effectsof TiO2(110) oxygen vacancies on metal cluster distribution,the partially annealed surface was imaged after Pd depositionat room temperature. Parts A and B of Figure 2 compare thesurfaces obtained with various Pd deposition times (1 min ofPd deposition for Figure 2A and 30 s of Pd deposition for Figure2B). The apparent dispersity of the Pd cluster distributionconfirms the effects of oxygen vacancies. The images in theinsets of Figure 2 further support this observation, i.e., fewclusters are located at the step edges. Contrary to previousimages of Pd deposited on defect-free TiO2(110),16 the imagesof Figure 2 show no preference for Pd cluster nucleation at stepedges.

To measure the density of Pd clusters, three STM images(100 nm× 100 nm) for each deposition time were acquired atdifferent areas of the substrate. The number of clusters in eachimage was then counted and the total counts from the threeimages averaged. Our results (0.029( 0.002 cluster/nm2 for a1-min deposition and 0.026( 0.001 cluster/nm2 for a 30-sdeposition) indicate that the cluster density is comparable forparts A and B of Figure 2. This suggests that, for deposition atroom temperature, Pd initially nucleates at oxygen vacancies.After saturation of the defect sites, additional Pd atomsagglomerate with the nucleated Pd to form clusters.

Parts C and D of Figure 2 show the surfaces of parts A andB of Figure 2 annealed for 3 min at 630°C, respectively. Adramatic change in both cluster size and density is evident dueto sintering of the Pd clusters. For sintering of the clusters inparts C and D of Figure 2, the cluster density is decreases by afactor of ∼16. The cluster density for the clusters of parts Cand D of Figure 2 is 0.00160( 0.00004 and 0.00112( 0.00006clusters/nm2, respectively.

Au deposition at room temperature on the partially annealedsubstrate also shows nucleation preference for the substrateterraces with no detectable nucleation at step edges (Figure 3A,1 min of Au deposition). Upon annealing the surface of Figure3A at 630 °C for 3 min, 3D growth of the Au clusters isobserved (Figure 3B). However, the extent of the Au cluster

growth is much less dramatic compared with that of Pd in Figure2. Subsequent annealing at 630°C for 6 min yields a surfacethat is virtually depleted of Au clusters (Figure 3C). This

Figure 2. (A) STM image of a partially annealed TiO2(110) surfaceafter 1 min of Pd deposition at room temperature (300 nm× 300 nm,Vs ) 2.11 V, I t ) 468 pA). (B) STM image of a TiO2(110) surfaceafter 30 s of Pd deposition at room temperature (300 nm× 300 nm,Vs

) 2.52 V, I t ) 73 pA). The insets in parts A and B are enlarged areas,respectively, of each image. (C and D) STM images (300 nm× 300nm, Vs (C) ) 2.18 V, I t (C) ) 534 pA,Vs (D) ) 2.52 V, I t (D) ) 54pA) of the surfaces in (A) and (B), respectively, after a 3 min annealat 630°C. This figure illustrates Route A in Table 1.

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phenomenon has been observed previously by Kiss et al. forAu on TiO2(110) and was purported to relate to sublimation ofAu when annealed at temperatures above 900 K.18 No evidencefor metal loss from Pd clusters was found under the sameannealing conditions.

In the following section, we use the partially annealedTiO2(100) exclusively as the starting substrate for Pd-Au clustersynthesis. We optimize the final cluster size and distributionby changing individual steps of the cluster preparation proce-dure.

Effects of Synthetic Routes on Pd-Au Cluster Size andDistribution. The aim of our experiment is to investigate theeffects of individual events, namely, sample annealing and metaldeposition time, of the Pd-Au cluster synthesis over the finalcluster size and distribution. The columns in Table 1 list thesequences of individual events from five synthetic routes. RouteA shows the preparation method for Pd only clusters (describedin previous section and shown in Figure 2). Routes B-E showthe preparation methods for Pd-Au clusters. Routes B and C

involve annealing after each deposition of each metallic species.Routes D1 and E involve annealing only after deposition ofboth Pd and Au. Route D2 involves the deposition of Pd andAu onto Pd-Au clusters already synthesized by Route D1. Hereagain, the final annealing is done only after the deposition ofboth metals. The final cluster size distributions and clusterdensities of these preparation routes are then measured, com-pared, and discussed to determine the optimum method.

Parts A-C of Figure 4 show a sequence of STM imagesillustrating the synthesis of Pd-Au clusters using Route B.Figure 4B shows an STM image after 1 min of Au depositiononto a surface decorated with preannealed Pd clusters (Figure4A). The sample was then annealed at 630°C for 3 min andthe image in Figure 4C was acquired. The cluster densitychanged little when comparing the images before the additionof Au (Figure 4A, 0.00112( 0.00006 clusters/nm2) and afterthe final annealing of the sample (Figure 4C, 0.00089( 0.00018cluster/nm2). Cluster growth is evident when comparing thesame two images. Clusters in both parts A and C of Figure 4show no adsorption preference for the substrate step edges.

The plot at the bottom of Figure 4 shows the measureddiameter of every cluster in Figure 4C, displayed in an ascendingorder from left to right. The staircase appearance of the plot iscaused by the STM resolution limit in thexy direction,determined by the pixel size of 0.59 nm (image size) 300 nm× 300 nm, at 512 pixels per line). The fact that Figure 4 displaysno dominant cluster diameter indicates that the size distributionis broad.

These data show that the use of partially annealed TiO2(110)directly affects final Pd-Au clusters. The presence of oxygenvacancies has allowed a disperse adsorption of the final Pd-Au clusters across the TiO2(110) terraces. Cluster densitymeasurements suggest that the final bimetallic cluster densityis determined by the density of the preannealed Pd clusters. Asevident in parts C and D of Figure 2, the preannealed Pd clusterdensity changes very little with the addition of Pd. However,the synthetic procedure described by Route B yields clustersthat have a very broad diameter distribution and a relativelylow cluster density. It should be noted that the exact chemicalcomposition of the Pd-Au clusters remains an outstandingquestion.

Figure 3. (A) STM image of a partially annealed TiO2(110) surfaceafter 1 min of Au deposition at room temperature (300 nm× 300 nm,Vs ) 2.61 V, I t ) 98 pA). (B) STM image of the same surface after 3min anneal at 630°C (300 nm× 300 nm,Vs ) 2.95 V, I t ) 117 pA).(C) STM image of the same surface after a 6-min additional anneal at630 °C (300 nm× 300 nm,Vs ) 2.03 V, I t ) 192 pA).

TABLE 1: Synthetic Routes A-E Used to Fabricate Pd andPd-Au Clustersa

a Every sample annealing process was carried out at 630°C for 3min. Each rectangle within the columns groups a set of processes thatyielded clusters suitable for STM analysis. The color of the rectanglecategorizes each route of the same Pd coverage.

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To improve the final cluster density and size distribution, thesynthetic methods described by Routes D and E in Table 1 areused. Here Pd-Au bimetallic clusters are synthesized bysequential deposition of both metallic species at room temper-ature before annealing. This deposition method enhances thetotal metal cluster density prior to thermal sintering to formbimetallic clusters. This higher initial cluster density, i.e., ashorter distance between clusters, in turn, increases the prob-ability of obtaining a higher final cluster density after sinteringand promotes a narrower cluster size distribution. Moreover,the use of small seed clusters can potentially promote thehomogeneity of the mixed-metal clusters synthesized, i.e., alloyclusters rather than core-shell clusters.

Pd was chosen as the first-deposited metal species becauseof its greater stability on TiO2(110) compared with Au. Figure5 compares the STM images from Pd-only deposition (Figure5A, 1 min of Pd deposition) and Pd-Au codeposition (Figure5B, 30 s of Pd deposition followed by 1 min of Au deposition).Although it is not possible to determine the chemical identityof the clusters in Figure 5B, the improvement in initial clusterdensity is evident. In the following we compare the Pd-Auclusters fabricated via Routes D and E.

Route D consists of two parts. The first step (Route D1) is a30-s Pd deposition, then deposition of Au for 1 min, followedby an anneal at 630°C for 3 min. The resulting surface wasthen imaged by STM (Figure 6A). The second step (Route D2)is completed by repeating the deposition/annealing process asecond time (Figure 6B). The average cluster diameters are 5.0( 1.2 nm for Figure 6A and 7.7( 1.4 nm for Figure 6B.

Route E consists of a 1-min Pd deposition, then depositionof Au for 2 min, followed by an anneal at 630°C for 3 min.STM imaging of the resulting surface (Figure 6C) indicates anaverage cluster diameter of 5.7( 0.9 nm.

To measure the cluster density after each process, 3 imagesper process (300 nm× 300 nm), acquired over macroscopically

different regions of the substrate, were used. The average clustercount in these three images is shown in Figure 6 with the

Figure 4. (A) STM image of a TiO2(110) with presintered Pd clusters (30 s of deposition) (300 nm× 300 nm,Vs ) 2.62 V, I t ) 74 pA). (B) STMimage of the same surface after a 1-min Au deposition at room temperature (300 nm× 300 nm,Vs ) 2.62 V, I t ) 74 pA). (C) STM image of thesame surface after a 3-min anneal at 630°C (300 nm× 300 nm,Vs ) 2.63 V, I t ) 77 pA). The bottom plot shows the diameter distribution of thebimetallic clusters of image C. This figure illustrates Route B in Table 1.

Figure 5. (A) STM image of a partially annealed TiO2(110) surfaceafter 1 min of Pd deposition at room temperature (100 nm× 100 nm,Vs ) 2.11 V, I t ) 468 pA). (B) STM image of a partially annealedTiO2(110) surface after 30 s Pd deposition, followed by 1 min Audeposition at room temperature (100 nm× 100 nm,Vs ) 2.83 V, I t )26 pA).

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corresponding standard deviations. These data indicate that theimages of Figure 6 are representative of the cluster density. Itshould be noted that small clusters (heighte 0.25 nm) can beseen in the images in Figure 6, especially in Figure 6B. Suchsmall clusters have been observed in previous sintering experi-ments on Au and Pd on TiO2(110) and have been attributed tobe the remnants of shrinking metal clusters caused by Ostwaldripening.24-26 These 2D clusters are excluded from our densityand size distribution measurements.

To analyze the cluster size distribution resulting from thedifferent deposition/annealing processes, the diameter and heightof every cluster from the three STM images shown in Figure 6were measured. The image of Figure 6C (Route E) will be usedin the following discussion to illustrate the data analysis.

The yellow triangles in Figure 7 show the measured clusterdiameters from Figure 6C. The diameter of every cluster in theimage is listed in an ascending order. All diameter measurementswere carried out in the fast scan direction of the image to reduceerror caused by tip drift. The staircase appearance of the datareflects the limit of the image resolution in thexy direction, setby the pixel size of 0.59 nm (300 nm× 300 nm image size, at512 pixels× 512 pixels). This resolution limit creates a naturalbinning effect of the data. To account for this, the error assuminga maximum error of 1.2 nm in the diameter measurement isshown as blue error bars. It should be noted that, in comparisonwith the plot shown in Figure 4, the cluster diameter distributionfrom Figure 7 is much narrower. The black dots in Figure 7show the cluster height measurements corresponding with eachdiameter measurements. The spread in cluster height gives anindication of the variation in the cluster aspect ratio.

To calculate the volumeV of the clusters, we approximatethe shape of the clusters as spherical caps by using the followingequation

wherer is the cluster base radius andh is the cluster height.The resulting cluster volumes are then listed in ascending orderalong thex axis of a bar chart for comparative purposes. Figure8 compares the cluster volume distribution resulted from RouteD1 and Route D2. The figure demonstrates that the secondcodeposition has little effect over the total number of imagedclusters. The integrated volume of the deposited metal resultedfrom Routes D1 and D2 is 1:2.4, compared with the expected1:2. It should be noted that the error in the STM volumemeasurements is influenced by several factors, namely, the tipaspect ratio, tip drift, and the conditions of the STM measure-ments. In this case, we believe that the tips used to image bothsurfaces have comparable aspect ratios.

Figure 9 compares the cluster volume distribution resultedfrom Route D2 (inset A) and Route E (inset B). This figuredemonstrates that the total amount of deposited metal beforeannealing has a dramatic influence over the final cluster density.At constant volume of deposited metal, a single codeposition(Route E) improves the final cluster density by 60% oversequential deposition (Route D2). Figure 9 also compares thetotal integrated metal volume for both processes. A volume ratiobetween Route D2 and Route E of 1:1.16 is measured andcompared with the anticipated 1:1 total volume ratio.

To acquire a better measure of the size distribution, the naturalbinning effect caused by the STM resolution limit in thexydirection (Figure 7) is exploited to categorize the cluster volumemeasurements. The image shown in Figure 6C (single codepo-sition) is used as an example to discuss the method of dataanalysis.

Figure 10 shows the cluster volume distribution extractedfrom the image of Figure 6C (Route E). The volume of eachcluster is listed in an ascending order of the correspondingmeasured diameter. The partitioning of the volume measure-ments is done according to the break points of the diametermeasurement of the same image (yellow triangles in Figure 7).The red line illustrates the partitions as well as the average valuefor the volume within each partition. The error bars representthe standard deviation of the average volume. The same analysisis performed for the image in Figure 6B (Route D2), and theresulting counts/partition plotted as the average cluster volumein Figure 11.

Figure 11 compares the cluster size distribution betweenRoute D2 (inset A) and Route E (inset B). This figure clearly

Figure 6. (A and B) Surface after sequential co-deposition describedin the text. (A) STM image of a TiO2(110) surface after deposition ofboth metallic species (30 s Pd, 1 min Au) followed by a 3-min annealat 630°C (300 nm× 300 nm,Vs ) 2.97 V, I t ) 110 pA). (B) Imageof the same surface after a second deposition of both metallic speciesand a 3-min anneal at 630°C (300 nm× 300 nm,Vs ) 2.72 V, I t )118 pA). (C) STM image of a TiO2(110) surface resulting from a singledeposition of both metallic species (1 min Pd, 2 min Au, 3-min annealat 630°C, 300 nm× 300 nm,Vs ) 2.54 V, I t ) 86 pA). The imagesin A, B, and C illustrate Routes D1, D2, and E in Table 1, respectively.

V ) (π/6) (3r2 + h2) h (1)

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Figure 7. Distribution of the STM-measured diameters and heights of the clusters of Figure 6C. The measurements correspond to specific clustersand are displayed in an ascending order of the diameter. The black solid line is the least-square fit of the height measurements.

Figure 8. Comparison of the cluster volume distribution resulted from Route D1 (A) and Route D2 (B) of the sequential deposition process. Thevolumes are displayed in an ascending order. The data represent the integrated cluster volumes from the STM measurements. The images A andB in the insets correspond to parts A and B of Figure 6, respectively.

Figure 9. Comparison of the final cluster volume distribution resulted from Route D2 (A) and Route E (B). The volumes are displayed in anascending order. The data represent the integrated cluster volumes from STM measurements. The images A and B in the insets correspond to partsB and C of Figure 6, respectively.

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illustrates the effects of the cluster preparation method on clustersize distribution. At equal quantities of deposited metal, thevolume distribution for Pd-Au bimetallic clusters obtained bysingle co-deposition (Route E) is almost three times as narrowas the volumes obtained by sequential deposition (Route D2).

To place our results in perspective, the normalized Pd-Aubimetallic cluster densities obtained by synthetic Routes A-Eare shown in Figure 12. The colors categorize each method withrespect to a particular Pd coverage. While metal coverage hasvery little effect on the final density obtained when annealingis performed after each deposition of each metallic species(comparing Routes B with Route C), its effect is dramatic forsequential deposition (comparing processes D1 with E). For bothsequential depositions (Route A followed by Route B as wellas Route D1 followed by Route D2), the second deposition has

no effect on the final density. Clearly, sequential depositionwithout annealing of the first metallic species enhances the finaldensity (comparing processes B with D1 and C with D2).

These results indicate that the final bimetallic cluster densitydepends strongly on the initial cluster density prior to bimetalliccluster formation by thermal sintering. Our STM images supportthis hypothesis. For depositions where each metallic species isannealed, the cluster density prior to annealing primarily is dueto Pd deposition at room temperature (Figure 4B), while forsequential depositions at room temperature, both metallic speciescontribute significantly to the initial cluster density (Figure 5B).This accounts for the higher final density for single codepositioncompared with sequential deposition.

In addition to controlling the initial cluster distribution, theevolution of the TiO2(110) surface morphology throughout thesynthesis process also influences the final cluster density. Forprocesses B and D2 in Figure 12, the substrate has alreadyundergone one anneal following the first depositions (namely,processes A and D1, respectively). This anneal reduces thesurface roughness caused by oxygen vacancies (Figure 1). Thereduction in surface defects facilitates metal mobility duringthe second deposition. Therefore, the evolution of the surfacemorphology explains the observation that the second depositionsin processes B-C and D1-D1 have no detectable effect onthe final cluster density.

The most dramatic effect shown in Figure 12 is thedependence of the final density on the metal coverage duringthe sequential depositions of both species at room temperature(processes D1 and E). This observation also supports the roleof surface defects in controlling the final cluster density. Sincethermally induced cluster growth implies metal transport, ahigher initial cluster density before sintering implies a shortertransport distance between clusters. This, in turn, increases thenumber of clusters that reach a critical size upon sintering. Thephysical meaning of this critical size depends on the mode ofmetal transport between clusters. Current views on metaltransport support two classes of mechanisms, namely, Ostwaldripening26 and coalescence sintering. The former involves metalsublimation from clusters smaller than a threshold size, followed

Figure 10. Binning of the volume measurements of the clusters imagedin Figure 6C. The binning uses the break points from the diametermeasurements (Figure 7) of the corresponding image. The clustervolumes are averaged within each category. The error bars show thestandard deviation of the volume averaging.

Figure 11. Cluster volume distribution comparing images A and B in the inset. The images A and B in the insets correspond to parts B and C ofFigure 6, respectively.

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by recondensation onto clusters larger than the threshold size.The latter case involves clusters merging through growth ordiffusion. Therefore, from the perspective of Ostwald ripening,the critical size can be understood as the threshold cluster size.Upon reaching this size, a particular cluster will continue togrow at the annealing temperature as long as metal donorclusters (those smaller than the threshold size) are available.From the perspective of coalescence sintering, the critical sizecan be viewed as a size large enough to prohibit thermaldiffusion of a particular cluster at the annealing temperature.Clusters that reach this size will continue to grow as long assmaller mobile clusters remain available.

Following this reasoning, the importance of the initial,presintering cluster density becomes clear. A higher initialdensity results in the formation of a higher number of clustersof the aforementioned critical size. This, in turn, highlights therole of the partially annealed TiO2(110), which directly affectsthe nucleation site distribution.

The difference in the width of the cluster size distribution(Figure 11) can also be explained by the initial cluster density.However, to pinpoint the reason for this effect, a more accuratepicture of the sintering mechanism is needed. Although theshapes of the distribution curves shown in Figure 11 supportthe dominance of coalescence sintering as the metal transportmechanism, the extent of the mechanistic contribution byOstwald ripening will require further work.

Previous work on the sintering process of Pd/TiO2(100)-(1× 2) has observed that the dominant sintering mechanism inthis system is Ostwald ripening.27 By use of STM and MonteCarlo simulation, the authors of ref 28 have demonstrated thatthe width of the Pd cluster size distribution broadens signifi-cantly and continuously with time. On the basis of this, it ispossible that the dominance of the coalescence sintering is onlytrue for the first stage of the sequential codeposition and thesingle codeposition (processes D1 and E in Figure 12). Ostwaldripening may control metal transport during the second stageof the sequential codeposition (process 5), where clustersdeposited at room temperature are considerably smaller thanthe presintered bimetallic clusters. This size disparity inducesa high cluster edge chemical gradient between clusterssa factthat is predicted to favor Ostwald ripening. If this reasoning istrue, the broader size distribution in image 6B is caused byOstwald ripening, and the shape of the distribution curve isdetermined by coalescence sintering during the first codeposition(process 4). Although the observation of small cluster remnants

in Figure 6 supports this hypothesis, future work involving insitu STM imaging is needed to confirm our observations.

Conclusions

A TiO2(110) substrate has been annealed to various degreesto control surface vacancy density and thus the size distributionand density of deposited Pd and Au clusters. This system wasused to synthesize Pd-Au bimetallic clusters by thermalsintering. By controlling the sequence and amount of depositedmetal, we have managed to control the density and sizedistribution of the final bimetallic clusters. The effects of thedifferent methods over the final density and size distributionare explained using current surface sintering mechanisms.Although clusters with average sizes of 5 nm with a final densityof 0.00347 clusters/nm2 were obtained, the metal compositionof the clusters is not well defined. Future work will involve thedetermination of the cluster metal composition. In situ STMduring sintering should provide insights into the mechanisticcontributions of Ostwald ripening and coalescence-sintering.

Acknowledgment. We gratefully acknowledge the supportfor this work by the Department of Energy, Office of BasicEnergy Sciences, and the Robert A. Welch Foundation.

References and Notes

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Figure 12. Final cluster density comparison between each processwhere all densities are normalized.

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