Self-Organization of Size-Selected Bare Platinum Nanoclusters

Apr 5, 2008 - Novel applications in catalysis rely on the design of tailored nanoarchitectures. In this field, we present a new physical route, based ...
0 downloads 0 Views 1MB Size
Download PDF
6842

J. Phys. Chem. C 2008, 112, 6842-6849

Self-Organization of Size-Selected Bare Platinum Nanoclusters: Toward Ultra-dense Catalytic Systems D. Tainoff, L. Bardotti, F. Tournus, G. Guiraud, O. Boisron, and P. Me´ linon* UniVersite´ de Lyon, F-69000, UniVersite´ Lyon 1, Laboratoire PMCN, CNRS, UMR 5586, F69622 Villeurbanne, Cedex, France ReceiVed: October 22, 2007; In Final Form: January 24, 2008

Novel applications in catalysis rely on the design of tailored nanoarchitectures. In this field, we present a new physical route, based on ultrahigh vacuum deposition of size-selected preformed clusters, leading to self-organization of platinum nanoparticles on the surface. The resulting array of “model” nanoclusters (i.e., size-selected and ligand-free) may provide a unique tool for future experimental and theoretical studies. A detailed analysis of the experimental and physical origins of the spontaneous organization of platinum nanoclusters is provided and points out the extreme importance of cluster-cluster and cluster-surface interactions.

1. Introduction Supported nanoparticles are extensively studied for the development of electronic, catalytic, mechanical, and magnetic systems. The interaction between clusters and substrates may be either weak or strong: in the former case, the properties of the cluster itself show up, whereas in the latter, a new property may be obtained. This is illustrated by the demonstration of gold catalytic activity. Indeed, catalytic properties of nanostructured gold catalysts are known to depend on the particle size and to be activated when their diameter decreases down to a few nanometers.1-4 This activity is due to the existence of a charge transfer at the interface between gold and substrate.5,6 Platinum/carbon is also a popular catalyst couple7 used in fuel cells, which directly transform chemical energy from the reaction between hydrogen and oxygen into electric energy and which are seen as one of the most promising energy sources of the future. A comprehensive study of catalytic processes needs a control of the size, the shape, the surface states, the distribution of the metallic particles, and of the metal-substrate interaction.8,9 For this purpose, two strategies, based on experiments and/or simulations, are developed. However, although first-principles and Density Functional Theory (DFT) calculations can be used to study supported metallic clusters, only very small systems can be investigated because such computer simulations are extremely time-consuming.10,11 Recently, a lot of experimental catalysis research has been directed toward model catalyst studies,12-14 in order to conduct controlled experiments testing each one of the catalyst’s properties and to gain insight into the details of the reaction mechanisms. The distance between catalyst nanoparticles supported on a surface may be one of the key parameters involved in particular in the so-called spillover processes (i.e., diffusion of chemical species), but is usually hard to control. Therefore, ordered arrays of nanoparticles have attracted attention as model catalysts for fundamental studies and have been under consideration for more than 10 years.15-18 Moreover, dense arrays of catalyst nanoclusters * To whom correspondence should [email protected].

be

addressed.

E-mail:

represent an extremely high level of integration, which should give rise to an exceedingly high catalytic activity. In this work, we report the formation of ordered arrays of bare platinum nanoparticles on a Highly Oriented Pyrolytic Graphite substrate (HOPG), following a physical route. These nanoparticles are well-defined (size-selected), ligand-free, deposited in an UltraHigh Vacuum (UHV) environment, and organized at an ultimate integration scale (the distance between two neighboring particles is of the same order of magnitude as the cluster diameter). These systems can be used as templates for a rigorous control of all parameters involved in catalytic properties. In addition, the formation of 2D arrays at room temperature avoids growth or sintering of nanoparticles, which is one of the origins of activity loss in catalytic processes. In this work, we show that the self-organized structure of the nanoclusters thin film is due to a strong interaction between platinum and carbon, while gold clusters only weakly interact with the same substrate, resulting in completely different morphologies. 2. Experimental Section Clusters are produced in a laser vaporization-gas condensation source similar to the one developed by Smalley, De Heer and Milani.19,20 Briefly, a plasma created by the impact of a Nd:YAG (Yttrium Aluminum Garnet) laser beam focused on a rod is thermalized by injection of a continuous flow of helium at low pressure (typically 30 mbar) inducing cluster growth. Clusters are subsequently stabilized and cooled down in a supersonic expansion taking place at the exit nozzle of the source. A low-energy cluster beam is then obtained, with clusters of different sizes, mostly neutral but also ionized (typically, 1 ion percent neutral clusters), allowing the growth of thin cluster films on a substrate. One of the main features of this source is the very high cooling rate of about 1010 K/s, which governs the formation of original nanoscale systems in nonequilibrium conditions. Moreover, the low kinetic energy of clusters (typically 0.1 eV/atom) gained during supersonic expansion ensures that their fragmentation upon impact on the substrate is avoided. This offers the opportunity to preserve the original

10.1021/jp710216s CCC: $40.75 © 2008 American Chemical Society Published on Web 04/05/2008

Bare Platinum Nanoclusters

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6843

Figure 1. TEM images of submonolayer thin films produced by platinum cluster deposition, without (a) and with (b) size selection. In both cases, the corresponding supported cluster size distribution is reported in the inset.

properties of the free phase clusters when a thin film of nanoparticles is produced. Until now, our experimental setup (fully described elsewhere)21 allowed deposition of the whole size distribution of free neutral clusters, in UHV conditions (base pressure 4 × 10-10 Torr). In this case, the incident average flux Fneutral and the equivalent thickness are controlled with a crystal quartz monitor. In order to determine the free-cluster size distribution, a small amount of platinum clusters has been deposited on a carbon-coated electron microscopy grid. Due to the absence of fragmentation and to limited cluster diffusion on amorphous carbon,22 the submonolayer thin film is composed of isolated particles, their size distribution reflecting that of incident clusters. As an example, Figure 1a presents a typical size histogram deduced from Transmission Electron Microscopy (TEM) image analysis, assuming that supported clusters have a spherical shape. Because cluster formation in the source is due to a random collision process, their size distribution is well described by a log-normal function, in this particular case with a mean diameter Dm of 1.6 nm and a relative standard deviation of 40%. This relatively broad size dispersion is not a worry for the study of marked size effects in clusters.23 However, more subtle size effects, as for instance structural transitions in clusters,24 need well-defined size-selected clusters. Thus, in order to lower the spread of cluster size distribution, the above setup has been refined by introducing a mass selection device (electrostatic quadrupole deviator) described elsewhere.25 In this device, charged clusters (with a charge q ) e for single cations) are deviated by application of a selected potential V chosen such as qV ) Ekinetic ) 1/2 mV2, where m is the clusters’ mass, and V their velocity. Since previous time-of-flight experiments have revealed that the free clusters’ velocity is almost independent of their mass,21 the energy selection corresponds to a mass selection of the ionized clusters. In addition, the cluster beam is controlled by a set of electrostatic lenses, allowing an optimization of the cluster flux and a good collimation of the beam on the substrate. The current corresponding to the massselected ionized cluster beam is then measured by a picoamperemeter connected to a Faraday cup. This allows us to determine the flux (Fion) of clusters reaching surface, before deposition on a substrate in UHV conditions. Note that, in such a setup, since the cluster source mostly produces neutral clusters, Fion is extremely small as compared to Fneutral.

Figure 1b shows the typical morphology of a thin film produced by deposition of a small amount of size-selected platinum clusters on amorphous carbon and the corresponding cluster size distribution. Since only a narrow part of the previously shown log-normal distribution is selected, the size histogram is in this case well-described by a Gaussian curve. From this fit, still with the spherical cluster approximation, a mean diameter of 2.2 nm and a relative standard deviation of 8% are derived. These results clearly point out the extreme efficiency of our recently developed setup to reduce the size dispersion of deposited clusters: this opens new perspectives for a precise study of the size dependence of nanocluster properties. In the following, all experiments are performed at room temperature and in UHV conditions, in order to avoid any pollution effect. Samples are observed after exposure to air; however, we have checked with in situ UHV STM observations that exposure to air has no influence on the films morphology. Two kinds of substrates are used in this study: an amorphous, holey carbon grid for direct TEM observation and a graphite substrate for direct observation using a Scanning Tunneling Microscope (STM) or indirect TEM observations. In the latter case, freshly cleaved HOPG substrates are annealed under UHV conditions at 770 K for 5 h prior deposition. This treatment results in atomically flat and clean surfaces extended over 1 µm between steps:26 it minimizes the presence of defects (natural defect density close to 108 cm-2) or morphological irregularities and of residual contamination. Indeed, the surface plays an important role in determining the catalytic and chemical properties of nanoparticles.27,28 Due to the lamellar structure of graphite, TEM observations are achievable by gluing a copper grid on the HOPG substrate and then lifting it off before observation, following a procedure fully described elsewhere.29 Cluster flows used in our experiments are of the order of 107 clusters per second for ionized clusters and 1010 clusters per second for neutral clusters. The coverage density of the sample is of the order of 104 clusters per µm2. 3. Results Figure 2a shows typical cluster “islands” (or “bunches” of clusters) generated by random deposition of platinum nanoparticles on graphite, with a 2.2-nm diameter. It is clear that, in order to explain aggregation of incident clusters in those islands,

6844 J. Phys. Chem. C, Vol. 112, No. 17, 2008

Tainoff et al.

Figure 2. (a-c) Typical TEM morphologies of platinum cluster arrays obtained at different magnifications; (d) distribution of the first neighbor shell distances for non size-selected (squares) and size-selected (triangles) platinum clusters; (e) Comparison between the experimental radial distribution function (solid curve) and the one expected for a perfect hexagonal array (Dirac peaks) of nanoparticles. The dashed curve represents an adjustment of the experimental curve taking into account a possible deviation of the (x,y) center of mass coordinates of each cluster, as mentioned in the text; (f) Autocorrelation function of one “bunch” of clusters: hexagonal order is observable.

we have to admit that clusters can easily move on the surface. This statement is not surprising since previous studies on metallic nanoclusters (gold or antimony for instance) deposited on graphite have already shown that the carbon π bonding is not efficient enough to pin clusters, despite their large size as compared to atoms.30 Moreover, we emphasize that the island density in the present experiment (and in the other experiments mentioned) is about 1 order of magnitude larger than the usual point defect (vacancies or contaminant atoms) density (108 defect.cm-2)31 on well-prepared HOPG. As a consequence, the observed morphologies on terraces result from a homogeneous nucleation rather than from a heterogeneous one (i.e., on defect). The role of natural point defect can thus be neglected. Furthermore, all of the following analysis has been performed on terraces far from steps in order to get rid of the influence of steps where preferential cluster growth takes place. It is now well-established that the large diffusion and the aggregation of clusters on a graphite surface lead to thin films composed of ramified structures formed by the juxtaposition of nanoparticles in contact with each other. Such morphologies are usually remarkably well-described by Monte Carlo simulations32 with a model that includes deposition, diffusion and aggregation of clusters as simultaneous processes (DDA model). Comparison between experimental results and theoretical predictions has allowed us to quantify the Brownian diffusion coefficient of nanoclusters, which surprisingly is extremely high (10-8 cm2s-1 for Sb2300 clusters at room temperature, for instance)33 and has provided a precise description of nanocluster behavior on graphite surface. Briefly, as a general rule, when an incident cluster arrives on graphite, it can either meet another cluster which is also diffusing on the surface and form a new island (homogeneous nucleation event) or be captured by an already existing island (growth process). During this growth process, an incident cluster sticks to the island irreversibly, without further diffusion along the island edges, so that ramified structures are obtained, the width of the ramifications being related to the magnitude of coalescence taking place when

clusters come into contact. Moreover, it has been established that the density of island formed on the surface, Nisland, scales with the following power law:

Nisland R (F/D)χ where F is the incident cluster flux, D the cluster diffusion coefficient, and χ is a parameter which depends on the detailed growth process as describe by Villain and Pimpinelli.34,35 In the present experiment, if we take a closer look at a typical platinum island structure (see Figure 2a-c) two unusual results appear as compared to studies mentioned above on neutral clusters (i.e., gold,28 antimony,32,36 and silver37). First, clusters composing the island stick together without contact. A quantitative image analysis reveals that there exists a well-defined distance d0 of 3.4 nm between the centers of mass of two neighboring clusters (see Figure 2d). Second, islands appear to be compact rather than ramified. Since the size of supported nanoparticles reflects exactly that of incident clusters, any coalescence process can be excluded and this compactness clearly evidences cluster diffusion along island edges during its growth. This edge diffusion enables an optimization of the interparticle interaction energy, and consequently of the compacity (number of nearest neighbors). This might be the driving force for a self-organization process. In addition, the computed radial distribution function (obtained by measuring all distances between the clusters’ centers of mass) displays a local order corresponding to a hexagonal array, up to the fifth neighbor shell. This can be seen in Figure 2e, where the experimental radial distribution function (solid curve) is compared to that expected for a perfect hexagonal lattice (Dirac peaks). The dashed curve in Figure 2e represents a hexagonal array where each position has been randomly modified (with a standard deviation ∆d0 around the perfect position). The best fit, obtained for ∆d0/d0 ) 10%, confirms the organization’s robustness. This local order is also evidenced by the autocorrelation function of the “bunch” of clusters of

Bare Platinum Nanoclusters

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6845

Figure 3. Morphology of a submonolayer thin film obtained by UHV deposition of platinum clusters, without size selection. A higher magnification image is shown in the inset.

Figure 4. Morphology of a submonolayer thin film obtained by UHV deposition of size selected gold clusters on graphite. A higher magnification image is shown in the inset.

Figure 2c (see Figure 2f). Remarkably, these clusters arrays are stable over 1 year at least. At first glance, we can see that bare (i.e., ligand-free) sizeselected platinum clusters self-organize on graphite, without any treatment. Our observations suggest that particles interact through a two-body potential with both a long-range attraction and a short-range repulsion. The origin of this kind of interaction has, to our knowledge, never been reported and will be analyzed in the following. Understanding the mechanisms leading to such a nanocluster self-organization on graphite is of primary importance both from the fundamental and application points of view. Spontaneous organization of preformed clusters (synthesized using physical processes) has already been observed on graphite by several authors,38,39 but unfortunately, with a set of widely varying experimental parameters (vacuum conditions, deposition rate, chemical nature of the particles, size distribution, charge of the clusters, etc.). As a consequence, the analysis of reported results cannot provide a definitive conclusion on the origin of organization. In this section, we will try to clarify the situation by an individual analysis of the influence of several parameters: (i) size selection (charge of ionized clusters, size dispersion, and magnitude of the cluster flux); (ii) chemical nature of clusters; (iii) vacuum conditions (with a possible contamination); and (iv) surface state of the HOPG substrate (possibly with localized defects). To this end, we have carried out additional experiments giving us insight into the respective influence of each of the above parameters. (i) Neutral platinum clusters have been deposited on HOPG, without any size selection (i.e., with the size distribution of Figure 1a), with a flux Fneutral equal to 300 Fion. As it can bee seen from Figure 3, the general thin film morphology is similar to that obtained with size-selected clusters. Nevertheless, the island density is larger for neutral clusters than for ionized clusters. Let us note that, according to eq 1, this observation can be assigned to a flux effect, since the deposition temperature remains unchanged between experiments, but such a study is not the purpose of the present work. More interesting here is the internal structure of islands. A careful analysis of Figure 3 reveals that, once more, “bunches” are composed by juxtaposition of noncontacting clusters. We emphasize that the size distribution of supported nanoparticles reflects that of incident clusters, so that coalescence between clusters can be excluded. Note that, while no hexagonal order is visible, there is always a gap between clusters, and the nearest-neighbor distance is relatively well-defined (with a value close to the d0 distance mentioned earlier, see Figure 2d). This result allows us to rule

out any critical influence of the clusters charge, size-dispersion, and flux on the fact that neighboring clusters do not touch each other. (ii) In order to investigate the effect of the chemical nature of particles, size-selected gold clusters have been deposited on graphite. This choice was motivated by the vicinity of gold and platinum in the periodic table, while their valence shells are different (noble metal and transition metal, respectively). Moreover, gold and platinum clusters are both under investigation for their catalytic properties, as mentioned in the Introduction. Finally, the choice of gold offers the opportunity to compare present results to previous studies already conducted in our group, using unselected neutral gold clusters, and by Pellarin et al.40 on size-selected clusters deposited in standard vacuum conditions. For this experiment, the size of incident gold clusters was centered on 2.4 nm in diameter, with a relative standard deviation of 10%. As it can be seen on Figure 4, the film is composed of large ramified structures. An examination of the islands’ morphology indicates that the width of the branches is larger than the incident clusters’ diameter. This means that a partial coalescence, limited by the kinetics of growth, has occurred between two or more adjacent clusters. In this case, gold clusters are thus able to stick together on the surface and then to partially coalesce after contact. Such structures are exactly the same as those obtained using unselected neutral gold clusters28 (i.e., with a rather wide size distribution). This result confirms that the mass-selection of clusters (and consequently the fact that they are ionized and that the cluster flux is very low) has no major influence on the morphology of 2D films obtained by cluster deposition (point i). However, these results suggest a strong influence of the chemical nature of nanoparticles, and a possible specificity of platinum nanoparticles. (iii) Because platinum particles are well-known for their catalytic properties, a pollution effect (caused for instance by a carbon monoxide residual pressure) cannot be excluded, even in UHV environment.41 As a consequence, one could argue that the presence of a thick contamination layer around clusters can explain the absence of contact between two neighboring supported clusters.28 Note that, with such an assumption, contamination should occur either in the free phase, before deposition on the substrate, or shortly after a cluster’s impact on the substrate (i.e., before a Pt cluster meets another one on the surface). In the present case, according to the edge-to-edge separation between platinum clusters (around 1.7 nm, corresponding to d0-Dm), the contamination layer should be at least 0.8 nm thick. Such a layer could theoretically be detected by

6846 J. Phys. Chem. C, Vol. 112, No. 17, 2008

Tainoff et al.

Figure 5. High-resolution TEM observations of mass-selected nanoparticles deposited on a holey amorphous carbon layer. (a) Gold clusters, some of them being partially coalesced. (b) Platinum clusters. Note that, in both cases, no thick contamination layer is observed around particles.

Figure 6. TEM images with different magnifications revealing the thin film morphologies obtained by successive deposition of size-selected gold and platinum clusters on graphite. Note that “bunches” of clusters marked by dotted rectangles are pure platinum islands.

TEM observations, but unfortunately, the graphite surface contribution in TEM images complicates the interpretation. To get rid of this carbon contribution, size-selected platinum and gold clusters have been deposited on ultrathin amorphous carbon holey TEM grids, allowing us to observe particles standing on the edge of holes in the carbon layer (see Figure 5). We emphasize that, even if the presence of an atomic layer of pollution around clusters cannot be excluded, TEM images attest without any doubt that there is no thick contamination layer, both in the gold and platinum case. Hence, the internal morphology of islands, with a gap between platinum clusters, cannot be explained by steric arguments involving a coating of clusters. (iv) The origin of platinum nanoparticle self-organization could be attributed to the pinning of clusters on a hexagonal array of natural traps (i.e., highly ordered concentration of single defects) present on the surface before deposition. Note that, in spite of extensive studies made on graphite, such a high concentration of point defects has never been observed to our knowledge.42 Further, this would mean that the cluster’s film morphology is highly dependent on the quality of the graphite surface, and one might imagine that some defects were present on the substrate used for Pt-clusters deposition while this was not the case for the deposition of Au-clusters. Even if such a trapping on natural defects appears unrealistic, since the

reproducibility of our results has been tested and since the selforganized platinum morphologies are observed over the entire sample, it is worth envisaging. To see if the difference in behavior between gold and platinum clusters remains when the very same substrate is used, we have successively deposited Au and Pt clusters on the same HOPG surface (the sample remaining under UHV). As can be seen in Figure 6, the morphology of the as-prepared thin film is similar to that obtained in the case of pure gold or platinum cluster deposition. As a matter of fact, gold clusters stick to each other, as usual, leading to ramified islands which then act as nucleation centers for Pt clusters. Once more, platinum nanoparticles stay close together without contact. This experiment discards a possible effect of localized surface defects. Furthermore, one can notice from Figure 6a that some “bunches” of clusters made only of platinum nanoparticles are localized between Au-Pt islands. This suggests that nucleation of platinum “bunches” of clusters does not occur exclusively around gold islands. In other words, this confirms that NPt islands > NAu islands. Since FPt ≈ FAu and since both depositions have been performed with the same substrate temperature, eq 1 suggests that DPt < DAu. A quantitative analysis of the diffusion coefficient of nanoclusters on graphite is not the purpose of this study, but this relative comparison should be kept in mind for the discussion in section 4.

Bare Platinum Nanoclusters Finally, the complementary studies reported in this section clearly point out that the astonishing morphology obtained by platinum clusters deposition on graphite seems to be specific to this element. They demonstrate that the hypothesis that various experimental parameters (points i, iii, and iv, above) give rise to the unusual behavior observed can be discarded. In the remainder of the present work, we will discuss some possible physical mechanisms which could be involved in the selforganization process of platinum nanoparticles on graphite. The systematic gap observed between platinum particles can either originate from a dissociation of clusters previously in contact (or having partially coalesced), or it can be due to a repulsive interaction which prevents nanoparticles to come into contact when they diffuse on the substrate’s surface. 4. Discussion Reversible sticking has been previously reported43 in the case of metallic clusters deposited on HOPG, where fractal islands grown by nanoparticle agglomeration evolve under heating. In this case, the Rayleigh instability leads to a fragmentation of the ramified structure’s arms, ending in individual clusters of well-defined diameter. However, such a phenomenon can be excluded in our case for several reasons: (i) with this physical mechanism, the diameter of the resulting isolated clusters would not be the one of the incident clusters; (ii) moreover, the interparticle distance would not be compatible with the firstneighbor distance observed in our samples, and this would not explain the appearance of a hexagonal order. The absence of contact between platinum clusters must then be due to a short-range repulsive interaction. Note that there always exists an attractive force (of van der Waals type) between clusters which is responsible for the formation of nanoparticles “bunches” or islands. Repulsion between platinum nanoparticles can either be a direct cluster-cluster interaction or it can be mediated by the substrate. Depositing clusters on various substrates would then be an opportune way to investigate the influence of surface. However, there is a limited choice of substrates where nanoparticles can easily diffuse and HOPG appears to be the only one, to our knowledge, allowing a convenient TEM observation after cluster deposition. This observation is necessary because near field microscopy (atomic force microscopy or STM) does not allow to precisely probe the structure inside clusters islands due to the unavoidable tipconvolution effect. Nevertheless, considering the specific case of a graphite surface, some considerations indicate that the substrate could indeed be involved in the self-organization process of platinum clusters. A strong cluster-surface interaction in the case of platinum is consistent with the fact that the DDA model, which assumes that such interactions are negligible, is unable to account for the morphology observed. On the contrary, the DDA model gives a successful description of Au-island morphology, which implies that interaction between gold clusters and graphite can indeed be neglected. The difference in morphology between Au and Pt clusters films could be traced back to a strong Ptgraphite interaction. Many X-ray Photoelectron Spectroscopy (XPS) studies have been conducted on Au and Pt, and on isoelectronic systems.44-47 They clearly indicate that, contrary to d10 metals, d9 metals, like Pt, strongly interact with graphite. This has been explained by the hybridization of the empty d state of d9 metals with the π band of graphite. Moreover, Yang et al.48 have shown that this hybridization was related to a decrease of the diffusion coefficient in the case of Ni clusters. This is consistent with our observations because existence of

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6847 islands made only of Pt nanoparticles, in the sample where Au and Pt clusters have been deposited on the same surface, indicates that diffusion is harder for platinum clusters than for gold clusters. In addition, the fact that, in this Au + Pt sample (see Figure 6), platinum nanoparticles are not in contact with gold islands, whereas gold clusters have partially coalesced suggests that the morphology is rather driven by a substratemediated force, than by a direct cluster-cluster interaction. Even though the physical mechanism leading to the observed morphology of 2D Pt clusters films is not fully understood, different phenomena involving an interaction between platinum nanoparticles and graphite can be envisaged. First, the mismatch between two graphene layers can create periodic oscillations of the electronic local density of states (LDOS), called moire´s, with a period of the same order of magnitude as d0. Sattler et al.49 have observed that Co nanoparticles can be trapped on these moire´s and more recently, N’Diaye et al.50 have demonstrated the possibility to obtain an ordered array of Ir nanoparticles on artificially created moire´s. However, while there are a huge number of possible values for the period of moire´ fringes,51 we always observe the same distance d0 between Pt clusters in all of our samples, and it seems highly improbable that the four graphite substrates we used have the same mismatch (and the same type of moire´ all along the surface). In addition, for unselected Pt clusters, this hypothesis is unable to account for the observation of a welldefined nearest neighbor distance, which is not accompanied by a local hexagonal order; and for size-selected Pt clusters, we have found that there is no correlation between the clusters organization in two distinct islands. This allows us to rule out the explanation of platinum nanoparticles self-organization by the presence of LDOS moire´ patterns. Second, interaction between Pt nanoparticles and graphite should modify the electron localization. On one hand, a charge transfer between the cluster and the substrate,52 together with an electronic cloud relaxation, can lead to a multipolar electrostatic interaction between particles (and a subsequent short-range repulsion). It is worth mentioning that interparticle electrostatic interactions, in particular dipole-dipole interactions, have been shown to be determinant for some nanoparticle superstructures.53 On the other hand, it has been demonstrated that interaction between an atom and a metallic substrate can create LDOS periodic oscillations around its adsorption site (called Friedel oscillations). This latter phenomenon can also lead to a self-organization of atoms in a hexagonal array with a lattice parameter very close to d0.54,55 Periodic oscillations of LDOS, evidenced by STM measurements, have also been reported56 on graphite near platinum nanoparticles (0.8 nm in diameter). However, in this case, the periodicity is not of the order of d0, and the system studied is very different from ours, in particular our particles are much bigger. In addition, in our case, no electronic density modulation could be observed by STM around platinum nanoparticles, which makes the usual Friedel oscillation model quite unlikely. Finally, we can envisage that the graphene sheet supporting clusters is distorted, so that, even though clusters are able to move on the surface, some kind of “bump” or deformation of the carbon lattice surrounds each platinum nanoparticle and accompanies it during its diffusion. This could then result in a short-range repulsion (due to a steric or an elastic effect) between Pt clusters, which is a necessary ingredient to explain their selforganization on graphite surface. Although ab initio calculations have predicted irreversible sticking of neighboring platinum clusters on frozen graphite57 (i.e., without any deformation), to

6848 J. Phys. Chem. C, Vol. 112, No. 17, 2008

Tainoff et al.

Figure 7. (a) Typical TEM morphology of an island formed by the deposition on graphite of size-selected platinum clusters of 1.6 nm in diameter. (b) Dependence of the interparticle distance [center-to-center (squares); edge-to-edge (triangles)] on the cluster diameter, in the case of platinum clusters deposited without size selection (see Figure 3). Error bars represent the standard deviation of interparticle distance for each diameter.

our knowledge, there is unfortunately no molecular dynamics study on this system where a distortion of graphite would be allowed. On a different but comparable system, a recent molecular dynamics study of Ni nanoparticles on a graphene sheet has shown that the carbon lattice gets twisted to finally coat the Ni cluster.58 This is consistent with the efficient use of Ni as a catalyst for carbon nanotube growth. Since platinum has the same catalytic properties and the same electronic configuration as Ni, it is likely that Pt cluster deposition on graphite will result in a local deformation of the carbon lattice. Such a phenomenon may be a sound trail to understanding of mechanisms leading to Pt cluster self-organization on graphite. As a matter of fact, electronic structure modification and structural distortion are both consequences of the electronic hybridization change due to Pt cluster adsorption. These effects then cannot be separated, and need to be both considered in a realistic molecular dynamics simulation allowing a full structural relaxation (cluster and substrate). Such a study may provide valuable information, allowing us to describe the impact of Pt cluster adsorption on the graphite surface structure, and on the electronic properties. As far as the interparticle distance is concerned, the various possible phenomena leading to short-range repulsion of Pt clusters could manifest themselves differently. Indeed, a process could result in a constant center-to-center nearest neighbor distance while another could result in a constant edge-to-edge distance. The latter case is what is expected in the case of a steric repulsion between particles, due to a pollution layer around particles. In the case of moire´ patterns on graphite, or a high concentration of ordered defects (present on the surface before cluster deposition), we expect that the center-to-center distance will be the constant one, but as discussed earlier, this hypothesis has been ruled out. The hypothesis of an interaction originating from an electronic modulation is more complicated, and we would rather expect a constant center-to-center distance than a constant edge-to-edge distance. Finally, the hypothesis of a deformation of the graphite surface around Pt nanoparticles is in favor of a constant edge-to-edge separation between clusters, even if, of course, the extension of the carbon “bump” around particles can slightly depend on their size. In order to try to discriminate between the previous models, we have deposited smaller size-selected Pt clusters (with a diameter of 1.6 nm) on graphite to see if any size effect would be visible. The cluster film morphology is the same as for Pt

clusters of 2.2 nm in diameter (see Figure 7). Unfortunately, a distance analysis does not allow us to draw a clear-cut conclusion: due to the dispersion and the measurement uncertainty, both center-to-center and edge-to-edge distances are very similar to those measured for clusters of 2.2 nm diameter. Besides, since there is significant size dispersion in the case of unselected Pt clusters, we can check if there is a detectable correlation between cluster size and separation (center-to-center or edge-to-edge) with the nearest neighbor. As can be seen from Figure 7b, although again the result is not obviously in favor of one of the two extreme possibilities (constant center-to-center or edge-to-edge distance), it seems that center-to-center distance in fact varies with cluster diameter. Therefore, we think that the physical process driving Pt cluster self-organization and short-range repulsion rather fixes the edge-to-edge distance. Since we have evidenced that there is no thick pollution layer around the Pt particles, this result makes the hypothesis of a substrate deformation around the nanoparticles a little bit more likely than that of an interparticle interaction purely originating from electronic modulations. 5. Conclusions We have evidenced for the first time that it is possible to organize “naked” (i.e., synthesized in UHV conditions, without any pollution or ligand) platinum nanoparticles in a regular hexagonal pattern. Moreover, these ultimate integration scale arrays of size-selected nanoparticles are obtained by selforganization, at room temperature. Using various experiments, with well-controlled conditions, we have shown that, while in the case of gold clusters it is impossible to self-organize bare nanoparticles, this phenomenon must be specific to platinum clusters and other isoelectronic elements. At present, experimental results have pointed out the role of the graphite substrate in interparticle interaction, even if the physical phenomenon responsible for short-distance repulsion is still unclear. Nevertheless, among all the possible explanations envisaged, a deformation of the substrate surface and/or an electronic structure modification appears to be the more relevant. A molecular dynamics investigation is in progress in our laboratory, and should shed light on the underlying physical mechanisms. We emphasize that arrays of bare nanoparticles are ideal model systems for fundamental catalysis investigations. Taking

Bare Platinum Nanoclusters advantage of the well-defined interparticle distance, we should be able to study interdiffusion of species between catalyst particles (spillover processes), and we plan to perform in situ hydrogen chemisorption experiments. Such dense arrays of sizeselected, ligand-free nanoparticles should exhibit a tremendous specific area for catalytic reactions. Acknowledgment. The authors are indebted to M. Pellarin for valuable discussion and to B. Kuhlmey for his reading of the manuscript. References and Notes (1) Hughes, M. D.; Xu, Y-J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2005, 437, 1132. (2) Haruta, M. Catal. Today 1997, 36, 153. (3) Haruta, M.; Yamada, N.; Kobayashi, T.; Ijima, S. J. Catal. 1989, 115, 301. (4) Weiher, N.; Bus, E.; Delannoy, L.; Louis, C.; Ramaker, D. E.; Miller, J. T.; Van Bokhoven, J. A. J. Catal. 2006, 240, 100. (5) Ichikawa, S.; Akita, T.; Okumura, M.; Haruta M.; Tanaka, K. Mater. Res. Soc. Proc. 2002, 727, 11. (6) Lopez, N.; Janssens, T. V. W.; Clausen, B. S.; Xu, Y.; Mavrikakis, C. M.; Bligaard, T.; Nørskov, J. K. J. Catal. 2004, 223, 232, and references therein. (7) Joo, S. H.; Jae Choi, S.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (8) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (9) Welch, C. M.; Compton, R. G. Anal. Bioanal. Chem. 2006, 384, 601. (10) Giordano, L.; Illas, F.; Ro¨sch, N. Surf. Sci. 2002, 499, 73. (11) Chia, H.; Cuonga, N. T.; Tuana, N. A.; Kima, Y-T.; Baoa, H. T.; Mitania, T.; Ozakib, T.; Nagaoc, H. Chem. Phys. Lett 2006, 432, 213. (12) Henry, C. R. Surf. Sci. Rep. 1998, 31, 231-325, and references therein. (13) Freund, H.-J.; Bau¨mer, M.; Libuda, J.; Risse, T.; Rupprechter, G.; Shaikhutdinov, S. J. Catal. 2003, 216, 223-235. (14) Somorjai, G. A.; Contreras, A. M.; Montano, M.; Rioux, R. M. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10577-10583. (15) Jacobs, P. W.; Ribeiro, F. H.; Somorjai, G. A.; Wind, S. J. Catal. Lett. 1996, 37, 131. (16) Jacobs, P. W.; Wind, S. J.; Ribeiro, F. H.; Somorjai, G. A. Surf. Sci. 1997, 372, L249. (17) Eppler, A. S.; Zhu, J.; Anderson, E. A.; Somorjai, G. A. Top. Catal. 2000, 13, 33-41. (18) Gustavsson, M.; Fredriksson, H.; Kasemo, B.; Jusys, Z.; Kaiser, J.; Jun, C.; Behm, R. J. J. Electroanal. Chem. 2004, 568, 371-377. (19) Dietz, T. G.; Duncan, M. A.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1981, 74, 6511-6512. (20) Milani, P.; deHeer, W. A. ReV. Sci. Instr. 1990, 61, 1835-1838. (21) Melinon, P.; Paillard, V.; Dupuis, V.; Perez, A.; Jensen, P.; Hoareau, A.; Broyer, M.; Vialle, J. L.; Pellarin, M.; Baguenard, B.; Lerme, J. Int. J. Mod. Phys. B 1995, 9, 339-397. (22) Pauwels, B.; Van Tendeloo, G.; Bouwen, W.; Kuhn, L. T.; Lievens, P.; Lei, H.; Hou, M. Phys. ReV. B 2000, 62, 10383-10393. (23) Paillard, V.; Me´linon, P.; Dupuis, V.; Perez, J. P.; Perez, A.; Champagnon, B. Phys. ReV. Lett. 1993, 71, 4170. (24) Nicolas, D.; Masenelli, B.; Melinon, P.; Bernstein, E.; Dujardin, C.; Ledoux, G.; Esnouf, C. J. Chem. Phys. 2006, 125, 171104.

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6849 (25) Alayan, R.; Arnaud, L.; Bourgey, A.; Broyer, M.; Cottancin, E.; Huntzinger, J. R.; Lerme, J.; Vialle, J. L.; Pellarin, M.; Guiraud, G. ReV. Sci. Instr. 2004, 75, 2461. (26) Me´tois, J. J.; Heyraud, J. C.; Takeda, Y. Thin Solid Films 1978, 51, 105. (27) Abbet, S.; Heiz, U.; Schneider, W. D.; Pacchioni, G.; Ferrari, A. M.; Ro¨sch, N. J. Am. Chem. Soc. 2000, 122, 3453. (28) Yang, D.-Q.; Sacher, E. Chem. Mater. 2006, 18, 1811. (29) Prevel, B.; Bardotti, L.; Fanget, S.; Hannour, A.; Melinon, P.; Perez, A.; Gierak, J.; Faini, G.; Bourhis, E.; Mailly, D. Appl. Surf. Sci. 2004, 226, 173. (30) Bardotti, L.; Pre´vel, B.; Treilleux, M.; Me´linon, P.; Perez, A. Surf. Sci. 2000, 164, 52. (31) Hahn, J. R. Carbon 2005, 43, 1506. (32) Jensen, P.; Barabasi, A. L.; Larralde, H.; Havlin, S.; Stanley, H. E. Phys. ReV. B 1994, 50, 15316. (33) Bardotti, L.; Jensen, P.; Hoareau, A.; Treilleux, M.; Cabaud, B. Phys. ReV. Lett. 1995, 74, 4694. (34) Pimpinelli, A.; Villain, J.; Wolf, D. E J. Phys. I 1993, 3, 447. (35) Villain, J.; Pimpinelli, A.; Tang, L.; Wolf, D. J. Phys. I 1992, 2, 2107. (36) Carlier, F.; Benrezzak, S.; Cahuzac, Ph.; Kebaili, N.; Masson, A.; Srivastava, A. K.; Colliex, C.; Brechignac, C. Nano Lett. 2006, 6, 1875. (37) Lando, A.; Kebaili, N.; Cahuzac, Ph.; Masson, A.; Brechignac, C. Phys. ReV. Lett. 2006, 97, 133402. (38) Couillard, M.; Pratontep, S.; Palmer, R. E. Appl. Phys. Lett. 2003, 82, 2595. (39) Alayan, R.; Arnaud, L.; Broyer, M.; Cottancin, E.; Lerme´, J.; Marhaba, S.; Vialle, J. L.; Pellarin, M. Phys. ReV. B 2007, 76, 075424. (40) Pellarin, M. private communication (in the case of size-selected gold clusters deposited on graphite in classical vacuum there is no contact between the particles). (41) Kalff, M.; Comsa, G.; Michely, T. Phys. ReV. Lett. 1998, 81, 1255. (42) Kushmerick, J. G.; Kelly, K. F.; Rust, H. P.; Halas N. J.; Weiss, P. S. J. Phys. Chem. B 1999, 103, 1619. (43) Brechignac, C.; Cahuzac, P.; Carlier, F.; Colliex, C.; Leroux, J.; Masson, A.; Yoon, B.; Landman, U. Phys. ReV. Lett. 2002, 88. (44) Fauth, K.; Schneider, N.; Hessler, M.; Schutz, G. Eur. Phys. J. D 2004, 29, 57. (45) Buttner, M.; Oelhafen, P. Surf. Sci. 2006, 600, 1170-1177. (46) Yang, D.-Q.; Zhang, G.-X.; Sacher, E.; Jose-Yacaman, M.; Elizondo, N. J. Phys. Chem. B 2006, 110, 8348, and references therein. (47) Cini, M.; Decrescenzi, M.; Pattela, F.; Motta, N.; Sastry, M.; Rochet, F.; Pasquali, R.; Balzarotti, A.; VerdozziI, C. Phys. ReV. B 1990, 41, 56855695 (48) Yang, D.-Q.; Sacher, E. J. Phys. Chem. B 2005, 109, 19329. (49) Xhie, J.; Sattler, K.; Ge, M.; Venkateswaran, N. Phys. ReV. B 1993, 47, 15835-15841. (50) N’Diaye, A. T.; Bleikamp, S.; Feibelman, P. J.; Michely, T. Phys. ReV. Lett. 2006, 97, 215501. (51) Pong, W. T.; Durkan, C. J. Phys. D: Appl. Phys. 2005, 38, R329R355. (52) Albe, K.; Nordlund, K.; Averback, R. S. Phys. ReV. B 2002, 65, 195124. (53) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Titov, A. V.; Kra´l P. Nano Lett. 2007, 7, 1213-1219. (54) Crommie, M. F.; Lutz, C. P.; Eigler, D. M. Nature 1993, 363, 524527. (55) Silly, F.; Pivetta, M.; Ternes, M.; Patthey, F.; Pelz, J. P.; Schneider, W. D. Phys. ReV. Lett. 2004, 92, 016101. (56) Xhie, J.; Sattler, K.; Mu¨ller, U.; Venkateswaran, N.; Raina, G. Phys. ReV. B 1991, 43, 8917. (57) Liem, S. Y.; Chan, K. Y. Surf. Sci. 1995, 328, 119-128. (58) Shibuta, Y.; Elliott, J. A. Chem. Phys. Lett. 2006, 427, 365-370.