Formation of Pt and Rh Nanoclusters on a Graphene MoirÃ© Pattern on

Article pubs.acs.org/JPCC

Formation of Pt and Rh Nanoclusters on a Graphene Moiré Pattern on Cu(111) Esin Soy, Zhu Liang, and Michael Trenary* Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States ABSTRACT: The formation of Pt and Rh nanoclusters on a graphene moiré pattern on Cu(111) was studied with ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Isolated graphene islands with different periodicities were successfully grown on the Cu surface. As a result of weak coupling to the Cu substrate, different graphene rotational domains were observed including one with a periodicity of 4 nm that has not been previously reported. Furthermore, our results have shown that Pt and Rh form dispersed nanoclusters on graphene/Cu(111) with Pt forming more regular structures trapped mostly at the hollow sites. This variation in growth behavior is mainly attributed to differences in Pt−carbon and Rh− carbon interaction energies. Thermal stability experiments were also performed, and Pt clusters were found to be structurally stable up to 700 K.

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INTRODUCTION Since its discovery in 2004, graphene has attracted a great deal of attention due to its novel chemical, electrical, and mechanical properties.1−5 Among many applications of graphene, which include electronics,6 sensors,7 biodevices,8 catalysis,9 and energy storage,10 one of the most promising is its use as a template for the formation of two-dimensional arrays of monodispersed metal nanoparticles. The lattice mismatch between the graphene layer and the underlying substrate surface results in moiré structures that offer a range of different binding sites for adatoms and small clusters. These binding sites were theoretically and experimentally shown to serve as a template for monodispersed nanoclusters that have high thermal stability and catalytic activity.11−14 It has also been reported that clusters of different sizes exhibit distinctly different catalytic activities.12 Templated metal nanoclusters therefore provide a unique opportunity for probing size-dependent reactivity in heterogeneous catalysis. As other researchers have found, in using graphene as a template, each combination of substrate and deposited metal yields distinct properties that are difficult to predict in advance. N’Diaye et al. first demonstrated that epitaxial graphene on Ir(111) can provide a novel template for the formation of twodimensional Ir, Pt, W, and Re cluster arrays.15 It was shown that Pt and Ir form long-range ordered epitaxial cluster superlattices, while W and Re sinter at the very early stages of growth.15 Zhou et al. explored the factors that govern the growth of Pt, Rh, Pd, Co, and Au clusters on graphene on Ru(0001).16 Their experimental results show that Pt and Rh form small dispersed nanoclusters, while Pd and Co form larger clusters at similar coverages. These results, coupled with previous observations that Ir and Pt are capable of forming regularly distributed clusters on a graphene moiré pattern with a narrow size distribution, suggest that both metal−carbon dissociation and © XXXX American Chemical Society

metal cohesive energies play significant roles in the cluster formation process. Furthermore, their experimental results show that Au behaves differently and forms a single-layer film on graphene, indicating that other factors such as growth temperature, metal substrate, and lattice matching should also be considered.16 To further explore the use of graphene templates for metalnanocluster growth, we have used graphene on Cu(111) as a substrate for the fabrication of Pt and Rh clusters. The use of Cu(111) instead of Ir(111) or Ru(0001) can potentially demonstrate that a lower cost metal like Cu can serve as the base for an array of nanoclusters of a precious metal with unique catalytic properties, such as Pt and Rh. Such a demonstration will motivate the search for ways to produce practical catalysts that make minimal use of rare and expensive metals. In addition, as copper is not a good catalyst for hydrogenation reactions whereas platinum and rhodium are, it is important to explore model systems consisting of cluster arrays of a metal with very different catalytic properties from those of the underlying substrate metal.

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EXPERIMENTAL METHODS The experiments were carried out in an UHV chamber with a base pressure of 5 × 10−10 Torr, and the STM images were obtained at room temperature with an Omicron variabletemperature SPM (scanning probe microscope) system. Images were acquired with Omicron SCALA PRO 4.0 software, and data processing was done with the WxSM program provided by Nanotech. The Cu(111) surface was cleaned by repeated cycles of sputtering with argon (4 × 10−6 Torr) and annealing to 1300 Received: July 6, 2015 Revised: September 16, 2015

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DOI: 10.1021/acs.jpcc.5b06472 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C K. The surface was verified to be clean and well ordered by lowenergy electron diffraction (LEED) and STM. Graphene was prepared on Cu(111) by first exposing to 1 × 10−5 Torr of ethylene for 5 min at room temperature, followed by a quick anneal to 1300 K. Then the sample was cooled to ∼500 K, and the process was repeated 15 times to achieve a high graphene coverage, which was found to vary across the Cu(111) surface based on the analysis of larger STM images. After a brief final anneal to 1100 K, the sample was transferred to the STM stage for data acquisition. All STM images were taken at room temperature. Pt and Rh were deposited on the graphene layer via evaporation from pure Pt and Rh rods by a triple electron beam evaporator (EFM 3T) with integral flux monitor. The evaporation rates for Pt and Rh were 0.96 × 10−3 ML/s and 6.13 × 10−3 ML/s, respectively. After careful analysis of the images, no visible impurities were detected either on graphene or on the deposited clusters. The sample was held at room temperature during deposition. Results were confirmed by reproducible values of particle heights and diameters that were measured with different STM tips. Diameter and height histograms were generated by the analysis of larger STM images and of areas of different sizes for the different domains.

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RESULTS Moiré Pattern of Graphene on Cu(111). Topographic STM images of three different areas containing bare Cu(111), as well as regions of monolayer graphene, are shown in Figure 1(a), (b), (c), and (d). These results reveal that isolated large graphene islands were successfully grown on the Cu surface with different periodicities. Four types of moiré superstructures with different orientations can be clearly seen in Figure 1. The four most prevalent moiré patterns have periodicities of 2, 4, and 5.6 nm, with rotational angles of 7°, 3°, and 1.2°, and 6.6 nm in which the lattice orientation of graphene is aligned with the Cu(111) lattice. Previously, the rotated and aligned phases of graphene on Cu (111) were observed with the periodicities of 1.5,17 2, 5.6, and 6.6 nm.18 The STM image shown in Figure 1(b) reveals a new rotational phase with a periodicity of 4 nm that has not been reported previously. The four moiré patterns in Figure 1 are of two types. In panels (a) and (d), the lattice appears as bright spots enmeshed in a dark background and in panels (b) and (c) as dark spots enmeshed in a light background. As shown below, the 2 and 6.6 nm moiré lattices of panels (a) and (d) in Figure 1 appear with the opposite contrast, possibly as a result of an occasional random change in the structure of the tip apex. Uncontrolled tip change during scanning has been found to cause contrast inversion in topographic images by Mallet et al.19 Contrast inversion can also occur for different bias voltages as observed for graphene on Ir(111) by N’Diaye et al.20 and explored in detail by Dedkov and Voloshina.21 The latter study demonstrates that contrast inversion originates predominantly from differences in tunneling bias and current and distance from the sample. Because there is not much difference in bias conditions between our images in Figure 1, a change in the tip seems a more likely origin for the contrast inversion that we observe. An atomic model of the moiré superstructure on Cu(111) is shown in Figure 1e. Three different sites, atop, fcc, and hcp, are marked by a circle, a triangle, and a square, respectively. Metal Nanoclusters on Graphene/Cu(111). To establish the properties of Pt and Rh on the bare batches of the graphene-covered Cu(111) surface, the images in Figures 2 and 3 were obtained for the surface without graphene. The

Figure 1. (a−d) STM topographic images of the four most commonly observed moiré domains. (a) STM image of the smallest observed moiré pattern with a periodicity of 2 ± 0.3 nm with rotational angle of 7° (Scale: 200 × 200 nm) (Inset: 10 × 10 nm). (b) STM image of a graphene island with a new rotational domain. The periodicity is 4 ± 0.2 nm with rotational angle of 3° (Scale: 200 × 200 nm) (Inset: 10 × 10 nm). (c) STM image of a moiré domain that has a periodicity of 5.6 ± 0.1 nm with a rotational angle of 1.2° (Scale: 100 × 100 nm). (d) STM image of the largest observed moiré pattern with a periodicity of 6.6 ± 0.2 nm that is aligned with the underlying Cu(111) lattice (Scale: 100 × 100 nm). The black rhombuses indicate the unit cells. The images were recorded at V = 0.50−0.70 V and I = 0.40−0.60 nA. (e) Model of the moiré superstructure on the Cu(111) surface (blue). The second and third layers of Cu are colored dark and light gray, respectively. The circle, triangle, and square mark atop, fcc, and hcp regions, respectively. This structural model follows that of ref 30.

deposited Pt forms brims and fingers on Cu step edges as can be seen in Figure 2(a). Previous studies indicated that Pt and Cu are capable of intermixing at 315 K and forming multiple metastable states.22 In addition, three-pointed isolated dendritic islands on wide terraces were observed at relatively high Pt coverages. Dendritic or fractal islands occur naturally during epitaxial growth in systems where island edge diffusion is restricted due to different step energies or adatom diffusion barriers between steps. It has been reported that the number of Cu atoms on the surface can be increased by exchange between the deposited Pt and the Cu substrate or by diffusion of Cu atoms from step edges. As a result, the islands consist of a mixture of Pt and Cu atoms.23 In contrast to the structure of deposited Pt, Rh does not form dendritic islands on Cu(111) but instead diffuses to the B


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The structures adopted by Pt and Rh on the bare Cu(111) surface are in marked contrast to the structures formed after deposition onto the graphene monolayer. Figure 4 shows STM

Figure 2. STM topographic images of (a) fingers/brims (the black circle indicates an elongated finger/brim structure formed along the Cu step edge) and (b) Pt-rich dendritic islands formed on terraces after deposition of 0.056 ML Pt on Cu(111) at 300 K. The images were recorded at V = 0.70 V and I = 0.40 nA (Scale: 200 × 200 nm).

Figure 4. STM topographic images (Scale: 60 × 60 nm) of (a) 0.075 ML, (b) 0.31 ML, and (c) 0.83 ML of Pt deposited on graphene/ Cu(111) at room temperature. The corresponding distribution of the cluster diameters and heights are given in (d, g) for 0.075 ML, (e, h) for 0.31 ML, and (f, i) for 0.83 ML. The images were recorded at V = 0.70 V and I = 0.40 nA.

topographs and size-distribution histograms for different Pt coverages on graphene/Cu(111), which reveal that at the later stages of cluster growth a broad range of cluster diameters and heights occur. At a coverage of 0.075 ML, the average cluster diameter was 2.8 nm, and 92% of clusters have heights corresponding to a single Pt-atom layer, indicating strong bonding between Pt and graphene as has been previously reported by other groups. Nakamura et al. showed that flat clusters with 1−2 Pt atoms in height and 1.5−5.0 nm wide can be formed on HOPG (highly oriented pyrolytic graphite) as a result of the strong hybridization between filled 5d orbitals of Pt and empty π* graphite states.26,27 At higher coverages of 0.31 and 0.83 ML, the average cluster diameters were 3.8 and 5.5 nm, respectively, and the heights correspond to three to four layers of Pt. The size and height distributions that we observe are similar to those reported by others. At 0.25 ML of Ni on graphene on a Rh(111) substrate, Sicot et al.28 showed histograms indicating an average diameter of 3.1 nm with a spread of about ±0.5 nm. Pan et al.29 constructed diameter histograms for Pt particles of a given height for Pt on graphene on a Ru(0001) surface and found a symmetric and narrow distribution with a spread of about ±1 nm. For the highly ordered Ir nanocluster arrays on graphene on Ir(111), N’Diaye et al.30 provide a histogram of cluster sizes expressed in number of Ir atoms for a coverage of 0.80 ML showing an average of about 70 atoms, with a distribution width of about ±10 atoms. Upon deposition of 0.31 ML of Pt, isolated Pt nanoclusters form on the different graphene rotational domains as labeled in Figure 5(a) and (b). The size and distribution of the Pt clusters

Figure 3. STM topographic images of Rh-rich step edges on Cu(111) formed after deposition of Rh at 300 K (Scale: 200 × 200 nm). (a) 2D and (c) 3D image of a Cu−Rh alloy formed along step edges by deposition of 0.012 ML at 300 K. (b) 2D and (d) 3D image taken after subsequent annealing to 500 K. Insets show zoomed images of the marked areas (Inset scale: 10 × 10 nm). The images were recorded at V = 0.70 V and I = 0.40 nA.

step edges where it appears to form a Rh−Cu alloy. Figure 3(a) shows a slight increase in brightness along the Cu step edges as a result of alloy formation. The behavior of Rh on Cu surfaces is qualitatively well explained by previous experimental and theoretical studies.24,25 These studies indicate that Rh should replace Cu below the surface so that the topmost atomic layer would consist predominately of Cu. The width of the Rh-rich step edges is found to be about 2 nm with a normal step height of one atomic layer. Although the brightness along the step edges remains intact up to 500 K, rough step edges become more distinct at higher temperatures as can be seen in Figure 3(b). Although not apparent in the figure, small clusters are also observed after annealing to 500 K. A strong tendency for Cu to remain on the surface as the Rh film grows was observed for higher temperatures, consistent with previous studies. Indeed, rapid diffusion of Cu to the surface of the Rh film occurs even at 300 K as a result of moderate solubility of Rh in Cu.25 C


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lower density (Figure 5e). The average cluster diameter is about 3.5 nm, and the height corresponds to 3−4 layers. Figure 6 shows an STM image and a diameter histogram for 0.41 ML of Rh on a graphene island on the Cu(111) surface. Compared to 0.31 ML Pt (Figure 4b and 4e), Rh clusters are noticeably larger at a given coverage. It is clear that Rh forms bigger clusters with average cluster sizes of ∼7 nm with heights of two to three layers. At a higher coverage of 3.5 ML, the average cluster diameter was 20 nm, and their heights correspond to 8−9 layers of Rh (results not shown). Compared to Pt on graphene/Cu(111), the high cluster sizes suggest that coalescence and coarsening are more important in Rh growth at higher Rh coverages as a result of a lower Rh−C dissociation energy (Table 1). Table 1. Cohesive Energy Ecoh and Metal−Carbon Dissociation Energies D(M−C) of the Pt and Rh15,16 metal

lattice parameter (nm)

Ecoh (eV)

D(M−C) (eV)

Pt Rh

0.277 0.269

5.84 5.75

6.32 6.01

Temperature Stability. For applications of nanoclusters in heterogeneous catalysis, thermal stability is a crucial factor. Figure 7 shows the effect on the Pt clusters of annealing to 700 and 800 K for a Pt coverage of 0.31 ML. No major changes in the size or density of the clusters for annealing temperatures below 700 K were observed. However, significantly lower densities and larger sizes were observed for annealing temperatures of 700 K or higher. The distribution of the clusters shifted toward larger sizes, and the coexistence of clusters with heights of 5 to 10 Pt atomic layers was observed for annealing to 800 K and above. This is a much greater degree of coarsening than observed as a function of increasing coverage at room temperature as presented in Figure 4. In that case, at a Pt coverage of 0.83 ML, the maximum cluster height was only three to four atomic layers.

Figure 5. (a,b) STM topographic images of 0.31 ML Pt showing clusters on graphene/Cu(111) with moiré periodicities of 2.3, 4.2, and 5.5 nm. (c−h) The corresponding distributions of the cluster diameters and heights are color coded (pink, blue, and green histograms show 2.3, 4.2, and 5.5 nm periodicities, respectively). The images were recorded at V = 0.70 V and I = 0.40 nA (Scale: 200 × 200 nm). Total counts are based on larger images but of different size for the different domains.

vary among the different graphene domains. An analysis of the distance between individual clusters revealed that they decorate the graphene moiré patterns in such a way that the spacing between more than 80% of individual clusters on the three different graphene domains is multiples of 2.3 ± 0.5, 4.2 ± 0.4, and 5.5 ± 0.4 nm, the moiré periodicities seen in Figure 1(a, b, and c). The distances between the clusters were determined from larger STM images than those shown in Figure 5. Figure 5(c), (d), and (e) shows the analysis of the cluster size distributions on different moiré patterns. The clusters on the 2.3 nm moiré lattice show a narrow size distribution in both diameter and height (Figure 5c and f). The height of the clusters is about two atomic layers. Moreover, the clusters on the 4.2 nm moiré pattern are slightly broader (Figure 5d). The heights correspond to 2−3 Pt layers. Additionally, on the 5.5 nm moiré pattern, the clusters tend to be larger with a much

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DISCUSSION Moiré Pattern of Graphene on Cu(111). The image in Figure 1(d) showing that a single rotational domain of graphene on Cu(111) can be as big as 100 nm is consistent with other reports of graphene on Cu(111).18,31 On metals other than copper, much larger domains have been observed. For example, N’Diaye et al.20 showed a 125 × 250 nm STM image of a single graphene domain that was structurally coherent over several terraces and step edges of an Ir(111) surface. A similar single graphene domain was also reported on Ru(0001) by Donner and Jakob.32 As Wintterlin and Bocquet33

Figure 6. (a) STM topographic image of 0.41 ML of Rh deposited on graphene/Cu(111). The image was recorded at V = 0.70 V and I = 0.40 nA. Scale: (60 × 60 nm). (b, c) Histograms of the number of particles as a function of their diameters and heights at 0.41 ML. D


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proposed, presumably the same considerations apply; therefore, the high regions would correspond to the graphene ring centered above atop Cu sites, and the low regions correspond to hcp- and fcc-type areas. For graphene on Ir(111) and on Ru(0001) there is a slight but observable contrast in the STM images between the hcp and fcc regions. Such contrast was not observed here nor in the previous reports of graphene/ Cu(111).18,31 Metal Nanoclusters on Graphene/Cu(111). It is clear that cluster diameter and height increase gradually with increasing coverage. However, no clear coalescence was observed with increasing coverage in contrast to the case of Co on graphene/Ru(0001).34 Coalescence was also observed for Ag clusters on HOPG where their mobility was attributed to a weak interaction with the substrate.35 Previous studies have shown that the diffusion and coalescence of clusters at room temperature alter the cluster sizes in such a way that a broad cluster size distribution and reduced cluster density can be observed, especially at high coverage.12,15 In contrast, an increase in cluster density with increasing coverage was seen for Pt on graphene/Ru(0001)29 and for Ni on graphene/ Rh(111).28 The Pt clusters on the graphene moiré patterns on Cu(111) appear to grow layer-by-layer (2D), followed by 3D clustering.36 In this case, however, the 2D growth is limited to small islands before additional Pt atoms are added to the top of the 2D islands. This suggests that the growth is nucleated at certain sites of the moiré lattice and that Pt atoms diffuse to these sites and attach to the edges of the existing Pt islands. However, once the islands reach a certain size, it is more favorable for additional Pt atoms to add to the tops of the islands to produce 3D clusters. This transition from 2D to 3D growth presumably occurs when the 2D islands reach a size where the Pt atoms at the island perimeters can no longer interact with the more favorable sites of the graphene. Growth of Ru nanoclusters on three coexisting moiré patterns with different periodicities has been previously reported by Sutter et al. on graphene on large Ru grains of (0001) orientation.37 They find that an ordered array of Ru nanoclusters with a narrow size distribution only forms on a moiré pattern of 2.5 nm periodicity, whereas large randomly arranged clusters formed for periodicities of 1.7 and 1.2 nm. DFT calculations revealed that the 2.5 nm moiré had a high projected density of states on the carbon atoms at the Fermi level for the fcc site, leading to a binding energy for Ru at those sites that was higher than the Ru−Ru bond energy. Nucleation on the fcc sites was therefore favored leading to templated growth. In contrast, for the smaller moiré periodicities none of the binding energies for Ru on positions in the moiré unit cells were larger than the Ru−Ru binding energies, which led to growth of randomly distributed large Ru clusters. It would thus appear that monolayer graphene on Cu(111) is similar to the 1.2 and 1.7 nm moiré lattices on Ru in not providing sites where the Pt binding energy is significantly higher than the Pt− Pt binding energy so that the tendency toward templated growth is weak. Although detailed theoretical calculations of Pt−C binding energies at different moiré sites on Gr/Cu(111) are not yet available, our observation that the cluster−cluster distances show some correlation with the moiré periodicity suggests that there are preferred nucleation sites, but the bonding to these sites is not strong enough relative to the Pt− Pt bond strength to give rise to well-ordered arrays of small clusters.

Figure 7. STM topographic images of 0.31 ML Pt on graphene/ Cu(1111) annealed to (a) 700 K and (b) 800 K.

note in their review of graphene layers on metal surfaces, these systems probably represent the highest quality surface layers known. Although our graphene domains do not cover the surface and are smaller and of different orientations, this provides an advantage: it allows us to correlate cluster growth with graphene domain orientation in one experiment. In addition it allows us to use deposition of metal on graphenefree areas of the substrate to estimate the coverage of the deposited metal. Donner and Jakob32 also used this approach to estimate their metal coverages. In cases where the carbon atoms are resolved within the graphene layer, structural models have been proposed that show the relationship between high (bright) and low (dark) regions of the moiré lattices and the positions of the carbon atoms with respect to the substrate metal atoms. This applies to the case of “normal contrast” where the graphene areas that are displaced further from the surface appear brighter. For both normal and inverse contrast, the high areas appear as discrete units, bright or dark, enmeshed in a background of the opposite shading. In the cases discussed here, we will refer to high and low areas, regardless of whether they appear bright or dark. In the case of graphene/Ir(111), the model shows that the high regions correspond to where the graphene ring is centered above an atop Ir site. The low regions are of two types corresponding to whether the center of the graphene rings are located above hcp or fcc 3-fold hollow sites. In these low regions the carbon atoms (as opposed to the centers of the graphene rings) are positioned above the Ir atoms. It has been reported that such a geometry would maximize π−d hybridization between the graphene and the substrate20,30,32 and lead to shorter Ir−C bond lengths. Thus, the electronic structure and the bonding strength of graphene are not uniform over the moiré structure. For this reason, the positions within the moiré unit cell are referred to as atop, hcp, and fcc regions, with the latter two regions closer to the metal substrate. In all cases, the metal clusters nucleate in the low fcc or hcp areas rather than the high atop areas. In the case of graphene/Ir(111), metal clusters nucleate at the hcp sites. However, metals nucleate at the fcc sites for graphene/Ru(0001). Although specific models of the atomic structure of graphene on Cu(111) have not been E


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Cu(111) is indicated by the A and B labels on the unit cell in Figure 8. The Pt clusters observed in Figure 8 are too large to allow their position within the moiré unit cell to be specified. These observations indicate that the Pt atoms have a high mobility on the graphene surface at room temperature, which results in bigger cluster sizes than the size of one fcc or hcp region. Adatoms and small clusters up to a certain size would be likely to leave their unit cell during growth, which would give rise to the observed distribution of cluster sizes. This would also explain the existence of a large number of empty moiré unit cells. Table 1 compares properties of Pt and Rh that are relevant to the current study. Both metals have very similar characteristics and form highly dispersed nanoclusters on Gr/Cu(111). The differences in growth behavior can be explained by two key parameters, the cohesive energy and the metal−carbon dissociation energy, as has been widely discussed in the context of metals deposited onto graphene on various metal substrates.15,16,38 A higher cohesive energy will favor agglomeration of the deposited metal into larger clusters rather than nucleation at preferred sites of the graphene moiré lattice. However, due to decreased mobility, small clusters are more likely to form at low temperatures (lower than room temperature) as there will be insufficient energy to overcome barriers to formation of larger clusters. As a result, a high density of clusters with a narrow size distribution is observed. However, since the surface free energies of Pt and Rh are much higher than that of graphene, both metals should form 3D clusters when the temperature is high enough to induce mobility on the surface. Thus, at high temperatures as at room temperature the metal atom mobility will be high enough that cohesive energy and/or surface free energy differences alone cannot explain the differences between Rh and Pt growth. Metals that have similar cohesive energies can behave differently in the formation of nanoclusters on graphene, as has been reported previously. One example is the growth of Au (with a cohesive energy of 3.81 eV) and Pd (with a cohesive energy of 3.89 eV) on Gr/Ru(0001).16 As one would expect, dispersed Pd clusters were observed by STM on Gr/Ru(0001). On the other hand, this was not the case for Au, where formation of a single-layer gold film instead of clusters was observed. The different behaviors of Au and Pd were attributed to a slight difference in cohesive energies where metals with low cohesive energy such as Au tend to wet the surface more efficiently. Additionally, it has been concluded that the differences in the lattice parameters of Ru (0.265 nm), overlayer graphene (0.245 nm), and Au (0.288 nm) would result in the formation of a Au film where Au atoms on the overlayer graphene would match the underlying Ru surface, which has a larger lattice parameter than that of graphene. In our case, however, Pt and Rh display very similar cohesive energies as well as lattice parameters, as listed in Table 1. Thus, the difference in cluster formation cannot be described only by the cohesive energies and lattice parameters for the Gr/ Cu(111) system. Alternatively, the difference might be due to differences in the metal−carbon dissociation energy, which has been found to play a significant role in cluster growth on graphene.15 A higher metal−carbon dissociation energy leads to lower mobility, thus producing finely dispersed clusters trapped at active sites of the graphene. Metal−carbon dissociation energies for Pt and Rh were reported to be 6.32 and 6.01 eV, which indicates a slightly stronger interaction between graphene and Pt than between

To determine the nucleation sites, it is necessary to obtain images of the metal clusters in which the graphene moiré lattice can be resolved. Although such images were relatively rare, one example is shown in Figure 8 of 0.83 ML of Pt on a 6.6 nm

Figure 8. 50 × 50 nm STM image of Pt clusters in which the 6.6 nm graphene moiré lattice is resolved but with the contrast inverted from that observed in Figure 1d. The superimposed grid is centered on the high points of the lattice. The hcp and fcc areas are indicated by the A and B labels on the moiré unit cell. The image was recorded at V = 0.70 V and I = 0.40 nA.

graphene moiré lattice. Compared to the image of the 6.6 nm lattice shown in Figure 1d, this image shows contrast inversion such that the high areas appear as dark objects enmeshed in a light background, most likely as a result of an occasional and spontaneous tip apex change during scanning. The points of the superimposed grid correspond to the moiré high points. The diameters of the clusters are approximately the size of one moiré unit cell, which obscures the exact nucleation point. However, it is clear that the clusters are centered in one unit cell, as has been reported for clusters on graphene on Ir(111) and on Ru(0001),15,16 which indicates that there is a preferred nucleation site. Given the high mobility of Pt clusters at room temperature and occasional tip effects where clusters may appear more elongated than they really are, the spillover of some clusters to adjacent unit cells is also observed. Nevertheless, from examination of several images such as the one shown in Figure 8, it is clear that there is a tendency for the clusters to be centered above the low points. Because the carbon atoms undergo local sp2 to sp3 rehybridization in the low regions, they can also form strong bonds to the deposited metal atoms, making these sites the favored ones for nucleation of Pt clusters on the Gr/Cu(111) system as they are on Gr/ Ru(0001). Likewise, the nature of the interaction between the clusters and the different regions of the underlying graphene was theoretically and experimentally established for Ir clusters on Gr/Ir(111). N’Diaye et al. reported that in the fcc and hcp regions three out of six C atoms in a carbon hexagon sit above an Ir atom and covalently bond to it through overlap of the C 2pz and Ir(d3z2−r2) orbitals. This strong interaction weakens the graphene π-bonds enabling the clusters to readily bind to the remaining three carbon atoms.30 The preference found in previous studies for nucleation at one low site (either fcc or hcp) over the other low site is less pronounced compared to the preference for low sites over high sites. The distinction between the two low sites for Gr/ F


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The Journal of Physical Chemistry C graphene and Rh.39 Therefore, it is very likely that the stronger metal−carbon interaction reduces the mobility of Pt atoms and clusters, which were thereby trapped at the active sites of the graphene lattice and formed dispersed clusters. This also explains why Rh clusters are bigger in size and lower in density compared to Pt on Gr/Cu(111). Temperature Stability. The high thermal stability of metal clusters on graphene is attributed to the relatively high metal− carbon bond dissociation energies, which ultimately reduces coarsening and desorption of clusters from the surface.40 Thus, Pt clusters trapped at hollow sites are inhibited from diffusing and coalescing into larger clusters until relatively high temperatures are reached. These higher temperatures do not appear to result in intercalation of the Pt into the region between the graphene layer and the Cu(111) substrate. Intercalation has been reported for Co34 and a range of other metals41 on graphene/Ru(0001). The key indicator of intercalation was observation of an intact moiré pattern for the graphene above islands of intercalated metal. As we do not observe the characteristic features of intercalation as described by Huang et al.,41 we conclude that it does not occur in our case.

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CONCLUSIONS We find that deposition at room temperature of both Pt and Rh on graphene moiré lattices on Cu(111) produces dispersed nanoclusters. Platinum nanoclusters are found to preferentially nucleate at the hcp or fcc regions with a narrow size distribution as a result of a high metal−carbon dissociation energy. On the other hand, Rh nanoclusters of larger sizes and lower densities are found for similar coverages. For the Pt nanoclusters, the different graphene rotational domains lead to differences in nanocluster size and density. Although dense and ordered arrays of metal nanoclusters were not observed, the results are similar to other cases of metals deposited onto graphene moiré lattices in that the clusters displayed a narrow size distribution and high thermal stability.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (312) 996-0777. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the National Science Foundation (CHE-1464816) and by an LAS Award for Faculty of Science from the University of Illinois at Chicago. The authors thank Dr. Randall J. Meyer for supplying the Rh rod for this work and Dr. Homa Khosravian for assistance with the STM system during the initial stages of this work.

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REFERENCES

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Formation of Pt and Rh Nanoclusters on a Graphene MoirÃ© Pattern on

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