Growth of Pd Nanoclusters on Single-Layer Graphene on Cu(111

Article pubs.acs.org/JPCB

Growth of Pd Nanoclusters on Single-Layer Graphene on Cu(111) Esin Soy,† Nathan P. Guisinger,‡ and Michael Trenary*,† †

Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States

J. Phys. Chem. B 2018.122:572-577. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/09/18. For personal use only.

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ABSTRACT: We report scanning tunneling microscopy results on the nucleation and growth of Pd nanoclusters on a single layer of graphene on the Cu(111) surface. The shape, organization, and structural evolution of the Pd nanoclusters were investigated using two different growth methods, continuous and stepwise. The size and shape of the formed nanoclusters were found to greatly depend on the growth technique used. The size and density of spherical Pd nanoclusters increased with increasing coverage during stepwise deposition as a result of coarsening of existing clusters and continued nucleation of new clusters. In contrast, continuous deposition gave rise to well-defined triangular Pd clusters as a result of anisotropic growth on the graphene surface. Exposure to ethylene caused a decrease in the size of the Pd clusters. This is attributed to the exothermic formation of ethylidyne on the cluster surfaces and an accompanying weakening of the Pd−Pd bonding.

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INTRODUCTION In recent years, metal nanoclusters (Ir, Ni, Ag, Pd, Pt, Au) formed on graphene or graphene-oxide templates have been widely investigated due to their numerous applications in catalysis, biosensing, semiconductor devices, hydrogen storage, and electron transport.1−8 Among these noble metals, Pd nanoparticles are particularly significant for their widespread catalytic applications such as for C−C coupling, the SuzukiMiyaura reaction, Wacker oxidation, the Stille reaction, and hydrogenation.9−13 However, in order to study the fundamental properties of Pd clusters, it is necessary to find a support material that promotes dispersion and stabilization of clusters under various conditions. Graphene is promising for its use as a template for the formation of three-dimensional metal nanoclusters.2,4,6,8,14 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. In some cases, the moiré structures can serve as templates for the growth of well-ordered arrays of metal nanoclusters of uniform size.2 Hence, such systems provide a new opportunity for investigating the fundamental properties of metals at the nanoscale. The adsorption and reactions of ethylene on transition-metal surfaces such as Pt and Pd have been extensively studied due to their relevance to various processes in heterogeneous catalysis.15 In general, two types of adsorption modes are identified for ethylene on metal surfaces: π-bonding, which is only stable at low temperature and di-σ bonding, which is the stable form up to ∼250 K.16,17 However, at higher temperatures, ethylene converts to the stable ethylidyne species, which decomposes only at temperatures above 300 K.17−19 Because the conversion of ethylene to ethylidyne is such a wellcharacterized reaction on flat single-crystal surfaces, it can be used as a probe reaction of graphene-supported transition metal © 2017 American Chemical Society

nanoclusters. In this study, we use STM to study the stability of Pd particles on graphene/Cu(111) by observing the sizedistribution of the nanoclusters before and after exposure to ethylene.

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EXPERIMENTAL SECTION The experiments were carried out in a UHV chamber with a base pressure of 5 × 10−11 mbar, and the STM images were obtained with an Omicron Variable-Temperature SPM (scanning probe microscope) system at 50 K. Images were acquired with Nanonis 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 mbar) and annealing to 900 K. The surface was verified to be clean and well ordered by STM. Graphene was prepared on Cu(111) by exposing to 1 × 10−5 mbar of ethylene at 1000 °C.20 Pd was deposited on the graphene layer via evaporation from a pure Pd rod by a single electron beam evaporator with integral flux monitor. The sample was held at room temperature during deposition. Diameter and height histograms were generated by the analysis of larger STM images than those shown.

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RESULTS AND DISCUSSION Graphene/Cu(111). Topographic STM images of a typical area containing bare Cu(111) as well as an island of monolayer graphene (outlined with a blue dashed line) are shown in Figure 1a. Randomly distributed dark features on the graphene are similar to those observed previously for graphene on

Special Issue: Miquel B. Salmeron Festschrift Received: May 24, 2017 Revised: July 3, 2017 Published: July 5, 2017 572

DOI: 10.1021/acs.jpcb.7b05064 J. Phys. Chem. B 2018, 122, 572−577

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The Journal of Physical Chemistry B

histograms of the distribution of shapes (b), diameters (c), and heights (d) for 0.90 ML of Pd deposited on graphene/Cu(111) at room temperature. At this coverage, the average cluster diameter was 7.8 ± 1.1 nm. The majority of the clusters have a height of 4−5 Pd layers. Three-dimensional Pd clusters have nucleated preferentially inside the graphene islands with mainly well-defined triangular shapes. Pd clusters decorate the terraces of graphene islands, consistently pointing either up or down as displayed in Figure 2a. As noted in previous studies, which of the two orientations forms depends on the initial nucleation site.25,26 Although only one orientation with the lowest energy should dominate, nucleation at hcp and fcc sites might occur with equal probability, which would lead to simultaneous growth of two different orientations of Pd clusters on the graphene surface. The tendency of Pd clusters to nucleate inside the graphene islands and form large clusters, rather than being trapped at the graphene edges, indicates that Pd atoms are highly mobile on the graphene at room temperature. Highly mobile Pd atoms can diffuse and nucleate at either the hcp or fcc sites. This also explains the coexistence with roughly equal abundance of two different orientations of Pd clusters. Figure 2a also shows that despite the absence of an ordered array of Pd clusters on graphene/Cu(111), the orientation and registration of the clusters are still preserved on the moiré structure. On the bare Cu patches, Pd forms dendritic islands, just as it does on a graphene-free Cu(111) surface.27 In contrast, well-defined triangular clusters were observed only on the graphene islands. The lack of long-range order was previously reported by Zhou et al. for Pd clusters on graphene/Ru(0001).8 The authors concluded that large Pd clusters (8−14 nm) develop at the earliest stages of growth as a result of a weak Pd−carbon dissociation energy. A lower metal−carbon dissociation energy leads to higher mobility, thus producing larger clusters on the graphene. The Pd−carbon dissociation energy was reported to be 436 kJ/mol, which indicates a weaker interaction between graphene and Pd than between graphene and other metals such as Pt and Rh.8 Therefore, it is very likely that the weak metal− carbon interaction increases the mobility of Pd atoms, which results in fewer and bigger clusters. Likewise, as a result of the weak interaction between the Pd clusters and graphene, the Pd clusters are generally larger than the moiré unit cells so that their nucleation site within a moiré unit cell cannot be specified. The high mobility of Pd on the graphene/Cu(111) surface would also account for the absence of metal atoms and clusters in many of the moiré unit cells. Figure 2b compares the relative frequency of the different types of islands. Following the example of Hershberger et al.,25 we classified the shapes of the islands into four groups: triangleup, triangle-down, spherical, and irregular. Irregular clusters, which were generally smaller than the triangular shapes, included a few merging triangular clusters and clusters with indistinguishable shape. Because of their larger size, the welldefined triangular clusters contained 92 ± 2% of the Pd atoms, although as indicated in Figure 2b, the triangular shapes comprised only about 80% of the observed clusters. The mechanism by which well-defined triangular metal islands are formed is well-known from previous experimental and theoretical studies.28−31 Anisotropy in diffusion around the corners and along the edges of islands is the main factor responsible for the development of triangular islands, with the exact shape formed dependent on temperature. Using kinetic Monte Carlo simulations, Ovesson et al. showed that while

Figure 1. (a) Typical STM topographic image of a graphene island on Cu(111) (scale, 100 nm × 100 nm). (b) Close up STM image from part a reveals a moiré periodicity of 2 nm with a rotational angle of 7° (scale, 10 nm × 10 nm). The black rhombus indicates the unit cell. The low hcp and fcc regions are indicated by the A and B labels, respectively, on the moiré unit cell.

Cu(111) where they were attributed to adsorbates.20 Figure 1b shows a moiré structure, with an approximate periodicity of 2.0 nm and a rotational angle of 7°, formed as a result of the lattice mismatch between graphene and the Cu(111) surface. Previously, the rotated and aligned phases of graphene on Cu (111) were reported with periodicities of 1.5 nm, 2 nm, 4 nm, 5.6 nm, and 6.6 nm.20−22 However, in the present study only the 2.0 nm periodicity was observed. 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.23,24 The low regions are of two types corresponding to whether the center of the graphene rings are located above hcp or fcc hollow sites. In these low regions, the carbon atoms (as opposed to the centers of the graphene rings) are positioned above the substrate atoms. Thus, the electronic structure and the bonding strength of graphene are not uniform over the moiré structure.22 Growth of Pd on Graphene/Cu(111). a. Continuous Deposition. Figure 2 shows an STM topograph (a) and

Figure 2. (a) STM topographic image of clusters formed from 0.90 ML of Pd deposited on graphene at room temperature with continuous deposition. A hexagonal graphene island is outlined with a blue dashed line (scale, 400 nm × 400 nm). (b) Frequency vs cluster-shape histogram. (c) Diameter and (d) height histograms of Pd clusters. 573

DOI: 10.1021/acs.jpcb.7b05064 J. Phys. Chem. B 2018, 122, 572−577

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The Journal of Physical Chemistry B

Figure 3. STM topographic images and corresponding diameter histograms of Pd clusters grown on graphene at room temperature after stepwise deposition, with coverages of (a,d) 0.29 ML, (b,e) 0.64 ML, and (c,f) 0.92 ML (scale, 100 nm × 100 nm). Inset in part a shows the moiré pattern of the graphene beneath the clusters for a Pd coverage of 0.29 ML (inset, 2 nm × 2 nm). Arrows in part c show nonspherical clusters. (g) Plot of island density versus Pd coverage.

islands do not attain their well-defined triangular shapes from stepwise deposition but agglomerate even at a very low coverage. Nevertheless, at higher coverages, clusters start to lose their spherical shape and have a shape intermediate between spherical and triangular as marked by arrows in Figure 3c. No apparent two-dimensional order of the clusters is observed with increasing Pd coverage. The size and distribution of the Pd clusters vary among the different coverages. At a coverage of 0.29 ML, the average cluster diameter was 3.7 ± 1.2 nm. The majority of the clusters have a height of three−four Pd layers. It is clear that the mobility of Pd is quite high since all the deposited atoms have moved to the clusters. At higher coverages of 0.64 and 0.92 ML, the average cluster diameters were 6.2 ± 1 nm and 8.9 ± 2.4 nm, respectively. The heights correspond to five layers of Pd for 0.64 ML and six-seven Pd layers for 0.92 ML. (Note that the actual size could be slightly smaller due to tip effects). Compared to 0.29 ML, there is a greater spread in cluster size for the higher coverages as shown by the corresponding histograms in Figure 3d−f. The height and diameter distributions that we observe are similar to those reported by others. Gotterbarm et al. constructed size and height histograms for 0.3 ML Pd clusters on graphene/Rh(111) and reported the average diameter of the clusters as 3 ± 0.3 nm.36 The height of the clusters were reported to be 6.6 ± 0.1 Å, which corresponds to three-four layers of Pd. Also, at 0.1 and 0.4 ML of Pd on graphene/ Ru(0001), Zhou et al. reported average cluster diameters of ∼8 and ∼14 nm, respectively.8 Compared to their results, Pd clusters on graphene/Cu(111) are substantially smaller in size. These results suggest that Pd is less mobile on the graphene/ Cu(111) surface than on graphene/Ru(0001). Figure 3g shows that as the coverage increases by a factor of 3 from 0.29 to 0.92 ML, the cluster density increases by a factor of 3. The increase in coverage produces both an increase in the size of existing clusters as well as the formation of new islands, which constitute the smaller clusters seen in Figure 3b,c. Increased cluster density with increased metal coverage was also observed for Ni clusters on graphene where their mobility

edge diffusion is necessary for the growth of metal islands at low temperatures, anisotropy in corner diffusion gives rise to triangular islands.28 Their results are found to be general for a wide class of metal systems such as Al(111), Pt(111), Ag(111), and so on.29−31 Other studies have attributed formation of welldefined triangular islands to edge diffusion anisotropy alone.32−35 Michely et al. demonstrated that fractal or dendritic shaped islands are formed in the homoepitaxial growth of Pt on Pt(111) at low growth temperatures, whereas higher temperatures give rise to compact shapes such as triangular, hexagonal, and inverted triangular islands.32 In their study, the process behind the formation of the compact islands is described by the anisotropy in edge diffusion along two types of steps. Since one type of the step will advance faster at a given temperature, islands will tend to be more triangular with the faster growing steps disappearing. Likewise, Prieto et al. studied with STM the atomic process for formation of triangular Co islands on Cu(111).35 Their results indicate that while Co forms dendritic islands at low temperature, well-defined triangular Co islands with two different orientations, rotated by 60°, are formed for deposition at room temperature. Our observation of triangular Pd islands on graphene on Cu(111) indicate that at our growth temperature of 300 K, diffusion of Pd atoms on graphene, as well as anisotropic edge diffusion, occur rapidly relative to the deposition rate. Figure 2c,d shows that the Pd clusters grow with a relatively narrow size and height distribution compared to our recent report on Pt and Rh clusters.22 The average height in Figure 2d corresponds to 3 to 4 layers of Pd and the average cluster diameter was 7.3 ± 1.8 nm. Since the surface free energy of Pd is much higher than graphene, Pd atoms form 3D clusters when the temperature is high enough (e.g., room temperature) to induce mobility on the surface.4 b. Stepwise Deposition. The morphology of the clusters for a final Pd coverage of 0.92 ML achieved by stepwise deposition is displayed in Figure 3 and can be compared to the 0.90 ML Pd coverage of Figure 2 achieved by continuous deposition. Figure 3 shows STM images and the corresponding size histograms of 0.29 (a, d), 0.64 (b, e), and 0.92 (c, f) ML of Pd deposited on graphene/Cu(111). These results reveal that the 574

DOI: 10.1021/acs.jpcb.7b05064 J. Phys. Chem. B 2018, 122, 572−577

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converts to ethylidyne at temperatures as low as 160 K and that the overall reaction, which occurs by a three-step mechanism, is determined to be exothermic by 49 kJ/mol. Their results suggest that a 45 L ethylene exposure yields up to 0.25 ML of ethylidyne on Pd(111).38 The stability of supported Pd and Ir clusters under ethylene exposure has been studied by Fahmi et al. and Uzun et al., respectively.39,40 In the former study, DFT calculations of the interaction of ethylene with Pd clusters showed that reconstruction of the clusters occurs when the bond between the two Pd atoms involved in the adsorption is broken and as a result the cluster structure changes. Similarly, Uzun et al. reported the first evidence of supported Ir cluster disintegration as a result of ethylene exposure. Their EXAFS result showed that during the ethylene exposure the Ir clusters break up, as indicated by the decreasing Ir−Ir coordination number, which was accompanied by low-Z scattering indicating bonding of molecules to Ir. Likewise, Frank at al. discussed the adsorption of ethylene on Pd clusters supported on a thin alumina film.6 Their study revealed that the surface will undergo reconstruction to optimize the bonding between ethylidyne molecules and Pd atoms as the coverage of ethylidyne is increased. In the light of those findings, we investigated the response of Pd clusters on graphene/Cu(111) to exposure of 45 L of ethylene at room temperature. After the exposure, the ethylidyne coverage is expected to be about 0.25 ML.38 Figure 5 shows the topographic STM images and corresponding

was attributed to a weak interaction with the underlying substrate.26 Hershberger et al. also observed differences in metal clusters formed by continuous versus stepwise deposition.25 They found that after stepwise deposition, Dy clusters on graphene on a SiC surface displayed a lower number density and were therefore larger for the same metal coverage than Dy clusters observed after continuous deposition. They attributed this to the high mobility of Dy on graphene, which caused the metal atoms to preferentially add to existing Dy clusters rather than to nucleate new islands. In their case, this had the additional effect of leading to clusters with incomplete top layers following stepwise deposition. In our case, our clusters are also generally larger for a given Pd coverage following stepwise deposition. In addition, the larger clusters from stepwise deposition do not show the distinct triangular shape observed following continuous deposition. This is attributed to kinetic effects, namely, that growth of the clusters occurred as a faster rate relative to the rate of migration of Pd atoms along the edges and around the corners of the islands, processes responsible for formation of the distinct triangular shapes seen in Figure 2 following continuous deposition. Since our samples were cooled to 50 K for imaging shortly after Pd deposition, structures formed at room temperature were preserved during image acquisition. Because Pd clusters are loosely bound to the graphene/ Cu(111) surface, it was also possible to manipulate them by decreasing the tip−sample distance. Figure 4 shows that when

Figure 4. STM topographic images of the manipulation of Pd nanoclusters on graphene/Cu(111) (scale, 100 nm × 100 nm). Weak interaction between the underlying graphene and the Pd allows the clusters to be moved by the STM tip. Figure 5. STM topographic images and cluster size distribution of Pd nanoclusters on graphene/Cu(111) before and after exposure to C2H4: (a) 0.64 ML of Pd on graphene/Cu(111), (b) after exposure to 45 L of C2H4 at PC2H4 = 1 × 10−6 mbar (scale, 100 nm × 100 nm). (c,d) Histograms of the number of particles as a function of their diameters before (c) and after (d) the ethylene exposure.

the tip−sample distance decreased, clusters within the white and yellow circles were relocated while the cluster within the purple circle disappeared, possibly by being picked up by the STM tip. This is another indication of the weak interaction between the Pd clusters and underlying graphene surface. Stability of Pd Clusters on Gr/Cu(111). The adsorption of ethylene has been extensively studied on real catalysts as well as on model systems. It is known that ethylene is di-σ- or πbonded on Pd(111) at low temperatures. Although the HREELS spectrum of adsorbed ethylene on Pd shown by Sock et al. indicates that the π- and di-σ bonded ethylene species indeed coexist on the surface at low temperature, several DFT calculations demonstrate that di-σ-bonding with an adsorption energy of −0.36 eV is favorable over π-bonding, which is found to be 0.1 eV less stable.37 However, adsorption of ethylene at room temperature yields the thermodynamically more stable species identified as ethylidyne, CH3−C≡. Stacchiola et al. reported that ethylene on Pd(111) rapidly

diameter analysis of the Pd clusters, which reveal that a broad range of cluster sizes appear after ethylene exposure. Before the exposure, the average cluster diameter was 6.2 nm and 92% of clusters had heights corresponding to five Pd layers. After the ethylene exposure, the average cluster diameter was found to be 4.2 nm, and the heights correspond to three to four layers of Pd. These results indicate that the formation of ethylidyne on the surfaces of the Pd nanoclusters induces their reconstruction. The formation of strong ethylidyne−Pd bonds provides the favorable thermodynamics for restructuring of the Pd surfaces, which may also be accompanied by the weakening of bonds within the Pd clusters. 575

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CONCLUSIONS The formation and growth of dispersed Pd nanoclusters on a graphene/Cu(111) template were studied with STM. The morphology of the Pd clusters was found to depend on whether the Pd was deposited continuously or stepwise. In both methods, the Pd clusters were found to not completely wet the graphene surface but tend to retain a 3D structure as a result of the weaker binding with the graphene compared to the Pd−Pd interaction. Exposure of the Pd clusters to ethylene induces their reconstruction, which is attributed to the exothermic formation of the stably adsorbed ethylidyne species.

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AUTHOR INFORMATION

Corresponding Author

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

Michael Trenary: 0000-0003-1419-9252 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 (Grant CHE-1464816). Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.

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