Article pubs.acs.org/JPCC
Atomic-Scale Investigation of Highly Stable Pt Clusters Synthesized on a Graphene Support for Catalytic Applications EunKyung Cho,*,† Esmeralda N. Yitamben,‡ Erin V. Iski,‡ Nathan, P. Guisinger,‡ and T. F. Kuech† †
Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
ABSTRACT: The synthesis and binding of highly stable platinum clusters supported on epitaxial graphene, grown on 6H-SiC(0001) substrates, were investigated by scanning tunneling microscopy (STM) and spectroscopy (STS). These Pt nanoclusters form uniformly on both mono- and bilayer sheets of graphene. Unlike most metals that simply diffuse on the inert graphene substrate, these Pt clusters appear to attach to the graphene through chemical bonding, as strong electron scattering and high thermal stability are observed. The thermal stability of the deposited Pt atoms was demonstrated by annealing the sample in situ up to 700 °C. The Pt clusters were not significantly changed by agglomeration or atom detachment. Further annealing at 1250 °C led to desorption of the Pt clusters from the monolayer of graphene, whereas the Pt clusters on the bilayer graphene remained and may have intercalated between the graphene sheets. The goal of this study was to explore the utilization of “ideal” graphene (not graphene oxide or reduced graphene) as a support for nanoparticle catalysts.
1. INTRODUCTION Graphene exhibits a unique structure of hexagonally packed sp2-bonded carbon atoms with single atomic thickness.1 Graphene has been extensively studied for its unique twodimensional electronic structure and its extremely high mechanical and chemical stability. Such properties make graphene an attractive supporting material for metal particle based catalysis. Preliminary studies utilizing graphene for catalysis have revealed the tremendous promise of graphenebased catalyst systems but were limited to multilayer graphene sheets and graphene−metal−oxide materials, which are not considered ideal graphene systems.2,3 In addition, metal adsorption on graphene is of interest in the fabrication of graphene-based transistors,4,5 because metal atom adsorption can be used to change the carrier density and tune the graphene Fermi level. For these applications, it is critical to understand the key factors that govern the growth and distribution of metal particles on graphene as well as the electronic and chemical interactions of the metal atoms with graphene. Even though there is a strong motivation to examine the metal−graphene system, the work has been limited, especially for catalysis.4−6 With that said, Pt nanoparticles supported on carbon materials are considered one of the best catalysts for hydrogen oxidation and oxygen reduction in a proton-exchange membrane fuel cell (PEMFC).7 Here, we have characterized the growth of platinum nanoparticles formed on “ideal” graphene surfaces at the atomic scale. This paper describes the preparation of the epitaxial graphene layers on the silicon face of 6H-SiC(0001) and the subsequent thermal deposition of Pt atoms on these surfaces. The evolution of the surface morphology with increasing Pt coverage and tunneling conductance spectra are presented. © 2012 American Chemical Society
These measurements are used to develop the deposition mechanism and to study the electronic properties of the Pt clusters and their interaction with the graphene support. A 5 min deposition of Pt resulted in a 13% surface coverage of Pt clusters. This sample was used in thermal annealing studies to determine the thermal stability of the nanoparticles, which remain up to 700 °C without any morphological changes. The surface features and cluster size were observed as a function of thermal annealing. The Pt electronic structure, measured from the Pt clusters before and after annealing, provides additional insight into the interaction between Pt and the graphene substrate.
2. EXPERIMENTAL SECTION The 6H-SiC(0001) single crystal samples were commercially purchased (Cree) and degassed at 600 °C overnight in a UHV system. Both mono- and bilayer graphene were prepared by the thermal decomposition of the SiC(0001) substrate surface through a series of annealing steps at a temperature of 1250 °C using direct heating within the UHV chamber. Pt atoms were thermally evaporated onto the sample at room temperature through the e-beam heating of a Pt crucible located in the UHV chamber. After each Pt deposition sequence, the sample was transferred and examined in situ by UHV STM and STS at 300 K. The thermal stability of the Pt clusters on the graphene/ SiC(0001) surface was investigated by annealing. The annealing was done in situ by resistive heating in the UHV chamber. The surface was characterized with STM and STS using an Received: September 25, 2012 Revised: November 9, 2012 Published: November 21, 2012 26066
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Figure 1. STM images of (a) clean graphene/SiC(0001) and (b) 1 min, (c) 3 min, (d) 5 min, (e) 10 min, and (f) 30 min doses of Pt deposited on graphene/SiC(0001) at sample bias V = −1.0 V and tunneling current I = 100 pA at 300 K ((a)−(f) 100 × 100 nm2, scale bar = 25 nm in (a) and 2.5 nm2 in (a) inset).
min doses of Pt deposited on graphene/SiC(0001) is shown in Figure 2. The Pt clusters are predominantly below 3 nm in
electrochemically etched W tip. In some cases, liquid helium was used to cool the sample to 55 K; otherwise all measurements were conducted at 300 K.
3. RESULTS AND DISCUSSION A topographic STM image of an area containing bare SiC(0001), as well as regions of monolayer and bilayer graphene is shown in Figure 1a. It is known that the SiC(0001) surface starts to form the (6√3 × 6√3)R30° reconstruction at about 1100 °C during annealing,8 and the growth of one to three layers of graphene can be induced by further heating to 1200−1350 °C. The prepared sample principally exhibits regions of either monolayer or bilayer graphene on the surface. The two regions can be distinguished via atomic resolution imaging depending on the voltage dependence and via signatures in the scanning tunneling spectroscopy (STS).9,10 The first layer of graphene was observed with higher resolution, and the characteristic hexagonal structure is clearly seen with atomic resolution at low bias (Figure 1a, inset). Once formation of clean graphene was confirmed by STM imaging, Pt was deposited on the prepared graphene/ SiC(0001) sample to study the growth behavior of Pt nanoparticles on graphene. Figures 1(b−f) display the STM images of Pt atoms deposited onto graphene for steadily increasing deposition times of 1, 3, 5, 10, and 30 min, respectively. All of the prepared surfaces had regions of the epitaxial graphene and the bare reconstructed SiC substrate. Initially, it is clear that the Pt clusters are highly dispersed on the graphene surface, with a few Pt clusters observed on the bare SiC areas. On the graphene surface, the Pt assembles into nanoclusters, indicating that the Pt atoms have sufficient mobility at room temperature to aggregate and form the clusters. The surface coverages of Pt clusters on one layer of graphene and on bilayer graphene appears to be similar. As expected, the Pt coverage increases with deposition time. The cluster growth becomes a competitive process between single atom adsorption on graphene versus attachment onto an already formed cluster. The size distribution of Pt clusters for 5
Figure 2. Cluter height distrubution as a function of the corresponding cluster diameter for 5 min doses of Pt deposited on graphene/ SiC(0001).
diameter and 0.1−0.3 nm in height. There is a linear correlation between cluster diameter and height with the following equation: y = 0.094x + 0.08 (R2 = 0.75). The Pt atoms form relatively flat clusters, meaning that the Pt atoms prefer to maximize their interactions with the C surface, instead with other Pt atoms. This may imply that the Pt−C interactions are stronger than the Pt−Pt interactions. It should be noted that different metals studied on graphene on SiC have exhibited different morphologies. For example, cobalt (Co) preferentially decorates the (6√3 × 6√3)R30° reconstructed SiC surface, rather than the monolayer graphene.6 Given these differing observations among metals on graphene, it is of interest to further develop an understanding of the deposition mechanism and the interaction between adsorbed metals and the graphene surface. Several theoretical studies2,11,12 have shown that Pt atoms, possessing a nearly filled d-shell, principally bind to the bridge (B) site of the graphene surface at the midpoint of a carbon−carbon bond. Adsorption on the B site involves hybridization between the C atom and the metal atom, and it is likely that the graphene sp2like orbital character partially shifts to the more covalently 26067
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Figure 3. (a) Schematic structure and STM topographic image of a Pt-deposited graphene/SiC(0001) sample. (b), (c) STM topographic images of the surface after a 5 min deposition of Pt at sample bias V = −1.0 V and tunneling current I = 200 pA, imaged at 300 K (scale bar = 5 nm in (b), 2 nm in (c)). (d) STS spectrum of clean monolayer graphene. (e) STS spectra of various Pt clusters. The corresponding measurement points are shown in (c).
Figure 4. STM images of the graphene/SiC(0001) sample after a 5 min deposition of Pt and anneal at (a) 400 °C, (b) 500 °C, (c) 600 °C, and (d) 700 °C each for 1 h and imaged at sample bias V = −0.3 V and tunneling current I = 100 pA at 55 K (scale bar = 10 nm).
reactive sp3-like character.12−14 The schematic structure of the Pt-deposited graphene surface is shown in Figure 3a. If Pt adsorption involves the hybridization of a Pt atom and a C atom, the electronic structure of the graphene would be significantly altered. Such changes in the graphene electronic structure were suggested in the STM images, which exhibit a modulation of the electronic state of the graphene in the vicinity of a Pt cluster. The modulation is almost identical to previously observed electron scattering from lattice defects in graphene.15 This hybridization can be used to rationalize the different behaviors of Co and Pt on graphene/SiC(0001) in terms of the metal−carbon (M−C) bond dissociation energies. It has been reported that the bond dissociation energy of Pt−C is 150 kJ/mol16 and that of Co−C is 37 kJ/mol.17 The stronger M−C bond would inhibit surface diffusion at a given temperature and flux on the graphene surface.18 The reduced surface transport, resulting from the reduction in the diffusion coefficient, leads to enhanced dispersion of the Pt clusters on graphene having stronger Pt−C bonds than Pt−Pt bonds. Therefore, Pt forms small, highly dispersed clusters on graphene in contrast to Co, which forms a lower density of larger clusters. To confirm the modulation of the electronic behavior of the graphene, STS spectra were taken on the monolayer graphene and Pt clusters (points indicated in Figure 3c) for a graphene/ SiC(0001) sample after a 5 min deposition of Pt and are shown in Figure 3d,e. Each spectrum was obtained by averaging
several bias sweeps with an initial tunneling current and bias voltage of 200 pA and 0.3 V, respectively. Figure 3d shows the dI/dV spectrum taken on the monolayer graphene region away from any Pt clusters. The spectrum is a minimum at zero bias and is similar to previously reported STS of monolayer graphene/SiC(0001) systems.9,10 From this similarity, we assume that the Pt deposition did not introduce Pt atoms below the first layer of epitaxial graphene. Figure 3e presents dI/dV curves taken and averaged on each Pt cluster as labeled in the corresponding STM image (Figure 3c). The peaks at the positive and negative bias voltages are attributed to the resonant electron transfer between the STM tip and the unoccupied and occupied electronic states of the Pt clusters, respectively. A peak in the positive bias voltage range appears at 0.49 V, and this peak is common to all Pt clusters, although the peak positions differ from cluster to cluster in the negative bias voltage range. The various peaks have their maxima at −0.26, −0.37, or −0.66 V. The height of the clusters named Pt1, Pt2, and Pt3 are ∼0.23 nm and that of Pt4 is ∼0.35 nm with a ∼1.5 nm width for both groups. Clusters with the same size on the surface do not necessarily possess the same electronic structure.19 This variation is likely to originate from the different atomic arrangements of the clusters and the different adsorption sites on the graphene surface below. Bettac et al. studied deposited Pt atoms on highly oriented pyrolytic graphite (HOPG) and reported sharp peaks at −0.20 and −0.40 V in the 0 to ∼−0.5 V range of the tunneling 26068
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conductance spectra.20 They stated that the discrete peaks in the STS spectra are due to quantum size effects. Kondo et al. observed a peak in the 0 to ∼−0.08 V range in the vicinity of Pt clusters on HOPG.21 They proposed that the peak is evidence of the nonbonding π electronic states due to the Pt−C hybridization. Because Pt atoms have no energy levels in the range investigated,20 the electronic structure observed in the present study could arise from either (i) quantization of the electronic density of states due to the reduced dimensions of the cluster20 or (ii) states associated with the carbon atoms due to the interaction between the Pt clusters and the graphene substrate.21 The thermal stability of the Pt clusters on the graphene/ SiC(0001) surface was investigated by annealing the 5 min Ptdeposited sample at elevated temperatures of 400, 500, 600, and 700 °C for 1 h. The annealing was done in situ by resistive heating in the UHV chamber. After annealing, the sample was cooled with liquid helium and examined by STM at 55 K with the resulting images shown in Figure 4. The images are characterized by terraces of mono- and bilayer graphene decorated with the Pt clusters. Regions of mono- and bilayer graphene are distinguished by the imaging of the SiC surface reconstruction features. The SiC reconstruction is observed beneath single layer graphene, but not beneath bilayer graphene when imaged at high tunneling bias, as shown in Figure 4. The graphene lattice structure is superimposed on the surface reconstruction of SiC(0001) with pyramidal clusters and hexagonal rings observed in the STM image at low tunneling bias (Figure 4a). These observed adatom features beneath the first graphene layer were reported previously.9 Interestingly, the STM images do not show any obvious increase in the Pt cluster size over the annealing range employed (up to 700 °C). This observation implies that there is no significant ripening or appreciable atom detachment from the Pt clusters at high temperatures. Quantitative analysis of Pt clusters performed on randomly selected STM images of the graphene/SiC(0001) sample after a 5 min deposition of Pt, after annealing at 400 and 700 °C, yielded the size histograms shown in Figure 5. The size interval of 0.2 nm was used in the histogram. A 90% amount of the Pt clusters was observed in the range 0.1−2.4 nm for the asdeposited sample as well as the samples after annealing at 400 and 700 °C. The Pt cluster size of the as-deposited sample generally shows a bimodal distribution and two maxima at the cluster diameters of 0.1 and 1.5 nm. After annealing, the size distribution of small Pt clusters (
