Structures of Pt clusters on graphene by first-principles calculations
Introduction
In a proton-exchange membrane fuel cell (PEMFC), the dissociation and oxidation of hydrogen molecules into protons at an anode and the formation of water molecules through reduction of oxygen molecules at a cathode are essential reactions, and platinum nano-particles supported on carbon materials are used as electrode catalysts. This is because Pt nano-particles are considered to be the best for both hydrogen oxidation and oxygen reduction at low temperature [1], [2]. In order to attain excellent catalytic activity, it is necessary to understand and control the microstructures of Pt/C electrode. For example, small Pt particles should be properly dispersed on carbon materials so as to attain large surface area of Pt and proper space for reaction and mass transport. For this purpose, we have to understand the interaction between Pt nano-particles and carbon materials. This point is also important for the deterioration problem. In life tests or acceleration deterioration tests, surface area loss of supported Pt occurs by agglomeration of Pt particles via particle migration and sintering [3] or dissociation and precipitation [4]. Akita et al. have clearly observed in an acceleration test that Pt dissolves and diffuses and that Pt crystals grow in the polymer electrolyte membrane, using the analytical transmission electron microscopy [5]. Therefore, it is important to investigate the stability of Pt nano-particles on carbon materials.
About the experimental study of the Pt–carbon interaction, there are several studies of Pt clusters on the basal plane of highly oriented pyrolytic graphite (HOPG), using scanning tunneling microscopy (STM) [6], [7] and X-ray photoelectron spectroscopy [7]. The interaction between Pt clusters and HOPG depends on the surface condition of HOPG. Carbon black is used as the support of the electrode catalysts in the PEMFC. Primary particles of carbon black consist of several graphene-like layers, which combine to form spherical or oval particles of powders [8], and thus we have to understand the Pt–graphene interactions. On the theoretical side, Okamoto investigated the interactions between a Pt(1 1 1) surface and graphene [9] and between a cluster and graphene [10] by first-principles calculations. He found that the Pt(1 1 1)–graphene interaction is very week and that carbon vacancies on a graphene sheet enhance the Pt–graphene interaction. We also examined the interactions of a single Pt atom, a cluster, and a Pt(1 1 1) surface with a graphene sheet using a similar theoretical scheme, and obtained consistent results [11].Kong et al. investigated the single Pt-atom adsorption on a graphene layer, a carbon nanotube, and graphene edges [12], and observed the strong adsorption on atomic vacant sites or graphene edges.
About the Pt-cluster–graphene interactions, there have been few ab initio studies of the dependence on the size and shape of Pt clusters, while there are classical molecular dynamics simulations of the stable configurations of Pt clusters on a graphite (0 0 0 1) surface using an embedded atom method [13] and using the Brenner force field and the Reax force field [14]. For free Pt clusters, the stable shape of a Pt cluster of up to 55 atoms [15] and of over 300 atoms [16] has been examined by first-principles calculations. It is observed that planar configurations are preferred for smaller clusters up to and that three-dimensional (3D) ones are more stable for larger clusters.
In this paper, we investigate the stability of clusters on a graphene sheet and the interfacial interaction between clusters and graphene using first-principles calculations. We analyze the effects of the size and shape of the Pt cluster and the manner of contact between the cluster and graphene. Preliminary results of have been briefly reported in Ref. [17].
Section snippets
Methods
All the calculations are carried out using the program code Simulation Tool for Atom TEchnology (STATE), which was successfully applied to Pt(1 1 1)/graphene [9], /graphene [10], and /graphene [17]. We adopt the generalized gradient approximation in density functional theory [18], [19] with the Perdew, Burke, and Ernzerhof formula [20] as the exchange-correlation energy functional. All pseudopotentials are generated from scalar relativistic [21] all-electron atomic calculations to
Results and discussion
First of all, we examined the single Pt-atom adsorption on graphene. We treated three adsorption sites for the non-defective graphene sheet; the hollow site surrounded by six carbon atoms (6H site), the top site above a carbon atom (T site), and the bridge site between two neighboring carbon atoms (B site). We also examined the adsorption on a single carbon-atomic vacancy (V) site. Results are shown in Table 1 and Fig. 1. For the non-defective graphene sheet, the most stable adsorption site is
Discussion and summary
We examined the stable atomic configurations and the electronic structures of the /graphene systems . For the small cluster with , it is the interfacial interaction energy, , to determine the stability of the /graphene system, resulting in the stabilization of the vertical adsorption of the planar cluster. For the planar cluster with of the parallel adsorption becomes similar to that of the vertical adsorption due to the formation of the
Acknowledgments
We thank Dr. Akita and Dr. Maeda for instructive discussion for experiments and Dr. Hyodo and Dr. Yamakawa for valuable discussion on calculations. This study was supported by Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST). And these calculations were carried out using AIST Super Cluster in National Institute of Advanced Industrial Science and Technology (AIST), Japan.
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