Binding of Pt Nanoclusters to Point Defects in Graphene: Adsorption, Morphology, and Electronic Structure

I Fampiou and A Ramasubramaniam, JOURNAL OF PHYSICAL CHEMISTRY C, 116, 6543-6555 (2012).

DOI: 10.1021/jp2110117

Graphene nanosheets serve as excellent support materials in the synthesis of advanced metal nanoparticle-graphene electrocatalysts. In this study, we employ a combination of density functional theory and bond-order potential calculations to perform a systematic investigation of the adsorption energetics, structural features, and electronic structure of platinum nanoclusters supported on both defective and defect-free graphene. We establish a hierarchy of point defects and their reconstructions that can act as strong trapping sites for platinum nanoclusters and inhibit catalyst sintering. We demonstrate that the preferred low-energy structure of supported platinum nanoclusters are neither high-symmetry structures (e.g., icosahedral, cuboctahedral) nor readily derived from moderate structural distortions of high-symmetry structures, as is often assumed in computational models. Rather, supported nanoparticles assume open, low-symmetry shapes much like those observed in earlier computational work on annealing of unsupported clusters in a vacuum. The formation of metal-carbon bonds at support defects influences the average bond length and thus the strain in the metal cluster, stronger binding correlating with larger average bond lengths. Additionally, stronger binding of the cluster to the support leads to increased charge transfer from the cluster to the substrate accompanied by a substantial downshift of the cluster d-band center; in several instances, the d-band center is shifted below that of a Pt(111) surface. Our study suggests possible avenues for enhancing the stability and CO tolerance of platinum nanoparticles on graphene through defect engineering.

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