Understanding Uptake of Pt Precursors During Strong Electrostatic Adsorption on Single-Crystal Carbon Surfaces

Abstract

The formation of Pt clusters deposited by strong electrostatic adsorption (SEA) has been studied on highly oriented pyrolytic graphite (HOPG) as well as on higher surface area graphene nanoplatelets (GNPs), which have well-defined surface structures. Both surfaces can be hydroxylated by treatment with hydrochloric acid, and in the case of HOPG, the surface functional groups are exclusively hydroxyls. When SEA is carried out with the anionic precursor PtCl6 2− under acidic conditions, the dominant cationic sites for electrostatic adsorption are protonated aromatic rings rather than protonated hydroxyl groups, and therefore the unfunctionalized surfaces exhibit high uptake compared to the HCl-treated surfaces. In contrast, for SEA with the cationic precursor Pt(NH3)4 2+ under basic conditions, surface hydroxylation leads to higher Pt uptake since deprotonated hydroxyl groups act as negatively charged adsorption sites for the cations. After reduction of the adsorbed ionic precursors by heating in H2, the metallic Pt clusters on HOPG produced from PtCl6 2− SEA form dendritic aggregates associated with high Pt diffusion rates and irreversible Pt–Pt bonding. For Pt(NH3)4 2+ SEA on HOPG, subsequent reduction resulted in clusters preferentially located at step edges on the untreated surface, whereas smaller clusters more uniformly distributed across the terraces were observed on the hydroxylated surface. Similar trends in particle sizes and densities were observed on the GNP surfaces, demonstrating that understanding of nucleation and growth on single-crystal HOPG surfaces can be extended to the high surface area GNPs. Oxidative adsorption to Pt4+ was observed for SEA of Pt2+ precursors on both HOPG and GNP surfaces.