The electrodeposition and electrochemistry of single nanoparticles supported on carbon nanoelectrodes

Abstract

A key goal of nanoparticle-based catalysis research is to correlate the structure of nanoparticles (NPs) to their catalytic function. To avoid the structural heterogeneity in NP ensembles, which are typically used for studying the structure-function relationships, we carried out analyses on individual NPs. This single-NP approach makes it possible to establish direct correlations between structures of single NPs and, in the cases reported here, their electrocatalytic properties. Shape-controlled single Pt NPs were deposited onto the carbon nanoelectrodes (CNEs) using an empirically developed square-wave potential program. The key advance is that the synthetic method makes it possible to produce ligand-free single, electrochemically active NPs with a vast range of crystal structures and sizes. In this project, single Pt NPs with {13 6 2}, {15 6 1}, {10 1 1}, and {11 5 1} surfaces were electrodeposited with reproducible sizes ranging from ~150 nm to ~500 nm. Equally important, the NPs can be fully characterized, and therefore the electrochemical properties of the NPs can be directly correlated to the size and structure of a single shape. The electrochemical properties of these shape-controlled single NPs were assessed using the formic acid electrooxidation (FAO) reaction. For comparison, single, spherical polycrystalline Pt NPs supported on CNEs were also prepared. The first study of the electrocatalytic analyses, on the shape-controlled single Pt NPs with {15 6 1} and {10 1 1} surfaces, is discussed in Chapter 4. In there, density functional theory, carried out prior to the electrochemical studies, was used to interpret the experimental results of the FAO experiments. Good agreement between the experimental results and theoretical predictions is observed. In the second study of the electrocatalytic analyses, which is presented in Chapter 5, we report on the enhanced electrocatalytic FAO activity using individual Cu-modified, Pt NPs with {11 5 1} surface. The results show that the Cu-modified Pt NPs exhibit significantly higher currents for FAO than the Pt-only analogs. The increased activity is enabled by the Cu submonolayer on the highly stepped Pt surface, which enhances the direct FAO pathway but not the indirect pathway (which proceeds via surface-absorbed CO*).