Size-Dependent Ripening of Gold Nanoparticles through Repetitive Electrochemical Surface Oxidation-Reduction Cycling

Here we describe the electrochemical size stability of 1.6, 4.1, and 15.1 nm diameter Au nanoparticles (NPs) supported on indium tin oxide-coated glass electrodes (glass/ITO). Anodic stripping voltammetry (ASV) and the electrochemically-measured total surface area-to-volume ratio (SA/V) provide the NP size following surface oxidation-reduction cycling from −0.2 to 1.6 V (vs Ag wire) in 0.1 M HClO4. After 1000 oxidation-reduction cyclic voltammetry (CV) scans, the relative size increases by a factor of 12, 7, and 2 for the 1.6, 4.1, and 15.1 nm diameter Au NPs, respectively. The relative size increase is largest for the smallest NPs, also confirmed by electron microscopy, indicating their lower size stability towards surface oxidation-reduction cycling. The size increase is fastest within the first 200 cycles, which decreases with a further increase in the number of cycles until the Au NP diameter stabilizes. The size transformation is more dramatic at higher Au NP electrode coverage and 30%-100% of the Au dissolves during cycling, depending on the coverage and NP size. Various potential cycling and holding profiles show that the Au NP size increases during reduction of the oxide layer, consistent with an electrochemical Ostwald ripening mechanism.