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

Abstract

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 HClO 4 . 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.

Electrochemical Cleaning Stability and Oxygen Reduction Reaction Activity of 1‐2 nm Dendrimer‐Encapsulated Au Nanoparticles

“…This is in contrast to other studies showing that such changes occur after hundreds to thousands of cycles. [7,8,24] Second, size changes are observed for G6-NH 2 (Au 55 ) DENs even before faradaic oxidation or reduction currents are detected in the CVs. Both of these observations suggest that the size of the G6-NH 2 (Au 55 ) DENs is particularly sensitive to even modest potential excursions.…”

“…The anodic current arises from oxidation of the AuNP surface, which may lead to formation of either AuO x or Au + in the case of smaller NPs. [7,8,57,58] When the potential is reversed, the reduction feature at~340 mV corresponds to reduction of oxidized Au products. This latter point will be discussed in more detail later.…”

“…[83,84] During the Ostwald ripening process, smaller and less stable NPs oxidize into soluble ions, which can then diffuse through solution before reducing onto the surface of larger, more stable NPs. [7,8,60,85] To investigate the possibility of Smoluchowski ripening, cleaning scans were carried out using higher-generation G8-NH 2 (Au 55 ) DENs. Previous studies have shown that highergeneration dendrimers having increased steric hindrance at their peripheries can suppress aggregation of Au 147 DENs during catalysis experiments.…”

“…In the field of electrocatalysis it is common practice to subject nanoparticles (NPs) to electrochemical processes intended to remove impurities from the catalyst surface. [1][2][3][4][5][6][7][8] This is often referred to as 'electrochemical cleaning', and its purpose is to expose a pristine NP surface so that electrocatalytic performance can be accurately assessed. [9][10][11][12] It is becoming increasingly clear, however, that these types of pretreatments can significantly alter the properties of the catalysts under study.…”

“…For example, following pretreatment, structured bimetallic NPs can undergo atomic rearrangements, [13][14][15][16] the crystal structure, shape, and facets can change, [4,[17][18][19] and atoms can undergo selective dissolution. [7,8,13,16,[20][21][22] Even for polycrystalline gold surfaces, the rate of anodic Au dissolution during potential cycling is highly dependent on the surface structure of the electrode. [23] These atomic-level structural changes are further exacerbated for very small (<~5 nm) NPs due to their inherent instability.…”