The Roles of Citrate and Defects in the Anisotropic Growth of Ag Nanostructures

Synthetic control of nanocrystal shape is often achieved by controlling the crystal structure of the seed crystals as well as through the use of additives that are thought to block atomic addition to certain facets. However, the effect of the crystal structure or additives on the rate of atomic addition to a specific facet is not usually quantified, making it difficult to understand and design nanocrystal syntheses. This article combines single-crystal electrochemistry measurements with measurements of anisotropic nanocrystal growth to quantify the roles of citrate and planar defects in anisotropic atomic addition. Citrate lowers the rate of atomic addition to Ag(100) and Ag(111) single crystals by 3.2 and 15 times, respectively. Citrate decreases the rate of ascorbic acid oxidation in a facet-selective manner, but citrate decreases the rate of silver ion reduction to roughly the same extent on Ag(100) and Ag(111) single crystals. The degree to which citrate passivates single-crystal electrodes at different citrate concentrations closely matches the facet-dependent growth rates for single-crystal seeds. In contrast, seeds with planar defects exhibit anisotropic growth that is 30–100 times greater than can be explained by the facet-selective passivation by citrate. Without citrate, more silver deposits on the edges of seeds with planar defects than in the middle, but the seeds do not exhibit anisotropic growth. Evidence suggests that citrate improves the stability of nanoplates bounded by large {111} facets by preventing diffusion to {111} facets.