The Roles of Capping Agents and Defects in the Anisotropic Growth of Ag Nanocrystals

Synthetic control of metal nanocrystal shape is a common strategy to control their properties. Shape control is often achieved by controlling the crystal structure of the seed crystals, as well as through the use of additives which are thought to block atomic addition to certain facets. However, the effect of crystal structure or additives on the rate of atomic addition to a specific facet is not usually quantified, making it difficult to design nanocrystal syntheses. This work combines seed-mediated growth, single-crystal electrochemistry measurements and Raman spectroscopy to understand the roles of capping agents and planar defects in the anisotropic growth of silver nanocrystals. The roles of citrate, polyvinylpyrrolidone (PVP), and halides have been investigated. Synthetic results show citrate is a {111} capping agent, PVP is a weak {111} capping agent, chloride and bromide are weak {100} capping agents. However, when chloride or bromide is added with PVP, they become strong {100} capping agents. Electrochemical measurements show the anisotropic growth is caused by capping agents selectively suppressing the oxidation of ascorbic acid (a reducing agent) on a specific crystal facet. The effect of capping agents on silver ion reduction is not facet-selective. Further comparison between the growth of single-crystal seeds and seeds with planar defects indicates defects can catalyze silver atom deposition by up to 100 times and cause greater anisotropic growth than can be explained by facet-selective passivation. Overall this work advances our understanding of nanocrystal chemistry, and informs the design of nanocrystal synthesis to obtain a desired nanocrystal morphology with a desired set of properties.