We investigate the photoconversion of aqueous 8 nm Ag nanocrystal seeds into 70 nm single crystal plate nanoprisms. The process relies on the excitation of Ag surface plasmons. The process requires dioxygen, and the transformation rate is first-order in seed concentration. Although citrate is necessary for the conversion, and is consumed, the transformation rate is independent of citrate concentration. We propose a mechanism that accounts for these features by coupling the oxidative etching of the seed and the subsequent photoreduction of aqueous Ag+. The reduced Ag deposits onto a Ag prism of specific size that has a cathodic photovoltage resulting from plasmon "hot hole" citrate photo-oxidation. This photovoltage mechanism also explains recent experimental results involving single and dual wavelength irradiation and the core/shell synthesis of Ag layers on Au seeds.