Self-Terminating Redox-Reactivity and Vacancy-Induced Void Formation during Electrochemical Collision Synthesis of Hollow Ag/Ag₂S Nanoparticles

We report a mechanistic study of the collision and selfterminating oxidation of individual Ag nanoparticles (NPs, 20 to 113 nm diameter) in aqueous HS- solution at a Au microelectrode, yielding hollow increases for 1 to 8 ms, reaches a maximum of 10 to 40 pA, and then precipitously decreases to zero current. The amount of Ag converted to Ag2S during the collision was determined by coulometric analysis of current transients during the oxidation of individual NPs. Transmission electron microscopy (TEM) images were used to correlate the change in the NP phase and size with the coulometric results. The thickness of the resulting Ag2S layer is independent of the original NP size. We quantitatively demonstrate that as the Ag2S layer grows on the Ag NP following collision, the ohmic potential loss in the film increases to several tenths of a volt, eventually cancelling the driving force required for continued growth and resulting in the sudden cessation of the oxidation reaction. This self-terminating redox reactivity acts as a switch to control the Ag2S layer thickness. On average, similar to 40% of the Ag NP precursor volume was converted to a void within the resulting hollow Ag/Ag2S NP, independent of the initial NP size. A vacancy-induced void formation mechanism is proposed that is consistent with the TEM, electrochemical data, and transport properties of Ag2S.