Surface passivation enabled-structural engineering of I-III-VI2 nanocrystal photocatalysts

Abstract

Cation exchange has evolved into a powerful tactic for synthesis of semiconductor nanocrystals (NCs) that are not readily accessible otherwise. Here we have investigated the In³⁺-for-Cu⁺ cation exchange in the dodecahedral-shaped Cu₇S₄ NCs and found that surface passivation, either caused by excess guest cations or ligand molecules, can be exploited to engineer the structural properties of the NCs. By monitoring the parallel reactions carried out under systematically varied conditions, together with the positron annihilation spectroscopy investigation, we have demonstrated that the key element accounting for the observed surface passivation is associated with the copper vacant sites on the surface of Cu₇S₄ NCs. The reduction in In³⁺/Cu⁺ ratio and the presence of strong-binding ligands could enhance the density of surface copper vacancies and boost cation exchange reaction, which therefore alters the competition between the in-diffusion of In³⁺ and out-diffusion of Cu⁺ ions. Such capability to control the inter-diffusion balance in cation exchange (and the accompanying Kirkendall effect) enables the creation of a series of CuInS₂ (and Cu₇S₄@CuInS₂) NCs with various exotic structures, which show different photocatalytic abilities in singlet oxygen generation. This study can not only add more structural complexity and diversity to the semiconductor NCs achievable by cation exchange, but also presents an important guideline for establishing a unifying mechanistic understanding of the reaction kinetics in cation exchange process.