Room temperature chemical transformation of SnSe to Ag2Se nanocrystals via cation exchange

Abstract

Atomic-scale control of the chemical composition of semiconductor nanocrystals through a cation exchange reaction affords greater tunability in the design of multifunctional semiconductor composite nanocrystals. Here, we report a facile route to SnSe–Ag₂Se composite nanocrystals using cation exchange at room temperature. Starting from freshly synthesized SnSe nanorods, we leverage the strong distortion of the Sn²⁺ octahedral coordination in SnSe and the hard–soft acid–base (HSAB) principle, to promote the exchange of an Sn²⁺ ion with two Ag⁺ ions in methanol leading to Ag₂Se/SnSe nanocomposites. The morphology and chemistry of the nanocrystals evolve from nanorods with SnSe@Ag₂Se (core@shell) structures for SnSe-rich composites to nanorods with a random distribution of Sn²⁺ and Ag⁺ ions for nearly equimolar composites, and finally to irregular fragmented nanocrystals for Ag₂Se-rich composites. A mechanistic understanding of the observed morphology evolution is discussed using the change in the cation coordination from octahedral (Sn²⁺) to tetrahedral (Ag⁺) geometry and the accompanying expansion of the hcp Se²⁻ sublattice. Interestingly, the synthesized composite nanocrystals exhibit an optical band gap value tunable within a wide energy range by increasing the Ag₂Se/SnSe ratio. This work provides a useful and facile strategy to modify the optical behavior of semiconductor nanomaterials, which can be leveraged for the design of better optical and/or photovoltaic devices.