Mechanistic Insight into the Topotactic Transformation of Trichalcogenides to Chalcohalides

Metal trichalcogenides (A2VB3VI) and chalcohalides (AVBVICVII) make up a class of semiconducting materials with a quasi-one-dimensional crystal structure. These low-symmetry semiconductors have shown favorable optoelectronic properties for photovoltaic and photo-electrocatalysis. Additionally, several chalcohalides, such as SbSI and SbSeI, were recognized as promising photoferroic materials due to their suitable bandgap and ferroelectric properties. A common synthetic strategy transforms trichalcogenides to chalcohalides. However, our understanding of the mechanism of this synthetic approach is limited. Herein, we have demonstrated that this route for transforming Sb2Se3 into SbSeI nanorods via SbI3 deposition and vapor iodination is a topotactic solid-state reaction that preserves the equivalent crystal orientation of the precursor crystal in the product. We have also found that in the process of transformation, the crystal expands, and an "unzipping" process progress along the rod's z-axis, which ultimately results in the breaking of the precursor crystal into nanorods with a diameter that is ∼25% of that of the original crystals. A detailed investigation of the transformation process reveals favorable thermodynamics that proceeds through a series of iodine exchange reactions between the Sb of SbI3 and the Sb of Sb2Se3 and due to the different Lewis basicity of Se ions in the Sb2Se3 ribbon. Density functional theory calculations suggest that this transformation can happen at a low temperature and requires overcoming a small activation energy barrier (10 kcal/mol) if the SbI3 molecules are present in the vicinity of each Sb2Se3 ribbon. A key step of this transformation, which is common to both synthetic routes, is achieved by facile intercalation of SbIx species through the center of the rods. The improved understanding of the A2VB3VI trichalcogenide and AVBVICVII chalcohalide transformation chemistry will facilitate their implementation in emerging applications.