Metastable Tetragonal Cu2Se Hyperbranched Structures: Large-Scale Preparation and Tunable Electrical and Optical Response Regulated by Phase Conversion

Despite the promising applications of copper selenide nanoparticles, an in-depth elucidation of the inherent properties of tetragonal Cu2Se (beta-Cu2Se) has not been performed because of the lack of a facile synthesis on the nanoscale and an energy-intensive strategy is usually employed. In this work, a facile wet-chemical strategy, employing HCOOH as reducing agent, has been developed to access single-crystalline metastable beta-Cu2Se hyperbranched architectures for the first time. The process avoids hazardous chemistry and high temperatures, and thus opens up a facile approach to the large-scale low-cost preparation of metastable beta-Cu2Se hyperbranched architectures. A possible growth mechanism to explain the formation of the beta-Cu2Se dendritic morphology has been proposed based on time-dependent shape evolution. Further investigations revealed that the metastable beta-Cu2Se can convert into the thermodynamically more stable cubic a-Cu2-xSe maintaining the dendritic morphology. An increase in electrical conductivity and a tunable optical response were observed under ambient conditions. This behavior can be explained by the oxidation of the surface of the beta-Cu2Se hyperbranched structures, ultimately leading to solid-state phase conversion from beta-Cu2Se into superionic conductor a-Cu1.8Se, which has potential applications in energy-related devices and sensors.