One-step hydrothermal synthesis and thermochromic properties of chlorine-doped VO₂(M) for smart window application

The monoclinic phase of VO₂ has promising application as a smart window material because it possesses a reversible metal-to-semiconductor transformation with a critical temperature of 68 °C. The high critical temperature must be lowered to achieve a possible application. Anion doping has been widely researched as possible doping of VO₂(M) with fluorine is the main option nowadays. However, other halogen elements such as chlorine have not been investigated albeit possessing possible advantages properties. In this work, we report the use of chlorine anion as doping for VO₂(M) to lower its critical temperature and to enhance its thermochromic performance. The synthesis was performed using a facile one-step hydrothermal reduction of vanadium pentoxide by hydrazine at 350-490 °C, using ammonium chloride as the source of the anion. The result showed that the optimum temperature to synthesize Cl-doped VO₂(M) was 490 °C. The lowest critical temperature that can be achieved by chlorine-doped VO₂(M) was at 59.9 °C. The thermochromic performance of Cl-doped VO₂(M) was improved compared to pristine VO₂(M) nanoparticle. This finding provides a novel use of chlorine-doped VO₂(M) with a facile one-step hydrothermal method to synthesize chlorine-doped VO₂(M) as well as the feasibility of chlorine-doped VO₂(M) as a smart window material.