Thermochromic Properties of Double-Doped VO₂ Thin Films Prepared by a Wet Coating Method Using Polyvanadate-Based Sols Containing W and Mo or W and Ti

Abstract

A mixture of metallic vanadium, tungsten and molybdenum powder was dissolved in a hydrogen peroxide solution, to yield a polyvanadate sol containing W and Mo. A thin film of V 1- x - y W x Mo y O 2 was fabricated on a fused silica substrate by spin-coating using such a sol followed by reduction in hydrogen and successive annealing in a nitrogen atmosphere. Thin films in the system V 1- x - z W x Ti z O 2 were similarly prepared using a sol containing W and Ti. All the films obtained in this study showed a clear thermochromic switching performance with a large transmittance change in the infrared region. For V 1- x - y W x Mo y O 2 , the switching temperature ( T t ) was lowered by doping at approximately T t = T t0 (=67°C)- ax - by , where a and b are the decrease rates of each individual dopant (about 1920 and 870°C, respectively, for 0≦ x , y ≦0.03). Doping of Ti had little effect on T t , but markedly reduced hysteresis of the transition. We demonstrated that a double-doped film of V 1- x - z W x Ti z O 2 ( x =0.018 and y =0.10) showed thermochromic switching almost without any hysteresis at a temperature as low as 45°C.