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

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-yW_xMo_yO₂ 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-zW_xTi_zO₂ 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-yW_xMo_yO₂, 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-zW_xTi_zO₂ (x=0.018 and y=0.10) showed thermochromic switching almost without any hysteresis at a temperature as low as 45°C.