Enhanced visible and tunable infrared transmittance of W-doped VO₂/SiO₂/PVP composite films for smart windows

As a thermochromic material, vanadium dioxide (VO₂) has great application potential in smart windows. It has always been the focus of research to improve the visible light transmittance (T_lum) and solar modulation ability (ΔT_sol) while reducing the phase transition temperature (T_c) of VO₂. To achieve this goal, tungsten-doped VO₂ nanoparticles (W-VO₂ NPs) with a T_c of 34 °C were used to form composite films with polyvinylpyrrolidone (PVP) by a simple method. High ΔT_sol of 14.2% was obtained at the optimized volumetric concentration of W-VO₂ and film thickness. The W-VO₂/SiO₂/PVP composite films were proposed to further improve T_lum and ΔT_sol. Compared with the W-VO₂/PVP composite film with the same thickness and volumetric concentration of W-VO₂, T_lum of the film is increased by 19.3% (from 54.5% to 65%) with the introduction of SiO₂ microspheres, while the ΔT_sol is maintained satisfactorily at 12.6%. This is mainly due to the monolayer dispersed SiO₂ microspheres providing optical pathways in the film, which greatly improves T_lum with a slight sacrifice of ΔT_sol. Indoor temperature of a scale-model room with W-VO₂/SiO₂/PVP film-coated glass was 9.1 °C lower than that with blank glass due to the isolation effect of metallic W-VO₂ on near-infrared radiation. This work provides a new strategy for promoting the development of VO₂ thermochromic smart windows.