Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings

A novel multifunctional window was demonstrated using VO2-based thermochromic films with a TiO2 antireflection coating. A strong enhancement in luminous transmittance of VO2 was calculated using either a single layer or a double layer of TiO2 for antireflection, with the double layer giving the better performance. A TiO2 (25 nm)/VO2 (50 nm)/TiO2 (25 nm) structure, of which the film thickness has been optimized to a maximum integrated luminous transmittance (T-lum) by calculation, was formed on SiO2 glass by sputtering in turn a V target in Ar+O-2 for VO2 and a TiO2 target in Ar for TiO2. Optical properties were measured with a spectrophotometer, and the integrated solar and luminous properties were calculated based on the measured spectra. A maximum increase in T-lum by 86% (from 30.9% to 57.6%) was obtained for VO2 after double-layer TiO2 antireflection coating. The deposited VO2 film on SiO2 exhibits good thermochromism with a sharp optical transition at 58.5 degreesC, which decreased slightly to 57.5 degreesC after TiO2 coating. The proposed window is the most advanced among those similarly reported in being multifunctional with automatic solar/heat control, ultraviolet stopping and possibly a wide range of photocatalytic functions in addition to a high value of the luminous transmittance.