Mesoporous SiO^sub 2^/VO^sub 2^ double-layer thermochromic coating with improved visible transmittance for smart window

Thermochromic vanadium dioxide (VO2) film is a potential material for smart-window. But the high refractive index (>2) of VO2 coating in visible band resulted in strong reflection and low visible transmission. If another coating with a definite low refractive index is laid on VO2 film to construct a gradient index multilayer system, the total reflection could be effectively reduced. Therefore in this paper we designed a mesoporous SiO2/VO2 double-layer system in which the mesoporous SiO2 layer possessed an index adjustable from 1.243 to 1.354 to reduce the surface reflection of VO2-based smart window. The mesoporous structure has been investigated through several techniques including 2D GISAXS, TEM and N2 ad/desorption. More important, a facile, safe and low-cost solution method was employed to prepare VO2 film with 60 nm thickness directly from ammonium citrato-oxovanadate (IV) compound that was reported by us previously. On this newly designed mesoporous SiO2/VO2 double-layer coating, the integral visible transmittance at 25 °C (Tvis,L) increased to 80.0% from 69.8% of pure VO2 coating while the corresponsive integral visible transmittance at 90 °C (Tvis,H) increased to 78.9% from 67.6% of pure VO2 coating, if the index of mesoporous SiO2 layer was optimized to 1.299. Simultaneously, the near infrared switching ability at 2000 nm (ΔT2000) reached 29.0% and solar energy modulation (ΔTsol) was maintained at 10.2%. The phase transition temperatures (Tt) for VO2 coating and the optimized SiO2/VO2 double-layer coating were 51.9 °C and 53.8 °C respectively, far below 68 °C of bulky VO2. This optical performance should be very attractive for application in the further smart window because of little increased cost and greatly enhanced property.