Porous W-doped VO2 films with simultaneously enhanced visible transparency and thermochromic properties

Porous thermochromic pure and tungsten (W)-doped vanadium dioxide (VO2) films have been prepared on silica substrates by spin coating via a sol-gel process and annealing in ammonia (NH3) atmosphere. NH3 with weak reducing capacity can prevent V4+ from further oxidization and contribute to the formation of porous structure. These films exhibit enhanced visible transparency and switching property at near-infrared wavelengths across the metal-insulator transition (MIT). The transmittance change in the VO2 film annealed at 2.0 x 10(3)Pa is as high as 52.9 % at lambda = 2000 nm, and its solar modulation efficiency reaches up to 9.4 %. W-doping shifts the MIT temperature of the VO2 films from 55 to 28 A degrees C, while the films remain the excellent modulating ability in near-infrared region, and the decreasing efficiency of V0.99W0.01O2 film can achieve to 20 K/at.%, which will greatly favor the practical application of VO2-based smart windows. Transmittance spectra for pure VO2 film in the range of 250-2500 nm and the recorded transmittance-temperature hysteresis loop in the range of 20-90 A degrees C of the W-doping VO2 films after annealing at 500 A degrees C for 30 min (middle inset).