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 V 4? from further oxidization and contribute to the formation of porous structure. These films exhibit enhanced visible trans- parency and switching property at near-infrared wave- lengths across the metal-insulator transition (MIT). The transmittance change in the VO2 film annealed at 2.0 9 10 3 Pa is as high as 52.9 % at k = 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 C, while the films remain the excellent modulating ability in near-infrared region, and the decreasing effi- ciency of V0.99W0.01O2 film can achieve to 20 K/at.%, which will greatly favor the practical application of VO2- based smart windows. Graphical Abstract Transmittance spectra for pure VO2 film in the range of 250-2500 nm and the recorded trans- mittance-temperature hysteresis loop in the range of 20-90 C of the W-doping VO2 films after annealing at 500 C for 30 min (middle inset).