Tungsten doped M-phase VO2 mesoporous nanocrystals with enhanced comprehensive thermochromic properties for smart windows

The phase transition temperature (similar to 68 degrees C) of M-VO2 film can be lowered significantly by tungsten (W) doping into the crystal lattice of VO2 due to the reduction of the strength of V-V pair interaction. However, W doping was always coupled with a serious weakening of luminous transmittance and solar modulation efficiency because W dopants can increase the electron concentration of VO2 film. Herein, the simultaneous introduction of W dopants and mesopores into M-VO2 nanocrystals was employed to prepare VO2 film. Interestingly, the obtained 0.4 at% W-doped mesoporous VO2 nanocrystals based composite films exhibited enhanced comprehensive thermochromic performance with excellent solar modulation efficiency (Delta T-sol = 11.4%), suitable luminous transmittance (T-lum = 61.6%) and low phase transition temperature around 43 degrees C, much lower than 65.3 degrees C of undoped VO2. It was demonstrated that the lower phase transition temperature of VO2 can be primarily attributed to abundant lattice distortion after W doping, whereas the mesoporous structure can facilitate the uniform distribution of W dopants in VO2 nanocrystals, enhance the luminous transmittance and guarantee enough VO2 nanocrystals in the composite film to keep relatively high solar modulation efficiency. Therefore, this work can provide a new way to balance the three important parameters for the thermochromic performance of VO2 film (Delta T-sol, T-lum and T-c) and probably promote the application of VO2 nanocrystals in the energy efficient windows.