W-Nb Co-Doped VO2 Films Realizing Near Room-Temperature Transition and Satisfactory Thermochromic Performance for Smart Window

The W-Nb co-doped VO2 films are prepared through hydrothermal method. The effects of the Nb and W dopants are investigated respectively on the phase transition temperature (θc) and optical properties of VO2 by keeping the concentration of partner dopant at 1.0 at.%. The Nb doping induces a reduction of θc at a rate of ~-13.0 ℃ per at.% Nb as Nb is less than ~1.5 at.%. For more than 1.5 at.% Nb, the θc shows a slight increase from ~23.0 ℃. The W doping leads to a linear decrease of θc with a rate of ~-17.2 ℃ per at.% W, more effective in reducing θc than the Nb dopant. However, the heavy W doping results in more serious deterioration of the solar energy modulation (ΔTsol) than the Nb doping. Therefore, taking use of the complementary advantages of W and Nb dopants, the 1.0 at.% W + 1.5 at.% Nb co-doped VO2 realizes the room-temperature transition at 23.0 ℃ with a satisfactory ΔTsol of ~9.6%, much better than the 1.5 at.% W + 1.0 at.% Nb co-doped which has a θc of ~22.1 ℃ and ΔTsol of ~5.3%. This work demonstrates the W-Nb co-doping is an effective doping formula in improving the performance of VO2 for smart window applications.