Acceleration of tungsten doping on vanadium dioxide (VO2) by alkali species

The monoclinic vanadium dioxide, VO2(M), undergoes reversible phase transition from monoclinic (semiconductor) to tetragonal (metal), which exhibits a good thermochromic property. VOSO4, as a vanadium source, is easy to handle produces VO2(M) under mild condition in the presence of alkali species. In this study, the effects of the additions of NH4HCO3, NH3·H2O, and NaOH on the VO2 crystal formation with/without tungsten doping for thermochromicity were investigated. NaOH, the strongest base of the three, provided the strongest and narrowest x-ray diffraction peak, while NH4HCO3, the weakest base, provided the opposite. Interestingly, for the tungsten doping to adjust the transition temperature, the use of NH4HCO3 was more suitable due to the possibility of a slow crystal frame formation as compared to the use of NaOH.