Phase- and shape-controlled synthesis of VO2 by a hydrothermal-calcination method

VO2(M) structures with various shapes were synthesized by vacuum annealed VO2 metastable phase, and this metastable phase (VO2(B/D)) were formed by hydrothermal method used H2C2O4 and V2O5 powders as starting reactants. The phase structure, morphology, surface composition and phase transition characteristics of VO2 were characterized by XRD, SEM, XPS and DSC. Obtained results show that the VO2's metastable phase structure and morphology strongly depended on the n-butanol/water ratio, molar ratio of V2O5 and H2C2O4 and molar concentration of V2O5, which leads a different synthesis pathway. The influence mechanism has also been studied. Low n-butanol/water ratio (2:45) and low molar ratio (1:1, 1:2) favor the formation of VO2(B) nanosheets/nanorods while high n-butanol/water ratio (45:2) and high molar concentration (0.22 mol/L) favor the formation of VO2(D) micro/nanospheres. Ultimately, the VO2(M) nanoparticles can be obtained at a relatively low vacuum annealing temperature of 300 degrees C, and with a lowest TC was as low as 55.43 degrees C. In addition, the effect of vacuum annealing temperature and morphology on the phase transition characteristic of VO2(M) has also been studied.