Effects of the annealing process on the structure and valence state of vanadium oxide thin films

[Display omitted] •TGA-DSC was used to determine the annealing temperature generally.•Pure VO2 (M) film was obtained with some annealing process.•The film surface transits from layer to grains as the annealing temperature increases.•Spans of O1s and V2p3/2 core level of V5+, V4+, V3+ were 12.75, 14.0 and 14.7 eV.•The Tc of pure VO2 (M) was 64.3 °C, its hysteresis loop width was 12.1 °C. V2O5 was used to prepare vanadium oxide films via the sol-gel method. The “n” in V2O5∙nH2O was determined to be approximately 1.67. After annealing at 390 °C for 4 h, a pure VO2 (M) phase was obtained and denoted sample Ⅴ. The crystallite sizes of VO2 (M) in samples Ⅱ, Ⅲ, Ⅳ and Ⅴ were 42, 41, 35 and 24 nm, respectively. The surface morphology of sample Ⅰ was flat; however, many small, dense particles covered the surface of sample Ⅴ. Sample Ⅱ had a layered structure, and samples Ⅲ and Ⅳ consisted of compact grains. The binding energy (BE) differences between the O1s and the V5+2p3/2, V4+2p3/2, and V3+2p3/2 core levels were 12.75, 14.0 and 14.7 eV, respectively. For the sample Ⅴ film, the maximum transmittance change at 2500 nm was 39%, the phase transition temperature (Tc) was 64.3 °C, and the hysteresis loop width was 12.1 °C.