Effects of annealing parameters on phase, structure and thermochromic properties of VO₂ (M) derived from nanostructured VO₂(B)

Synthesis of thermochromic VO₂ (M) was successfully done by annealing hydrothermally-prepared VO₂ (B) at different temperatures and times. Conversion of the metastable VO₂ (B) to the thermochromic VO₂ polymorph was studied using thermogravimetric analyzer (TGA) under N₂ atmosphere. Moreover, the phase and morphology of the synthesized samples were studied using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), respectively. Accordingly, the XRD scans of all the annealed samples exhibited the presence of monoclinic VO₂ (M), while the FE-SEM images of the samples showed the formation of nanorods and nanospheres, particularly those heated at high temperatures (650 °C and 700 °C). Meanwhile, differential scanning calorimetry (DSC) was used to measure the phase transition temperature (τ_c), hysteresis, and enthalpy of the prepared VO₂. Based on these results, all samples displayed a τ_c of about 66 °C. However, the hysteresis was high for the samples annealed at lower temperatures (550 °C and 600 °C), while the enthalpy was very low for samples heated at lower annealing time (1.5 h and 1 h). These findings showed that crystallinity and nanostructure formation affected the thermochromic properties of the samples. In particular, the sample annealed at 650 °C showed better crystallinity and improved thermochromic behavior.