Hydrothermal synthesis of VO₂(B) and its phase transformation to VO₂(M): Investigating metal-insulator transition behavior

In this study, VO_2 (B) nanostructures were synthesized via a stabilizer-free hydrothermal route and successfully converted to VO_2 (M) at 350°C in just 30 minutes. This process offers a reduced thermal budget compared to established protocols, which generally require temperatures above 400°C and durations exceeding one hour. The resulting urchin-like VO_2 nanostructures were characterized using a variety of techniques such as XRD, Raman Spectroscopy, SEM and DSC to investigate their structural evolution, surface morphology, and metal-insulator transition characteristics. The experimental results reveal that VO_2 (B) predominantly transforms to VO_2 (M) upon annealing at 350ºC, with minor secondary oxide phases. Furthermore, the annealing at 450ºC led to the complete phase transformation of VO_2 to V_2O_5 , indicating the sensitivity of VO_2 to annealing temperature. This shows the insulator-to-metal transition temperature to be ~65ºC, which is lower than the bulk VO_2 (~68ºC), indicating modified transition behavior in the nanostructured samples. These results demonstrate that hydrothermally synthesized VO_2 (B) can be converted to predominantly VO_2 (M) by annealing at 350°C for 30 min, enabling observation of a reversible metal-insulator transition near 65°C.