Recovery of monoclinic phase of VO2 nanoparticles from oxidized samples: Assessment of reusability and optimization of re-annealing conditions
Abstract
Vanadium dioxide (VO2) nanoparticles in the monoclinic (M) phase exhibit a reversible metal–insulator transition (MIT) at ~68 °C, underpinning thermochromic applications such as smart windows and thermal sensors. However, prolonged atmospheric exposure causes progressive surface oxidation that converts V4+ to V5+, leading to phase degradation and loss of thermochromic function. This study demonstrates that oxidized VO2 nanopowders can be fully recovered by thermal re-annealing in an inert argon atmosphere. Starting from VO2(M) nanoparticles synthesized via a hydrothermal route, controlled oxidation under ambient conditions produced a degraded sample rich in V2O5. Systematic re-annealing at 450-600 °C for 2-3 h identified 500 °C for 2 h as the optimal recovery condition. XRD, Raman spectroscopy, and UV-Vis-NIR transmittance measurements confirm complete phase restoration and recovery of thermochromic switching performance equivalent to the as-synthesized material, establishing a practical strategy for reusing degraded VO2 nanopowder stocks.