An abnormal phase transition behavior in VO2 nanoparticles induced by an M1–M2–R process: two anomalous high (>68 °C) transition temperatures

Abstract

Vanadium dioxide (VO₂) has a reversible metal–insulator transition (MIT) at 68 °C and can be used to develop thermally and electrically sensitive devices. In this study, an abnormal phase transition behavior of VO₂ nanoparticles was discovered during the comparison of pristine nanoparticles without and with high temperature thermal treatment. The single phase transition temperature at 65.1 °C for the pristine VO₂ nanoparticles split into two temperatures at approximately 74 °C (T₁) and 84 °C (T₂) after thermal treatment at 400 °C for 6 h. Both temperatures are much larger than 68 °C. Through characterization by Raman and transmission electron microscopy (TEM), the two higher transition temperatures could be well explained by the formation of VO₂ (M2). Grain boundaries were observed during the merger and fusion processes of VO₂ nanoparticles at high temperatures. The grain boundaries and interfacial defects resulted in the dislocation of the lattice structure and produced stress and strain in the VO₂ nanoparticles. Consequently, VO₂ (M2) with a higher temperature formed in the heating process and the initial MIT (M1–R) became an M1–M2–R transition. Moreover, the thermal treatment improves the phase transition enthalpy (ΔH) of VO₂, which promotes the increase in the solar modulation ability (ΔT_sol) of VO₂–PET composite film from 12.8% to 15.2–17.0% without loss in the luminous transmittance. These findings are of great significance to the deep understanding of MIT and the development of VO₂ smart windows.