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 (ΔTsol) 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.