Ultra-long VO2 (A) nanorods using the high-temperature mixing method under hydrothermal conditions: synthesis, evolution and thermochromic properties

Abstract

Well-crystallized, ultra-long VO2 (A) nanorods were synthesized using a facile high-temperature mixing method (HTMM) under hydrothermal conditions. The as-obtained products were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-Vis-NIR, differential scanning calorimetry and Fourier transform infrared spectroscopy. The effect of W doping on the phase-transition properties of VO2 (A) was also studied. The results indicate that the optimal synthesis condition of VO2 (A) using the HTMM is hydrothermal treatment at 240 °C for 48 h with a 3 : 1 molar ratio of the reducing agent to the vanadium source. The reason why the polymorphic forms of VO2 show different colors is that the light in the visible region reflected by the samples is different. The phase-transition temperature of the pure VO2 (A) is 154.75 °C, which is significantly lower than the 162 °C reported previously. When a small amount of W is doped, VO2 (A) will be transformed into other polymorphic forms, which indicates that the crystal structure of VO2 (A) is highly sensitive to limited doping. Importantly, the as-obtained pure VO2 (A) shows good thermochromic properties and optical-switching characters. A crystal growth mechanism for VO2 (A), oriented-attachment–exfoliation–recrystallization–oriented-attachment, is proposed and described in detail.