Controllable synthesis of VO2(D) and their conversion to VO2(M) nanostructures with thermochromic phase transition properties

Abstract

VO₂(M) nanostructures of various shapes were synthesized by a hydrothermal-calcination method. First, VO₂(D) nanoparticles were synthesized by the surfactant-free hydrothermal reduction of ammonium metavanadate by oxalic acid at 160–220 °C. Then, the produced VO₂(D) was further calcined at 250–600 °C to obtain the VO₂(M) nanoparticles. To understand the hydrothermal reduction processes, both in situ powder X-ray diffraction (PXRD) and ex situ characterization were carried out. The results indicate a sequential process starting from the reduction of ammonium metavanadate and nucleation of the vanadium precursor, followed by the formation of intermediate VO₂(B) nanosheets or nanorods, and finally phase transformation from VO₂(B) to VO₂(D) with a variety of morphologies. A crystal growth mechanism based on self-assembly and Ostwald ripening was proposed to explain the formation process of these unique nanostructures. The as-prepared VO₂(M) nanoaggregates exhibited a lower thermochromic phase transition temperature (41.0 °C) and a narrower thermal hysteresis width (6.6 °C) than those nanopowders prepared by other methods.