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

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 degrees C. Then, the produced VO2(D) was further calcined at 250-600 degrees 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 degrees C) and a narrower thermal hysteresis width (6.6 degrees C) than those nanopowders prepared by other methods.