Two-Step Synthesis of VO 2 (M) with Tuned Crystallinity.

Highly crystallized monoclinic vanadium dioxide, VO2 (M), is successfully synthesized by a two-step thermal treatment: thermolysis of vanadyl ethylene glycolate (VEG) and postannealing of the poorly crystallized VO2 powder. In the first thermolysis step, the decomposition of VEG at 300 °C is investigated by X-ray diffraction and scanning electron microscopy (SEM). A poorly crystallized VO2 powder is obtained at a strict time of 3 min, and it is found that the residual carbon content in the powder played a critical role in the post crystallization of VO2 (M). After postannealing at 500 and 700 °C in an oxygen-free atmosphere, VO2 particles of various morphologies, of which the crystallite size increases with increasing temperature, are observed by SEM and transmission electron microscopy. The weight percent of crystalline VO2 , calculated using the Fullprof program, increases from 44% to 79% and 100% after postannealing. The improved crystallinity leads to an improvement in metal-insulator transition behaviors demonstrated by sharper and more intense differential scanning calorimetry peaks. Moreover, V2 O3 and V2 O5 with novel and particular microstructures are also successfully prepared with a similar two-step method using postannealing treatment under reductive or oxidizing atmospheres, respectively.