Carbon-reduction as an easy route for the synthesis of VO2 (M1) and further Al, Ti doping

Abstract

A low-cost and facile method to synthesize highly crystallized VO₂ (M1) particles is proposed, using carbon black as the reducing agent mixed with V₂O₅ nanopowders comparing two types of vacuum systems for the thermal activation. In a sealed vacuum system, CO gas is generated in the first reductive step, and continues to reduce the new born VO₂, until all the V (+4) is reduced to V (+3), resulting in V₂O₃ formation at 1000 °C. In contrast, in a dynamic vacuum system, CO gas is ejected through pumping as soon as it is generated, leading to the formation of pure VO₂ (M1) at high temperatures (i.e. in the range 700 °C ≤ T ≤ 1000 °C). The evolution of the carbon content, determined by CHNS, of each sample versus the synthesis conditions, namely temperature and type of vacuum system, confirms that the transformation of V (+5) into V (+4) or V (+3) can be controlled. The characterization of the morphologies and crystal structures of two synthesized VO₂ (M1) at 700 °C and 1000 °C shows the possibility to tune the crystallite size from 1.8 to more than 5 μm, with a uniform size distribution and highly crystallized powders. High purity VO₂ (M1) leads to strong physical properties illustrated by a high latent energy (∼55 J g⁻¹) during the phase transition obtained from DSC as well as high resistivity changes. In addition, with this method, dopants such as Ti⁴⁺ or Al³⁺ can be successfully introduced into VO₂ (M1) thanks to the preparation of Al or Ti-doped nano-V₂O₅ by co-precipitation in polyol medium before carbon-reduction.