Formation mechanisms and crystallographic characteristics of metastable VO2(A) nanofiber hydrothermally synthesised in V2O5–H2C2O4–H2O system

Abstract

There are many similarities between VO2(B) and VO2(A) from crystallographic view. However, missing of VO2(A) during the preparation of VO2 polymorph confused many researchers. Here, the preparation of VO2(A) was studied systemically via a hydrothermal method in V2O5–H2C2O4–H2O system. As a metastable phase, it can be transferred from VO2(B) by assembling process. Usually, poly-crystal VO2(A) nano-fibers are formed by this process. On contrast, owing to the small energy gap between meta-stable VO2(A) and stable VO2(R), single crystal VO2(A) with regular shape can also be obtained by exfoliating some parts of VO2(R) during non-equilibrium cooling process. VO2(A) has higher phase transition temperature than stable VO2(R). The hysteresis in this phase transition can be observed by DSC measurement and the phase transition temperature of VO2(A) can be tuned down by tungsten doping.