Low temperature selective growth of VO2 polymorphs by one-step hydrothermal method

Abstract

Phase pure vanadium dioxide (VO₂) polymorphs, namely, VO₂(A), VO₂(M1), and VO₂(B), were successfully prepared via low temperature ($\le {250}^{\circ }C)$ hydrothermal technique that is often a challenge. Using Taguchi method of design of experiment (DOE) processing conditions for direct synthesis of said polymorphs based on growth temperature, filling ratio, and molar ratio of reactant were established. Qualitative and quantitative analyses were performed using x-ray diffraction, differential scanning calorimetry, RAMAN, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscope. Further, it has been always difficult to coat vanadium oxide nanoparticle on an amorphous substrate. However, by adopting supersonic cold spray technique using de Laval nozzle, obtained nanoparticles were coated on soda lime silica glass substrate as thin film, and subsequent temperature dependent electrical characterizations were carried out. Finally, feasibility of formation of various polymorphs was investigated by calculating Gibbs free energy for each reaction. The obtained ∆G⁰ was (−521 kJ/mol) for VO₂(A), (−694 kJ/mol) for VO₂(B), and (−750 kJ/mol) for VO₂(M1) and was further related to the order of formation of phases. Additionally, an estimation was made on the significance and percentage contribution of each growth parameter, that is, growth temperature, filling ratio, and molar ratio using the algorithm of DOE.