D. Teixeira, R. Quesada-Cabrera, M.J. Powell, G.K.L. Goh, G. Sankar, I. Parkin, R. Palgrave
University College London,
Keywords: VO2, pH, morphology, thermochromic
Summary:Vanadium dioxide (VO2) it is a thermochromic material with a reversible transition phase from metal to insulator (MIT) at a 68 °C. The MIT transition temperature of the VO2 near room temperature makes this material a great candidate for smart windows applications. Vanadium dioxide has a wide and well known range of stable phases such VO2(M) and VO2(R) and meta-stable phases as VO2(A), VO2(B) and VO2(C). Newly reported VO2(D) meta-stable phase and Tungsten doped VO2(D) is presented and analysed in this work as an easy, cost effective and reproducible way to obtain thermochromic VO2(R) and also to be used for many applications like batteries or supercapacitors due to different morphologies. A variety of morphologies of this new phase VO2(D) have been synthesized via hydrothermal synthesis using ammonium metavanadate and V2O5 as a precursors. The pH of the starting solution was adjusted from 0.6 to 6.9 using NaOH (1M) . The different pH in the reaction solution determines the morphology and size of the final VO2(D) product. In this work a flower shape morphology is reported when the pH is acid (0.65) passing through star shape particles of 1 µm at pH 2.5 and finally obtaining homogeneous round balls of around 3 µm at 6.9 pH. Nanoparticles between 20-50 nm were obtained at solution pH 6.50 using V2O5 . VO2(D) can be transformed into VO2(R) (with thermochromic properties) annealing the sample in vacuum at 165°C, for samples prepared using V2O5, or at 300°C, for samples prepared using NH4VO3 (lower temperature than reported in literature). The MIT transition temperature (depending on the particle size) reported after annealing the samples is between 61 to 51 °C ± 1 °C for the heating cycle and 53 to 41 °C ±1 °C for the cooling cycle (lower than reported in the literature for VO2(R) to VO2(M) transition). Tungsten doped VO2(D) samples were also successfully synthesized (presenting different sizes and morphology depending on the solution pH) and annealed in vacuum to obtain VO2(R) showing a metal to insulator transition temperature from 60 – 20 °C depending on the percentage of dopant.