Particle size, morphology and phase transitions in hydrothermally produced VO2(D)

Abstract

Different morphologies and sizes of VO₂(D) particles were synthesised via hydrothermal synthesis using ammonium metavanadate (NH₄VO₃) or vanadium pentoxide (V₂O₅) as a vanadium precursor. By adjusting the concentration of vanadium precursors and the pH of the starting solution, a variety of morphologies and sizes of VO₂(D) particles from 20 nm to 3 μm could be produced. A flower-shape morphology was obtained under strongly acidic conditions, passing through star-shape particles of 1 μm at pH 2.5 and finally obtaining homogeneous round balls of around 3 μm at pH 6.9. Nanoparticles were produced hydrothermally using V₂O₅ as a precursor and hydrazine as a reducing agent. The transition from VO₂(D) to thermochromic VO₂(R) in micron scale particles occurred at 350 °C under vacuum. However, the nanoparticles of VO₂(D) had a significantly lower VO₂(D) to thermochromic VO₂(R) transition temperature of 165 °C after annealing for only a few minutes. This is, to our knowledge, the lowest annealing temperature and time reported in the literature in order to obtain a thermochromic VO₂ material via another VO₂ phase. After the conversion of VO₂(D) microparticles to thermochromic VO₂(R), the metal to insulator transition temperature is 61 ± 1 °C for the heating cycle and 53 ± 1 °C for the cooling cycle. However, VO₂(R) nanoparticles showed a significantly reduced metal insulator transition temperature of 59 ± 1 °C and 42 ± 1 °C for the cooling cycle lower than that reported in the literature for bulk VO₂. This is important due to the need for having a compound with a switching temperature closer to room temperature to be used in smart window devices for energy consumption. W-VO₂(D) star shape microparticle samples were prepared using 2–7 at% of the dopant (using ammonium metavanadate as a precursor), although unexpectedly this does not seem to be a viable route to a reduced metal to insulator transition in this system.