Direct and continuous hydrothermal flow synthesis of thermochromic phase pure monoclinic VO2 nanoparticles

Abstract

Monoclinic vanadium(IV) oxide [VO₂(M)] is a widely studied material due to its thermochromic properties and its potential use in energy-efficient glazing applications. VO₂(M) nanoparticles can be a great advantage for energy-efficient glazing as below 50 nm the nanoparticles poorly interact with visible wavelengths – resulting in an increase in visible light transmittance whilst maintaining the thermochromic response of the material. The direct synthesis of VO₂(M) nanoparticles with effective thermochromic properties will be a step forward towards industrial applications of this material. Unfortunately, many of the synthesis processes reported so far involve multiple steps, including post-treatment, and the synthesis is not always reproducible. In this study, we present the first direct synthesis of pure monoclinic VO₂ nanoparticles by continuous hydrothermal flow synthesis (CHFS). TEM images showed that nanoparticles in the size range of 30–40 nm were produced. The VO₂(M) nanoparticles also showed good thermochromic properties with a solar modulation (ΔT_sol) of 3.8%, as established by UV/Vis spectroscopy. A range of analytical methods was used to characterise the materials, including X-ray absorption spectroscopy (XANES and EXAFS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The influence of niobium (Nb) doping on the physical and thermochromic properties of the VO₂ nanoparticles was also explored. Previous work has shown a sharp metal-to-semiconductor transition of VO₂ upon incorporation of a Nb dopant. The results of the current work suggested that these changes are likely due to changes on the local structure of the oxide.