Hydrothermal synthesis and energy saving potential of thermochromic VO2 nanocomposite window coatings in temperate North American climates

Abstract

Buildings in climate zones with both heating and cooling demands could significantly benefit from adopting dynamic windows. Thermochromic VO₂-based nanocomposite coatings have gained attention for their ability to enhance visible transmittance and solar modulation compared to continuous films. However, processing VO₂ nanoparticles with the desired thermochromic properties remains challenging. This article details the hydrothermal synthesis of VO₂ structures and systematically investigates several experimental parameters, including reaction temperature and duration, tungsten doping level, reductant type, vanadium concentration, and precursor pH. The results indicate that the best-performing film was obtained using rod-like particles processed at 280 °C for 72 h, with tartaric acid as a reducing agent. Doubling the initial vanadium concentration improved the crystallinity of the particles and VO₂ (M/R) phase content. Tungsten doping played crucial role in fabricating the desired M/R phases in a single-step synthesis, while lowering the precursor solution’s pH produced uniform VO₁.₇(OH)₀.₃ particles, which were successfully transformed into VO₂ (M/R) after a brief low-temperature annealing at 250 °C for 30 min in air. The best-performing film exhibited a phase transition temperature of 40 °C, a solar modulation ability of 7 %, a visible transmittance of 44 %, and potential annual energy savings of up to 23 % in temperate climates, depending on the local weather conditions.