Facile synthesis of VO2 (D) and its transformation to VO2(M) with enhanced thermochromic properties for smart windows

Abstract Pure VO2(M) is usually synthesised by the hydrothermal method and a subsequent annealing process at high temperatures (>400 °C). However, the high-temperature annealing process always leads to large sized VO2(M) particles without reduced phase transition temperature. In this work, VO2(D) spherical particles composed of nanotablets were synthesised by the hydrothermal process and then converted into VO2(M) nanoparticles by annealing at 250–400 °C for 0.5–5 h. The obtained VO2(M) grains exhibited a grain size of 20–30 nm and a reduced phase transition temperature of ~62 °C compared with that of bulk M − VO2 (~68 °C). With an increase in the annealing temperature from 250 °C to 400 °C, the grain size increased from 20 nm to 30 nm, the phase transition temperature increased slightly from 62.17 °C to 62.70 °C, and the hysteresis width increased from 5.72 °C to 7.51 °C. This was possibly attributable to the grain size effect and the interface effect. In addition, the VO2(M) nanoparticle-based film showed an initial increase and then a decrease in the visible transmittance (Tlum) and solar modulation efficiency (ΔTsol). When the annealing time was extended from 0.5 h to 5 h, Tlum of the VO2(M) nanoparticle-based film is firstly increased and then decreased while ΔTsol is increased from 5.74% to 11.90%. Therefore, this work provides a facile way to synthesise pure VO2(M) nanoparticles with reduced phase transition temperature and enhanced thermochromic performance.