Surface modification-assisted solvent annealing to prepare high quality M-phase VO₂ nanocrystals for flexible thermochromic films

Well-crystalline and stoichiometric M-phase vanadium dioxide (VO₂ (M)) ultrafine nanoparticles are desired in fabricating high-efficient VO₂-based flexible thermochromic films. In this study, a novel solvent annealing technique is developed to avoid growth and agglomeration of VO₂ (M) nanoparticles in the post-annealing process. Oxygen-deficient VO₂ (D) nanoparticles (~36 nm) are firstly prepared by a facial hydrothermal method. A double-layered reactor is designed to separate the surfactant-modified VO₂ (D) nanoparticles physically from H₂O₂ which serves as the oxidant. The unique reactor enables the formation of a moderate oxygenation environment in the solvent annealing process, making the transformation of oxygen-deficient VO₂ to VO₂ (M) more controllable. The growth and agglomeration of VO₂ nanoparticles are effectively suppressed due to the surface modification by polyvinyl pyrrolidone, liquid environment and low temperature. The obtained VO₂ (M) nanoparticles have an average particle size of ~41 nm. The flexible film prepared with the VO₂ (M) nanoparticles exhibit excellent thermochromic properties with ΔT_sol reaching 18.03% and T_lum up to 58.33%. This amazing performance is attributed to the high quality of VO₂ (M) nanoparticles and ultrafine particle size that makes the surface plasmon absorption take effect in the solar light region. This work demonstrates the proposed solvent annealing technique is an effective approach to prepare high-quality ultrafine VO₂ (M) nanoparticles for achieving efficient thermochromic performance of the flexible smart windows.