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

Abstract

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 ΔTsol reaching 18.03% and Tlum 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.