Preparation of the VO2@SiO2@TiO2/PU composite film. Preparation of VO2 nanoparticles. The VO2 nanoparticles were synthesised using the hydrothermal method according to a previously reported protocol30. In a typical procedure, vanadium pentoxide (V2O5, analytically pure) powder was added to an oxalic acid dehydrate aqueous solution to form a yellowish slurry. Then, the slurry was trans ferred to a 50 mL Teflon – lined stainless-steel autoclave, which was maintained at 240 u C for 24 h, and was then air-cooled to room temperature