Fast synthesis of monoclinic VO2 nanoparticles by microwave-assisted hydrothermal method and application to high-performance thermochromic film

Monoclinic-phase vanadium dioxide (VO2 (M)) has attracted much attention due to the potential for energyefficient smart window applications, owing to its thermochromism in the infrared region induced by insulator to metal phase transition. The composite coatings fabricated by dispersing VO2 (M) nanoparticles in a polymer matrix are superior to continuous VO2 (M) coatings due to their advantages in large solar modulation ability and high luminous transmittance. Ultrafine high-quality VO2 (M) nanoparticles are desired to achieve the excellent thermochromic performance of the VO2-based composite films. In this study, we present a fast synthesis of ultrafine VO2 (M) nanoparticles by a microwave-assisted hydrothermal method. The crystal phase and morphology evolution along the synthesis process was studied in detail. Low-reaction temperature (200 degrees C) favored the formation of intermediate nanosheets of VO2 center dot xH(2)O with a lateral size of 50-500 nm. As the temperature increased, the edges of the nanosheets were gradually consumed and grew into nanoparticles with imperfect crystallinity. At higher temperature of 300 degrees C, the VO2 center dot xH(2)O nanosheets collapse and eventually transform into pure single VO2 (M) nanoparticles with average size similar to 20 nm. The optimal duration for the synthesis of VO2 (M) nanoparticles in the microwave hydrothermal reaction at 300 degrees C was only 8 min. The flexible thermochromic films fabricated with the VO2 (M) nanoparticle-polymer composite coating exhibited excellent thermochromic performance with a solar modulation efficiency of 20.73% and luminous transmittance of 55.04%, the values being one of the best reported thermochromic performance of hydrothermally synthesized VO2 (M).