Freeze-drying induced nanocrystallization of VO2 (M) with improved mid-infrared switching properties

Abstract

Vanadium dioxide (VO2) with reversible metal-semiconductor transition is one of the most researched smart materials due to its near room-temperature transition at around 68 °C. Coating VO2 (M) nanoparticles onto substrate is superior to fabrication of VO2 (M) thin films owing to its advantages on large-scale preparation and flexible shaping. In this study, freeze-drying has been applied in a convenient precipitating and following annealing process of fabricating VO2 (M) nanoparticles. More importantly, effects of freeze-drying and pre-freezing temperature on phase composition, size distribution and phase-transition temperature of synthesized particles have been systematically researched by varying the pretreatment conditions. It is observed that the application of freeze-drying is crucial to the formation of pure VO2 (M), meanwhile, phase-transition temperature and size distribution of VO2 (M) nanoparticles can be effectively reduced with lowered pre-freezing temperatures. Besides, the influence of different annealing temperatures on nanocrystallite size has been investigated, and it has been found that both the crystallinity and particle size increased as the annealing temperature was increased. Finally, the as-fabricated VO2 (M) particles were coated onto microscopic glass substrate to characterize mid-infrared modulation performance. The coated film (made from sample pre-frozen at −50 °C) exhibits an excellent mid-infrared transmittance change of 23.3% at a fixed wavelength of 4 μm. This work demonstrates the effect of freeze-drying on the fabrication of VO2 (M) nanoparticles and provides a facile method for large-scale preparation of VO2 (M) nanoparticles.