The preparation and characterization of transparent nano-sized thermochromic VO2–SiO2 films from the sol–gel process

VO2, as a promising material for its thermochromic characteristics, is widely used for various thermal and optical applications due to the presence of multiple thermal hysteresis loops resulting in various fabrication conditions. In this study, VO2–SiO2 films were prepared using the sol–gel method from the precursors of VO(OR)3 and Si(OR)4 in various Si/V volumetric mixing ratios ranging from 0 to 1. The spin-coated film was then heat-treated at 600 °C for 15 min in reducing atmospheres. The presence of SiO2 in the VO2 films enhances the luminous transmittance at the spectra of visible-light without compromising the thermochromic characteristics of the films. In addition, the optical contrast of the as-prepared films in the mid-infrared region attains levels up to 50%, a remarkable figure when compared to that of plain VO2 films. Microstructure and morphology studies show that the introduction of silica sols in the preparation of composite films limits the growth of VO2 particles, leading to increasingly wider hysteresis loops as the particles become smaller. Furthermore, the existence of amorphous structure of SiO2 among VO2 crystalline lattices reduces the asymmetric shear stress in cooling, which results in a more symmetric appearance on IR transmittance–temperature hysteresis loops. The relationships between the Si/V mixture ratios and their corresponding thermochromic mechanisms become essential data for material design and future applications.