Crystallised mesoporous TiO2(A)–VO2(M/R) nanocomposite films with self-cleaning and excellent thermochromic properties

Abstract

VO₂(M/R) (Monoclinic/Rutile) exhibits reversible insulator–metal transition (IMT) near room temperature, and is a widely used film material for thermochromic smart windows. Recent studies have shown that VO₂-based nanocomposite coatings show better optical performance (luminous transmittance, T_lum and solar energy modification ability, ΔT_sol) than pure VO₂ thin films. In previous studies, we succeeded in preparing VO₂-based organic–inorganic (O–I) composite coatings and optimising their optical performance to a level higher than ever reported previously. Compared to O–I composite coatings, inorganic–inorganic (I–I) composite coatings show advantages in weatherability and multifunctions (for example, self-cleaning TiO₂–VO₂ composites). In this work, we succeeded in dispersing VO₂ nanoparticles (NPs) in TiO₂ sol by surface modification, and prepared high quality I–I nanocomposite films with comparable optical performance to that of the best pure VO₂ thin film. With the help of thermal analysis, a two-step annealing process was developed, and crystallised TiO₂(A)(anatase)–VO₂(M/R) composite films were obtained. By introducing mesopores, the optical performance was improved to T_lum = 62.0% (20 °C), 60.5% (90 °C) and ΔT_sol = 14.6%, which was better than that of any reported VO₂-based pure films. In addition, due to the crystallised TiO₂(A), these composite films showed self-cleaning properties (a low contact angle and photocatalytic decomposition of organic contaminants).