Self-Densified Optically Transparent VO₂ Thermochromic Wood Film for Smart Windows

Optically transparent wood has emerged as a promising glazing material. Thanks to the high optical transmittance, strong mechanical properties, and excellent thermal insulation capability of transparent wood, it offers a potential alternative to glass for window applications. Recently, thermo-, electro-, and photochromic transparent woods that dynamically modulate light transmittance have been investigated to improve building energy efficiency. However, it remains challenging to widely replace windows with transparent wood because of its poor weather resistance. In this study, an environment-friendly thermochromic transparent wood film (TTWF) with thermal switching of transmittance is proposed and demonstrated. To achieve thermochromism, the bleached wood is impregnated with the vanadium dioxide (VO₂)/polyvinyl alcohol composite. Due to the self-densification of cellulose microfibrils during the evaporation of solvents, the transparent wood is in the form of thin films, which can be attached on the inner face of a window to protect it from severe weather conditions, making the installation convenient and low-cost. Furthermore, the surface of VO₂-TTWF is modified by octadecyltrichlorosilane to enhance the waterproof ability and achieve self-cleaning and antidust functions. The proposed VO₂-TTWF shows great potential for application in energy-efficient buildings using sustainable materials with advanced optical properties (i.e., T_lum = 50.5%, ΔT_sol = 3.4%, and haze = 70%) that are mechanically robust (i.e., σ = 130.6 MPa along the wood growth direction), have low-thermal conductivity (i.e., K = 0.29 W m^-1 K^-1 along the perpendicular direction to the wood fibers), and demonstrate hydrophobic self-cleaning and antidust functions (i.e., contact angle: 121.9°). An experiment, using a model house, showed that the VO₂-TTWF attached on the inner face of the window could significantly reduce the indoor air temperature by 33.9 °C compared with a bare glass panel, proving that VO₂-TTWF has potential to be applied as a new-generation energy-efficient material for smart windows.