Tungsten-doped vanadium dioxide thin films as smart windows with self-cleaning and energy-saving functions

A simple and cost effective sol-gel process is developed for producing thermochromic tungsten-doped thin films of vanadium dioxide (VO2). The precursor is first prepared by the reaction of vanadyl acetylacetonate, methanol, and tungsten chloride in a beaker. The precursor is then spin-coated on the substrate and, finally, annealed at 600 °C in argon gas. The resulting vanadium dioxide thin films are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, variable temperature UV/Vis spectroscopy, and contact angle measurements. The results indicate that the particles of tungsten-doped vanadium dioxide thin films range from 30 to 150 nm, and the thin films show excellent hydrophilicity with a water contact angle (WCA) of 12°. According to optical tests, the tungsten-doped vanadium dioxide thin films exhibit satisfactory optical properties with an applicable integrated luminous transmittance (Tlum,s = 80.75%, Tlum,m = 79.24%) and excellent solar regulation efficiency (△Tsol = 9.10%, from Tsol,s = 81.40% to Tsol,m = 72.30%). The VO2 films exhibit an applicable transition temperature of the semiconductor-to-metal phase change: at a W-doping level of 2 at.%, the transition temperature was measured to be 32 °C. The simple and low-cost method is a noteworthy addition to literature on the synthesis of nanostructured materials towards applications in energy-saving smart windows.