Electrochromic Properties of Sol-Gel Synthesized Macroporous Tungsten Oxide Films Doped with Gold Nanoparticles

Macroporous tungsten oxide thin films were prepared by using a polystyrene microsphere template and the peroxitungstic acid precursor. Gold nanoparticles were introduced into the tungsten oxide film by two different methods, either in the polystyrene template, through a self-assembly process, or they were cast on the surface of the annealed film. Raman imaging experiments showed an enhancement effect of the intensity of the major tungsten oxide bands by the gold nanoislands. The electrochromic properties of gold-doped macroporous tungsten oxide are described and compared to those of the standard porous sol-gel film. The films with gold nanoparticles on the surface of the tungsten oxide film were found to have better electrochromic properties than the standard sol-gel tungsten oxide films and the ones doped with gold in the polystyrene template. The diffusion coefficients of protons in the gold-doped tungsten oxide films were determined from both Cyclic Voltammetry (CV) and Electrical Impedance Spectroscopy (EIS) measurements. The results indicate a faster diffusion of the proton in the porous film, compared with a compact tungsten oxide film. The model that evolves from the EIS data, involves fast charge transfer rates at the electrolyte/tungsten oxide film interface. The results demonstrated an enhancement of the coloration efficiency of the macroporous tungsten oxide films with Au nanoparticles on the surface of the film. The possible involvement of plasmon resonance of Au nanoparticles in the electrochromic process is discussed.