Microstructure and electrochromic properties of nanostructured tungsten oxide thin films prepared by the vacuum filtration and transfer method

Nanostructured tungsten oxide films for electrochromic applications are fabricated by using a new method - vacuum filtration and transfer onto Indium Tin Oxide (ITO) substrates. It is found that the thickness and morphology of the film depend on the size of nanoparticles, the amount of the filtrated material, the pore size of the membrane, and the temperature of the post-deposition heat-treatment. The films show interesting and quite unexpected electrochromic properties, especially regarding the response time. Compared to films fabricated through sol-gel methods, the response time, especially, the bleaching time, is considerably shorter, in spite of the larger thickness of the filtrated films. As shown by chronoamperometry measurements, the bleaching process is considerably faster than the coloring. This characteristic is reinforced by the diffusion coefficients calculated from both cyclic voltammetry and electrical impedance spectroscopy data. The vacuum filtration method proved to be well-suited for the fabrication of electrochromic films based on single materials and can be easily extended to doped materials and mixtures.