An Efficient Route to Nanostructured Tungsten Oxide Films with Improved Electrochromic Properties

Abstract

An effective chemical route to nanostructured tungsten oxide films derived from a peroxopolytungstic acid (PTA)/thiourea precursor solution is demonstrated. The conventional procedure of preparing the precursor needs more than 24 h for well-mixing and refluxing the PTA-based solution, while the thiourea-assisted approach takes less than 1 h to prepare the precursor solution because the excess hydrogen peroxide can be efficiently eliminated by oxidation of thiourea. With the precursor solution, tungsten oxide films are deposited by spin coating followed by high temperature annealing. The film annealed at 400 °C possesses a porous nanostructure of nanocrystalline tungsten oxide embedded in an amorphous tungsten oxide matrix, which arises from the gaseous species released through decomposition of thiourea oxides during annealing. The 400 °C-annealed, thiourea-assisted tungsten oxide film exhibits electrochromic (EC) properties superior to those of the film prepared without thiourea, including large transmittance modulation and coloration efficiency, fast response time and adequate reliability. When increasing the annealing temperature to 450 °C, the thiourea-assisted tungsten oxide film is also porous but well-crystallized and shows inferior EC properties. Electrochemical impedance spectroscopy analysis indicates that, in addition to the porous structure, a fast charge-transport rate within the solid portion of the 400 °C-annealed nanostructured film plays a crucial role in enhancing EC performances of the thiourea-assisted tungsten oxide film.