Mesoporous semiconducting films consisting of preferentially orientated monoclinic-phase nanocrystals of tungsten trioxide have been prepared using a novel version of the sol-gel method. Transformations undergone by a colloidal solution of tungstic acid, stabilized by an organic additive such as poly(ethylene glycol) (PEG) 300, as a function of the annealing temperature have been followed by means of a confocal Raman microscope. The shape and size of WO3 nanoparticles, the porosity, and the properties of the films depend critically on preparation parameters, such as the tungstic acid/PEG ratio, the PEG chain length, and the annealing conditions. Well-crystallized WO3 films combine excellent photoresponse to the blue region of the solar spectrum, up to 500 nm, with good transparency at wavelengths larger than 550 nm. Particular applications of these nanocrystalline WO3 films include photoelectrochemical and electrochromic devices.

The photoelectrochemical characteristics of highly transparent nanoporous WO3 films are described. The photocurrent versus excitation wavelength spectra of these photoelectrodes exhibit a maximum close to 400 nm and a significant photoresponse to the blue part of the visible spectrum. The observed conversion efficiencies attain 75% for the photogeneration of oxygen from 1 M aq HClO4 and reach 190% in the presence of methanol in the solution, denoting in the latter case the occurrence of a perfect photocurrent doubling. Experiments conducted under simulated solar AM 1.5 illumination resulted in steady-state anodic photocurrents of the order of several mA/cm2.