Microwave-assisted nonaqueous synthesis of WO3 nanoparticles for crystallographically oriented photoanodes for water splitting

Nanostructured WO3 photoanodes with crystallographic orientation along the [001] direction were fabricated via doctor blading nanoparticles synthesized through a microwave-assisted nonaqueous sol-gel route. Monoclinic WO3 platelets with a size ranging from 20 to 40 nm and a thickness of 3 nm were obtained after a short reaction time of 10 minutes under microwave irradiation. The films consisted of a porous network of nanoparticles and their photoelectrochemical activity was tested. After cathodic polarization of the photoanodes in the dark which led to a significant increase of 65% of the photocurrent, the films exhibited initially a maximum photocurrent of 2.7 mA cm(-2) at 1 V vs. reversible hydrogen electrode (RHE) in 3 M H2SO4 under simulated AM 1.5 G illumination (100 mW cm(-2)) comparable to the best photocurrents reported for WO3 photoanodes. However oxygen evolution measurements showed that the faradaic efficiency dropped after the cathodic polarization and other products than O-2 might be formed. In comparison to the chemical solution growth of films from molecular precursors, the use of preformed nanoparticles in the form of powders is not only more robust and easier to up-scale, but also offers many opportunities to improve the photoelectrochemical performance by tailoring the nanoparticle size, the shape, and their arrangement on the substrate.