Photoelectrochemical Properties of WO3/BiVO4 junctions and their application to the degradation of emerging contaminants in aqueous solution.

The photoelectrochemical properties of electrodeposited WO3/BiVO4 junctions were considered for photoassisted electrolysis in the presence of various electrolytes containing sulfate, phosphate and borate salts. It was found that, with respect to sulfate salts, the anodic overpotential in the presence of weak bases was reduced by several hundred millivolts, while the oxygen production yield was ca. 5 fold larger than WO3 and nearly doubled in phosphate buffers with respect to sulfate electrolytes. This effect was also accompanied by a reduced production of hydrogen peroxide. This evidence points to a proton coupled electron transfer rate determining step for a multielectron oxygen evolution mechanism on the BiVO4 surface. Consistently, Electron Paramagnetic Resonance carried out on illuminated BiVO4 powders was unable to detect the presence of OH radicals resulting from the monoelectronic oxidation of water. On the contrary, ·OH where primary products following illumination of WO3 in otherwise identical conditions. This points to a limitation of WO3/BiVO4 junctions to the photoelectrochemical remediation of water from emerging contaminants, like antibiotics and their metabolic products. Indeed, while WO3 is able to lead, under visible illumination to a complete mineralization of the organics, thanks to ·OH mediated oxidation, WO3/BiVO4 only achieve a partial degradation of these, with the harder to oxidize degradation byproducts surviving in aqueous solution beyond tens of hours of illumination. Nevertheless, WO3-BiVO4 could be exploited with some advantage in photo-(electro)-catalytic cycles where the milder oxidative power of holes confined into BiVO4 could be exploited, possibly in combination to catalytic substrates, to produce added-value chemicals, without triggering undesired ·OH mediated side reactions.