Photoelectrochemical Transients for Chlorine/Hypochlorite Formation at Roll-On Nano-WO3 Film Electrodes

Commercial nano-WO3 (10-100 nm average particle size) is dispersed in n-hexanol and applied to tin-doped indium oxide (ITO) substrates in a simple roll-on process. As-deposited and annealed (500 degrees C) films are compared and shown to be photoactive for the formation of hypochlorite in neutral aqueous NaCl (e.g., seawater). Annealing at 500 degrees C in air improves photocurrents most likely due to improved interparticle charge transport (e.g., removal of hydration or n-hexanol surface layers). In phototransients (interpreted here in the limiting case of a weakly associated nanoparticle aggregate as opposed to the limiting case of a single-crystal semiconductor), evidence for the presence of both holes (as O(-I), fast moving) and electrons (as W(V), slow moving) is obtained in particular in as-deposited films. Bipotentiostat experiments reveal the presence of chlorine as a reaction intermediate close to the photoanode when immersed in 3 M NaCl. A molecular conduit effect with adsorbed Co(II/III) sepulchrate is observed to significantly enhance the photocurrents at as-deposited electrodes (but not at annealed electrodes).