Photo-Chlorine Production with Hydrothermally Grown and Vacuum-Annealed Nanocrystalline Rutile

Photo-generated high-energy surface states can help to produce chlorine in aqueous environments. Here, aligned rutile (TiO 2 ) nanocrystal arrays are grown onto fluorine-doped tin oxide (FTO) substrates and activated either by hydrothermal Sr/Ba surface doping and/or by vacuum-annealing. With vacuum-annealing, highly photoactive films are obtained with photocurrents of typically 8 mA cm −2 at 1.0 V vs. SCE in 1 M KCl (LED illumination with λ = 385 nm and approx. 100 mW cm −2 ). Photoelectrochemical chlorine production is demonstrated at proof-of-concept scale in 4 M NaCl and suggested to be linked mainly to the production of Ti(III) surface species by vacuum-annealing, as detected by post-catalysis XPS, rather than to Sr/Ba doping at the rutile surface. The vacuum-annealing treatment is proposed to beneficially affect (i) bulk semiconductor TiO 2 nanocrystal properties and electron harvesting, (ii) surface TiO 2 reactivity towards chloride adsorption and oxidation, and (iii) FTO substrate performance. Graphical abstract Photo-chlorine production is possible in aqueous chloride media under neutral conditions with rutile nanocrystal arrays under illumination from band gap–matched LEDs.