“…Since the pioneering work of Asahi et al in 2001 [29] there has been significant work devoted to studying substitutional doping of TiO2 to induce visible light absorption [19,21,26,30,31,32,33,34] ; the majority of work on doping has been with metal cations and more recent work has been on non-metal anion doping [19,20,21,29,30,31] . Since 2009, doping with both cations and anions, socalled co-doping [19,20,32,33,34] , has been well studied.…”