“…It was also suggested that despite the decrease in band gap energy, metals were not always successfully introduced into the TiO 2 framework, and that the remaining metals on the outside surface sheltered the photoreaction sites [44]. Significant attention has been directed at reducing the band gap and in turn increasing the visible light activity of TiO 2 by doping with a range of non-metal elements including nitrogen [34,[45][46][47][48][49], carbon [43,[50][51][52][53], sulphur [54][55][56], phosphorous [57], boron [58,59], oxygen [60], and fluorine [61,62], along with heterojunctions [63][64][65][66][67] and combinations of these elements [68][69][70][71][72][73].…”