“…In order to increase the photocatalytic activity under visible irradiation, extensive studies have focused on doping metals (Fe, Cu, Cr, etc. ) or non-metals (carbon and nitrogen) into TiO 2 to narrow the bandgap of TiO 2 (Zhang and Lei, 2008;Hua et al, 2015;Yang et al, 2014;Chen et al, 2009;Lan et al, 2013). As an alternative, some novel photocatalysts with response to visible light, such as Fe 2 O 3 , WO 3 and metal-free g-C 3 N 4 , have been developed (Zhang et al, 2010;Hepel and Hazelton, 2005;Cheng et al, 2007;Shinde et al, 2013Shinde et al, , 2016.…”