“…An appended benefit of transition metal doping like Mn, Cr, Ni, Cu is to improve the trapping of electron to inhibit the e + /h − recombination during irradiation of light [6,7]. "Transition metal doped TiO 2 nanoparticles demonstrated that Ni or Mn doped TiO 2 possess better absorption ability of visible light and Ni/Mn either substitutes Ti 4+ site or embeds in the vacancy of TiO 2 lattice" [8].…”