He, Z.Q., et al. “Activity and Selectivity of Cu and Ni Doped TiO2 in the Photocatalytic Reduction of CO2 With H2O Under UV-Light Irradiation”. Asian Journal of Chemistry, vol. 26, no. 15, July 2014, pp. 4759-66, doi:10.14233/ajchem.2014.16199.