Effects of Coapplication of Rh-Doping and Ag-Substitution on the Band Structure of Li₂TiO₃ and the Photocatalytic Property

Formation of impurity levels by doping and control of valence bands by substitution with suitable elements into photocatalyst materials are useful strategies for sensitization of metal oxide photocatalysts with a wide band gap to visible light. In the present study, we demonstrated energy gap narrowing of a metal oxide photocatalyst by coapplication of the doping and the substitution. Rh-doped and Ag(I)-substituted Li₂TiO₃ (AgLi_1/3Ti_2/3O₂:Rh, 2.2-2.3 eV) was successfully synthesized from Rh-doped Li₂TiO₃ (Li₂TiO₃:Rh, 3.0-3.2 eV) with a molten AgNO₃ treatment. The AgLi_1/3Ti_2/3O₂:Rh possessed a narrower energy gap than either Li₂TiO₃:Rh or Ag(I)-substituted Li₂TiO₃ (AgLi_1/3Ti_2/3O₂, 2.8 eV). The AgLi_1/3Ti_2/3O₂:Rh showed the activity for photocatalytic O₂ evolution from an aqueous AgNO₃ solution under irradiation of light with wavelength longer than 480 nm under which AgLi_1/3Ti_2/3O₂ did not show the activity. The photocatalytic activity of AgLi_1/3Ti_2/3O₂:Rh for the sacrificial O₂ evolution became ∼4 times greater by Sb-codoping. The optimum doping levels of Rh and Sb were 1% and 2% in Li_1/3Ti_2/3O₂ layers of AgLi_1/3Ti_2/3O₂:Rh,Sb, respectively.