Enhancement Effects of Cobalt Phosphate Modification on Activity for Photoelectrochemical Water Oxidation of TiO2 and Mechanism Insights

Cobalt phosphate-modified nanocrystalline TiO2 (nc-TiO2) films were prepared by a doctor blade method using homemade nc-TiO2 paste, followed by the post-treatments first with monometallic sodium orthophosphate solution and then with cobalt nitrate solution. The modification with an appropriate amount of cobalt phosphate could greatly enhance the activity for photoelectrochemical (PEC) water oxidation of nc-TiO2, superior to the modification only with the phosphate anions. It is clearly demonstrated that the enhanced activity after cobalt phosphate modification is attributed to the roles of cobalt(II) ions linked by phosphate groups with the surfaces of nc-TiO2 mainly by means of the surface photovoltage responses in N-2 atmosphere. It is suggested that the linked cobalt(II) ions could capture photogenerated holes effectively to produce high-valence cobalt ions, further inducing oxidation reactions with water molecules to rereturn to cobalt(II) ions. This work is useful to explore feasible routes to improve the performance of oxide-based semiconductors for PEC water splitting to produce clean H-2 energy.