Two-dimensional titanium oxide nanosheets rich in titanium vacancies as an efficient cocatalyst for photocatalytic water oxidation

Water oxidation is a crucial step in photocatalytic water splitting and CO2 reduction with H2O, but generally hindered by slow kinetics. Herein, we demonstrated that titanium oxide nanosheets with abundant Ti vacancies (Ti-defected TiO2) efficiently promoted photocatalytic water oxidation, exhibiting even superior performance as compared with RuO2 and IrO2. Along with the decrease of layer thickness, photocatalytic O2 evolution over Ti-defected TiO2/WO3 composite was gradually enhanced, while the highest promotion effect with enhancement factor reached 2.6 was achieved over Ti-defected TiO2 with hierarchal layer thickness. The cocatalyst effect of defected TiO2 nanosheets was validated by experimental and theoretical study. Photoluminescence and ESR measurements showed effective charge carrier separation and transfer in composite photocatalyst. Theoretical simulation demonstrated that H2O adsorbed onto two-fold-coordinated O sites through H-bonding other than O-bonding onto Ti sites over normal TiO2 surface. The corresponding H2O adsorption energy dramatically decreased from −0.37 to −0.82 eV, which was supported by diffuse-reflectance FTIR spectroscopy measurement. Furthermore, the presence of Ti vacancy was found to be energetically favorable for the activation of water molecules, which is manifested as relatively low apparent activation energy (6.8 kJ·mol−1). Our discovery provides a novel and nonprecious cocatalyst candidate for efficient water oxidation and is also a beneficial attempt to explore the potential application of metal vacancies in photocatalysis.