Activating Layered Perovskite Compound Sr2TiO4 via La/N Codoping for Visible Light Photocatalytic Water Splitting

Solar water splitting into H2 and O2 upon a particulate photocatalyst relies on significant advances in material engineering where a number of important properties such as optical absorption, charge transportation, defect level, etc. can be manipulated. In this work, we have gained control over these properties for the wide band gap semiconductor Sr2TiO4 and successfully actualized water splitting under visible light illumination. This has been realized by codoping La/N into the laminated perovskite structure of Sr2TiO4. Strong visible light absorption as far as 650 nm can be tailored by varying the La/N content in Sr2–xLaxTiO4–yNy (0 ≤ x ≤ 0.5). Optimal photocatalytic H2 and O2 production has been achieved at x = 0.2, 0.3 and outweighs that of a number of typical perovskite oxynitrides. These activities are a function of several important parameters for photogenerated charges (e.g., concentration, mobility, and lifetime) which are all linked to La/N codoping levels. More strikingly, overall water splitting has been achieved at x = 0.2 with Rh/Cr2O3 as a cocatalyst. Defects such as Ti3+ species play a negative role in the photocatalytic activity, as they strongly promote charge recombination and shorten the electron lifetime. Theoretical calculations reveal the crucial role of N in uplifting the valence band maximum of Sr2TiO4 by hybridization with O 2p orbitals. La, therefore, balances the charge discrepancies induced during N/O replacements which would otherwise be unfeasible for substantial doping. Our calculations also suggest that Sr2TiO4 has a 2D charge transportation character which is extremely useful for charge separations.