Improving photo-oxidation activity of water by introducing Ti₃₊ in self-ordered TiO₂ nanotube arrays treated with Ar/NH₃

The synthesis of TiO2 nanostructures with visible light absorption is critical for realizing low-cost and efficient photoelectrochemical hydrogen generation from water splitting. In this work, we report an Ar-NH3-Ar treatment method capable to generate Ti3+ species in anodic self-ordered TiO2 nanotube arrays for the synthesis of substoichiometric TiO2 nanotube arrays. The presence of Ti3+ is verified by XPS, EPR and EDS measurements while no detectable N is observed in the material. The modified TiO2 exhibits a 5-fold enhancement in photocurrent compared to pristine TiO2 at 1.23V vs. RHE due to the increased absorption of the solar spectrum. For the 4-h treated sample, a photocurrent of 1 mAcm−2 is obtained at 1.18V vs. RHE, owing to the reduced flat band potential and the increased density of charge carriers evaluated using EIS measurements. The photocatalytic activity can be further optimized by increasing the layer thickness, leading also to a cathodic shift of the onset potential. Furthermore, the modified TiO2 remains intact in air for no less than 1 week. This enhancement in photo-catalytic activity is proven to be closely relevant to the Ti3+ induced by the Ar-NH3-Ar treatment