Construction of multi-homojunction TiO₂ nanotubes for boosting photocatalytic hydrogen evolution by steering photogenerated charge transfer

As an effective means to improve charge carrier separation efficiency and directional transport, the gradient doping of foreign elements to build multi-homojunction structures inside catalysts has received wide attentions. Herein, we reported a simple and robust method to construct multi-homojunctions in black TiO₂ nanotubes by the gradient doping of Ni species through the diffusion of deposited Ni element on the top of black TiO₂ nanotubes driven by a high temperature annealing process. The gradient Ni distribution created parts of different Fermi energy levels and energy band structures within the same black TiO₂ nanotube, which subsequently formed two series of multi-homojunctions within it. This special multi-homojunction structure largely enhanced the charge carrier separation and transportation, while the low concentration of defect states near the surface layer further inhibited carrier recombination and facilitated the surface reaction. Thus, the B-TNT-2Ni sample with the optimized Ni doping concentration exhibited an enhanced hydrogen evolution rate of ~ 1.84 mmol·g⁻¹·h⁻¹ under visible light irradiation without the assistance of noble-metal cocatalysts, ~ four times higher than that of the pristine black TiO₂ nanotube array. With the capability to create multi-homojunction structures, this approach could be readily applied to various dopant systems and catalyst materials for a broad range of technical applications.