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Surface modification has been used as a method to create defects on ${\mathrm{\text{TiO}}}_2$ materials, which can improve their desirable properties. In this paper, defected ${\mathrm{\text{TiO}}}_2$ nano-powder was successfully synthesized by chemical reduction using ${\mathrm{\text{NaBH}}}_4$ as the reducing agent at 300–400${}^{\circ }\mathrm{\text{C}}$ under argon atmosphere. High defect concentration can be produced by increasing process temperature. The modified ${\mathrm{\text{TiO}}}_2$ shows good visible light absorption and photocatalytic activity on degradation of Rhodamine B (4–9 times higher than the pristine ${\mathrm{\text{TiO}}}_2$) with the visible light irradiation. Further XPS analysis and theoretical studies using full potential linearized augmented plane wave (FP-LAPW) method as implemented in wien2k code revealed the existence of oxygen vacancy and ${\mathrm{\text{Ti}}}^{3+}$ in the modified samples. These types of defects were responsible for the modifications of the electronic and optical properties of ${\mathrm{\text{TiO}}}_2$, resulting in the improved photocatalytic activity in visible light irradiation.

TiO₂ is a promising photocatalyst in the reaction of water splitting for hydrogen. Here, we used NaBH₄ reduction reaction to introduce ${\text{Ti}}^{3+}$/V_O defect states into Si-doped TiO₂ nanotubes and investigated the photoelectrochemical water splitting properties. It was found that the photocatalytic activity was improved through NaBH₄ reduction because of the existence of ${\text{Ti}}^{3+}$/V_O. The defect states (${\text{Ti}}^{3+}$/V_O) could play a role of capture trap and accelerate the separation of photogenerated electrons and holes. The photocurrent density of the Si-doped TiO₂ nanotubes reduced for 3h was 1.5mA/cm² compared to 0.7mA/cm² of the unreduced nanotubes. The conversion efficiency was 0.7%, which was almost 4 times than that of pure TiO₂. On the other hand, reduction for a long time would generate excess defect states and thus cause the decrease of photocurrent density. Excess defect states could act as the recombination centers of photogenerated electrons and holes.