Enhanced Visible-Light-Driven Photoelectrochemical Activity in Nitrogen-Doped TiO₂/Boron-Doped Diamond Heterojunction Electrodes

Doping and constructing heterojunctions have been demonstrated to be effective for improving the catalytic activity of titanium dioxide (TiO2); however, better performance is still widely expected for practical applications in renewable energy, environmental issues, etc. TiO2/boron-doped diamond (BDD) heterojunction is a promising approach that has been extensively studied in recent years. Herein, nitrogen-doped TiO2 (N-TiO2)/BDD heterojunction is proposed by employing N-TiO2 instead of TiO2. The N-TiO2/BDD heterojunctions were fabricated and systematically characterized to make a detailed comparison between N-TiO2/BDD and TiO2/BDD. The photoelectrochemical (PEC) activity was tested under 1 sun and visible light irradiation, respectively. N-TiO2 showed stronger photoresponse with the extended absorption spectrum that covered both UV and visible ranges. Besides, N-TiO2 also showed higher electrical conductivity due to the higher carrier concentration introduced by N doping. Therefore, larger current density and more efficient charge transport were demonstrated in N-TiO2/BDD heterojunctions, achieving enhanced PEC activity. Interestingly, compared with that of TiO2/BDD, the PEC activity of N-TiO2/BDD was weak at a low applied bias potential (<1.6 VRHE), but it increased dramatically and became much stronger than that of TiO2/BDD at a higher potential (>2.1 VRHE). This was suggested to be caused by the higher carrier concentrations and variation of electronic structures in N-TiO2. The PEC activity could be further promoted if the bias potential was further improved, resulting in excellent PEC performance that could not be realized by TiO2/BDD. Moreover, the doping concentration of N-TiO2 exhibited complicated influences on the PEC performance and needed to be elaborately controlled. Based on the optimized conditions, the largest current density achieved was 0.51 mA/cm2 at 2.8 VRHE and 1 sun irradiation, which was 2.22 times that of TiO2/BDD (0.23 mA/cm2). The carrier transport mechanism was discussed based on the experimental results. The N-TiO2/BDD electrodes showed degradation efficiency 2 times that of tetracycline hydrochloride (TCH) compared with that of undoped TiO2/BDD, which demonstrated promising applications of N-TiO2/BDD in the treatment of organic pollutants.