Photoelectrochemical oxidation of azo dye and generation of hydrogen via CN co-doped TiO2 nanotube arrays

• C, N co-doped TiO2 nanotube arrays works as a photoanode in a PEC system.
• Recombination of photo-generated holes and electrons were greatly reduced.
• Synergetic effect was quantified in PEC degradation.
• PEC oxidation and reduction abilities were enhanced in this study concurrently.

Carbon and nitrogen were co-doped into TiO2 nanotube arrays (C–N-TNTAs) to extend their light response region using the chemical bath deposition method. The as-synthesized C–N-TNTAs were employed as the working anode in photoelectrochemical (PEC) experiments. Using the applied bias potential, the recombination of photo-generated holes and electrons was reduced significantly. The crystalline, optical properties, surface morphology, and structure of the C–N-TNTAs were characterized by XRD, UV–vis absorbance edges, SEM, and XPS, respectively. The XRD results showed the C–N-TNTAs were dominated by the anatase phase after sintering at 450 °C with significant visible light response. XPS analyses indicated nitrogen doping was mainly responsible for reducing the band gap as evidence of 0.82% N doping into the structure via the linkage of the TiON and NTiO bond. SEM images illustrated the diameter of the supported TiO2 nanotubes was approximately 90–100 nm with a length of approximately 400 nm. After carbon and nitrogen co-doping, the nanotubular structure of TiO2 nanotube kept its integrity with no significant morphological change, which was beneficial for PEC applications. The degradation efficiency of methyl orange (MO) was examined by photoelectrochemical, photocatalytic, electrochemical and photolysis methods for comparison in terms of pseudo-first-order reaction rate. The PEC method had the best MO degradation efficiency with a rate constant of 2.3 × 10−3 s−1 at a bias potential of 1.0 V (vs. SCE) under illumination, that was consistent with results of IPCE (%) measurements (the maximum IPCE up to 30.02% at 325 nm wavelength). The synergetic effect was quantified at current/time curves at bias potentials of 0.03 mA/0 V and 3.0 mA/1.0 V, respectively. Electrochemical impedance spectroscopy (EIS) measurements revealed the electron lifetime τel of photoexcited electrons in photoanodes was increased about 3.2 times after CN doping treatment. The bias potential could separate photo-generated holes and electrons effectively and enhance the electrochemical-oxidation of MO. Hydrogen generation was concurrently conducted in the cathodic chamber. After 180 min of reaction time, the amount of H2 reached 3.2 mmol by employing C–N-TNTAs as the photoanode.

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