Photoelectrical and charge transfer properties of hydrogen-evolving TiO2 nanotube arrays electrodes annealed in different gases

TiO2 nanotube arrays were fabricated by sonoelectrochemical anodic oxidation and calcined in nitrogen, air, or 5% hydrogen/nitrogen which was denoted as TNT-A, TNT-N, and TNT-H, respectively. All annealed TiO2 nanotube arrays samples exhibited similar surface morphology. With UV illumination (365 ± 15 nm), the photocurrent density of the TNT-A, TNT-N and TNT-H was about 0.27 mA/cm2, 0.45 mA/cm2 and 0.60 mA/cm2, respectively. The trapped electron at the Ti4+ center of TiO2 nanotube arrays shows absorption at around 500–700 nm. From the XPS measurement, it was found that annealing in 5% hydrogen/nitrogen helped the sample obtain a greater defect density. Because of the reduction of Ti4+ and the formation of oxygen vacancies, the charge transfer resistance appeared in this order: TNT-A > TNT-N > TNT-H. Thus TNT-H harvested the greatest charge carrier density of 9.86 × 1020 cm−3, TNT-N and TNT-A obtained a charge carrier density of 1.38 × 1020 cm−3 and 1.06 × 1020 cm−3, respectively. Accordingly, the hydrogen production rate by water splitting over TNT-A, TNT-N and TNT-H (320–780 nm irradiation, 3 h) was about 120 μL/h cm2, 159 μL/h cm2 and 231 μL/h cm2, respectively.