TiO2 nanotube arrays sensitized by copper (II) porphyrins with efficient interfacial charge transfer for the photocatalytic degradation of 4-nitrophenol

In this paper, photocatalysts based on TiO2 nanotubes (TNTs) and TiO2 nanotube arrays (TNTAs) sensitized by Cu(II) meso-tetrakis(N-ethylpyridinium-4-yl) porphyrin (CuTEPyP) were synthesized and their structures were characterized by various analytical methods. The photocatalytic activities of both composites were then investigated through degradation of 4-nitrophenol (4-NP) in aqueous solutions under visible light irradiation. It was found that CuTEPyP/TNTAs could eliminate 95% 4-NP within 4 h, which was considerably higher than the yield obtained with CuTEPyP/TNTs (56%) under the same conditions. Compared to CuTEPyP/TNTs, the improved photocatalytic activity of CuTEPyP/TNTAs can be ascribed to increased light absorption, high separation rate of photo-generated charge pairs, and efficient charge transfer. A plausible photocatalytic degradation mechanism involving hydroxyl radicals, superoxide radical anions and singlet oxygen species was also proposed. This work presents an efficient paradigm for eliminating 4-NP under visible light irradiation.