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.

MoS2 quantum dots@TiO2 nanotube composites with enhanced photoexcited charge separation and high-efficiency visible-light driven photocatalysis

MoS₂ quantum dots (QDs) that are 5 nm in size were deposited on the surface of ultrathin TiO₂ nanotubes (TNTs) with 5 nm wall thickness by using an improved hydrothermal method to form a MoS₂ QDs@TNT visible-light photocatalyst. The ultrathin TNTs with high percentage of photocatalytic reactive facets were fabricated by the commercially available TiO₂ nanoparticles (P25) through an improved hydrothermal method, and the MoS₂ QDs were acquired by using a surfactant-assisted technique. The novel MoS₂ QDs@TNT photocatalysts showed excellent photocatalytic activity with a decolorization rate of 92% or approximately 3.5 times more than that of pure TNTs for the high initial concentration of methylene blue solution (20 mg l^-1) within 40 min under visible-light irradiation. MoS₂ as the co-catalysts favored the broadening of TNTs into the visible-light absorption scope. The quantum confinement and edge effects of the MoS₂ QDs and the heterojunction formed between the MoS₂ QDs and TNTs efficiently extended the lifetime of photoinduced charges, impeded the recombination of photoexcited electron-hole pairs, and improved the visible-light-driven high-efficiency photocatalysis.