Porous carbon nanofiber/porphyrin-based metal organic frameworks/TiO2 nanoparticles ternary materials for selective detection and photoreduction of Cr(Ⅵ)

For the heavy metal ion Cr(Ⅵ), which is widely present in wastewater with acute toxicity, mutagenicity and carcinogenicity, we successfully constructed a ternary material of PCNFs/CuMOF/TiO2 for rapid detection and photoreduction of Cr(Ⅵ). The material has obvious advantages: on the one hand, ZIF-8/polyacrylonitrile (PAN) nanofibers and their derived porous carbon nanofibers (PCNFs) were synthesized by electrospinning and high temperature carbonization, which provided a loading platform for CuMOF and TiO2, effectively inhibited the agglomeration of CuMOF and TiO2, and ensured the exposure of active sites. Meanwhile, its excellent conductivity can promote the effective migration of photogenerated carriers in the photocatalytic process. On the other hand, CuMOF constructed with tetra-carboxyphenylporphyrin (TCPP) as the linking arm and Cu(NO3)2·3 H2O as the metal node was directly loaded on the surface of porous carbon nanofibers (PCNFs), and then TiO2 nanoparticles were introduced by hydrothermal method. CuMOF ensures the responsiveness of the composite to visible light. Moreover, the energy level structure of TiO2 matches with CuMOF, which effectively inhibits the recombination of photogenerated electron-hole pairs (e--h+). The results show that the colorimetric detection of Cr(Ⅵ) was achieved by using 3,3′,5,5′-tetramethylbenzidine (TMB) as the sensing probe. The method was highly selective for Cr(Ⅵ) ions with a limit of detection (LOD) of 24.1 nM. Encouragingly, PCNFs/CuMOF/TiO2 was further used as a photocatalyst. The reduction rate of Cr(Ⅵ) under visible light irradiation is 96.1 % in 60 min. The active species trapping experiments confirm the key role of e- photocatalytic reduction, upon which a possible photocatalytic mechanism is proposed. The degradation rate of Cr(Ⅵ) is still 86.4 % after the five times cycle, and the good stability and reproducibility allow it to be used as a potential green photocatalyst for the conversion of Cr(Ⅵ) to Cr(Ⅲ).