Marigold-flower-like TiO2/MIL-125 core−shell composite for enhanced photocatalytic Cr(VI) reduction

Series TiO2/MIL-125 core-shell composites were fabricated from MIL-125 via in-situ post-solvothermal method. The composites composed of TiO2 as shell and MIL-125 as core demonstrated some advantages like abundant linker defects, mesoporous structure and unsaturated titanium-oxo clusters, which enabled it to be a desired photocatalyst. The photocatalytic Cr(VI) reduction performances of as-prepared TiO2/MIL-125 composites with different treatment times were evaluated under Xe lamp irradiation. The results indicated that optimal marigold-flower-like MT-2 (TiO2/MIL-125 composite produced by treating MIL-125 for 2 h) exhibited superior photocatalytic Cr(VI) reduction activity than those of pristine MIL-125 precursor, TiO2/MIL-125 composites produced by treating MIL-125 for 1 h, and the completely derived TiO2. The TiO2 nanosheet formed from the outer-to-inner loss of organic ligands in the MIL-125 precursor can boost the separation efficiency of photo-induced carriers. Within this work, the influence of operation factors including pH values, foreign ions and organic acids with small molecular weight on photocatalytic Cr(VI) reduction were explored. The apparent quantum efficiency of optimal MT-2 was calculated under different monochromatic light. As well, the possible mechanism of photocatalytic Cr(VI) reduction over MT-2 was proposed and confirmed by electrochemical determination, active species trapping experiments and electron spin resonance analyses.