of main observation and conclusion The AL-P25 composites (AL = alizarin, P25 = nano titanium dioxide) with oxygen vacancies were synthesized by a simple hydrothermal method and applied for the photoreduction of Cr(VI) under visible light. The AL-P25 composites were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy analysis, Fourier transform infrared spectrometer, thermo-gravimetric analysis, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy and electron paramagnetic resonance spectroscopy. The results suggested that the AL-P25 composites were successfully prepared, and alizarin was linked to P25 via Ti-O-C bonds. AL-P25 composites represented excellent photoreduction efficiency on Cr(VI) under visible light, which was 99.4% after 60 min of illumination. Finally, a photocatalytic mechanism of AL-P25 composites was proposed that ligand-metal charge transfer (LMCT) process between alizarin and P25 effectively enhances separation efficiency of photogenerated electrons and holes.

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Water contamination caused by highly toxic Cr(VI) should be resolved imminently with a more efficient photocatalytic approach. The interaction between photocatalyst heterostructure is critical for charge separation which largely affects the photocatalytic efficiency. Herein, TiO2 hollow sphere/UiO‐66‐NH2 (TiO2/UiO‐66‐NH2) heterostructures were successfully synthesized by an adsorption‐induced in‐situ solvothermal growth strategy. The UiO‐66‐NH2 particles were well dispersed on TiO2 hollow spheres by solvothermal treatment of Zr4+ adsorbed TiO2 hollow spheres with NH2‐BDC ligand. The photocatalytic activities of TiO2/UiO‐66‐NH2 heterostructures were investigated by reduction of Cr(VI) with visible light irradiation. With the introduction of UiO‐66‐NH2, the reduction efficiency toward Cr(VI) by TiO2/UiO‐66‐NH2 heterostructures reached 89% after 180 min of irradiation, which is much higher than those of TiO2 hollow spheres (10%) and UiO‐66‐NH2 crystals (29%). Moreover, the TiO2/UiO‐66‐NH2 heterostructures display good regeneration ability and can still maintain 70% of Cr(VI) removal ability after four cycles. The integration of TiO2 and UiO‐66‐NH2 could construct a type‐II heterojunction to broaden the absorption range of light and suppress the recombination of photoproduced electrons and holes. As a result, such unique composite structure significantly enhance the photocatalytic performance towards Cr(VI) reduction. This work not only inspires a new approach for in‐situ solvothermal synthesis of semiconductor@MOFs composites, but provides a novel photocatalyst for highly efficient removal of Cr(VI) from wastewater.