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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