Glucose-TiO₂ charge transfer complex-mediated photocatalysis under visible light

Glucose adsorbed-TiO₂ nanoparticles show photoactivity under visible light (λ>420nm) through the ligand-to-metal charge transfer (LMCT) mechanism. Although glucose has been often utilized as a hole scavenger in TiO₂-based photocatalytic systems, the fact that TiO₂-glucose can form a LMCT complex that absorbs visible light has not been recognized. The TiO₂-glucose LMCT complex induced a marked red-shift in the absorption spectrum which extended to 600nm, and the visible light absorption gradually decreased with decreasing the concentration of glucose. The TiO₂-glucose complex exhibited remarkable visible light activities for the reduction of Cr(VI) to Cr(III) and the reduction of O₂ to H₂O₂. The observed visible light activities were significantly inhibited when the TiO₂ surface was fluorinated, because the surface fluorides inhibited the formation of LMCT complex of glucose. The electrode coated with TiO₂-glucose complex generated a significant level of photocurrent under visible light. The ATR-FTIR spectra showed that glucose forms a surface complex on TiO₂ through the hydroxyl linkages. The evidences for the formation of the TiO₂-glucose complex and the experimental parameters affecting the visible light-induced activities are discussed in detail.