Charge-transfer surface complex of EDTA-TiO2 and its effect on photocatalysis under visible light

Abstract

▶ EDTA forms the charge-transfer complex on the surface of TiO 2 that absorbs visible light. ▶ The EDTA-TiO 2 complex has activities for the reduction of Cr(VI) under visible light. ▶ The visible light activity is mediated by ligand-to-metal charge transfer. We investigated the formation of surface complex between TiO 2 and common electron donors (methanol, formic acid , acetic acid, triethanolamine and EDTA) that can absorb visible light through ligand-to-metal charge transfer (LMCT) mechanism. The visible light activity of TiO 2 –EDTA complex was outstanding among all tested TiO 2 –substrate complexes. The complexation of EDTA on TiO 2 induced a visible light absorption up to 550 nm and exhibited a marked visible light activity for both the reductive conversion of Cr(VI) and the production of H 2 . The optimal concentration of EDTA was 100 μM for the reduction of Cr(VI) but much higher (10 mM) for the production of H 2 . The EDTA-adsorbed TiO 2 electrode also generated a significant level of photocurrent under visible light irradiation. The LMCT-excited electrons (in TiO 2 conduction band) are subsequently transferred to electron acceptors such as Cr(VI) and protons. The degradation of EDTA on TiO 2 under visible light was significant only in the presence of Cr(VI) that should scavenge electrons with inhibiting the recombination. The surface CT-complex formation between TiO 2 and EDTA was significantly inhibited when the surface of TiO 2 was fluorinated. The isoelectric point of TiO 2 particles suspended in water was gradually shifted to lower pH values with increasing the concentration of EDTA, which supported the formation of surface complex between TiO 2 and EDTA anions. The FT-IR spectra of TiO 2 –EDTA complex showed a band centered at 1409 cm −1 , which is ascribed to the carboxylate group complexed with TiO 2 surface. The complex IR band was diminished upon fluorinating the surface of TiO 2 . The fact that common electron donors like EDTA can form CT-complexes on the surface of TiO 2 and contribute to the visible light activity should be recognized in assessing and understanding the overall photocatalytic activity.