Visible Light driven Photocatalysis of Amino Acid Complex on TiO2 Surface

Abstract

Although amino acid has been widely investigated as a target pollutant and hole scavenger with irradiated TiO2 under UV light, its utilization as a visible light sensitizer has not been reported. In this study, we investigated amino acid as a sensitizer and achieved simultaneous degradation of amino acid and aqueous pollutants under visible light (λ > 420 nm). Serine and histidine were employed and characterized with TiO2 by various spectroscopic methods (TEM, EELS, and DRS) because they exhibited the highest visible light activity among various amino acids. The visible light absorption was increased through the ligand-to-metal charge transfer (LMCT) between amino acid (especially, serine and histidine) and TiO2. However, some kinds of amino acid (e.g., alanine, glycine) could not increase the visible light absorption because of its weak binding on TiO2 surface. Due to increased visible light absorption and efficient charge transfer, the complexed TiO2 by amino acid enhanced simultaneous degradation of aqueous pollutants (e.g., chromate (Cr(VI)) and 4-chlrophenol (4-CP)) through reductive pathway and amino acid via LMCT mechanism (oxidative pathway) under visible light compared to naked TiO2. The observed visible light activity of amino acid complexed-TiO2 as a function of irradiation wavelengths was slightly overlapped with the absorption spectrum of prepared photocatalyst, which demonstrated LMCT mechanism based photocatalysis. Furthermore, we investigated pH effect on LMCT between amino acid and TiO2. The photocatalytic activity of TiO2-serine was higher than that of TiO2-histidine under acidic condition, whereas the result was reversed under neutral pH. This result was described to that histidine has its own pKa value in side chain (pKa = 6.0). At neutral pH condition, the imidazole side chain of histidine can interact with the Ti atoms via the π–orbitals and also the amine group interact with the surface O atoms by H-bonds. The formation of LMCT complex of histidine is increased at neutral pH so the photocatalytic activity was reversed.