Two metal-quinoline complexes, Fe(III)-8-hydroxyquinoline-5-sulfonic acid (Fe-HQS) and Er(III)-8-hydroxyquinoline-5-sulfonic acid (Er-HQS), were used as sensitizers of TiO2 for improving visible light photocatalytic activity of TiO2. UV-vis spectra, X-ray Diffraction, X-ray Photoelectron Spectroscopy, Scanning Electron Microscope and Transmission Electron Microscope were put to use to characterize the structure and morphology of Fe-HQS-TiO2 and Er-HQS-TiO2. The effects of interfacial electron transfer and energy level matching between each complex and TiO2 were researched intensively. Er-HQS-TiO2 and Fe-HQS-TiO2 exhibited enhanced photocatalytic activity on photodecomposition of phenol in aqueous solution under visible light irradiation comparing to that of pure TiO2. According to photoelectrochemical response property and theoretical calculation of energy levels, two possible energy matching modes and photoelectron transmission pathways were proposed to explain the effects of Fe-HQS and Er-HQS on the interfacial interaction between metal-HQS complex and TiO2, and their different photocatalytic activities under visible light irradiation. The work in this paper indicated that metal-HQS complex with appropriate electronic structure and HOMO-LUMO energy levels relative to the band gap of TiO2 played a major role in improving TiO2 photocatalytic activity. (C) 2016 Elsevier B.V. All rights reserved.