Effects of the conjugated structure of Fe-bipyridyl complexes on photoinduced electron transfer in TiO2 photocatalytic systems

[Fe-II(dcbpy)(2)(H2O)(2)] (dcbpy = 2,2'-bipyridine-4,4'-dicarboxylic acid) complex was synthesized through a simple method and used as a photosensitizer to improve the photocatalytic activity of TiO2 in the photodegradation of phenol under visible light irradiation. X-ray diffraction, X-ray photoelectron spectroscopy, UVvis spectroscopy, Raman spectra, scanning electron microscopy, and transmission electron microscopy were used to characterize the structure and morphology of [Fe-II(dcbpy)(2)(H2O)(2)]/TiO2. It exhibited enhanced photocatalytic activity in photodecomposition of phenol in aqueous solution under visible light irradiation compared with that of pure TiO2. When one of ligand dcbpy was replaced by bpy (2,2'-bipyridine) or phen (1,10-phenanthroline), two novel composite catalysts, [Fe-II(dcbpy)(bpy)(2)]/TiO2 and [Fe-II(dcbpy)(phen)(2)]/TiO2, were obtained and exhibited excellent photocatalytic activity like that of [Fe-II(dcbpy)(2)(H2O)(2)]/TiO2. Their different performance in the degradation of phenol, derived from their different interfacial interaction and electron transfer between complexes and TiO2 surface, are discussed in detail. Based on research on the dynamical process of photocatalytic degradation of phenol, recycling stability, absorption spectra, photoelectrochemistry performance and cyclic voltammetry, trapping experiments, and density functional theory calculations of quantum chemistry, possible electron transfer mechanism are proposed, which can reasonably explain the different photocatalytic activities of these three novel catalysts. (C) 2017 Elsevier Inc. All rights reserved.