Abstract
The use of inexpensive semiconducting materials and abundant natural sunlight can deliver the sustainable as well as cost-effective cleaning of contaminated water. In the current effort, the low photocatalytic activity of ZnO in sunlight exposure has been addressed by the slow deposition and after calcination, the deposition pattern of Fe3+ ions was investigated. The appearance of distinct multiple absorption edges in the optical spectra verified the composite nature of the synthesized materials whereas the absorption edge at similar to 2.5 eV indicated the formation of visible light responsive structures other than Fe2O3. The additional reflections, besides the characteristic reflections of ZnO and Fe2O3, in the X-ray diffraction patterns and the appearance of discrete assortments in HRTEM analysis exposed the formation of surface ZnFe2O4 in the interfacial region between the ZnO and Fe2O3. The X-ray photoelectron analysis revealed the minor transitions in the oxidation state, more pronounced at higher Fe3+ loadings, during the synthetic route. The electrochemical characterization revealed the p-type nature of the oxides of Fe3+ whereas the flat band potentials of the composites were assessed by Mott-Schottky analysis. As compared to pure ZnO, the composites loaded with lower concentrations (0.5% and 1%) of Fe3+ showed substantially high activity for the removal of 2,4-dichlorophenoxy acetic acid (2,4-D) in the complete spectrum whereas 3% Fe3+ loading exhibited optimum activity in the exposure of the visible region of natural sunlight. The higher Fe3+ loadings revealed the detrimental effect on the photocatalytic removal process. The variations in the photocatalytic activity with the increasing Fe3+ were discussed in correlation with electrochemical properties. The key intermediates were identified and the plausible mechanisms of the removal of 2,4-D were proposed by correlating the evidence from various experimental tools such as HPLC, IC, and TOC. The probable contribution of the reactive oxygen species (ROS) involved in the degradation process was estimated and discussed. The validity of the Langmuir-Hinshelwood kinetic model was also examined for the degradation as well as mineralization process. (C) 2016 Elsevier B.V. All rights reserved.