Visible light driven photo-degradation of Congo red by TiO2-ZnO/Ag: DFT approach on synergetic effect on band gap energy

In this paper, we report the combination of two metal oxides (TiO2-ZnO) that allows mixed density of states to reduce band gap energy, facilitating the photo-oxidation of Congo red dye under visible light. For the oxidation, a possible mechanism is proposed after analyzing the intermediates by GC-MS, and it is consistent with Density Functional Theory (DFT). The nanohybrids were characterized comprehensibly by several analytical techniques such as X-Ray diffraction (XRD), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS). For the addition of ZnO to TiO2, a dominance of anatase phase was found rather than other phases (rutile or brookite). A broad band (similar to 550 nm) is observed in UV-Visible spectra for TiO2-ZnO/Ag NPs nm because of Surface Plasmon properties of Ag NPs. The band gap energy was calculated for TiO2-ZnO/Ag system, and then it has been further studied by DFT in order to show why the convergence of two semiconductors allows a mixed density of states, facilitating the reduction of the energy gap between occupied and unoccupied bands; ultimately, it improves the performance of catalysts under visible light. Significantly, the interaction of crystal planes (0 0 1) of TiO2 anatase and (0 0 1) of ZnO crucially plays as an important role for the reduction of energy band-gap. Additionally, TiO2-ZnO-Ag NPs were used recognize Saccharomyces cerevisiae cells by con-focal fluorescence microscope, showing that it develops bright bio-images for the cells; while for TiO2 or ZnO or TiO2-ZnO NPs, no fluorescent response was seen within the cells. (C) 2018 Elsevier Ltd. All rights reserved.