UV and visible photodecomposition of organic pollutants over micro arc oxidized Ag-activated TiO2 nanocrystalline layers

Although titania is one of the most important metal oxide photocatalysts, its practical applications are to some extent limited by its relatively wide band gap. Doping TiO₂ by metallic species has been expressed as a promising solution to decrease its band gap energy and enhance its photocatalytic performance. In this research, we employed micro arc oxidation technique to grow Ag-doped TiO₂ porous layers where influence of the electrolyte composition and the applied voltage on the surface morphology, topography, phase structure, chemical composition, and optical properties was investigated for the first time. The photocatalytic efficiency of the layers was studied using different model materials namely methylene blue and 4-chlorophenol solutions as well as gaseous benzene. A porous morphology with a rough surface was revealed by the SEM and AFM techniques. The pore size varied depending on the voltage and the electrolyte concentration. Compositional studies, conducted by XRD and XPS methods, showed that the fabricated layers mainly consisted of anatase phases. A small amount of rutile phase was also detected at high voltages, i.e. 500 V. Considering the XRD patterns, the anatase average crystalline size was determined as 67.8 and 21.5 nm for the pure TiO₂ and silver doped TiO₂ layers, respectively. A red-shift was observed in the absorbance edge of the layers when Ag was introduced into the titania lattice giving rise to a visible-light response. Comparing photocatalytic reaction rate constants of pure and Ag-doped titania, the doped layers exhibited more favorable potential to decompose the pollutants in aqueous phase under ultraviolet and visible lights. Moreover, the doped layers were useful to decompose the pollutants in gaseous phase under ultraviolet light.