A new photocatalyst (Ag−S/PEG/TiO 2 ) was synthesized by adding polyethylene glycol (PEG) to an efficient Ag−S/TiO 2 photocatalyst, to obtain a photocatalyst that is highly active under visible light. In addition to Ag−S/PEG/TiO 2 , Ag−S/TiO 2 and pure TiO 2 were prepared to compare their properties and activities. Specifically, the morphologies and microstructures of the nanophotocatalysts were characterized by means of powder X-ray diffraction (XRD), N 2 adsorption−desorption measurements, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) microanalysis, transmission electron microscopy (TEM), UV−visible diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Moreover, to evaluate their activities, the synthesized powders were used for the degradation of acid orange 7 (AO7) and methylene blue (MB) azo dyes in aqueous solution under low-voltage lightemitting diodes (LEDs) as the visible-light source. It was found that addition of PEG to Ag−S/TiO 2 increases the photodegradation of AO7 and MB by about 28.82% and 24.24%, respectively, in comparison with Ag−S/TiO 2 without PEG addition.

In this study, for the first time a statistical analysis based on the response surface methodology (RSM) was employed to investigate individual and interaction effects of key operating parameters of the photocatalytic degradation under visible-light irradiation using Ag-S/PEG/TiO 2 . Ag-S/PEG/ TiO 2 is a visible-light-driven photocatalyst and was synthesized (based on earlier research) by co-doping of TiO 2 with silver and sulphur and addition of polyethylene glycol (as a reagent template). In addition, the model pollutant was methylene orange (MO) and the studied operating parameters included the photocatalyst loading, initial concentration of the pollutant, and pH of the solution. The statistics-based experimental design and RSM was utilized to find a quadratic model as a functional relationship between the degradation efficiency and the three operating parameters. The regression analysis with R 2 value of 0.9678 showed a close fit between the model prediction and experimental data of the degradation efficiency. The analysis of variance based on the model indicated that pH of the solution was the most influential factor, while the two other operating parameters were also significant. The efficiency of MO degradation reached 94.0 % under the optimum conditions (i.e. photocatalyst loading of 1.20 g/L, MO concentration of 5 mg/L, and pH of 2).

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To overcome the drawback of poor solar light utilization brought about by the narrow photoresponse range of TiO₂, a silver and sulfur co-doped TiO₂was synthesized. Using the prepared catalyst, solar photocatalytic degradation of 2-nitrophenol (2-NP) by a TiO₂-based catalyst was studied for the first time. Effects of the co-doping on the structural, optical and morphological properties of the synthesized nanoparticles were investigated by different characterization methods: X-ray diffraction, N2 adsorption-desorption measurements, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, UV-visible diffuse reflectance spectroscopy and Fourier transform infrared spectroscopy. Solar experiments showed that the co-doping with silver and sulfur significantly increased the photocatalytic activity. In various initial concentrations of 2-NP more than 99% of the contaminant was decomposed by Ag-S/TiO₂in less than 150 minutes, while the degradation efficiency was much less in the presence of bare TiO₂. Kinetic studies suggested that solar photocatalytic degradation of 2-NP is consistent with the Langmuir-Hinshelwood model. The rate constant of the reaction and adsorption constant of the modified photocatalyst were found to be 2.4 and 4.1 times larger than that of bare TiO₂, respectively.

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