Abstract
Alkaline hydrogen peroxide treatment was proposed as a simple and green way to improve the performance of commercial TiO2 powder for water-dispersibility and visible-light photocatalytic activity on the degradation of organic dyes. The performance of treated TiO2 (AHP-TiO2) was evaluated as a function of NaOH concentration, H2O2 concentration, and treatment time. The optimal conditions were determined to be 24 h in 100 mM H2O2 and 8 M NaOH. The treated samples were characterized by Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), and ultraviolet-visible spectrophotometry. The analysis revealed that the crystal structure, morphology, and absorption band gap were retained, but the surface of AHP-TiO2 was dramatically changed. AHP-TiO2 could be highly dispersible with a uniform hydrodynamic size of 41 ± 12 nm and stable over months in acidic water without any stabilizing ligand. It could also significantly enhance the visible-light photodegradation of dye pollutants. The superior performance was attributed to the formation of abundant surface hydroxyl groups, estimated to 12.0 OH/nm2. Effect of Fe3+ ion on the photocatalytic activity of the treated TiO2 was studied. The results show that Fe3+ accelerated the photodegradation of dyes in aqueous AHP-TiO2 dispersions with one order of magnitude larger than that of commercial P-25. This may be ascribed to the complexation of the surface hydroxyl groups of AHP-TiO2 with Fe3+ to form Fe(OH)2+. A plausible reaction mechanism for this system was proposed. The apparent quantum efficiency of hydroxyl radical formation was calculated for different TiO2 suspensions by methanol oxidation and coumarin derivatization. The experimental observations suggest that Fe3+ ion could accelerate the generation rate of hydroxyl radical species in AHP-TiO2 and the system oxidation should be caused by adsorbed hydroxyl radical species, rather than free hydroxyl radical species under visible light irradiation. Keywords: TiO2, Dispersion, Dye photosensitization, ferric ions, Reactive oxygen species