Effects of Ce addition on the properties and photocatalytic activity of TiO2, investigated by X-ray absorption spectroscopy

TiO2, 0.1, 1, 3 and 8 wt% Ce-doped TiO2 photocatalysts were prepared by sol-gel method. All prepared samples were characterized by X-ray diffraction (XRD), N-2 adsorption-desorption, Scanning electron microscopy (SEM), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), Raman spectroscopy and Xray absorption spectroscopy (XAS). Upon Ce addition into TiO2, the phase structure of doped samples were similar to that of pure TiO2. The expansion of unit cell parameters and the shifting of Raman peaks to higher wavenumbers indicated to the incorporation of Ce ions into TiO2 structure resulting in the formation of lattice defects. The optical properties indicated that the addition of Ce into TiO2 enhanced the visible light absorption of the photocatalyst due to a reduction of band gap energy. For XANES study, it was found that the oxidation state of Ce in Ce-doped TiO2 showed the mixture of Ce3+ and Ce4+ which confirmed the formation of oxygen vacancies or lattice defect. The EXAFS results indicated that larger Ce ions were incorporated in TiO 2 lattices and the structure of Ce-doped TiO2 deviated from the octahedral symmetry on the Ti site surrounded by six O atoms. The photocatalytic activity of all samples were investigated for degradation of methyl orange (MO) dye under UV light irradiation with a catalyst dosage of 0.3 g L-1, pH 2 and light intensity of 125 W. The Methyl orange removal percentage of TiO2 was 80.4% for TiO2 which was increased up to 86.9% for 0.1 wt% Ce-doped TiO2. The enhancing of photocatalytic activity was possibly due to the formation of energy levels by Ce3+/Ce4+ and oxygen vacancy under the conduction band of TiO2 which can trap electrons and resulting in a reduction of charge recombination. However, further upon increase of Ce content, the removal percentage decreased due to the formation of recombination centers from an overabundance of oxygen vacancies in the TiO2 lattice. (C) 2018 Elsevier B.V. All rights reserved.