Selenium modified oxalate chelated titania: Characterization, mechanistic and photocatalytic studies

Abstract

In order to enhance the visible photocatalytic activity of titanium dioxide (TiO2), selenium (Se) doped TiO2 was prepared by sol–gel method using oxalic acid as a chelating agent. To characterize and describe the effect of Se+4 ions on the structural, optical and photocatalytic properties of TiO2, the powders were characterized by XRD, FTIR, UV-DRS, HR-TEM, PL, Raman, XPS and BET measurements. XRD analysis showed that doped powders consist of anatase phase, while undoped TiO2 is a composite mixture of anatase and rutile at a calcination temperature of 550°C. Williamson–Hill plot results indicated the presence of tensile strain on the addition of 0.02, 0.04 and 0.06mol% Se content, while compressive strain was specified on the addition of 0.08, 0.1 and 0.2mol% Se content in TiO2 lattice. FTIR analysis revealed the chelating behaviour of oxalic acid with titanium ions in forming titanium oxalate stretches. Diffuse reflectance measurements showed that visible light absorption is induced by dopant localized/intermediate electronic states, probably originating from Se 4s and 4p states within the band gap of TiO2. Raman peak broadening and the small frequency shifting for the peaks (1-Eg and B1g) were attributed to oxygen vacancies and crystal lattice distortions. Photoluminescence (PL) analysis showed that electron hole recombination was decreased with the incorporation of Se+4 ions in TiO2, which act as trapping sites for the photogenerated electrons. Moreover, PL analysis demonstrated that Se doping produces Ti+3 centers, oxygen defects and localized electronic states in TiO2 band gap, which become responsible for the TiO2 response towards the visible light. The activity of the prepared photocatalysts powder were tested by investigating the kinectics of the photocatalytic decoloration of methylene blue (MB) under UV irradiations and decomposition of 4-chlorophenol (4-CP) under visible light irradiation. It was observed that the TSe0.08 powder with Brönsted acid sites, calcined at 550°C showed highest photocatalytic activity under both UV and visible light irradiations for the selected pollutants. In the decoloration of MB, TSe0.08 powder attain a rate constant (kapp) value of 0.045min−1 which is higher than Degussa P25 (0.033min−1) as well two times higher than undoped TiO2 (0.021min−1). Based on the obtained results, a UV/vis light irradiated photocatalytic mechanism has been proposed for the Se+4 doped anatase TiO2.