Study of the photocatalytic activity of Fe³⁺, Cr³⁺, La³⁺and Eu³⁺single-doped and co-doped TiO₂catalysts produced by aqueous sol-gel processing

An aqueous sol-gel process, previously developed for producing undoped and Cu²⁺, Ni²⁺, Zn²⁺or Pb²⁺doped TiO₂photocatalysts with remarkably high photocatalytic activity without requiring any calcination step, has been adapted to produce Fe³⁺, Cr³⁺, La³⁺or Eu³⁺single-doped TiO₂photocatalysts as well as La³⁺-Fe³⁺and Eu³⁺-Fe³⁺co-doped TiO₂catalysts. The physicochemical properties of the obtained catalysts have been characterized using a suite of complementary techniques, including ICP-AES, XRD, UV–Vis spectroscopy, nitrogen adsorption-desorption and Fe-57 Mössbauer. The active crystalline phase is obtained without requiring any calcination step and all the different catalysts are composed of nanocrystallites of anatase with a size of 6–7 nm and a high specific surface area varying from 181 to 298 m²g⁻¹. In this study, the effect of the NO₃:Ti(IV) mole ratio used to induce the peptisation reaction during the synthesis has been studied and the results revealed that this ratio can influence significantly the textural properties of the resulting catalyst. A screening of the photocatalytic activity of the undoped and Fe³⁺, Cr³⁺, La³⁺or Eu³⁺single-doped and co-doped photocatalysts has been performed by evaluating the degradation of 4-nitrophenol under UV–Visible light (330 nm < λ < 800 nm). This study suggests that the photocatalytic activity is significantly influenced by the dopant nature and content with an optimal dopant content being observed in the case of Fe³⁺or La³⁺single-doped as well as in the case of La³⁺-Fe³⁺and Eu³⁺-Fe³⁺co-doped catalysts. In the case of Cr³⁺single-doped catalysts, a detrimental effect of the dopant on the photocatalytic degradation of 4-nitrophenol has been observed while no significant influence of the dopant has been detected in the case of Eu³⁺single-doped catalysts. The role of the different dopants in modulating the photocatalytic activity is discussed.