Synthesis of Cr 3+ -doped TiO 2 nanoparticles: characterization and evaluation of their visible photocatalytic performance and stability.

Cr3+ -doped TiO2 nanoparticles (Ti-Cr) were synthesized by microwave-assisted sol-gel method. The Ti-Cr catalyst was characterized by X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, N2 adsorption-desorption analysis, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy (XPS) and zetametry. The anatase mesoporous Ti-Cr material exhibited a specific surface area of 54.5 m2 /g. XPS analysis confirmed the proper substitution of Ti4+ cations by Cr3+ cations in the TiO2 matrix. The particle size was of average size of 17 nm for the undoped TiO2 but only 9.5 nm for Ti-Cr. The Cr atoms promoted the formation of hydroxyl radicals and modified the surface adsorptive properties of TiO2 due to the increase in surface acidity of the material. The photocatalytic evaluation demonstrated that the Ti-Cr catalyst completely degraded (4-chloro-2-methylphenoxy) acetic acid under visible light irradiation, while undoped TiO2 and P25 allowed 45.7% and 31.1%, respectively. The rate of degradation remained 52% after three cycles of catalyst reuse. The higher visible light photocatalytic activity of Ti-Cr was attributed to the beneficial effect of Cr3+ ions on the TiO2 surface creating defects within the TiO2 crystal lattice, which can act as charge-trapping sites, reducing the electron-hole recombination process.