Photocatalytic degradation of rhodamine B in the visible region using nanostructured CoAl_2−xLa_xO₄ (x = 0, 0.01, 0.03, 0.07, and 0.09) series: Photocatalytic activity and DFT calculations

In the present research, we focused on the study of the photocatalytic activity of nanostructured CoAl_2−xLa_xO₄ (x = 0, 0.01, 0.03, 0.07, and 0.09) as photocatalysts synthesized by the citric sol–gel method which were used for the photocatalytic degradation of rhodamine B (RhB) under visible light. Doping of lanthanum ion, La³⁺ in the cobalt aluminate matrices were characterized by XRD, XPS, FT-IR, Raman, Dif-UV, TEM, and N₂ adsorption/desorption isotherms using Brunauer-Emmett-Teller (BET). Also, The first principle DFT calculations were performed to study the band structure and charge distribution of the spinel compounds, and the results showed the presence of a direct bandgap at the G symmetry point. The Optoelectronic properties revealed that the energy bandgap values of CoAl_2−xLa_xO₄ decreased from 3.7 to 2.8 eV upon increasing the La ratio. The photocatalytic activities were evaluated in the removal of the textile dye (RhB) from aqueous solutions. Several conditions such as initial dye concentrations and contact times were investigated. The mechanism of photocatalytic activity of CoAl_2−xLa_xO₄ (x = 0.00 and 0.07) substitution on the RhB above the liquid surface make clear that both hydroxyl and superoxide anions radicals are the main active species that play the main role in removing RhB from solutions. The PL spectra were measured to suggest the impact of La dopant on the photoactivity of the CoAl₂O₄. The blue shift in the visible emission peak was caused by the quantum size effect in cobalt aluminate nanostructures due to the substitution of Al³⁺ with La³⁺ ions. Also, PL intensity decreased dramatically when the La³⁺ substitution with the Al³⁺ ions.