Modified Sol-gel synthesis of LiNbO3: Rietveld refinement, morphology, optical properties and high photocatalytic performance over RhB dye

In this study, we report for the first time that a high photocatalytic performance of multiphase mixed LiNbO₃ was successfully obtained using a modified Pechini method, varying the calcination temperatures between 500 °C (LiNb500) and 800 °C (LiNb800), against the Rhodamine B (RhB) dye under simulated ultraviolet light. The produced materials were characterized by Thermogravimetric analysis/Difference thermal gravimetry (TG/DTG), X-ray diffraction (XRD) and Rietveld refinement, Raman spectroscopy, UV–Vis by diffuse reflectance, Scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), Photoluminescence (PL) spectroscopy, Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and the surface area and pore volume by BET. Based on the results, it is confirmed that the influence of heat treatment leads to the obtaining of a pure phase for LiNb500. At temperatures equal to or higher than 600 °C, minor secondary phases of LiNb₃O₈ (2.82 % to 4.2 %) were observed. The optical characterization revealed strong luminescent emission for the LiNb500 sample, resulting from the synergistic effect of the matrix with the residual carbon, and suppression of the luminescent effect for the other samples caused by the elimination of carbon by increasing the heat treatment temperature. On the other hand, the XPS analysis of the chemical states of the elements present in the LiNbO₃ matrix revealed that the LiNb600 sample has a higher percentage of Nb⁴⁺ than Nb⁵⁺ species on the surface of the nanocrystals, compared to the others, as well as chemical states related to oxygen vacancies. These characteristics provided improvements in photocatalytic performance. In this case, it promoted the discoloration of 100 % of RhB (10 mg.L⁻¹) after 60 min of exposure time under UVc light. Through the scavenger study for reactive species photogenerated, it is confirmed that the superoxide radicals and holes are the main species involved in the discoloration of RhB dye molecules, where the absence of oxygen in the photocatalyst test resulted in a decrease of 35.39 %. In addition, after the five consecutive cycles of 90 min in the reuse test, the efficiency exceeded 91 %. Thus, the LiNbO₃ polymorphs exhibit a synergistic and promising performance in the photodegradation of persistent organic pollutants.