Alto desempenho fotocatalítico e piezocatalítico de Li1-xCuxNbO3 (0 ≤ x ≤ 0,025) na degradação de poluentes orgânicos persistentes

Abstract

In this study, we report for the first time that a high photocatalytic performance of LiNbO3 was successfully obtained using a modified Pechini method, varying the calcination temperatures between 500 °C (LiNb500) and 800 °C (LiNb800), against the Rodamina B (RhB) dye under simulated UV light. In addition, Cu-doped LiNbO3, Li1-xCuxNbO3 (0 ≤ x ≤ 0.025 mol) was used as a piezocatalyst in the reduction of p-nitrophenol (PNF) into p-aminophenol (PAF). The produced materials were characterized by thermogravimetric/ derivative analysis (TG/DTG), X-ray diffraction (XRD) and Rietveld refinement, Raman spectroscopy, UV-Vis by diffuse reflectance, scanning electron microscopy (SEM), photoluminescence spectroscopy (PL), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and surface area and pore volume by BET. Based on the results, it was confirmed that the influence of heat treatment leads to obtaining a pure phase for LiNb500. At temperatures equal to or greater than 600 °C, minor secondary phases of LiNb3O8 (2.82% to 4.2%) were observed. Furthermore, copper ion doping led to the formation of the minority phase of CuNb2O6 (3.11% to 5.29%) in samples with x ≥ 0.005. The XPS analysis of LiNbO3 revealed that among all, the LiNb600 sample has the higher percentage of Nb4+ species than Nb5+, in addition to oxygen vacancies on the surface of the nanocrystals, providing 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, whose superoxide radicals and holes acted as the main species involved in the discoloration of the RhB dye. In addition, the catalyst obtained > 91% efficiency, after five consecutive cycles of 90 min in the reuse test. Moreover, LiNbO3 polymorphs exhibit a synergistic and promising performance in the photodegradation of persistent organic pollutants. In the reaction of reduction of p-nitrophenol into p-aminophenol, the results indicated that pure LiNbO3 was not capable to reduce PNF, and that doping was essential in the process, in this case, x = 0.025 sample showed better efficiency, and was able to completely reduce PNF (50 mg·L-1) in 4 min, with t1/2 = 0.05 min. The radical scavenger tests revealed that the main species acting in the reduction of the PNF are electrons. Finally, piezocatalyst showed ~ 93% efficiency after 4 cycles of reuse, in 10 min.