Desenvolvimento de nanomaterial à base de NiO/δ- FeOOH para aplicações fotocatalíticas: um estudo teórico e experimental

Abstract

Modified photocatalysts by heterojunction formation between semiconductors are of great interesting in increase their efficiency. This work proposed the obtaining of a new photocatalyst composed of δ-FeOOH, an iron oxide-hydroxide, and nickel oxide, NiO. The materials FN5, FN10 and FN20 were obtained using a ball mill, in which 5, 10 and 20% of pure NiO were mixed, respectively, in relation to the mass of δ-FeOOH. The materials and their precursors were characterized by x-ray diffraction, Fourier transform infrared spectroscopy, BET surface area, diffuse reflectance in the ultraviolet-visible region, x-ray spectrometry and scanning and high-resolution transmission electron microscopy techniques. It was confirmed that pure NiO and δ-FeOOH nanoparticles were obtained and that the milling did not alter the structure of the respective precursors. It was assumed that the NiO nanoparticles occupied the δ-FeOOH mesopores and that the crystallographic planes of both semiconductors overlapped, forming a heterostructure. This resulted in a new arrangement of the electronic bands, evidenced by the band gap value, especially for FN20, of 2.40 eV, when compared to the precursors, 1.90 eV for δ-FeOOH and 3.65 eV for NiO. The photocatalytic tests were carried out with 25W and 8W LED lamps, for the removal of the methyl orange dye, and the FN20 was the most efficient, about 72% of removal. A theoretical study of the reaction mechanism of this process was proposed by calculating the positions of the valence (VB) and conduction bands (CB), for the evaluation of charge movement. Electrons and holes transitions occur from δ-FeOOH CB to the NiO CB and from the NiO VB to δ-FeOOH VB, respectively. From these positions, it was admitted that the oxidation reactions of water molecules to obtain •OH radicals would occur on the surface of δ-FeOOH and through a thermodynamic analysis, that the process would be energetically favorable. Starting from the FN20, the influence of the experimental parameters light, concentration and pH of the solution and photocatalyst mass were studied, as a function of the percentage response of dye removal in the presence and absence of the oxidizing agent peroxymonosulfate, PMS. Trough the response surface methodology, it was evaluated that the pH was the most significant parameter in both situations. PMS, in turn, had a greater effect on dye removal time than on efficiency, accelerating the process when present.