Síntese de filmes de óxido de zinco nanoestruturados para aplicações em fotocatálise

Abstract

The development of nanostructured films has been widely studied for several applications, including photocatalysis. Currently, one of the great challenges in sustainability is to develop strategies for the treatment of effluents in water bodies contaminated by toxic substances from industrial activities. Among the Advanced Oxidative Processes (AOP's), photocatalysis is a widely studied alternative due to its efficiency in degrading pollutants. Zinc oxide is one of the semiconductors most used as a catalyst in photocatalytic processes. Considering these factors, this work aimed to produce nanostructured ZnO films by hydrothermal microwave synthesis. The studied variables were: the type of substrate, the synthesis time and the thermal treatment in the formation of the films. The films were deposited on glass substrates and on a fluorine-doped tin oxide (FTO) substrate. Syntheses were made with 10 minutes and 30 minutes in duration. Part of the samples were subjected to heat treatment at 600ºC and the others were dried in an oven. The films were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (SEM-FEG), atomic force microscopy (AFM) and UV spectroscopy -Vis, in diffuse reflectance (DRS) mode. The films were used in photocatalytic tests in the degradation of the dye Rhodamine B, under UVC radiation. Reuse tests were also carried out. All films showed hexagonal wurtzite crystal structure. Films synthesized on a glass substrate presented 3,3 to 3,7 eV bandgap. In the case of these films, the variation in the synthesis time was crucial for defining the structure of the films: films synthesized in 30 minutes present a better agglomerate distribution with the substrate filled in a more uniform way. The photocatalytic activity on Rhodamine B degradation of these films ranged from 38% (synthesis in 10 minutes) to 48% (synthesis in 30 minutes) in the case of the oven dried samples. With heat treatment the efficiency increases to 68% (10 minutes synthesis) to 76% (30 minutes synthesis). For films synthesized on FTO substrate, bandgap values ranged from 3,3 to 3,5 eV. Initially, the structure obtained presented a lamellar shape. After thermal treatment at 600ºC, both in the synthesis lasting 10 minutes and in the 30 minutes, the films presented a surface completely filled with ZnO. Thus, in the case of films deposited in FTO, the difference was the post-synthesis heat treatment. The photocatalytic activity on Rhodamine B degradation of these films was 34% (10 minutes synthesis) and 43% (30 minutes synthesis) in the case of oven dried samples. With heat treatment the efficiency increases to 59% (10 minutes synthesis) and 53% (30 minutes synthesis). The films obtained on glass substrate with post-synthesis heat treatment were used in reuse tests for the degradation of Rhodamine B. It was concluded that after 5 cycles the films maintain good photocatalytic activity, above 50%.