Avaliação fotocatalítica de nanoestruturas de ZnO obtidas pelo método solvotérmico assistido por micro-ondas

Abstract

Textile industries are largely responsible for the pollution of rivers and lakes by spilling large amount of organic dyes per day into the water bodies. To minimize problems caused by this type of pollution, several works using photocatalyst materials capable of degrading dye molecules under solar irradiation have been developed. The search for materials with semiconductor characteristics to be used as photocatalysts is becoming more common every day. Thus, herein, nanostructured crystalline materials based on ZnO were synthesized by microwave-assisted solvothermal method in different reaction media (aqueous, ethanolic and 50:50 mixture of water and ethanol). This allows us to study the influence of particle morphology in the photodegradation of an organic dye, methylene blue (MB). The synthesized materials were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), ultraviolet-visible spectroscopy (UV-Vis), Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) with elemental mapping by energy dispersive X-ray spectroscopy (EDX) and paramagnetic resonance spectroscopy (EPR). According to the XRD results, single-phase ZnO samples with good crystallinity were obtained; IR vibrational modes characteristic of the oxide were observed in the IR spectra; the band gap energy values of 3.30 eV estimated for the materials by UV-Vis spectra are in agreement with the literature data; FE-SEM images in combination with TEM indicated that all ZnO samples are formed by nanoparticles with distinct morphologies and sizes that vary according to the synthesis conditions. The ZnO materials presented different photocatalytic potentials, presenting a desired efficiency in the degradation of MB dye when the sample synthesized in aqueous medium was used as photocatalyst. This sample presented the highest photocatalytic efficiency reaching 65 and 91% of MB degradation in 4 h under UV-C and visible light exposure, respectively. This activity was related to the flower-like morphology of the ZnO particles and to the balance of electronic defects present in the material evidenced by EPR.