Ariadne Catto1,Luis da Silva1,Mateus Ferrer2,Osmando Lopes3,Valmor Mastelaro3,Elson Longo1
Federal University of São Carlos1,University of São Paulo State–UNESP2,University of São Paulo3
Nowadays, a concern over environmental problems has enhanced our awareness of the need to develop alternative and environmentally friendly process aiming at increase environmental protection. For the treatment of waste water and pollutant degradation, photocatalysis is considered one of the most promising and sustainable way due to its high-efficient, eco- friendly, low toxicity and low cost.<br/>Zinc oxide (ZnO) exhibiting different morphologies and high surface/volume ratio exhibit many favorable optical, electronic and surface properties leading it a promising candidate in field of photocatalysis as a sustainable and eco-friendly technology. Motivated by these considerations, the goal of this study consisted to investigate the influence of zinc precursor on the structural, microstructure, surface and photocatalytic properties of nanostructured ZnO films grown under hydrothermal conditions.<br/>The nanostructured ZnO films were grown via hydrothermal method using different zinc salts, including acetate (Zn(CH<sub>3</sub>COO)<sub>2</sub> 2H<sub>2</sub>O), nitrate (Zn(NO<sub>3</sub>)<sub>2</sub> 6H<sub>2</sub>O), and sulfate (ZnSO<sub>4</sub> 7H<sub>2</sub>O) under identical conditions. The crystalline structure and morphological features were investigated by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) measurements, while the surface properties were probed by using X-ray photoelectron spectroscopy (XPS).<br/>XRD patterns of the ZnO films were indexed to the wurtzite ZnO structure, however the sample synthesized from zinc sulfate also presented additional peaks assigned to the zinc hydroxy-sulfate-hydrate<i> </i>phase. FE-SEM analyses reveal that the morphology of the ZnO structures is strongly dependent on the zinc salt. The zinc nitrate provides the formation of the same morphology of the acetate, with an anisotropic growth preferentially along the (0001) direction resulting in hierarchical nanorod-like structures with an average diameter of ca. 90 nm. The average rod diameter was larger for ZnO samples synthesized using zinc acetate (ca. 2 times). Regarding the zinc sulfate, the change in microstructure was more significant, where SEM images revealed the formation of hexagonal microplate-like structures. In this case, the growth along (0001) was dramatically suppressed, producing a micrometric 2D morphology.<br/>With respect to photocatalytic activity, the ZnO samples were studied for the degradation of methylene blue (MB), and rhodamine B (RhB) dyes under ultraviolet light. The ZnO film prepared from zinc nitrate exhibited the best photocatalytic activity compared to other samples, followed in turn by the sample prepared with acetate, and sulfate. The enhancement of photocatalytic activity was attributed to structures size, surface morphology including the higher fraction of exposed (0001) ZnO polar facets, as well as the presence of oxygen defects.