ZnO Low-Dimensional Thin Films Used as a Potential Material for Water Treatment

In this paper the properties of zinc oxide (ZnO) low-dimensional conductive oxide nanostructures in the aspect of their potential applications in microelectronics, in toxic gas sensors as well as checking whether they can be also used in water treatment has been determined. ZnO nanostructured porous thin films deposited by DC reactive sputtering (RS) have been deposited on Si substrates at different temperature conditions. For the surface properties and chemical morphology analysis the X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) have been used. Thanks to those methods, it is possible to obtain information on changes in the structure caused by the adsorption of various gases from the atmosphere, mainly C pollution from air, but also from the water. Investigated ZnO thin films were also tested for their photocatalytic properties performed in UV light. For this purpose the methylene blue as a dye pollutant to evaluate the activity of the nanostructures has been used. Within this work it has been observed that the ZnO thin films, which were used, react in the selected environment and their presence reduces the amount of dye. This shows that in addition to sensor properties, properly selected zinc oxide nanostructures, used so far in toxic gas sensors, can also be used in the process of water purification and treatment, which is crucial in protecting the natural environment primarily from various types of dyes or also medicines

Novel insight on the local surface properties of ZnO nanowires.

Abstract Novel insight on the local surface properties of ZnO nanowires (NW) deposited by the evaporation-condensation method on Ag-covered Si substrates is proposed, based on the results of comparative studies by using the scanning electron microscopy (SEM), x-ray photoemission spectroscopy (XPS) and thermal desorption spectroscopy (TDS) methods, respectively. SEM studies showed that ZnO nanowires (nanoribbons) are mostly isolated and irregular, having the average length μm and the average at the level of tens nm, respectively. Our XPS studies confirmed their evident surface non-stoichiometry, combined with strong C surface contaminations, which was related to the existence of oxygen-deficient regions. Additionally, TDS studies showed that undesired surface contaminations (including C species and hydroxyl groups) on the surface of ZnO NWs can be removed almost completely, leading to an increase of the final non-stoichiometry. Both effects are of great importance when using ZnO NWs for the detection of oxidizing gases, because the undesired C contaminations (including C–OH species) play the role of undesired barriers for the gas adsorption, especially at the low working temperature, additionally affecting the uncontrolled sensor ageing effect

Photocatalytic degradation of methylene blue at nanostructured ZnO thin films

The photocatalytic degradation of the wastewater dye pollutant methylene blue (MB) at ZnO nanostructured porous thin films, deposited by direct current reactive magnetron sputtering on Si substrates, was studied. It was observed that over 4 photocatalytic cycles (0.3 mg · l−1 MB solution, 540 minUV irradiation), the rate constant k of MB degradation decreased by ∼50%, varying in the range (1.54 ÷ 0.78) · 10–9 (mol·l−1·min−1). For a deeper analysis of the photodegradation mechanism, detailed information on the nanostructured ZnO surface morphology and local surface and subsurface chemistry (nonstoichiometry) were obtained by using scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) as complementary analytical methods. The SEM studies revealed that at the surface of the nanostructured ZnO thin films a coral reef structure containing polycrystalline coral dendrites is present, and that, after the photocatalytic experiments, the sizes of individual crystallites increased, varying in the range 43 ÷ 76 nm for the longer axis, and in the range 28 ÷ 58 nm for the shorter axis. In turn, the XPS studies showed a slight non-stoichiometry, mainly defined by the relative [O]/[Zn] concentration of ca. 1.4, whereas [C]/[Zn] was ca. 1.2, both before and after the photocatalytic experiments. This phenomenon was directly related to the presence of superficial ZnO lattice oxygen atoms that can participate in the oxidation of the adsorbed MB molecules, as well as to the presence of surface hydroxyl groups acting as hole-acceptors to produce OH· radicals, which can be responsible for the generation of superoxide ions. In addition, after experiments, the XPS measurements revealed the presence of carboxyl and carbonyl functional groups, ascribable to the oxidation by-products formed during the photodegradation of MB