TiO2 and ZnO metal oxides lattice defects driven magnetism and their contribution in discoloring of dyes
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Date
2017
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University of Zululand
Abstract
We demonstrate that even wide band gap nonmagnetic materials as simple as ZnO and TiO2 with small concentrations of ionized oxygen vacancies and interstitial zinc or Ti can exhibit extraordinary properties. We show that the presence of oxygen vacancies turns the d10 nonmagnetic oxide into a ferromagnet. The study further shows an existence of a correlation between interstitial zinc on the magnetization hysteresis reversal. The observed reversal in hysteresis loop known as exchange coupling existing in a ferromagnet and antiferromagnet/ferrimagnet interface is currently observed only in similar morphological nanostructures. This remarkable observation suggests an existence of mixed and itinerate magnetism in one material. We have found a consistent correlation between the crystallite sizes and the shift in the exchange bias. Our ZnO hierarchical nanostructures are of “triangular- or samosas-like”, “platelets-like”, “hierarchical flowers-like” and “hexagonal open cups-like” structures possessing hexagonal internal structure of ZnO as demonstrated in our structural analysis. These ZnO nanostructures were synthesized in a shape-selective manner using simple microwave assisted hydrothermal synthesis. The direct correlation between the observed ferromagnetism and the relative concentration of oxygen and zinc vacancies that exist, suggest their vital role in modulating ferromagnetic behavior. These defects combination correspond to a g-factor value of > 2.1 depending on each structure. In addition, a novel low field microwave absorption signal was observed resulting from large surface areas due to nanostructuring. Moreover, there is an interesting and consistent twinning phenomena observed. The “samosas” tend to twins together with “hierarchical flowers” while “platelets” twin with “cups” nanostructures. Furthermore, we have synthesized ZnO nanorods. These nanorods seem to nucleate from one single rod and subsequently grow in numbers facing all different directions making like flowers but originating from one single point. These flowers-like rods were tested for photocatalytic activity under ultra-violet light. On the other hand, pure and manganese (Mn) ion-doped TiO2 films were grown by pulsed laser deposition under vacuum at different substrate temperatures 350 and 580 ºC for 50 min each. The concentration of the Mn in TiO2 films was varied from 10 to 30%. The prepared films were then characterized by various techniques such as X-ray diffraction spectroscopy, scanning electron microscopy, and atomic force microscopy among others. A close connection between photocatalytic degradation performance and photo-excited electron recovery/relaxation was observed. It was found that longer photo-excited electron recovery/relaxation leads to poor photocatalytic degradation performance while shorter or ever collapsing photo-excited electron recovery/relaxation improves the photocatalytic degradation performance. These photo-degradation measurements were observed with UV light. The 20% Mn doped TiO2 film improved the photo-degradation performance by scavenging the photo-generated electrons more than its counterparts. Moreover, the formation of more short-range than long-range order crystal characteristics induced by the oxygen deficient environment and low substrate temperature during vacuum laser deposition resulted in the formation of oxygen vacancies. This further enhanced the photocatalytic performance of the TiO2-x films. The films thicknesses reduced drastically by almost one order of magnitude after the photocatalytic activities of methylene blue. These thin films grown on corning glass substrate were further tested for magnetic properties and photocatalytic activities. The results revealed that the films grown at 580 ºC photo-degrade [Rhodamine-6G (Rh-6G) and Methyl Orange (MO)] faster than those grown at 350 ºC with the 10% Mn films showing an enhanced performance. On the contrary, the 350 ºC grown films photo-degrades [Methylene Blue (MB)] much faster than the 580 ºC films with the 20% Mn doped film being the most efficient. At both temperatures, the 20% Mn films possess the highest MB degradation rate and spin concentration at 150 and 310 mT fields. The 10% Mn film at 580 ºC possesses the highest degradation rate of both Rh-6G and MO but has the lowest spin concentration at 150 mT field. In addition, it was also found that the non-crystalline formation further enhances the degradation of MB dye. Moreover, we performed photoactivities on the ZnO hierarchical structures morphology control which were synthesized at different times, 15, 30 and 60 minutes. We have observed rather a non-increasing trend but quite constant as the UV irradiation time is increased to maximum of 2 hours. However, the 30 and 60 minutes’ samples showed a low increasing trend to below 30 % of decoloratization. These low photoactivities results are attributed to low or poor crystallinity.
Description
A thesis submitted to the Faculty of Science and Agriculture in partial fulfillment of the requirements of the Degree of Doctor of Philosophy in the Department of Physics and Engineering at the University of Zululand, 2017
Keywords
zinc oxide(ZnO) --titanium dioxide(Ti02) --photo-catalytic activities --lattice defects --photoluminescence --ferromagnetism --electron spin resonance (ESR) --vibrating sample magnetometer (VSM) --X-Ray photoelectron spectroscopy