Aqueous Solution Preparation, Structure, And Properties Of Ferrites And ZnO Films

With the rapid development of modern communication technology, electronic devices have become more miniaturized and integrated, which makes spinel ferrite films become increasingly important. This trend and interest are further motivated by potential applications such as high-density perpendicular magnetic recording media, magnetic heads, transformer, inductor, multilayer devices, biotechnology, medical diagnosis and magnetic drug. Structural and magnetic properties of spinel ferrites MeFe2O4 depend upon the nature, concentration, and distribution of the substituted Me cations on A and B sites as well as the method of preparation. In the traditional process of preparation, post heat-treatments or high deposition temperatures (>600℃) required to induce the desired crystalline phases. The high temperature would deteriorate the non-heat-resistant substrates, such as GaAs integrated circuits, plastics and biomaterials. Abe et al. developed a widely used spin-spray method. In the process, long deposition time and complex equipment are needed; and a great deal of reactants is wasted. Izaki et al. developed a wet chemical preparation method which can overcome the drawbacks. However, there is little information on the preparation, structure and magnetic of complex spinel ferrite films prepared by Izaki's method.The main findings are listed as follows:(1) Ni0.11ZnxCo0.03Fe2.86-xO4 spinel ferrite films with x=0.00,0.23,0.34,0.43 and 0.51 were prepared on Ag-coated glass substrates from nitrate and dimethylamine borane (DMAB) solution at 80℃for the first time. The Zn content x is very sensitive on the amount of Zn2+ ions in the solution.(2) The Ni0.11Zn0.51Co0.03Fe2.35O4 ferrite film is about 500 nm in thickness and is composed of uniform equiaxed granules of about 40-50 nm in size. The surface of the film is very rough and has a 200 nm roughness (200 nm thickness difference between "hill" and "valley").(3) With the Zn content x increasing from 0 to 0.51, the lattice constant of the ferrite films increases from 8.383 to 8.425 A. The average grain size for all the ferrite films estimated from the diffraction peak widths by Scherrer equation is about 40 nm. The Raman modes of 680 cm-1 for Ni0.11Zn0.51Co0.03Fe2.35O4 film was fitted by two Gauss peaks of 655 cm-1 and 687 cm-1, which can be attributed to the O breathing vibrations against Zn and the O vibrations against Fe. It implies that the Zn2+ ions are incorporated into the A sites of the spinel lattice and affect the AO4 breathing vibrations.(4) The magnetic properties of the Ni0.11ZnxCo0.03Fe2.86-xO4 spinel ferrite films could be controlled by x. Saturation magnetization increases with x increasing from 0 to 0.35, reaches a maximum value of 460 kA/m at x=0.35, and then decreases with further increase of x. Coercivity decreases monotonically from 12.3 to 1.7 kA/m with x increasing from 0 to 0.51. The change in magnetic properties can be explained by the decrease of A-B interactions and the anisotropy constant due to the incorporation of non-magnetic Zn2+ ions.One dimensional (1D) ZnO nanostructures have been attracting considerable attention owing to their unique electrical, piezoelectric, optoelectronic and luminescence properties as well as promising applications in energy scavenging, light-emitting diodes and gas sensors. Various 1D ZnO nanostructure arrays with different morphologies have different properties. Morphology-controlled synthesis of multiple 1D ZnO nanostructures and studying their morphology-related properties become topics of extensive research. Many methods, such as metal-organic chemical vapor deposition, thermal evaporation and vapor phase transport technique and aqueous solution method, have been used to prepare 1D ZnO nanostructures. The physical methods require high temperatures and complicated equipments and have a low yield. In contrast, the chemical methods can be carried out in soft environments on a large scale. Till now, no systematic work on synthesis of aligned ZnO nanoneedle arrays, nanorod arrays and polycrystal films by Izaki's method has been studied.The main findings are listed as follows:(1) Aligned ZnO nanoneedle array, nanorod array and polycrystal films have been prepared on the Ag-coated glass substrates in aqueous Zn(NO3)2 and DMAB solutions for the first time. By adjusting the Zn(NO3)2 concentration, the morphology of the ZnO nanostructures can be tuned from nanoneedles array through nanorod array to polycrystal ZnO films. Low Zn(NO3)2 concentrations (2-15 mM) result in ZnO nanoneedle arrays.(2) With the increase of reaction time, The growth process of the ZnO at low Zn(NO3)2 concentrations includes the formation of the equiaxial ZnO granules, the growth of the equiaxed granules into small nanorods, the growth of the small nanorods into nanoneedles, the diameter and length growth of the nanoneedles, the growth of the nanoneedles into nano-obelisks and the growth of the nano-obelisks into thick hexangular rods.(3) The TEM and XRD results confirm that ZnO nanoneedles grow along the c axis. The nanoneedles arrays have the largest surface area by estimating from SEM results. XPS results proved that the zinc hydroxide species and oxygen-deficient states co-exist in the surface of the ZnO nanostructures.(4) The ZnO nanoneedle arrays have the largest number of zinc hydroxide species and oxygen-deficient states on the surface which result in the largest ratio of the visible emission to ultraviolet emission. The ZnO nanoneedles also have the highest photocatalytic activity.