Preparation And Characterizations Of Several Kinds Of Photocatalytic Nanoparticles And Magnetic Nanocomposite Films

Due to the surface and interface effect, quantum size effect, macroscopic quantum tunnel effect and so on, nanomaterials have exhibited huge prospects in applications such as energy, environment, information, and biology. The study on the preparation and properties of nanomaterials has become one of the most active fields internationally.Environmental pollution from the industry has seriously threaten human being's health and existence. Semiconductor photocatalytic oxidation technology can effectively degrade the hazardous industrial waste water and toxic exhaust gas using solar light energy, which opens a potential approach to purify environment. Simultaneously, with the coming of the information age, the area density of magnetic recording media continuously increases at a tremendous high rate, which has tended to its physical limitation. Nowadays, it is a tough challenge to develop new ultra-high density magnetic recording materials and systems.In this dissertation, focusing on the novel semiconductor photocatalytic nanomaterials and ultra-high density magnetic recording media, we explored the photocatalytic degradation of the organic dyes and the fabrication technology of the magnetic nanocomposite films. The Ta-doped ZnO and ZnTiO3 nanopowders have been synthesized by the wet chemical method, and FePt/Al2O3 nanocomposite films have been prepared via a special approach which combines the atomic layer deposition (ALD) and FePt self-assembly. And the impact of processing parameters on the structural, morphological, photocatalytic and magnetic properties has been studied systematically. Some relative mechanisms were also discussed deeply.Main achievements are summaried as follows:1. Ta-doped ZnO nanoparticles (NPs) with different Ta-doping contents have been prepared through a modified Pechini method, using the home-made water-soluble peroxo-citrato-tantalum precursor as the source of Ta. A systematic study on the effect of Ta-doping content and processing on the structure, morphology, defect, and visible-light photocatalytic properties has been carried out by means of various analytical techniques. It is found that the Ta-doped ZnO samples exhibit the excellent photocatalytic performance for degradation of the dye methlene blue (MB), and 1 mol % Ta-doped ZnO annealed at 700℃shows the optimal photocatalytic activity. It is closely related to the smallest crystalline size, highest concentration of surface hydroxyl groups and active hydrogen-related defect sites, and largest specific surface area. The photodegradation of MB using 1% Ta-doped ZnO at 700℃follows the first order kinetics according to the Langmuir-Hinshelwood model with first-order rate constant (k) and half-life (t1/2) of 4.01×10-2 min-1 and 17.2 min, respectively. The maximum photocatalytic degradation rate is achieved at pH= 8, which is attributed to the electrostatic interactions and hydroxylation of the catalyst surface. The mechanism of the photodegradation of MB using Ta-doped ZnO photocatalyst is proposed. Ta5+ ions enter into the crystal lattice of ZnO by replacing the Zn2+, producing more zinc vacancy defects (VZn") along with some oxygen vacancy (Vo(?)). This leads to the formation of intermediate defective band in bandgap, playing an important role in the photocatalytic degradation of MB. In comparison, the oxygen vacancies have stronger photocatalytic degradation of MB than the zinc vacancies.2. ZnTiO3 nanoparticles have been prepared through a modified alcoholysis method. The effect of annealing conditions on the structure, morphology, optical bandgap and photocatalytic properties of ZnTiO3 samples have been investigated systematically. With the increasing of the annealing temperature and time, the perovskite-type cubic to ilmenite-type hexagonal transformation occurs with the increasing grain sizes and the decreasing BET specific surface areas gradually. Pure hexagonal-phase ZnTiO3 can be achieved at 800℃for 3 h, while the hexagonal-phase ZnTiO3 decomposes to the cubic Zn2TiO4 at 900℃. Experimental results indicate that the relation between the hexagonal phase and the photocatalytic performance is close-knit in the ZnTiO3 system. The more hexagonal phase the ZnTiO3 system has, the higher photocatalytic efficiency the catalyst exhibits. And the pure hexagonal-phase ZnTiO3 annealed at 800℃shows the excellent photocatalytic degradation of the dye methyl violet (MV). For the pure hexagonal-phase ZnTiO3, the proper processing parameters such as concentration of catalysts and pH value also plays an important role in accelerating the photocatalytic activity. The optimal pH value of the catalyst ZnTiO3 annealed at 800℃for 3 h is around 8 with the optimal catalyst amount of 4 g L-1 and the H2O2 concentration of 0.102 mol/L at a 10 mg/L aqueous solution of the azo dye MV. Over loading of catalyst may reduce the photocatalytic degradation rate, similar to higher pH value and H2O2 concentration. The density of state (DOS) of the ilmenite-type hexagonal ZnTiO3 is calculated by first-principle calculation, the optimized lattice parameters are slightly smaller than the experimental data. The bandgap is about 2.75 eV, which is smaller than the experimental value (Eg=3.05 eV).3. A new fabrication technique for the FePt/Al2O3 nanocomposite films for ultrahigh density magnetic memory applications has been developed by combination of ALD technology and FePt self-assembly firstly. It is found that the amorphous Al2O3 layer deposited by ALD can prevent FePt NPs from sintering and keep the position order of the FePt NPs assembly, producing the L10-FePt/Al2O3 nanocomposite films with well dispersion and high coercivity. With increasing the Al2O3 layer thickness, on one hand FePt NPs display evidently improved dispersion and reduced exchange coupling, on the other hand the chemical ordering parameter S and coercivity of FePt/Al2O3 nanocomposite films decreases gradually. It can be attributed to two factors. The smaller FePt particle size (close to critical size) the composite film has, the more difficult the fcc→fct phase transition is. The other is that the residual strain at the FePt/Al2O3 interfaces could decrease the ordering parameter. When the thickness of amorphous Al2O3 capping layer is 10 nm, the nanocomposite films exhibit optimal dispersion and magnetic property after high-temperature annealing. The fct-FePt NPs/10 nm-thick Al2O3 sample possess a coercivity of 5.9 kOe and magnetocrystalline anisotropy constant Ku of 3.86 MJ/m3. The thermal factorηfor above sample is 68.5, meeting the industry requirement (η≥50). The effect of in-situ applied magnetic field on the structure, morphology and properties of the nanocomposite film during the FePt NPs assembly and ALD Al2O3 deposition has also been explored. And the in-situ magnetic field perpendicular to the substrate shows certain influences on the L10 phase transition and magnetic property for FePt-Al2O3 films with better dispersion of FePt NPs and in-plane orientation of magnetic moments. The carbon nanotube loaded FePt NPs catalysts for the oxidative dehydrogenation of ethylbenzene have been characterized. The carbon nanotube loaded 6% FePt NPs sample show higher catalytic property and better stability.In summary, we have studied the preparation of several kinds of nanopowders and nanocomposite films and their photocatalytic and magnetic properties systematically and deeply and made some significant progress. Based on the ALD technology, a feasible potential approach to fabricate the ultrahigh bit-patterned magnetic recording media has been explored. This work is of importance and value to facilitate the development of ultra-high density magnetic memory devices (1Tbit/inch2) with China own intellectual property rights...