Preparation and Characterization of Different Iron Oxide Phases for Advanced Applications

Abstract

Research work reported in this thesis involved optimization of three important phases of iron oxide i.e. magnetite (Fe3O4), maghemite (γ-Fe2O3) and hematite (α-Fe2O3) through low cost and application oriented sol-gel method. Main emphasis of this research was to develop a single route to prepare the above-mentioned three phases of iron oxide. Important parameters of sol-gel synthesis, including choice of precursor, pH and sol concentration, had initially been optimized for the preparation. Iron chloride and iron nitrate were selected as precursor because of low decomposition temperature and controlled hydrolysis rates. First route (R-I) for synthesis of iron oxide sol included iron chloride (FeCl3.6H2O) as precursor and oleic acid as surfactant. R-I was sub divided in 2 parts a and b. In R-Ia, water and ethanol were used as solvent whereas ethylene glycol was used as solvent in R-Ib. Shelf life of R-Ia was longer as compared to R-Ib that’s why R-Ia was selected for further investigations. Sol concentration of R-Ia was varied as 0.2mM to 2.0mM. Sols were spin coated onto copper (Cu) substrates for preparation of thin films. Films were studied under as-synthesized conditions and after annealing at 300˚C in vacuum under 500Oe externally applied magnetic field. XRD results confirmed the formation of Fe3O4 phase of iron oxide at sol concentration 1.4mM. All other values of sol concentration resulted in mixed iron oxide phases. Formation of different phases for various sol concentrations had been confirmed using Raman spectroscopy. In addition to variation in solvent, ratio of Fe3+/Fe2+ cations during sol synthesis was also varied as 1.0, 1.25, 1.50, 1.75 and 2.0. Mixed iron oxide phases with reduced crystallinity were observed under all the conditions. Mixed phases lead to reduction in magnetic as well as dielectric properties. In the second route (R-II), iron chloride (FeCl3.6H2O) was used as precursor, water and ethanol as solvent whereas no surfactant was used during sol synthesis. Sol concentration was varied as 0.2mM to 2.0mM. Sol concentration 0.6mM to 1.0mM resulted in formation of γ-Fe2O3 phase while formation of α-Fe2O3 phase was observed at sol concentration 1.6mM-2.0mM. Mixed phases were observed for rest of the sol’s concentrations. Formation of Fe3O4 phase was not observed for the entire sol concentration range studied. Third route (R-III) was also surfactant free whereas iron nitrate was used as precursor. RIII was sub divided in 2 parts a and b. In R-IIIa, water and ethanol were used as solvent whereas ethylene glycol was used as solvent in R-IIIb. Shelf life of R-IIIa was shorter as compared to R-IIIb that’s why R-IIIb was selected for further investigations. Sol concentration was varied as 0.2mM-2.0mM. XRD results confirmed the formation of γ-Fe2O3, Fe3O4 and α-Fe2O3 phases at sol concentrations 0.2-0.6mM, 1.2-1.4mM and 1.8-2.0mM, respectively. Whereas, mixed iron oxide phases were observed for sol concentrations 0.8-1.0mM. Changes in crystallite size, strain and dislocation density were consistent with variation in different phases of iron oxide. Cole-Cole plots fitted using Zview software indicated that changes in dielectric constant along with variation in saturation magnetization strongly depend on different phases of iron oxide. pH of R-IIIb sol was varied from 1 to 11. Strengthened iron oxide phases were observed at pH 1. Optimized molarities (0.6mM, 1.4mM, 2.0mM) using R-IIIb route, with pH 1, were used for the synthesis of doped iron oxide sols. Cobalt (Co), nickel (Ni), and chromium (Cr) were used as dopants. Dopant concentration was varied as 2-10wt %. No phase transition was observed for all dopant concentrations. However, variations of magnetic and dielectric properties were observed at particular dopant content. γ-Fe2O3 thin films showed high saturation magnetization and high dielectric constant for 6%, 8% and 4% Co, Ni and Cr doping, respectively. Fe3O4 thin films, showed high saturation magnetization and high dielectric constant for 8%, 6% and 4% Co, Ni and Cr doping, respectively. α-Fe2O3 thin films showed high saturation magnetization and high dielectric constant for 4%, 8% and 8% Co, Ni and Cr doping, respectively.