Experimental Investigations of Iron Oxide Based Nanostructures and their Correlation with Theory

Abstract

The present study deals with synthesis of different iron oxide (magnetite, maghemite and hematite) based nanostructures using sol-gel method. The main emphasis is to experimentally synthesize iron oxide based nanostructures and to correlate these results with theory. Molarity, pH, temperature and surfactant of the sol are important parameters to control morphology. Hence, in this research work all of the 4 parameters were optimized to study their effect on structural, morphological, optical and magnetic properties. In the first step seven different sols with varying concentrations from 1.8 to 0.6 mM are synthesized. The concentration of 1.4 mM results in a pure magnetite phase whereas others show mixed magnetite and maghemite phases. Free growth of iron oxide nanostructures, including nanoneedles, nanorods, nanospheres and nanobrushes, are observed in scanning electron microscope images. In the second step 1.4mM sol concentration is used and nine sols are synthesized with pH 1 to 9. Samples exhibit magnetite phase with superparamagnetic nature at low pH (1, 2 & 6) with 50nm diameter nanoparticles. For pH 3-5 hematite phase is observed while with further increase in pH (7&8) maghemite phase is achieved. Annealing for pH 1-8 only strengthened the existing phases rather than transformation. Maghemite and hametite phases are observed at pH 9 with annealing at 200 oC and 300 oC respectively. In the third step effect of surfactants was studied in detail. PVA, triton X-100 and oleic acid are used as surfactants. Sols thus prepared are analyzed magnetically before and after room temperature aging. Superparamagnetic behaviour is observed for iron oxide sol synthesized using oleic acid as surfactant, and therefore was selected for further studies. The amount of oleic acid is varied as 5%, 10% and 15% by volume. Shape and morphology of iron oxide nanoparticles strongly depend on calcination temperature, which is varied from 300˚C to 900˚C. Iron oxide sol with 15% by volume show superparamagnetic behavior while sols prepared with 5% and 10% oleic acid show dia-ferromagnetic and para-ferromagnetic mixed behavior. Two types of NPs are observed in SEM images; one with shell and one without shell with 10% oleic acid. Cubic NPs with size less than 25nm and highest dielectric constant of ~107.5 (log f = 5.0) is observed with 15% oleic acid at 500˚C. ZnO is selected for iron oxide based nanostructures. Once again sol-gel method has been employed for synthesis of iron oxide added ZnO nanoparticles. The dopant concentration is varied as 1wt% to 5wt%. Ferromagnetic behavior of Fe added ZnO nanoparticles arise due to the presence of long range oscillating interactions among the free charge carriers. The band gap of these iron oxide based nanostructures is in the range of 3.05eV to 3.48eV. Mn/Fe co-doped ZnO structures are also prepared by simple sol-gel and spin coating method. Five different sols with the change in concentration (1-5wt%) of both Mn and Fe are synthesized. Molar ratio of Mn and Fe is kept constant, i.e., 1:1. Sols are spun onto glass and copper substrates by spin coating method followed by the post magnetic field annealing at 300 ˚C for 1 h. XRD results show incorporation of Mn and Fe in the host lattice up to a dopant concentration of 4wt%. Small crystallites of Mn and Fe2O3 are observed by increasing the dopant concentration to 5wt%. VSM results indicate room temperature ferromagnetism in all samples. Moreover, Mn/Fe co-doped thin films show magnetic hysteresis equivalent to that of multilayered structure, indicating that such complex structures can be replaced by a single ZnO layer with co-doping of Mn and Fe. Density functional theory is used for the theoretical investigation of iron oxide based nanostructures. Amsterdam Density Functional (ADF) software with BAND tool is used. Generalized Gradient Approximation (GGA) and Local Density Approximation (LDA) are used in order to correlate structural, optical and magnetic properties of iron oxide based nanostructures. With basis set of TZ2P geometry optimization is achieved. Underestimation of electronic properties of all phases is observed by GGA and LDA. While, improved value of band gap is obtained by GGA+U and LDA+U. Exchange correlational potential is also optimized in case of GGA+U calculations. Hubbard potential (U eV) is optimized and lowest value of U i.e. 0.6 eV is used for all calculations for wüstite, hematite, maghemite and magnetite. Total DOS and partial density of states for iron and oxygen are also studied for both approximations. Analysis of the density of states confirms the strong hybridization between Fe 3d and O 2p states in iron oxide. In all cases (magnetite, maghemite and hematite) density of states plots confirm that the main reason for the magnetic properties in iron oxide based nanostructures is the d orbital electrons. As a result, a good correlation of theory with experiment is being reported in this thesis.