Monitoring And Controlling Of The Micromorphology Of Iron And Cobalt Based Transiton Metal Oxides And Their Performance

A series of iron and cobalt based transiton metal oxides involving ?-Fe2O3,Fe3O4 and Co3O4 with different morphologies have been fabricated using ions and inorganic small molecules as well as suitable polar solvent as structure-directing agents without using any surfactant. We investigated the morphology formation mechnism and made deep understanding of the correlation between the performance and the micro-nanostructure. The specific research contents as follows:1. A series of sequential morphological changes of ?-Fe2O3 nanocrystals were fabricated via an environmental friendly hydrothermal route under the system of Fe(NO3)3, ammonia(NH3·H2O) and ethylene glycol(EG). The morphology can be tuned from nanoplates to nanodisks, nanodrums, pseudo-spindles and finally to nanospindles with the(001) facets disappeared gradually simply by increasing the ratio of NH3·H2O to EG. Discussion is focused on the fundamental understanding of morphology evolution based on the characterization of nanostructure and the surface functional groups analyse along with the crystal structure. The oriented adsorption of NO3- ions and NH3 molecules on ?-Fe2O3 nuclei facets at elevated temperature is the key factor in the formation of different hematite morphologies. Simultaneous, the presence of NH3 molecules guarantee the NO3- ions presenting strong oxidizing ability that make the product is ?-Fe2O3 rather than Fe3O4. Morphology-dependent properties were demonstrated. The decreased visible photocatalytic efficiency from nanoplates to nanospinles under same conditions indicated that the increase of(001) facets exposed promoted photocatalytic performance. The as-synthesized ?-Fe2O3 nanoplates showed high ferromagnetism at roomtemperature and are easy to recycle after use.2. Iron oxides with different morphologies were successfully fabricated via an environmental friendly hydrothermal route by applying alkaline earth metal ions along the row from Mg2+ to Ba2+ ions as structure-directing agents. It was found that the ionic radii mismatch degree between the alkaline earth metal ions and Fe3+ ions played a vital role in determining the crystalline phase and morphology of the iron oxide. Pure ?-Fe2O3 crystals were created with Mg2+, Ca2+ and Sr2+ ions(30.9%, 81.8% and 114.5% ionic radii mismatch degree with that of Fe3+) as additives while goethite iron oxide nanoparticles were obtained with Ba2+ ions(145.5% ionic radius mismatch degree with that of Fe3+) as additives. Mg2+ ions induced ?-Fe2O3 crystals were quasi-cubes with rough surface while Ca2+ induced ?-Fe2O3 crystals were quasi-cubes with very smooth surface, and the edge length of which is much smaller than that of Mg2+ ions controlled ?-Fe2O3 crystals. However, when Sr2+ ions were introduced into the reaction system, the product tended to be plate-like structures. The effect of alkaline earth metal ions on the crystal growth rate and final morphology through anisotropic incorporation mechanism was proposed in this paper. XRD characterization indicates that the incorporation of alkaline earth metal ions increased the lattice parameters. Moreover, the effect of incorporation of alkaline earth metal ions on the magnetic properties of ?-Fe2O3 crystals were investigated.3. A solvothermal method has been developed to synthesize rhombic dodecahedral(RD) Fe3O4 single crystal structures bound by high-energy active {110} facets using N, N-dimethylformamide(DMF) as solvent and structure-directing agents. The growth process and formation mechanism of the RD Fe3O4 structures was investigated in detail by changing the reaction time and experimental parameters. The formation of Fe3O4 RDs underwent the generation of rod like organic precursors and subsequently decomposition into Fe3O4 monomers and finally gone through the nucleation and growth process according to the characterization of the morphology and composition for different reaction stage along with the surface functional groups analyse of the final product. During the growth process, Fe3O4 RDs was obtained by oriented adsorption of DMF molecules on the {110} facets of the Fe3O4 nuclei. Compared with the Fe3O4 microstructures with other mophologies, the RD Fe3O4 structures possessing active basal facets showed excellent peroxidase-like catalytic activity toward oxidation of methylene blue dye in the presence of H2O2 indicating the high surface energy do endows them with exceptionally high activity. There was no obvious decrease of the peroxidase-like catalytic activity of the RD Fe3O4 structures after being used even for six cycles. The as-synthesized products exhibit high magnetic properties with saturation magnetization value of 80 emu/g and are easy to recycle after use.4. Co3O4 nanostructures from 1D nanochains to a mixture of nanochains and nanosheets and finally to 2D nanosheets were obtained by tailoring the morphology of Co(CO3)0.5(OH)·0.11H2 O and subsequently calcination process under certain temperatures. The precursor of Co(CO3)0.5(OH)·0.11H2 O was obtained by a facile and environmentally-friendly hydrothermal method without any addition of CO32- sources. The results showed that different Co3O4 nanostructures can be tailored by controlling the calcination of Co(CO3)0.5(OH)·0.11H2 O nanobelts at different temperatures. It is found that Co3O4 nanochains with different individual crystalline grain size can be created in the range of 350-600 °C while porous Co3O4 nanobelts and irregular nanoparticles formed at 300 and 700 °C, respectively. We proposed that the formation of different Co3O4 nanostructures under different temperatures is a synergetic result of reaction kinetics and gas promotion. Simply by changing the volume ratio of NH3·H2O to EG, the morphology of precursor can be tuned from one dimensional nanobelts to a mixture of nanobelts and nanosheets and finally to two dimensional nanosheets. After calcination, corresponding Co3O4 with porous nanostructures obtained. Importantly, when evaluated as electrode materials for supercapacitors, compared with porous nanobelts and nanosheets, the as-prepared Co3O4 nanochains exhibit superior supercapacitive performance with the capacitance of as high as 800 F/g which is higher than most of previous reports which possibly due to its special short range order and long range disorder structures and its average feature size.