| Spinel ferrites have attracted great attention due to the wide applications in magnetic material, adsorption, photocatalysis, and solar cells. Superparamagnetic ZnFe2O4 nanomaterials have great potentials in magnetic resonance imaging (MRI), separation and purification, drug delivery, and magnetically induced hyperthermia. And forming magnetic colloidal nanocrystal clusters (MCNCs) has proved to be an effective way to retain superparamagnetic behavior with high magnetization. A polyol method was developed for the preparation of ZnFe2O4 MCNCs through the hydrolysis and reduction of iron chloride and zinc chloride in ethylene glycol at high temperatures. But the Zn content could not be controled accurately by the traditional preparation method.In this paper, the chlorates were replaced by the nitrates as the reactants for the preparation of stoichiometric ZnFe2O4 MCNCs. And the formation mechanism was studied by in situ microcalorimetry technique. Moreover, a series of zinc doped Fe3O4 (ZnxFe3-xO4) MNCs were synthesized by this method.The main works are listed as the following:1. Stoichiometric ZnFe2O4 magnetic colloidal nanocrystal clusters (MCNCs) with an average diameter of 127 nm have been successfully synthesized by using a one-step solvothermal method without any additives, where nitrates (Fe(NO3)3 and Zn(NO3)2) were used as reactants. However, when FeCl3 and ZnCl2 were used as reactants replaced the nitrates at the same experimental conditions, ZnFe2O4-40Fe3O4 was synthesized. Different characterization techniques were used, such as XRD, ICP-AES, TEM, HRTEM, XPS, and SQUID. The results demonstrate that the stoichiometric ZnFe2O4 is superparamagnetic, and the saturation magnetization is higher than that reported in references. A possible mechanism for the stoichiometric ZnFe2O4 formation was proposed and discussed based on the time-resolved experiments.2. In situ microcalorimetry was used to investigate the energy evolution during ZnFe2O4 colloidal nanoparticles growth process synthesized by a one-step solvothermal method without any additives, where nitrates (Fe(NO3)3 and Zn(NO3)2 were used as reactants. And different characterization techniques were used, such as XRD, ICP-AES, AAS, XPS, FTIR, UV and SEM. The results demonstrate that when the experimental temperature was smaller than 134℃, the Fe2(C2H4O2)3 and ZnC2H4O2 gel were generated. And the transformation processes were α-(Fe,Zn)OOHâ†'α-Fe2O3â†'Fe3O4 when the temperature was higher than 159℃.3. A series of Zn-doped Fe3O4 magnetic nanoparticles (MCNCs) with different Zn content represented as ZnxFe3-xO4 (0<x<1) were synthesized by a solvothermal method. The ratio of Zn to Fe contained in the as-prepared samples was measured by ICP-OES, which demonstrated that this ratio can be accurately controlled. The structure of the sample characterized by XRD showed that the Zn2+ion replaced the Fe3+at the A site and the Fe2+ ion changed to Fe3+ at the B site when x≤0.4, and replaced the Fe2+ ion at the B-site when x>0.4. The morphology of the sample characterized by TEM showed that magnetic nanocrystal clusters (MNCs) assembled by the MNPs were synthesized, the sizes of the MNCs and the MNPs decreased with the increase of x in the range from 0 to 0.5, the size of the MNCs increased and the size of the MNPs kept about a constant with x increase from 0.5 to 1.0. The magnetic properties of the sample characterized by VSM showed that the saturation magnetization of the samples increased with x increase from 0 to 0.2 and reached a maximum 108 emu/g at x=0.2, then decreased with x increase from 0.2 to 1.0, which can be explained by the occupation of the Zn2+ ion in the sample. |
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