Synthesis And Characterization Of Hierarchically Nanostructured Iron Chalcogenides

As an important class of inorganic functional materials, Iron selenides and sulfides have unique optical, electronic, magnetic and transport properties. However, reports on synthesis of iron selenides and sulfides such as FeSe, FeS₂ and FeSe₂ are very rare in comparison with that for other metal chalcogenides. In additional, synthesis of three-dimensional hierarchically nanostructured materials has become a hot spot in the field of research on nanomaterials. These materials with complex structures usually exhibit special properties correlated with their structures and morphologies. The most promising and now extensively used method for achieving novel structures is the self-assembly process. Although the major advances have been made in"building"the novel structures, fully understanding and exploiting the self-assembly process to develop facile and simple methods to synthesize various novel structures are still highly desired. In this thesis, hierarchically nanostructured FeSe, FeS₂ and FeSe₂ with defined morphologies were selectively produced employing polyol as the solvent. Moreover, we have investigated the morphology, structure, and formation process of the products. Several important results were summarized as follows:(1) Microwave-assisted synthesis of flower-likeβ-FeSe microstructuresFlower-like tetragonal iron selenide (β-FeSe) microstructures assembled by nanoplates have been successfully prepared using Se powder, FeCl3·6H2O and NaOH as reactant in 1,2-propylene glycol (1,2-PG) solvent system by microwave irradiation for 1h. The product was characterized by means of X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDX). In addition, ethylene glycol (EG) was also used as the reaction medium to investigate the influence of the different polyols on the size and structure of the product. The reaction mechanism and assembly process of the flower-likeβ-FeSe microstructures have been investigated and discussed, and a novel Se disproportionation-based reaction mechanism was proposed. This microwave-assisted polyol approach can be potentially extended to synthesis of other iron chalcogenides, such as FeS, FeTe, (FeSSe) and Fe(SeTe).(2) Microwave-assisted synthesis of monodisperse pyrite FeS₂ microspheres in ethylene glycol Uniform and monodisperse pyrite FeS₂ microspheres were synthesized by microwave-assisted method in ethylene glycol for the first time. FeSO4·7H2O and S powder were used as elemental precursors. The reaction process for the synthesis involved the reduction of sulfur and the reaction between intermediate sulphur species and Fe2+. Polyvinyl pyrrolidone (PVP) was added to the system which played an important role in control the phase and morphology of the product. The sample was characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and selected area electron diffraction (SAED) technique, revealing that formation of pyrite FeS₂ microspheres was via an aggregation-based mechanism. The time dependent experiments also demonstrated that primary FeS₂ nanocrystals were formed at the early stage and then aggregated into large spheres. The influence of microwave powder on the size and morphology of the products was investigated. Moreover it was found that the microwave heating method was definitely helpful in regard to fast synthesis of the uniform and monodisperse pyrite microspheres in comparison with the traditional oil bath heating.(3) Solvothermal method synthesis of FeSe₂ flower-like nanostructuresIn this section, flower-like FeSe₂ nanostructures were successfully prepared by a simple one-step solvothermal method using ethylene glycol as the solvent and Se powder, NaOH and FeCl₂·4H₂O were used as reactants, which provides a novel method for the synthesis of the material. The products were characterized by means of XRD and SEM. It was found that the reaction temperature and quantity of NaOH added to the reaction system have an important effect on the phase of the final products.