Shape-Controlled Synthesis Of Iron Oxides And Prussian Blue Analogues And The Study On Nonclassical Crystallization Mechanism

To fabricate novel materials and enhance the properties of materials have drawn much attention during the past decades. Shape-controlled synthesis is considered to be an important avenue to acquire materials with excellent properties. In addition, by using uniform nanoparticles with different kinds of shapes, multiple functional composites may be fabricated. With the development of the shape-controlled synthesis of materials, significant developments in multiple fields such as materials science, materials chemistry, materials physics, crystallography and biomimetic syntheis have been achieved. However, there is still much work need to be done, such as developing a general wet-chemical synthetic method to control the morphology or composition of materials, fabricating ordered hierarchical materials, understanding the assembling manner and mechanism, studying non-classical crystallization and mechanism.In this thesis, we aim to develop a general shape-controlled synthesis method, to fabricate ordered hierarchical inorganic materials and to clarify the mechanism of classical and non-classical crystallization phenomena. We used hydrothermal method, solid-state thermal decomposition and centrifugation-assisted crystallization at the interface between oil and water phases to fabricate inorganic materials with different morphologies. The crystal structures, the compositions, the morphologies, the assembling manner of the hierarchical structures, the magnetic properties and the porosity of the samples were characterized by X-ray diffraction analysis (XRD), Mossbauer spectrum analysis, selected area electron diffraction (SAED), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectra (FT-IR), a vibrating sample magnetometer (VSM) and a surface area analyzer. The main results and conclusions are listed as follows:1. Iron oxides and Prussian blue analogues (PBAs) with different morphologies were successfully fabricated through the disassociation of K4[Fe(CN)6] by hydrothermal method.(1) When the concentration of NaOH was 0.1 mol/L, the products were sheetlike magnetite. The sheets were ultra large with a length of about several tens micrometers, and were formed by oriented aggregation of nanoparticles. When the concentration of NaOH was 1 mol/L, the products were pseudo-octahedron with a size of about tens micrometers. SAED illustrated these pseudo-octahedrons were single crystals. In addition, the pseudo-octahedral magnetite were formed via the dissolution of sheetlike magnetite and the subsequent recrystallization.(2) Prussian white crystals were successfully fabricated in neutral environment. Mossbauer spectrum indicated that the Prussian white were air-stable which had not been achieved by other groups before. The Prussian white particles were microcubes with a size of about 2-5μm. In addition, some of the microcubes were hollow with cubic holes. A layer-by-layer growth mechanism was proposed to explain the formation of microcubes, and the dissolution effect caused the formation of holes.(3) Prussian blue mesocrystals were successfully fabricated under acidic environment. When the concentration of HCl was higher than 1 mol/L, the products were octahedron with size of about 7μm in the long orientation axes and about 4μm in the edges. Moreover, the octahedrons were formed by aggregation of nanoparticles sized of about several hundreds nanometers. Both the SEM and SAED analysis indicated the octahedron crystals were ordered hierarchical structures. Owing to the etching effect of HCl, the concentration of Prussian blue nanoparticles was decreased to an appropriate value, and resulted in oriented aggregation of nanoparticles to form mesocrystals.(4) Prussian blue mesocrystals were successfully fabricated with the addition of HCl and polyethylene glycol (PEG). The products were pseudo truncated cubes, with a size of about 5-6μm. Such an exquisite structure was formed by the assembling of both nanoparticles and sub-microcubes.(5) Prussian blue microcrystals were successfully fabricated with the addition of HCl and CTAB. The products were elongated cubes sized about 2-3μm between two parallel faces and the square faces are about 1μm in length. CTAB played two roles in the fabrication of elongated cubes. First, CTAB could almost divided the nucleation and growth of PB by inhibiting the formation of PB crystal, leading to nearly uniform PB microcrystals; Second, CTAB could protect the PB cubes, which result in selective etching of PB cubes at the sides and corners. 2. Magnetite crystals were successfully fabricated through the disassociation of K3[Fe(CN)6], with the addition of hydrazine and surfactant by hydrothermal method. Through the addition of cetyltrimethylammonium bromide (CTAB), dendritic microcrystals were able to be fabricated. The selective adsorption of CTA+ on the surface of magnetite induced the oriented growth of magnetite, resulted in single crystalline dendritic crystals. When the additives were the mixture of CTAB and sodium citrate, the products were spheres which aligned in lines.3. A solid state method was developed to fabricate porous iron oxides. The Prussian blue elongated cubes were used as precursor to fabricated elongated cubic iron oxides through thermal decomposition. Such iron oxides were composed ofβ-Fe2O3 andγ-Fe2O3. During the thermal treatment process, C and N were oxidized into gases and escaped, resulting in mesoporous iron oxides.4. Prussian blue nanocrystals were generated at the interface between oil (mixed ethanol and n-hexane) and water with the assistance of centrifugation. By choosing different centrifugation rates, the crystallization process of Prussian blue was under control. When the centrifugation rate was high (5400 r/min), the products were amorphous nanoparticles with a size smaller than 5 nm; when the centrifugation rate was low (3600 r/min), the products were crystalline nanocubes with a size of about 10-30 nm. The products obtained at different centrifugation represented the different growth stage of Prussian blue.5. Two kinds of assembling manners were found to generate Prussian blue mesocrystals. Without the addition of surfactants, near-uniform nanocubes were aggregated together to form octahedral mesocrystals in the acidic environment. With the addition of PEG, Prussian blue nanoparticles and submicrocubes, as multiple sized building blocks, were aggregated together to form pseudo truncated cubic mesocrystals.6. The transition process of amorphous Prussian blue was investigated by centrifugation-assisted crystallizaiton. The amorphous precursors aggregated together, crystallized, self-regulated the crystallographic orientation of crystalline part, and finally generated Prussian blue crystal. The observed core-shell structure during the transition process could be the characteristic of such nonclassical transition of amorphous precursors.