Solution-based Synthesis And Properties Investigation Of Iron Compound

This dissertation is focused on the solution-based synthesis of Fe-based nanomaterials,including hollow spindles,hollow triangular pyramid structures,and nanocrystals.Investigations are based on several aspects including synthetic procedure,formation mechanism,properties,and applications.The contents comprise solvothermal synthesis of hollow spindle-like iron oxide nanoparticles via a template-free method,synthesis of triangular pyramid shells via a solution-based anisotropic template route,facile synthesis of plate-like magnetic iron oxide nanocrystals.The formation mechanism and magnetic properties of all the products were investigated in detail.This work is not only enriching the Fe-compound investigations,but also beneficial to the fundamental research of the formation and growth of nanomaterials.1 Fabrication,characterization,formation mechanism,and magnetic property investigation of hollow spindle-like iron oxide nanoparticlesUniform hollow spindle-likeα-Fe₂O₃ nanoparticles were synthesized by solvothermal treatment of the precursor-a kind of Fe(OH)₃-SDBS composite-in dimethylbenzene at 200℃.These hollow spindles were 220-300 nm in length, 70-100 nm in diameter,and 18 nm in thickness.Crystal structure characterizations indicated they were formed by oriented attachment of nanocrystals about 30 nm along the[001]direction.The IR,XPS,and TGA results indicated that there was about 3.5 %SO₄^2- groups in the product,bidentate coordinating to the surface Fe³⁺ cations of the spindles.By tracking the formation procedure of the hollow spindles,it is found they were transformed from the pre-formed solid spindles by selective dissolution of the interiors.When the interiors of solid spindles were preferred dissolved to form cavities,the exteriors maintained and went on growing.A small amount of H₂O and SDBS molecules in the reaction system were found crucial in the system.The formation mechanism was proposed as follows.After the solid spindles were generated,water derived from the precursor and the by-product of the transformation from Fe(OH)₃ to Fe₂O₃ were mixed with dimethylbenzene to form a microemulsion-like system,in which DBS surfactants concentrated in the oil/water interface to stabilize the system.As the reaction proceeded,SO₄^2- anions were generated from the decomposition of DBS and coordinated to the surface Fe³⁺ atoms of nanopartieles.The exteriors of spindles were protected by SO₄^2- and DBS,so the dissolution preferred occurred in the interiors.The solvated Fe³⁺ cations diffused to the exterior and reerystallized to form new particles on the spindles' surface.Finally hollow spindles formed.Hollow spindle-like magnetite and maghemite were obtained by reducing hollow spindle-likeα-Fe₂O₃ under H₂ and then re-oxidizing them in air. Their magnetic properties investigation demonstrated high coercive which might related to their quasi 1-dimensional(1D) morphology.2 Synthesis of triangular pyramid shelled Fe-polymer and magnetic porous carbon and their property investigationTriangular pyramid shelled Fe-polymer was synthesized via a solution-based anisotropic triangular pyramid template route.Triangular pyramid shells were 350-600 nm in side-edge length and 55-90 nm in shell thickness,with 70.6 m²/g of the BET surface area and 3.9 nm mean size of mesopores in the shells.The FT-IR spectrum indicated that ascorbic acid was oxidized to form furfural molecules,the main oxidation-decomposition product of ascorbic acid,which further polymerized to form triangular pyramid shelled structures with Fe³⁺ ions incorporated in this polymer. Element analyses demonstrated the product was consisted of C(41.95%),Fe (10.90%),O(42.30%),H(3.85%) and S(1.00%).Magnetic porous carbon was obtained by calcining the pyramid polymer shells under N₂ atmosphere.The components of the carbonized product were mainlyα-Fe and amorphous carbon,with the contents of 41.7%and 58.3%respectively.TEM and HR-TEM images further indicated that they are constructed byα-Fe nanoparticles of ca.5.0 nm embedded in amorphous carbon matrix.N₂ adsorption-desorption isotherms indicated the BET surface area increased to 399.1 m²/g,and the present of mesopores with the mean size of ca.3.8 nm and micropores with the average size of 0.6 nm.The saturation magnetization value of magnetic porous carbon was ca.39.3 emu/g.The dye adsorption experiments showed they have higher adsorption rate of cationic dyes than that of anionic dyes.The final adsorption capabilities were 72.56 mg/g for RB,69.79 mg/g for MO and 81.43 mg/g for DB-78.After releasing these dyes in ethanol, repeating adsorption test showed>90%adsorption capabilities were maintained and a slightly decreases of the saturation magnetization value to 34.4 emu/g was observed after 11 times of adsorption-desorption cycles.The triangular pyramid templates with the edge lengths of 400-600 nm were formed by oriented attachment of nanoplates. They gradually shrunk to form amorphous structures under the irradiation of electron beam,and could be dissolved in water.The corresponding FT-IR spectrum showed characteristic vibration adsorptions of SO₄^2- anions.Based on the elemental analysis result,it is speculated they were Fe-Na hydroxysulfate.The whole formation procedure was:first,the anhydrous FeCl₃ reacted with SDBS to form NaCl bulk crystals and ethanol solution of DBS salts of Fe³⁺ and Na⁺ at 90℃;second,the Fe³⁺ partially hydrolyzed to form DBS salts of nanoparticles,the DBS groups in these nanoparticles decomposed to form the Fe-Na hydroxysulfate nanoplates,which further oriented-aggregated to form the triangular pyramids;then,the ascorbic acid molecules enriched on the template's side surfaces and decompose-polymerized to form shell structured Fe-polymer due to the different crystallographic planes of the base and side surface of the template.3 Facile synthesis,property investigation and self-assembly of plate-like magnetic iron oxide nanocrystalsPlate-likeγ-Fe₂O₃ nanocrystals coated by hydrophilic PVP were synthesized in a simple system.They were 30-40 nm in side length and 10-13 nm in thickness,with the(111) planes as the basal planes.After theseγ-Fe₂O₃ nanocrystals were reduced by the hydrazine hydrate,Fe₃O₄ nanoplates with the same size and morphology maitained were obtained.Althoughγ-Fe₂O₃ and Fe₃O₄ have similar crystal structures, they could be distinguished by XPS and TGA methods.Based on the IR and XPS results,the PVP molecules coordinated withγ-Fe₂O₃ via its C=O groups rather than the N atoms,γ-Fe₂O₃ nanoplates could be well dispersed in various dipolar solvents except water.When they dispersed in water,part of the PVP molecules absorbed on the surface were released,resulting the aggregation of nanoplates.Strong dipolar interactions between nanoplates,which derived from their large size and anisotropic shape,lead them to self-assemble into 1D chain-like structure on substrate.The formation mechanism investigation revealed that the plate-like nanocrystals were formed by a kinetic shape control process,and PVP has two important effects:1) selectively coordinated with the(111) facets ofγ-Fe₂O₃ nanocrystals to reduce the growth rate along the[111]direction,and 2) increase the viscosity of the system to tune the growth rate ofγ-Fe₂O₃.γ-Fe₂O₃ nanoplates were obtained when the nucleation and growth rates were on a proper proportion.Magnetic investigation indicated these nanoplates were ferromagnetic at room temperature,with strong interparticle interaction.Weak applied magnetic field could induce theseγ-Fe₂O₃ nanoplates self-assemble into micrometer-scale 1D bundles.