【作者】 卢健；

【导师】 陈代荣；

【作者基本信息】 山东大学 ， 纳米材料化学， 2009， 博士

【摘要】 本论文主要采用液相法合成了中空纺锤形纳米结构、空心三角锥结构、纳米晶的铁基纳米材料,并分别从纳米材料的制备、形成机理以及性质表征和应用研究几个方面进行论述。内容涉及溶剂热条件下无模板法合成空心纺锤形氧化铁纳米颗粒,利用各向异性模板液相合成空心三角锥结构,利用简单体系合成板状磁性氧化铁纳米晶,对产物的形成机理和磁性质进行了细致的研究。论文不仅丰富了铁基纳米材料的研究,对纳米材料的合成与生长的基础研究也具有一定的现实意义。1空心纺锤形氧化铁纳米颗粒的制备、表征、形成机理与磁性质研究以Fe（OH）₃和十二烷基苯磺酸钠（SDBS）形成的复合物为前驱体,以二甲苯为溶剂,经过200℃下的溶剂热反应,制备了尺寸均匀的空心纺锤形α-Fe₂O₃纳米颗粒。空心纺锤形颗粒的长度为220-300 nm,直径为70-100 nm,壁厚约为18 nm。晶体结构表征表明空心纺锤形颗粒是由30 nm左右的纳米晶沿着[001]方向定向聚集而成。结合红外光谱（IR）、X射线光电子能谱（XPS）、热重分析（TGA）等分析手段,表明样品中含有大约3.5%的硫酸根,以双齿配位的形式与氧化铁表面的Fe原子发生配位。通过跟踪空心纺锤的形成过程,发现空心纺锤是由先形成的实心纺锤经过选择性的内部溶解转变而成。在转变过程中,纺锤的内部优先溶解形成空腔,外部得到保持并继续生长。通过几个对比实验,我们发现体系中的少量水与十二二烷基苯磺酸根（DBS）对纺锤内部空腔的形成起关键作用,并提出了空心纺锤的形成机理。当实心纺锤形成以后,前驱体中含有的水以及Fe（OH）₃转变成α-Fe₂O₃生成的水进入体系中,二甲苯和水形成一个类似于微乳液的体系,DBS富集在油/水界面起稳定作用。从DBS分解得到的硫酸根优先与Fe₂O₃的表面Fe原子配位,随着反应的进行,纺锤外层由于有DBS阴离子和SO₄^2-根的保护使得溶解过程主要在纺锤的内部发生。被溶解了的Fe³⁺阳离子扩散到纺锤的表面,重新生成小颗粒,使纺锤外层继续生长,最终形成了空心纺锤。利用H₂还原以及再氧化过程可以得到空心纺锤形Fe₃O₄和γ-Fe₂O₃。对产物进行了光学性质与磁性质的研究,发现产物具有较高的矫顽力,这可能与颗粒的准一维形貌有关。2空心三角锥形含Fe聚合物和磁性多孔碳的制备与研究利用具有各向异性的三角锥形颗粒作模板,液相合成了空心三角锥形的含Fe聚合物。空心三角锥颗粒边长大约为350-600 nm,壳层厚度约为55-90 nm,颗粒具有平均孔径为3.9 nm的介孔和70 m²/g的比表面积。红外结果表明产物是由抗坏血酸分子的主要降解物糠醛聚合而成。聚合物的组成元素为:C-41.95%;H-3.85%;Fe-10.9%;O-42.3%;S-1.00%（重量百分比）。将聚合物在N₂气氛下烧结,可以得到磁性多孔碳,它是由41.7%的α-Fe和58.3%的无定形碳组成。透射电镜（TEM）表明粒径约为5 nm的α-Fe纳米颗粒均匀地分散在无定形碳基体中。磁性多孔碳具有级次孔结构,比表面积为399.1 m²/g,其中包括孔径为3.8 nm的介孔和0.6 nm的微孔。磁性多孔碳的饱和磁化强度为39.3 emu/g,对水溶液中染料的吸附研究表明它对阳离子型染料的吸附速度要高于阴离子染料,对罗丹明B、甲基橙、直接耐晒蓝的饱和吸附量分别是72.56 mg/g、69.79 mg/g、81.43 mg/g。对罗丹明B吸附-脱附循环11次后,磁性多孔碳的重复吸附量仍可以达到饱和吸附量的90%,饱和磁化强度降低到34.4 emu/g。三角锥模板的边长在400-600 nm之间,结构表征表明它是由更小的纳米片定向聚集而成。模板颗粒由于结构不稳定,在透射电镜电子束连续的照射下,会逐渐收缩并最终消失。它的IR光谱显示出明显的硫酸根振动吸收峰。结合元素分析,推断出模板是一种Fe和Na的羟基硫酸盐。整个反应过程及反应机理为,首先SDBS和FeCl₃形成NaCl沉淀和DBS的乙醇溶液;然后Fe³⁺部分水解得到十二烷基苯磺酸盐的纳米颗粒,随着DBS分解产生硫酸根,形成的Fe-Na羟基硫酸盐的纳米片,定向聚集成三角锥形貌;接着,由于三角锥模板具有不同晶面特性,使得抗坏血酸只在模板的侧面降解聚合得到空心三角锥形纳米结构。3板状磁性氧化铁纳米晶的合成、性质研究及其自组装利用简单的液相体系,合成了表面包覆亲水性聚乙烯吡咯烷酮（PVP）的γ-Fe₂O₃板状纳米晶。颗粒边长为30～40 nm,厚度为10～13 nm,其上下底面为（111）晶面。用水合肼还原得到同样尺寸与形貌的Fe₃O₄板状纳米晶。IR和XPS结果表明,PVP通过分子上的C=O而不是N原子与颗粒表面的Fe原子配位。γ-Fe₂O₃可以稳定地分散在除水以外的大多数极性溶剂中。颗粒分散在水中后,由于表面部分的PVP分子解离而发生聚集。颗粒之间具有强偶极作用,使得纳米板在基底上自发形成一维链状结构。这种作用是由于颗粒具有较大的尺寸和各向异性的形貌。对γ-Fe₂O₃的形成机理研究发现,板状形貌的形成是动力学生长控制的结果,PVP在体系中主要起两种作用:1)选择性的与γ-Fe₂O₃的（111）面配位,降低了[111]方向的生长速度;2)调节体系粘度以及γ-Fe₂O₃生长速率。只有γ-Fe₂O₃的成核与生长速率合适时,才能形成γ-Fe₂O₃板状纳米晶。磁性质研究表明γ-Fe₂O₃和Fe₃O₄在室温下具有明显的铁磁性,以及较强的颗粒间偶极作用。γ-Fe₂O₃板状纳米晶在弱磁场的诱导下,可以自组装成微米级别的一维束状结构。更多还原

【Abstract】 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.更多还原