【作者】 张嵩波；

【导师】 温戈辉；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2014， 博士

【摘要】 铁钴镍系过渡金属氧化物纳米材料由于具有一系列良好的磁学、光学、电学等性能，被广泛的应用在磁存储器件、核磁共振成像、催化剂、敏感器件、吸波材料、光电器件等领域。纳米材料的物理和化学性质很大程度上与其形貌、尺寸和结构有着密切的关系。因此，发展能够有效控制合成具有特定尺寸和形貌、结构良好、性能优越的铁钴镍系金属氧化物纳米结构的新方法具有重要意义。由于单一组分的纳米粒子表面极不稳定，容易产生团聚，且暴露在外部环境中容易被氧化或腐蚀，导致其性能大大降低。为了有效的阻止纳米金属氧化物颗粒的团聚，避免其被氧化或腐蚀，找到一种步骤简单、合成周期短、成本低廉、无污染的合成方法制备出具有核壳结构的铁钴镍系金属氧化物纳米复合材料，对进一步的研究有着重要的理论和实际应用价值。近年来，人们在非磁性和反铁磁性物质中发现了室温铁磁性后，随着化学合成方法的发展和测试手段的进步，有关NiO和CoO纳米颗粒室温铁磁性的研究引起了许多科研工作者的关注。然而，目前的工作中大多是有关NiO及CoO纳米颗粒的研究，很少有对具有各向异性形状的NiO及CoO纳米材料的室温铁磁性的报道。本论文以FexOy、NiO和CoO为研究对象，在FexOy核壳结构的合成新方法，NiO和CoO各向异性结构的合成及形成机制和所获得的铁钴镍系新型纳米结构的磁学性能等方面进行了研究。主要的工作如下：1.以Fe(NO3)3·9H2O为铁源，PVA为软模板，首次在不需要添加任何催化剂和表面活性剂的条件下，采用简单的水热法制备了FeOOH@PVA纳米棒，研究了PVA、反应温度及反应时间对样品结构及形貌的影响。分析表明，在FeOOH@PVA纳米棒的形成过程中，PVA起到阻止FeOOH向α-Fe2O3转变和生长导向的作用。反应温度对样品的结构及形貌有重要影响，随着反应温度的升高，样品逐渐由棒状变为球状。反应时间对样品的形貌也有一定影响，随着反应时间增加样品由棒状变为无规则形貌。因此，温度为160℃，反应12h为合成FeOOH@PVA纳米棒的最佳反应条件。采用水热法制备了Fe3O4@PVA纳米颗粒，研究了反应温度、不同铁源对产物结构和形貌的影响。分析了不同形貌样品的生长机理，并研究了样品的室温磁性。结果表明，以FeCl2为Fe2+源，反应温度为160℃时，样品为非晶态；随着反应温度升高，样品向八面体形状转化，温度升高至200℃时，样品完全转化为八面体结构Fe3O4@PVA纳米颗粒。实验中其它条件不变时，使用FeSO4代替FeCl2，得到的产物为立方体形状Fe3O4@PVA纳米颗粒。使用振动样品磁强计（VSM）测量了八面体结构和立方体结构Fe3O4@PVA样品的室温磁学性质，研究了样品形貌对磁性的影响。2.首次采用高温高压方法，以FeOOH@PVA纳米棒为前驱体，合成了γ-Fe2O3@C纳米棒。研究了HPHT过程中，合成温度对样品的物相和形貌的影响，得到了最佳制备条件。实验结果表明：反应温度为400℃，压强为1Gpa条件下制备的γ-Fe2O3@C纳米棒具有较高的长径比（直径约为20nm，长度约为150nm），室温矫顽力可达到330Oe。反应温度过高，会导致γ-Fe2O3@C纳米棒的核壳结构被破坏。HPHT方法与传统合成方法相比，反应环境密闭，不易引入杂质，反应时间短，该方法为制备具有核壳结构的一维纳米材料提供了新思路。3.以FeOOH@PVA纳米棒为前驱物，在H2环境中，400℃下热处理1h得到了具有较高矫顽力的Fe3O4@C纳米粒子，研究了反应温度及反应气体环境不同对样品的物相及形貌的影响。随着反应温度的升高，产物结晶度提高，但反应温度过高时，Fe3O4@C纳米颗粒的内核与壳层之间形成狭小缝隙。在室温下，Fe3O4@C纳米粒子的饱和磁化强度约为38emu g-1，矫顽力约为521Oe，大矫顽力的来源可能是由于核壳结构所引起的粒子表面磁各向异性导致。4.以硝酸镍、PVA水溶液为反应前驱物，通过水热法在高温高压条件下一步合成了具有花状结构的NiO样品。研究了反应温度和反应时间对其物相和形貌的影响，发现合成花状NiO结构的最佳条件为在300℃条件下，反应24h。利用振动样品磁强计对样品进行了磁学性质研究，发现其具有铁磁性质，来源可能是由于样品表面具有大量未抵消的净自旋磁矩，从而表现出铁磁性质。5.以PVA水溶液为反应介质，硝酸钴为原料，通过高压水热法一步合成了具有一维结构的CoO纳米线。通过研究反应温度和反应时间对其物相和形貌的影响，发现反应温度升高会导致CoO由纳米线生长为片状；相同反应温度下，随反应时间延长，氧化钴由最初的颗粒状生长为棒状，再由棒状组装成纳米线状。合成CoO纳米线的最佳条件为300℃条件下，反应24h。通过研究样品的磁学性能发现其具有铁磁性质，样品表现出较大的矫顽力（1100Oe），这可能是由于CoO纳米线的形状各向异性所导致。更多还原

【Abstract】 Due to the wide rang of magnetic, optical and electronic properties, Iron, cobaltand nickel transition metal oxides have been widely used in the field of high densitymagnetic storage, magnetic resonance imaging, catalysis, sensors, electronic andoptical devices. Since the morphology, structure, and size of nanostructures are vitalparameters for their properties. It is of great significance to develop methods toeffectively synthesize iron, cobalt and nikel metal oxide nanostructures with specificsize, morphology and superior performance.However, there are major drawbacks in the application of iron, cobalt and nickelmetal oxide nanostructures, which originate from the magnetically-inducedaggregation, surface oxidation, and instability under physiological conditions andacidic environments. In order to solve these problems, it has important theoretical andpractical value for further research to design simple, economic, green and effectivemethods of preparation of core-shell nanostructures.With the development of chemical synthesis methods and the progress of testmeans, antiferromagnetic oxides have attracted much attention. Since researchersfound that small particles of an antiferromagnetic material might exhibit magneticproperties such as superparamagnetism or weak ferromagnetism. In the present work,however, most studies are about NiO and CoO nanoparticles, and few papers reportthe room-temperature ferromagnetism about the NiO and CoO nanomaterials withother anisotropic morphology.In this paper, FexOy、NiO and CoO nanomaterial have been selected as the research objects. New synthetic strategies to synthesize FexOycore-shellnanostructures have been expored. The synthesis of NiO and CoO anisotropicnanostructures as well as their formation mechanism and the magnetic properties ofthe as-obtained nanostructures have been studied. The main contents are as follows:1. FeOOH@PVA nanorods were fabricated using Fe(NO3)3·9H2O as iron sourceand PVA as soft-template without any hydrolysis-controlling agent and surfactant. Theeffects of PVA, reaction temperature, reaction time on the products are studied. Theresults show that PVA play an important role in formation of FeOOH nanorods. Theeffects of recation temperature and time on the structures and morphology of thesamples are important. With the increase of reaction temperature, the nanorods aretransformed to sphere-like nanoparticles. With the extension of reaction time, theshapes of samples are converted form rod to granular. The best conditions for thepreparation of FeOOH@PVA nanorods are that the reaction temperature is160℃andthe reaction time is12h.Fe3O4@PVA nanoparticles were prepared by a simple hydrothermal method.The effects of reaction temperature and different iron source on the nanostructureswere systematically studied. The growth mechanism and the room temperaturemagnetic properties of samples with different morphologies were also discussed. Theresults show that the sample is amorphous with FeCl2as Fe2+source when thereaction temperature is160℃. With the increasing of reaction temperature, the smallnanoctystal has been converted to octahedral structure. When the reaction temperatureis200℃, the transformation finished completely. When other experiment conditionsare constant, the shape of obtained products is cube with the FeSO4as Fe2+source.The reason is that different anion in the solution lead to the different growthorientation of crystal nucleus. VSM was used to measure the magnetic properties ofmaterials as a function of magnetic field at room temperature. The effects ofmorphology on the magnetic properties are mainly investigated.2. γ-Fe2O3@C core-shell nanorods with average diameter of20nm and length of150nm are synthesized by transforming FeOOH@PVA nanorods under the conditionof high pressure and high temperature (HPHT). The best synthesis condition for transforming FeOOH@PVA core-shell nanorods into γ-Fe2O3@C nanorods is400℃under1GPa. Owing to high aspect ratios, the γ-Fe2O3@C nanorods present a highcoercivity of330Oe. Compared with the traditional synthesis method, HPHT reactionprocess has a unique advantage in the preparation process which has a short reactiontime and is not easy to introduce impurities. The HTHP method can provide a newway for preparing of one-dimensional core-shell nanostructures.3. Fe3O4@C nanoparticles with high coercive force were obtained usingFeOOH@PVA nanorods as precursors under400℃in H2environment. The effectsof reaction temperature and gas environment on the structures and morphology werestudied. The crystallinity of products increases as the reaction temperature increases.When the reaction temperature is too high, there is a narrow gap between the core andshell. The magnetic property of samples were studied, it proves that these samples areferromagnetic at room temperature. The coercive force and saturation magnetizationof Fe3O4@C nanoparticles are about521Oe and38emu g-1, respectively, the highvalue of coercive force may be caused by surface magnetic anisotropy of core-shellstructure particles.4. Flower-like NiO was obtained by a one step hydrothermal process withNi(NO3)2and PVA aqueous solution as precursor under high temperature highpressure. The morphology and microstructure of as-synthesized NiO werecharacterized. The results demonstrated that the best synthesis condition is that thereaction temperature and time are300℃and24h, respectively. The flower-like NiOnanostructures exhibit ferromagnetic at room temperature, which may be due toexchange coupling of the uncompensated spins on NiO surface.5. CoO nanowires were prepared by a one step hydrothermal process withCo(NO3)2and PVA aqueous solution as precursor under high temperature highpressure. Magnetic measurements reveal the presence of ferromagnetic at roomtemperature in the CoO nanowires. The morphology of samples are relate to thereaction temperature and reaction time. As increasing the reaction temperature, CoOnanowires are transformed into nanoflakes. When the hydrothermal time is prolonged,CoO nanoparticles are transformed into nanowires finally. The CoO nanowire exhibits a high coercive force（1100Oe）, the reason for this may be due to the shapeanisotropy of CoO nanowires.更多还原