几种纳米晶的制备及其磁学、光学与低温磁卡性质的研究

IMR OpenIR

	几种纳米晶的制备及其磁学、光学与低温磁卡性质的研究
	张强
学位类型	博士
导师	张志东
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	尺寸依赖性吸波光催化低温磁制冷局域等离子体共振 Size-dependence Microwave Absorption Photocatalysis Cryogenic Magnetic Refrigeration Localized Surface Plasmon Resonance
摘要	"近年来，纳米材料因其在光学、磁学、电学、电磁学、催化等方面巨大的潜在应用价值而成为研究的焦点。我们采用机械化学法、溶胶-凝胶法、水热法、化学共沉淀法等方法，分别制备了金属/半导体型多重微波共振吸波材料、具有优异可见光催化性能的半导体材料、具有低温磁卡效应的纳米胶囊、具有局域等离子体共振性能的球状银纳米颗粒、单分散的磁性纳米颗粒，利用X射线衍射、透射电镜、紫外-可见光谱、超导量子干涉仪等测试手段对上述材料的结构、形貌、光学、磁学、光催化、低温磁卡等性能进行了表征，发现了电磁波吸收以及光催化性能对纳米颗粒尺寸的依赖性、直流电弧放电法制备低温磁卡材料DyNi5的条件、小尺寸单分散纳米颗粒的制备条件等现象，探索了这些现象背后的规律。总结如下：（1）使用机械化学法制备了具有多重微波共振的金属/半导体型复合材料Fe/TiO2。研究了在2-18 GHz波段的介电共振以及多重磁共振现象，发现Fe晶粒的尺寸对金属/半导体型复合材料的介电共振具有决定性的作用。材料的极化机制是金属/半导体的界面极化。多重磁共振分别是自然共振与交换共振，后者可以用Aharoni的交换共振理论进行解释。（2）使用溶胶-凝胶法制备了尺寸在78 nm-14 nm之间的单晶Bi2Fe4O9纳米颗粒。样品的磁学性能展现出了对尺寸的依赖性，这可归因于反铁磁纳米颗粒表面未补偿的自旋，这种可调控的磁性不仅具有深刻的理论意义而且在自旋电子学方面有着广泛的潜在应用价值。随着尺寸的减小，Bi2Fe4O9半导体纳米颗粒的能隙值由1.91 eV增加到2.04 eV，这是可以用纳米颗粒的量子限制效应解释。在可见光辐照下Bi2Fe4O9纳米颗粒展现出了对亚甲基蓝优秀的降解性能。有意思的是，我们发现Bi2Fe4O9纳米颗粒的光催化活性对纳米颗粒尺寸也具有依赖性。这是因为随着纳米颗粒的减小，光激发的电子/空穴对在颗粒体内复合的几率也减小，不仅如此，随着比表面积的增加，纳米颗粒对光的吸收能力也会增加。（3）使用直流电弧放电法制备了不同成分的纳米胶囊。根据蒸发碰撞理论，详细讨论了纳米胶囊的形成机制。使用高分辨透射电镜分析了DyNi5@NiO的核壳结构。详细讨论了DyNi5@NiO与Ni@NiO纳米胶囊的包括冻结温度与铁磁到顺磁转变在内的磁学性质。在温度为12.5 K、外加磁场为7 T时，纳米胶囊的最大磁熵变为11.1 J kg-1· K-1。顺磁性DyNi5@NiO与铁磁性Ni@NiO纳米胶囊之间的相互作用对磁熵变的提升作用也进行了详细的讨论。（4）使用水热法合成了单分散、颗粒尺寸在4.1 nm-11.6 nm之间的银纳米颗粒。详细讨论了银纳米粒子的形成机制。十二胺一方面作为弱还原剂用来控制粒子的生长速率从而有利于控制粒子的粒径分散度，另外一方面由于其具有较长的碳链可以防止粒子团聚。使用透射电镜表征了银纳米粒子的形貌。有意思的是，我们观察到在非极性溶剂甲苯中尺寸较小的银纳米粒子排列成为密排六角形结构。银纳米粒子紫外可见光吸收谱的峰位在407.4 nm-409.4 nm之间，随晶粒尺寸增大峰位红移。（5）以聚乙烯醇（PVA）做络合剂使用溶胶-凝胶法制备了纯相的BiFeO3纳米颗粒。使用XRD与TEM分别表征了样品的物相与形貌。研究了金属离子/羟基比例对物相形成的影响，我们发现当金属离子/羟基比例为1：1时在600 ℃能够得到纯相。当金属离子/羟基比例为2：1时由于高分子的量较少不能形成足够的网络固定住金属离子而使化学计量比失衡所致；当金属离子/羟基比例为1：2时，多余的高分子碳化形成的碳化物也会导致化学计量比失衡从而导致杂相的出现。同时总结了不同络合剂对BiFeO3纯相的影响，发现羟基有利于而羰基不利于纯相BiFeO3的形成。（6）使用化学共沉淀法制备了单分散的CoFe2O4球状磁性纳米颗粒。我们发现表面活性剂聚乙二醇对单分散CoFe2O4球状磁性纳米颗粒的形成起到了重要的作用。CoFe2O4纳米颗粒的饱和磁化强度为60.4 emu/g，几乎不存在矫顽力。这种方法简单、廉价、效率高可以用来制备其它结构的纳米颗粒。"
其他摘要	"Recently, vigorous research efforts have been devoted to nanocrystals for their widely potential application in optics, magnetism, electronics, catalysis and so forth. In this thesis, various methods, including mechanochemical method, sol-gel approach, DC arc discharge method, hydrothermal synthesis and chemical co-precipitation, were employed to fabricate several nanocrystals: metal/semiconductor composites with multiple microwave resonances, semiconductors with highly photocatalytic activity under visible irradiance, nanocapsules with cryogenic magnetocaloric effect, silver nanospheres with localized surface Plasmon resonance, phase-pure ferroic nanocrystals, monodisperse magnetic nanospheres. Structures, morphologies, optical properties, magnetic properties, photocatalytic properties and cryogenic properties were characterized by X-Ray Diffraction(XRD), Transmission Electron Microscope(TEM), UV-Visble spectra and Superconducting Quantum Interference Device(SQUID), respectively. Size-dependent microwave absorbing, magnetic, photocatalytic properties were found. The principles behind these phenomena were also investigated, which include: (1) The dielectric resonance and multiple magnetic resonances which correspond to multiplemicrowave absorptions in the 2-18 GHz range have been studied in thecomposite Fe/TiO2. The Fegrain size is found to have great impact on the dielectric resonance in this metal-semiconductorcomposite. The polarization mechanism is attributed to interfacial polarization. The multiplemagnetic resonances can beascribed to the natural resonance and exchange resonances, which canbe explained by Aharoni’s exchange resonance theory. (2)Single-crystalline Bi2Fe4O9 nanocrystals (NCs) of various sizes (78 nm to 14 nm), synthesized using sol-gel method, exhibit size-dependent magnetic properties, which can be attributed to the uncompensated spins on the surface of the antiferromagnetic NCs.These tunable magnetic properties have a profound significance and can be widely applied in spintronics . As the crystal size decreases, the energy gap of the Bi2Fe4O9semiconductor NCs increases from 1.91 eV to 2.04 eV, which results from quantum confinement effect. The Bi2Fe4O9 NCs exhibit excellent photocatalytic oxidation of methylene blue (MB) under visible light irradiation with the assistance of a small amount of H2O2. Interestingly, smaller NCs exhibit higher photocatalytic activities. The size-dependence of photocatalytic properties of the Bi2Fe4O9 NCs were ascribed to the lower recombination rate of photogenerated electron/hole pair within smaller NCs and higher aspect ratios, which allow for stronger photon absorption on the surface of smaller crystals. (3)The three nanocapsules with different phase components were synthesized by the arc-discharge technique evaporating DyxNi100-x (x = 17, 30, 40) alloys. The formation mechanism of the nanocapsules was discussed in detail according to the evaporating and bumping principle. The core/shell structure of DyNi5@NiOnanocapsules were analyzed according to the observation of the high-resolution transmission electron microscope (HRTEM). The magnetic properties of mixture of DyNi5@NiO and Ni@NiOnanocapsules, including the blocking temperature and a phase transition from ferrimagnetic to paramagnetic state, were discussed. The maximum magnetic entropy change ΔSm (11.1 J kg-1· K-1) of the mixture was found at 12.5 K in an applied-field change of 7 T. The enhancement of the ΔSmdue to the interactions between paramagnetic DyNi5@NiO and ferromagnetic Ni@NiOnanocapsules was also discussed in detail. (4) Monodispersed silver nanocrystals with radius between 4.1 nm and 11.6 nm were synthesized via chemical co-precipitation method. The formation mechanism was discussed in detail. Dodecylamine played an important role in nanocrystal size controlling for its weak reductiveness, as well as in preventing nanocrystals agglomeration for its long alkyl chains. HRTEM was employed to characterize the morphologies of silver nanocrystals. Interestingly, silver nanocrystals dissolved in toluene were found to exhibit close-packed hexahedron. UV-visible absorbance spectra peaks of silver nanoprticles in toluene were in 407.4 nm-409.4 nm, which show red-shift with nanocrystal size increasing. (5) Phase-pure BiFeO3nanocrystals were synthesized by a sol-gelprocess using polyvinyl alcohol (PVA) as a complexing agent.Phase and morphologies of samples were characterized by XRD and TEM, respectively. The effect of the ratios of positivelycharged valences to hydroxyl groups of PVA (Mnn+/-OH) onthe formation of BiFeO3 was investigated.When the ratio was 1:1, phase-pure BiFeO3 was synthesized. But when the ratio was 2:1, PVA can’t provide enough network to hold the metal ions and thus the stoichiometry was imbalanced and as a result, impurities appear. When the ratio was 1:2, the PVA content is too high, and an excess ofcarbonaceous materials leads to the formation of impurity phasesduring the calcination procedure. (6) Monodispersed magnetic CoFe2O4 nanospheres were prepared unsing chemical co-precipitation.Polyethylene glycol(PEG), as a surfactant, was found to play an important role in the formation of dispersed CoFe2O4 nanospheres. The saturation magnetization of CoFe2O4 nanospheres was 60.4 emu/g and magnetic coercivity was negligible.This approach provides a onestep,simple, general, and inexpensive method for thepreparation of monodisperse magnetic microspheres. We believe thatthese microspheres will haveimportant applications not only in advanced magneticmaterialsand ferrofluid technology, but also in biomedical fieldssuch as biomolecular separations, targeted drug delivery,cancer diagnosis and treatment, as well as magnetic resonance imaging."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64478
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	张强. 几种纳米晶的制备及其磁学、光学与低温磁卡性质的研究[D]. 北京. 中国科学院金属研究所,2012.