【作者】 匡勤；

【导师】 郑兰荪； 谢兆雄；

【作者基本信息】 厦门大学 ， 无机化学， 2008， 博士

【摘要】 众所周知,许多金属氧化物是具有特殊物理化学性能的功能材料,在催化剂、微电子器件、能量储存与转化等诸多领域有着广泛的应用。氧化锌（ZnO）和氧化锡（SnO₂）以其优异的光电性质以及高灵敏的气敏性质一直被认为是金属氧化物中最有应用前景的两种功能材料,它们都是典型的n型宽禁带直接半导体,其室温禁带宽度分别为3.37 eV和3.65 eV。最近几年,科研人员投入了相当大的精力对各种类型ZnO和SnO₂纳米材料的合成、表征以及它们在纳米器件领域中的应用进行了深入研究。鉴于它们在未来功能器件中的重要性,ZnO和SnO₂这两种金属氧化物被确定为本人博士论文期间的研究内容。本博士论文将主要围绕三大研究方向进行:（1）单一ZnO和SnO₂纳米材料的合成和表征;（2）ZnO和SnO₂单根纳米线器件的研制和应用;（2）核壳型ZnO/SnO₂复合材料及其相应SnO₂空心材料的合成与表征。研究工作共分为十章,研究结果可以概括如下:第一章:简要地对ZnO和SnO₂这两种金属氧化物的基本特性以及在纳米材料领域的相关研究进行综述,阐明我们的选题依据以及研究内容。第二章:以锡烷（SnH₄）为气态前驱体,运用化学气相沉积法在喷金的硅片基底上沉积得到了高产率的SnO₂纳米线。电镜测试结果表明这些SnO₂纳米线的生长符合气-固-液（VLS）机理,其阴极荧光测试结果表明纳米线的发光性质与晶体内部中的氧缺陷浓度密切相关。此外,通过一种自催化的VLS机制,进一步合成得到了由SnO₂纳米棒构成的花状聚集体。第三章:运用不同的合成方法获得了三种典型ZnO纳米结构（四角叉、纳米棒和纳米线）。基于各种电镜分析结果并结合ZnO晶体结构的自身特点,我们对这些ZnO纳米结构的生长机理进行了深入的探讨,并利用光致发光和阴极荧光两种表征手段对它们的光学性质进行了细致的研究。第四章:以单根ZnO纳米线和SnO₂纳米线为结构基元,利用光刻、聚焦离子束沉积等微加工技术在硅片基底上构筑了简单的纳米线器件。基本电学测试结果表明金属氧化物半导体纳米线与金属电极的接触情况可以通过用聚焦离子束沉积金属Pt的方式显著改善,而与它们的功函数大小无关。热敏测试结果表明,SnO₂纳米线属于负温度系数热敏材料,但是其热敏常数B仅为433 K,不适合作为热敏电阻。光电导测试结果表明,对于365 nm的紫外光,ZnO纳米线的响应灵敏度远远大于SnO₂纳米线。这种响应灵敏度上的差别来源于两种纳米线的光电导效应产生机制的不同。场效应晶体管测试结果则提供了跨导、电子迁移率等SnO₂纳米线器件的基本性能参数。纳米线器件的成功构筑为我们进一步研究纳米线在纳米传感器领域的应用打下了良好的基础。第五章:以ZnO和SnO₂纳米线为例,探索了单根金属氧化物半导体纳米线构筑而成的纳米器件在湿度传感和气体传感两个领域的应用。湿敏传感测试结果表明,SnO₂纳米线器件对被测环境的相对湿度存在线性的响应,空气中水分子和O₂分子在半导体表面的竞争物理吸附是可能的响应机制。同时,利用两根独立的ZnO纳米线和SnO₂纳米线在同一硅片基底上成功构建了一个简单的二元气敏传感器阵列,并初步探讨了此阵列传感器在混合气体检测领域中的应用。此外,我们通过在金属氧化物纳米线表面修饰金属或氧化物颗粒大大改善了纳米线气敏传感器的灵敏度和选择性。这些研究结果将推动单根金属氧化物半导体纳米线湿敏/气敏传感器向实用化方向发展。第六章:以核壳型ZnO/SnO₂四角叉为例,在两种存在较大结构差异的晶体之间实现了三维外延生长,并且发现了外延界面对金属氧化物半导体发光性质的调制作用。电镜表征结果证实了合成得到的产物是以ZnO四角叉为内核,厚度为15～30 nm的SnO₂外延层为外壳的核壳型复合结构,它们之间的外延关系为:（010）SnO2‖（01（?）0）ZnO和[100]SnO2‖[0001[ZnO。光致发光和阴极荧光分析结果表明在ZnO/SnO₂外延界面的诱导下,四角叉的光学性质发生重大改变,在450～600nmn范围内出现了强烈的绿光发射。这项研究工作为今后类似的异质外延结构的合成以及光学性质的调制开辟了一条新的道路。第七章:以ZnO六棱柱和六棱锥为生长模板,系统考察了SnO₂在ZnO晶体的三组典型晶面（{01（?）0}、{10（?）1}、{0001}）上的外延生长。通过控制合适的实验条件,SnO₂在ZnO晶体表面可以形成纳米颗粒、定向生长的纳米棒/线和连续单晶薄膜三种纳米结构形态。晶体结构分析表明ZnO和SnO₂之间的晶格失配在SnO₂外延生长中起着重要作用:SnO₂总是沿着晶格失配度最小的方向（即[2（?）0]ZnO‖[001]SnO₂）优先生长,而其径向上的生长则受到失配应力的束缚。阴极荧光分析结果表明,与核壳型ZnO/SnO₂四角叉的发光性质一样,复合之后的ZnO/SnO₂六棱锥也表现出了不同于单一ZnO和SnO₂的强烈的绿光发射,为此我们提出了外延界面缺陷调制半导体发光的机制。对ZnO/SnO₂这一特殊外延体系深入而彻底的认识,有可能为其它类似的金属氧化物半导体复合纳米材料的合成和应用提供理论上的指导。第八章:以预先制备好的ZnO纳米棒阵列为牺牲模板,经过SnO₂包覆和盐酸刻蚀两步合成得到了一系列SnO₂空心纳米管阵列。通过选择合适尺寸的初始模板并控制合适的实验条件,我们可以得到四种类型的氧化锡纳米管阵列:纳米颗粒组成的纳米管、纳米棒组成的纳米管、纳米环组成的纳米管以及单晶薄膜组成的纳米管。SnO₂与ZnO之间的外延生长在纳米管管壁的形成过程中,扮演了非常重要的角色。我们相信这些SnO₂纳米管将会在高灵敏气敏传感器、光催化剂等领域有着良好的应用前景。第九章:以SnH₄为气态前驱体,运用化学气相沉积法成功地实现了SnO₂纳米颗粒在多壁碳纳米管（MWCNTs）表面的负载,SnO₂纳米颗粒的尺寸和覆盖度可以通过精确控制SnH₄前驱体的供应（气体流速、沉积时间）加以调控。因此,我们的研究为制备MWCNTs-SnO₂复合纳米材料提供了一条好的合成路径。此外,以MWCNTs为牺牲模板,在较高的沉积温度（730℃）合成得到由纳米颗粒连接而成的一维链状SnO₂纳米线。第十章:以四氯化碳为原料、以金属钾为还原剂,运用溶剂热的方法在相对较低的温度下（60～100℃）合成得到了厚度仅为3～8 nm的碳纳米薄膜。X射线粉末衍射和拉曼光谱表征结果表明这些膜状产物是石墨化的碳;BET测试结果表明碳纳米薄膜具有较大的比表面(97.2 m².g^-1)。此外,以六氯苯为原料、以金属钠为还原剂,运用相似的方法成功地合成得到纳米球、纳米管等各种碳空心结构。这些碳空心结构将在催化剂载体、药物传输等许多领域具有潜在的应用。更多还原

【Abstract】 As is well known,many metal oxides are functional materials with excellent physical and chemical properties,and widely applied in many fields including catalysts,microelectronic devices,energy storage and conversion.Among these metal oxides,ZnO and SnO₂ have been considered as the most promising functional materials due to highly sensitive gas sensing property and excellent photoelectrical property.Both of them are n-type wide direct band-gap semiconductors（Eg = 3.37 eV for ZnO and Eg = 3.65 eV for SnO₂,respectively）.Recently,much effort has been devoted into the fabrication and their potential application in nanodevices based on various nanostructures of ZnO or SnO₂.Considering their potential importance in future functional materials,ZnO and SnO₂ are selected as research targets of my thesis.In this thesis,our research focus on three fields including:（1） fabrication and characterization of sole ZnO or SnO₂ nanostructures;（2） construction and application of single ZnO or SnO₂ nanowire-based nanodevices;（3） fabrication and characterization of core-shell ZnO/SnO₂ nanocomposites with epitaxial relation and related hollow SnO₂ nanostructures.Major results have been summarized as follows:Chapter 1.Briefly review the basic characteristic and recent development of ZnO and SnO₂ nanostructures and clarify my research significance and detailed plan.Chapter 2.By using tin alklane（SnH₄） as the gaseous precursor,high-yield SnO₂ nanowires are successfully synthesized on the Au-coated Si substrates by means of chemical vapor deposition.Electronic microscope characterization results demonstrate that the growth of these SnO₂ nanowires follows the vapor-liquid-solid （VLS） mechanism.In addition,a kind of flower-like SnO₂ nanorod assembly can be acquired via a self-catalytic VLS mechanism.Cathodoluminescence analysis indicates the luminescence properties of the SnO₂ nanowires are related with the concentration of oxygen vacancies in the nanocrystals.Chapter 3.Three typical morphologies of ZnO nanostructures including tetrapods, nanorods and nanowires are successfully fabricated by means of various synthetic strategies.Combining electronic microscope characterization results with intrinsic features of wurtzite-type crystal,possible growth mechanisms of these ZnO nanostructures are proposed and deep discussed.In addition,their optical properties are carefully investigated by means of photoluminescence and cathodoluminescence.Chapter 4.Using ZnO or SnO₂ nanowires synthesized in chapter 2 and 3 as building blocks,single metal oxide semiconducting nanowire-based devices are successfully fabricated on the silicon substrates via photolithography and focused ion beam deposition（FIB）.Measurement results indicate that the contact behavior between metal oxide nanowires and metal electrodes are markedly improved by FIB deposition.And SnO₂ nanowires belong to NTC thermal-sensitive material while they don’t suit thermal-sensitive resistor because its calculated thermal-sensitive constant B is only 433 K.In addition,it is found that the photoconductance of single ZnO nanowire is more sensitive to 365 nm UV light than that of single SnO2 nanowire due to the difference between their generating mechanisms.Finally,transconductance（g_m） and electron mobility（μ_e） of single SnO₂ nanowire are successfully acquired based on field-effect-transistor measurement.This research work laid a good foundation for further investigation of nanowire-based devices on the application in chemical nanosensors.Chapter 5.Single ZnO and SnO₂ nanowire-based devices are applied for the detection of relative humidity and toxic gas in the environment.Humidity-sensing results indicate that the resistance of a single SnO₂ nanowire has linear response to the relative humidity of atmospheres.Humidity-sensing properties of the nanowire should originate from competitive physical adsorption between water molecular and oxygen molecular on the surface of nanowires.At the same time,a simple gas sensor array constructed with a ZnO nanowire-based sensor and a SnO₂ nanowire-based sensor on the same substrate is successfully built up by double FIB process although this sensor array failed in the detection of CO and H₂S mixture gas.In addition,it is found that sensitivity and selectivity of single ZnO or SnO₂ nanowire-based gas sensor could be improved by surface functionalization.This research work will accelerate the practicability of metal oxide semiconductor nanodevices. Chapter 6.Using a ZnO/SnO₂ core-shell heterostructure as an example,we demonstrate the possibility of establishing a three-dimensional epitaxial interface between two materials with different crystal systems for the first time and show possible tailoring optical properties by building the heteroepitaxial crystal interface. The electron microscopy characterization results reveal that as-prepared ZnO/SnO₂ heterostructure has a tetrapod-like ZnO core and a SnO₂ shell with 15-30 nm,and their special epitaxial relation is（010）_SnO2|（01（?）0）_ZnO and[100]_SnO2|[0001]^ZnO. In addition,a strong green luminescence in the 450～600 nm is induced by epitaxial interface.This research work will break a new path to fabricate epitaxial hetero -structures and tailor the luminescence properties of metal oxide semiconductors.Chapter 7.Using ZnO hexagonal micro-prisms and micro-pyramids as two typical templates,epitaxial growth of SnO₂ on three typical ZnO crystal planes including {01（?）0},±（0001） and {10（?）1} is systematically investigated.Various nanostructures of the epitaxial SnO₂ from nanoparticles,to self-assembled nanowire arrays,and to continuous single-crystalline thick films,are controlled prepared on ZnO template under appropriate kinetics conditions.Structural analysis reveals that lattice-mismatch between two epitaxial planes plays a crucial role in the growth of self-assembled SnO₂ nanowire arrays on the ZnO surface.SnO₂ has a preferential growth direction along the minimum lattice-mismatch direction(i.e.[2（?）0]ZnO| [001]_SnO2),and the width of SnO₂ nanowires is strictly confined by the accumulated strain which is induced by large lattice-mismatch.Cathodoluminescence characterization indicates that as-prepared ZnO/SnO₂ composite nanostructures exhibit different optical properties from the original ZnO templates or SnO₂ hollow shells.A clear understanding of the role of lattice-mismatch-strain in ZnO / SnO₂ system will inspire great interest in exploring other epitaxial metal oxide heterostructures and their potential applications.Chapter 8.Using prefabricated ZnO nanorod arrays as sacrificial templates, various SnO₂ nanotube arrays are successfully synthesized via two-step process including SnO₂ coating and hydrochloric acid etching.Depending on the size of original ZnO templates and experimental parameters,these SnO₂ nanotubes are constructed with four possible types:nanoparticles,nanorods,nanorings and single-crystalline film.Epitaxial growth of SnO₂ on ZnO surface plays a key role on the formation of SnO₂ walls.These hollow SnO₂ nanostructures are believed to have potential application in high-sensitive gas sensors and photocatalysts.Chapter 9.A simple and efficient approach for coating multiwalled carbon nanotubes（MWCNTs） with size-controllable SnO₂ nanoparticles by chemical vapor deposition has been developed using SnH₄ as the source of SnO₂ at 550℃.The size and coverage of SnO₂ nanoparticles can be adjusted by simply controlling the deposition time and the flow rate of SnH₄/N₂ mixture gas during the CVD procedure. In addition,by using the MWCNTs as a sacrificial template,a kind of one-dimensional chain-like SnO₂ nanostructure has been synthesized by increasing the deposition temperature to 730℃.This technique may provide a good way to produce tunable SnO₂-MWCNT composites.Chapter 10.Crumpled carbon nanosheets with 3～8 nm thickness have been successfully synthesized via a catalyst-free solvothermal route at a very low temperature range（60℃～100℃）,using tetrachloromethane（CCl₄） as carbon source and potassium as the reductant.The X-ray powder diffraction pattern and Raman spectrum indicates that the products are hexagonal graphite.The result of BET experiment shows carbon nanosheets have a large surface area(97.2 m²·g^-1).In addition,various hollow carbon nanostructures were prepared by means of similar method where C₆Cl₆ acted as carbon source and metal sodium acted as reductant. Such hollow carbon nanostructures should have potential application in many fields including catalysts supporter and drug delivery.更多还原