【作者】 孙士斌；

【导师】 邹增大； 朱艳秋；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 博士

【摘要】 近年来,低维纳米材料作为一个新兴的材料家族,以其独特的物理、化学、电子学和力学等性能,引起人们的广泛关注。其中,低维纳米结构氧化钨是一种非常重要的功能材料,具有电致变色、气致变色、热致变色和光致变色等特性,在平板显示器、灵巧窗口、传感器和催化剂等领域有广泛的应用前景。一维纳米结构氧化钨还可以作为前驱体制备二硫化钨纳米管。作为无机类富勒烯家族中重要的一员,二硫化钨纳米管在纳米半导体电子器件、催化、固体润滑和高性能复合材料等领域都具有潜在的应用前景。本文在低维纳米结构氧化钨和二硫化钨的液相和气相合成方法、低维纳米材料的微观结构以及所获纳米材料的物性等方面进行有益的探索和系统的研究,同时详细分析了热处理条件下一维纳米结构氧化钨的形貌、微观结构、相组成和气敏特性的转变特征及转变机制。采用X射线衍射仪、扫描电镜、透射电镜、傅立叶变换红外光谱仪、差热分析仪和BET（Brunauer-Emmett-Teller）比表面积分析仪等对不同产物的形貌、相结构和性能进行了系统的分析。采用WCl₆为前驱体,以环已醇为溶剂,通过水热合成法成功制备出W₁₈O₄₉纳米线束。研究表明,随着前驱体WCl₆浓度的增加或反应时间的延长,纳米线束直径逐渐增大,最后出现块状的不规则颗粒。通过优化工艺参数,最后得到单根长度大于2μm、直径为2-15nm的超细W₁₈O₄₉纳米线。高分辨透射电镜图片、选区电子衍射和X射线衍射图谱分析均表明,纳米线束中存在堆垛层错、位错和氧原子空位等晶体结构缺陷。BET氮吸附脱附实验结果表明,W₁₈O₄₉纳米线具有高的比表面积和孔隙率,分别为151m²/g和0.51cm³/g。热处理实验结果表明,W₁₈O₄₉纳米线在热处理过程中形貌和相结构均发生了变化。随着热处理温度的升高,纳米线束变短变粗,单根纳米线也变短变粗。400℃和450℃热处理后,原始超细纳米线的比表面积分别降低至110m²/g和66m²/g,孔隙率也分别降至0.23cm³/g和0.13cm³/g;经过高温热处理（900-1000℃）后,一维纳米结构消失,出现大尺寸的不规则微米级颗粒。气敏性实验结果表明,W₁₈O₄₉纳米线在室温对乙醇气体有很好的灵敏度,当乙醇的浓度为2.7ppm时便可以产生140%的灵敏度。但经热处理后的W₁₈O₄₉纳米线对乙醇的灵敏度急剧降低,即使在高浓度的乙醇气氛中（27ppm）,最大灵敏度也仅为20%左右。本研究中水热合成制备的W₁₈O₄₉纳米线,对于在比表面积参数要求比较高的纳米器件和纳米技术应用领域来说,400℃应该是其应用的温度上限。500℃热处理后,W₁₈O₄₉完全转变为WO₃,WO₃在高温下（500-1000℃）始终保持单斜晶结构,没有发生任何相变。以原始W₁₈O₄₉纳米线和400℃热处理的W₁₈O₄₉纳米线为前驱体,通过气-固相合成反应后,反应产物为纯WS₂,反应产物形貌相似,均为WS2纳米棒、纳米管和纳米颗粒的混合物。以400℃热处理的W₁₈O₄₉纳米线为前驱体时,反应产物中存在许多完整的中空WS₂纳米管,纳米管形状各异。另外,WS₂纳米管中存在原子空位、层错、分枝层和波浪层等明显的晶体缺陷。以600℃热处理的W₁₈O₄₉纳米线为前驱体时,反应产物为不规则的纳米颗粒和短棒的混合物,团聚现象严重。X射线衍射结果表明,硫化反应不充分,产物中残留有WO₃。以纯酒精为溶剂,通过超声化学方法获得了大量WO₃·H₂O纳米须,纳米须的形成与WO₃在溶液中不断变化的过饱和度有关。500℃热处理以后,纳米须转变成直径约为10nm的纳米棒,同时正交晶WO₃·H₂O转变为单斜晶WO₃。以蒸馏水和酒精的混合物为溶剂时,最终产物变成厚度为60-100nm的纳米板。以纯蒸馏水为溶剂时,产物为超薄的纳米片。在热处理过程中,纳米板和纳米片经历脱水、缩聚和晶粒长大的过程,尺寸均明显增大。热处理后,正交晶WO₃·H₂O转变为六方晶WO₃。以仲钨酸铵为前驱体,采用高温气相沉积方法成功制备出WO₃纳米颗粒。气流量、反应温度和沉积基体的温度对WO₃的形态都有重要的影响。当气流量较低时（2L/min）,在1300-1400℃的反应温度范围内,随着沉积区域温度的降低,产物分别呈现晶须、短棒、准球形颗粒和多面体的结构。在相同气流量的条件下,随着反应温度的升高,不同区域产物尺寸变大;而当反应温度相同时,随着气流量的增加,产物尺寸明显减小,粒度分布也逐渐均匀。当反应温度为1350℃、气流量为6L/min时,可以得到直径约为20-100nm的纳米颗粒。更多还原

【Abstract】 In recent years, one-dimensional （1-D） nanostructured materials have attracted tremendous research interest owing to their unique physical, chemical and optical properties. Among all these kinds of nanomaterials, tungsten oxides are of much importance because of their outstanding electrochromic, gaschromic, thermochromic and optochromic properties as well as their widely applications in electrochromic display, semiconductor gas sensors and photocatalysts. Particularly, 1-D tungsten oxide can be used as precursor for the preparation of tungsten disulfide nanotubes. As one important part of the inorganic fullerene-like materials, tungsten disulfide nanotubes find many potential applications in electron device, catalyst, superior solid lubricant and high performance composite.In this dissertation, systematic research has been focused on the synthetic strategies of low-dimensional tungsten oxide and tungsten disulfide, their formation mechanisms and physical properties. In addition, the morphology, structure and phase transition behaviour of the as-synthesized one-dimensional nanostructured tungsten oxide under thermal processing were demonstrated and possible trasition mechanisms were proposed. Thermal treatment-dependent gas-sensing characteristics were also examined. The as-synthesized products were characterized by using scanning electron microscopy （SEM）, field emission scanning electron microscopy （FE-SEM） equipped with energy-dispersive X-ray spectroscopy （EDX）, X-ray diffractometer （XRD）, transmission electron microscopy （TEM）, Fourier infrared spectrophotometer （FTIR）, differential thermal analyzer （DTA） and Brunauer-Emmett-Tettler （BET） specific surface area analyser.Bundled W₁₈O₄₉ nanowires were successfully synthesized by a simple solvothermal method with tungsten hexachloride （WCl₆） as precursor and cyclohexanol as solvent. With increasing concentration of WCl₆ in cyclohexanol and increasing reaction time of the hydrothermal process, the bundles became larger and shorter, and finally block-shape product occurred. Ultra-thin W₁₈O₄₉ nanowires with diameter ranging from 2 nm to 15 nm and length of more than 2μm can be finally obtained by modifying the main experimental parameters. TEM, SAED and XRD analysis showed that the ultra-thin W₁₈O₄₉ nanowires exhibit many intrinsic defects such as stacking faults, dislocations and oxygen vacancies. The calculated BET specific surface area and pore volume of the ultra-thin W₁₈O₄₉ nanowires are 151m²/g and 0.51cm³/g.The nanostructured W₁₈O₄₉ bundles underwent a series of morphological evolution with increased annealing temperature, becoming straighter, larger in diameters and smaller in aspect ratio, and eventually becoming irregular particles with size up to 5μm after processing at 1000℃. The calculated specific surface areas dropped to 110m²/g and 66 m²/g after annealing at 400℃and 450℃, equivalent to a moderate decrease of 27.8% and a drastic decrease of 56.2%, respectively. Accordingly, the pore volumes reduced to 0.23cm³/g and 0.13cm³/g. Sensing properties of thin film sensors made from original and thermally processed W₁₈O₄₉ nanowires have been tested with respect to ethanol and a response value as high as 140% can be obtained to 2.7ppm of ethanol even at room temperature. However, the sensitivity of 400℃or 450℃thermally-processed nanowires is not satisfactory, even exposing to high concentration of ethanol. This result is indicative that the original surface character of the nanowires at room temperature is mainly preserved at 400℃, and this temperature can be considered as a top temperature limit in design for high temperature nanodevice and nanotechnology applications where high specific surface areas are important, such as in sensor and fuel cell applications. At 500℃, the monoclinic W₁₈O₄₉ was completely transformed to monoclinic WO₃ phase, which remains stable at high processing temperature.With the original and 400℃thermally-processed W₁₈O₄₉ nanowires as precursor, pure tungsten disulfide can be obtained by means of gas-solid phase reaction. Both of the as-synthesized products are composed of nanotubes, nanorods and nanoparticles. Additionally, intact tungsten disulfide nanotubes with different morphology can be observed when thermally processed nanowires at 400℃were used as precursor, and these nanotubes possess many obvious crystal defects such as layer discontinuities, waving layers, branched layers and dislocations. When thermally processed nanowires at 600℃were used as precursor, irregular nanoparticles mixed with few short nanorods were finally formed. XRD result showed that the resulting product from those thermally processed at 600℃is a mixture of WS₂ and WO₃, indicating the insufficient oxide-to-sulfide conversion. Numerous nano-whiskers have been obtained with ethanol as solvent by using a sonochemical method. The continuous changing supersaturation of tungsten trioxide in the solution may account for the formation of the nano-whiskers. After thermal processing at 500℃, the as-synthesized nano-whiskers transformed to short rods with diameter of about 10nm. Interestingly, the morphological evolution was accompanied by phase transformation, from orthorhombic tungsten trioxide hydrate to monoclinic WO₃. Nano-plates with depth of 60-100nm were formed from ethanol-water mixed solvent, while ultra-thin nano-sheets were synthesized when only water was used as the solvent. Due to a combination of the loss of crystalline water and crystal growth, sizes of both of the nano-plates and nano-sheets were apparently increased. Meanwhile, orthorhombic tungsten trioxide hydrate was transformed to hexagonal WO₃.WO₃ nanoparticles were synthesized by using a high-temperature vapour deposition method with APT·4H₂O as raw material. The morphology of the as-synthesized products was strongly affected by the temperature of the deposition substrate, the reaction temperature and the gas flow rate. At a low gas flow rate, products collected from different areas of the glass tube exhibit various morphologies, from whiskers, rods and quasi-spherical particles to irregular polyhedral particles, depending on the temperature of the deposition substrate. At the same gas flow rate, the product became larger with increased reaction temperature. When the reaction temperature remained unchanged, however, the sizes of the obtained particles decreased with increasing gas flow rate and also the size distribution became more homogenous. Eventually, nanoparticles with diameters ranging from 20nm to 100nm can be obtained when the Ar flow rate reached 6L/min and the reaction temperature was kept at 1350℃.更多还原