Sn(OBu~n)_4溶胶-凝胶-气凝胶体系的结构深化及其表征
文献类型：学位论文
| 作者 | 黄瑞安 |
| 学位类别 | 博士 |
| 答辩日期 | 2005 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 侯立松 |
| 关键词 | 溶胶-凝胶法 锡醇盐 水解和缩合 二氧化锡 气凝胶 |
| 其他题名 | Structural Evolution and Characterization of Sn(OBu~n)_4 Sol-Gel-Aerogel Systems |
| 中文摘要 | SnO2是一种n－宽带半导体材料，具有特殊结构的纳米Sn02材料表现出独特的物理和化学性质，有望在电子学·光学·电化学·催化和气体传感器等领域有新的应用。溶胶一凝胶法在制备新型纳米材料方面占有越来越重要的地位，在分子层次上设计和剪裁出具有特殊结构的SnO2基纳米材料方面有独特的优势。本文对溶胶一凝胶材料制备技术的原理、化学和物理过程、研究现状、气凝胶的制备和应用等方面进行了全面综述。利用无水SnC14为原料，在冰浴条件下，通过氨法成功合成了sn（0Bun），把它作为前驱体，通过溶胶一凝胶工艺制备SnO2材料，对sn（0Bun）溶胶一凝胶一气凝胶过程中的胶团的结构演化及其表征进行了研究。Sn（oBun）4的化学活性非常高，遇水会发生剧烈的水解反应并产生沉淀。通过乙酞丙酮（AcAc）化学改性Sn（OBun）4，控制水解率，添加催化剂和交换溶剂等方法调整体系参数，制备了稳定的溶胶体系。实验表明，在［AcAc］／ISn（OR）n］＝2．0情况下，水解率「HZO］／［sn（oR）n］＝2．0的溶胶体系非常稳定，而在水解率较高时，即使添加更多的AcAc，体系也会产生沉淀；氢氟酸、盐酸和氢漠酸等催化剂可以加快改性Sn（OBun）4的水解缩合反应速度，但氢氟酸催化的体系会产生沉淀，而盐酸和氢溟酸催化的体系可以得到稳定的溶胶体系，体系凝胶时间为数百个小时不等。改变溶剂也会使改性Sn（OBun）4溶胶体系稳定性发生改变，不同溶剂的体系稳定性大小依次为Sn（OEt）＞Sn（OBun）4＞Sn（OBus）4＞sn（OBut）4，体系的凝胶时间在接近2000小时到仅仅几个小时之间发生变化。在不同的催化剂条件下，经AcAc化学改性的Sn（oBun）4，在水解率为2．00的溶胶一凝胶体系中，SnO2高聚物分子的生长方式不同，并表现出不同的流变学性质。经AcAc化学改性的Sn（OBun）4前驱体，各反应基团的水解和缩合反应几率不同，（OBun）基团上反应几率大于AcAc基团，反应几率的各向异性产生聚合物溶胶胶团的近似线形链结构，符合近似线性生长模型；聚合物溶胶胶团的近似线形链具有统计意义上的自相似性，其分形维度为1．76－2.05；Sn（oBun）4溶胶－凝胶体系聚合物的生长机制是介于扩散置限团簇凝聚（diffusion一limitedclustersaggregation，DLCA）和反应置限团簇凝聚模型（reaction－imitedclustersaggregation，RLCA）之间，但大体上可以用前者来描述。小角X射线散射实验表明，在没有催化剂的情况下，经AcAc化学改性的Sn（oBun）4在水解率为2．05的溶胶一凝胶体系，在溶胶陈化的早期阶段，体系中即存在尺度为30－Onm的较大溶胶团簇，他们具有表面分形结构，表面分形维度约为2．4，此外，还存在尺度约为10nm的结构较疏松的小团簇，质量分形维度sn（OBun）4溶胶一凝胶一气凝胶体系的结构演化及其表征／黄瑞安约为2．6；在发生凝胶之前，随着陈化时间的增长，较大溶胶团簇保持几乎不变的的表面分形结构，而溶胶小团簇的表面光滑化，这是由于前驱体可以深入小团簇并与之进行水解和缩合反应；动态光散射实验表明，溶胶体系内尺度约为30-60nm的较大团簇是由尺度约为10nm的小团簇聚集而成的二级结构，与小角X射线散射的实验结果相一致。此外，体系中还存在着更大尺寸结构溶胶团簇；体系凝胶变为以后，经过陈化的凝胶骨架具有光滑的表面，这是由于小团簇或小团簇在骨架内扩散运动并与之反应使其表面平整化的结果；溶胶体系的动态行为可以用一个单一指数和一个扩展指数之和来描述。随着陈化时间的增长，快弛豫时间变化不大，而慢弛豫时间按幂律发散，扩展指数也随着陈化时间的增长而变小。SnO2湿凝胶在干燥过程中极容易开裂，但经过N，N-二甲基甲酞胺处理后再进行缓慢干燥至120℃，可以得到没有开裂的snoZ干凝胶块体；sno2湿凝胶的固化过程大致可以分为三个阶段，即溶剂的解吸脱附的快速失重阶段，有机物的氧化分解的线性失重阶段和SnO2晶粒长大、凝胶致密化的无失重阶段。用超临界干燥法合成了具有高比表面积的透明和半透明SnO2气凝胶，表观密度分别为0．34和o．47g／cm3，比表面积分别为357和306m2／g。snoZ气凝胶由各种纳米和微米尺度的棉花状聚集物块体堆积而成，棉花状聚集物块体之间具有大量的微米级的大孔，而棉花状聚集物块体内部存在大量的细小孔隙；SnOZ气凝胶的孔隙分布非常宽，孔隙跨越微孔、介孔和大孔各种尺度，是结构非常复杂和不规则的分形结构。SnO2气凝胶经200-400℃温度热处理时，随着凝胶内部的有机物分解，并开始出现SnO2结晶，气凝胶骨架表面逐步粗糙化，具有表面分形结构；在500℃温度下热处理，SnO2长成粒径约为7．3nm的晶粒，并使气凝胶骨架表面失去分形结构，表面光滑化；在600℃温度下热处理，SnO2Z长成晶粒约为10．7nm的晶粒，该晶粒表面粗糙，具有表面分形结构，其分形维度为2．6：在200-600℃热处理温度下，凝胶骨架在微米尺度上表现为Sn02聚集体和氧化物聚集体之间的孔隙的细化和均匀化。本研究的创新点在于：①简化了锡的金属醇盐的传统合成方法，在无气氛保护条件下成功合成了Sn（0Bun）4；②解决了sn（oBu）4溶胶一凝胶系的稳定性，实现了体系的凝胶时间从几个小时甚至几分钟到几千个小时可控；③探索出了一条通过溶胶的流变性质来求出SnOZ近似线性高聚物分子的分形维度的新途径；④利用小角x射线散射技术和动态光散射技术研究了sn（OBun)4溶胶团簇和sno2气凝胶的静态／动态结构，得出了溶胶胶团和气凝胶不同层次的结构；⑤首次利用锡的金属醇盐成功合成了具有高比表面积的无色透明SnO2气凝胶。 |
| 英文摘要 | SnO2 is an n-type broad band semiconductor. SnOi-based materials with special structures possess unique physical and chemical properties and have many extraordinary applications in the area of electronics, optics, electrochemistry, catalysis and gas sensors. So-gel processing plays more and more important role in the preparation of new nano-materials and has the advantage in designing and tailoring special structures at the molecular level for the synthesis of SnCVbased nano-materials. In this dissertation, the principle, chemistry and physics of the sol-gel process, as well as the present status, synthesis and applications of aerogels are extensively reviewed. Sn(OBun)4 has been susessfully synthesized by ammonia method in ice-bath using SnCl4 as starting compound. As a precursor, Sn(OBun)4 was employed to prepare SnO2 mateials by sol-gel processing. Emphasis was laid on the structural evolution and characterization of SnC>2 polymers (clusters) and glomerates in the sol-gel-aerogel processes. Sn(OBun)4 is highly chemically reactive and sensitive to moisture resulting in precipitation by drastic hydrolysis. A stable sol system can be obtained by modifying Sn(OBun)4 with acetylacetone (AcAc), controlling hydrolysis rate, catalyzing, and/or exchanging solvents. It is experimentally showed that a hydrolysis ratio ( /z=[H2O]/[Sn(OR)n] ) of 2.0 and modification ratio ( w=[AcAc]/[Sn(OR)n] ) of 2.0 help to make a very stable sol system. However, precipitation will occur under high hydrolysis ratio even in excess of AcAc. Hydrofluoric, hydrochloric, hydrobromic, nitric and acetic acids and ammonia were used as catalysts in the modified Sn(OBun)4 sol-gel systems whose gelation times vary within several hundreds hours. However, precipitation will occur in a system in which hydrofluoric acid is added. Stability of the modified Sn(OBu")4 sol systems can be drastically affected by solvent exchanging, and their gelation times vary from about 2000 hours to several hours in the sequence of EtOH> Bull0H>Bus0H>But0H. In the systems of both the hydrolysis and AcAc-modification ratios being 2.0 and under different catalytic conditions, hydrolysis and condensation can preferentially take place on the OBun sites in the modified precursor mpleeules. Tin oxide polymers formed in these systems are nearly linear and reinaif Statistically self-similar in a certain region. Their fractal dimensions vary between Ji'&S fggg} JLO5 under different conditions. The growth process can be mainly descriiN&#v%.'.#|ft (jifiusion-limited clusters aggregation (DLCA) regime. ........ It is experimentally indicated by small-angle X-ray scatteiingfSAXS) that there exist larger clusters about 30~60nm in diameter with surface fractal dimension of 2.4 and smaller clusters about lOnm in diameter with mass fractal dimension of 2.6 in the early stage of the modified Sn(OBu")4 sol-gel system with the hydrolysis ratio being 2.05 and without any catalyst. The larger clusters would keep their surface fractal structure unchanged during aging. However, the smaller would clusters become more and more compact with the time evolution because of the penetration of monomers into the clusters and smoothing their surfaces. Dynamic light scattering (DLS) experiments also show that the smaller clusers about 10 nm in diameter act as building blocks and build up larger clusters of about 30~~60nm. Althougt the two-level structures from DLS are in good agreement with that of SAXS experiments, there also exist large clusters with the size exceeding 60 nm in this sol-gel system. The skeleton of the aged gel has smooth surfaces owning to the diffusion of oligomers and smaller clusters with unreacted groups among the network. The dynamic behaviour of the clusters in this sol-gel system can be described by a monoexponential plus a stretched exponential. As time proceeds, the slow relaxation times diverge with a power law, however, the fast relaxation times do not. Furthermore, the stretched exponent decreases during ageing. SnC>2 wet gels are apt to crack during drying, however, a crack-free xerogel has been obtained by handling SnO? wet gel with N,N-dimethylformamide and slowly o drying up to 120 C. Three stages are included in the solification process of SnC>2 wet gels, namely the desorption of solvent from gels accompanied by a large percent of weight loss, the burning of organic groups and crystallization of SnOi with a linear weight loss, and the densification of xerogel without weight loss. Transparent and translucent SnCb aerogels with high specific surface area were also obtained. Their apparent densities are 0.34 and 0.47g/cmJ, and specific surface areas 357 and 306m7g, respectively. These aerogels are composed of cottonlike oxide glomerates with a large number of pores and have complex fractal structure. The pore size distribution of these aerogels is pretty wide spanning mesopore and macropore especially in the case of the translucent aerogel. When heating the aerogels at temperatures from 200 to 400 C, the decomposition of organics from the aerogel and crystallization of SnC>2 would occur along with the coarsening of aerogel skeleton which is surface fractal. The average grain size of 7.3 nm was obtained by heating the aerogel at the temperature of 500 °C for two hours, with the surface of the aerogel smoothing. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15387]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 黄瑞安. Sn(OBu~n)_4溶胶-凝胶-气凝胶体系的结构深化及其表征[D]. 中国科学院上海光学精密机械研究所. 2005. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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