【作者】 易志刚；

【导师】 周祚万；

【作者基本信息】 西南交通大学 ， 材料学， 2013， 博士

【摘要】 ZnO是一种典型的宽禁带直接带隙半导体材料(约3.37eV),因其具有良好的生物相容性和环境安全性,作为一种很有前景的光催化材料而受到环境污染治理领域的广泛关注。众所周知,催化剂材料的结构、形貌、表面状态等因素对其性能和应用有重要的影响。近年来,已能控制制备多种不同形貌的ZnO,但深入分析形貌对其光催化活性影响的研究工作却开展较少。本论文针对催化剂形貌对其光催化活性影响的相关科学问题,选择不同形貌的ZnO开展光催化实验,深入分析了ZnO形貌对其光催化活性的影响。此外,考虑到ZnO对有机物的吸附行为对其光催化活性有显著影响,选用高吸附性能的SiO2气凝胶(英文缩写SiO2(AG))作为改性材料,制备了SiO2(AG)/ZnO的复合材料,以提高ZnO对有机物的吸附性能,探讨光催化剂吸附性能对其光催化活性的影响。以四针状ZnO晶须(T-ZnO)、纳米ZnO (n-ZnO)和微米ZnO (c-ZnO)三种不同形貌的ZnO为催化剂,硝基苯、苯酚、亚甲基蓝三种不同性质的有机物为目标物,进行了光催化降解对比实验。结果表明,三种不同形貌的ZnO对硝基苯、苯酚、亚甲基蓝的光催化降解活性均表现为T-ZnO最好,c-ZnO次之,n-ZnO最差。导致这一结果的原因主要有三方面：一是不同形貌的ZnO,其结构中的氧空位浓度存在差异,导致典型活性氧.OH的产率不同,表现在其光催化活性的显著差异。本课题组前期研究表明,T-ZnO氧空位浓度明显高于c-ZnO和n-ZnO,能产生更多的.OH,使其具有更好的光催化活性。二是生成.OH的过程中对光生空穴的消耗,可抑制光生空穴与ZnO表面的氧原子发生反应溶出Zn2+, T-ZnO光催化过程中产生的.OH最多,光催化反应过程中溶出Zn2+浓度最低,其光稳定性最好。三是T-ZnO具有特殊四针状结构,针尖尺寸为纳米级,使其具有纳米材料的活性,同时又克服了纳米材料容易发生团聚现象的缺陷,具有很好的分散性。n-ZnO的氧空位浓度高于c-ZnO,但光催化活性却更差,这是因为两种ZnO的氧空位浓度差异不大,但颗粒尺寸更小的n-ZnO在反应过程中更易发生团聚现象,导致光催化活性降低。采用溶胶凝胶法,在常压条件下通过溶剂交换法制备了SiO2(AG)及不同负载量的SiO2(AG)/T-ZnO复合材料。材料的表征结果表明,SiO2(AG)样品为由细微颗粒形成的疏松多孔结构,样品粒径分布均匀,比表面积为902m2/g,平均孔径为8.91nm,孔体积为2.01ml/g,具有很好的疏水性能,经500℃热处理,SiO2(AG)可以由疏水性转化为亲水性；SiO2(AG)/T-ZnO复合材料的BET和孔体积随着SiO2(AG)负载量的增加而增大,复合材料保持了SiO2(AG)和T-ZnO原有的材料学特性,SiO2(AG)/T-ZnO对紫外光的吸收强度与T-ZnO相比,无明显变化。以硝基苯、苯酚、亚甲基蓝三种不同性质的有机物为吸附质,SiO2(AG)、T-ZnO及SiO2(AG)/T-ZnO为吸附剂,系统研究了材料对有机物的吸附性能。结果表明,SiO2(AG)对硝基苯、苯酚、亚甲基蓝的吸附过程,吸附剂和吸附质之间的亲疏水作用起主导作用,疏水的SiO2(AG)对疏水的难溶有机物硝基苯有很好的吸附性能,而对亲水的易溶有机物苯酚和亚甲基蓝吸附会比较困难；500℃热处理后,疏水性SiO2(AG)转变为亲水性,对苯酚和亚甲基蓝表现出了较好的吸附性能,对硝基苯的吸附量明显降低。SiO2(AG)/T-ZnO复合材料对硝基苯的吸附性能相对于T-ZnO,明显改善,经500℃热处理后,对苯酚和亚甲基蓝的吸附性能明显改善。以硝基苯、苯酚、亚甲基蓝三种不同性质的有机物为目标物,T-ZnO及SiO2(AG)/T-ZnO为光催化剂,开展了有机物光催化降解动力学研究,系统探讨了材料吸附性能改善对有机物光催化降解效果的影响。结果表明,不同负载量的SiO2(AG)/T-ZnO复合材料相比,SiO2(AG)负载量为8.0%的SiO2(AG)/T-ZnO对硝基苯的光催化活性最好,经500℃热处理后,对苯酚和亚甲基蓝表现出更好的光催化活性。SiO2(AG)/T-ZnP、500℃热处理SiO2(AG)/T-ZnO和T-ZnO对硝基苯、苯酚、亚甲基蓝光催化反应过程的动力学研究结果表明,三种有机物在催化剂上的光催化降解过程符合准一级反应动力学方程。在考虑初始反应动力学时,硝基苯和苯酚的反应符合Langmuir-Hinshelwood动力学模型,催化剂对有机物吸附性能的改善使光催化剂对有机物具有更好的光催化活性；随着亚甲基蓝浓度增大,考虑初始动力学时,光催化初始反应速率与初始浓度线性相关性变差,亚甲基蓝浓度增大,使其色度增加,影响光源的吸收和利用,使光催化活性明显降低；此时,催化剂对有机物吸附性能的改善对其光催化活性的发挥不起作用。这一结果说明光催化技术不适宜处理高浓度的染料废水。以硝基苯溶液为模拟废水,SiO2(AG)负载量为8.0%的SiO2(AG)/T-ZnO为催化剂,对硝基苯废水光催化反应影响因素进行了深入分析。结果表明,常温条件下,硝基苯的表观反应速率常数随着其初始浓度的增大而降低；对硝基苯初始浓度为24.0mg/1的溶液,SiO2(AG)/T-ZnO的最佳投放量为2.0g/1,硝基苯去除率达82.1%；反应温度对光催化去除率无明显影响；按1.5ml/1的量向反应体系中加入30.0%的H2O2,硝基苯的去除率可由82.1%提高到85.8%；工业废水常见的阴离子中,Cl-、SO42-使硝基苯的光催化去除率明显降低,且SO42-的抑制作用比Cl-更大,而NO3-对硝基苯的光催化去除率基本没有影响；在处理硝基苯和亚甲基蓝的混合废水时,亲水性的亚甲基蓝对硝基苯的光催化降解影响较小；相反,亚甲基蓝的光催化降解受到明显抑制,其原因是SiO2(AG)/T-ZnO表面负载了疏水性SiO2,能优先吸附疏水性的硝基苯,实现有机物的选择性降解。可重复性实验结果表明,SiO2(AG)/T-ZnO重复使用三次,仍能保持较好的光催化性能。更多还原

【Abstract】 ZnO is a semiconductor with direct wide band gap of3.37eV. Owing to its good biocompatibility and environmental safety, ZnO has received enormous attention to be a promising photocatalyst for degradation of environmental pollutants. It is well known that the structure, morphology and sufacial character, et al. of the catalyst are confessed to have great effects on their properties and applications. In recent years, different morphologies of ZnO have been synthesized, but there are few report about the effects of the morphology of ZnO on the photocatalytic activity of ZnO. In this dissertation, we systematically investigated this kind of effects on the photocatalytic activity of ZnO due to the morphological differences. Considering that the adsorption process of organic compounds in ZnO had a significant impact to the photocatalytic activity during the process of the ZnO photocatalytic degradation of organic compounds, we prepared SiO2aerogel/ZnO (SiO2(AG)/ZnO) composite materials to improve the ZnO adsorption to the organic compounds. Additionally, it was of important to investigate the effect of the adsorption of organic compounds in the photocatalyst on photocatalytic activity.We conducted the comparison experiments of photocatalytic degradation of different kinds of organic compounds such as nitrobenzene, phenol and methylene blue using tetrapod-like ZnO whisker (T-ZnO), nano-sized ZnO (n-ZnO) and micro-sized ZnO (c-ZnO) as the catalysts which had different morphologies. The results showed that T-ZnO had the best photocatalytic activity against the mentioned organic compounds, and c-ZnO exhibited the second, n-ZnO displayed the worst activity. Firstly, ZnO with different morphologies had different oxygen vacancy concentration in their crystals, which directly caused different amount of-OH generation and lead different photocatalytic activity. The oxygen vacancy concentration of T-ZnO was significantly higher than those in c-ZnO and n-ZnO. Accordingly, T-ZnO generated more-OH in its suspension, displaying better photocatalytic activity than the other two kinds of ZnO. Secondly, the process of-OH generating could restrain the dissolving Zn2+which was reacted by the photogenerated holes and the oxygen atoms of the ZnO surface. T-ZnO generated the most amount of-OH, and had the lowest Zn2+concentration in photocatalytic reaction process, resulting in the best light stability. Thirdly, T-ZnO had a special four needle-like structure, whose needles’tip were in nano-scale, which let it exhibit features of nanomaterials. Meanwhile, it overcame the shortcomings that nanomaterials usually had such as aggregation and worse dispersal. The n-ZnO had higher amount of oxygen vacancy concentration than c-ZnO, but it displayed worse photocatalytic activity. This was because n-ZnO had smaller particle size, resulting in stronger inclination of aggregating, which lead to decrease of photocatalytic activity, even the oxygen vacancy concentrations of these two types of ZnO were similar.By using sol-gel method, we prepared SiO2(AG) and SiO2(AG)/T-ZnO with different loading amount by solvent exchange under ambient pressure. The result of material characterization shows that SiO2(AG) sample is the micro-porous structure formed by numerous fine particles, whose particle size distributed evenly, Its specific surface area is902m2/g, and the average pore size is8.91nm, and the pore volume is2.01ml/g. Further experiment illustrates SiO2(AG) is hydrophobic. SiO2(AG) can be transformed from hydrophobic to hydrophilic after500℃treatment. The specific surface area and the pore volume of SiO2(AG)/T-ZnO composite materials increase as the amount of the loaded SiO2(AG) increase. The composite materials preserve the original material characteristics of SiO2(AG) or T-ZnO. As it is compared to T-ZnO, SiO2(AG)/T-ZnO have no obviously change in the UV absorptions.We systematically studied the materials’absorption performance of organic compounds.The results show that, in the absorption of nitrobenzene, phenol and methylene blue, the hydrophobic and hydrophilic interaction of adsorbent and adsorbate plays a dominant role, and the SiO2(AG) which is hydrophobic has good adsorption capability on hydrophobic and insoluble organic compounds nitrobenzene, though it shows few absorption capabilities on hydrophilic and soluble organic compounds phenol and methylene blue. After500℃treatment, SiO2(AG) turns hydrophilic, and it shows good adsorption capacities on phenol and methylene blue, and the adsorption capacity on nitrobenzene drops significantly. The adsorption performance of SiO2(AG)/T-ZnO for nitrobenzene improved significantly compared to T-ZnO. After500℃treatment, its adsorption performance on phenol and methylene blue also improved significantly.We studied the photocatalytic degradation kinetics of organic compounds, and explored the effect of the improvement of adsorption performance for organic compounds photocatalytic degradation of nitrobenzene, phenol and methylene blue. The result shows that SiO2(AG)/T-ZnO having8.0%loading of SiO2(AG) on T-ZnO has the best photocatalytic activity for nitrobenzene. SiO2(AG)/T-ZnO having8.0%loading of SiO2(AG) shows better photocatalytic activity for phenol and methylene blue after500℃treatment. The process of SiO2(AG)/T-ZnO and T-ZnO photocatalytic reactions for nitrobenzene, phenol and methylene blue display a pseudo first-order kinetics process. Considering the initial reaction kinetics, the reactions of nitrobenzene and phenol are fitted to the Langmuir-Hinshelwood kinetics model, but methylene blue is unfitted. The experimental result also shows that catalyst improves the performance of organic compounds adsorption which beneficial to the photocatalytic degradation of organic pollutants. The high colorimetry of dye wastewater with high concentration affects the light absorption and utilization of catalyst, which makes photocatalytic activity decrease.We deeply analyzed the factors that affect the photocatalytic reaction of nitrobenzene wastewater using SiO2(AG)/T-ZnO having8.0%loading of SiO2(AG) on T-ZnO as the catalyst. The results show that, at the room temperature, the apparent rate constant of nitrobenzene decreases as the initial concentration increases. The best quantity of SiO2(AG)/T-ZnO is2.0g/1for the nitrobenzene solution of24.0mg/1. And the degradation ratio of nitrobenzene is as high as82.1%and there are few effects on degradation ratio by reaction temperature. The nitrobenzene degradation ratio increases to85.8%from82.1%by adding30.0%H2O21.5.0ml/1. It is also found that the common anions of industrial wastewater, such as Cl-,SO42-make nitrobenzene degradation ratio decrease significantly, SO42-exhibit greater restraints than Cl-, but NO-3has no obvious effects on nitrobenzene degradation ratio. As to the treatment of mixed wastewater containing nitrobenzene and methylene blue, SiO2(AG)/T-ZnO preferentially adsorbs hydrophobic nitrobenzene, and makes it degrade preferentially. The hydrophilic methylene blue has small effects on the photocatalytic degradation of nitrobenzene. Conversely, the photocatalytic degradation of methylene blue is restrained significantly. In addition, it is experimentally proved that, after3times of usage, SiO2(AG)/T-ZnO exhibits photocatalytic performance almost the same as the original sample.更多还原