| 题名 | 二氧化钛光催化体系活性调控机制及稳定性研究 |
| 作者 | 孙静 |
| 学位类别 | 博士 |
| 答辩日期 | 2014-05 |
| 授予单位 | 中国科学院研究生院 |
| 授予地点 | 北京 |
| 导师 | 郭良宏 |
| 关键词 | 纳米TiO2 光催化 聚集 界面过程 腐殖酸 nano-TiO2 photocatalysis aggregation interface process natural organic matter |
| 其他题名 | Mechanism Research of Modulation Photocatalytic Performances and Stability with Titanium Dioxide |
| 学位专业 | 环境科学 |
| 中文摘要 | 半导体纳米材料的表面效应和小尺寸效应，使其具备优异的光学及光电特性，有力地推动了光催化技术的发展。在众多光催化材料中，二氧化钛（TiO2）由于高效、低成本、低毒性等的性质，因此在环境污染修复、太阳能电池和表面自清洁材料领域被广泛使用。但是由于TiO2 自身存在光生载流子易复合，量子产率低等不足，限制了其在光催化方面的进一步应用。发展高效的光催化材料可以通过构建具有异质结构的复合光催化体系来实现。异质结构可以有效驱动光生载流子在界面间的转移。通常情况下，TiO2 的光催化活性与其晶粒大小、晶体结构、缺陷结构、比表面积和表面官能团性质等密切相关。对于具有高光活性的纳米材料，纳米TiO2 在长时间光照过程中很容易发生物理化学转化，改变其表面性质，从而影响其光催化活性的稳定性。因此有必要研究TiO2 纳米颗粒在长时间使用过程中的稳定机制，以及环境水体中纳米TiO2 的光稳定性。围绕以上内容，主要开展了以下工作： （1）构建了零维-二维（0D-2D）P25/石墨烯、一维-二维（1D-2D）TiO2 纳米管/石墨烯以及TiO2 纳米片/石墨烯二维-二维（2D-2D）三种异质结构光催化体系。通过电镜及光谱表征表明三种光催化体系的有效构建。特别是TiO2 纳米片/石墨烯二维-二维异质结构，二者之间通过化学相互作用紧密结合。与零维-二维（0D-2D）P25/石墨烯复合材料以及一维-二维（1D-2D）TiO2 纳米管/石墨烯复合材料相比较，TiO2 纳米片/石墨烯在紫外光照下降解罗丹明B 和2,4-二氯酚的效率最高。自由基捕获实验以及ESR 表征发现，2D-2D 异质结构能够有效提高·OH和O2·−的产生量。通过拟合激发态TiO2 的荧光寿命，计算得到0D-2D，1D-2D 以及2D-2D 异质结构界面间的电子迁移速率分别为1.15×108 s-1, 3.47×108 s-1 和1.06×109 s-1。因此，TiO2 纳米片/石墨烯构建的多维异质结构界面更有利于光生载流子的分离，从而有效提高活性氧自由基的产生量，加快光催化降解污染物的反应速率。 （2）研究纳米TiO2 在水环境下光照引起的环境转化行为，考察了该转化对TiO2 的聚集行为以及光催化活性的影响。发现紫外光照能够引起P25（TiO2）的聚集，同时该聚集行为随着光照时间的延长而加剧。光照50 小时后，聚集速率从光照前的<0.001 nm s-1 增长到0.027 nm s-1，且聚集体的平均水动力学直径达到623 nm。考察Zeta 电位发现光照后等电点发生左移，表明TiO2 表面正电荷量减少，因此更易于聚集行为的发生。ATR-FTIR 表征发现紫外光照后TiO2 表面羟基官能团增加，可能会导致TiO2 表面电荷的变化，从而引起聚集速率的改变。同时，紫外光照也会导致TiO2 光催化降解罗丹明B 的效率随光照时间呈现先增加 后减小的趋势。基于荧光寿命及光电流的表征，发现催化效率的改变与颗粒间电荷转移能力的改变有关。光催化降解效率与聚集体尺寸之间存在着以下关系：小尺寸聚集体颗粒间的紧密接触有利于电荷转移，而大尺寸聚集体由于提高了电子-空穴的复合几率，从而降低了光催化效率。 （3）考察了模拟环境水体中TiO2 纳米颗粒的光致聚集现象，研究有机质或盐离子与光照共同作用下的TiO2 聚集行为。发现光照会加剧TiO2-盐离子体系的聚集行为，并且该现象与光照时间有相关性。在该体系中，TiO2 与盐离子之间的静电相互作用对聚集起着关键作用。同时，TiO2-CaCl2 体系中，可能还存在着Ca2+与TiO2 表面羟基官能团的相互作用，导致临界聚集浓度（CCC）值与理论值相比偏小。在TiO2-盐离子-腐殖酸体系中，光照会削弱腐殖酸的稳定作用，并且体系的聚集行为随着光照时间的延长而加剧。该现象可能是由于光照后的TiO2对腐殖酸的吸附能力减弱，从而导致腐殖酸的空间位阻及静电相互作用力减弱， 引起体系的聚集。 以上结果表明，通过构建纳米材料不同的空间维度界面，可以实现对光生载流子迁移速率的调控，该研究可以为指导设计合成高活性的光催化剂提供新思路。同时，对于纳米TiO2 的光致聚集行为的考察，不仅有助于纳米光催化剂的实用化及稳定性的提高，而且可以为评估纳米TiO2 在天然环境中的迁移和转化过程及暴露风险奠定基础。 |
| 英文摘要 | Semiconductor nanomaterials have attracted intensive interest due to their ability to simultaneously harvest solar irradiation and drive photochemical reactions by photogenerated charge carriers. Especially, TiO2 nanoparticles are widely used in photocatalytic devices to achieve environmental recombination and water splitting because of its low-cost, low toxicity and robust performance. However, the intrinsic fast recombination of charge carriers in their defect-rich bulk can be problematic. Generally, the effectiveness of photocatalytic devices is dictated to a great extent by three processes: optical absorption, charge separation and catalytic reaction on semiconductor surface. Semiconductors coupled photocatalytic systems represent a typical strategy for enhancing charge carriers separation and thus enhance the photocatalytic performance. It is noted that the photoreactivity of nano-TiO2 is significantly related to its physicochemical properties, such as crystalline phase and facet, defect sites, and surface groups. As a photoactive nanomaterial, UV light induced photochemical transformation could be a critical fate process of nano-TiO2 in aquatic environment after long-term irradiation. Thus, it is essential to explore the effect of long-term irradiation on the stability of nano-TiO2 as well as the mobility and transformation under natural environment. In this dissertation, several works have been done as follows: (1) TiO2 nanosheet/graphene composite based 2D−2D heterojunctions were fabricated by a solvothermal process. Microscopic and spectroscopic characterization revealed a homogeneous sheetlike morphology with intimate interfacial contact between the TiO2 nanosheet and graphene due to chemical interactions. Compared with 0D−2D Degussa P25 (TiO2)/ graphene and 1D−2D TiO2 nanotube/graphene composites, the 2D−2D TiO2 nanosheet/graphene hybrid demonstrated higher photocatalytic activity toward the degradation of Rhodamine B and 2,4-dichlorophenol under UV irradiation. Radical trapping and ESR experiments revealed the enhanced generation of ·OH and O2·− in the 2D−2D heterojunction system. By analyzing TiO2 excited state deactivation lifetime, the interfacial electron transfer rates determined for 0D−2D, 1D−2D, and 2D−2D TiO2/graphene composites were 1.15 × 108 s−1, 3.47 × 108 s−1, and 1.06 × 109 s−1, respectively. It was therefore proposed that the fast charge separation in the TiO2 nanosheet/graphene photocatalyst promoted the generation of reactive oxygen species and enhanced the photodegradation reactions. (2) UV irradiation induced transformation of TiO2 nanoparticles in aqueous solution was demonstrated, and its effect on the aggregation and photocatalytic reactivity of TiO2 were investigated. UV irradiation of pristine P25 TiO2 nanoparticle suspension was found to accelerate nanoparticle aggregation that is dependent on the duration of irradiation. The aggregation rate increased from <0.001 nm/s before irradiation to 0.027 nm/s after 50 hr irradiation, with an average hydrodynamic radius of 623 nm for the aggregates. In Zeta potential measurements the isoelectric point of the suspension was lowered from 7.0 to 6.4 after irradiation, indicating less positive charges on the surface and thus higher tendency to aggregate. ATR-FTIR spectra displayed successive growth of surface hydroxyl groups with UV irradiation which might be responsible for the change of surface charge and aggregation rate. UV irradiation also changed the photocatalytic degradation rate of Rhodamine B by TiO2, which initially increased with irradiation time, then decreased. Based on the photoluminescence decay and photocurrent collection data, the change was attributed to the variation in interparticle charge transfer kinetics. The unusual pattern of change in photoreactivity with aggregate size was rationalized by evoking a model in which the photoreactivity of TiO2 is enhanced in smaller sized aggregates due to interparticle charge transfer, and is reduced in larger aggregates due to increased electron-hole recombination. (3) The aggregation kinetics of nano-TiO2 before and after UV irradiation was investigated in aqueous solutions varying in ionic strength, ionic composition, and humic acid. In NaCl, Na2SO4, and CaCl2 solutions, the growth of surface hydroxyl groups with UV irradiation remarkably induced nano-TiO2 aggregation due to the less positively charged. It is dependent on the duration of irradiation. Furthermore, the critical coagulation concentration (CCC) of TiO2 in CaCl2 solution is lower than the normal range. It is indicated that Ca2+ probably specifically interacts with the hydroxyl groups of the irradiated nano-TiO2 surface. Although humic acid greatly enhanced the stability of pristine nano-TiO2 via the steric hindrance effect, UV irradiated-TiO2 was found to accelerate nanoparticle aggregation in ionic and humic acid solutions. That is possibly due to the reduced adsorption of humic acid on the surface of nano-TiO2. As demonstrated in this study, nanomaterial dimensionality plays an important role in interfacial charge transfer processes and thus the photocatalytic performance. It could stimulate the development of a photocatalytic system with a well-defined nanocomposite interface to fulfill their potential in the photocatalytic devices. Furthermore, findings from this study underscore the significance of UV irradiation induced nano-TiO2 aggregation behavior in engineered applications as well as in natural systems. A better understanding of environment-induced changes in TiO2 surface chemistry is crucial to predict its environmental transport, fate and potential risks. |
| 公开日期 | 2015-07-07 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.rcees.ac.cn/handle/311016/15739]  |
| 专题 | 生态环境研究中心_环境化学与生态毒理学国家重点实验室 |
| 推荐引用方式 GB/T 7714 | 孙静. 二氧化钛光催化体系活性调控机制及稳定性研究[D]. 北京. 中国科学院研究生院. 2014. |
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