|关键词||可见光光催化 熔盐法 微观结构 氧空位 光催化性能 光催化机理|
以太阳光为驱动力的光催化一直是国际前沿研究，其核心是半导体材料。半导体光催化技术问世以来已经开发了很多具有光催化活性材料，但是目前该技术利用太阳光效率依然不高，实际应用受两大瓶颈的制约：材料带隙宽引起的光吸收不足及光生载流子快速复合引起的量子效率低，而这些因素与催化剂结构和表界面过程紧密相关。利用材料对可见光的充分吸收和载流子强化分离而提高光转化效率一直是国际研究热点，同时也是尚未解决的、具有巨大挑战性的国际难题。因此，深入研究光催化机理、微观结构调控和光生电荷传输行为，高效的可见光活性的光催化剂依然是光催化领域今后一段时间乃至很长时间的研究重点。本论文立足于光催化材料形貌和缺陷效应引起的光生载流子的强化分离，以密度泛函理论模拟为基础，设计、合成、探索具有不同微观结构和富含氧空位的新型钛基钙钛矿催化剂，并寻找探索所制备的可见光活性催化材料潜在的环境应用。论文中主要制备了含氧空位和平面四配位CuO4构筑单元的CaCu3Ti4O12、微观结构可调的层状钙钛矿Pb2Bi4Ti5O18和新型钛基催化剂CeTiO4等三种具有可见光吸收的材料。在熔盐合成中通过调控熔盐种类、成分、煅烧温度等实现了催化剂微观结构的调控；通过调整不同缺陷位点调节了光催化剂的能带结构，成功实现了带隙工程调控和载流子的强化分离。基于材料晶体结构、可见光吸收、可调的微观结构和缺陷效应分别选择抗生素、氮氧化物以及二氧化碳为探针分子进行光催化活性的评估，深入研究了光致电荷传输与分离效率及活性物种相关的反应机制和催化机理。在此基础上，基于材料模拟计算揭示了材料结构和性能关系。具体如下：1. 在一步熔盐合成中，通过改变熔盐成分制备了具有立方、多面体、纳米棒和八面体等不同微观结构和含氧空位的双钙钛矿结构的CaCu3Ti4O12材料，为钛基钙钛矿材料的设计、合成、形貌和氧空位调控提供了简单的方法。根据熔盐中阴阳离子在物理化学性质的不同，与材料表面的相互作用差异等因素初步推测形貌和缺陷产生的原因。借助密度泛函理论第一性原理 (DFT) 首次计算了结构中氧空位的形成能。基于强可见光吸收、特定的微观结构以及富含氧空位等特点，以抗生素为探针分子系统地评价了材料的催化性能。光催化降解结果表明，不同形貌的CaCu3Ti4O12在抗生素降解中表现明显的活性差异，其中八面体结构的样品性能最佳。循环光催化测试表明该材料不仅在单次光催化降解抗生素中表现出高效催化活性，还可以循环使用多次，依然表现高效的光催化活性，反映了该化合物在环境治理中潜在应用价值。结合光催化结果，利用密度泛函理论DFT计算分析并揭示催化剂高效且稳定的光催化性能的根本原因。结果表明，材料光催化活性的提高主要来自于富含的氧空位、表界面性质、电荷传输和各种活性物种引起的载流子强化分离作用。此外，用DFT计算了四环素和材料之间相互作用，对实验结果进行了很好的印证。这是关于双钙钛矿材料CaCu3Ti4O12在熔盐中微观结构和缺陷调控以及光催化性能的首次系统性研究。2. 利用熔盐合成的逐步离子化、成分可调等特性并借助晶体生长方法，在卤素钾盐 KX (X = F，Cl，Br) 中首次制备了含氧空位的、具有立方、八面体以及纳米颗粒等不同微观结构的CaCu3Ti4O12材料。基于所选择熔盐的物化性质，首次讨论了不同微观结构产生的机制、弱的相互作用对材料光学性质、缺陷的产生和缺陷浓度的调控以及表界面的影响。材料的强可见光吸收与钛晶格紊乱和氧空位相关。基于良好的可见光吸收，独特的表界面特点以及富含的氧空位，选择抗生素为目标污染物进行了可见光催化性能的评价。研究表明，微观结构不仅与缺陷浓度紧密相关，并且直接影响了材料的表面性质、催化活性、活性物种的产生以及反应动力学。基于微观结构和缺陷特性探索了催化剂缺陷依赖的高效催化活性起源，揭示了材料的构效关系。材料的高效可见光光催化活性主要来自于材料良好的微观结构、有效的载流子传输、增强的表界面性质，氧空位以及活性物种的协同作用。这是关于弱的相互作用对纳米材料微观结构和缺陷调控以及光催化性能影响的的首次系统性研究。3. 在一步、简单的熔盐合成中，仅靠熔盐成分的简单调节，实现了一个非中心对称的钛酸铋铅Pb2Bi4Ti5O18层状钙钛矿材料维度和光学性质的调控。基于独特的晶体结构、不同微观结构和强的可见光吸收优势，以氮氧化物 (NO) 为探针分子考察了材料的催化活性。结果表明，制备的催化剂对NO转化不仅具有比商业化P25高的光催化转换活性、最终变成NO3?，而且产生的中间产物NO2甚少。五次循环光催化结果和反应前后XRD测试表明材料在连续循环光催化中均表现高的活性和非常好的稳定性。基于光催化活性测试、微观结构、光学性质和不同寻常的晶体结构，讨论了上述因素对光催化活性的影响和相关反应机理。据我们所知，在环境治理中具有氮氧化物转化活性的含铅催化剂尚未报道，尤其是微观结构可调的可见光活性钙钛矿材料尚未应用，而这个工作是在气固反应中进行，因此可以避免铅带来的二次污染，是首次报道的第一例具有空气污染物处理活性的高效、稳定的含铅光催化材料。4. 借助晶体生长的可控条件，首次在熔盐合成中将具有可见光活性的Ce2O3半导体引入到紫外活性的明星光催化材料TiO2中，成功地合成了具有较强可见光吸收的CeTiO4催化剂。通过熔盐种类的改变实现了CeTiO4光催化剂微观结构的有效调控，获得了纳米短棒、多面体以及立方结构，同时实现了带隙工程的裁剪，拓展了催化剂对光的吸收范围。基于材料光学性质上的差异，首次揭示了CeTiO4材料微观结构和光吸收关联。以CO2为探针分子考察了材料的催化活性，结果表明所制备的催化剂具有明显的催化活性，优先形成选择性还原产物CO，甚至好于已报道的材料活性。在此基础上，详细地讨论了光催化机理。该部分研究中仅靠少许无机盐实现了具有不同微观结构的可见光活性材料的设计和合成，这样材料的光吸收比较灵活，催化活性不会受到通过金属掺杂的方法拓展光吸收范围带来的不良效果，同时赋予材料优良的微观结构和高的催化活性。
Solar light driven photolysis has been considered as a frontier research, the core of which is semiconductor materials. Many successful state-of-the-art catalysts have been developed for decades, though, the solar conversion efficiency of photocatalytic reactions are undesirable. The practical applications of catalysts are hindered by two major bottlenecks: infinite solar light absorption and dissatisfactory reaction yield due to inappropriate electronic band gap and rapid carrier recombination, which are closely associated with crystal structures and surface-interface properties of semiconductors. It is found that sufficient visible light absorption and enhanced charge carrier separation are the most promising ways to improve light harvesting efficiency, which is most desired, obscure and challenging topic. Therefore, it is utmost desirable and challenging task to establish deeper understanding of reaction mechanisms, effective tuning microstructures, charge transfer, and better to develop photocatalysts with high effectiveness as well as robust stability, so that to achieve enhanced solar conversion efficiency.(i) benefitted from the advantages of effective carrier separation induced by microstructure and crystal defects, (ii) based on density functional theory (DFT) theoretical investigation, this dissertation aims, mainly, to design, synthesis, microstructure regulation, defect tuning and exploring photocatalytic functionality of titanium-based visible light active perovskites as environmental catalysts. Double perovskite CaCu3Ti4O12 with oxygen vacancy and planar tetra-coordinated CuO4 cluster, layered structured perovskite Pb2Bi4Ti5O18 with tunable microstructures and novel Ti-based CeTiO4 with visible light absorption were synthesized. By varying salt compositions, reaction time, calcination temperature and molar ratio of salts during a molten salt synthesis, micro-structure regulation was achieved; by tuning defect concentrations and distributions, the ban gap engineering and effective charge carrier separation were realized. Based on the unique crystal structures, strong visible light responses, regulated microstructures and oxygen vacancies, their photocatalytic activity were evaluated by chosing an antibiotic, nitrogen oxide and carbon dioxide as probe molecules, respectively. The corresponding reaction mechanisms and structure property correlations were disclose with the conjunction of theoretical investigations. The research is highlighted with several aspects as following:1. A strategy towards morphology tailoring in conjunction with oxygen vacancy in the structure of double structured Ti-based perovskite CaCu3Ti4O12 was developed. By varying the salt composition during a molten salt synthesis, cube, polyhedron, nanorod and octahedron shaped crystal CaCu3Ti4O12 microstructures were obtained. Based on the salt compositions, surface-interface interactions of desired catalysts, formation mechanisms of microstructure and oxygen defects in as-obtained catalysts were speculated. With density functional theory (DFT) theoretical investigation, a formation energy of oxygen vacancy was calculated for the first time. Based on strong visible light responses, well-tuned microstructures and high abundant oxygen vacancies, an antibiotic, tetracycline was chosen as a model pollutant to investigate sample's visible-light photocatalytic capacity. The degradation results showed that as-obtained samples exhibited assorted photoactivity for tetracycline decomposition, in which excellent photooxidation performances were observed for octahedron and nanorod shaped CaCu3Ti4O12. Successive degradation tests demonstrated that as-obtained sample not only acquire high efficiency, robust stability during single run photocatalytic tests, it also display superior efficiency for long time stability, implying practical application in the field of environmental science. The improved photoefficiency could be attributed to high abundant oxygen vacancies, surface properties, charge transfer and enhanced carrier separation due to the synergetic roles of active species, as supported by active species trapping experiments and theoretical simulations. The enhanced efficiency was further elucidated by theoretical investigations of DFT calculations on the adsorption between tetracycline and CaCu3Ti4O12. This is the first systematic investigation about microstructure regulation, defect tuning and corresponding visible light driven photocatalytic performances of double structured perovskite catalyst CaCu3Ti4O12.2. Taking advantages of stepwise ionization, tunable compositions of molten salts and combined with the synthetic techniques of crystal growth process, a novel oxygen-deficiency contained perovskite CaCu3Ti4O12 with diverse microstructures of cube, nanoparticle and octahedron was synthesized by a feasible molten salt reaction approach depending on halogen anions of employed salts KX (X = F, Cl and Br) for the first time. On the basis of innate characters of the selected halide anions, growth mechanisms of the different microstructures were discussed, meanwhile, affects of ionic interactions on optical property, defect generation and vacancy concentration on the surface of as-obtained catalysts were revealed. Assorted shapes displayed strong visible light absorption, which was interconnected with Ti displacement and oxygen deficiency. On the bases of strong visible light absorption, surface-interface properties and oxygen deficiencies, photocatalytic performances of those three homologues were evaluated by an antibiotic degradation under visible light. Results demonstrated that microstructures of catalysts not only closely related to oxygen defect concentration, but directly determined the surface properties, formation of photocatalytically active species, decomposition efficiencies and reaction kinetics. The corresponding reaction mechanisms were disclosed. The enhanced photocatalytic performance was ascribed to the synergetic roles of well-defined microstructures, effective charge transfer, oxygen deficiencies and critical roles of active radicals, all which were the main determinants for antibiotic degradation with the assistance of assorted morphological CaCu3Ti4O12. This is the first investigation of weak interaction and halogen anion effects of employed salts on morphology tailoring and defect regulations of tunable structured inorganic nano-materials and its defect induced photocatalytic activity.3. Through one-step, facile molten salt synthetic approach, morphology tailoring and optical property regulation of non-centrosymmetrical Ti-based catalysts Pb2Bi4Ti5O18 were achieved simply by altering salt composition during the synthesis. Based on unique crystal structures, distinct microstructures and strong optical responses, the nitrogen oxide (NO) was selected as a probe molecule for photocatalytic activity evaluation. Results demonstrated that as-obtained samples not only displayed high efficiency on NO conversion, even better than commercially available famous star catalyst P25 (TiO2), to generate final photooxidation product NO3?, also produced trace amounts of intermediate NO2, which is more toxic. As-obtained samples exhibited high efficiency and outstanding robustness during five runs successive tests under visible light irradiation as reveled by cyclic photocatalytic experiments and XRD tests. Based on experimental results of assorted microstructures, optical properties and unique crystal structures of as-obtained samples, detailed reaction mechanisms were revealed. As we know, photocatalytic NO removal over Pb-based catalysts has not been reported, especially with controlled microstructures and strong visible light responses. This is the first example of Pb-based photocatalyst working in gaseous system so as to avoid the secondary contamination of Pb and acquire high efficiency in the field of environmental remediation.4. Taking advantages of single crystal growth process, visible light driven catalysts CeTiO4 were synthesized for the first time by introducing visible light sensitive Ce2O3 into UV active famous star catalyst TiO2 during the molten salt synthesis. By merely changing salt composition during the synthesis, morphology tailoring and band gap engineering were achieved, and diverse morphological CeTiO4 of nanorods, polyhedrons and cubic with enlarged visible light absorption ranges were obtained, respectively. Based on diverse optical properties of samples, a detailed correlation between light absorption and microstructures was discussed. CO2 was selected as a probe molecule for the activity evaluation. Results showed that as-obtained samples displayed exciting results in CO2 photoreduction by preferable formation of CO than CH4, even higher than previously reported results. On the bases of these experimental results, corresponding reaction mechanisms were elucidated. This study demonstrates a remarkably different construction principle and strategy for the rational design and fabrication of visible light driven catalysts with controlled microstructures merely depending on trace amounts of inorganic salts, by which light absorptions become more flexible and efficiencies are no longer suffered from undesired effects caused from metal doping process.
|热沙来提·海里里. 可见光活性钛基钙钛矿材料的设计、合成及其增强的光催化性能[D]. 北京. 中国科学院大学,2018.|
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