【作者】 王杨；

【导师】 刘博；

【作者基本信息】 北京交通大学 ， 光学， 2021， 博士

【摘要】 金属-有机框架材料（Metal-Organic Frameworks,MOFs）是基于金属阳离子与有机配体通过配位键形成的多孔聚合物,具有高比表面积、高孔隙率且结构可剪裁、功能可调节的优势,在诸多领域,如气体储存与分离、荧光传感、药物传送以及催化等得到了广泛应用。而MOFs的半导体性质自2007年首次报道后,其作为新型光催化剂也得到了快速发展,特别是在光降解染料、光还原二氧化碳、光解水制氢等领域展现出了诱人的前景。然而,与传统半导体光催化剂相比,MOFs光生电子-空穴复合效率仍相对较高,并导致其光活性较低。因此,探索合理、可行的方法来提高MOFs电子-空穴分离效率,增强其光催化活性成为目前的研究重点之一。本论文以Zr-MOFs为研究对象,从其能带位置,晶体结构以及基本光催化机理出发,深入研究其电子-空穴复合效率高等问题,从不同角度,提出了三种改善电子-空穴分离效率的结构优化方法,通过系统的光学表征和光催化活性评价等手段,证明了三种方法的可行性;并对相关机理进行了详细的解释与论证。主要研究内容及结果如下:（1）构建Zr-MOFs与半导体间的电子转移通道:选择无毒、低成本以及能带位置与Zr-MOFs匹配的半导体Ti O₂,合成并优化Zr-MOFs/Ti O₂异质结,获得了利于电子传递的结构。光降解染料评价结果表明,当Ti:Zr摩尔比为49时,与纯Zr-MOFs和Ti O₂相比,Zr-MOFs/Ti O₂（UT49）的降解效率提升最为明显,且四次循环后染料降解效率仍超80%。通过活性物种湮灭实验揭示了直接参与氧化染料分子的有机物的活性物种并非·OH,而是O₂^·-和h⁺。进一步研究还发现,该类异质结材料表现出了双功能性,如当Ti:Zr摩尔比为0.05时,Zr-MOFs/Ti O₂（UT0.05）除光催化能力增强外,在二氧化碳和甲烷气体分离方面也存在一定潜力。（2）构建Zr-MOFs多向电子转移通道:引入电子导体石墨烯,精准调控石墨烯与Zr-MOFs外表面的相互作用模式,利用静电作用使两者形成有效的包裹接触结构（RGOWU6N）,实现光电子可沿多个接触面进行多方向传输,提升Zr-MOFs自身的光生电子-空穴分离效率。结果显示,与原Zr-MOFs,石墨烯与Zr-MOFs物理混合（RCGO/U6N）以及石墨烯与Zr-MOFs进行单点或单面接触（RDGO/U6N）相比,包裹结构的形成可使Zr-MOFs荧光寿命明显提高,激子复合效率显著降低。在光解水制氢评价中,其产氢效率提高约2倍,且4次循环后效率仍保持稳定。（3）构建Zr-MOFs内外共修饰电子转移结构:在上述石墨烯包裹MOFs表面研究的基础上,进一步将助催化剂Pt封装于Zr-MOFs晶体内部,形成内外共修饰结构。结果表明,外修饰结构是通过石墨烯表面上C=O与Zr-MOFs有机配体的-NH₂作用形成,内修饰结构则由Zr-MOFs以Pt纳米颗粒为晶核进行成核生长形成。光学表征结果显示,内外共修饰结构实现光电子沿Zr-MOFs近表面进行多方向传输的同时,还可加快Zr-MOFs晶体体相内电子向助催化剂的转移速率。所表现出的协同作用有效改善了整个Zr-MOFs骨架内的电子-空穴分离效率。光催化产氢实验结果表明,内外共修饰结构与单纯引入石墨烯或铂相比,光解水制氢效率分别提升了21倍和32倍,且外修饰结构在一定程度上还可减少助催化剂的流失,并有利于保持材料的循环稳定性。更多还原

【Abstract】 Metal-organic frameworks（MOFs）are a kind of porous polymers composed by metal cations coordinating with organic ligands.They have been widely used in some fields such as gas storage and separation,fluorescence sensing,drug delivery and catalysis due to their characteristics of high specific surface area,porosity,structure tailoring and function tunable.Since the semiconductor-like behavior of MOFs was first reported in2007,MOFs as a new type of photocatalyst has been developed rapidly,especially in the fields of photodegradation of dyes,photoreduction of carbon dioxide,photolysis of water for hydrogen production and etc.However,compared with traditional semiconductor photocatalysts,the efficiency of photogenerated electron-holes recombination of MOFs is still relatively higher,which leads to the lower photocatalytic activity.Therefore,exploring rational strategy to improve the efficiency of electron-hole pairs separation and enhance the photocatalytic activity of MOFs has become one of the hot topics.Herein,with the understanding of energy band position,crystal structure and basic photocatalytic mechanism of Zr-MOFs,we proposed three structural optimization strategies to improve the efficiency of electron-hole separation for improving their photoactivities.The feasibility of the three methods was proved by systematic optical characterization and photocatalytic activity evaluation.Accordingly,the relevant mechanism was explained and demonstrated in detail.The main strategies and results are listed as follows:（1）Construction of electron transfer channels between Zr-MOFs and semiconductors:Non-toxic,low-cost semiconductor of Ti O₂with energy band position matching with Zr-MOFs was selected to synthesize and optimize Zr-MOFs/Ti O₂heterojunctions for falicitating electrons transfer.The photoactivity evaluation results of photodegradation dyes showed that when the molar ratio of Ti:Zr was 49,the degradation efficiency of Zr MOFs/Ti O₂（UT49）presented significant improvments compared with pure Zr-MOFs and Ti O₂,and the dye degradation efficiency still exceeded 80%after four cycles.The active species annihilation experiments revealed that O₂^·-and h⁺directly involved in the oxidation of dye molecules instead of·OH.Interestingly,it was also found that these heterojunction materials exhibited bifunctionality.For example,when the molar ratio of Ti:Zr is 0.05,Zr-MOFs/Ti O₂（UT0.05）not only showed enhanced photocatalytic ability,but also has a certain potential in the separation of carbon dioxide and methane.（2）Construction of multi-directional electrons transfer channels of Zr-MOFs:By introducing graphene,an electronic conductor,we precisely manipulated the interaction mode between graphene and the outer surface of Zr-MOFs to synthesize an effective wrapping structure（RGOWU6N）through electrostatic interaction method,realizing the multi-directional transportation of photoelectrons along the multiple contact surfaces,and improving the intrinsic photogenerated electron-hole separation efficiency of Zr-MOFs.The results showed that compared with the original Zr-MOFs,the physical mixing of graphene and Zr-MOFs（RDGO/U6N）and the single-spot or single-face contact（RCGO/U6N）of graphene and Zr-MOFs,the formation of the wrapping structure can significantly increase the fluorescence lifetime of Zr-MOFs and significantly reduce the excitons recombination efficiency.In the hydrogen production tests of water splitting,the hydrogen generation efficiency was increased by about 2times and maintained stable after 4 cycles.（3）Construction of inside-outside co-decoration structure of Zr-MOFs for electron transfer:On the basis of the above-mentioned surface interaction study of graphene coated MOFs,the cocatalyst of Pt was further encapsulated into Zr-MOFs crystals to form the internal and external comodified structure.The results showed that the outer modified structure was formed by the interaction of C=O of graphene oxide and-NH₂of Zr-MOFs,while the inner modified structure is formed by the nucleation growth of Zr-MOFs with Pt particles as the nucleus.The optical characterization results showed that the inner and outer comodified structure can realize the multi-directional transportation of photoelectrons along the surface of Zr-MOFs,and simultaneously accelerate the electron transfer rate from the inner of Zr-MOFs crystals to the Pt.The synergistic effect effectively improves the electron hole separation efficiency in the whole Zr-MOFs framework.The experimental results of photocatalytic hydrogen production showed that the efficiency of hydrogen production from water was increased by 21 times and 32 times respectively compared with the introduction of only graphene and Pt.Moreover,the external modification structure can reduce the loss of cocatalyst to a certain extent,which is conducive to maintaining the high stability of the materials.更多还原