掺杂对金属有机框架材料吸附性能及石墨烯电子结构影响的第一性原理研究

发布时间：2018-03-26 01:47

本文选题：金属有机框架(MOFs)　切入点：吸附　出处：《辽宁大学》2017年硕士论文

【摘要】：金属-有机框架材料(Metal-Organic Frameworks,MOFs)是由无机金属离子或团簇与有机连接体自组装形成的、一类具有周期性拓扑结构的多孔晶体材料。由于MOFs材料具有较大的比表面积和可调控的三维孔洞结构,使得其在气体吸附和分离、储氢、催化、传感器及药物释放等诸多领域存在广泛的应用前景。近年来,越来越多的科研组开始关注对MOFs的金属位点及其有机连接体进行的掺杂或改性,通过掺杂或改性不仅可以改变MOFs的电荷分布,还可以提高其对气体的吸附能力等。同时,掺杂或改性也是在MOFs中引入新官能团的重要手段。石墨烯作为新型二维碳基纳米材料,所呈现出的独特性质使其成为凝聚态物理和材料科学领域炙手可热的明星。尤其是在电化学储能应用,石墨烯表现出显著优于其他储能材料的电化学特性,因此,系统研究电化学掺杂对石墨烯电子结构的影响,对于进一步优化和提高其电化学储能特性十分重要。本文采用基于密度泛函理论的第一性原理计算,系统研究了不同结构改性对CO_2在MOF-74中的吸附行为和吸附强度的影响,以及电化学掺杂对单层石墨烯原子结构、晶格振动和电子结构的影响。通过对Co-MOF-74的金属位点进行碱土金属掺杂改性,研究了具有混合金属位点的MOF-74的电子结构、CO_2吸附性能、活化催化CO_2的作用机理。同时,通过对单层石墨烯进行电化学掺杂模拟,研究了不同浓度下电子和空穴掺杂的石墨烯的平衡碳-碳键长、电子结构、电荷转移、振动频率及能隙的改变。具体研究内容包括:(1)利用VASP方法研究了1-6个Mg原子对Co-MOF-74进行金属位点掺杂而得到具有混合金属位点的Mg-Co-MOF-74对CO_2的吸附影响,通过对吸附能及弱相互作用的比较发现,具有混合金属位点的功能化Mg-Co-MOF-74对CO_2的吸附能力显著加强,这表明对金属位点的改性可以提高其对气体的吸附作用,且发现吸附作用的主要贡献源自范德华力。其中掺杂一个Mg和两个Mg原子的Mg-Co-MOF-74对CO_2的吸附能较大,而其余五个体系的吸附能则呈现这样的趋势:6Mg5Mg4Mg3Mg6Co。我们的这一计算结果与Yang Jiao课题组的相关实验结果一致。表明我们的计算方法和计算结果合理、可靠。通过Bader charge analysis(BCA)和Electron density difference(EDD)分析,我们发现吸附作用显著提高的主要贡献是源自Mg金属位点及CO_2中靠近Mg位点的O1原子,并发现正电荷积累在Mg原子周围,负电荷则富集在O1原子周围。此外,我们还系统研究了Mg-Co-MOF-74对CO_2的催化作用。计算发现,CO_2的分子构型发生弯曲,不再呈现为线性构型,O-C-O键角小于180°,介于177.53°到177.79°之间,且C=O键长也不再对称(由于受到Mg的吸引,使得靠近金属原子Mg位点的C=O键增长。基于上述理论计算,我们发现采用碱土金属Mg掺杂或改性,可有效提高MOFs材料对气体分子的吸附强度,并可为基于类似的功能化处理提高MOFs材料对气体的吸附作用提供理论依据。(2)基于DFT方法,我们首先研究了不同类型(n型和p型)掺杂的石墨烯的原子结构和电子结构。选取包含两个碳原子的原包作为石墨烯的结构模型,采用改变原包中总的电子数方式来模拟石墨烯的电化学掺杂(如增加总电子数为n型掺杂,反之为p型掺杂),在电子掺杂和空穴掺杂两个情况下,分别考虑了十个不同掺杂浓度,从而系统研究电化学掺杂对石墨烯C-C键长、费米能级及晶格振动频率的影响。计算结果发现,n型掺杂使得C-C键长逐渐增长,费米能级也逐渐往高能方向移动,且振动频率呈现减小的趋势。对于p型掺杂,情况则较为复杂,随着掺杂浓度的增加,C-C键长先减小后增大,费米能级则单调地向低能方向移动,而振动频率则与C-C键长呈现相反的趋势,出现先增大后减小的现象。分析原因,我们认为这主要是由掺杂浓度增加引起库伦排斥作用和库伦吸引作用之间的重新平衡所致。其次,我们进一步研究了碱金属、碱土金属及三氯化铁掺杂的单层石墨烯,对于Li和Ca原子,我们选取包含6个碳原子的单胞,即C_6X。对于K原子,我们选取的掺杂构型为一个K原子对应8个碳原子,即C_8K。结构弛豫后,我们发现掺杂原子Li、Ca和K均稳定吸附在六元环的中空位点。由于碱金属和碱土金属均属于n型掺杂,因此,我们也对上述性质进行了相应的研究。其中,电荷转移规律为:C_8KC_6CaC_6Li。最重要的是,由于破坏了子晶格的对称性,这三个体系均打开了带隙,使得石墨烯由半金属性转变为半导体特性。对于FeCl_3掺杂的石墨烯,选取的晶胞分别为5×5的石墨烯和2×2的三氯化铁,这使得二者的晶格失配度约为0.7%。不同于三氯化铁插层的双层石墨烯,对于三氯化铁掺杂的单层石墨烯来说,由于对称性未改变,因此并没有打开带隙,从而保留了其半金属特性。
[Abstract]:Metal organic frameworks (Metal-Organic Frameworks MOFs) is composed of inorganic metal ions or clusters and organic linkers self-assembly, a kind of porous crystalline material with periodic structure. The MOFs material has larger surface area and adjustable three-dimensional pore structure, makes the gas adsorption and separation, hydrogen storage, catalysis, has wide application prospect in many fields such as sensors and drug release. In recent years, more and more research group began to focus on metal sites on MOFs and organic linkers were doped or modified by doping or modification of the charge distribution can not only change the MOFs, but also can improve its adsorption capacity the gas. At the same time, an important means of doping or modification is the introduction of new functional groups in MOFs. As a new type of two-dimensional graphene based on carbon nano materials, showing the unique nature of the Hot condensed state physics and material science in the field of stars. Especially can be used in the electrochemical performance of graphene, electrochemical properties, significantly better than other energy storage materials. Therefore, systematic research on influence of electrochemical doping on the electronic structure of graphene, to further optimize and improve the electrochemical energy storage characteristics is very important. This paper uses calculation the first principle based on the density functional theory, modification of different structure influence on the adsorption behavior and adsorption strength in MOF-74 CO_2 were studied, and the electrochemical doping of graphene atomic structure, effects of vibration and electronic structure. Modified by alkaline earth metal doped by metal sites on the Co-MOF-74, electronic structure study with mixed metal sites of MOF-74, CO_2 adsorption, mechanism of catalytic activation of CO_2. At the same time, through the power of graphene Study on the simulation of chemical doping, carbon balance of electron and hole doped graphene under different concentrations of carbon bond length, electron structure, charge transfer, vibration frequency and energy gap change. The research contents include: (1) of 1-6 Mg atoms were doped in Co-MOF-74 and metal sites with mixed adsorption effect the metal site of Mg-Co-MOF-74 on CO_2 by using VASP method, the adsorption energy and weak interaction shows that adsorption capacity of functionalized Mg-Co-MOF-74 with mixed metal sites of CO_2 significantly enhanced, which indicates that the metal sites of modification can improve the adsorption of gas, and found that the main contribution of adsorption from van Edward. The adsorption of a Mg doped and two Mg atoms of Mg-Co-MOF-74 CO_2 can be larger, and the remaining five adsorption energy of the system is showing this trend: 6Mg5Mg4Mg3Mg6Co. I The relevant experimental results the calculation results are consistent with the Yang Jiao task group. Our calculations show that the method and the calculation results are reasonable and reliable. The Bader charge analysis (BCA) and Electron density difference (EDD) analysis, we found that the main contribution of adsorption increased from Mg metal sites and CO_ 2 near Mg the sites of O1 atoms, and found that the positive charge accumulation around the Mg atom, the negative charge is enriched in around O1 atoms. In addition, we also studied the catalytic effect of Mg-Co-MOF-74 on CO_2. The calculation shows that the molecular configuration of CO_2 bending, no longer appear as linear configuration, O-C-O bond angle less than 180 degrees, between 177.53 degrees at 177.79 degrees, and the C=O bond length is no longer symmetric (due to attraction, by Mg the C=O bond near the metal atom site of Mg growth. Based on the above theoretical calculation, we find that the alkaline earth metal doped Mg Or modification, can effectively improve the adsorption strength of MOFs material of gas molecules, and similar functional treatment and provide a theoretical basis to improve the adsorption effect on gas based on MOFs material. (2) based on the DFT method, we first studied the different types (N and P) atomic structure and electronic structure of graphite graphene doped. Selected contains two carbon atoms in the original package as a structural model of graphene, graphene using electrochemical doping to simulate the change of electron number means the total in the original package (such as increasing the total number of electrons for n doped, and P doped), in electron doping and hole doping two a case, considering ten different doping concentration, so the system study on electrochemical doping on the graphene C-C bond length, the Fermi level and the effect of lattice vibration frequency. The results show that n doped C-C bond length gradually increased, the Fermi level is gradually to high Can the direction of movement, and the vibration frequency decreases. For P doping, the situation is more complicated, with the increase of doping concentration, the bond length of C-C decreased firstly and then increased, the Fermi level is monotonically shifts to lower energy, and the vibration frequency and the bond length of C-C showed the opposite trend, after the first increases the phenomenon of the decrease. To analyze the reasons, we believe that this is mainly caused by the re balance caused by Kulun and Kulun attraction repulsion between the doping concentration increasing. Secondly, we further study the alkali metals, alkaline earth metals and graphene doped ferric chloride, for Li and Ca atoms, we selected contains 6 carbon atoms in a single the cell, namely C_6X. for K atom doping configuration chosen as the one of the K atoms corresponding to 8 carbon atoms, namely C_8K. structural relaxation, we found that doping Li, Ca and K were stable adsorption sites in the hollow six membered ring Because of alkali metals and alkaline earth metals belong to the N type doping, therefore, we are of the nature of the corresponding study. Among them, the charge transfer rule is: C_8KC_6CaC_6Li. is the most important, because the symmetry of the sub lattice, these three systems are opened the gap, make the transition from graphene semi metal semiconductor properties. For graphene doped FeCl_3 crystal were selected, graphene 5 x 5 and 2 x 2 ferric chloride, which makes the two lattice mismatch is about 0.7%. different bilayer graphene in FeCl3 intercalation, the graphene doped iron chloride three due to symmetry, has not changed, and therefore did not open the gap, which retains its half metallic properties.

【学位授予单位】：辽宁大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O647.3;O613.71

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