硼化锂低维结构与储氢性能预测

发布时间：2018-04-19 18:06

本文选题：纳米材料 + 密度泛函理论　；参考：《河北师范大学》2015年博士论文

【摘要】：硼元素与碳元素非常类似,它也具有丰富的低维构形,包括二维平面、一维纳米管和零维笼状结构。这些结构通过某些金属修饰后,具有良好的储氢性能,可以在常温常压下实现氢的存储,而备受关注。另外,硼元素由于具有良好的化学活性,可以同所有的金属化合形成形态各异的化合物。本论文,我们通过不同的方法预言了一系列的硼化锂二维结构。基于密度泛函理论计算,研究了这些结构的电学特性和储氢性能。主要内容如下:(1)基于一个稳定的结构单元构建了两种稳定的、构形类似的准二维平面结构。由于其化学当量为Li2B5,因此将其命名为Li2B5-I和Li2B5-II。将Li2B5-II沿着两个垂直的方向卷曲,可以得到直径在4.48-19.59?之间的一系列稳定的纳米管。和平面结构相比,管状结构具有更高的热力学稳定性。随着纳米管直径的增加,结构的平均原子能先减少后增加,最终与平面结构的平均原子能趋近。对平面结构和几个典型的管状结构储氢性能进行了分析。结果显示,每个Li原子周围最多可以吸附两个H2分子,储氢质量密度达10.6wt%。由于Li原子与B原子之间通过离子键结合,离子化的Li原子周围分布较强的电场,对H2分子具有较强的极化作用,从而实现对H2分子的吸附。同时,H2分子与Li之间也有轨道杂化,并且B原子也参与了与H2分子间的相互作用。(2)基于粒子群优化算法的晶体结构分析,对化学当量为Li2B5二维构形进行广泛搜索,得到了两个能量最低的异构体。这两种结构的平均原子能相差只有0.0007e V,在计算允许的误差范围内。这两种准二维平面结构,一种B原子框架为平面命名为Planar-sheet,另外一种B原子框架为有起伏的准平面命名为Buckled-sheet。两种结构都呈现出金属性。储氢性能计算显示,Buckled-sheet中每个Li原子周围以分子形式吸附3个H2,平均吸附能为0.19e V,储氢质量密度为15.1wt%。差分电荷密度和Bader电荷分析表明,储氢作用主要来源于极化作用和轨道杂化作用。计算结果显示,该结构有望成为高性能的储氢材料。(3)预测了三种Mg B2的金属夹层硼化锂结构(分别命名为MS-1、MS-2和MS-3),并对这三种构形的结构稳定性进行了研究。三种构形中的B平面均呈现出规则的几何构形,一种为已知的β-sheet和一种新的类B2C-sheet。形成能、Bader电荷分析和差分电荷密度均显示,当金属Li原子位于B平面的六角环结构的中心顶端位置时,结构更为稳定。这同六角环结构的中B原子的电子缺失有关。基于最稳定构形(MS-1),对大量金属硼化物进行了结构预测,发现四价金属的夹层结构较为稳定,而最为稳定的结构是硼化铁。
[Abstract]:Boron element is very similar to carbon element, it also has a rich low-dimensional configuration, including two-dimensional plane, one-dimensional nanotubes and zero-dimensional cage structure.After some metal modification, these structures have good hydrogen storage performance, which can realize hydrogen storage at room temperature and atmospheric pressure.In addition, because of its good chemical activity, boron can be combined with all metals to form compounds of different shapes.In this paper, we predict a series of two-dimensional structures of lithium boride by different methods.Based on density functional theory (DFT), the electrical properties and hydrogen storage properties of these structures are studied.The main contents are as follows: (1) two stable quasi-two-dimensional planar structures with similar configuration are constructed based on a stable structural unit.Because its chemical equivalent is Li _ 2B _ 5, it is named as Li2B5-I and Li _ 2B _ 5-II.By curling Li2B5-II in two vertical directions, the diameter of Li2B5-II is 4.48-19.59?Between a series of stable nanotubes.Compared with planar structure, tubular structure has higher thermodynamic stability.With the increase of the diameter of nanotubes, the average atomic energy of the structure first decreases and then increases, and finally approaches the average atomic energy of the planar structure.The hydrogen storage properties of planar structure and several typical tubular structures are analyzed.The results show that at most two H2 molecules can be adsorbed around each Li atom and the mass density of hydrogen storage reaches 10.6 wts.Due to the ionic bonding between Li atom and B atom, the electric field around the ionized Li atom has a strong polarization effect on H _ 2 molecule, thus realizing the adsorption of H _ 2 molecule.At the same time, there is orbital hybridization between H _ 2 molecule and Li, and B atom is also involved in the interaction with H _ 2 molecule.Two isomers with the lowest energy were obtained.The average atomic energy difference between the two structures is only 0.0007e V, within the allowable error range of calculation.The two quasi-two-dimensional planar structures, one of which is named Planar-sheetplane, and the other is Buckled-sheet.The other is the undulating quasi-plane.Both structures exhibit a gold attribute.The calculation of hydrogen storage performance shows that three H _ 2 are adsorbed in molecular form around each Li atom in Buckled-sheet, the average adsorption energy is 0.19 EV, and the mass density of hydrogen storage is 15.1wt.Differential charge density and Bader charge analysis show that hydrogen storage mainly comes from polarization and orbital hybridization.The calculated results show that the structure is expected to be a high performance hydrogen storage material. It can predict the structure stability of three kinds of mg B2 intercalated lithium boride structures (named MS-1MS-2 and MS-3N respectively).The B-plane of the three configurations presents a regular geometric configuration, one is a known 尾 -sheet and the other is a new B2C-sheet-like configuration.The formation energy Bader charge analysis and differential charge density show that the metal Li atom is more stable when it is located at the top of the center of the hexagonal ring structure in the B plane.This is related to the electron deletion of the B atom in the hexagonal ring structure.Based on the structure prediction of a large number of metal boride based on the most stable configuration, it is found that the interlayer structure of tetravalent metal is relatively stable, and the most stable structure is iron boride.
【学位授予单位】：河北师范大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ131.11

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