Stability of Three-Hydrogen Clusters on Graphene
Stability of Three-Hydrogen Clusters on Graphene
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DOI：
10.1143/jpsj.78.035002
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发表时间：
2009-03
影响因子：
1.7
通讯作者：
T. Roman;H. Nakanishi;H. Kasai;Kunihiro Nobuhara;Tsuyoshi Sugimoto;Kyouichi Tange
中科院分区：
文献类型：
--
作者：
T. Roman;H. Nakanishi;H. Kasai;Kunihiro Nobuhara;Tsuyoshi Sugimoto;Kyouichi Tange
实现表面的高氢吸收率是固态材料实际储氢的重要方面之一。为了实现这一目标，从所涉及的物理机制来看，了解表面吸附饱和的道路是如何的，以及我们如何根据这些知识控制所需的反应，总是有益的。在这个主题上，已经开展了关于最简单的氢基团-对-在石墨/石墨烯上的行为的工作。该研究表明，氢对相互作用不能用吸附间分离的简单函数来描述，并且只有表面上的某些配对几何形状在能量上有利（我们注意到，没有发现底物原子的完全弛豫改变这些结论）。由于检测和区分单个吸附和小群吸附是了解如何达到饱和的重要步骤，我们随后展示了如何探测表面电子态来识别原子氢吸附，并将其与表面上紧密间隔的氢对区分开来，肯定了先前发表的关于这一主题的实验工作，特别是参考文献3。在本文中，我们评论了迈向饱和的下一步：在石墨烯上形成三个氢簇，即三氢。讨论了从石墨烯上吸附氢对得到的结果。利用VASP代码通过几何优化计算确定稳定的氢吸附构型，该代码实现了基于密度泛函理论的投影增强波方法的电子结构计算。所有的计算都是自旋极化的，并利用基于PBE版本的广义梯度近似的交换相关泛函。在不影响计算精度的情况下，我们采用了400 eV截止来限制平面波基集，并采用了4 4 1 Monkhorst-Pack特殊k点网格进行布里渊区采样。在48-C原子单片上的3个H原子组成了单元胞，其C-C最近邻距离为松弛前的1.42 a。在几何优化中，所有原子完全不受限制。A 15.0 A真空分离相邻的薄片。图1显示了本研究计算中包含的三个氢原子系统的不同簇。我们特别选择了14个最紧密排列的3个原子的组合，它们吸附在C原子的“顶部”位置。重构后氢原子的横向位置与图1中接收C原子时的位置相差不大。一个三元组是根据其最小的H对分量（o = ortho, m = meta, p = para）和第三个成员到对中心的距离来命名的。这意味着，例如，标记为to1的三重奏是由一对相邻吸附的（邻位）氢组成的三氢团簇，以及吸附在上述对最近距离上的第三个H原子。表1显示了按吸附能排列的三位一体，从最稳定的几何形状开始。表值使用以下表达式计算：Eads 1⁄4 Egr+3H（ads） ðEgr þ 3EH(g)Þ；Es 1 / 4 Eads=3；Eo 1 / 4 Egr+3H(ads) or Egr+2H（ortho） þ EH(g)Þ；Em 1⁄4 Egr+3H（ads） ðEgr+2H（meta） þ EH(g)Þ；和Ep 1 / 4 Egr+3H(ads) or Egr+2H（para） þ EH(g)Þ，其中Egr、EH(g)、Egr+3H（ads）和Egr+2H分别是石墨烯片、气相H原子、由吸附H三元组和石墨烯组成的体系以及由吸附H对和石墨烯组成的体系的总能量。孤立的H原子在石墨烯上的吸附能Eiso 1⁄4 Egr+H（ads） ðEgr þ EH(g)Þ为0:77 eV，由于这些研究中使用的吸附质覆盖范围不同，该值与先前的计算略有不同。由于同样的原因，本文报道的对吸附能和三人吸附能也与参考文献报道的相应值略有不同。2和8。吸附的三氢基团通常是稳定的：Eads（和Es）值都是负的，这意味着所有吸附的三氢基团相对于远离石墨烯表面的氢原子是稳定的。如果给定三元组的Es小于Eiso，则该三元组比由三个分离吸附在石墨烯上的H组成的体系更稳定。换句话说，在能量上，氢原子聚集在一起比在表面上彼此分离更有利，也就是说，净相互作用表现出“吸引”的特征。只有一个三重奏tm1是1比4的
Realizing high hydrogen uptakes on surfaces is one of the essential aspects of practical hydrogen storage in solid-state materials. To achieve this, it is always beneficial to know how the road to adsorption saturation on the surface looks like in terms of the physical mechanisms involved, and how we can control required reactions given this knowledge. On this topic, work has been carried out on how the simplest groups—pairs—of hydrogen behave on graphite/graphene. In that study it was shown that hydrogen pair interaction cannot be described by a simple function of interadsorbate separation, and that only certain pairing geometries on the surface are energetically favored (we note here that full relaxation of the substrate atoms was not found to change these conclusions). As detecting and discriminating singly adsorbed and small groups of adsorbates is an essential step to knowing how saturation can be reached, we have subsequently shown how probing surface electronic states can be used to identify an atomic hydrogen adsorbate, and distinguish it from the closely-spaced hydrogen pairs on the surface, affirming previously published experimental work on this subject, particularly that in ref. 3. In this paper we comment on the next step towards saturation: the formation of hydrogen clusters of three, i.e. hydrogen trios, on graphene. Results are discussed with respect to results obtained from hydrogen pairs adsorbed on graphene. Stable hydrogen adsorption configurations were determined through geometry optimization calculations using the VASP code, which implements the projector augmentedwave method for density functional theory-based electronic structure calculations. All calculations were spin-polarized, and utilized the exchange–correlation functional based on the PBE version of the generalized gradient approximation. We applied a 400 eV cutoff to limit the plane-wave basis set without compromising computational accuracy, and a 4 4 1 Monkhorst–Pack special k point grid for Brillouin zone sampling. Three H atoms on a 48-C atom single sheet comprise the unit cell, with C–C nearest-neighbor distances of 1.42 A before relaxation. All atoms were completely unrestricted in the geometry optimization. A 15.0 A vacuum separating adjacent sheets was used. Figure 1 shows the different clusters of three hydrogen atoms systems included in the computations of this study. We specifically choose the fourteen most closely-packed combinations of three atoms adsorbed on C atom ‘‘top’’ sites. Upon reconstruction hydrogen atom lateral positions generally don’t deviate much from the positions on receiving C atoms shown in Fig. 1. A trio is named based on its smallest H pairing component (o = ortho, m = meta, p = para) and distance of the third member of the trio from the pair center. This means, for example, that the trio labeled to1 is the three-hydrogen cluster comprised of a pair of adjacently adsorbed (ortho) hydrogen, and a third H atom adsorbed in the closest possible distance from the aforementioned pair. Table I shows the trios arranged by adsorption energy, starting with the most stable geometry. Table values were computed using the following expressions: Eads 1⁄4 Egr+3H(ads) ðEgr þ 3EH(g)Þ; Es 1⁄4 Eads=3; Eo 1⁄4 Egr+3H(ads) ðEgr+2H(ortho) þ EH(g)Þ; Em 1⁄4 Egr+3H(ads) ðEgr+2H(meta) þ EH(g)Þ; and Ep 1⁄4 Egr+3H(ads) ðEgr+2H(para) þ EH(g)Þ, where the terms Egr, EH(g), Egr+3H(ads), and Egr+2H are the total energies for the graphene sheet, a gas phase H atom, the system comprised of an adsorbed H trio and graphene, and the system comprised of an adsorbed H pair and graphene, respectively. The adsorption energy of an isolated H atom on graphene Eiso 1⁄4 Egr+H(ads) ðEgr þ EH(g)Þ is 0:77 eV, a value which differs slightly from previous calculations due to the different adsorbate coverage used in these studies. For the same reason pair and trio adsorption energies reported here also differ slightly from corresponding values reported in refs. 2 and 8. Adsorbed trios are generally stable: the Eads (and Es) values are all negative, meaning all adsorbed three-hydrogen groups are stable with respect to hydrogen atoms located far from the graphene surface. If Es for a given trio is less than Eiso, the trio is more stable compared with a system comprised of three isolated adsorbed H on graphene. In other words it would be more energetically favorable for the hydrogen atoms to clump together than to separate from each other on the surface, i.e., the net interaction shows an ‘attractive’ character. Only one trio— tm1— is found to1 to4