| Boron similar to carbon can exhibit multiple forms of low-dimensional allotropic structure, such as quasi-2D sheets, quasi-1D nanotubes, or quasi-0D cage molecules. Metal-decorated boron nanomaterials have attracted broad attention as potential storage media at ambient temperature. In addition, boron can compound with all metals due to high chemical activity. In this paper, we predict a series of lithium boride sheets according to different methods. Based on the framework of the first-principles density-functional theory, we investigated the electronic properties, the hydrogen adsorption and storage mechanism for these nanostructures. Following are the main contents:(1) We created two quasi-2D lithium boride sheets from a stable unit with Li2B5 stoichiometry. These two sheets have similar geometries which are named as Li2B5-I and Li2B5-II respectively. By rolling up the Li2B5-II sheet along different axes, we can obtain a series of nanotubes, varying in diameter in the range 4.48-19.59?. The calculation results show that all these nanotubes are more stable than the original sheet and that the cohesive energies per atom for the two nanotube types first decrease with the diameter, and then as the diameter further increases, begin to drop and approach that of the sheet.The application of both the sheets and nanotubes to hydrogen storage has been investigated and it has been found that both of them can adsorb two H2 molecules around each Li atom with an average binding energy of 0.152-0.194 e V/H2, leading to a gravimetric density of 10.6 wt%. The enhanced electrostatic field around the Li atom originating from the charge transfers from Li to B frameworks increases the polarization of H2 molecules and accounts for the high capacity. In addition, the binding of the attached H2 molecules comes from not only the polarization mechanism but also from orbital hybridization, and all the substrate material including the B atoms takes part in the hydrogen adsorption.(2)Using the particle swarm optimization(PSO) algorithm combined with first-principles methods, we found two new stable quasi-two-dimensional structures with almost the same total energies(just 0.0007 e V/atom difference within the margin of error). We refer to one structure as the buckled sheet due to the fact that the geometry of the boron framework without lithium atoms is buckled and the other as a planar sheet because the boron framework is flat. Calculated electronic band structures and densities of states indicate that the sheets are metallic.The hydrogen adsorption properties were investigated and it was found that the buckled sheet can adsorb three H2 molecules around each Li atom, leading to the gravimetric density of stored hydrogen as high as 15.1wt% with the average adsorption energy lying in the range 0.1-0.2e V/H2. The Bader analysis and the partial density of states show that both hybridization and polarization contribute to the hydrogen bonding.(3) Three meta-sandwich(MS) crystal structures of lithium boride(named MS-1, MS-2, and MS-3) were predicted using the first principle method. All B planes in the three configurations are showing regular geometric configurations, which are known as β-sheet and B2C-sheet. The calculated formation energy, Bader charge analysis and the deformation electron density showed that, it will be more stable when Li atoms on top of the center of the B hexagonal ring structure. This phenomenon may be related to the electron deficiency of the hexagonal ring. We calculate the formation energy of a large library of alkali, alkaline and transiton metals in the MS-1 configuration. The results show that it is more stable for tetra-valent metal borides and Fe borides is the most stable configure. |
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