DFT and TB study of the geometry of hydrogen adsorbed on graphynes

Using density-functional calculations (DFT) and a tight-binding model, we investigate the origin of distinct favorable geometries which depend on the type of graphyne used. The change in the H geometry is described in terms of the tuning of the hopping between sp(2)-bonded C atoms and sp-bonded C atoms hybridized with the H atoms. We find that the different preferred geometry for each type of graphyne is associated with the electronic effects due to different symmetries rather than a steric effect minimizing the repulsive interaction between the H atoms. The band gaps are significantly tuned as the hopping varies, except in alpha-graphyne, in agreement with the result of our previous DFT study (Koo J et al 2013 J. Phys. Chem. C 117 11960). Our model can be used to describe the geometry and electronic properties of hydrogenated graphynes

Exotic Geometrical and Electronic Properties in Hydrogenated Graphyne

On the basis of first-principles calculations, we present exotic geometrical and electronic properties in hydrogenated graphyne, a 2D material of sp-sp2 hybrid carbon networks. Hydrogen atoms adsorbed onto sp-bonded carbon atoms can form both sp2-and sp3- hybridized bonds and can exist in three different geometries: in-plane, out-of-plane, and oblique-plane; this is in sharp contrast to hydrogenated graphene, which has only one hydrogenation geometry. The band gaps of hydrogenated graphyne can vary by ???3 eV as the geometry changes. We also find that change in the hydrogen concentration allows a large band-gap tuning of ???5 eV. Unlike hydrogenated graphene, in which H atoms show a tendency to cluster, H atoms tend to be dispersed in graphyne, making band-gap tuning feasible. These exotic properties in hydrogenated graphyne indicate that the band gap of hydrogenated graphyne can be tailored for new device applications. Furthermore, the composite of fully hydrogenated graphyne is C1H 1.75, which has a hydrogen-to-carbon ratio greater than that of graphane (C1H1). This large hydrogen capacity (???13 wt % H) suggests that graphyne also can be used as a high-capacity hydrogen storage material.close7

Tailoring the electronic band gap of graphyne

We report a first-principles study on tuning the electronic band gap of graphyne, consisting of two-dimensional sp-sp2 hybrid carbon atoms, by chemical functionalization. Halogen atoms form a sp2 hybridization with sp-bonded carbon atoms. This is in sharp contrast to the adsorption of halogen atoms onto graphene: fluorine atoms on graphene form sp3 bonds, while chlorine, bromine, and iodine atoms do not form any bond to graphene. The band gaps of graphyne increase by ???3 eV as the halogen concentration varies, comparable to the ???3.4 and ???2.7 eV engineered band gaps of graphene by hydrogenation and fluorination, respectively. We also find that the mixture adsorption of hydrogen and halogen atoms is favorable compared with the segregation of the hydrogen-attached phase and the halogen-attached one and that the band gaps are tunable by ???1.5 eV as the hydrogen-halogen concentration varies. We also consider sp3 hybrid bonds by halogenation to sp-bonded carbon atoms.close

Multilayer Graphynes for Lithium Ion Battery Anode

Graphynes, two-dimensional layers of sp- and sp(2)-bonded carbon atoms, have recently received considerable attention because of their potential as new Dirac materials. Here, focusing on their large surface area, we explore the applicability of graphynes as lithium ion battery anodes through the first-principles density functional calculations. We have found that Li potential energies are in the range suitable to be used as anodes. Furthermore, the maximum composite of Li-intercalated multilayer alpha- and gamma-graphynes is found to be C(6)Li3, which corresponds to a specific capacity of 1117 mAh g(-1), twice as large as the previous theoretical prediction for graphynes. The volumetric capacity of Li-intercalated multilayer alpha- and gamma-graphynes is 1364 and 1589 mAh cm(-3), respectively. Both specific and volumetric capacities of Li-intercalated graphynes are significantly larger than the corresponding value of graphite, from which we conclude that multilayer graphynes can serve as high-capacity lithium ion battery anodes.close1

Graphene-templated directional growth of an inorganic nanowire

Assembling inorganic nanomaterials on graphene1-3 is of interest in the development of nanodevices and nanocomposite materials, and the ability to align such inorganic nanomaterials on the graphene surface is expected to lead to improved functionalities4, as has previously been demonstrated with organic nanomaterials epitaxially aligned on graphitic surfaces5-10. However, because graphene is chemically inert, it is difficult to precisely assemble inorganic nanomaterials on pristine graphene2,11-16. Previous techniques2,3 based on dangling bonds of damaged graphene11,17-20, intermediate seed materials11,15,16,21,22 and vapour-phase deposition at high temperature12-14,23-25 have only formed randomly oriented or poorly aligned inorganic nanostructures. Here, we show that inorganic nanowires of gold(I) cyanide can grow directly on pristine graphene, aligning themselves with the zigzag lattice directions of the graphene. The nanowires are synthesized through a self-organized growth process in aqueous solution at room temperature, which indicates that the inorganic material spontaneously binds to the pristine graphene surface. First-principles calculations suggest that this assembly originates from lattice matching and ?? interaction to gold atoms. Using the synthesized nanowires as templates, we also fabricate nanostructures with controlled crystal orientations such as graphene nanoribbons with zigzag-edged directions.close1