Hydrogen adsorption on boron doped graphene: an ab initio study

Abstract

(i) The electronic and structural properties of boron doped graphene sheets, and (ii) thechemisorption processes of hydrogen adatoms on the boron doped graphene sheets havebeen examined by ab initio total energy calculations. In (i) we find that the structuraldeformations are very localized around the boron substitutional sites, and in accordancewith previous studies (Endo et al 2001 J. Appl. Phys. 90 5670) there is an increase of theelectronic density of states near the Fermi level. Our simulated scanning tunnelingmicroscope (STM) images, for occupied states, indicate the formation of bright (triangular)spots lying on the substitutional boron (center) and nearest-neighbor carbon (edge) sites.Those STM images are attributed to the increase of the density of states withinan energy interval of 0.5 eV below the Fermi level. For a boron concentration of∼2.4%, we find that two boron atoms lying on the opposite sites of the same hexagonal ring(B1–B2 configuration) represents the energetically most stable configuration, which is incontrast with previous theoretical findings. Having determined the energeticallymost stable configuration for substitutional boron atoms on graphene sheets,we next considered the hydrogen adsorption process as a function of the boronconcentration, (ii). Our calculated binding energies indicate that the C–Hbonds are strengthened near boron substitutional sites. Indeed, the bindingenergy of hydrogen adatoms forming a dimer-like structure on the boron dopedB1–B2 graphene sheet is higher than the binding energy of an isolatedH2 molecule. Since the formation of the H dimer-like structure may represent the initial stageof the hydrogen clustering process on graphene sheets, we can infer that the formation of Hclusters is quite likely not only on clean graphene sheets, which is in consonancewith previous studies (Hornekær et al 2006 Phys. Rev. Lett. 97 186102), but also onB1–B2 boron doped graphene sheets. However, for a low concentration of boron atoms, theformation of H dimer structures is not expected to occur near a single substitutional boronsite. That is, the formation (or not) of H clusters on graphene sheets can be tuned by theconcentration of substitutional boron atoms.