Domain formation on oxidized graphene

Using first-principles calculations within density functional theory, we demonstrate that the adsorption of a single oxygen atom results in significant electron transfer from graphene to oxygen. This strongly disturbs the charge landscape of the C-C bonds at the proximity. Additional oxygen atoms adsorbing to graphene prefer always the C-C bonds having the highest charge density and, consequently, they have the tendency to form domain structure. While oxygen adsorption to one side of graphene ends with significant buckling, the adsorption to both sides with similar domain pattern is favored. The binding energy displays an oscillatory variation and the band gap widens with increasing oxygen coverage. While a single oxygen atom migrates over the C-C bonds on the graphene surface, a repulsive interaction prevents two oxygen adatoms from forming an oxygen molecule. Our first-principles study together with finite-temperature ab initio molecular dynamics calculations conclude that oxygen adatoms on graphene can not desorb easily without the influence of external agents.

Figure 1

Various critical sites of adsorption on the 2D honeycomb structure of graphene and an oxygen atom adsorbed on the bridge site, which is found to be the energetically most favorable site. Carbon and oxygen atoms are shown by gray and red balls, respectively. (b) Variation of energy of oxygen adatom adsorbed to graphene along $H\to T\to B$ directions of the hexagon. The diffusion path of a single oxygen adatom is shown by stars. (c) Charge density isosurfaces, isovalues, and contour plots of oxygen-adsorbed graphene in a plane passing through C-O-C atoms. (d) Same as (c) on the lateral plane of honeycomb structure. (e) Total and partial density of states projected to carbon and oxygen atoms. Calculations are carried out for the supercell presented in (c) where O-O interaction is significantly small.

Figure 2

(a) Charge density isosurfaces in a rectangular supercell containing 128 carbon atoms and a single adsorbed O atom shown by a red dot. $B_{i,j}$ identifies a specific C-C bond, where $i$ indicates the total number of oxygen atoms in the supercell and $j$ labels some of the bonds around adsorbed oxygen atom(s). (b), (c), and (d) are the same as (a), except that 2, 3, and 4 oxygen atoms are adsorbed to the sites, which are most favorable energetically. (e) Energetically favorable configurations up to 11 oxygen atoms adsorbed on graphene deduced from charge. (f) Energetically favorable configuration (left) and less energetic, ordered configuration (right) for 12 O atoms. (g) Variation of the binding energies of the last oxygen adatom up to 12.

Figure 3

Adsorption of two oxygen atoms on one surface of graphene with buckling of 0.88 Å. (b) Adsorption of two oxygen atoms at both sides with a buckling of 0.25 Å. (c) Isosurfaces of bond charge densities after the adsorption of two oxygen atoms, each one adsorbed to different sides of graphene. Some of the C-C bonds of graphene, which are deprived of regular charges upon oxidation, are highlighted with arrows.

Figure 4

(a) The bonding configuration of a single carbon atom on graphene and the resulting redistribution of bond charges shown by isosurfaces. (b) The bonding configuration of a single fluorine adatom on graphene with energetically favorable top site. The resulting charges of C-C bonds at close proximity are shown by isosurfaces. (c) The growth pattern in the course of the fluorination of graphene.

Figure 5

(a) The interaction energy between two free oxygen atoms approaching each other from a distance. The distance between them is $d_{\mathrm{O}-\mathrm{O}}$. (b) The interaction energy between a single oxygen atom adsorbed at the bridge site and a free oxygen atom approaching from the top. Different positions of approaching O atom are shown in the side views. (c) Variation of the energy between two oxygen adatoms on graphene. Some of the positions of the approaching oxygen atom on the path of minimum energy barrier are labeled by numerals (I–VII). Top and side views of the configuration of two oxygen atoms are shown by insets.

Figure 6

(a) Bare graphene and its typical density of states with zero-state density at the Fermi level $E_F$. The constant energy surfaces of conduction and valence bands are shown on the left-hand side. (b) A four-adatom domain corresponding to lowest total energy configuration and has a band gap of 70 meV. (c) Another adsorption configuration of four oxygen adatoms resulting in a band gap of 127 meV. (d) A uniform and symmetric configuration of adsorbed oxygen atoms with zero density of states at $E_F$. (e) A sizable band gap is opened when the symmetry of oxygen decoration is broken by the removal of a single oxygen atom. (f) The wide band gap of 3.25 eV is opened for coverage corresponding to $\Theta =N_{\mathrm{O}}/N_{\mathrm{C}}=0.5$ at one side [N${}_{\mathrm{O}}$ and N${}_{\mathrm{C}}$ are the numbers of oxygen and carbon atoms in the ($4\times 4$) supercell].