Graphene coatings: An efficient protection from oxidation

We demonstrate that graphene coating can provide efficient protection from oxidation by posing a high-energy barrier to the path of oxygen atom, which could have penetrated from the top of the graphene to the reactive surface underneath. A graphene bilayer, which blocks the diffusion of oxygen with a relatively higher energy barrier, provides even better protection from oxidation. While an oxygen molecule is weakly bound to a bare graphene surface and hence becomes rather inactive, it can easily dissociate into two oxygen atoms adsorbed to low-coordinated carbon atoms at the edges of a vacancy. For these oxygen atoms the oxidation barrier is reduced and hence the protection from oxidation provided by graphene coatings is weakened. Our predictions obtained from the state-of-the-art first-principles calculations of the electronic structure, phonon density of states, and reaction path will unravel how graphene can be used as a corrosion-resistant coating and guide further studies aimed at developing more efficient nanocoatings.

Figure 1

(a) Atomic configuration of an oxygen atom adsorbed to an Al(111) surface. Oxygen and Al atoms are illustrated by small (red) and large (blue) balls, with numerals indicating their layer numbers from the top. (b) Density of states projected to $s$ and $p$ orbitals of adsorbed O and surface and subsurface layers of an Al(111) slab.

Figure 2

(a) Variation of the energy of adsorbed oxygen atom along the T (top)–H (hollow)–B( bridge) site directions of a hexagon showing that the B site is energetically most favorable. Stars indicate a favorable path for the diffusion of oxygen atom on the graphene surface. (b) Atomic configuration for an oxygen atom adsorbed at the bridge site on a ($4\times 4$) supercell of graphene consisting of 32 carbon atoms. (c) Electronic energy band structure together with charge densities of specific conduction and valence bands states. The crossing of π and ${\pi }^{*}$ bands is clarified in the text. (d) Calculated density of phonon modes of a pristine graphene (shaded area) and those of oxygen adsorbed to the bridge site of the ($4\times 4$) supercell of graphene [solid (red) line]. Insets: Relevant localized phonon modes.

Figure 3

Calculation of energy barriers of O${}_2$ and O passing from the top to the bottom side of suspended graphene along various paths. (a) Energy barriers for triplet (magnetic) and singlet (nonmagnetic) $O_2$, which are forced to pass from the top to the bottom side of graphene. (b) Energy barrier for an O atom along the same path as in (a). (c) The path of the minimum energy barrier for an O atom, which is initially adsorbed at the bridge site above the graphene plane, is forced to pass to the bottom side. Positions of C atoms, as well as the lateral $x$ and $y$ coordinates of O, are optimized for each value of indentation.

Figure 4

(a) Variation of the total energy for an O atom (red ball) passing (indenting) from the top side of a single graphene layer to its bottom side and eventually adsorbing to the Al(111) surface (blue balls) underneath. The O atom follows the path of the minimum energy barrier ($Q_{\mathrm{ox}}=5.93$ eV). (b) Snapshots of atomic configurations corresponding to various stages between the initial stage A, starting from the bridge site of graphene, and the final stage E, ending with the adsorption of the O atom on Al(111). (c) Protection of an Al(111) surface from oxidation by a graphene bilayer and variation of energy for an O atom penetrating from the top side of the outermost graphene layer and eventually adsorbing to Al(111). The greatest barrier to be overcome by a diffusing O atom is $Q_{\mathrm{ox}}=6.81$ eV along the the path to reach the Al(111) surface. (d) Atomic configurations of various stages of case (c).

Figure 5

(a) Evolutions of energetics and atomic structure with the indentation of the oxygen atom, which is initially adsorbed at the edge of a single vacancy. (b) Evolutions of energetics and atomic structure of two oxygen atoms resulting from the dissociation of an O${}_2$ molecule at the edge of a single vacancy and the indentation of one of the oxygen atoms from the top to the bottom site of suspended graphene. (c) Diffusion of one of the adsorbed oxygen atoms in (b) toward the Al(111) surface, resulting in its oxidation. Red, gray, and blue balls indicate O, C, and Al atoms, respectively.