Two-Step Mechanism for Low-Temperature Oxidation of Vacancies in Graphene

We study the oxidation of vacancies in graphene by ab initio atomistic thermodynamics to identify the dominant reaction mechanisms. Our calculations show that the low-temperature oxidation occurs via a two-step process: Vacancies are initially saturated by stable O groups, such as ether (C-O-C) and carbonyl ($\mathrm{C}\text{}\text{}=\text{}\text{}\mathrm{O}$). The etching is activated by a second step of additional ${\mathrm{O}}_2$ adsorption at the ether groups, forming larger O groups, such as lactone ($\mathrm{C}\mathrm{\text{−}}\mathrm{O}\mathrm{\text{−}}\mathrm{C}\text{}\text{}=\text{}\text{}\mathrm{O}$) and anhydride ($\mathrm{O}\text{}\text{}=\text{}\text{}\mathrm{C}\mathrm{\text{−}}\mathrm{O}\mathrm{\text{−}}\mathrm{C}\text{}\text{}=\text{}\text{}\mathrm{O}$), that may desorb as ${\mathrm{CO}}_2$ just above room temperature. Our studies show that the partial pressure of oxygen is an important external parameter that affects the mechanisms of oxidation and that allows us to control the extent of etching.

Figure 1

Upper panel: The adsorption energy for dissociative ${\mathrm{O}}_2$ adsorption on the perfect basal plane of graphene (route G) and at a bare four-atom (V4) vacancy (route I). Red lines indicate ${\mathrm{O}}_2$ adsorption and blue lines O-atom diffusion. Lower panel: The corresponding geometries of the stable intermediate adsorption structures along route I, summarizing the first step of the oxidation mechanism. Oxygen adsorption at a pentagon leads to formation of two parallel carbonyl ($\mathrm{C}\text{}\text{}=\text{}\text{}\mathrm{O}$) groups. The adsorption energy for O atoms forming ether groups is larger than for carbonyl groups, such that O atoms are expected to diffuse along the perimeter of the vacancy to form another ether group. All molecular images were done with VMD [22].

Figure 2

Upper panel: The adsorption energy for dissociative ${\mathrm{O}}_2$ adsorption at an oxygen-saturated $\mathrm{V}4\mathrm{\text{:}}3\mathrm{O}$ vacancy along route II. Red lines indicate ${\mathrm{O}}_2$ adsorption and black lines ${\mathrm{CO}}_2$ desorption. Lower panel: The corresponding geometries of the stable intermediate adsorption structures. The first event is the formation of a lactone ($\mathrm{C}\mathrm{\text{−}}\mathrm{O}\mathrm{\text{−}}\mathrm{C}\text{}\text{}=\text{}\text{}\mathrm{O}$) group, which can either desorb as ${\mathrm{CO}}_2$ or form an anhydride ($\mathrm{O}\text{}\text{}=\text{}\text{}\mathrm{C}\mathrm{\text{−}}\mathrm{O}\mathrm{\text{−}}\mathrm{C}\text{}\text{}=\text{}\text{}\mathrm{O}$) group by additional adsorption of an ${\mathrm{O}}_2$. The latter can finally disintegrate by releasing ${\mathrm{CO}}_2$.

Figure 3

The minimal formation energy $\Delta G_{\mathrm{form}}$ for the oxidized multiple vacancies V1–V6 as a function of the oxygen chemical potential $\Delta {\mu }_{\mathrm{O}}={\mu }_{\mathrm{O}}-{\mu }_{\mathrm{O}}^{\mathrm{DFT}}$ at a typical oxidation temperature $T=923\mathrm{K}$ [1, 4, 5]. ${\mu }_{\mathrm{O}}^{\mathrm{DFT}}$ is the chemical potential of an O atom in the free molecule calculated by DFT. The slope of the lines is given by the number of O atoms adsorbed at the vacancy, and the black squares indicate the transition value of the ${\mu }_{\mathrm{O}}$, where the configuration changes. The upper scale converts ${\mu }_{\mathrm{O}}$ into a pressure scale.

Figure 4

A phase diagram for a V4 vacancy in graphene, showing how the oxygen concentration changes as a function of $T$ and $p$. $\mathrm{V}4\mathrm{\text{:}}\mathrm{nO}$ denotes how many O atoms are adsorbed at the vacancy. $\Delta G_{\mathrm{form}}$ is negative below the dotted black line. The hashed regions indicate coexistence regions, where the energy difference between two configurations is below the numerical resolution.

Figure 5

The binding energy and stability range of the O groups with respect to CO or ${\mathrm{CO}}_2$ desorption. $\Delta G_{\mathrm{bind}}(T,p)$ is negative below the critical temperature $T_c$, indicated by the lines. The inset shows the binding energy $E_{\mathrm{bind}}$ for CO or ${\mathrm{CO}}_2$ desorption at multiple vacancies V1–V5 in graphene.