Oxidation is the basic and important process for the carbon-based materials as an initial process for producing very thin graphitic materials composed of a single or several graphene layers [1]. Recently, it has been reported that the epoxy group on the graphene breaks carbon-carbon bonds, followed by the formation of ether groups [2]. Such ether groups tend to align one-dimensionally, resulting in the introduction of cracks of the graphite.
In this study, structural and electronic properties for oxygen-adsorbed graphene sheets have been explored using first-principles total-energy calculations within the local spin density functional theory. In particular, we focus on initial adsorption processes of oxygen on graphene. We have prepared two types of configurations for the oxygen adsorption on the graphene sheet; one is the homogeneously-arranged (2D) model and the other is the one-dimensionally (1D) aligned one. As for the 1D model, we have found the structural bistability with regard to the oxygen adsorption. This bistability corresponds to the formation of epoxy group or ether group, where the ether group phase is more stable than the epoxy group one. Energy barrier for the transformation between these phases decreases with increasing separation length between oxygen rows. On the other hand, the 2D model prefers to form the epoxy group on the graphene sheet. Further, we have evaluated the adsorption energy for the epoxy group as a function of the oxygen coverage for the 2D model. The results show that isolated oxygen atoms are highly mobile and incline to condense on the graphene sheet. We will also discuss the structural stability for the oxygen alignment onto the wall of carbon nanotubes.
[1] S. Horiuchi et al., Appl. Phys. Lett. 84, 2403 (2004).
[2] J.-L. Li et al., Phys. Rev. Lett. 96, 176101 (2006). |