Influence of in-plane Stone–Thrower–Wales defects and edge functionalisation on the adsorption of CO2 and H2O on graphene

Abstract

There is now increasing recognition of the potential of graphene membranes for gas separation, with the application to CO₂ capture being one of specific interest; however, the co-adsorption of H₂O which saturates flue-gas remains a major impediment. Towards enhancing hydrophobic characteristics of graphene while increasing specificity to CO₂, we investigate here the adsorption characteristics of CO₂ and H₂O on four different kinds of graphene sheet – namely, hydrogen-terminated and fluorine-terminated pristine sheets, and the corresponding Stone–Thrower–Wales (STW) defect-incorporated sheets using density functional theory methods. Our results reveal that fluorine termination enhances hydrophobicity and favours the adsorption of CO₂, while reducing that of H₂O, in comparison to hydrogen termination. On the other hand, H₂O adsorption affinity is increased on introducing the Stone–Thrower–Wales defect in both H-terminated and F-terminated sheets, while for CO₂ the affinity change is more marginal, evidenced from the change in height of the adsorbed molecule above the surface, and of the adsorption energy. The Henry law constant for H₂O is reduced by 54% on F-termination, for both pristine and defective H-terminated graphene sheets, while for CO₂ it is increased by 12% and reduced by 18% respectively, on F-termination of the two sheets; indicating the pristine F-terminated sheet as the preferred option. From the density of states analysis, the Fermi level shows a 0.7 eV shift towards the valence band for fluorine termination in both pristine and STW sheets, but is not influenced by CO₂ and H₂O adsorption. Fluorine termination is shown to have a significant effect on the valence band, and offers a convenient route for tuning the electronic structure of graphene.