Temperature and pressure driven functionalization of graphene with hydrogen and oxygen via ab initio phase diagrams

Abstract

Functionalized graphene has numerous applications due to its remarkable and tunable physicochemical properties. However, precise control of functionalization remains challenging as direct connections between defect and functional group stability on graphene as a function of treatment conditions are missing. Here, a systematic study of graphene functionalization with hydrogen and oxygen is performed using a combination of density functional theory and ab initio phase diagrams. The interplay between carbon defects (e.g. vacancies, Stone-Wales, 555–777) and functionalization (e.g. R-H, R-H2, R-O, R-OH, and R-COOH) is examined. Generally, the process of functionalizing graphene led to significantly more stable carbon defects. By calculating Gibbs free formation energies with varying temperatures and partial pressures of H2 and H2O, ab initio phase diagrams were constructed. These diagrams revealed that the applied hydrogen and oxygen chemical potentials can be balanced during treatment to generate six distinct structures: R-H, R-O, R-OH, and R-COOH on the basal plane; and R-O and R-H2 on a double carbon vacancy. Through this systematic study, both the treatment conditions conducive to the formation of specific functional groups on graphene are pinpointed and a range of stable and metastable graphene-based structures with applications in various fields are identified.