X-ray absorption spectroscopy studies of electronic structure recovery and nitrogen local structure upon thermal reduction of graphene oxide in an ammonia environment

Annealing graphene oxide under an ammonia environment provides a facile approach to defunctionalise this material while simultaneously enabling nitrogen incorporation en route to the preparation of chemically derived graphene. Here, we use X-ray photoemission spectroscopy (XPS) in conjunction with near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy to probe both the global recovery of electronic structure in this material as well as to monitor evolution of the local structure of incorporated nitrogen atoms when graphene oxide is reduced under an ammonia gas environment at ambient and low pressures in the temperature range between 250 and 1000 °C. The local structure and extent of recovery of the π-conjugated framework is correlated to electrical conductivity measurements. Angle-resolved C K-edge NEXAFS spectra along with O K-edge NEXAFS and C 1s high-resolution XPS spectra suggest that hydroxyl and epoxide functional groups on the basal plane of graphene oxide are eliminated upon annealing to a temperature of 250 °C, bringing about substantial restoration of the π-conjugated framework of graphene. Furthermore, an increase in the in-plane orientation of constituent graphene oxide flakes is observed up to a temperature of 750 °C for annealing under both sets of conditions and is manifested as a greater spread in the intensity of the C K-edge π* resonance as a function of angle of incidence of the X-ray beam. Angle-resolved N K-edge NEXAFS spectra and high-resolution N 1s XPS spectra supplement the global view of recovery of π-conjugation with a local perspective of the chemical bonding environments of incorporated nitrogen atoms. Three distinct modes of nitrogen incorporation are evidenced: amine or nitrile like (N1), pyridinic (N2), and substitutional/graphitic (N3). The data suggest that nitrogen is initially incorporated as nitrile like functionalities at lower temperatures with these moieties protruding above and below the graphene basal plane; however, the nitrile and amine groups are subsequently transformed at higher temperatures through the elimination of oxygenated functional groups and reconstitution of the sp2-hybridized network to in-plane pyridinic and graphitic moieties. The latter two configurations are seen to substantially enhance the conductivity of reduced graphene oxide.