Imaging functional groups in graphene oxide at atomic resolution

M.S. MORENO; C.B. BOOTHROYD; M. DUCHAMP; A. KOVÁCS; N. MONGE; G. MORALES; C.A. BARBERO; R. DUNIN-BORKOWSKI

Congreso; 18th International Microscopy Congress; 2014

Graphene oxide is a form of graphene with its surface modified by the addition of functionalgroups such as carboxylic groups, ketones and hydroxyl. The structure and distribution of thefunctional groups depends on the synthesis method used and they affect its chemical,electrical and mechanical properties. Of particular interest is the chemical composition andspatial distribution of these functional groups. Direct identification of the groups byhigh-resolution imaging is not possible at present, but they can be made visible by bondingheavy atoms, such as Ba, to selected groups and imaging the distribution of the heavy atoms.High-resolution images of Ba doped graphene oxide taken at 80kV in a Cs and Cc correctedtransmission electron microscope (TEM) show the structure of the graphene oxide withminimal electron beam damage but are not able to identify the Ba atoms. We attempted touse energy-filtered electron microscopy to image the Ba M5 and M4 edges at 781eV butradiation damage over the long exposures required prevented location of the Ba atoms tobetter than a few nm. Compositions measured using energy loss spectroscopy (EELS) revealeda progressive loss of O with increasing temperature and that 1% O is retained at 800°C.High-resolution scanning transmission electron microscopy (STEM) at room temperatureproved to be impossible due to contamination from migration of the functional groups on thegraphene surface. Contamination was overcome by imaging with the graphene oxide above400°C in a stable heating holder.Simultaneous STEM energy-loss spectrum images (SI) and high-angle annular dark-field(HAADF) and bright-field (BF) images acquired at a specimen temperature of 800°C allowed usto correlate the location of the Ba atoms with features in the high-angle dark-field images atatomic resolution.Simultaneously acquired HAADF and BF-STEM images (Fig. 1) show bright and dark dots at thesame positions. These bright dots are identified as Ba atoms by EELS as shown in Fig. 2a. Thecorresponding elemental maps extracted from the SI are shown in Fig. 2b-d. The maps alsoshow that Ca and O occur together and that Ba is not associated with a significantconcentration of O. The positions of Ba atoms attached to functional groups on graphene oxidecan therefore be mapped with atomic spatial resolution by using a combination of STEM andTEM techniques. The fact that Ca was observed to correlate strongly with O suggests that itcould be used as a marker for the positions of the O-containing groups.