Reconstructed Ribbon Edges in Thermally Reduced Graphene Nanoribbons

Graphene oxide nanoribbons (GONRs) with a high aspect ratio and high gravimetric density of their edges relative to those of graphene flakes are promising platforms for graphene-based devices. Since the edge chemistry of GONRs determines their final electronic and transport properties, it is important to understand the interactions of oxygen with the edges of the ribbons. Although oxidative unzipping of carbon nanotubes has been studied by Dai's(1) and Tour's(2) groups for GONR production, the role of oxygen concentration, the nature so, of edge oxygen groups, and their effect on the ribbon-edge geometry is still not well understood. We have therefore studied thermal annealing of GONRs, obtained by unzipping few-walled or multi-walled carbon nanotubes, focusing on the reduction process. For this purpose, in situ infrared absorption spectroscopy is used to monitor the edge reconstruction during the thermal reduction process. The ribbon edges of reduced graphene nanoribbons (rGNRs), initially functionalized with carboxyls, are found to convert to edge carbonyls during annealing at high temperatures (similar to 850 degrees C). The formation of these highly stable carbonyl species therefore leads to edge reconstruction of rGNRs after high temperature anneals. The concentrations of initial hydroxyl, edge carbonyl, and carboxyl are found to be key factors that determine the resulting oxygen concentration in rGNRs after annealing. Although the initial concentration of these oxygen groups introduced during unzipping is associated with the concentration of oxidant (KMnO4 or H3PO4), the resulting amount of total oxygen in rGNRs upon thermal reduction is found to be independent of the wall thickness of the starting carbon nanotubes (i.e., number of original unzipped layers of GONRs).