Global and local oxidation behavior of reduced graphene oxide

The oxidation and etching of reduced graphene oxide (RGO) by thermal oxidation in air, microwave oxygen plasma, ultraviolet-generated ozone, and scanning tunneling microscopy (STM) lithography have been investigated. This type of graphene exhibited a higher reactivity toward oxidation than that of pristine graphite (taken as a reference carbon material), which could be related to its intrinsically defective structure. Etching of RGO as a result of thermal oxidation in air was started at as low a temperature as 400 °C, as compared to ∼500 °C for graphite, indicating that the defects present on the former are different in nature from those found on the surface of pristine graphite. The morphological evolution of individual RGO sheets upon oxidative attack was consistent with the sheets being essentially a patchwork of minute domains (a few to several nanometers large) with two distinct reactivities, higher (lower) reactivity associated with defective (defect-free) domains. The introduction of oxygen functional groups on the basal plane of RGO was revealed directly by X-ray photoelectron spectroscopy and indirectly through STM. STM lithography enabled discrimination between oxidation proper (introduction of oxygen groups) and etching (desorption of the groups as CO or CO₂) of the RGO sheets through control of the applied bias voltage. The former was visualized in the STM images as locally depressed features of electronic origin, whereas etching led to the generation of actual trenches on the sheets. Taken together, the present results provide insight into the reactivity of RGO and suggest potential practical applications involving controlled oxidation of this type of graphene.