In situ scanning tunneling microscopy of the anodic oxidation of highly oriented pyrolytic graphite surfaces

The scanning tunneling microscope (STM) was used to follow the in situ anodic oxidation of highly oriented pyrolytic graphite (HOPG) in 0.1 M H₂SO₄. Prior to electrochemical cycling, the HOPG surface appeared atomically flat and atomic structure was observed with the surface under potential control. Cycling between 0.0 and 1.8 V vs AgQRE (silver wire quasi-reference electrode) resulted in the formation of an amorphous graphite oxide overlayer structure exhibiting reduced barrier heights to tunneling, which grew anisotropically into the basal graphite plane. With continued cycling this structure completely covered the surface, forming a rough amorphous film. These images suggest that graphitic oxide overlayer formation occurs according to a nucleation and growth mechanism.