Effects of carbonization atmosphere and subsequent oxidation on pore structure of carbon spheres observed by scanning tunneling microscopy

The surface of carbon spheres was studied by using field-emission scanning electron microscopy and scanning tunneling microscopy paying particular attention to the effects of atmosphere during carbonization and of subsequent oxidation on shape and size of the entrance of micropores. Commercial spheres of glasslike carbon prepared by carbonization of phenol resin spheres in either N-2, or CO2 atmosphere were subjected to the oxidation by immersing into nitric acid and then heating in air at 400 degrees C. The size distribution of pore entrance at nanoscopic scale was determined from scanning tunneling microscope images. Carbon spheres prepared in CO2 atmosphere had predominantly ultramicropores, but those prepared in N-2, had a very low porosity. The behavior during the oxidation process in air was found to be quite different on these two carbon spheres; spheres carbonized in N-2, were oxidized heterogeneously, but those in CO2 showed homogeneous oxidation, giving a high density of ultramicropores.