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 N₂ or CO₂ atmosphere were subjected to the oxidation by immersing into nitric acid and then heating in air at 400 °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 N₂ 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 N₂ were oxidized heterogeneously, but those in CO₂ showed homogeneous oxidation, giving a high density of ultramicropores.