Evidence of selective oxidation in surface layers of graphite-like thin sheets by mild oxidation

Graphite-like thin sheets (GLSs) contained in globular aggregates of carbon nanohorns have few oxygenated groups; therefore, they are suitable for studying how oxidation can be finely controlled. We found that mild oxidation in GLSs with H2O2 solution at room temperature for 7–28 days enabled GLS surface layers to be selectively oxidized, where carboxyl, quinone, carbonyl, and hydroxyl groups were created. The inner layers were little oxidized and almost no exfoliation occurred as was suggested by the lack of change in the layer–layer distances and the histogram of layer numbers in the GLS. The other evidence was that the quantity ratio for the surface and inner layers, viz., oxidized and not-oxidized layers, was estimated to be about 2:1 from thermogravimetric analysis, and this value largely coincided with the surface and inner ratio of layer numbers estimated from the histogram for the layer number.