Close-open-close evolution of holes at the tips of conical graphenes of single-wall carbon nanohorns

We previously reported that holes opened at the tips of conical single-graphenes of single-wall carbon nanohorns were closed during heat treatment at 1200 degrees C in an argon atmosphere. Here, we report phenomena related to the behavior of these holes: the holes closed quickly at first, then reopened, and then slowly closed again as the heat treatment period increased. This close-open-close evolution occurred only when the initial hole size was medium (0.7-0.9 nm). Smaller and larger holes did not reopen after they thermally closed. We discuss the close-open-close mechanism by considering the functions of oxygenated groups existing at the edges of the holes.