Professor, Meijo University
Director, Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology (AIST)
Nanotube and nanohorn
— Future real key player of nanotechnology —
“I really don’t want people to exaggerate carbon nanotubes too much. I want them to leave the tubes alone a little more,” says Prof. Iijima, the discoverer of carbon nanotubes. He now thinks that commercial applications of carbon nanohorns will be realized much earlier than those of carbon nanotubes.
Unlike carbon nanotubes, carbon nanohorns can be made simply without the use of a catalyst. Carbon nanohorn aggregates can be produced with a yield of more than 90% through laser vaporization of carbon at room temperature. These aggregates have a dahlia-like shape with a large number of horn-shaped short single-layered nanotubes that stick out in all directions. The tips of these short nanotubes are capped with five-membered rings. Carbon nanohorns’ key characteristic is high adsorbability, due to their large surface area — about 400 square meters per gram, but as Prof. Iijima says, “Adsorbed atoms tend to slip easily from the surface of the carbon nanohorns because of their complete graphite surface structure. To hold atoms on the carbon nanohorn surface, either the carbon nanohorns must be modified chemically or their structures must be partially damaged. Various potential characteristics of carbon nanohorns can be displayed by modifying their surface.”
Researchers have high expectations for applying carbon nanohorns to fuel cells as their electrode material, among other applications under consideration. Fuel cell electrodes made of carbon nanohorns are expected to help improve the cells’ power-generation capacity and extend their lifetime because platinum catalyst nanoparticles disperse among carbon nanohorns and do not aggregate. Carbon nanohorns are also expected as gas storage material, making use of their high adsorbability. Carbon nanohorns have for the first time cleared the United States Department of Energy threshold of commercial reality as methane gas storage material. Carbon nanohorns have also been found to selectively adsorb DNA fractions. Inorganic materials are now used in selecting DNA fractions. However, it is believed that carbon with a high biocompatibility may be a better material than inorganic substances. The Japan Science and Technology Agency (JST) has adopted a project to promote the application of carbon nanohorns in the biotechnology field as one of its “Solution Oriented Research and Technology” projects. This project started in January 2003 for a better understanding of the adsorption to carbon nanohorns, as well as for studying surface modification methods for controlling their selective adsorbability.
Prof. Iijima has not forgotten carbon nanotubes entirely; he has been studying how they grow. Carbon nanotubes will not be used commercially unless they can be mass-produced. He says, “Real nanotechnological progress is to develop nanomaterials, which can be used in fuel cells, field effect transistors and other useful products.” Such developments have not been achieved yet at this stage. Prof. Iijima says, “People are exaggerating carbon nanotubes too much. However, I can say with confidence that carbon nanotubes have made great contributions to basic science.” They do play a significant role in verifying the quantum effect. Prof. Iijima thinks that the real value of carbon nanotubes is their contribution to basic science.
He does not want to hear that what he has achieved in his research is the discovery of carbon nanotubes. He says, “I had conducted research using electron microscopy for 30 years before I discovered carbon nanotubes, so discovering them is just one of the results of my research based on electron microscopy.” He obtained a Ph.D. in studying filament-shaped silver bromide. His experience conducting structural analyses at that time helped him find carbon nanotubes. He says, “When you do not have any clue as to how to start new research, you cannot rely on anyone but yourself. What you can rely on when you face a serious difficulty is nothing but your experience.” This is his empirical rule.
