Functionalization ratio of isocyanate groups on plasma-processed multiwalled carbon nanotubes

Abstract

This article reports the functionalization ratio of isocyanate (NCO) groups on multiwalled carbon nanotubes after exposure of plasma, which was generated with the gas mixture of nitrogen and carbon dioxide. The ratio was derived from measurements with an organic fluorescence, so-called acridine yellow G (AYG), which identifies the modified groups on carbon nanotubes (CNTs). First, the authors made several microscopic observations with the fluorescence. The observation showed that the fluorescence was especially intense from the area where small-sized CNTs exist, indicating that plasma-treated CNTs have more NCO density in small size than in large size. Second, the authors made a size selection on the CNTs with centrifugal forces and investigated the ratio of the groups over the total number of carbons in the CNTs to confirm the first observation and indication. This second investigation showed that the ratio increases as the degree of centrifugal force increases, i.e., the size of the nanotubes gets smaller, supporting the authors’ first microscopic observation. The authors also took the diameter distributions of the CNTs with an observation of an electron microscope, which proved that the centrifugation decreases the size distribution of the CNTs. As a result of overall experiments, the small diameter of plasma-processed CNTs has more NCO number density than the large diameter of the nanotubes in functionalizing isocyanate groups with the plasma.