This article reports a possible functionalization mechanism of isocyanate (NCO) groups on multiwalled carbon nanotubes (CNTs) with low-temperature plasma. The mechanism was clarified according to the analysis with two plasmas generated with the gas mixture of (1) nitrogen and carbon dioxide and (2) nitrogen and oxygen. We analyzed the mechanism through optical emission spectroscopy from these plasmas and the NCO functionalization ratio measured with the fluorescent method after plasma exposure over CNTs. The optical emission gave us information on the quantitative analysis of the gas species of atomic nitrogen (N), atomic oxygen (O), and carbon monoxide (CO) and the qualitative analysis of carbon nitride (CN) species in the plasma. Compared with our results from the gas species in the plasma and the NCO functionalization ratio on CNTs, CO and CN species in the gas phase in plasma are less likely to contribute to forming NCO groups on CNTs. Rather, the equal densities of atomic nitrogen and oxygen species in the plasma could be effective in forming NCO groups on the CNT surface: the NCO groups should form by N, O, and carbon (C) species on the CNT surface. The groups likely build up gradually by N, O, and C individually reaching a CNT surface, or the NCO radicals form in the gas phase and then attach to the CNT surface.

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