Surface modification of multi-wall carbon nanotubes by nitrogen attachment

Commercial multiwall carbon nanotubes (MWCNT-s) were treated by RF activated N2 gas plasma at (nominally) room temperature. Treatment time of 5 to 10 min was applied at negative bias varying in the 0–300 V range. Surface chemical alterations were followed by X-ray photoelectron spectroscopy (XPS). All the applied treatments resulted in a significant build-up of nitrogen in the surface of MWCNT-s. The amount of nitrogen varied between 19 and 25 at.% depending on the treatment time and, in a lesser extent, also on biasing conditions. Interestingly, the nitrogen attachment was also significant (20 at.%) when the treatment commenced without bias. Evaluating the high-resolution N1s XP spectral region, typically three different chemical bonding states of the nitrogen was delineated. Peak component at 398.3 ± 0.3 eV is assigned to CNC type, at 399.7 ± 0.3 eV to sp2 N in melamine-type ring structure and at 400.9 ± 0.3 eV to N substituting carbon in a graphite-like environment. Identical chemical bonding of the nitrogen was detected on the surface of highly oriented pyrolytic graphite (HOPG) and on microcrystalline graphite surfaces treated in the same way for comparison. Estimating the penetration depth of the nitrogen atoms by the SRIM program it was concluded that at the applied DC bias energy range the implanted nitrogen is incorporated in the top 2–4 monoatomic layers of the samples. A model for the distribution of the chemically bonded nitrogen on the outer walls of the MWCNT-s is proposed.

Research highlights
► Large amount of nitrogen, 19–23 atomic % could be incorporated into the carbon matrix at short 5–10 min treatment time.
► Three chemical states of N were delineated as CNC in pyridine, diazine/triazine and N in a graphite type environment.
► N is incorporated in the top 2–4 monoatomic layers, according to calculated penetration depth at the applied bias.