Oxygen-containing functional groups on single-wall carbon nanotubes: NEXAFS and vibrational spectroscopic studies

Single-walled nanotubes (SWNTs) produced by plasma laser vaporization (PLV) and containing oxidized surface functional groups have been studied for th e first time with NEXAFS. Comparisons are made to SWNTs made by catalytic s ynthesis over Fe particles in high-pressure CO, called HiPco material. The results indicate that the acid purification and cutting of single-walled na notubes with either HNO3/H2SO4 or H2O2/H2SO4 mixtures produces the oxidized groups (O/C = 5.5-6.7%), which exhibit both pi*(CO) and sigma*(CO) C K-edg e NEXAFS resonances. This indicates that both carbonyl (C=O) and ether C-O- C functionalities are present. Upon heating in a vacuum to 500-600 K, the p i*(CO) resonances are observed to decrease in intensity; on heating to 1073 K, the sigma*(CO) resonances disappear as the C-O-C functional groups are decomposed. Raman spectral measurements indicate that the basic tubular str ucture of the SWNTs is not perturbed by heating to 1073 K, based on the inv ariance of the ring breathing modes upon heating. The NEXAFS studies agree well with infrared studies which show that carboxylic acid groups are therm ally destroyed first, followed by the more difficult destruction of ether a nd quinone groups. Single-walled nanotubes produced by the HiPco process, a nd not treated with oxidizing acids, exhibit an O/C ratio of 1.9% and do no t exhibit either pi*(CO) or sigma*(CO) resonances at the detection limit of NEXAFS. It is shown that heating (to 1073 K) of the PLV-SWNTs containing t he functional groups produces C K-edge NEXAFS spectra very similar to those seen for the HiPco material. The NEXAFS spectra are calibrated against spe ctra measured for a number of fused-ring aromatic hydrocarbon molecules con taining various types of oxidized functional groups present on the oxidized SWNTs.