Raman spectra of MWCNTs and MWCNT-based H2-adsorbing system Original Research Article

Raman spectra were taken with an UV–vis Raman system on two types of multiwalled carbon nanotubes (MWCNTs, CH4-based) and MWCNTs (CO-based), grown catalytically with CH4 and CO as the carbon sources, respectively, and their H2-adsorbing systems. The strongest peak at 1580 cm−1 and the second-strongest band at 1416 cm−1 could be attributed to the fundamental frequency modes E2g and D (induced by defects) analogous to those of graphite, and the bands at 2832, 2996, 3160 cm−1 and the region 4410–4422 cm−1 may be due to their second- and third-order combination frequencies: 2D, D+E2g, 2E2g, and 2D+E2g, respectively. By a comparative study with the different laser excitation lines (325 nm vs. 514 nm), it is found that the position of the D-band depends strongly on the laser excitation frequency. The Raman-spectroscopic investigation of the H2–MWCNTs adsorption systems showed that adsorption of H2 on the MWCNTs could occur in associative and dissociative forms. The observed Raman peaks at 2856, 2967 and 3950 cm−1 may be assigned to symmetric C–H stretch of surface CH2, asymmetric C–H stretch of surface CH3, and H–H stretch of molecularly adsorbed H2(a), respectively. It was also shown that the concentration of the observed hydrogen-containing ad-species on the MWCNTs (CH4-based) was higher than those on the MWCNTs (CO-based), and that doping of a proper amount of K+ in the MWCNTs enhanced somewhat H2 adsorption. Adsorption of trace amount of moisture on the purified MWCNTs was below the detectable limit, but pronounced on the K+-doped MWCNTs. On the H2/K+–MWCNTs (CH4-based) adsorption system, the observed strong Raman bands in the region 3370–3470 cm−1 may be ascribed to O–H stretching of surface hydroxyls generated from adsorption of the trace amount of moisture.