Raman spectra of MWCNTs and MWCNT-based H 2-adsorbing system

Abstract

Raman spectra were taken with an UV–vis Raman system on two types of multiwalled carbon nanotubes (MWCNTs, CH 4-based) and MWCNTs (CO-based), grown catalytically with CH 4 and CO as the carbon sources, respectively, and their H 2-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 E 2g 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: 2 D, D+ E 2g, 2 E 2g, and 2 D+ E 2g, 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 H 2–MWCNTs adsorption systems showed that adsorption of H 2 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 C H 2, asymmetric C–H stretch of surface C H 3, and H–H stretch of molecularly adsorbed H 2(a), respectively. It was also shown that the concentration of the observed hydrogen-containing ad-species on the MWCNTs (CH 4-based) was higher than those on the MWCNTs (CO-based), and that doping of a proper amount of K + in the MWCNTs enhanced somewhat H 2 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 H 2/K +–MWCNTs (CH 4-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.