Raman spectra of MWCNTs and MWCNT-based H-2-adsorbing system
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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 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, 3 160 cm(-1) and the region 441614422 cm(-1) may be due to their second- and third-order combination frequencies: 2D. D + E-2g 2E(2g), and 2D + 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) under barH(2). asymmetric C-H Stretch of Surface (C) under barH(3), and H-H stretch of molecularly adsorbed H-2(a), respectively. It was also shogun 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 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 (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. (C) 2002 Elsevier Science Ltd. All rights reserved.