Raman spectraof MWCNTs and MWCNT-based H_2-adsorbing system

摘要

Raman spectra were taken 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 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:2D,D+E_(2g),2E_(2g),and 2D+E_(2g),respectivley.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 thelaser 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 CH_2,asymmetric C-H stretch of surface CH_3,and H-H stretchof molecularly adsorbed H_2(a),respectivley.It was also shown that the concentration of the observed hydrogen-containing ad-species on the MWCNTs (CH_4-based) was higher than thsoe 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 purfied 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.