Figure 2: (a) FT-IR spectra for the -MWCNTs, -MWCNTs, and -MWCNTs and (b) intensity ratios of the FT-IR absorbance peak at ca. 1720 to the most intense peak in the range 1520–1590 cm−1 and the content of carboxyl groups, (mole%), estimated from the absorbance peak intensities at ca. 1720 cm−1 using the calibration curve drawn for benzoic acid.