Raman characterization, electron microscopy analysis and length distribution of MWCNTs. Figure 1A. The figure is a histogram of the Raman spectra of pristine (black) and one hour acid-washed carbon nanotubes (red). Four independent bands have been identified for both samples, i.e., D band around 1350 cm-1, G band at 1585 cm-1, G’ band around 2690 cm-1, and an additional band around 2930 cm-1. Shifts in these bands are noticed for samples that have been treated with acid for 1 h. Figure 1B. Histograms of length distribution of pristine (a) and 1 h acid-washed MWCNTs (b) as identified by tapping mode Atomic force microscopy (AFM). At least 30 nanotubes have been analyzed for each one of the samples. Figure 1C, D, E, F: Figure 1C shows a representative bright-field image and Figure 1D shows the corresponding dark-field image of the MWCNT sample. The images demonstrated that the MWCNTs have a diameter of 10–20 nm and a typical multi-walled tubular morphology. Figure 1D shows representative dark-field STEM (DF-STEM) image of the native MWCNT sample that was acquired. The analysis demonstrated low amounts of the iron catalyst. Figure 1E shows a representative bright-field image and Figure 1F shows the corresponding dark-field image of the MWCNT sample. The dark-field image provides atomic number contrast information. The bright 10 nm particle at the end of the MWCNT in Figure 1F is a catalyst particle. Energy dispersive X-ray spectroscopy (EDS) showed that the catalyst particle was iron-rich. Further analysis of the MWCNT sample identified low amounts of the iron catalyst.