Exfoliation of single-wall carbon nanotubes in aqueous surfactant suspensions: A Raman study

Aqueous suspensions of bundles and individual single-wall carbon nanotubes were prepared with the help of surfactants. We study the changes in the Raman spectra of the suspensions with respect to powders, and of exfoliated tubes with respect to bundles. The radial breathing modes (RBM) upshift in suspensions because of the internal pressure of the liquid. By contrast, no shift is observed in the RBM spectra after exfoliation in the suspensions. However, we demonstrate a selectivity of the exfoliation process for tubes of small diameters.

Figure 1

High resolution transmission electron microscopy from raw electric arc samples: (a) bundles and (b) isolated tubes. See details in the text.

Figure 2

Raman and photoluminescence spectra for powders, suspensions of bundles, and suspensions of individual tubes, as labeled in the figures, for (a) electric arc samples and (b) HiPCO samples, excited with a 1.16 eV laser line. The spectra were normalized and shifted along the vertical axis for clarity. The broad and intense signal in the ranges 3200–3500 and $2300\text{–}2700{\mathrm{cm}}^{-1}$ is due to OH or OD stretchings from ${\mathrm{H}}_2\mathrm{O}$ or ${\mathrm{D}}_2\mathrm{O}$, respectively. The signal fluctuations in the range $2100\text{–}2300{\mathrm{cm}}^{-1}$ are due to absorption by atmospheric water.

Figure 3

RBM Raman spectra for (a) electric arc samples and (b) HiPCO samples for three differents laser lines, as labeled in the figures. The spectra were normalized and shifted along the vertical axis for clarity. The vertical dotted lines indicate the maxima of intensity in the spectra of powders. The arrows mark the RBM peaks whose intensity increase strongly after exfoliation (see text).

Figure 4

Shift of the RBM in the suspensions with respect to powders as a function of RBM frequency for powders (inset: shift of the RBM after exfoliation in the suspensions). The solid and open symbols are for SB and SIS, respectively. The squares, circles, and triangles are for laser energies 2.41, 1.92, and 1.16 eV, respectively. The error bars correspond to 95% confidence limits. The solid line is the best linear fit for the whole data (see text). The dotted lines are guides for the eyes.