Dispersions of individual carbon nanotubes (CNTs) are crucial for nanodevices and polymer/CNTs nanocomposites. In this paper, stable and homogenous dispersions of individual multiwalled carbon nanotubes (MWCNTs) have been synthesized. The factors influencing the dispersibility mechanism, including the surfactant concentration and the pH value, have been investigated. SEM images display the impurities sticking on MWCNTs which have been removed. The oxygen-containing groups on the surface of MWCNTs sample have been detected through FT-IR and Raman spectra. All experimental results illustrate that using fullerenols as surfactant can greatly improve the dispersibility of MWCNTs. Moreover, the prepared dispersions exhibit good stability that the sediment percentage of fullerenols-MWCNTs is only 5.2% after 5 days.
As a new type of one-dimensional nanomaterials, CNTs have attracted considerable attention due to its novel structure and unique physical and chemical properties in recent years [
Generally, CNTs can be synthesized by arc discharge, laser ablation, and chemical vapor deposition (CVD), which inevitably contained amorphous carbon, carbon nanoparticles (CNPs), and metal catalyst particles [
In most common solvents, van der Waals’ force holds MWCNTs together and severely impedes its manipulation and application. To resolve this problem, various types of surfactants have been proposed to improve the dispersion properties, such as sodium dodecylbenzene sulfonate (NaDDBs) whose dispersibility is limited and polyvinylpyrrolidone (PVP) which is hard to be removed [
In this work, MWCNTs were purified by two-step liquid phase oxidation with NaOH and a mixture of nitric and sulfuric acids (H2SO4/HNO3, volume 1 : 3). Fullerenols serving as surfactants were used to disperse MWCNTs. The effects of fullerenols concentration and solution pH on the dispersibility of MWCNTs were investigated, respectively. Based on the experimental results, a possible mechanism of improvement of dispersibility was proposed.
The N-doped MWCNTs produced by chemical vapor deposition (CVD) were supplied by Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. Fullerenes C60 were purchased from Suzhou Dade Carbon Nano Science and Technology Co. Ltd. NaDDBs and PVP were bought from Aladdin Reagent. All other reagents were of analytical reagent (AR) pure grade and were purchased from Sinopharm Chemical Reagent Co. Ltd.
To obtain the purified MWCNTs (P-MWCNTs), raw MWCNTs (R-MWCNTs) were refluxed in 2 mol/L NaOH for 12 h. Then, MWCNTs were sonicated (400 W, KQ-500DE) for 30 min in a mixture of nitric/sulfuric acids (volume 1 : 3) and washed with deionized water until the eluate reached neutral. Finally, P-MWCNTs were dried at 80°C under vacuum.
The fullerenols were synthesized from powdered Fullerenes C60 by a novel method reported by Chiang et al. [
The morphologies and chemical compositions of the P-MWCNTs were characterized by scanning electron microscopy (SEM, JEOL JSM-7001F), atomic force microscope (AFM, Bruker Veeco Dimension-5), and Fourier transform infrared spectrometer (FT-IR, Bruker Tensor 27).
The optical absorbance of dispersions was measured by UV-Vis spectrum (METASH UV-8000A), and then the suspensions were collected, dried, and weighed to obtain the quality and concentration. Standard spectra of optical absorbance and the MWCNTs concentration can be obtained since the dispersion concentration is proportional to the dispersion absorbance.
Figure
SEM images of MWCNTs: (a) R-MWCNTs; (b) P-MWCNTs.
Figure
FT-IR spectra of (a) R-MWCNTs; (b) P-MWCNTs.
Differences of Raman spectra between R-MWCNTs and P-MWCNTs are illustrated in Figure
Raman spectra of (a) R-MWCNTs; (b) P-MWCNTs.
Stability of the MWCNTs dispersion is of great importance in its applications. Dispersion 1 of P-MWCNTs was synthesized by using fullerenols as the surfactant. Figure
UV-Vis spectra of dispersion 1 with different concentrations in methanol; the inset illustrates the linear dependence of absorption intensity on suspension concentration.
The absorbance peak at 260 nm of all dispersions could characterize the concentration of MWCNTs seeing that the concentration of MWCNTs is proportional to the absorbance intensity of UV-Vis spectra. Generally, the sediment percentage used to characterize the suspension stability could be calculated by the following equation:
Variation of sediment percentage of dispersion 1, dispersion 2, and dispersion 3. The inserts are the optical images of P-MWCNTs dispersions with fullerenols, NaDDBs, and PVP after 120 h standing (from left to right), respectively.
The results show that the sedimentation percentage of the dispersion 1 is the minimum, which is only 5.2% after 5 days standing. However, the sedimentation percentage of the dispersion 2 and dispersion 3 is 47.5% and 83.1%, almost 10 and 20 times as much as that of dispersion 1. In right inset, it is also clearly observed that NaDDBS-MWCNTs and PVP-MWCNTs dispersions contain coagulated bundles at the bottom while fullerenols-MWCNTs dispersions are homogeneous.
Fullerenols are soluble in polar solvents such as water and alcohol because of decorating of polar hydroxyl groups on the carbon cages. Further experimental results illustrated by SEM images in Figure
In the experiment, the fullerenols concentrations in dispersion 1-1, dispersion 1-2, and dispersion 1-3 were 250 × 10−6 g/ml, 500 × 10−6 g/ml, and 1000 × 10−6 g/ml, respectively. Accordingly, the concentrations of P-MWCNTs in the above dispersions were about 420 × 10−6 g/ml, 1080 × 10−6 g/ml, and 1250 × 10−6 g/ml. Figures
(a) SEM image of dispersion 1-1; (b) FESEM image of dispersion 1-2; (c) FESEM image of dispersion 1-3; (d) a further magnified SEM image from the area marked in image (b); (e) AFM image of dispersion 1-2; (f) cross sections along the solid lines in the image (e).
The quantity of P-MWCNTs which adheres to the substrate is proportional to the concentrations of fullerenols mentioned above. The phenomenon is consistent with the results of UV-Vis investigation. Figure
The dispersibility also could be tuned by solutions pH. In this work, the pH values of dispersion 1 have been adjusted by HNO3 and NaOH. Figure
SEM images of dispersion 1-2: (a) acidity (pH = 5); (b) alkalinity (pH = 9).
P-MWCNTs have been produced using two-step liquid phase oxidation treatment with NaOH and a mixture of nitric and sulfuric acids (1 : 3 by volume). SEM images have shown that oxidative treatment could remove the impurities in MWCNTs. Moreover, hydroxyl groups and carboxyl groups have also been identified on the surface of MWCNTs applying FT-IR and Raman spectroscopy. The results indicated that an effective route has been developed to disperse P-MWCNTs in methanol using the fullerenols as a surfactant. The conspicuously enhanced homogeneity and stability of dispersions can be attributed to excellent electronic absorption ability and weak acid dissociation properties of fullerenols in the solution. This approach of preparing dispersions of MWCNTs is promising to be used in the area of nanodevices and polymer/CNTs nanocomposites fabrication.
The authors declare that they have no competing interests.
This work is supported by the Basic Research Project of Shanxi Province (2015021092), National Science Foundation of China (nos. 61471255, 61474079, 61501316, 51505324, and 51622507), and 863 project (2015AA042601).