We investigated the purity and defects of single-wall carbon nanotubes (SWCNTs) produced by various synthetic methods including chemical vapor deposition, arc discharge, and laser ablation. The SWCNT samples were characterized using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Raman spectroscopy. Quantitative analysis of SEM images suggested that the G-band Raman intensity serves as an index for the purity. By contrast, the intensity ratio of G-band to D-band (G/D ratio) reflects both the purity and the defect density of SWCNTs. The combination of G-band intensity and G/D ratio is useful for a quick, nondestructive evaluation of the purity and defect density of a SWCNT sample.
Evaluating the quality of single-wall carbon nanotubes (SWCNTs) is very important, both in basic research and industrial application. To evaluate quality, we must consider two independent parameters: purity and defect density. Purity can be defined as a content ratio of SWCNTs to impurities, and the defect density can be defined as the abundance of structural defects on the nanotube walls. Raman spectroscopy has often been applied for the purity evaluation because a Raman mode around 1350 cm−1 (the “D-band”) is sensitive to structural defects in the graphitic sp2 network typical of carbonaceous impurities, such as amorphous carbon particles [
The Raman intensity of the G-band and radial breathing modes (RBMs) has been applied to the purity evaluation of SWCNTs [
The SWCNT samples used in this work and the measurement conditions for the purity evaluation were as follows. The six SWCNT samples are referred to as (1) HiPco (raw material produced by the HiPco process, lot no. R0546, Carbon Nanotechnologies Inc.), (2) CoMoCAT (purified material produced by the CoMoCAT method, Southwest Nanotechnologies Inc.), (3) Meijo (raw material produced by arc discharge, AP-J grade, Meijo Nano Carbon Inc.), (4) Carbolex (raw material produced by arc discharge, Carbolex Inc.), (5) Laser (raw material produced by laser ablation in our laboratory), and (6) DIPS (raw material produced by direct injection pyrolytic synthesis (DIPS), Nikkiso Co., Ltd.). Raman spectra were measured in the back-scattering geometry using a single monochromator with a microscope (LabRam Aramis, Horiba Jobin Yvon) equipped with a charge-coupled device detector and a notch filter. The sample was excited by the continuous wave second harmonic of an Nd:YAG laser at 2.31 eV (532 nm). To avoid laser heating, a 0.1 mW laser beam was focused onto the sample using an objective lens (×10). Raman spectra were obtained by averaging 20~30 spectra obtained from different locations on the sample. All Raman measurements were carried out under the same conditions for all samples to maintain high uniformity of the intensity. Thermogravimetric analysis (TGA) profiles were recorded from room temperature to 900°C in air flow (50 ccm) at a heating rate of 10°C/min with a microthermobalance (TGA-50, Shimadzu). Samples weighing 1~2 mg were used for the TGA measurements. The corresponding SEM images of the samples were obtained using JEOL JSM-7500F, operated at 15 kV.
The purity of the samples was first characterized quantitatively using SEM, which provided visual information of the ratio of SWCNTs to carbonaceous impurities. The SEM images and corresponding image analysis results of the samples are shown in Figure
Relative area of SWCNTs in a SEM image,
Sample | |||||
---|---|---|---|---|---|
DIPS | 98 | 95 | 2.0 | 1 | 164 |
HiPco | 83 | 73 | 1.0 | 0.60 | 25 |
Meijo | 57 | 73 | 1.5 | 0.53 | 31 |
Laser | 48 | 91 | 1.4 | 0.41 | 60 |
Carbolex | 47 | 53 | 1.5 | 0.21 | 13 |
CoMoCAT | — | 84 | 0.8 | 0.14 | 20 |
SEM images and corresponding image analysis results of (a)-(b) DIPS, (c)-(d) HiPco, (e)-(f) Meijo, (g)-(h) Laser, (i)-(j) Carbolex, and (k) CoMoCAT samples. In the image analysis, the carbonaceous impurities were painted green and background was painted blue for the area counting.
(a) relative area of SWCNTs in an SEM image, (b) normalized G-band Raman intensity, (c) G/D ratio, and (d) weight ratio of carbon in a sample for DIPS, HiPco, Meijo, Laser, Carbolex, and CoMoCAT samples.
Image analysis of the as-purchased CoMoCAT sample was unsuccessful because the SWCNTs were fully covered with carbonaceous impurities, probably due to the purification process. As a result, the SEM image could not be separated into SWCNT and impurity regions for analysis. The Raman and TGA results from the CoMoCAT sample were nevertheless included for reference.
It is well known that the G-band of SWCNTs shows multipeaks around 1580 cm−1. For purity evaluation using Raman spectroscopy, the G-band peak around 1593 cm−1, which derives from the longitudinal optical (LO) phonons of semiconducting SWCNTs [
Raman spectra of the samples are presented in Figure
Raman spectra of (a) DIPS, (b) HiPco, (c) Meijo, (d) Laser, (e) Carbolex, and (f) CoMoCAT samples.
In Figures
In contrast to the poor relationship between the G/D ratio and G-band intensity, the G-band intensity was well associated with the SWCNT purity estimated by SEM image analysis, as shown in Figures
The G-band intensity may also be affected by additional factors such as excitation laser wavelength and nanotube diameter. It is known that the Raman intensity of SWCNTs is significantly enhanced by a resonance effect when the excitation energy matches the absorption bands of SWCNTs [
Although we primarily discuss as-grown samples, it is noteworthy that the Raman intensity of SWCNTs is sensitive to their aggregation [
Once the purity of a sample is assessed from the G-band intensity, the G/D ratio can be used to discuss the relative abundance of SWCNT defects. The mismatch between the purity (Figure
Raman and SEM were used for purity evaluation and could be associated with the ratio of SWCNTs and carbonaceous impurities in a sample. On the other hand, TGA was used to measure the content ratio of noncarbonaceous impurities in a sample by monitoring the weight decrease of samples during combustion in air. TGA profiles of the samples are presented in Figure
TGA profiles of (a) DIPS, (b) HiPco, (c) Meijo, (d) Laser, (e) Carbolex, and (f) CoMoCAT samples.
A comparative study of purity and Raman intensity was carried out for SWCNTs produced by various synthetic methods. The detailed comparison of Raman intensity and SEM observations suggested that the G-band intensity is a better indicator of SWCNT purity than the G/D ratio. The G/D ratio is not suitable for determination of the purity of various samples produced by different synthetic methods. The present results indicate that a combination of the G-band intensity and the G/D ratio is useful to evaluate the purity and defect density of SWCNTs. The evaluation conditions employed in this study can be applied to SWCNTs with an average diameter of 1~2 nm, which covers most commercially available SWCNTs. If a consistent standard sample were available, evaluation based on the G-band and the D-band intensities could provide an easy means of determining the quality of SWCNTs for all users and suppliers of SWCNTs.
The authors thank K. Ogura of JEOL Ltd., Y. Nakata and H. Matsumoto of HORIBA Ltd., and Y. Suzuki of SHIMADZU CO. for providing experimental data; S. Shiraki and Y. Nakagawa of NIKKISO CO., LTD., for supplying the DIPS sample; and T. Okazaki, T. Saito, and D. Nishide of AIST for helpful discussions. They acknowledge the support from the Nanotechnology Program “Carbon Nanotube Capacitor Development Project’’ (2006–2010) by the New Energy and Industrial Technology Development Organization (NEDO).