Effect of sulfur on enhancing nitrogen-doping and magnetic properties of carbon nanotubes
© Cui et al; licensee Springer. 2011
Received: 4 September 2010
Accepted: 12 January 2011
Published: 12 January 2011
Sulfur (S) is introduced as an additive in the growth atmosphere of carbon nanotubes (CNTs) in the range of 940-1020°C. CNT products with distorted sidewalls can be obtained by S-assisted growth. Moreover, many fascinating CNT structures can also be found in samples grown with S addition, such as bamboo-like CNTs, twisted CNTs, arborization-like CNTs, and bead-like CNTs. Compared with CNTs grown without S, more nitrogen-doping content is achieved in CNTs with S addition, which is beneficial for the properties and applications of nitrogen-doped CNTs. In addition, S can also enhance the encapsulation of ferromagnetic materials and thus improve the soft magnetic properties of CNTs, which is favorable to the applications of CNTs in the electromagnetic wave-absorbing and magnetic data storage areas.
Sulfur (S) is an important additive for controlling the structures and properties of carbon nanotubes (CNTs). Many interesting carbon nanostructures, such as Y-junction CNTs , sea-urchin-like CNTs , long single-walled CNT (SWCNT) strands , double-walled CNT films , amorphous CNTs , large-diameter SWCNTs , ultra-short CNTs , and small-diameter multi-walled CNTs  can be obtained by S-assisted growth. Meanwhile, enhanced mechanical properties , water-solubility , catalyst support performance , electrochemical properties , and photovoltaic performance  can all be achieved by CNT products with S-assisted growth. Therefore, the introduction of S into growth atmosphere of CNTs has attracted much research interest because of their promotional effect on synthesis of carbon nanostructures with novel morphologies and properties.
Nitrogen-doping (N-doping) is an effective way to modify the properties of CNTs. Nitrogen-doped (N-doped) CNTs demonstrate negative differential resistance behavior , high electrocatalytic activity for oxygen reduction , and can also be used as suitable support for uniform distribution of Pt catalyst [12, 13]. In addition, N-doping has a significant effect on controlling chirality and crystallinity of CNTs , leading to n-type CNTs , and improving the field emission performance of CNTs [16–18]. The effective doping of nitrogen into CNTs is mainly achieved in situ during CNT growth [12–16, 19], and N-doping content is usually controlled by adjusting the ratio of carbon source to nitrogen source [16, 19], which is disadvantageous for increasing the doping content of CNTs produced with liquid carbon source. For example, N-doped CNTs can be synthesized by mixture of xylene and pyridine, and CNTs with different N-doping contents are obtained from different xylene/pyridine ratios, but their doping contents cannot be larger than that with pure pyridine . Therefore, it is crucial to develop an efficient way to improve the N-doping content in CNTs from the view of both theoretical and applied research.
In this study, S was introduced as an additive in the growth of CNTs. CNTs with distorted walls were obtained, which might be good supports for catalyst nanoparticles. Many other fascinating CNT structures were also obtained in this study's samples, such as bamboo-like CNTs, twisted CNTs, arborization-like CNTs, and bead-like CNTs. When S was used as growth promoter, the improved CNTs with high N-doping content could be achieved. In addition, the effect of S addition on enhanced soft magnetic properties of CNTs was also demonstrated.
Experimental setup and procedure are similar to that described in the previous report by the authors about N-doped CNT arrays . Ferrocene and pure S powders were dissolved in acetonitrile to form a solution (20 mg/ml), and fed into chemical vapor deposition (CVD) reactor by a syringe pump at a constant rate of 0.4 ml/min for 0.5-1 h. A mixture of Ar (2000 sccm) and H2 (300 sccm) acts as the carrier gas. In order to investigate the effect of S on CNT growth at a relatively low temperature, the temperature is set in the range of 940-1020°C. The S concentration in the catalyst (atomic ratio of S:Fe) is 1:10. CNTs without S additive are also prepared for comparison.
The resulting CNTs were characterized using scanning electron microscope (SEM, JOEL JSM-6460 LV SEM), transmission electron microscope (TEM, JEM-200 CX), microscopic confocal Raman spectrometer (Renishow RM 2000, using 632.8-nm laser excitation), Auger electron spectroscopy (AES, PHI-700), and X-ray diffractometer (XRD, Bruker D8 advence). The magnetization measurements were performed on a vibrating sample magnetometer (LakeShore VSM-7307) at room temperature. Thermogravimetric analysis (TGA) results were obtained using 6-mg samples in air flow at a heating rate of 20°C/min.
Results and discussion
The improvement of N-doping content is also confirmed by AES. Figure 3c shows the surface AES results of the two CNTs samples produced at 980°C with S and without S, respectively. Almost no signals of Fe were detected in both of the samples, indicating that Fe catalyst particles are fully covered by carbon layers. The CNTs produced with S consist of C (95.2 at.%), N (2.4 at.%), and O (2.4 at.%), while CNTs without S consist of C (96.9 at.%), N (1.2 at.%), and O (1.9 at.%). The presence of O can be attributed to the exposure of the CNTs in the air atmosphere [18, 24]. It can be seen that S has an effect on increasing N-doping content in CNTs. This might be because S induces pentagon and heptagon in sp 2 carbon lattice [1, 2], and these heterocyclic rings are supposed to be favorable to the enhancement of N-doping.
The saturation magnetization (M s, emu/g) of CNTs produced at different conditions
CNTs with distorted sidewalls are synthesized by the S-assisted CVD method. S is favorable to the formation of pentagon and heptagon carbon rings, and these rings are supposed to result in the formation of CNTs with distorted sidewalls. This kind of CNTs might supply more active positions for catalyst nanoparticles and have potential usage in catalyst support area. It is also found that S addition can enhance the N-doping content of CNTs, resulting from heterocyclic rings, which are favorable for the enhancement of N-doping. The effect of S on enhancing the soft magnetic properties of CNTs is also demonstrated, and the enhanced saturation magnetization can be attributed to the enhanced encapsulation of ferromagnetic material. Enhanced soft magnetic properties of CNTs are favorable for the applications of CNTs in the electromagnetic wave-absorbing and magnetic data storage areas.
Auger electron spectroscopy
chemical vapor deposition
scanning electron microscope
transmission electron microscope
The authors are grateful for the financial support received from China Postdoctoral Science Foundation (Grant No. 20090450021) and the National Natural Science Foundation of China (Grant No. 50902080, 50632040).
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