Synthesis and Enhanced Field-Emission of Thin-Walled, Open-Ended, and Well-Aligned N-Doped Carbon Nanotubes
Thin-walled, open-ended, and well-aligned N-doped carbon nanotubes (CNTs) on the quartz slides were synthesized by using acetonitrile as carbon sources. As-obtained products possess large thin-walled index (TWI, defined as the ratio of inner diameter and wall thickness of a CNT). The effect of temperature on the growth of CNTs using acetonitrile as the carbon source was also investigated. It is found that the diameter, the TWI of CNTs increase and the Fe encapsulation in CNTs decreases as the growth temperature rises in the range of 780–860°C. When the growth temperature is kept at 860°C, CNTs with TWI = 6.2 can be obtained. It was found that the filed-emission properties became better as CNT growth temperatures increased from 780 to 860°C. The lowest turn-on and threshold field was 0.27 and 0.49 V/μm, respectively. And the best field-enhancement factors reached 1.09 × 105, which is significantly improved about an order of magnitude compared with previous reports. In this study, about 30 × 50 mm2 free-standing film of thin-walled open-ended well-aligned N-doped carbon nanotubes was also prepared. The free-standing film can be transferred easily to other substrates, which would promote their applications in different fields.
KeywordsCarbon nanotubes Thin-walled open-ended and aligned Thin-walled index Bamboo-shaped carbon nanotubes Field emission Free-standing
Since the discovery in 1991 , carbon nanotubes (CNTs) have attracted much attention due to their unique electronic and mechanical properties . Numerous articles have reported studies on their field-emission properties [3, 4, 5, 6, 7, 8]. Previous study of our group had shown that thin-walled CNTs possessed better field-emission properties than thick-walled ones . Quantitative analysis and experiment showed that open-ended CNTs had better field-emission properties than closed-ended ones [8, 9], and it was also found that aligned CNTs had better field-emission properties than random ones . Nowadays, the synthesis of N-doped CNTs has attracted considerable attention. There were many articles reported on the synthesis and properties of N-doped CNTs [10, 11, 12, 13]. It was found that doping nitrogen into CNTs could improve their field-emission properties [14, 15]. Thus, thin-walled open-ended well-aligned N-doped CNTs are expected to have excellent field-emission properties; however, there are few reports on the synthesis of this kind of CNTs. In this study, floating catalyst CVD method was used to synthesize thin-walled open-ended N-doped CNT arrays by using acetonitrile as the carbon source. As-obtained products are multi-walled CNTs and have a large thin-walled index  (TWI, defined as the ratio of inner diameter and wall thickness of a CNT). Furthermore, enhanced field-emission properties were also demonstrated in this study.
The synthesis of vertically aligned CNT arrays was investigated by many researchers [3, 4, 6, 14, 17, 18]; however, it is still a challenge to obtain free-standing membranes of CNTs without destroying their aligned structure. The fabrication of flexible free-standing CNT membranes has been reported by many publications [18, 19, 20, 21, 22, 23, 24, 25, 26, 27]. The applications of the free-standing membranes are in diverse fields, such as lithium ion batteries [21, 25], electromechanical actuators , electron-emitting cathodes , sensor devices , hydrogen fuel cells , and so on. Up to now, the most frequently used method for the fabrication of free-standing membranes is transferring CNTs onto plastic substrates by photolithograph or spin-coating methods [18, 21], and filtration of CNT suspension . However, these methods are somehow limited due to the expensive experimental set-up and/or complex processes. In this study, a simple method was proposed to obtain free-standing membranes of as-synthesized N-doped CNTs, which might be helpful to their applications in many fields.
The experimental setup and procedure are similar to that described in our previous report about Fe-filled CNTs , but we use acetonitrile rather than chlorine-containing benzene as carbon source. Ferrocene powders were dissolved in acetonitrile to form solutions with concentration of 20 mg/ml, and fed into CVD furnace by a syringe pump at a constant rate of 0.4 ml/min for 30 min. A mixture of Ar and H2 was flowing through the system at 2,000 and 300 sccm, respectively. A quartz slide was put into the middle of furnace to collect CNTs at a reaction temperature. In our previous study, we found the suitable reaction temperature for aligned carbon nanotube was 800–840°C using xylene as the carbon source . The reaction temperature in present case is thus set in the range of 780–860°C for investigation.
The scanning electron microscope (SEM) images were obtained by a JOEL JSM-6460 LV SEM. The transmission electron microscope (TEM) images were taken by a TEM with a model of JEM-200 CX, using an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) results were obtained by measuring 6 mg samples in air flow at a heating rate of 20°C/min. The X-ray photoelectron spectroscopy (XPS) spectra were obtained by PHI Quantera. The XPS measurements were carried out in a vacuum chamber of 1.4 × 10−8 Torr, using Al Kα (1486.7 eV) laser excitation. Raman spectra were performed on microscopic confocal Raman spectrometer (Renishow RM 2000) using 632.8 nm (1.96 eV) laser excitation. The field-emission measurements were carried out in a vacuum chamber of 2.2 × 10−6 Torr with CNT samples on silicon wafer as cathode. A glass plate with transparent indium tin oxide (ITO) electrode and phosphor was used as both an anode to collect electrons and a display screen. Distance between anode and top of CNT samples was kept at 2.0 mm.
Results and Discussion
It can be seen from Fig. 2a–2c that the diameter and TWI become larger as temperature rises. The effect of temperature on diameter in present study is similar to that reported by Yadav, et al. , but no obvious temperature effect on TWI was shown in their case. In present study, a possible explanation for temperature effect on TWI is that larger-sized catalyst particles lead to wider inner cavity of the CNTs, and therefore larger TWI is obtained.
The ID/IG ratio of the CNTs produced at the three different temperatures
Field-emission data of different samples, here Eto (V/μm) and Eth (V/μm) are turn-on electric field and threshold electric field, respectively; β is the field-enhancement factor
ZnO nanoneedle arrays
Aligned N-doped CNTs
1.86 × 104
CuO nanoneedle arrays
2.48 × 104
6.41 × 104
7.79 × 104
1.09 × 105
where φ is the work function of CNTs (=5.0 eV ), d is the emitting distance (=2.0 mm), and B = 6.83 × 109 V/(eV3/2 m−1) . The field-enhancement factors were calculated and listed in Table 2, and the results showed that the field-enhancement factors had been significantly improved.
Thin-walled, open-ended, and well-aligned N-doped CNTs were synthesized by using acetonitrile as a carbon source. Temperature effects on diameter, TWI of as-produced CNTs and Fe encapsulation in the CNTs were also investigated. The resulting CNTs grown at 860°C exhibited much enhanced field-emission properties with a low turn-on field (0.27 V/μm), threshold field (0.49 V/μm), and high field-enhancement factor (1.09 × 105). A simple method is proposed to obtain free-standing membranes of this kind of CNTs. The free-standing membranes may find their applications in the supercapacitors, alkaline fuel cells, lithium ion batteries, and heat conductive material.
The authors are grateful to the financial support from the National Natural Science Foundation of China (Grant No. 50902080, 50632040) and China Postdoctoral Science Foundation (Grant No. 20090450021).
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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