Nitrogen-doped carbon nanotubes synthesized with carbon nanotubes as catalyst

Nitrogen-doped carbon nanotubes synthesized with carbon nanotubes as catalyst

Diamond & Related Materials 20 (2011) 1353–1356 Contents lists available at SciVerse ScienceDirect Diamond & Related Materials journal homepage: www...

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Diamond & Related Materials 20 (2011) 1353–1356

Contents lists available at SciVerse ScienceDirect

Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond

Nitrogen-doped carbon nanotubes synthesized with carbon nanotubes as catalyst Can Wang a, Zhenghong Huang b, Liang Zhan a,⁎, Yanli Wang a, Wenming Qiao a, Xiaoyi Liang a, Licheng Ling a,⁎ a

State Key Laboratory of Chemical Engineering, Key Laboratory for Specially Functional Polymers and Related Technology of Ministry of Education, East China University of Science and Technology, Shanghai 200237, PR China Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China

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a r t i c l e

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Article history: Received 11 March 2011 Received in revised form 5 July 2011 Accepted 25 August 2011 Available online 3 September 2011 Keywords: Carbon nanotubes Nitrogen-doped carbon nanotubes Catalyst

a b s t r a c t Nitrogen-doped carbon (CNx) nanotubes were synthesized with carbon nanotubes (CNTs) as catalyst by detonation-assisted chemical vapor deposition. CNx nanotubes exhibited compartmentalized bamboolike structure. Electron energy loss spectroscopy and elemental mapping studies indicated that the synthesized tubes contained high concentration of nitrogen (ca. 17.3 at.%), inhomogeneously distributed with an enrichment of nitrogen within the compartments. X-ray photoelectron spectroscopy analysis revealed the presence of pyridine-like N and graphitic N incorporated into the graphitic network. The catalytic activity of CNTs for CNx nanotube growth was ascribed to the nanocurvature and opening edges of CNT tips, which adsorbed Cn/CN species and assembled them into CNx nanotubes. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction Doping carbon nanotubes (CNTs) with nitrogen is a practical way to tailor the electronic, chemical and mechanical properties of CNTs [1]. This has led to an increasing interest in the synthesis and potential applications of nitrogen-doped carbon (CNx) nanotubes. So far, many research groups have reported the CNx nanotube growth by chemical vapor deposition (CVD) using C/N sources [2–4]. Generally, the catalysts adopted are iron-group metals (Fe, Co and Ni) and their alloys (Fe/Co/Ni with Mo/Mg) [5–7]. The unavoidable metal species remaining in the products would result in obvious disadvantages for both intrinsic property characterization (e.g. chemical and magnetic properties) [8,9] and application exploration (e.g. catalysts and biosensors) [10,11] of CNx nanotubes. Despite such problems, it is found that the nitrogen doping level and bonding configuration differ depending on the used catalyst [12,13]. These have promoted the motivation for exploring nonmetal catalysts for the synthesis of CNx nanotubes. Lu et al. [14,15] reported the synthesis of CNTs by detonationassisted CVD, and validated the self-catalytic ability of CNTs [16]. The above studies inspire us to investigate the possibility of growing CNx nanotubes with CNTs as catalyst. In this account, compartmentalized CNx nanotubes were expectably produced with CNTs as catalyst by detonation-assisted CVD. The nitrogen concentration, distribution and bonding configuration of CNx nanotubes were characterized by Transmission electron microscopy (TEM), Electron energy loss spectroscopy

⁎ Corresponding authors. Fax: + 86 21 64252914. E-mail addresses: [email protected] (L. Zhan), [email protected] (L. Ling).

(EELS) and X-ray photoelectron spectroscopy (XPS). Finally, a growth mechanism of CNx nanotubes is proposed. 2. Experimental CNTs as catalyst were purchased commercially (Chengdu Organic Chemicals Co., Ltd., CAS), which have been purified with HCl solution to eliminate the metal catalysts. In the detonation-assisted CVD, the starting materials (2 g picric acid, 0.02 g CNTs and 0.5 g melamine) were physically pre-mixed and then put in a sealed stainless steel pressure vessel (10 ml). The detonation was induced by external heating (20 °C/min) to 310 °C. The detonation reaction, occurring on a microsecond scale, generated about 40 MPa pressure (shock wave) and 900 °C temperature inside the vessel. After the reaction, the vessel was cooled in air and emptied of gaseous products, and then the solid products were collected. TEM was carried out on a FEI TECNAI G2 S-TWIN operated at 200 kV. EELS was obtained by using a JEOL-2010 F equipped with an electron energy loss spectrometer (Gatan Image Filter 200). XPS was performed on a Kratos AXIS Ultra using Mg anode. 3. Results and discussion CNTs as catalyst (Fig. 1a) exhibited hollow cores, with diameters of 10–20 nm and tube wall thickness of about 7 nm. Unlike the CNTs, the newly formed CNx nanotubes (Fig. 1b) derived from the detonation of mixture containing 2 g picric acid, 0.02 g CNTs and 0.5 g melamine, exhibited compartmentalized bamboo-like structure. The diameters of CNx nanotubes were in a range of 40–70 nm and the thickness of tube wall was about 3 nm. As a reference, eliminating

0925-9635/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.08.013

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Fig. 1. (a) TEM image of CNTs used as catalysts. (b), (c) TEM images of CNx nanotubes. The arrowed objects are the remaining CNTs. (d) EELS spectra of CNTs and CNx nanotubes.

CNTs from the explosive mixture led to a material containing only amorphous carbon spheres and debris, which indicated that the presence of CNTs was crucial for CNx nanotube formation. The large difference in the diameters and channel structures of CNTs and CNx nanotubes suggested that CNx nanotubes were the catalytic product of CNTs, rather than the result of CNT amination. On the other hand, the original CNTs were sheared into short nanotubes by the shock wave and left behind in the detonation product (Fig. 1c). These short nanotubes normally disjointed with the newly formed CNx nanotubes, similar to the result observed in the self-catalytic synthesis of CNTs [16]. This phenomenon is interesting and different from the metal catalysis in which CNTs usually contain metal particles at the ends. We speculate that the CNTs may be ejected from the CNx nanotubes by the newly formed compartment layers in the channels of CNx nanotubes. The detailed catalytic mechanism of CNTs for the CNx nanotube growth will be discussed later. In order to confirm the compositions of CNx nanotubes and CNTs, EELS measurement was performed. EELS spectrum of CNx nanotubes (Fig. 1d) showed the presence of ionization edges at ca. 284.5 and 400 eV corresponding to the C–K and N–K edge, respectively, in contrast to that of CNTs with only C–K edge at ca. 284.5 eV [5]. Quantitative analysis revealed that the nitrogen concentration was ca. 17.3 at.%. Based on the above results, the distinct structure and composition of CNTs and CNx nanotubes unambiguously validate the catalytic activity of CNTs for CNx nanotube growth by detonation-assisted CVD. The distribution of nitrogen in the CNx nanotubes was examined by mapping the elements using TEM with a Gatan image filter. The zero-loss image of CNx nanotubes (Fig. 2a) showed a bamboo-like structure. The carbon and nitrogen mapping images (Fig. 2b and c)

revealed that nitrogen was incorporated in the CNx nanotubes with nonuniform distribution. It was observed that the compartment layers were brighter than the tube wall, suggesting increased N content in the former. Due to the high concentration N-doping, the contrast of carbon and nitrogen mapping images was reduced. To obtain a more accurate value of the atomic distribution of nitrogen in CNx nanotubes, we measured the EELS spectra (Fig. 2d) at the tube wall and compartment layer of a single CNx nanotube. Both the spectra had distinct absorption features corresponding to C-K and N-K edge. The nitrogen concentration at the tube wall and compartment layer was calculated to be ca. 15 and 19.6 at.%, respectively, agreeing well with the mapping results. The bonding character within the CNx nanotubes could be reflected by the fine EELS edge structure [17]. The C-K edge (Fig. 2d) revealed the distinguished π* and σ* features, suggesting an sp 2 hybridization state for carbon. Likewise, the welldefined π* absorption feature for the N-K edge onset at 396 eV was assigned to sp 2-bonded nitrogen within hexagonal network. The triangle-shaped σ* absorption feature for the N-K edge was a signature of pentagonal defect and corrugations with sp 3-bonding character. No evidence of the existence of N2 was observed in the spectra. The similarity of the position and line shape of N-K edge for the tube wall and compartment layer suggested that the nitrogen bonding configurations were the same but their abundance was larger in the compartment. High resolution TEM image of a single CNx nanotube (Fig. 3a) showed that the graphene layers were corrugated and turbostratic, indicating that defects induced by nitrogen doping were formed within hexagonal graphitic network. To further investigate the element composition and bonding configurations of CNx nanotubes,

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Fig. 2. (a) TEM image of CNx nanotubes. Corresponding energy-filtered TEM: (b) carbon and (c) nitrogen mapping images of CNx nanotubes. (d) EELS spectra taken from the tube wall and compartment layer of a single CNx nanotube.

XPS analysis was carried out. Fig. 3b shows the full-range XPS spectra of CNx nanotubes, depicting the existence of C1s, N1s and O1s signals corresponding to the main peaks centered at 284.6, 399.0 and 531.8 eV, respectively [18]. The strong C peak was assigned to the π* feature associated with sp 2-hybridized carbon. The O peak might be originated from the adsorbed oxygen or water on the CNx nanotubes surface. The nitrogen concentration was ca. 18 at.%, consistent with the EELS result. The N1s peak was asymmetric, indicating the existence of at least two components. The N1s peak could be deconvoluted

into three peaks at 398.9, 400.5 and 405.4 eV, corresponding to the pyridine-like N, graphitic N and the physically adsorbed molecular N2, respectively. The pyridine-like N versus graphitic N gave a ratio of about 1:1, commensurate with the incorporation of ca. 9 at.% pyridine-like N within the sp 2 carbon network. The high concentration of pyridine-like N induced the deformation and corrugation of graphene layers, as demonstrated by DFT calculations [19]. Since the building blocks of CNx nanotubes, i.e., Cn/CN species, can't be dissolved in CNTs, the growth of CNx nanotubes with CNTs

Fig. 3. (a) High resolution TEM image of a CNx nanotube. (b) N1s spectrum of CNx nanotubes. The inset shows the full-range XPS spectrum of CNx nanotubes.

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as catalysts did not follow the vapor–liquid–solid mechanism, which is well recognized for CNT/CNx nanotube growth with iron-group metals as catalysts [12,20,21]. Associated with the recent reports about metal-catalyst-free growth of multi-walled/single-walled CNTs with nanodiamond, nanocrystalline carbon, semiconductor nanoparticles and oxide nanoparticles as catalysts, the catalytic function of catalysts is mainly size dependent [22–25]. Accordingly, we suggest that the CNT tips may have the catalytic ability for the nucleation and growth of CNx nanotubes, which is supported by the disjunctive feature of CNTs and CNx nanotubes and the compartmentalized structure of CNx nanotubes. The detailed growth mechanism of CNx nanotubes is described as follows. In the detonation, an extremely fast decomposition of explosives generated enormous thermal energy and powerful shockwave on a microsecond timescale. Melamine was decomposed into Cn/CN species and CNTs were sheared into short nanotubes with open tips as previously described (Fig. 1c). In the absence of metal catalysts, these short CNTs served as catalyst to catalyze the growth of compartmentalized CNx nanotubes. It was speculated that the nanocurvature and opening edges of the open tips of short nanotubes adsorbed Cn/CN species and assembled them into CNx nanotubes. The compartmentalized structure of CNx nanotubes was induced by the high concentration of Cn/CN species supplied by the detonation on a microsecond timescale and the high nitrogen doping level of CNx nanotubes. The CNTs as catalysts were ejected from the CNx nanotubes by the newly formed compartment layers, resulting in the disjunctive feature of CNTs and CNx nanotubes. Further study is under way to investigate why the nitrogen is inhomogeneously distributed in the tube wall and compartment and the detail information about how the CNT catalysis performs. 4. Conclusions CNTs were demonstrated to be an effective nonmetal catalyst for the synthesis of CNx nanotubes by detonation-assisted CVD. The synthesized CNx nanotubes exhibited compartmentalized structure and contained high concentration of nitrogen (ca. 17.3 at.%). XPS revealed the presence of graphitic N and pyridine-like N incorporated into the graphitic network. Unlike the metal catalysts, CNTs as catalyst have no carbon solubility and only surface diffusion is involved in the growth of CNx nanotubes, which are useful for understanding the growth mechanism of CNx nanotubes in-depth. The synthesis of CNx

nanotubes without metal catalysts will facilitate the applications of CNx nanotubes. Acknowledgments This work made use of the resources of the Beijing National Center for Electron Microscopy. We thank the National Natural Science Foundation of China (NO. 50730003, 50672025, 20806024 and 51002051), the Research Fund of China 863 program (No. 2008AA062302) and Shanghai Leading Academic Discipline Project (B502) for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]

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