Large-scale and controllable synthesis of metal-free nitrogen-doped carbon nanofibers and nanocoils over water-soluble Na2CO3
© Ding et al.; licensee Springer. 2013
Received: 11 July 2013
Accepted: 17 December 2013
Published: 27 December 2013
Using acetylene as carbon source, ammonia as nitrogen source, and Na2CO3 powder as catalyst, we synthesized nitrogen-doped carbon nanofibers (N-CNFs) and carbon nanocoils (N-CNCs) selectively at 450°C and 500°C, respectively. The water-soluble Na2CO3 is removed through simple washing with water and the nitrogen-doped carbon nanomaterials can be collected in high purity. The approach is simple, inexpensive, and environment-benign; it can be used for controlled production of N-CNFs or N-CNCs. We report the role of catalyst, the effect of pyrolysis temperature, and the photoluminescence properties of the as-harvested N-CNFs and N-CNCs.
KeywordsCarbon materials Chemical vapor deposition Water-soluble Nitrogen-doped
Since Iijima’s paper on helical carbon nanotubes, carbon nanomaterials (CNM) such as carbon nanotubes (CNT) and carbon nanofibers (CNF) have attracted great attention for their unique and outstanding electrical and mechanical properties [1–4]. The helical CNT are composed of five-membered or seven-membered rings, having carbon atoms of sp2 and sp3 hybridization [5, 6]. It is envisaged that helical CNT exhibit novel and peculiar properties that are different from those of linear CNT. It has been suggested that CNM can be utilized in hydrogen storage [7, 8], microwave absorption , and field emission [10, 11]. Using CNM, scientists tried to fabricate nanosized electromagnetism devices [12–14] such as solenoid switch [15, 16], miniature antenna [17, 18], energy converter [19, 20], and sensor [21, 22].
For CNM generation, methods such as arc discharge, laser ablation, hydrothermal carbonization, solvothermal reduction, and chemical vapor deposition (CVD) are used [23–28]. Nonetheless, it is common to have metal impurities in the products, and the intrinsic properties of the as-obtained CNM are uncertain. The problem of metal impurities hinders further researches on CNM especially those related to electromagnetism features [29, 30]. It is tedious and costly to remove metal impurities such as those of iron-group elements or their alloys . Furthermore, unexpected defects or contaminants could be introduced into the CNM during purification procedures.
As a traditional method, CVD has its advantages [32, 33]. By regulating parameters such as catalyst amount, reaction temperature, source flow rate, one can obtain different kinds of CNM. It is possible to control the CVD process for a designated outcome by adopting a particular set of reaction conditions [34, 35]. Using acetylene as carbon precursor, Amelinckx et al. , Nitze et al. , and Tang et al.  obtained CNM with high purity and selectivity. Nevertheless, there are disadvantages such as high reaction temperature and outgrowth of desired product [28, 39]. As for the growth mechanism of CNT in CVD processes, there are still controversies [40, 41].
By doping foreign elements such as nitrogen and boron into the graphite lattices of CNM, Wang et al. , Ayala et al. , and Koós et al.  induced crystal and electronic changes to the structures of CNM [42–44]. It is noted that as support for palladium nanoparticles, helical CNM show excellent properties in electro-catalytic applications [45, 46]. According to Franceschini et al.  and Mandumpal et al. , the introduction of nitrogen restrains the aggregation of vacancies, resulting in defects and dislocations, as well as amplified curvature of graphite planes. The results of both experimental and theoretical studies demonstrate that compared to pure CNT, nitrogen-doped CNT show enhanced field emission properties and there is a shift of the dominant emission towards lower energies [49–51]. Through theoretical studies of heteroatom-substituted graphite systems, Hagiri et al. suggested that different heteroatom arrangements cause different spin-stable singlet and triplet states and that the substituted nitrogen atom as a spin cap induces the π electron excess . When it comes to CNT utilization, high incorporation of nitrogen is desirable in promoting porosity and electrochemical reactivity of CNT. On the other hand, if CNT are supposed to be applied in semiconductor technology, low nitrogen-doping density is necessary.
Recently, we reported the large-scale synthesis of various kinds of non-doped CNM that are metal-free [53–55]. Herein, we report the use of Na2CO3 as catalyst for the selective formation of nitrogen-doped CNF (N-CNF) and nitrogen-doped CNC (N-CNC). We used Na2CO3 because it is water-soluble and can be removed from N-CNM through steps of water washing. We found that the Na2CO3 catalyst prepared by us is active and selective for mass formation of N-CNF and N-CNC. By means of CVD using Na2CO3 as catalyst, high-purity N-CNM can be obtained after washing the products with deionized water and ethanol. The approach is simple, inexpensive, and environment-benign, and can be used for mass production of high-purity N-CNF and N-CNC.
All materials used were commercially available and analytically pure. In the present study, we employed Na2CO3 as catalyst. First, we mixed 10 g of Na2CO3 (in powder form) in 200 ml of deionized water at room temperature (RT) with continuous stirring. Once a transparent solution was obtained, the solution was kept at 80°C for several hours and allowed to cool down to RT for the precipitation of a white powder. The powder was filtered out, dried, and ground into tiny particles.
We placed 0.5 g of catalyst at the center of a ceramic boat with two open ends. The boat was then put inside a quartz tube with a thermocouple attached to its center. For the CVD reaction, we used acetylene as carbon source and ammonia as nitrogen source. After the reaction chamber was purged with argon for the elimination of oxygen, the sources were introduced into the system at either 450°C or 500°C at a C2H2/NH3 flow rate ratio of 1:1 for 6 h. To study the effect of changing the flow rate ratio, we also introduced acetylene and ammonia at a C2H2/NH3 flow rate ratio of 5:1 at 450°C for 6 h. After the reaction, argon was again introduced to protect the product from oxidation until the system was cooled down to RT. To remove the catalyst and to avoid organic outgrowth, the as-obtained products were repeatedly washed with deionized water and ethanol. Compared to the methods commonly used for CNM purification, the one used in the present study causes no damage to the desired product.
The morphologies of samples were examined using a transmission electron microscope (TEM) operated at an accelerating voltage of 200 kV and a field emission scanning electron microscope (FE-SEM) operated at an accelerating voltage of 5 kV. Fourier transform infrared (FTIR) spectroscopic studies of samples (in KBr pellets) were conducted over a Nocolet 510P spectrometer (Thermo Nocolet, Stanford, CT, USA). The surface analysis of products was carried out by means of X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, UIVAC-PHI Inc., Chigasaki, Kanagawa, Japan). The products were examined on an X-ray powder diffractometer (XRD) at RT for phase identification using CuKα radiation (model D/Max-RA, Rigaku Corporation, Tokyo, Japan). Raman spectroscopic investigations were performed over a Jobin-Yvon Labram HR800 instrument (Horiba, Ann Arbor, MI, USA) with 514.5-nm Ar laser excitation. The photoluminescence (PL) spectra were collected at RT over a spectrofluorophotometer (Shimadzu RF-5301 PC; Shimadzu Co. Ltd., Beijing, China) using a Xe lamp as light source. For PL investigation, about 0.1 mg of sample was ultrasonically dispersed in 5 ml of deionized water. Thermoanalysis was carried out using a thermal analysis system (NETZSCH STA 449C; NETZSCH Company, Shanghai, China) with the sample heated in air at a rate of 20°C/min.
Results and discussion
Preparation summary of samples
Reaction temperature (°C)
Flow rate ratios (C2H2/NH3)
Nitrogen content of samples
Nitrogen content (at.%)
The I G / I D intensity ratios of all samples
Compared with C5N1, C450N is lower in IG/ID value. The C2H2/NH3 flow rate ratio for the formation of C5N1 is 5:1 whereas that of C450N is 1:1. In other words, with a source flow richer in nitrogen, there is rise of nitrogen content, and with more defects or vacancies in N-CNM, there is decline of IG/ID value. With the rise of reaction temperature from 450°C to 500°C, there is slight decrease of nitrogen content but enhanced formation of amorphous carbon, and the net result is the further decline of IG/ID value.
By controlling the acetylene decomposition temperature, N-CNF and N-CNC can be selectively synthesized in large scale over Na2CO3. Due to the water-soluble property of NaCO3, the products can be obtained in high purity through steps of water and ethanol washing. The CVD process using Na2CO3 as catalyst is simple, inexpensive, and environment-benign. We detect graphitic, pyridine-like as well as pyrrole-like N species in the nitrogen-doped CNM. Compared to the non-doped pristine CNM, the nitrogen-doped ones show enhanced UV PL intensity.
This work was supported by the National Natural Science Foundation of China (grant no. 11174132), the National Key Project for Basic Research (grant nos. 2010CB923402 and 2011CB922102), and PAPD, People’s Republic of China.
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