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Carbon

Volume 48, Issue 11, September 2010, Pages 3033-3041

Carbon journal image

Comparison of structural changes in nitrogen and boron-doped multi-walled carbon nanotubes

  1. ⁎ Corresponding author.
  2. ⁎⁎ Corresponding author.
  3. a Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
  4. b A.M. Prokhorov General Physics Institute RAS, 38 Vavilov Str, 119991 Moscow, Russia

Research highlights

Abstract

We investigated the effect of the reaction parameters on the structure of multi-walled carbon nanotubes containing different concentrations of nitrogen and boron. The nanotubes were produced using a ‘standard’ aerosol chemical vapour deposition technique in conjunction with benzylamine, triethylborane, hexane and toluene mixtures. These precursors were thermally decomposed between 800 and 1100 °C under argon at atmospheric pressure. By varying the precursor concentrations, the nitrogen and boron content of the nanotubes could be altered between 0–2.2 and 0–0.5 at.% respectively. Using a typical laboratory-sized 50 cm long tube furnace, yields between 0.3 and 1.5 g of nanotubes/10 min were relatively easily achieved. Moreover, we show that doping carbon nanotubes with heteroatoms, such as B and N, can be used to control nanotube diameters, change their defect density, and manipulate their oxidation resistance within a range of ca. 170 °C. Hence, we show that it is possible to tune nanotube properties within a certain interval and to produce nanotubes with relatively well defined properties in quantities usable for further characterisation and for studying their viability in applications such as composite materials, gas sensors, capacitors, and electronic components.

1. Introduction

Carbon nanotubes (CNTs) [1] are the subject of widespread research due to their outstanding properties [2,3] but precise control of their properties has yet to be realised. Moreover, a vast number of potential applications of conventional and doped nanotubes can be found in the literature, however, these are limited by the low yields and general availability of nanotubes possessing well defined properties. Accurate control over the nanotube properties is essential if the many applications envisaged are indeed to be realised. Theoretical [4] and experimental studies [5–7] have shown that it is possible to tailor the electronic properties of the nanotubes by replacing some of the carbon atoms with heteroatoms [8]. Furthermore the incorporation of these heteroatoms also changes the nanotube structure [9,10], chemical reactivity [11] and mechanical properties [12], presenting the possibility of controlling nanotube properties.

Previously, we reported that by using N as a dopant the morphology of the CNTs could be manipulated [13] and here, we describe how we extended the interval in which fine tuning of the nanotube properties is possible. For example, B acts as a p-type dopant, while N is n-type [8]. It has been reported that B promotes nanotube growth [14], whilst N inhibits growth [15] and additionally that B increases the oxidation resistance [16], as N decreases it [17].

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Therefore, it appears that B and N possess complementary properties with regard to controlling nanotube morphology and properties. The atom sizes of boron and nitrogen are similar to that of carbon; hence, it is relatively easy to incorporate them into the graphitic network.

It was shown that the incorporation of nitrogen or boron atoms within the graphitic carbon network strongly depends on the choice of precursor, catalyst, reaction temperature, reaction time, gas flow rate and pressure [18–21]. Therefore, nanotubes with well defined properties can only be produced if the effects of each of these experimental parameters on the nanotube properties are understood. Although several articles have been published about the synthesis of doped nanotubes [19–35], comparison of the results obtained is difficult as different experimental setups were used, thus the influence of one chosen parameter on the morphological changes and properties of the nanotubes cannot be determined. The dependence of the nanotube structure on individual synthesis parameters can only be understood if one parameter is changed at a time. Therefore, we investigated the parameter–property relationship by individually varying parameters to identify general trends in order to develop a route to controlling nanotube properties.

Practical applications of nanotubes necessitate fairly large amounts of material with well defined properties at commercially viable prices [36]. Aerosol chemical vapour deposition (CVD) appears to be the most feasible synthesis method for industrial scale production of CNTs. It is suitable for the continuous injection of catalyst precursor and carbon feedstock, it requires no additional catalyst preparation step and the samples need minimal or no purification [37]. Using aerosol CVD it is also possible to produce doped nanotubes without flammable or corrosive gases [9].

This study herein was carried out in order to identify and compare structural trends in nitrogen- and boron-doped carbon nanotubes produced via aerosol CVD as a function of the synthesis parameters. The nanotubes were analysed using the following characterisation techniques: scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and thermogravimetric analysis (TGA).

2. Experimental

Conventional carbon nanotubes (MWCNTs), N-doped (CNX), B-doped (CBX) and N–B-doped (CNXBY) carbon nanotubes were produced using an aerosol-based CVD system consisting of a piezo-driven aerosol generator connected to a quartz tube (2.2 cm inner diameter) placed in a 50 cm long horizontal electrical furnace [13]. Solutions of 5 wt.% ferrocene (Fe(C5H5)2, Aldrich 98%) in mixtures of toluene (T) (C6H5CH3, Fluka 99.7%), benzylamine (BA) (C6H5CH2NH2, Fluka 99%) or 1 M solutions of triethylborane in hexanes (TEB) ((CH3CH2)3B, CH3(CH2)4CH3, Aldrich) were prepared in an ultrasonic bath. Toluene was used as precursor for pure MWCNTs. CNX nanotubes were produced from 5:95, 10:90 and 100:0 volume percentage mixtures of benzylamine and toluene, while for CBX nanotubes mixtures of TEB and toluene (5:95 and 10:90) were used. CNXBY nanotubes were synthesised with 5:1:94, 10:2:88 and 10:90:0 mixtures of TEB, benzylamine and toluene. For CNT and CNX nanotubes the furnace was operated at 800 and 900 °C, whilst the CBX and CNXBY nanotubes were generated at temperatures between 900 and 1100 °C. All experiments ran for 10 min while the argon flow was kept at 2500 sccm. During heating and cooling of the furnace the aerosol generator and the quartz tube were flushed with 100 sccm argon. The soot was collected from the inner wall of the quartz tube with a sharp metal tool.

3. Results and discussions

3.1. Overall structural investigation via SEM and TEM

SEM studies revealed that the CNTs and CNX nanotubes grew perpendicularly to the quartz substrate, forming flakes of “parallel” aligned nanotubes. In contrast to CNX nanotubes, CBX and CNXBY nanotubes were not attached to the quartz surface and they did not form flakes. Pure CNTs produced from toluene contained many kinks and were undulating (wavy). However, the number of kinks in CNTs could be reduced by adding 5% of N containing hydrocarbon (benzylamine) to the precursor. The number of kinks in CNX nanotubes decreased as more nitrogen was added to the precursor. Above 10% benzylamine in the precursor the nanotubes were free of kinks. In contrast to that, the presence of boron in the precursor did not change the number of kinks observed in the nanotubes. Generally, the density of kinks in CNTs, CBX and CNXBY nanotubes was found to be comparable. Representative SEM micrographs of the CNTs, CNX and CBX nanotubes are shown in Fig. 1.

Figure 1

Fig. 1: SEM images of nanotubes made from toluene (a and b), benzylamine (c and d), mixture of 10 TEB:90 toluene (e and f), and 10 TEB:2 benzylamine:88 toluene (g and h). It is noteworthy, that the nanotubes produced from toluene only and TEB:toluene mixture contained many knees, while the tubes made from benzylamine were straight.

The length of pure CNTs produced from toluene at 800 °C was ca. 110 ± 11 μm whereas CNX nanotubes synthesised from benzylamine were 17 ± 2 μm long, i.e. only 15% of the length of the CNTs synthesised from toluene. This result shows that N slows down the nanotube formation significantly, which leads to shorter tubes [15]. Unfortunately, the length of the CBX and CNXBY nanotubes could not be identified since they occurred in cotton wool-like bundles only and did not form flakes consisting of aligned tubes. However it was observed that CBX and CNXBY nanotubes formed a several mm thick elastic soot layer, which may be an indication that B doping considerably increased the length of carbon nanotubes [14].

TEM investigations showed that pure CNTs and CNX nanotube samples were ‘clean’, i.e. they contained less than 5 wt.% amorphous carbon and almost no polyhedral carbon particles. CBX and CNXBY nanotube samples produced above 1000 °C were also clean, yet samples synthesised at 900 °C contained more than 10 wt.% amorphous carbon. Fe catalyst particles encapsulated inside the nanotube core or attached to the nanotube surface were also observed.

The weight of the all samples produced using different concentrations of benzylamine was similar (ca. 0.3 g/experiment), despite the change in nanotube length and diameter. For the B-containing samples made from 10 TEB:90 toluene mixtures it was observed that the weight of the samples increased with temperature to more than 1.5 g nanotubes per 10 min experiment at 1100 °C.

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At concentrations above 10% TEB in the precursor nanotube production ceased. The influence of B may be explained by the decrease of C solubility in Fe–C–B system as the temperature decreases and B content increases [38].

Apart from changing the overall morphology of the nanotubes, e.g. the number of kinks present in the nanotubes, the incorporation of nitrogen and boron in the carbon lattice also altered the inner structure of the nanotubes. TEM revealed (Fig. 2 depicts TEM images of samples synthesised at 800 and 900 °C) that the samples contained nanotubes exhibiting stacked-cone structure, nanotubes consisting of corrugated walls, and more crystalline nanotubes, i.e. nanotubes which exhibit fewer defects in the carbon lattice. The high-resolution TEM images show the differences in the nanotube wall structure which ranges from fairly crystalline pure CNTs (Fig. 2b) to the typical stacked-cone structure found in CNX nanotubes (Fig. 2d). Corrugation of the walls already occurs in tubes made from precursors containing 5% benzylamine. Similarly the walls of CBX nanotubes were also corrugated, but the walls were less undulated for CNXBY nanotubes than in CNX nanotubes (Fig. 2h). The formation of nanotubes with corrugated walls is caused by the presence of N or B in the graphitic network, which induces curvature of the graphitic layer [15,39]. When both N and B were incorporated in carbon nanotubes their effect attenuated each other.

A summary of the outer diameter distribution of the different sets of nanotube samples, based on several hundreds of measurements of TEM images, is represented by the graph depicted in Fig. 3.

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Figure 2

Fig. 2: TEM images of nanotubes made from toluene (a and b), benzylamine (c and d), 10:90 TEB:toluene mixture (e and f) and 10:90 TEB:benzylamine mixture (g and h). CNTs contained defects typical to CVD, but had fairly crystalline structure (b). The N-doped nanotubes were straight, while their walls were corrugated (d). However the B-doped nanotubes contained many knees and corrugated walls, but the CNXBY nanotube walls were less undulated (h).

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These measurements revealed that the average diameter of the pure CNTs was 62 nm, 52 nm for 5% benzylamine, and it decreased to 30 nm for CNX nanotubes grown from benzylamine only (please see also the comparison on Fig. 6). Concurrently, the change in the FWHM was also found to be significant. Hence the nanotube diameters appear to be highly dependent on the N.

Figure 3

Fig. 3: Outer diameter distribution of the nanotubes. Compared with the CNTs, the diameter of the CNX nanotubes decreased with the increase of benzylamine content of the precursor. About 5% TEB in toluene increased, while 10% TEB decreased the diameter of CBX nanotubes. The CNXBY nanotubes were thinner than CNTs. (Tol = toluene, BA = benzylamine, TEB = 1 M solution of triethylborane in hexanes).

Figure 6

Fig. 6: Comparison of the nanotube diameter distribution, Raman intensity ratios, and oxidation resistances.

The addition of B-containing compounds also resulted in changes of the average nanotube diameter. However, depending on the TEB content, it was possible to increase or decrease the diameter of CBX nanotubes. Based on TEM images, 5% TEB in precursor increased the average diameter of CBX nanotubes to 77 nm, while 10% TEB decreased to 47 nm. The combination of boron and nitrogen together reduced the diameter of the CNXBY nanotubes compared to pure CNTs. The average diameter of the CNXBY nanotubes produced from 5:1:94 and 10:2:88 mixtures of TEB, benzylamine and toluene were 58 and 49 nm, respectively. The most frequent diameter was nearly the same, 46 and 44 nm, respectively. So, using precursors with the same B/N ratio it is possible to grow nanotube samples with similar most frequent diameters, but different properties. However, the average diameter of the nanotubes produced from 10 TEB:90 benzylamine mixtures was only 27 nm and the most frequent diameter was reduced to 22 nm. Consequently, the doping with nitrogen and boron gives the possibility to tune the diameter of carbon nanotubes in a broad range (Fig. 6). Moreover combining B and N doping results in nanotubes with less corrugated walls.

During nanotube growth, N appears to be more stable at the growing edges of the tube than within the hexagonal carbon lattice. Therefore, it is assumed that the nanotube edges are likely to be N-saturated. Consequently, the incorporation of carbon atoms within the nanotube is most likely hindered. Therefore, at high N concentrations in the precursor, the growth of large-diameter nanotubes was inhibited [15]. Contrary to N, and B does not interfere with formation of the nanotubes directly, but alters the substrate–catalyst interaction and impedes the attachment of the catalyst particles on the quartz surface. Hereby, B changes the catalyst diameter distribution and consequently the nanotube diameter distribution.

3.2. Raman spectroscopy

Raman spectroscopy was used to compare the structural disorder of the nanotubes [40,41]. Representative spectra are shown in Fig. 4, while the D/G and 2D/G intensity ratios are compared in Fig. 6.

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The MWCNTs contained defects typical to MWCNTs grown via CVD methods, which can be observed both on TEM and SEM micrographs. As expected, the introduction of heteroatoms increased the defect density in all cases, i.e. increased the intensity of the D line and decreased that of the 2D line. For CNX nanotubes the defect density increased together with the benzylamine ratio of the precursor, showing significant increase at low nitrogen content and slower increase at high benzylamine concentrations. The CBX nanotubes contained even more defects, but the defect concentration did not depend on TEB content of the precursor. In the case of CNXBY nanotubes produced from precursor containing 1% and 2% benzylamine the presence of nitrogen decreased slightly the D/G intensity ratio, but it remained approximately 1 for nanotubes made from 10 TEB:90 benzylamine mixtures.

Figure 4

Fig. 4: Representative Raman spectra of carbon nanotubes. The doping increased the defect density.

The D/G intensity ratios measured with Raman spectroscopy confirmed the increase of the disorder and the corrugated nature of the nanotube walls observed with TEM (Fig. 2). The D/G intensity ratios close to 1 showed that the incorporation of heteroatoms decreased significantly the crystallinity of the nanotubes. It is important to note that the influence of boron was especially strong, since less than 0.1 wt.% B in the precursor increased considerably the defect concentration. Contrarily, at low concentrations nitrogen does not cause as significant increase in the D/G intensity ratio in the same way boron does. Therefore, the doping of carbon nanotubes increased the defect density in all cases [42], yet it was possible to tune it to a certain extend (Fig. 6).

3.3. Oxidation resistance measured via TGA

The oxidation resistance of the nanotubes was determined using TGA (Figs. 5 and 6). In order to compare the samples with the highest possible N and B content, we choose the nanotubes made at the lower end of the optimum parameter interval. We measured the samples produced at 800 °C from toluene and benzylamine, and at 900 °C from TEB containing precursors.

Figure 5

Fig. 5: TGA measurements. Nitrogen doping decreases while boron doping increases the oxidation temperature of carbon nanotubes. In CNXBY nanotubes the effects of nitrogen and boron doping are compensated.

The oxidation resistance of the nanotubes decreased as the benzylamine content of the precursor increased, but interestingly the samples produced from precursors containing 5% benzylamine were more inert than pure CNTs. Differently to CNX nanotubes, the oxidation resistance of CBX samples increased with higher TEB concentrations in the precursor. For CNXBY, produced from 5:1:94 and 10:2:88 mixtures of TEB, benzylamine and toluene the oxidation temperature did not appear to depend on the boron or nitrogen content and was similar to the CBX samples made for 5% TEB solution. However, the oxidation resistance of CNXBY nanotubes produced from precursors with high benzylamine concentrations, e.g. mixtures of 10 TEB:90 benzylamine, was comparable to that of pure MWCNTs. Hence, the oxidation temperature of the nanotubes could be tuned within a ca. 170 °C interval (Fig. 6).

CNX nanotubes exhibited an increasing density of ‘defects’ and wall reactivity due to the N incorporation within the carbon network. A sharp weight decrease can be observed in the TGA spectra of samples produced with high benzylamine concentrations, which indicates that the CNX nanotubes are likely to react and oxidise along their entire surface, rather than only at their tips or randomly distributed defects as is commonly observed for pure MWCNTs. This increased reactivity and uniform defect distribution of CNX may help the sidewall functionalisation of the N-doped nanotubes and improve their interaction with other materials, such as matrix in composites [43,44].

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In contrast to nitrogen, boron tends to inhibit the oxidation process, similar to the more traditional B-doped carbon fibers [16]. Generally, boron is known to protect carbonaceous materials by carbon active sites poisoning (for low percentages of boron) or by forming a B2O3 film at the surface of the carbonaceous material (for high percentages of boron) [45], the boron oxide coated graphite specimens were impervious to oxygen at temperatures below 815 °C [46]. In the case of CNXBY nanotubes the effect of nitrogen and boron compensated for each other, i.e. the presence of nitrogen weakened the protection of boron. This fact provides ground for the possibility to accurately tune the reactivity and therefore oxidation resistance of the nanotubes.

3.4. Characterisation of the nitrogen content via XPS and EELS

The incorporation of nitrogen and boron heteroatoms within nanotubes was confirmed by XPS and EELS measurements. According to XPS the CNX samples produced at 800 °C from toluene precursors mixed with 5% and 10% benzylamine exhibited 0.28 ± 0.09 and 0.49 ± 0.16 at.% nitrogen, respectively. The nitrogen content increased to 2.2 ± 0.3 at.% in samples produced from benzylamine only. EELS showed that the CNX nanotubes made from benzylamine contained approximately 2.5 at.% nitrogen. Very unfortunately, our EELS measurements were not sensitive enough to quantify the nitrogen concentration of CNX nanotubes produced from precursors with lower benzylamine content. An increase of the synthesis temperature of only 100 °C, i.e. from 800 to 900 °C, resulted in an overall lower nitrogen content in the nanotubes, e.g. the nitrogen concentration was roughly reduced by half. XPS measurements indicated that the CBX nanotubes made from 10 TEB:90 toluene mixtures at 900 °C contained approximately 0.25 at.% B, while the CNXBY nanotubes made from 10 TEB:2 benzylamine:88 toluene mixtures contained the same amount of N and B, i.e. ca. 0.1 at.%. Therefore it can be concluded that the presence of N decreased the B content of the nanotubes. The doping of CBX and CNXBY nanotubes was also confirmed by EELS, but the quantification was not accurate.

4. Conclusions

Multi-walled carbon nanotubes with different concentrations of nitrogen and boron were produced by aerosol CVD from toluene, benzylamine and triethylborane mixtures at high yields, e.g. 0.3 and 1.5 g of nanotubes/10 min. According to TEM and SEM studies the increasing benzylamine content in the precursor decreased the number of kinks incorporated into CNX nanotubes, decreased their length and diameter. In contrast to benzylamine, TEB did not decrease the number of kinks incorporated into boron-doped nanotubes, and increased or decreased the diameter depending on experimental parameters. The nitrogen and boron content of the samples could be tuned; the highest nitrogen content was 2.2 at.%, while the highest boron content was 0.5 at.%, respectively. TGA measurements showed that the presence of nitrogen decreased whilst the presence of boron increased the oxidation resistance of the nanotubes. So it was possible to tune the oxidation temperature of the nanotubes in 170 °C wide interval.

We compared N- and B-doped nanotubes made in the same experimental setup at similar experimental parameters, which allowed accurate comparison of the samples. Using a cheap and scalable method, we were able to produce nanotubes with well defined properties at high yields. Our results show the relationship between the experimental parameters and nanotube properties, and reveal the possibility to easily control the properties by changing the precursors. These results can be used to produce nanotubes with desired properties and to extend the range of their application, especially in composite materials, gas sensors, and electronic components.

Acknowledgements

This work was supported by the European Union FP6 Project BNC nanotubes 033350 (NG, AAK, FD)the Royal Society (NG), ERC Starting Grant (ERC-2009-StG-240500) (NG), BegbrokeNano. The authors would like to thank Michael Dowling for his help with the initial synthesis experiments.

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