High pressure study of nitrogen doped carbon nanotubes using Raman spectroscopy and synchrotron X-ray diffraction

1 Introduction

Carbon nanotubes, star materials, have already shown the most active research fields and promising potential applications in wide range of fields due to their unique molecular and nano-structures, chemical and physical properties of high electronic conductivity, low density, corrosion resistance, excellent mechanical performance and the low cost (Fan et al., 1999, de Jonge et al., 2002). Similar to new carbon materials, substitutional doping of carbon nanotubes may lead to chemical activation of the passive surface of the CNTs and add additional electronic states around Fermi level. Extrinsically doped tubes, such as nitrogen doped carbon nanotubes (CN_x-NTs) owing to the enhanced π bonding and the strong electron donor behaviors, are considered as excellent starting candidates for a new generation of controlled chemically functionalized nanotubes-based materials (Ayala et al., 2010).

High pressure as an external force is used widely to explore new structures, new properties of current existing materials. Carbon has rich forms, such as graphite, diamond, carbon nanotubes (CNTs), amorphous carbon, carbines, fullerences (e.g. C60). So far, there have been a lot of studies on all the forms of carbon and novel high pressure structures (e.g., bct-C, H-, S-, M-, W-, and Z-carbon and so on) with special properties were found in addition to the known forms mentioned above (Kumar et al., 2007; Li et al., 2009; Umemoto et al., 2010; Sakurai and Saito, 2011; Wang et al., 2011; Amsler et al., 2012; He et al., 2012; Wang et al., 2012; Xu et al., 2015) Without exception, CNTs with different wall thickness were studied extensively under high pressure (Tang et al., 2000; Tangney et al., 2005; Yusa and Watanuki, 2005; Caillier et al., 2008; Yao et al., 2008; Abouelsayed et al., 2010; Kuntscher et al., 2010; Aguiar et al., 2011; Sakurai and Saito, 2011; Lu et al., 2013; Xu et al., 2015; Pashkin et al., 2016). High pressure treatment of CNTs may active C—C as a partial pyramidalization of the sp² carbon configuration (Meyyappan, 2004), which may provide unpredictable products with unknown properties. In recent years, performances of CNTs under high pressure have been briefly studied by several groups using diamond anvil cells (Tangney et al., 2005; Yusa and Watanuki, 2005; Maldonado et al., 2006; Kawasaki et al., 2007; Kumar et al., 2007; Papagelis et al., 2007; Nakayama et al., 2008; Yao et al., 2008; Merlen et al., 2009; Sakurai and Saito, 2011; Pashkin et al., 2016). Results show a reversible sp² to sp³ transformation in carbon, polymerization and a structural transition associated with a G-band plateau phenomenon in their Raman shift change, which represents a circular to an ellipse shape change and then to a peanut/flatten-shaped at 6.5–10 GPa by Raman spectroscopy (Yusa and Watanuki, 2005; Yao et al., 2008). In strong contrast to extensive study of CNTs, element doped CNTs, for example CNTs with nitrogen doped, were barely studied. However, in the case of CN_x-NTs, even if the degree of structural disorder with increase in nitrogen content from 0 to 10% exhibit exceptional mechanical properties (Maldonado et al., 2006). The doping of nitrogen changes electronic structures, and induces more defects to CNTs, therefore whether they could be deformed, collapsed, or transformed new carbon phases with novel electronic and mechanical properties under high pressure is still unclear. So far there is no available reported data on polygonization, phase transformation or new carbon forms for CN_x-NTs under high pressure up to 20 GPa so far. Such studies may also be helpful to gain a better understanding of the effect of minor amounts of elements on the detailed phase relations and properties (Shao et al., 2008; Merlen et al., 2009; Tohru et al., 2012).

In this work, high pressure behaviors of CN_x-NTs in terms of phase transformations and compressibility were investigated in comparison with none nitrogen doped CNTs using both Raman spectroscopy and synchrotron X-ray diffraction (XRD). Effect of N doped CNTs on high pressure behaviours of carbon nanotubes were discussed based on the results as well as reference values. This study not only helps to understand the properties of CN_x-NTs under high pressure, but also helps to rich high-pressure phase diagram of graphite-related carbon materials. Moreover, this study could also shines light into CNTs designing with new features by studying how doped elements affect the properties of CNTs, for example the mechanic properties.

2 Material and methods

CN_x-NTs was successfully synthesized via a floating catalyst CVD method, which has been reported elsewhere (Liu et al., 2010). The products were synthesized on silicon wafers under an argon/ethylene flow with ferrocene as the catalyst precursor and melamine as the nitrogen source. Briefly, the silicon substrates were located in the center of the reaction chamber while the melamine and ferrocene mixture was placed at the entrance of the furnace. The mixture was heated up to 350 °C to let both precursors sublimate. Meanwhile, ethylene was introduced into the system and maintained for 15 min. Then, the system was cooled down naturally to room temperature in the flowing Ar gas. In order to get rid of catalyst particles, the as-synthesized CN_x-NTs were washed and stirred in a mixture of 1 M HCl and 1 M HNO₃ at 80 °C for 2 h. The samples were then characterized by Philips CM10 transmission electron microscopy (TEM) operated at 80 kV and Kratos Axis Ultra Al (alpha) X-ray photoelectron spectroscopy (XPS) operated at 14 kV to quantify the N content and assumptions on the chemical state of nitrogen in carbon nanotibes. The details of characterization were described in elsewhere (Liu et al., 2010). The CN_x-NTs sample used in this works had a nitrogen content of 8.4%.

High pressure experiments of both CN_x-NTs and CNTs were carried out in a diamond anvil cell (DAC). The culet size of the diamond anvils was 300 μm. A hole with diameter of 90 μm was drilled at the center of the stainless steel gasket and used as the sample chamber. Then the sample was loaded in the sample chamber together with a ruby ball, the fluorescence of which was used as the pressure calibrant (Mao et al., 1986). For both CN_x-NTs and CNTs, a 4:1 methanol to ethanol mixture was used as the pressure-transmitting medium. High pressure Raman measurements were carried out using a customized micro-Raman system. A monochromatic argon laser was used as the excitation source with a 514.5 nm wavelength. The Raman system was at the backscattered geometry, and the scattering signal was collected by a 0.5 m spectrometer equipped with a liquid-nitrogen cooled CCD detector at a resolution of 1 cm⁻¹. Each Raman spectrum was collected for 1800 sec.

High pressure Angle-dispersive X-ray diffractions (XRD) were performed at beamline X17B3, at National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). A monochromatic X-ray which had a wavelength of 0.3866 Å was employed, and its beam size was reduced to 15 × 25 µm² by a KB mirror. A MAR imaging plate was used to collect the sample’s 2D Debye-Scherrer patterns. The experimental parameters were calibrated using the CeO₂ standard diffraction. For each pressure point, the XRD pattern was collected for 60 mins. Then the data collected were processed using the Fit2D program for further analysis.

Moreover, structures of the retrieved samples were also examined by a JEOL 2010F high resolution transmission electron microscopy (HRTEM) operated at 200 kV.

3 Results

3.1

3.1 Morphology characterization

Morphology of the synthesized CNT and CN_x-NTs were examined by TEM as shown in Fig. 1. The TEM images show the CNTs have perfect cylindrical structures with a outer tube diameter around 300 nm. In strong contrast to CNTs, the CN_x-NTs (with x = 8.4% nitrogen content) exhibit an abundance of curved graphite layer step edges in cylindrical tubes, and its tube diameter was similar as the CNTs.

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Fig. 1

TEM images of CNTs (a) and CN_x-NTs (b) before compression. The scale bars are labeled at bottom of each figure.

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3.2

3.2 Raman results of CNTs and CN_x-NTs under high pressure

For comparison purpose, both carbon nanotubes without and with nitrogen doped were investigated under high pressure using Raman spectroscopy and XRD with synchrotron radiation, and the results were shown in Fig. 2. As shown in Fig. 2a, two peaks are observed at 1584 cm⁻¹ and 1630 cm⁻¹ at ambient condition. The peak originally located at 1584 cm⁻¹ is assigned to the G band of CNTs, which is attributed to the tangential mode vibrations of the C atoms in graphite or multiwall carbon nanotubes (Dresselhaus et al., 2005). The other peak initially positioned at 1630 cm⁻¹ is associated with the defect induced Raman mode (D′) (Ferrari 2007; Pimenta et al., 2007). Upon compression, the G band Raman profile becomes weaker and broader, which also shifted to higher frequency with increasing pressure. In comparison with the G band, the D′ peak almost remained unchanged, resulting in a peak merging at 10.4 GPa and then separating at 22.4 GPa. At 28.0 GPa, both peaks are significantly broadened that are hard to identify, which indicated pressure induced disordering occurred during the compression. However, after pressure was completely removed from the sample, the vanished two peaks reappeared suggesting the retrieved sample has the same structure as the original sample, and the pressure induced sample disordering is reversible.

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Fig. 2

Raman spectra of CNTs (a) and CN_x-NTs (b) collected under high pressure. Peak assignments as well as pressure for each spectrum are labeled beside. The spectrum on the top shows the Raman spectrum of the retrieved sample. The spectra below are those taken upon compression.

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As shown in Fig. 2b, the two peaks associated with the G and D′ bandswere found in CN_x-NTs as well at 1580 cm⁻¹ and 1630 cm⁻¹ respectively. The G-band in CN_x-NTs is 4 cm⁻¹ lower than the nitrogen-free CNTs, and its full width at half maximum (FWHM) is also broadened by 55 cm⁻¹ (45 cm⁻¹ for CNTs vs 100 cm⁻¹ for CN_x-NTs). As mentioned above, the G band presents the tangential mode vibration of carbon atoms in the carbon nanotubes. The red shift of the G band may be due to the C—C expansion and the changes of electronic structures in CN_x-NTs in comparison with nitrogen free CNTs (Casiraghi et al., 2005; Liu et al., 2010). The broadness of the G band is related to the increasing defect content in the carbon nanotube induced by the doped nitrogen which introduced more defects into the crystals (Liu et al., 2010). In addition, intensity of the defected induced D′ band in CN_x-NTs was almost the same as the G band, which further supported the increased defects in CN_x-NTs in comparison with CNTs. This was also in good agreement with the observation from TEM images in Fig. 1 and previous reports (Maldonado et al., 2006; Koós et al., 2009). Unlike the CNTs, no emerging of two peaks is observed in CN_x-NTs upon compression. Instead, the intensity of the G band decreases dramatically as the pressure increases, and eventually becomes completely undiscerned at 20 GPa. In strong contrast, the D′ peak is still clearly seen. After pressure was released, the Raman spectrum of retrieved sample shows a peak recovery, but the Raman shift of the G band was found ∼10 cm⁻¹ higher than that of the original sample which might due to the pressure induced C—C bond reduction. Such recovery suggested the retrieved CN_x-NTs have similar structure as the original sample but with certain structural modification caused by the pressure.

3.3

3.3 XRD results of CNTs and CNx-NTs under high pressure

In order to investigate the structural evolution, both CNTs and CN_x-NTs samples were studied by synchrotron XRD up to around 20 GPa. The corresponding XRD results are shown in Fig. 3. As shown in Fig. 3(a), two reflections are observed at ambient pressure in CNTs, which are assigned to reflections (0 0 2) and (1 0 0), respectively. The (0 0 2) reflection is assigned to the interlayer spacing along the radial direction (c-axis), while the (1 0 0) reflection is along the in-plane direction (a-axis).(Yusa and Watanuki 2005) Upon compression, the (0 0 2) reflection exhibits a great shift towards higher angle, while the (1 0 0) reflection is almost unchanged as the pressure increases. Therefore, such observation indicates that interlayer relation along the radial direction (c-axis) is more sensitive to pressure than that along the in-plane direction (a-axis). Upon further compression, the XRD patterns of the CN_x-NTs remained almost unchanged up to 6.8 GPa, beyond which two new reflections appeared. Similar diffraction patterns have been found in high pressure studies of crystalline graphite. (Li et al., 2009, Wang et al., 2012) In their studies, the M-carbon was identified by the appearance of two new reflections located to the left and to the right of the (1 0 0) peak. In this study, same scenarios were found in the XRD pattern suggesting the new high pressure phase can be attributed to monoclinic M-carbon phase. This high pressure M-carbon phase persisted to the highest pressure achieved in this study. However, after the pressure was completely removed, the M-carbon phase was no longer existed in the XRD profile indicating such high pressure phase is only stable under high pressure. The retrieved XRD pattern was the same as the one taken before compression, suggesting the structural phase transition is reversible and the M-carbon phase reverts back to graphite-like one or undergoes partial amorphization upon pressure release.

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Fig. 3

XRD patterns of CNTs (a) and CN_x-NTs (b) collected under high pressure upon compression. Pressure for each pattern is labeled beside.

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4 Discussions

Although all Raman peaks for both samples became weak and broad under high pressure, we managed to obtain the Raman shifts by fitting the corresponding Raman peaks with Voigt components. Raman shifts as a function of pressure for CNTs and CN_x-NTs were plotted and shown in Fig. 4. In CNTs, both the D′ and G bands showed linear tendency without any abrupt slope change upon the whole compression range indicating there was no phase transformation happened in CNTs during compression. However, the shift rates for the D′ and G bands were dramastically different (0.24 cm⁻¹/GPa for D′ band vs 3.39 cm⁻¹/GPa for G band) suggesting the G band is more sensitive to the pressure than the D′ band. In strong contrast, the G band in CN_x-NTs exhibited a more complex pressure response, although its D′ band showed a linear tendecy at a rate of 0.33 cm⁻¹/GPa as well. The G band first shifted to lower frequency at a rate of −4.20 cm⁻¹/GPa, and then a slope platform was found between 1.0 GPa and 7.3 GPa. The 7.3 GPa was extrapolated by fitting the experimental data linearly as seen in Fig. 4. Beyond 7.3 GPa, the Raman shift rate dropped to −2.52 cm⁻¹/GPa. Such abrupt slope changes implied two possible phase transformations might occur at 1.0 GPa and 7.3 GPa, respectively, in CN_x-NTs during compression.

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Fig. 4

Raman shifts as a function of pressure for CNTs and CN_x-NTs. Solid square symbols represent the Raman shift of G-band obtained from compression, while the solid circle ones are obtained from the defects of diamonds. Error bar for each data point is also added. The solid lines are drawn only for eye guidance.

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Similar phase transformations below 3 GPa have been found in single wall carbon nanotubes as well as double wall carbon nanotubes (Peters et al., 2000, Caillier et al., 2008, Abouelsayed et al., 2010, Kuntscher et al., 2010, Aguiar et al., 2011). It has been concluded that this phase transition was due to tube cross-section change from circle to an oval or elliptical shape. However, in the double wall carbon nanotubes such tube cross-section change happened initially to the outer tube shape change and then to the inner tube shape by further compression (Aguiar et al., 2011). In this study, it is more reasonable to attribute the first phase transition to the outter tube shape change rather the inner tube shape change since the CN_x-NTs was a multiwall carbon nanotube (Liu et al., 2010). However, no such phase transition was observed in none-nitrogen dopened multiwall CNTs. As shown in Fig. 1, the wall thickness of CNTs is much thicker than CN_x-NTs, which might enhance the stability of carbon tube that no such tube shape change occured in lower pressure range. The second phase transformation is observed at 7.3 GPa in the Raman results. Similar phase transition was also found in single wall nanotubes studied indicated by the flatform of G band’s Raman shift but at a relative lower pressure of 5 GPa (Lu et al., 2013). Such phase transition is believed to relate to a flattened cross-section forming. However, in this study, the phase transition pressure was enhanced which is 2.3 GPa higher than that for single wall CNTs. Such enhancement is due to the increase in wall thickness that the outer tubes are mechanically supported via their interaction with the inner tubes, which increases the pressure required to collapse the tubes in comparison with single wall CNTs (Aguiar et al., 2011).

One dramatically different observation for the second phase transition in CN_x-NTs is the appearance of the high pressure M-carbon phase, which was not evidenced in either single wall or multiwall CNTs. However, the M-carbon phase has been reported in graphite at 14 GPa or higher pressure both theoretically and experimentally. (Li et al., 2009, Wang et al., 2012) Their studies also show that the high pressure M-carbon phase is a dynamical stable phase which is only stable above 13.4 GPa (Li et al., 2009). In strong contrast, the appearance of this M-carbon phase was found around/below 10 GPa which is significantly lower than the stable pressure of M-carbon phase in graphite. Since no such structural change has been found in CNTs, it is reasonable to speculate that the nitrogen doping in CNTs might help to lower barrier/condition to form M-carbon phase. It is well known that the wall thickness plays an important role in altering stability of carbon nanotubes (Xiao et al., 2006, Pashkin et al., 2016). However, by comparing high pressure behaviours of CN_x-NTs with those of carbon in forms of single-wall CNTs, multi-wall CNTs, and graphite, the nitrogen doped CNTs behaved like none of them. Therefore, the incorporation of nitrogen atoms makes high pressure behaviours of carbon nanotubes more complex and unpredictable, and the doping of nitrogen in addition to the wall thickness ruled the high pressure behaviours of CN_x-NTs. Especially the doping of nitrogen could also help to reduce the formation pressure of M-carbon phase required.

In addition, in this work the G-band of CN_x-NTs showed a blue shift upon compression. According to previous high pressure studies of CNTs, it has been concluded that the blue shift of the G-band represented the change of the double degenerate (E_2g) phonon and the SP² to SP³ C—C bonding of CNTs during the shape shift from circular to ellipse-like shape involving band distance extension, which further confirmed the phase transformation is due to the tube shape change interpreted above (Prawer et al., 2000, Liu et al., 2010). Moreover, the nitrogen doped in the CN_x-NTs studied in this work existed as C—N⚌C bonds (Liu et al., 2010). Therefore, the absence of the red shift of the G band instead of blue shift in CN_x-NTs might also be ascribed to the breakdown of N⚌C double bond which resulting in extension in the bond distance slightly. Furthermore, in comparison with CNTs, the overall sensitivity of the G band is significantly reduced by two times, which might also be attributed to the enhanced bond strength in CN_x-NTs induced by the replacement of C atoms by N atoms.

The d-spacings of reflections (0 0 2) and (1 0 0) were also plotted against the pressure as seen in Fig. 5. The (0 0 2) reflection is more compressible than the (1 0 0) reflection, where the (1 0 0) remained almost unchanged upon the compression process. The (0 0 2) reflection has been assigned to the interlayer spacing in the radial direction (c-axis), while the d-spacing of (1 0 0) reflection is in the in-plane direction (a-axis). Therefore, it suggests that both carbon nanotubes with or without nitrogen doping are mainly compressed along the c-axis. Both samples are extremely incompressible along the a-axis, which is consistent with results found in the single wall carbon nanotube, multiwall carbon nanotube as well as graphite (Shen et al., 2000, Tang et al., 2000, Yu et al., 2000, Yusa and Watanuki 2005, Xiao et al., 2006). It is more interesting that carbon nanotube without nitrogen doped is more single direction preferable as its (1 0 0) reflection is almost unchanged. In strong contrast, the (1 0 0) reflection in CN_x-NTs shows slightly decreasing tendency, which indicated the carbon nanotubes with nitrogen doped was also possibly compressed along the axial direction. Such increasing in compressibility is highly likely attributed to the increased defects in CN_x-NTs due to nitrogen doping, since defects is also one of the important factor that affact materials’ compressibility (Dong and Song 2009). Above 10 GPa, two reflections of the new high pressure M-carbon phase were also plotted in Fig. 5, both of which show extremely incompressible as the (1 0 0) reflection. Such observation indicates the M-carbon phase has the potential as a superhard material. This is consistent with the references that the M-carbon phase is considered as the superhard polymorph phase of carbon which is even harder than c-BN (Li et al., 2009, Wang et al., 2012).

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Fig. 5

Comparison of d-values of (0 0 2) and (1 0 0) between CNTs and CN_x-NTs upon compression. The solid circles are data of CNTs, and the open circles are data of CN_x-NTs. The solid and dashed lines are only for eye guidance.

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To further study the compressibility, unit cell volumes of CN_x-NTs and CNTs upon compression were plotted vs pressure and shown in Fig. 6. Then the experimental data were fitted using the third order Birch-Murnaghan Equation by fixing the B₀′at 4. The obtained bulk moduli B₀ for CNTs and CN_x-NTs are 48.9 (0.7) GPa and 67.0 (0.4) GPa, respectively. The B₀ value of CNTs in this work is similar to values of multiwall carbon nanotube and Graphite considering the uncertainty for the reference, which further confirmed the conclusion that multiwall carbon nanotube act more likely as graphite (Hanfland et al., 1989, Zhou et al., 1994, Yusa and Watanuki 2005). The bulk modulus of CN_x-NTs is higher than both CNTs and Graphite. It should be noted in this study although both CNTs and CN_x-NTs are found much more compressible in comparison with diamond (441 GPa), they do show high hardness along the a-axis direction as well as the new M-carbon phase as shown in Fig. 5.

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Fig. 6

The EOS of CN_x-NTs and CNTs upon compression. Solid square and circle symbols denote the experimental data for CNTs and CN_x-NTs, respectively. The EOS was obtained by fitting the third order Birch-Murnaghan equation.

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The retrieved CN_x-CNT sample after compression was also examined by the high resolution transmission electron microscope (HRTEM) and the results were shown in Fig. 7. It was clearly shown that the compressed nanotube recovered their original tube shape and structures, which was consistent with the Raman results. However, the magnified image in Fig. 7(b) also shows that some structures failed to retain their initial structures and resulted in graphite like structures which are clearly amorphourized. These observations attributed to the background increase in both Raman and XRD experiments. Moreover, such amorphous carbons are highly possible attributed to the quenched phase from the new high pressure phase observed upon compression indicated by XRD. Such observation further confirmed that the new high pressure phase was an unstable phase since it was unattainable at ambient pressure.

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Fig. 7

The high resolution TEM images of retrieved CN_x-CNTs sample after compression with different magnifications. The image b was the magnified TEM of the circled part in image a.

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5 Conclusions

In this work, high pressure behaviours of both CN_x-NTs and CNTs were studied and compared using Raman spectroscopy and synchrotron X-ray diffraction. Upon compression, two phase transitions were found in CN_x-NTs by examining the Raman shift of G band against the pressure, while no such phase transformations were identified in CNTs. In comparison with references, the two phase transformations can be assigned to tube shape change from circular to ellipse-like and then to flatten shape. Moreover, a new high pressure M-carbon phase was evidented by the synchrotron XRD. In addition, it has been revealed that both CNTs and CN_x-NTs are more compressible along the radial direction, indicating the tube collapse (shape change) mainly happens in such direction. Both samples imply extraordinary high stiffness along the axial direction. By comparing high pressure behaviors of CN_x-NTs with Carbon in single wall, multiwall CNTs and graphite, it was found the doping of nitrogen into carbon nanotubes make high pressure behaviours of CNTs more complex and the doping of nitrogen as well as the wall thickness together affect high pressure behaviors of CN_x-NTs. Moreover, the doping of nitrogen could also help to reduce the formation pressure of M-carbon phase required. Finally, the TEM images also show the retrieved CN_x-NTs were in a partially amorphousized state, which was believed due to new high pressure phase formed at high pressure.

Conflicts of interest

The authors declare no conflicts of interest.

Declarations of interest

None.