From isotope labeled CH₃CN to N₂ inside single-walled carbon nanotubes

Abstract

The observation of one-dimensional N₂ inside single-walled carbon nanotubes raises the questions, how are the N₂ molecules formed and how do they manage to make their way to this peculiar place? We have used N¹⁵ and C¹³ isotope labeled acetonitrile during the synthesis of single-walled carbon nanotubes to investigate this process. The isotope shifts of phonons and vibrons are observed by Raman spectroscopy and X-ray absorption. We identify the catalytic decomposition of acetonitrile as the initial step in the reaction pathway to single-walled carbon nanotubes containing encapsulated N₂.

---

1 Introduction

The ability of multi-walled carbon nanotubes (MWNTs) to encapsulate N₂ was first demonstrated in ref. 1. If N₂ is enclosed in the as-synthesized MWNTs, the nanotubes exhibit a compartmentalized bamboo structure that traps pockets of N₂.^2,3 The detailed distribution of encapsulated N₂ and N incorporated into the walls of MWNTs has been directly imaged by polarized scanning transmission X-ray microscopy.⁴ The alignment of N₂ in the confining walls has also been observed by X-ray absorption.^5,6 A commonly observed effect of using N-containing precursors in the growth of single-walled (SW) and MWNTs by chemical vapor deposition (CVD) is the increased curvature in the emerging sp² networks. In MWNTs this is seen as a topological curvature resulting in the formation of separate compartments. In SWNTs, however, reducing diameters increase the geometrical curvature.^7–11

In the case of MWNTs, the N₂ gas is partially aligned on the walls of the nano-compartments. For a truly one-dimensional phase of nitrogen, much more narrow single-walled carbon nanotubes with diameters smaller than one nanometer are required. Such configurations of aligned N₂ chains have been predicted and expounded using density functional theory.¹² Only recently have one-dimensional, highly aligned N₂ inside SWNTs been confirmed by polarized X-ray absorption spectroscopy.¹³ The commonly accepted mechanism for the encapsulation of N₂ has been put forth in ref. 14. Very briefly, C≡N radicals react inside the catalyst particle and release N₂, which then emerges into the currently growing compartment. While the correlation of encapsulated N₂ with the use of N containing precursors is generally accepted, direct identification of the reaction pathway by isotope labeling has not yet been achieved. Moreover, it remains an open issue whether or not the proposed C≡N radicals come from intermediate gas-phase HCN or if they are formed during the catalytic decomposition of CH₃CN.

Here we utilize either CH₃CN¹⁵ or CH₃C¹³N to label the pathways of C and N in the proposed intermediate C≡N units. We see that the two inequivalent carbons in CH₃CN are incorporated into the sp² network of the SWNT in equal proportion, while the N₂ is formed from the N¹⁵. We deduce that the direct catalytic processing of entire CH₃CN molecules at the catalyst surface is the initial step in the reaction pathway leading to N₂-filled SWNTs.

2 Materials & methods

Vertically aligned single-walled carbon nanotubes (VA-SWNTs) were grown by no-flow chemical vapor deposition.^10,15 We used either 1.5 vol% CH₃CN¹⁵ added to ethanol feedstock or pure CH₃C¹³N to synthesize vertically aligned SWNTs containing N¹⁵₂ or N¹⁴₂. The CH₃CN¹⁵ and CH₃C¹³N were purchased from Cambridge Isotope Laboratories, Inc. The isotope purity is given to be >98%. X-ray photoelectron spectra (XPS) were recorded with a PHI 5000 VersaProbe setup. X-ray absorption spectroscopy (XAS) was conducted at beamline BL27SU at the SPring-8 synchrotron facility. The beamline is dedicated to soft X-ray absorption spectroscopy.^16,17 To maximize the signal intensity from the encapsulated N₂ molecules (∼600 ppm) we used a slit-limited resolution of 80 meV, a stepsize of 10 meV and an acquisition time of 3 s. Additionally, all spectra were recorded at 75° grazing incidence using p-polarized X-rays for maximum attainable peak intensities.¹³ We normalized the XAS spectra by dividing the drain current of the samples by the simultaneously recorded drain current at the last focusing mirror. To obtain the best possible comparison, N¹⁵₂ and N¹⁴₂@VASWNTs were always mounted onto the same sample holder and were always measured without any delay using identical settings.

3 Results & discussion

3.1 Stoichiometry of N and C

The high yield of N₂ over pyridinic or substitutional N bonding was confirmed by XPS. Fig. 1 shows the survey scan of clean, as-synthesized N¹⁵₂@VA-SWNTs in the lower panel. During the root growth process the CoMo catalyst particles stay attached to the substrate and only pure SWNTs emerge upwards. Hence, the single prominent spectral feature is the C1s at 284.6 eV. At this scale the O1s at 530 eV and N1s at 404 eV are hardly discernible. Detailed scans of the N1s and C1s region are shown above the survey scan. The C1s shows an asymmetric Doniach Šunjić lineshape with the secondary π electron shake up. The lineshape is typical for metallicity-mixed SWNTs.^18,19 The N1s shows a single prominent peak of molecular N₂ at 404.5 eV. The N1s and C1s were measured with identical resolution settings and can be directly compared. The atomic cross-section of nitrogen is 1.8 times that of carbon,²⁰ therefore the abundance of N as compared to C is ∼600 ppm. If we assume a typical macroscopic density of VA-SWNT material to be 50 μg cm⁻³, the density of stored N₂ would be equivalent to that of N₂ gas at 340 mbar and 0 °C. Inside densely packed SWNT bundles (1.3 g cm⁻³) the stored density of N₂ would correspond to that of N₂ gas at 8.8 bar and 0 °C. The N/C stoichiometry of 600 ppm is approximately 10 times lower than the 1.5 vol% of CH₃CN in C₂H₅OH used during SWNT synthesis. The noisy signal just above 401 eV may be tentatively assigned to traces of NO_x. The signal from substitutional or pyridinic N is expected to be in the range from 398–400 eV, and is evidently not detectable.

Fig. 1 XPS spectra of N1s (top left), C1s (top right) and survey scan (bottom) of N¹⁵₂@VA-SWNTs.

3.2 Fractionation of C¹³ and C¹²

Isotope labeling of the carbons in the feedstock is a viable technique for investigating reaction pathways. We grew SWNTs from pure CH₃C¹³N and regular CH₃CN. The Raman spectra of these two samples are shown in Fig. 2.

Fig. 2 Raman spectra of SWNTs made of natural C¹³ : C¹² = 99 : 1 (blue) and C¹³ : C¹² = 48 : 52 (red). The excitation wavelength is 633 nm.

The high-frequency optical phonon modes undergo a softening and broadening due to the increased mass and phonon–phonon scattering rates.^22–24 The G mode of SWNTs synthesized from pure isotopic CH₃C¹³N reveals a significant downshift. The actual concentration of C¹³ can be readily calculated from eqn (1)

here ω_N is the G-band frequency of naturally occurring C¹² material, which contains 1.1% C¹³. The observed maxima of the tangential G modes ω_N = 1594.8 and ω(c) = 1564.3 yield an abundance c = 48% C²³ in the nanotube walls. The C¹² and C¹³ in CH₃C¹³N have—under the current conditions—roughly the same probability to contribute to growth. The reaction pathways in ethanol have only been recently elucidated in a study using site-selectively isotope-labeled C₂H₅OH, where either one or both of the C atoms were replaced with C¹³. In particular it was demonstrated that under equivalent synthesis conditions the reaction pathway for ethanol is dominated by catalytic decomposition, which has a balanced isotope fractionation.²⁵

The even fractionation of both inequivalent C atoms strongly suggests that the entire CH₃CN molecule arrives at the catalytic site, where it is then further processed. We conclude that at 800 °C thermal decomposition is negligible since the initial dehydrogenation step is thermally inaccessible.²¹ Detailed reaction processes on the catalyst particle during the synthesis are, however, obscured from macroscopic post-synthesis investigations.

In our specific case a scenario may be devised by considering the binding energies. The binding energy of C≡N is 891 kJ mol⁻¹ and that of free C₂ is calculated to be on the order of 300 kJ mol⁻¹.^21,26 We suggest that during dissolution the C–C bond and all H bonds in CH₃CN break, after which both C and C≡N diffuse into or on the metal catalyst particle. The inert N₂ is released when two C≡N react. The individual steps are illustrated in Fig. 3. This pathway implies that all the N from CH₃CN is processed via the catalyst particle before it is released as inert N₂.

Fig. 3 Steps in the proposed reaction pathway from CH₃CN to dissolved C₂ and inert N₂.

In the case of pure CH₃CN feedstock there is clearly not enough room inside the SWNT to accommodate all the N₂ that is expelled from the active catalyst particle. Even at sufficiently low concentrations of N (e.g., less than 3 vol% would roughly correspond to the saturation value of 1 at%), most of the N₂ is released into the environment. The low trapping probability is in accordance with reduced nanotube diameters rooting from larger sized catalyst particles.¹⁰ In the perpendicular growth mode, most of the catalyst particle surface area is not covered by the growing nanotube.²⁷ Therefore the size of the catalyst particle determines the trapping efficiency of N₂, but it does not determine the diameter of the growing nanotubes.

3.3 Isotope shift in N¹⁵₂

X-ray absorption is the method of choice to investigate the possible isotope shift in the vibron energy of N₂. With the natural isotope abundance of over 99% N¹⁴, the equidistant satellites in the XAS spectrum are separated by 233 ± 2 meV.²⁸ For pure N¹⁵ a spacing of 225 ± 2 meV is expected from scaling with . The isotope shift in the vibrons of isotope mixed N¹⁴N¹⁵ was observed by Raman spectroscopy to be 4 meV.²⁹ At a full width at half maximum of 140 meV and a typical uncertainty of ±2 meV, very smooth spectra from a ∼600 ppm fraction in the specimen are required to reveal the subtle isotope shift. The XAS spectra in Fig. 4 show the normalized data before and after background subtraction. The lowest resonance energy corresponds to the direct N1s absorption event, and higher resonance energies correspond to the concomitant excitation of one or more vibrons. The isotope shift between N¹⁴₂ and N¹⁵₂ may already be visible to the trained eye in the original spectra. After subtracting the background, it is much easier to recognize the equidistant series of four well-resolved peaks. A fifth peak is included in the fitting procedure, but its relative weight is considered insufficient for quantitative comparison.

Fig. 4 High resolution XAS of the vibron satellites in the N1s resonance of N¹⁴₂ (black) and N¹⁵₂ (red)@VA-SWNTs. Raw data are circles, solid lines are fits and dashed lines are individual peaks or the background, respectively.

The four Voigtian peaks that are resolved with reliable peak positions reveal an increasing offset due to the isotope shift. The peak positions are marked by vertical lines, and seem to be systematically shifted to lower resonance energies with increasing vibron numbers. We find an average vibron energy of 228 ± 2 meV, which is in reasonable agreement with the expected value. We are surely at the limit of how accurately the peak positions could be determined from the actual line shapes in one individual spectrum, yet the direct comparison of the N1s region of N¹⁴₂ and N¹⁵₂ shows a noticeable isotope shift. The biggest isotope shift is expected for the fourth peak, and would be 24 meV (or halfway between 2 and 3 data points). This corresponds to the shift in peak position that is visible in the actual data and is within experimental accuracy.

The obtained N1s XAS spectra confirm the expected isotope shift of N¹⁴₂ and N¹⁵₂. Moreover, following the synthesis and XPS measurement the samples were stored for over six months under ambient conditions until the scheduled beamtime at SPring-8. The extended period over which N¹⁵₂ was retained proves that the as-grown capped SWNTs are tight molecular containers.

4 Conclusions

We used isotope-labeled acetonitrile CH₃C¹³N and CH₃CN¹⁵ to trace the reaction pathway from acetonitrile to SWNTs containing encapsulated N₂. The balanced incorporation of both inequivalent C atoms into acetonitrile, as evidenced by Raman spectroscopy, is the signature of a catalytic dissolution of the entire molecule. The very strong C≡N bond further suggests that all N molecules are present during dissolution in the catalyst metal. The geometry of a perpendicular growth mode limits the area for encapsulating N₂ that is released when two C≡N react at the catalyst particle. The vibron shift in the N1s X-ray absorption spectra clearly identifies stably encapsulated N¹⁵₂ from CH₃CN¹⁵ inside SWNTs.

Acknowledgements

Beamtime at SPring-8 was granted for proposal 2013A1107. CK is an APART fellow (Grant no. A-11456) of the Austrian Academy of Sciences. TT is supported by the office of the higher education commission of Thailand.

References

1. M. Terrones, R. Kamalakaran, T. Seeger and M. Ruhle, Chem. Commun., 2000, 2335–2336.
2. H. C. Choi, S. Y. Bae, W.-S. Jang, J. Park, H. J. Song, H.-J. Shin, H. Jung and J.-P. Ahn, J. Phys. Chem. B, 2005, 109, 1683–1688.
3. J. Liu, Y. Zhang, M. I. Ionescu, R. Li and X. Sun, Appl. Surf. Sci., 2011, 257, 7837–7844.
4. J. G. Zhou, J. A. Wang, H. Liu, M. N. Banis, X. L. Sun and T. K. Sham, J. Phys. Chem. Lett., 2010, 1, 1709–1713.
5. A. V. Okotrub, L. G. Bulusheva, A. G. Kudashov, V. V. Belavin, D. V. Vyalikh and S. L. Molodtsov, Appl. Phys. A: Mater. Sci. Process., 2009, 94, 437–443.
6. A. V. Okotrub, M. A. Kanygin, L. G. Bulusheva and D. V. Vyalikh, Fullerenes, Nanotubes, Carbon Nanostruct., 2010, 18, 551–557.
7. P. Ayala, A. Grüneis, C. Kramberger, M. H. Rümmeli, I. G. Solorzano, F. L. Freire Jr and T. Pichler, J. Chem. Phys., 2007, 127, 184709.
8. E. M. M. Ibrahim, V. O. Khavrus, A. Leonhardt, S. Hampel, S. Oswald, M. H. Rummeli and B. Buchner, Diamond Relat. Mater., 2010, 19, 1199–1206.
9. P. Ayala, R. Arenal, A. Loiseau, A. Rubio and T. Pichler, Rev. Mod. Phys., 2010, 82, 1843–1885.
10. T. Thurakitseree, C. Kramberger, P. Zhao, S. Aikawa, S. Harish, S. Chiashi, E. Einarsson and S. Maruyama, Carbon, 2012, 50, 2635–2640.
11. T. Thurakitseree, C. Kramberger, A. Kumamoto, S. Chiashi, E. Einarsson and S. Maruyama, ACS Nano, 2013, 7, 2205–2211.
12. R. E. Barajas-Barraza, J. A. Ramirez-Ruiz and R. A. Guirado-Lopez, J. Comput. Theor. Nanosci., 2008, 5, 2255–2263.
13. C. Kramberger, T. Thurakitseree, H. Koh, Y. Izumi, T. Kinoshita, T. Muro, E. Einarsson and S. Maruyama, Carbon, 2013, 55, 196–201.
14. J. H. Yang, D. H. Lee, M. H. Yum, Y. S. Shin, E. J. Kim, C. Y. Park, M. H. Kwon, C. W. Yang, J. B. Yoo, H. J. Song, H. J. Shin, Y. W. Jin and J. M. Kim, Carbon, 2006, 44, 2219–2223.
15. H. Oshima, Y. Suzuki, T. Shimazu and S. Maruyama, Jpn. J. Appl. Phys., 2008, 47, 1982–1984.
16. H. Ohashi, E. Ishiguro, Y. Tamenori, H. Okumura, A. Hiraya, H. Yoshida, Y. Senba, K. Okada, N. Saito, I. H. Suzuki, K. Ueda, T. Ibuki, S. Nagaoka, I. Koyano and T. Ishikawa, Nucl. Instrum. Methods Phys. Res., Sect. A, 2001, 467, 533–536.
17. H. Ohashi, E. Ishiguro, Y. Tamenori, H. Kishimoto, M. Tanaka, A. Irie, T. Tanaka and T. Ishikawa, Nucl. Instrum. Methods Phys. Res., Sect. A, 2001, 467, 529–532.
18. C. Kramberger, H. Rauf, H. Shiozawa, M. Knupfer, B. Büchner, T. Pichler, D. Batchelor and H. Kataura, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 75, 235437.
19. P. Ayala, Y. Miyata, K. De Blauwe, H. Shiozawa, Y. Feng, K. Yanagi, C. Kramberger, S. R. P. Silva, R. Follath, H. Kataura and T. Pichler, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 80, 205427.
20. J. Scofield, J. Electron Spectrosc. Relat. Phenom., 1976, 8, 129–137.
21. A. Lifshitz and C. Tamburu, Int. J. Chem. Kinet., 1998, 30, 341–347.
22. F. Simon, C. Kramberger, R. Pfeiffer, H. Kuzmany, V. Zólyomi, J. Kürti, P. M. Singer and H. Alloul, Phys. Rev. Lett., 2005, 95, 017401.
23. C. Kramberger, M. Löffler, M. Rümmeli, A. Grüneis, R. Schönfelder, O. Jost, T. Gemming, T. Pichler and B. Büchner, Phys. Status Solidi B, 2006, 243, 3050–3053.
24. S. D. Costa, C. Fantini, A. Righi, A. Bachmatiuk, M. H. Rümmeli, R. Saito and M. A. Pimenta, Carbon, 2011, 49, 4719–4723.
25. R. Xiang, B. Hou, E. Einarsson, P. Zhao, S. Harish, K. Morimoto, Y. Miyauchi, S. Chiashi, Z. Tang and S. Maruyama, ACS Nano, 2013, 7, 3095–3103.
26. S. Shaik, D. Danovich, W. Wu, P. Su, H. S. Rzepa and P. C. Hiberty, Nat. Chem., 2012, 4, 195–200.
27. M. F. C. Fiawoo, A. M. Bonnot, H. Amara, C. Bichara, J. Thibault-Penisson and A. Loiseau, Phys. Rev. Lett., 2012, 108, 195503.
28. C. T. Chen, Y. Ma and F. Sette, Phys. Rev. A: At., Mol., Opt. Phys., 1989, 40, 6737–6740.
29. M. Musso, F. Matthai, D. Keutel and K. L. Oehme, J. Chem. Phys., 2002, 116, 8015–8027.

---

Footnotes

† Present address: Maejo University, Chiang Mai, Thailand.

‡ Present address: University at Buffalo, Buffalo, USA.

---