From Isotope Labeled Ch 3 Cn to N 2 inside Single-walled Carbon Nanotubes

The observation of one-dimensional N 2 inside single-walled carbon nanotubes raises the questions, how are the N 2 formed and how do they manage to make their way to this peculiar place? We have used N 15 and C 13 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 N 2 .


Introduction
The ability of multi-walled carbon nanotubes (MWNT) to encapsulate N 2 was first demonstrated in Ref. 1 .If N 2 is enclosed in as-synthesized MWNT, the nanotubes exhibit a compartmentalized bamboo structure that traps pockets of N 2 . 2,3The detailed distribution of encapsulated N 2 and N incorporated to the walls of MWNT has been directly imaged by polarized scanning transmission x-ray microscopy. 4The alignment of N 2 in the confining walls has also been observed by xray absorption. 5,6A commonly observed effect of using Ncontaining precursors in the growth of single-walled (SW) and MWNT by chemical vapor deposition (CVD) is increased curvature in the emerging sp 2 networks.In MWNT this is seen as topological curvature resulting in the formation of separate compartments.[9][10][11] In the case of MWNT, the N 2 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 N 2 chains have been predicted and expounded using density functional theory. 12nly recently was one-dimensional, highly aligned N 2 inside SWNT confirmed by polarized x-ray absorption spectroscopy. 13The commonly accepted mechanism for the encapsulation of N 2 has been put forth in Ref. 14  Here we utilize either CH 3 CN 15 or CH 3 C 13 N to label the pathways of C and N in the proposed intermediate C − − − N units.We see that the two inequivalent carbons in CH 3 CN are incorporated into the sp 2 network of the SWNT in equal proportion, while the N 2 is formed from the N 15 .We deduce that the direct catalytic processing of entire CH 3 CN molecules at the catalyst surface is the initial step in the reaction pathway leading to N 2 -filled SWNT.

Materials & Methods
Vertically aligned single-walled carbon nanotubes (VA-SWNT) were grown by no-flow chemical vapor deposition. 10,15We used either 1.5 vol.%CH 3 CN 15 added to ethanol feedstock or pure CH 3 C 13 N to synthesize vertically aligned SWNT containing N 15 2 or N 14 2 .The CH 3 CN 15 and CH 3 C 13 N were purchased from Cambridge Isotope Laboratories, Inc.The isotope purity is given to be >98%.X-ray photoelectron spectra (XPS) were measured with a PHI 5000 Ver-saProbe 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. 16,17To maximize the signal intensity from the encapsulated N 2 molecules (∼600 ppm) we used a slit-limited resolution of 80 meV, a stepsize of 10 meV and acquisition time of 3 s.Additionally, all spectra were recorded at 75 • grazing incidence using p-polarized x-rays for maximum attainable peak intensities. 13We 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, N 15 2 and N 14 2 @VASWNT were always mounted onto the same sample holder and were always measured without any delay using identical settings.

Stoichiometry of N and C
The high yield of N 2 over pyridinic or substitutional N bonding was confirmed by XPS. Figure 1 shows the survey scan of clean, as-synthesized N 15 2 @VA-SWNT in the lower panel.During the root growth process the CoMo catalyst particles stay attached to the substrate and only pure SWNT 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 metallicitymixed SWNT. 18,19The N1s shows a single prominent peak of molecular N 2 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 N 2 would be equivalent to that of N 2 gas at 340 mbar and 0 • C. Inside densely packed SWNT bundles (1.3 g/cm 3 ) the stored density of N 2 would correspond to that of N 2 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 CH 3 CN in C 2 H 5 OH 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 in the range from 398-400 eV, and is evidently not detectable.

Fractionation of C 13 and C 12
Isotope labeling of the carbons in the feedstock is a viable technique for investigating reaction pathways.We grew SWNT from pure CH 3 C 13 N and regular CH 3 CN.The Raman spectra of these two samples are shown in Figure 2.
3][24] The G mode of SWNTs synthesized from pure isotopic CH 3 C 13 N, reveals a significant Here ω N is the G-band frequency of naturally occurring C 12 material , which contains 1.1% C 13 .The observed maxima of the tangential G modes ω N = 1594.8and ω(c) = 1564.3yield an abundance c = 48% C 13 in the nanotube walls.The C 12 and C 13 in CH 3 C 13 N have-under the current conditionsroughly the same probability to contribute to growth.The reaction pathways in ethanol were only recently elucidated in a study using site-selectively isotope-labeled C 2 H 5 OH, where either one or both of the C atoms was replaced with C 13 .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. 25he even fractionation of both inequivalent C atoms strongly suggests that the entire CH 3 CN 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. 21etailed reaction processes on the catalyst particle during the synthesis are, however, obscured from macroscopic postsynthesis investigations.
In our specific case a scenario may be devised by considering binding energies.The binding energy of C − − − N is 891 kJ/mol and that of free C 2 is calculated to be on the order of 300 kJ/mol. 21,26We suggest that during dissolution the are illustrated in Fig. 3.This pathway implies that all the N from CH 3 CN is processed via the catalyst particle before it is released as inert N 2 .
In the case of pure CH 3 CN feedstock there is clearly not enough room inside the SWNT to accommodate all the N 2 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 N 2 is released to the environment.The low trapping probability is in accordance with reduced nanotube diameters rooting from larger sized catalyst particles. 10In the perpendicular growth mode, most of the catalyst particle surface area is not covered by the growing nanotube. 27Therefore the size of the catalyst particle determines the trapping efficiency for N 2 , but it does not determine the diameter of the growing nanotubes.X-ray absorption is the method of choice to investigate the possible isotope shift in the vibron energy of N 2 .With the natural isotope abundance of over 99% N 14 , the equidistant satellites in the XAS spectrum are separated by 233±2 meV. 28For pure N 15 a spacing of 225±2 meV is expected from scaling with √ 1/m.The isotope shift in the vibrons of isotope mixed N 14 N 15 was observed by Raman spectroscopy to be 4 meV. 29t 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 concomitant excitation of one or more vibrons.The isotope shift between N 14 2 and N 15 2 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.
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 are seen 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 of N 14 2 and N 15 2 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 N 14 2 and N 15 2 .Moreover, following synthesis and XPS measurement the samples were stored for over six months at ambient conditions until the scheduled beamtime at SPring-8.The extended period over which the N 15 2 was retained proves that as-grown capped SWNT are tight molecular containers.

Conclusions
We used isotope-labeled acetonitrile CH 3 C 13 N and CH 3 CN 15 to trace the reaction pathway from acetonitrile to SWNT containing encapsulated N 2 .The balanced incorporation of both inequivalent C atoms in 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 is present during dissolution in the catalyst metal.The geometry of a perpendicular growth mode limits the area for encapsulating N 2 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 N 15 2 from CH 3 CN 15 inside SWNT.
Very briefly, C − − − N radicals react inside the catalyst particle and release N 2 , which then emerges into the currently growing compartment.While the correlation of encapsulated N 2 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 gasphase HCN or if they are formed during the catalytic decomposition of CH 3 CN.