Small Carbon Nano-Onions: An Ion Mobility Mass Spectrometric Study

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Small Carbon Nano-Onions: An Ion Mobility Mass Spectrometric Study Ryoichi Moriyama,† Jenna W. J. Wu,† Motoyoshi Nakano,†,‡ Keijiro Ohshimo,† and Fuminori Misaizu*,†,§ †

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai 980-8576, Japan § New Industry Creation Hatchery Center, Tohoku University, 6-6-4 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ‡

S Supporting Information *

ABSTRACT: Structures and charges of nanocarbon cluster ions, Cnz+ (100 ≤ n ≤ 800, z = 1 and 2), have been determined using ion mobility mass spectrometry. For singly charged ions, a compact cluster ion series was observed in addition to monolayer fullerene ions for n = 260−700 continuously. Previous electron microscopic observations indicated that the compact clusters were bilayer fullerenes (nano-onions), in which the inner and outer layers grow from a structure close to [C30@C230]+ at n = 260. The present study also suggests that several combinations of inner and outer layer fullerenes were produced. The results indicated that the interlayer distance depended on different combinations of inner and outer layers and that the observed lower limit of the interlayer distance agreed well with that of graphite (3.35 Å). The upper limit corresponded to bilayer structures in which the number of atoms of the inner layer was constant at about 30, the smallest fullerene size observed in this study. Series of monolayers and compact bilayers of doubly charged ions with cross sections that coincided with those of monocations were observed in nearly the same size region as monocations. (IM-MS).26 They suggested that this isomer might be composed of nested double-wall fullerenes with an inner layer of 30−40 and outer layer of 260−270 carbon atoms. In addition, bilayer structures with high symmetry, such as C60@ C180 and C60@C240, were reported from theoretical calculation studies.21,27−29 In contrast to the magic numbers of the singlelayer fullerenes, such as n = 60 and 70 observed for small size region (n < 100),9 specific magic numbers were not reported in cluster size regions with n > 100.30,31 Therefore, discovering the combinations of inner and outer layer fullerenes in the stable bilayer structures is important. A combination of ion mobility spectrometry (IMS) and mass spectrometry (MS), IM-MS, can determine mass, charge, and a rough structure of the ions in the gas phase.32,33 Information about such geometrical structures of ions can be obtained as collision cross sections (CCSs). In this study, structural isomers of carbon cluster cations in the size region of n = 100−800 were investigated by IM-MS. Structures and charges of bilayer fullerene ions determined from experimentally determined CCSs were compared with those of monolayer fullerenes.

1. INTRODUCTION Carbon cluster ions, Cn±, grow and increase in size (n < 100) from linear to two-dimensional (2D) cyclic and then to cagelike structures typified by buckminsterfullerene, C60±.1−4 In contrast, nanometer-scale carbon materials form structures such as carbon nanotubes,5 graphene,6 and multilayer nested fullerenes7 called “carbon nano-onions”. The carbon nanoonions were first discovered by Iijima in 1980,8 five years earlier than the discovery of C60.9 Kroto and co-workers also proposed that the nested structures of the nano-onions have full Ih point group symmetry and 60m2 carbon atoms (m = 1, 2, ...), i.e., C60@C240@[email protected] After the discovery of carbon nanoonions, multiple-layered nano-onions were actively investigated using electron microscopy,11−17 and information about separated multishell fullerenes was obtained through theoretical calculations.18−22 Three-dimensional helical structures (spiroid structures) have also been proposed23−25 as models of nanoonion structures in addition to the separated layered fullerenes. Among the multilayered structures observed through electron microscopy, bilayer structures have been reported only for C60@C240, C240@C540,16 and [email protected] However, the actual numbers of constituent carbon atoms are uncertain because they were estimated from electron microscopic images. Hunter and Jarrold also reported a compact ion isomer, rather than singly charged monolayer fullerene cations, in the arrival time distribution of C300+ by ion mobility mass spectrometry © XXXX American Chemical Society

Received: January 18, 2018 Revised: February 19, 2018 Published: February 21, 2018 A

DOI: 10.1021/acs.jpcc.8b00597 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. EXPERIMENTAL AND THEORETICAL METHODS The IM-MS experiments were performed using a home-built vacuum apparatus composed of a cluster ion source, ion-drift cell for IMS,34 and a reflectron time-of-flight (TOF) mass spectrometer.35 Details of the experimental setup and procedures for IM-MS have been reported previously.36,37 Briefly, carbon cluster cations, Cnz+, were generated by a laser vaporization method in which the second harmonic of a Nd:YAG laser was focused onto a rotating and translating carbon disk. The microplasma containing carbon vapor produced by laser vaporization was cooled by He gas expanded from a pulsed valve with a stagnation pressure of 0.3 MPa. The carbon cluster ions grew in a channel (20 mm long; 3 mm inner diameter) of the cluster source to enhance the collisions and generate larger clusters than those generated in a previous study of small carbon clusters.38 Cluster ions were injected into the ion-drift cell by a pulsed electric field with 50 V potential difference at a given time (t = t0). Therefore, the injection energy of monocation was 50 eV, and that of dication was 100 eV. The cell was 100 mm long and was filled with He buffer gas at a pressure of 3.0 Torr at room temperature, and a drift electric field of 9.95 V/cm was applied to guide the ions downstream. After passing through the cell, the ions were reaccelerated by pulsed electric fields in an acceleration region of the TOF mass spectrometer at a specific time from the first pulse, t = t0 + Δt. A series of TOF mass spectra were obtained sequentially by scanning the “arrival time”, Δt, between the injection pulse and ion acceleration pulse. The time spent in the cell was related to the CCS between the ion and the buffer gas, based on the ion transport theory;39 therefore, the CCS was deduced from the measured arrival time distribution. In IM-MS, the ratio of the drift electric field, E, to the number density of buffer gas, N, needs to be kept low for deducing CCSs using ion transport theory from IMS measurements.40 In the present experiments, the conditions were optimized with E/ N = 10 Td (1 Td = 10−17 V cm2), whereas the E/N values were 1.5−10 Td in typical low-field conditions.41−43 In our apparatus, the mass resolution was m/Δm ∼ 230, and the ion mobility resolution was Ω/ΔΩ ∼ 20. The latter was estimated from the arrival time distribution of a chromium oxide cluster anion, Cr4O10−.44 The CCSs of carbon cluster ions were also determined using theoretical calculations with the trajectory method (TM) in the MOBCAL program.41 For the present calculations, values of ε = 1.34 meV and σ = 3.04 Å were used for the Lennard-Jones parameters, from a previous study on carbon clusters.41 To estimate the theoretical CCSs of the large carbon cluster ions, the structures of giant fullerene ions and the inner and outer shell structures of nano-onions were assumed to be the same as the optimized neutral structures in the database45 and a previous report.21 The charge of each carbon atom was also assumed to be 1/n.

Figure 1. 2D plot of carbon cluster cations measured by ion mobility mass spectrometry, (A) without and (B) with the channel in the cluster source. (a)−(c) correspond to different structures or charges. The He buffer-gas pressure, applied electric field, and cell temperature were (A) 0.8 Torr, 7.5 V/cm, and 180 K and (B) 3.0 Torr, 9.95 V/cm, and 293 K, respectively.

However, as observed in the previous study by Bowers and coworkers,1,2 the geometry changes from linear to ring and finally to fullerene were clearly observed for Cn+ in the 2D plot: The linear structures were observed for n ≤ 10, whereas cyclic isomers, which have slightly smaller CCSs than linear structures, became predominant for n = 10−30. Finally, fullerene structures were exclusively observed for n ≥ 30. It should be noted that only the even-sized ions were observed for the fullerene ions with magic numbers at n = 60 and 70 in Figure 1A. Geometric structures of larger cluster and nanoparticle ions, Cnz+, can also be investigated using the correlation between arrival time and n/z in the 2D plot as shown in Figure 1B. In contrast to small clusters, three different series [(a)−(c)] were observed in the region representing larger clusters. In this condition optimized to the production of cluster ions with large size region, cluster ions with n < 80 were not observed. Series (a), which had the longest arrival times (largest CCSs) at a given m/z value in the 2D plot, was determined to be monolayer fullerene monocations1,2 because: • Each peak of this series was separated by two carbon atoms in the mass spectrum (Figure 2a), obtained by projecting the corresponding series in the 2D plot on the horizontal axis, although the precise mass of the peak could not be determined for the large size region due to the limited mass resolution. This result agrees with the predominantly even number property of carbon atoms found in fullerenes.30,31 • Experimental CCSs determined from the arrival times coincided with the theoretical CCSs of fullerene monocations. Series (b) was composed of two different structural series from the mass separation shown in Figure 2b. Series (b-1) contained a weak flat segment near n/z ≈ 200, and series (b-2) had a broad distribution peak near n/z ≈ 500. The ions in series (b-2) also appeared to be singly charged ions because they were also composed of cluster ions separated by two carbon atoms (Figure 2b). The ions in series (b-1) were

3. RESULTS AND DISCUSSION 3.1. Assignments of Size and Charge. From the sequential measurement of TOF mass spectrum at each arrival time, 2D plots of arrival time vs cluster size to charge ratio (n/ z) were obtained as shown in Figure 1. Singly charged carbon cluster cations Cn+ with n = 3−78 were observed in Figure 1A. This data was obtained without the channel of the cluster ion source. In this figure, the arrival times, which are proportional to CCSs, were found to increase with increasing cluster size. B

DOI: 10.1021/acs.jpcc.8b00597 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. Mass spectra of series (a) and (b) selected from the 2D plot (Figure 1B). Insets show enlarged detail around the maximum. Figure 3. Fourier transform results for the mass spectra of each component.

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assigned as monolayer fullerene dications. This series had more compact structures than did the monolayer fullerene ions [series (a)], since the arrival times were shorter than those of series (a). Nested bilayer fullerenes or spiroid structures23−25 were considered as candidates for the structures of series (b-2). If the structures were spiroid, the series in the 2D plot should branch out from the monolayer series [series (a)] because spiroid structures are formed by partial cleavage of the fullerenes at some size. However, series (b-2) was found only after n ≥ 260 in the present ion source, and thus spiroid structures were ruled out. Based on these considerations and former observations, series (b-2) of n/z = 260−700 was assigned to be carbon nano-onions or double-layered fullerenes. This is the first clear observation of a nano-onion series with a continuous size distribution of carbon cluster ions by IM-MS; this series was not clearly reported as a continuous distribution in reports of previous studies using IM-MS.26,46 Finally, series (c), which had the shortest arrival times (corresponding to highest mobility), was observed in the size range of n/z = 150−700. This series was also not observed in previous reports on IM-MS.26,46 The structures and charges of this series were assigned to be doubly charged nano-onions from the CCSs. To confirm the charge of each cluster series [(a), (b-1), (b2), and (c)] from the mass separation, a Fourier transform (FT) was performed on each series. As shown in Figure 2, cluster ions of series (a) and (b-2) were separated by two carbon atoms, but those in series (b-1) and (c) were not clearly separated. Therefore, FT was used here because of the insufficient resolution of the TOF mass spectrometer in IMMS. In general, the FT spectrum provides the frequency component spectrum of the original function. Therefore, the horizontal axis of the present FT spectrum shown in Figure 3 corresponds to the frequency of the mass spectrum function, i.e., is approximately the inverse of the interval of the size-tocharge ratio (n/z) of the mass spectrum, [Δ(n/z)]−1 or z/Δn, shown in Figure 2. Because the fullerene Cn series are stable for even numbers of carbon atoms (Δn = 2), the monocation (z = 1) and dication (z = 2) have intervals of Δ(n/z) = Δn/z = 2 (C2/1) and 1 (C2/2), respectively. In Figure 3, the peak at z/Δn = 0.5 was assigned as the Cn+ monocation series, and the peak at z/Δn = 1 was assigned as the Cn2+ series. As a result, both series (a) and (b-2) [Figure 3, (1) and (3)] correspond to monocations, while series (b-1) and (c) [Figure 3, (2) and (4)] correspond to dications.

Experimental CCSs can be derived from the 2D plot of arrival time vs cluster size to charge ratio (n/z). Figure 4 shows

Figure 4. Experimental and theoretical CCSs of carbon cluster cations as a function of the number of carbon atoms. Experimental CCSs determined that series (a) and (b-2) were monocations and series (b1) and (c) were dications.

the experimental CCSs as a function of the number of carbon atoms, n, in the cluster. Based on the assignment of the cluster charges above, experimental CCSs were estimated for series (a) and (b-2) as monocations and series (b-1) and (c) as dications. The CCSs of monocation series (a) agreed well with those of dication series (b-1). In addition, the CCSs of monocation series (b-2) coincided well with those of dication series (c). Therefore, the CCSs of carbon clusters were minimally affected by the charge. Based on the results, the observed cluster ions were assigned as follows: singly charged monolayer fullerenes for series (a), doubly charged monolayer fullerenes for series (b-1), and singly and doubly charged nano-onion structures for series (b-2) and series (c), respectively. Under the present experimental conditions, the minimum cluster size of the nanoonion structures was n ≈ 260. 3.2. Collision Cross Sections of Carbon Nano-Onion Ions. Details of the nano-onion structures were also determined. The structure assignments were confirmed by comparing the experimental CCSs obtained with the theoretical CCSs of the monolayer fullerenes equal to or larger than C60 and bilayer fullerenes. Figure 4 also shows theoretical CCSs of C

DOI: 10.1021/acs.jpcc.8b00597 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C larger fullerenes45 and those of reported bilayer fullerene structures such as C60@C180 and [email protected] In this figure, theoretical CCSs of monolayer fullerenes of C60+−C540+ were nearly the same but were slightly larger than the experimental CCSs [series (a) and (b-1)]. For the nano-onions, theoretical CCSs of C60@C180 and C60@C240 appear on the same line with the bilayer fullerene series (b-2) in Figure 4, although the former species was smaller than the minimum nano-onion size observed in series (b-2). Therefore, the ions in the compact series [series (b-2) and (c)] have bilayer fullerene structures. The sizes of the inner and outer fullerene layers of the nanoonion series were also investigated. They were estimated based on the assumption that the CCS of the bilayer structure depended only on the size of the outer layer. Casella et al. reported DFT calculation results that indicated a slightly smaller radius for C240 in the C60@C240 bilayer fullerene (7.097 Å) than that for the monolayer C240 (7.100 Å).21 The experimental CCSs of the dications were also slightly smaller than those of the monocations, as shown in Figure 4. However, this small difference was negligible because the effect due to this difference on the CCSs was ≈0.2 Å2. Figure 4 shows that the slope of the CCSs of the bilayer series was less than that of the monolayer series, indicating that the fullerene inner layer grows along with the outer layer. The ratio of the slopes, bilayer/ monolayer = 1.4, suggested that each time the number of carbon atoms in the outer layer of bilayer fullerenes increased by 22, the number of atoms in their inner layer increased by 8. According to this relationship, the inner and outer fullerene layer structure for n = 260 was [C30@C230]+, 300 was [C40@ C260]+, 600 was [C120@C480]+, and so on. The smallest nanoonion size at n ≈ 260 was reasonable from the inner layer size of n ≈ 30 because n ≈ 30 was the smallest size of monolayer fullerene observed (Figure 1A) in this study and in a previous one.1,2 The present experimental result also indicated that the observed bilayer fullerene ion at n = 300 was [C40@C260]+. Yet, the experimental CCS agreed well with the calculated CCS of C60@C240, perhaps because the charge effect was not factored in. The exact number of carbon atoms in each layer was difficult to determine because the effect of the charge on the structures was uncertain. A former theory of carbon nano-onions by Kroto and McKay assumed that the very stable C60 should always be found inside the onions.10,23 If the bilayer fullerenes always contained a particular fullerene such as C60 inside, the slope of CCS as a function of total cluster size of the bilayer fullerene would be the same as that of monolayer series. However, the observed result was different and showed that a variety of fullerenes could be present in the inner layer of the nanoonions. In addition, the arrival time of the series (b-2) showed a broad distribution (Figure 1B), which indicates several structural isomers for each size of the nano-onion and which was mentioned in previous studies. Hunter and Jarrold reported that the C300+ nano-onion was composed of 30−40 inner atoms and 260−270 outer atoms from IM-MS.26 Bilayer fullerenes were also observed as small as C50@C230 from electron microscopy studies.12 Thus, the smallest bilayer structures can have inner layers with a size smaller than 60. The interlayer distance of the bilayer fullerenes was examined. As reported by Kroto et al., the interlayer distances of the C60@C240@C540@C960... series were about 3−4 Å and were independent of size.10 The distance was comparable to the interlayer distance of bulk graphite (3.35 Å). For the present bilayer fullerenes, the interlayer distance increased monotonically with cluster size, e.g., 4.6 Å for [C40@C260]+ and 5.1 Å for

[C120@C480]+, where the fullerene radii were estimated from a known formula.47 As noted above, several combinations of inner and outer layer sizes were observed in the nano-onion series shown in Figure 1B. This result also indicates that a wide range of interlayer distances were observed in the bilayer fullerene ions, depending on the combinations of inner and outer layer fullerenes. Figure 5 shows the 2D plot as Figure 1B

Figure 5. 2D plot of carbon cluster cations measured by ion mobility mass spectrometry (Figure 1B) with lines indicating arrival time peak tops and upper and lower limit arrival time approximations. The red and blue lines corresponded to CCS series of monolayer and bilayer fullerene monocations. The white solid and dotted lines were the approximations for upper and lower limit of the arrival time of bilayer monocations.

with colored lines that represent the cluster series and upper and lower limit arrival time approximation. The red and blue lines mark across the peak tops for the CCSs of monolayer and bilayer fullerene monocation series, respectively. The white solid line corresponds to the upper limit of the arrival time for the bilayer fullerene monocations, which is obtained from shifting the red line to the right by the size of 30, representing the smallest fullerene (Figure 1A). On the other hand, the white dotted line is the lower limit of the arrival time, which is determined so that the interlayer distance is constant at 3.35 Å. According to a paper by Voytekhovsky,47 the radius rn of a fullerene, Cn, can approximately be written as rn = a(0.1034n − 0.4245)1/2

where a is the bond length between two carbon atoms, 1.44 Å. By using this formula, the numbers of carbon atoms were obtained for inner and outer layers of the specific nano-onions in which the radius differences between the inner and outer layers are 3.35 Å. The white dotted line was deduced from the following steps: (1) Calculate the radius of inner layer, rin, using the formula with the number of carbon atoms of the inner layer as nin. (2) Calculate the radius of outer layer, rout, as rin + 3.35 Å. (3) Calculate the number of carbon atoms of the outer layer nout. (4) Total cluster size is nin + nout, and CCS is π(rout + rC + rHe)2, where rC is the radius of carbon atom (1.55 Å) and rHe is the radius of buffer gas helium atom (1.15 Å). (5) Estimate the arrival time of the ion with total size nin + nout and collision cross section π(rout + rC + rHe)2 using the ion transport theory.39 From the analysis of the interlayer distances as a function of total cluster size, the lower limit of the distance agreed well with that of bulk graphite, whereas the upper limit corresponded to D

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(3) Shelimov, K. B.; Clemmer, D. E.; Jarrold, M. F. Structures and Isomerization of LaCn+ Clusters. J. Phys. Chem. 1995, 99, 11376− 11386. (4) Van Orden, A.; Saykally, R. J. Small Carbon Clusters: Spectroscopy, Structure, and Energetics. Chem. Rev. 1998, 98, 2313−2358. (5) Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56−58. (6) Allen, M. J.; Tung, V. C.; Kaner, R. B. Honey Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132−145. (7) Zeiger, M.; Jäckel, N.; Mochalin, V. N.; Presser, V. Review: carbon onions for electrochemical energy storage. J. Mater. Chem. A 2016, 4, 3172−3196. (8) Iijima, S. Direct observation of the tetrahedral boning in graphitized carbon black by high resolution electron microscopy. J. Cryst. Growth 1980, 50, 675−683. (9) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162−163. (10) McKay, K. G.; Kroto, H. W.; Wales, D. J. Simulated Transmission Electron Microscope Images and Characterisation of Concentric Shell and Icospiral Graphitic Microparticles. J. Chem. Soc., Faraday Trans. 1992, 88, 2815−2821. (11) Ugarte, D. Curling and closure of graphitic networks under electron-beam irradiation. Nature 1992, 359, 707−709. (12) Ugarte, D. Canonical Structure of Large Carbon Clusters: Cn, n > 100. Europhys. Lett. 1993, 22, 45−50. (13) Ajayan, P. M.; Ichihashi, T.; Iijima, S. Distribution of pentagons and shapes in carbon nano-tubes and nano-particles. Chem. Phys. Lett. 1993, 202, 384−388. (14) Kuznetsov, V. L.; Chuvilin, A. L.; Butenko, Y. V.; Mal’kov, I. Y.; Titov, V. M. Onion-like carbon from ultra-disperse diamond. Chem. Phys. Lett. 1994, 222, 343−348. (15) Banhart, F.; Ajayan, P. M. Carbon onions as nanoscopic pressure cells for diamond formation. Nature 1996, 382, 433−435. (16) Mordkovich, V. Z. The Observation of Large Concentric Shell Fullerenes and Fullerene-like Nanoparticles in Laser Pyrolysis Carbon Blacks. Chem. Mater. 2000, 12, 2813−2818. (17) Du, A. B.; Liu, X. G.; Fu, D. J.; Han, P. D.; Xu, B. S. Onion-like fullerenes synthesis from coal. Fuel 2007, 86, 294−298. (18) Tománek, D.; Zhong, W.; Krastev, E. Stability of multishell fullerenes. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 15461− 15464. (19) Tang, A. C.; Huang, F. Q. Theoretical studies of multishell fullerenes. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 17435− 17438. (20) Todt, M.; Rammerstorfer, F. G.; Fischer, F. D.; Mayrhofer, P. H.; Holec, D.; Hartmann, M. A. Continuum modeling of van der Waals interactions between carbon onion layers. Carbon 2011, 49, 1620−1627. (21) Casella, G.; Bagno, A.; Saielli, G. Spectroscopic signatures of the carbon buckonions C60@C180 and C60@C240: a dispersion-corrected DFT study. Phys. Chem. Chem. Phys. 2013, 15, 18030−18038. (22) Todt, M.; Bitsche, R. D.; Hartmann, M. N.; Fischer, F. D.; Rammerstorfer, F. G. Growth limit of carbon onions − A continuum mechanical study. Int. J. Solids Struct. 2014, 51, 706−715. (23) Kroto, H. W.; McKay, K. The formation of quasi-icosahedral spiral shell carbon particles. Nature 1988, 331, 328−331. (24) Zhang, Q. L.; O’Brien, S. C.; Heath, J. R.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Smalley, R. E. Reactivity of Large Carbon Clusters: Spheroidal Carbon Shells and Their Possible Relevance to the Formation and Morphology of Soot. J. Phys. Chem. 1986, 90, 525− 528. (25) Ozawa, M.; Goto, H.; Kusunoki, M.; O̅ sawa, E. Continuously Growing Spiral Carbon Nanoparticles as the Intermediates in the Formation of Fullerenes and Nanoonions. J. Phys. Chem. B 2002, 106, 7135−7138. (26) Hunter, J. M.; Jarrold, M. F. Drift Tube Studies of Large Carbon Clusters: New Isomers and the Mechanism of Giant Fullerene Formation. J. Am. Chem. Soc. 1995, 117, 10317−10324.

bilayer structures in which the number of inner layer carbon atoms was constant at about 30, the smallest fullerene size observed in this study (Figure 1A). Therefore, the bilayer fullerene ions observed were generated within a size range that maintained the stability of each layer.

4. CONCLUSION In conclusion, monolayer and bilayer fullerene ion series with a cluster size n ≤ 800 were analyzed by ion mobility mass spectrometry. This is the first systematic observation of bilayer fullerene ion series with a continuous distribution at n ≥ 260; only few bilayer fullerenes with specific compositions have been reported. Several combinations of inner and outer layers were predicted at a given size-to-charge ratio of the nano-onions. The observed bilayer fullerenes had a variety of inner and outer fullerene combinations with a broad range of interlayer distances between lower and upper limits. The lower limit corresponded to the interlayer distance of bulk graphite, whereas the upper limit corresponded to a bilayer structure in which the inner layer involved the smallest fullerenes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00597. Numerical data of experimental and theoretical collision cross sections of monolayer and bilayer fullerene monocation series (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax + 81 22 795 6580; Tel +81 22 795 6577 (F.M.). ORCID

Motoyoshi Nakano: 0000-0002-9615-2206 Fuminori Misaizu: 0000-0003-0822-6285 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Professors Yutaka Takaguchi, Eiji O̅ sawa, Hitoshi Goto, Takeshi Kodama, and Yoji Achiba for their helpful comments. This work was supported by Steel Foundation for Environmental Protection Technology and The Salt Science Research Foundation, Grant Nos. 1418, 1524, and 1621. Theoretical calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan. F.M. acknowledges support from the “Center for Fundamental and Applied Research of Novel Nanocarbon Derivatives, Center for Key Interdisciplinary Research”, and “Creation of International Research Center for AtomEndohedral Fullerene Nanobiotronics, Program for Key Interdisciplinary Research”, Tohoku University, Japan.

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REFERENCES

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DOI: 10.1021/acs.jpcc.8b00597 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b00597 J. Phys. Chem. C XXXX, XXX, XXX−XXX