Single-Walled Carbon Nanotube Reactor for ... - ACS Publications

Single-Walled Carbon Nanotube Reactor for Redox Transformation of Mercury Dichloride Yuliya V. Fedoseeva,*,†,‡ Andrey S. Orekhov,§,∥ Galina N. Chekhova,† Victor O. Koroteev,†,‡ Mikhail A. Kanygin,†,‡ Boris V. Senkovskiy,⊥,# Andrey Chuvilin,△,¶ Daniele Pontiroli,□ Mauro Riccò,□ Lyubov G. Bulusheva,†,‡ and Alexander V. Okotrub†,‡ †

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia Novosibirsk State University, 2 Pirogova Street, Novosibirsk 630090, Russia § Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium ∥ National Research Center, Kurchatov Institute, Moscow 123182, Russia ⊥ II Physikalisches Institut, Universität zu Köln, 77 Zülpicher str., 50937 Köln, Germany # St. Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia △ CIC nanoGUNE Consolider, 76 Tolosa Hiribidea, Donostia-San Sebastian 20018, Spain ¶ IKERBASQUE Basque Foundation for Science, 3 Maria Diaz de Haro, Bilbao E-48013, Spain □ Dipartimento di Scienze Matematiche, Fisiche e Informatiche, Università di Parma, Parco Area delle Scienze 7/a, 43124 Parma, Italy ‡

S Supporting Information *

ABSTRACT: Single-walled carbon nanotubes (SWCNTs) possessing a confined inner space protected by chemically resistant shells are promising for delivery, storage, and desorption of various compounds, as well as carrying out specific reactions. Here, we show that SWCNTs interact with molten mercury dichloride (HgCl2) and guide its transformation into dimercury dichloride (Hg2Cl2) in the cavity. The chemical state of host SWCNTs remains almost unchanged except for a small p-doping from the guest Hg2Cl2 nanocrystals. The density functional theory calculations reveal that the encapsulated HgCl2 molecules become negatively charged and start interacting via chlorine bridges when local concentration increases. This reduces the bonding strength in HgCl2, which facilitates removal of chlorine, finally leading to formation of Hg2Cl2 species. The present work demonstrates that SWCNTs not only serve as a template for growing nanocrystals but also behave as an electron-transfer catalyst in the spatially confined redox reaction by donation of electron density for temporary use by the guests. KEYWORDS: single-walled carbon nanotubes, mercury chlorides, HRTEM, mechanism of Hg(II) reduction

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are difficult or impossible to produce in open-space conditions. Polarizability of a CNT, as shown theoretically, can influence the reactions occurring in the internal volume.11 Nonbonding interactions of reagents with the CNT electronic cloud may increase12 or lower13 the activation barrier with respect to the corresponding gas-phase reaction. An enhanced interaction of iron oxide nanoparticles with the concave CNT surface facilitated their reduction at a lower temperature as compared to that for outside nanoparticles.14,15 Spontaneous formation of platinum nanoparticles inside the nanotube channels after addition of a chloroplatinic acid precursor was assigned to an

arbon nanotubes (CNTs) easily absorb various molecules and materials into the cavity, where the guests can be effectively stored owing to protection by mechanically and chemically stable shells.1,2 Liquids and volatile compounds diffuse into the opened CNTs due to capillary forces,3 and the nonfunctionalized concave surface ensures their rapid transport.4 Recent studies have demonstrated that CNTs not only can be used as a simple container but also can be utilized as a nanoreactor, providing a confined space for chemical transformations.5 The pathway of the transformation is influenced by an available volume and in rare cases by pairwise host−guest interactions.6 The effective pressure arising inside of a vacuum-sealed CNT governs coalescence of organic species into one-dimensional structures such as diamond-like wires,7,8 narrow graphene ribbons,9 and carbyne chains,10 which © 2017 American Chemical Society

Received: June 22, 2017 Accepted: August 7, 2017 Published: August 7, 2017 8643

DOI: 10.1021/acsnano.7b04361 ACS Nano 2017, 11, 8643−8649

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Figure 1. (a) HAADF/STEM images and (b) EDX spectrum of filled SWCNTs. (c) XRD patterns of filled (1) and initial SWCNTs (2), polycrystalline Hg2Cl2 (3), mercury chloride recrystallized without addition of SWCNTs (4), and polycrystalline HgCl2 (5).

electron transfer from CNTs to platinum cations.16 Charging the CNT cavity with iodine molecules gave a reaction with cofilled metal hexacarbonyls, yielding anionic nanoclusters stabilized by CNT counterions.11 A subsequent sulfurization converted these nanoclusters to zero-charged nanoribbons, and this multistep synthesis revealed the potential of CNTs as electrically active host structures. The participation of CNTs in redox reactions by providing electron density to guest molecules or atoms for temporary use could be especially useful for the controlled synthesis of inorganic nanocrystals. Continuous encapsulated nanocrystals grow best from the melts if the compounds possess appropriate surface tension and melting temperature.2 The stoichiometry of inorganic compounds introduced into CNT channels is usually close to the starting stoichiometry before introduction into the channel. A deviation from the stoichiometry of the bulk materials is commonly attributed to lattice terminations of the confined nanocrystals17 or strong metal−carbon interactions.18 For some compounds crystallization inside CNT channels is hindered, and this means it is impossible to determine their atomic arrangement.19,20 Typically, CNTs act as a template, which directs growth along a particular crystallographic orientation and limits the size of the growing nanocrystal.10,21 Mercury dichloride is a toxic water-soluble bioaccumulating pollutant, which is present in flue gases.22,23 Activated carbon materials are high-capacity and selective sorbents for the removal of mercury compounds from the environment.24 In an aqueous medium, such materials may reduce the adsorbed Hg(II) ions to Hg(I).25,26 This process is related to the functional groups present on the carbon surface and is accompanied by oxidation of carbon.27,28 Single-walled CNTs (SWCNTs) used in the present study possess atomically smooth and clean interiors, which excludes participation of any active groups in the reduction reaction. The absence of chemical modification of the SWCNT surface after the filling reveals a mechanism for redox transformation of the HgCl2 in the nanotube cavity, which is completely different from that proposed for activated carbon. Transfer of electron density

from CNTs to encapsulated molecules is a crucial stage in this transformation, as is shown by density functional theory (DFT) calculations. We demonstrate here that CNTs can play at least two roles in this process, particularly, facilitating the redox transformation of mercury dichloride (HgCl2) and templating the crystallization of the obtained dimercury dichloride (Hg2Cl2).

RESULTS AND DISCUSSION To provide an optimal filling, preopened purified SWCNTs and HgCl2 were taken in a weight ratio necessary for close packing of guest molecules in the nanotube channels. The powders were mixed and put into a glass ampule, which after sealing was heated to 290 °C, 17 °C above the melting temperature of HgCl2. High-angle annular dark-field scanning transmission electron microscopy (HAADF/STEM) data of the product revealed that the guest molecule was effectively incorporated into the nanotube channels (Figure 1a). Since mercury and chlorine are much heavier than carbon, they have brighter Zcontrasts on HAADF/STEM images, and this allows clear discrimination between the filled (bright) and empty (dark) SWCNTs. The filling ratio of SWCNTs, presented in Figure 1, is between 20% and 50%, while the length of encapsulated crystals varies from 1 to 150 nm (see Supporting Information (SI) for the details and examples of filling ratio estimation). We also observed a notable amount of dense particles on the external surface of the tubes (Figure S1), which according to XRD data cannot be anything other than Hg2Cl2. An energy-dispersive X-ray (EDX) spectroscopy study of filled SWCNTs detected signals from mercury, chlorine, carbon, oxygen, and copper (Figure 1b). The signal from copper is a typical artifact usually observed due to the TEM support grid, which is made of copper. Oxygen originates from the oxidized copper surface layer or oxygen-containing groups on the nanotubes, developed during the tip-opening procedure. The presence of mercury and chorine is unambiguous, but the exact Hg/Cl ratio cannot be determined precisely due to a low signal-to-noise ratio. 8644


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Figure 2. (a, b) HRTEM images of filled SWCNTs viewed in [100] Hg2Cl2 (a) and [110] Hg2Cl2 (b) projections. (c, d) Filtered and simulated HRTEM images of a selected area (white box in (a) and (b)). (e, f) Diffractograms and calculated electron diffraction patterns for a Hg2Cl2 crystal along the [100] and [110] directions. (g) Atomic model of Hg2Cl2@SWCNT viewed in [100] Hg2Cl2 and [110] Hg2Cl2 projections and cross section of filled nanotube.

The structure of the encapsulated nanocrystals was evaluated using high-resolution TEM (HRTEM) images acquired with a short exposure time of 0.4 s for the detection of possible nanocrystal rotation and motion under an electron beam. HRTEM analysis found two orientations of Hg2Cl2 nanocrystals with respect to the beam (Figure 2a,b). We employed fast Fourier transform (FFT) analysis to estimate lattice parameters and angles between reciprocal vectors of the 2D lattice (Figure 2e,f) using (110) graphene reflections of SWCNTs as internal calibration standard. Periodicities of encapsulated nanocrystals along and perpendicular to the SWCNT axis are d1 = 0.271 nm and d2 = 0.248 nm (the ratio d1/d2 = 1.09), respectively, for the first Hg2Cl2 orientation (Figure 2a,c,e) and d3 = 0.269 nm and d4 = 0.324 nm (the ratio d3/d4 = 0.83) for another Hg2Cl2 orientation (Figure 2b,d,f). Assuming limited accuracy of HRTEM measurements, these values are in a good agreement with the tabulated bulk structure of the tetragonal phase of Hg2Cl2 (ICSD 65441). Two observed orientations are the [100] and [110] zone axis of Hg2Cl2, while the (001) direction is oriented along the tube axis in both cases. The simulated HRTEM images of the atomic model of the (13, 13) SWCNT

X-ray diffraction (XRD) analysis of the product obtained after the reaction of SWCNTs with HgCl2 showed a set of narrow peaks along with a broad [002] reflection from the graphitic planes (Figure 1c), which could arise from admixed double- and triple-walled CNTs. The narrow peaks coincide well with the reflections from the Hg2Cl2 crystallites (ICSD 65441). To reveal the role of SWCNTs in the observed reduction of HgCl2 to a lower-valence compound, we have carried out reference experiments by heating HgCl2 in sealed ampules without any carbon additive and with natural graphite taken in the same weight ratio as it was in the case of SWCNTs. The XRD analysis detected a simple recrystallization of HgCl2 in the former experiment (Figure 1c) and very little transformation of HgCl2 to Hg2Cl2 on the graphite surface in the second case (Figure S2 in the SI). The active sites in graphite plates could be developed during the graphite−HgCl2 mixture grinding before the reaction. On the basis of these experiments, we conclude that the presence of SWCNTs is a necessary condition for the complete reduction of HgCl2 to Hg2Cl2. 8645


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Figure 3. (a) XPS Cl 2p and (b) Hg 4f spectra of the product of SWCNT reaction with HgCl2 at 290 °C for 16 h. (c) NEXAFS C K-edge spectra and (d) XPS C 1s spectra of SWCNTs before and after the filling.

filled with Hg2Cl2 crystals in these two orientations are shown in Figure 2c and d, respectively, and they are in good agreement with experimental images. The changes in composition and electronic state of SWCNTs after the filling procedure were monitored by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy with a probing depth for graphitic-like materials of ∼1.5 and ∼10 nm, respectively. An overview XPS spectrum of the opened SWCNTs showed the presence of carbon as the dominant element and an admixture of oxygen at no more than 3 at. %. After the SWCNTs were filled, the concentration of oxygen did not change and the spectrum revealed an additional 6 at. % of mercury and 6 at. % of chlorine (Figure S3 in the SI), thus supporting the Hg2Cl2 composition of guest nanocrystals. The XPS Cl 2p line of the filled SWCNTs was fitted by a 2p3/2−2p1/2 spin−orbit doublet with a component separation of 1.6 eV (Figure 3a). The Cl 2p3/2 component is located at 198.2 eV, which is closer to the component energy for Hg2Cl2 (198.5 eV) than that for HgCl2 (198.7 eV).29,30 The XPS Hg 4f spectrum was also presented by a single doublet with the position of the 4f7/2 component at 100.7 eV (Figure 3b), corresponding to the Hg2Cl2 compound.29 The NEXAFS spectrum measured at the C K-edge of SWCNTs exhibited two main resonances located at 285.4 and 291.8 eV and assigned to electron transitions from C 1s core levels to respectively π* and σ* states (Figure 3c). Weak features between 286 and 289 eV correspond to oxygenated carbons,31 which can arise due to opening of SWCNT tips by oxidation. The spectrum of filled SWCNTs is similar in shape to the spectrum of empty SWCNTs, with the exception of enhanced intensity in the spectral range between π* and σ* resonances and the appearance of a shoulder at 283.9 eV. Since the XPS analysis revealed no increase of oxygen in the SWCNT sample after the filling, the enhanced NEXAFS intensity around 288.5 eV could be related to an interaction between carbon shells and guest crystals. DFT calculations of empty and filled SWCNT models show that such an interaction

is actually possible when the distance from guest atoms to nearest carbon atoms is about 0.18 nm or less (see details in Figure S4, SI). The features below the π* resonance peak have been previously observed in the spectra of SWCNTs filled by metal halides, including chlorides of copper and silver, and they are commonly attributed to the intercalation-induced charge transfer.19,20 We found that the XPS C 1s spectrum of Hg2Cl2@SWCNTs is shifted by ∼0.2 eV toward low energy as compared to the spectrum of the empty SWCNTs (Figure 3d). This downshift corresponds to the lowering of the Fermi level of SWCNTs due to electron density transfer from nanotubes to the encapsulated or deposited compounds.19,32 An electron depletion of SWCNTs is supported by Raman spectroscopy that detected an upshift of the G band33,34 by 17 cm−1 after the nanotube filling (Figure S5, SI). Our results demonstrate a complete spontaneous transformation of HgCl2 to Hg2Cl2 inside SWCNTs. The chemical state of SWCNTs does not change significantly; therefore they play a catalytic role in this process. This behavior is completely different from that observed for activated carbon,25,35 whose reducing activity toward the metal ions in high oxidation states is mainly associated with the edges decorated by hydrogen or basic functional groups participating in redox reactions.27 The reaction can be presented by a following scheme: T

2HgCl2 + SWCNT → Hg 2Cl 2@SWCNT + Cl 2

The liberated chlorine may leave the nanotubes through the open ends or be trapped between the encapsulated Hg2Cl2 nanocrystals. An appearance of a Raman band at 519 cm−1 corresponding to the stretching vibration in the Cl2 molecule in the spectrum of the freshly prepared sample (Figure S5, SI) confirms the reaction. To shed light on the mechanism of the observed redox reaction, we have invoked DFT calculations. The accurate representation of chemical behavior of the mercury compounds requires accounting for relativistic effects,36 and the corresponding approaches are computationally very time-consuming. 8646


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Figure 4. Results of optimization with the UPBE0/LAV3P method of a free HgCl2 molecule (a), one (b) and three (c) HgCl2 molecules inside a (6,6) SWCNT, and Hg3Cl4 associated inside the SWCNT (d) obtained by removal of two chlorine bridges from the model (c).

EXPERIMENTAL SECTION

The long-range-corrected Perdew−Burke−Ernzerhof hybrid functional37 (UPBE0) with the effective core potential LAV3P gives suitable structural parameters for mercurycontaining species at reasonable computational time. The obtained bond length for an isolated HgCl2 molecule (Figure 4a) is 2.36 Å; this is 0.07 Å larger than the value in the crystallized compound. Inside an SWCNT, the molecule bends and the distance between the atoms slightly increases (Figure 4b). This is due to attraction of the metal cation to the electron-rich SWCNT surface. Electron density donated to the guest molecule is localized on the chlorine atoms. The estimated energy gain from the encapsulation of a single HgCl2 molecule is 0.78 eV. An increase of local concentration of HgCl2 in the nanotube results in chain-like structures, where the molecules are linked by chlorine bridges (Figure 4c). Coordination of a mercury atom by four chlorine atoms causes a further elongation of Hg−Cl bonds. We propose that at increased temperatures the weak bonds are broken and the released chlorine species combine to form Cl2 molecules. The gap in the chain-like HgxCly structure after the removal of Cl2 (Figure S6a, SI) disappears owing to the Hg−Hg bond formation (Figure 4d). The structure obtained during the optimization process contains a Cl−Hg−Hg−Cl unit, which can be a source of Hg2Cl2 molecules. Actually, the calculations show that the separation of Hg2Cl2 and HgCl2 (Figure S6b, SI) is energetically more favorable than the metastable structure of the mixture of these units (Figure 4d). The encapsulated Hg2Cl2 molecule has a smaller negative charge than the HgCl2 molecule. Hence, after the redox reaction the excess electron density should be transferred back to the SWCNT. A similar process may potentially take place to a lesser extent on the outer surface of the SWCNTs, leading to the larger particles we observe by STEM.

Sample Preparation. The TUBALL brand name product of CNTs was purchased from OCSiAl Company. The sample consisted of SWCNTs with an average diameter of 1.7 nm and an admixture of double-walled and triple-walled CNTs. As-received powder was placed into a flask containing concentrated hydrochloric acid diluted with distilled water (1:1 by volume) for 24 h to remove metal-containing impurities. During this treatment the dispersion was sonicated three times for 30 min each. After that, the sample was washed by distilled water to pH = 6 and dried at 100 °C for 24 h. In order to open the nanotube tips, the sample was heated to 500 °C for 30 min in an air atmosphere. HgCl2 was obtained through a chemical reaction between mercury(II) oxide and hydrochloric acid. SWCNT powder was mixed with HgCl2 in the weight ratio of 1.2:1, which was calculated from a close-packed arrangement of HgCl2 crystals inside the nanotube channels. The HgCl2/SWCNT mixture and HgCl2 only were placed into Pyrex ampules, which were heated in a furnace at 290 °C for 16 h and then cooled to room temperature. The heating temperature was taken to be slightly higher than the melting temperature (277 °C) of HgCl2. Characterization Methods. XRD analysis was carried out on a Bruker D8 Discover diffractometer operating in Debye-Scherrer geometry equipped with a double Göbel mirror monochromator, a GADDS position-sensitive detector, and a Cu Kα X-ray radiation source. The sample was sealed in a 0.7 mm quartz capillary. The TEM study was carried out on a Titan 60-300 TEM/STEM microscope (FEI, The Netherlands) equipped with an x-FEG monochromator and an image side Cs spherical aberration corrector. SWCNTs were dispersed in ethanol using an ultrasonic treatment, and a drop of suspension was put on a copper TEM grid with a holey carbon film. The images were acquired at an 80 kV accelerating voltage for TEM and 300 kV for STEM and processed in Digital Micrograph software using custom-made scripts. HRTEM image simulations were performed by a standard multislice algorithm (the Musli38 package) utilizing typical parameters for Titan 60-300 TEM with Cs of 20 μm. XPS and NEXAFS experiments were done at the Helmholtz Zentrum Berlin using radiation from the Russian−German beamline installed at the electron storage ring BESSY II. Samples were not annealed or irradiated before the experiment to avoid their destruction. Monochromatized synchrotron radiation at 830 eV was used for excitation of XPS spectra, which were collected with a VG CLAM-4 hemispherical analyzer with an energy resolution of 0.4 eV. The binding energy was calibrated with a Au 4f7/2 peak. XPS spectra were fitted by Gaussian/Lorentzian symmetric line shape after subtraction of a Shirley-type baseline. NEXAFS spectra near the C K-edge were recorded in total-electron yield mode with a probing depth of ∼10 nm. The π* resonance energy of the SWCNTs at 285.4 eV was taken as an internal standard.

CONCLUSIONS To summarize, we have observed the reaction of molten mercury dichloride with SWCNTs and its spontaneous transformation to dimercury dichloride in the nanotube cavity. The presence of solely Hg2Cl2 inside the SWCNTs was confirmed by XRD, TEM, XPS, and NEXAFS. The SWCNTassisted reduction of very toxic HgCl2 to insoluble and, hence, less reactive Hg2Cl2 can be potentially useful for environmental applications, e.g., for extraction of Hg(II) from water solutions and flue gases without the necessity for an expendable reducing agent. After the reaction, SWCNTs remain intact except for pdoping by the encapsulated Hg2Cl2 nanocrystals and thus serve as the electron-donating catalyst in the redox process. The DFT calculations revealed negative charging of HgCl2 molecules inside SWCNTs and elongation of Hg−Cl bonds. SWCNTs promote reduction of HgCl2 to Hg2Cl2 with formation of chlorine. The study here provides insights into this fascinating chemistry, which takes place inside CNTs and opens a route to the syntheses of various functional nanomaterials.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04361. Determination of SWCNT filling ratio; additional HAADF/STEM image of the product of reaction of SWCNTs with HgCl2; reference reaction of graphite 8647


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with HgCl2; overview XPS spectrum of the reaction product; quantum-chemical modeling of NEXAFS C Kedge spectra; Raman scattering; quantum-chemical modeling of the formation of Hg2Cl2 from HgCl2 molecules inside an SWCNT (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Yuliya V. Fedoseeva: 0000-0003-1681-1708 Victor O. Koroteev: 0000-0002-4473-9689 Boris V. Senkovskiy: 0000-0003-1443-6780 Mauro Riccò: 0000-0002-6879-2687 Lyubov G. Bulusheva: 0000-0003-0039-2422 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Collaboration between partner institutions was partially supported by European FP7 IRSES project 295180. We are grateful to the bilateral Program “Russian-German Laboratory at BESSY II” for the assistance in XPS and NEXAFS measurements. We acknowledge C. Tollan for proofreading the manuscript. We are grateful to Dr. Y.V. Shubin for XRD measurements of graphite with HgCl2. REFERENCES (1) Khlobystov, A. N.; Britz, D. A.; Briggs, G. A. D. Molecules in Carbon Nanotubes. Acc. Chem. Res. 2005, 38, 901−909. (2) Eliseev, A. A.; Kharlamova, M. V.; Chernysheva, M. V.; Lukashin, A. V.; Tretyakov, Yu. D.; Kumskov, A. S.; Kiselev, N. A. Preparation and Properties of Single-walled Nanotubes Filled with Inorganic Compounds. Russ. Chem. Rev. 2009, 78, 833−854. (3) Monthioux, M. Filling Single-Wall Carbon Nanotubes. Carbon 2002, 40, 1809−1823. (4) Axet, M. R.; Serp, P. Carbon Nanotube Nanoreactors for Chemical Transformations. In Organic Nanoreactors. From Molecular to Supramolecular Organic Compounds; Sadjadi, S., Ed.; Elsevier Inc., 2016; pp 111−157. (5) Khlobystov, A. N. Carbon Nanotubes: From Nano Test Tube to Nano-Reactor. ACS Nano 2011, 5, 9306−9312. (6) Miners, S. A.; Rance, G. A.; Khlobystov, A. N. Chemical Reactions Confined Within Carbon Nanotubes. Chem. Soc. Rev. 2016, 45, 4727−4746. (7) Zhang, J.; Zhu, Z.; Feng, Y.; Ishiwata, H.; Miyata, Y.; Kitaura, R.; Dahl, J. E. P.; Carlson, R. M. K.; Fokina, N. A.; Schreiner, P. R.; Tománek, D.; Shinohara, H. Evidence of Diamond Nanowires Formed inside Carbon Nanotubes from Diamantane Dicarboxylic Acid. Angew. Chem., Int. Ed. 2013, 52, 3717−3721. (8) Nakanishi, Y.; Omachi, H.; Fokina, N. A.; Schreiner, P. R.; Kitaura, R.; Dahl, J. E. P.; Carlson, R. M. K.; Shinohara, H. Template Synthesis of Linear-Chain Nanodiamonds Inside Carbon Nanotubes from Bridgehead-Halogenated Diamantane Precursors. Angew. Chem., Int. Ed. 2015, 54, 10802−10806. (9) Chuvilin, A.; Bichoutskaia, E.; Gimenez-Lopez, M. C.; Chamberlain, T. W.; Rance, G. A.; Kuganathan, N.; Biskupek, J.; Kaiser, U.; Khlobystov, A. N. Self-Assembly of a Sulphur-Terminated Graphene Nnanoribbon Within a Single-Walled Carbon Nanotube. Nat. Mater. 2011, 10, 687−692. (10) Giusca, C. E.; Stolojan, V.; Sloan, J.; Börrnert, F.; Shiozawa, H.; Sader, K.; Rümmeli, M. H.; Büchner, B.; Silva, S. R. P. Confined Crystals of the Smallest Phase-Change Material. Nano Lett. 2013, 13, 4020−4027. 8648


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