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Cite this: Chem. Commun., 2015, 51, 5017 Received 29th December 2014, Accepted 12th February 2015

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Reductive dismantling and functionalization of carbon nanohorns† Damien Voiry,a Georgia Pagona,b Elisa Del Canto,a Luca Ortolani,c d d b Vittorio Morandi,c Laure Noe ´, Marc Monthioux, Nikos Tagmatarchis* and a Alain Penicaud*

DOI: 10.1039/c4cc10389k www.rsc.org/chemcomm

Reduction of carbon nanohorn (CNH) aggregates with potassium naphthalenide resulted in their dismantling and individualization. Furthermore, the reduced CNHs were functionalized via addition of electrophiles.

Beyond carbon nanotubes and graphene, carbon nanohorns (CNHs) are original carbon-based nanomaterials made of a single sheet of graphene typically wrapped into a long conical tip.1 Raw CNHs synthesized via laser ablation or an electric arc without using any metal catalyst, under the form of spherical aggregates, possess average sizes ranging from 50 to 100 nm. Furthermore, CNHs possess a high specific surface, which makes them interesting as materials for gas storage,2 supercapacitors,3 or catalyst supports.4 Notably, the solubilization of the otherwise completely insoluble CNHs was aided by numerous functionalization strategies,5 leading to CNH-based hybrid materials for energy conversion6 and drug delivery.7 However, regardless of the diverse functionalization methodologies followed, the nanoscale morphology of CNHs as imaged using TEM always remained unchanged, i.e. a dahlia-like superstructure, while smaller CNH aggregates were obtained when the raw material was sonicated in aqueous sodium dodecylbenenzesulfonate.8 Furthermore, sucrose density gradient centrifugation of oxidized CNHs allowed the isolation of individualized CNHs; however, the method was neither scalable due to its low yield and long washing process nor applicable to raw CNHs due to their lower density.9 On the other hand, reductive dissolution is an efficient way of dissolving carbon nanotubes10 or graphite.11 Charges are used

not only to exfoliate the carbon nanostructure but also to further induce chemical modification via covalent functionalization. Hence, starting from reduced carbon nanotube (or graphene) solutions, the degree of alkylation can be adjusted by tuning the charge extent.12 In this communication, it has been shown that aggregated CNHs spontaneously dismantle in organic solvents by reductive dissolution. Moreover, reduced CNHs can be quenched by electrophiles. Following literature procedures, raw CNHs were chemically reduced using a solution of potassium naphthalenide in THF.12 Briefly, under an inert atmosphere inside a glovebox, when the CNH powder was mixed with the green solution of reduced naphthalene, the reaction mixture became rapidly transparent due to the deactivation of the naphthalenide radical anion reducing CNHs and forming neutral naphthalene, upon contact with CNHs. After 12 hours of reaction at room temperature, the suspension was filtered inside the glovebox, and the residue was washed several times with THF and dried, yielding CNH salt 1 as a black powder (Scheme 1). The reduced CNHs were spontaneously soluble (Fig. 1a–c) in a variety of organic solvents, such as acetone, acetonitrile, benzonitrile, DMSO, DMF and NMP (Fig. 1d). Importantly, in all these solvents, the dissolution of CNHs proceeds quite fast (ESI,† Video), while no sonication is needed. It was found that DMSO is by far the best solvent, with a solubility reaching 20 mg mL 1, which is four times higher than the solubility

a

CNRS, Centre de Recherche Paul Pascal (CRPP), UPR 8641, F-33600 Pessac, and University of Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France. E-mail: [email protected]; Fax: +33 556845600 b Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 11635, Greece. E-mail: [email protected]; Fax: +30 210 7273794 c CNR-IMM Section of Bologna, via Gobetti 101, 40129 Bologna, Italy d CEMES, UPR-8011 CNRS, BP 94347, F-31055 Toulouse Cedex 4, France † Electronic supplementary information (ESI) available: Experimental, spectral and TEM details. See DOI: 10.1039/c4cc10389k

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Scheme 1 Dismantled and individualized CNHs upon reduction by potassium naphthalenide in DMSO.

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Fig. 1 (a–c) Digital photographs, taken with 30 s intervals, showing the fast and spontaneous dissolution of reduced CNHs 1 in acetone. (d) UV-visible relative absorbance at 400 nm of 1 in different solvents. The absorbance of SWCNT solution in DMSO measured for the same conditions is shown for comparison.

measured for reduced carbon nanotubes.10 The extinction coefficient for CNHs in DMSO at 400 nm was estimated to be 6.5  10 3 L mg 1 cm 1. Fig. 2a displays a representative TEM image of raw CNHs, forming large dahlia-shaped aggregates with diameters ranging from 50 to 100 nm. In stark contrast, TEM images of reduced CNHs 1 revealed the presence of disassembled CNHs in the form of individual straight and/or branched tubules (Fig. 2b). To reduce the contribution of the supporting membranes and to enhance the visibility of CNHs in the TEM, solutions of reduced CNHs were drop-cast on copper grids covered with graphene membranes. The dimensions of the tubular objects observed, around 5 nm in diameter for lengths of B50 nm, match perfectly well that of individual CNHs (Fig. 2c and d). Fourier analysis (ESI,† Fig. S1) reveals lattice reflections from the CNHs, confirming the graphenic nature of their walls. Notably, some CNHs were found half-opened (Fig. 2d) and appear to be promising for carrying nanoparticles or molecules since their inner cavity is now fully accessible. At this point, it should be stated that the reductive treatment of CNHs with potassium naphthalenide is not harmful to CNHs; hence, the open end imaged under the TEM study is original. The Raman spectra of CNH salts 1 compared with raw and re-oxidized CNHs 2 are shown in Fig. 3a. The Raman spectrum of raw CNHs shows an intense disorder-induced mode (D-band), most likely associated with the presence of free graphene edges on the opposite side of the conical tip of CNHs. In comparison, the Raman spectrum of reduced CNHs 1 revealed an upshift of the G-band (i.e. 14 cm 1) ascribed to n-doping.13 It should be pointed out that the latter upshift of the G-band in 1 was varied, most likely

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Fig. 2 (a) High-magnification TEM image of raw CNHs. (b) Low-resolution TEM image of reduced CNHs 1. (c, d) High-magnification TEM images of individualized CNH salts 1. The amorphous material apparently surrounding the CNHs is brought by the supporting graphene membrane itself.

Fig. 3 (a) Raman spectra (514 nm) normalized at the G-band of raw CNHs (black), reduced CNH salts 1 (green), re-oxidized CNHs 2 (red) and functionalized CNHs 3a (blue). (b) TGA of raw CNHs (dotted), re-oxidized CNHs 2 (black) and functionalized CNH-based materials 3a–c (red, blue and green, respectively), obtained under nitrogen.

due to the inhomogeneity of the sample. Removal of the charges, i.e. re-oxidation, promotes flocculation. The latter process does not induce additional sp3 defects as shown by the constant ID/IG ratio observed in the Raman spectra (Fig. 3a and ESI,† Table S1). It is also worth noting that Raman spectra of re-oxidized CNHs 2, obtained from different positions on the sample, were highly uniform, proving that the material homogeneity was restored. Then, moving a step forward, a variety of electrophiles were added to the reduced CNHs 1, yielding CNH-based materials 3a–c (Scheme 2). As a possible mechanism for the quenching of reduced CNHs with the electrophiles, it is reasonable to believe that electron transfer occurs.14 The functionalized CNHs 3a–c were found to be soluble in various organic solvents, including dichloromethane and acetone (ESI,† Table S2). Morphological examination of functionalized CNHs 3, based on TEM studies, revealed that individualization was maintained (ESI,† Fig. S2). Raman spectra of re-oxidized 2 and functionalized CNHs 3, all show a non-vanishing G-band upshift, far less pronounced than that for reduced CNH salts 1 (Fig. 3a). As described earlier,

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This work has been performed within the framework of the GDR-I 3217 ‘‘Graphene and Nanotubes’’.

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Notes and references

Scheme 2 Dried-air oxidation of CNH salts 1 removes the excess of negative charges along the graphene walls generating re-oxidized CNHs 2. Electrophilic addition to CNH salts 1 furnishes functionalized CNH-based materials 3a–c.

the ID/IG ratio remains constant after re-oxidation, confirming that the reduction–dissolution–re-oxidation process does not create further defects on CNHs (ESI,† Table S1). In contrast, functionalized CNHs 3 exhibit an enhanced ID/IG ratio, as expected due to the additional anchorage of organic addends onto the skeleton of CNHs (ESI,† Table S1). The number of substituents added onto modified CNHs 3 was calculated from TGA performed at 750 1C under nitrogen. Since raw CNHs are thermally stable under an inert atmosphere, the weight loss observed for materials 3a–c (12%, 14% and 12%, respectively, Fig. 3b) was ascribed to the thermal decomposition of the organic addends. Furthermore, a weight loss of 10% was observed for re-oxidized CNHs 2, which is ascribed to the decomposition of the individualized CNHs due to the presence of free graphene edges on the opposite side of the conical tip of CNHs. Then, considering that individualized CNHs are thermally labile and lose weight during the TGA measurements, for the calculation of the number of carbon atoms per which a substituent was grafted in modified CNHs 3a–c, the former weight loss must be taken into account before attempting to calculate the latter. Thus, it was estimated that one substituent was grafted per every 55, 202 and 282 carbon atoms for 3a–c, respectively (ESI,† Table S1). These values are in good agreement with those reported previously for similarly modified carbon nanotubes.12,15 All in all, reductive dissolution of CNH aggregates by potassium naphthalenide was achieved leading to their dismantling and individualization of CNHs under a reduced form. Reduced CNHs were further functionalized upon quenching by electrophiles, while the individualization was maintained. Considering the 3-D nanosized dimensions of CNHs, the availability of such individualized graphene-based objects is expected to enhance their applicability towards medicinal oriented applications, with respect to raw CNH aggregates or 2D-nanosized carbon nanotubes. Raw CNHs were kindly provided by Dr M. Yudasaka and Prof. S. Iijima (Nanotube Research Center, National Institute of Advanced Industrial Science and Technology – AIST, Japan). ´gion Aquitaine, CNRS (BDIR grant to DV) and Support from Re the Agence Nationale de la Recherche (GRAAL) is acknowledged.

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Chem. Commun., 2015, 51, 5017--5019 | 5019

Reductive dismantling and functionalization of carbon nanohorns.

Reduction of carbon nanohorn (CNH) aggregates with potassium naphthalenide resulted in their dismantling and individualization. Furthermore, the reduc...
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