Local “repristinization” of oxidized single-walled carbon nanotubes by laser treatment

CARBON

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Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment Chiara Fabbro a b

a,b,* ,

Tatiana Da Ros

a

Dipartimento di Scienze Chimiche e Farmaceutiche, Universita` di Trieste, Trieste 34127, Italy Dipartimento di Scienze Molecolari e Nanosistemi, Universita` Ca’ Foscari di Venezia, Venezia 30123, Italy

A R T I C L E I N F O

A B S T R A C T

Article history:

Though having passed the second decade since their discovery, carbon nanotubes (CNTs)

Received 16 December 2013

still hold promise in different fields, and many steps forward have been done. However,

Accepted 15 April 2014

one major limit is given by their lack of solubility, which still represents a challenge, given

Available online xxxx

the need to properly handle the material for many applications. The most efficient strategies to solve this problem often rely on a covalent chemical modification of CNT structure, in order to prevent inter-tube interactions. However, many applications, being based on CNT peculiar electronic properties, require structural integrity, and they are therefore incompatible with such strategies. We prepared strongly oxidized single-walled carbon nanotubes, with a dramatically improved dispersibility, and we performed a subsequent localized repristinization by means of laser-induced heating of the sample, during Raman analysis, consisting in both removal of the amorphous material and healing of structural defects. Our finding highlights an effect which all researchers in the field must be aware of, when using Raman spectroscopy for characterization purposes. Moreover, this local laser effect was systematically investigated in order to gain a deeper understanding of the phenomenon.  2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNTs)-based technology experienced a very rapid development in many different fields, ranging from biomedical research to material science [1,2], thanks to the fascination resulting from their unique electronic properties, their chemical and thermal stability, together with the extremely high tensile strength and elasticity. Nevertheless, they still present to date an important drawback that needs to be addressed, namely their poor dispersibility. In fact, pristine single-walled carbon nanotubes (SWCNTs) are insoluble

in water and in any organic solvent, strongly hindering their usefulness. For this reason, in most cases, a chemical modification is needed prior to application, to reduce the strong inter-tube interactions. The reactivity of CNTs in terms of covalent chemistry on the C backbone is due to local strain, which is caused by two main reasons: the first one is the curvature-induced pyramidalization of the conjugated carbon atoms, important mostly for CNT caps, and the second is the p orbital misalignment between adjacent pairs of conjugated carbon atoms, that accounts for the strain in CNT sidewall [3]. Since both effects scale inversely with the diameter of the

* Corresponding author at: Dipartimento di Scienze Molecolari e Nanosistemi, Universita` Ca’ Foscari di Venezia, Venezia 30123, Italy. E-mail address: [email protected] (C. Fabbro). http://dx.doi.org/10.1016/j.carbon.2014.04.054 0008-6223/ 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Fabbro C, Da Ros T. Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.04.054

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tubes, a higher strain-induced reactivity is expected for smaller tubes than for larger ones. Among the different available treatments for a covalent modification of the unsaturated carbon network of CNTs [4], oxidation is one of the most effective. CNTs are typically synthesized with poly-disperse micrometer lengths, bound together into entangled ropes, and they contain metallic impurities, deriving from the catalyst used during their synthesis. Many applications, however, require individual short CNTs, void of metallic impurities. Oxidative treatments reduce metal content due to the removal of the amorphous carbon usually covering metallic particles, or the etching of CNT caps when the particles are still inside them, and the subsequent oxidation of the metal to soluble species, which are then easily removed. Moreover, as a result of chemical oxidation, the ends and the sidewalls of CNTs are covered with oxygen containing groups, mainly carboxylic groups, that increase the solubility and allow further derivatization [4–6]. The most common way to oxidize CNTs involves acid treatments, with sonication of CNTs in concentrated acid mixtures, where nitric acid or hydrogen peroxide plays the role of the oxidative agent [7,8]. Depending on the strength of the conditions, oxidation can attack only already existing reactive sites, or proceed with the generation of new defects, introducing hydroxyl groups, then further oxidized to carboxyl groups, and finally with the cutting of the nanotubes. Covalent functionalization drawback is represented by the loss of the high conductivity and of the remarkable mechanical properties of CNTs, since it generates sp3 carbon sites on CNTs, which disrupt the band-to-band transitions of p electrons. Therefore, this kind of modification, though being very effective, is mandatorily avoided when the final application requires CNT structural integrity. Alternatively, a subsequent healing step can be envisaged, to remove the defects and the groups introduced by the oxidation. In this context, CNTs thermal annealing treatments on the bulk, at different temperatures, for different time, and in different atmospheres, have been reported for oxidized SWCNTs [9–14]. In the present work SWCNTs obtained by the high-pressure carbon monoxide disproportionation process (HiPCO) have been subjected to a very harsh oxidative treatment, adapting a procedure described by Tour and co-workers [15]. Nitric acid was employed as the oxidizing agent, following an intercalation step in oleum, to obtain a homogeneous distribution of CNTs in solution [16]. The mechanism for this intercalation consists in the protonation of the tubes and in their surrounding by sulfuric acid anions, thus disrupting van der Waals inter-tube interactions, and it is therefore possible only in superacids, where no competing base is present [17]. The treatment led to a purification from metal particles and to a dramatically improved dispersibility, allowing for an easy handling of the SWCNTs. Afterwards, a local repristinization of the CNTs by means of laser-induced heating was carefully investigated as a new methodology for removing in

situ the amorphous carbonaceous material and the structural defects derived from the oxidation.

2.

Results and discussion

2.1.

Oxidation of SWCNTs

HiPCO SWCNTs were first treated with oleum, under Ar atmosphere, and then oxidized with a mixture of oleum and aqueous nitric acid, exposing the mixture to air at 65 C (Fig. 1). The oxidized SWCNTs 1 thus obtained, after having been thoroughly washed, were fully characterized with different complementary techniques. The dispersibility in both polar organic solvents and water highly increased with respect to the pristine SWCNTs,1 and transmission electron microscopy (TEM) images clearly show shorter and less aggregated CNTs with respect to the pristine ones (Fig. 2). Nevertheless, they also indicate that a certain amount of amorphous carbonaceous material was present in the sample, as a result of the fragmentation caused by the quite aggressive oxidative treatment. From TEM images, it is also possible to appreciate the expected decrease in the iron content of the sample, confirmed by thermogravimetric analysis (TGA) in air (Supplementary data, Fig. S1a). FT-IR spectroscopy indicated the formation of carboxylic acid groups (Fig. 3a and b), considering the carbonyl stretching peak at 1726 cm 1, as already observed for oxidized SWCNTs [18]. Moreover, a shoulder appearing in the 3000– 3300 cm 1 region could be ascribed to the stretching of carboxylic O–H bonds. Comparing SWCNTs 1 TGA under N2 atmosphere with the one of the pristine SWCNTs (Fig. 4a), a big increase in the weight loss (almost 40%) was observed at temperatures below 800 C, ascribable to the amorphous carbon seen in the TEM pictures and to the oxidized groups introduced with the cutting, mainly carboxylic acids, but also some sulfonic acids, as appreciable through mass spectrometry coupled to TGA (TGA–MS) of SWCNTs 1 (Supplementary data, Fig. S1b). Major changes are observed also in the UV–Vis–NIR (Supplementary data, Fig. S2) and Raman spectra (Fig. 4b), due to covalent modification of the sp2 network of the CNTs. Concerning Raman spectra, the oxidative treatment led, as expected, to a big increase in the D/G ratio, due to modified sp3/sp2 carbon atoms ratio. The significant presence of amorphous material was confirmed by the increased full-width-athalf-maximum (FWHM) intensity of the D-band, that is in general much broader for the various carbon impurities than for CNTs [19], together with the appearance of a background fluorescence, raising the spectrum baseline. In the radial breathing mode (RBM) zone, a loss of most of the bands was observed, since the smallest and most reactive tubes were completely destroyed. The sample was analyzed using the 532, 633 and 785 nm lasers (Figs. 4b and S3 in Supplementary data), that gave similar results, with slight differences associated with the different populations of tubes in resonance at the energy of the specific laser used.

1 Colloidal suspensions of SWCNTs 1 in DMF and NMP (1 mg/mL), obtained with the aid of 15 min ultrasonic bath treatment, are stable indefinitely and can cross a 0.22 lm pore size PTFE filter (while they are retained by a 0.1 lm one).

Please cite this article in press as: Fabbro C, Da Ros T. Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.04.054

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Fig. 1 – Preparation of SWCNTs 1.

Fig. 2 – Comparison between pristine SWCNTs (a) and SWCNTs 1 (b). DMF dispersions and representative TEM images at different magnifications. Lower panels show enlargements of the area indicated by the square in the upper ones. Arrows point at metal particles.

In summary, the different techniques used to characterize SWCNTs 1 showed the dual effect of the treatment, responsible for the dispersibility improvement: (i) the covalent disruption of the sp2 network of carbons, and the formation of oxidized functions, (ii) the formation of amorphous carbonaceous fragments, acting as surfactants.

2.2.

Repristinization of the oxidized SWCNTs

During the Raman analyses of SWCNTs 1, we observed a pretty particular behavior, which might help in better understanding the nature of the product obtained. Quite surprisingly, increasing laser power, a sort of repristinization of

Please cite this article in press as: Fabbro C, Da Ros T. Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.04.054

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Fig. 3 – FT-IR spectra of pristine SWCNTs (a), SWCNTs 1 (b) and SWCNTs 2-t(1) (c). (A color version of this figure can be viewed online.)

Fig. 4 – TGA of pristine SWCNTs, SWCNTs 1, SWCNTs 2-t(1) and SWCNTs 3, performed in N2 (a). Raman spectra (785 nm laser) of pristine SWCNTs, SWCNTs 1 and SWCNTs 3 (b). In the inset, the enlargement of the RBM zone is shown. (A color version of this figure can be viewed online.)

the sample occurred (Fig. 5), as can be clearly observed in the spectra depicted in Fig. 6. Working with the 785 nm laser, the starting laser power used was 0.57 mW/lm2. Laser power density was then gradually increased. At 2.86 mW/lm2 we started to observe a decrease in the D/G ratio, a lower baseline and a better definition of RBM bands. Afterwards, we exposed the sample to a laser power density of 5.73 mW/lm2, which gave a too high response, corresponding to a saturated spectrum (not reported), but resulted in a complete repristinization of the

sample, appreciable when the analysis was repeated again on the same spot at lower laser power (Fig. 6a). These CNTs, deriving from SWCNTs 1 by laser treatment, will be referred to as SWCNTs 2-l(1). By comparison of this spectrum with the one of the pristine SWCNTs, one can clearly notice that the two profiles are pretty similar, apart from the RBM zone, where an enrichment of certain tubes with respect to others is evident (Fig. 6b). Also, an overall increase in the Raman cross section was observed (note that all the spectra are normalized to the G-band maximum), and in fact we needed to

Please cite this article in press as: Fabbro C, Da Ros T. Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.04.054

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Fig. 5 – Preparation of SWCNTs 2 and 3.

Fig. 6 – Raman repristinization process of SWCNTs 1 (785 nm laser) (a). In the inset, the enlargement of the G-band zone is shown; Raman spectra of SWCNTs 1 (785 nm laser), before and after (SWCNTs 2-l(1)) repristinization of the sample, in comparison with the pristine SWCNTs (b). In the inset, the enlargement of the RBM zone is shown. (A color version of this figure can be viewed online.) lower the laser power density to 0.06 mW/lm2 to be able collect the spectrum without saturating the signal after the repristinization took place (Fig. 6a). The same phenomenon was observed using the 532 and 633 nm lasers, with laser power of 2.07 and 2.55 mW/lm2, respectively (Supplementary data, Fig. S4). In both cases, the main difference between pristine SWCNTs and SWCNTs 2l(1) were in the RBM zone and in the shape of the G-band, both correlated with different composition of the SWCNT sample, in terms of diameters. We believe that the phenomenon is due to thermal effects. Treating CNTs with a high power laser resulted in a local increase of temperature, to which we ascribe an annealing of amorphous carbon and of structural defects, and, consequently, a re-establishment of the sp2 network. Of course, the tubes that were completely destroyed during the oxidation were not present anymore, therefore leading to an

enrichment of the sample with the remaining tubes, as confirmed by the changes in the RBM and G-band patterns. In particular, the loss of the higher wavenumber RBM bands, observed with all the lasers, confirmed the selective destruction of smaller diameter tubes by the aggressive acid treatment. A confirmation for the thermal origin of the effect arises from the reversible shift in the G-band to lower wavenumbers (from approximately 1593 to 1584 nm) upon increasing of laser power density (Fig. 6a), since this downshift has been correlated with a lengthening of the C–C distances as the nanotube undergoes thermal expansion [20]. Very few papers appeared in the literature so far, reporting a similar behavior, and they all refer to pristine CNTs. In two cases, pristine SWCNTs were treated with the 633 nm laser, at laser power densities of 1 or 0.1 mW/lm2, and the authors observed a decrease in the D/G ratio, a reversible shift of the

Please cite this article in press as: Fabbro C, Da Ros T. Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.04.054

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Fig. 7 – Raman spectra (785 nm laser) of SWCNTs 2-t(1) (a) and of SWCNTs 3, before and after (SWCNTs 2-l(3)) repristinization of the sample (b). In the inset, the enlargement of the RBM zone is shown. (A color version of this figure can be viewed online.)

Fig. 8 – Representative TEM images of SWCNTs 3 (a) and SWCNTs 2-t(1) (b), at different magnifications.

G peak to lower wavenumbers, and a change in the RBM peaks, together with an increase in their intensities [21,22]. Also a general increase in the Raman cross section was reported, explained as a result of the higher purity of the sample, since more CNTs produce higher Raman response. In both cases, the authors explained the fact as a thermal effect, resulting in annealing of the CNTs. They calculated the temperature reached in their sample, at the higher laser power density used, which was in the range 300–450 C. Nevertheless, it should be pointed out that this value should be considered just as indicative, since it is calculated by the ratio between Stoke and anti-Stoke intensities, which, in resonance Raman spectroscopy, is not a reliable method. In fact

different kind of CNTs are resonant in the Stoke and antiStoke spectra [21]. Also other reports appeared, regarding pristine MWCNTs, where a laser treatment resulted in lowering of the D/G ratio, further confirming a sort of annealing and purification effect [23–26]. Regarding oxidized SWCNTs, no similar results are present in the literature, to the best of our knowledge. On the contrary, a paper published in 2000 describing a similar laser treatment (633 nm, 2.2 mW/lm2) for oxidized SWCNTs, reported just a change of the line shape of the G bands, that the authors ascribed to degassing of the sample due to heating effects, but not to annealing of defects, since they did not observe any decrease in the D/G ratio, nor any change in the

Please cite this article in press as: Fabbro C, Da Ros T. Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.04.054

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RBM zone [27]. Another report on a 633 nm laser treatment of pristine SWCNTs at 1 mW/lm2 describes, contrary to our findings, the selective destruction of metallic tubes, together with an increase in the D/G ratio, and a decrease in the Raman cross section [28]. To confirm that the phenomenon was actually thermal, we prepared a reference sample, obtained by thermal annealing at 500 C of SWCNTs 1. The temperature was selected after a set of experiments at different temperatures. While the treatment at 450 C did not lead to healing of the sample, as confirmed by Raman analysis (data not shown), for the sample treated at 500 C (SWCNTs 2-t(1)), indeed, the Raman profile was the same as for SWCNTs 2-l(1) (Figs. 7a and S5 in Supplementary data). The laser-induced phenomenon is therefore the same operating during the thermal annealing, with the difference of happening in situ, during the Raman analysis, as a consequence of the local heating induced by the laser, and thus allowing a precise spatial control (laser spot size on the sample was 2 lm). Further confirmations of the repristinization process come from TGA profile (Fig. 4a), showing the disappearance of the low-temperature burning material, and, more importantly, from FT-IR analysis (Fig. 3c), where the disappearance of the carboxylic acid-related peaks was observed. With comparative purposes, an alkaline wet purification of SWCNTs 1 was also performed, by treatment with sodium hydroxide (NaOH) at 100 C (Fig. 5) [29,30]. TGA of the purified SWCNTs 3 showed a big decrease in the weight loss with respect to SWCNTs 1 (Fig. 4a), and the Raman profile became much more similar to the one of pristine SWCNTs (Figs. 4b and S3 in Supplementary data). Coherently with these observations, TEM images showed much cleaner CNTs (Fig. 8a). These findings together indicate that the alkaline treatment effectively removed from the sample the major amorphous impurities deriving from the strong oxidative treatment, found instead in the filtrate (Supplementary data, Fig. S6). Nevertheless, as a consequence, the dispersibility of SWCNTs 3 strongly decreased with respect to SWCNTs 1, as TEM images confirmed, showing many bundles and big aggregates (Fig. 8a). On the contrary, TEM analysis of SWCNTs 2 showed that the repristinized CNTs are not only cleaner, void of amorphous carbon contamination, but also well-dispersed (Fig. 8b). Finally, the laser repristinization experiment was performed for SWCNTs 3 as well, resulting in SWCNTs 2-l(3), that gave the same Raman profile previously reported for SWCNTs 2l(1) (Figs. 7b and S7 in Supplementary data). Our hypothesis is that, while the NaOH washing was effective in purifying the sample from the amorphous carbon content, it did not induce any structural healing, that was instead achieved by the following laser treatment, thus completing the repristinization process.

3.

Conclusions

In summary, an oleum/nitric acid treatment of SWCNTs resulted in the production of shortened and disentangled CNTs, with a reduced metal content, bearing carboxyl and sulfonic groups, together with a significant amount of amorphous carbonaceous material, with a dramatic improvement

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in dispersibility. While performing Raman analysis on this sample, we serendipitously observed and then systematically investigated a laser-mediated repristinization of the SWCNT tubular structure that, besides burning the amorphous fragments, was also able to effectively heal the defects introduced by the oxidation. To the best of our knowledge, this is the first time that such a laser-induced repristinization phenomenon is reported for oxidized SWCNTs. We believe that being aware of such an effect is quite important for different reasons. First of all, it should always be kept in mind by researchers in the field how the laser power could affect the outcome of a Raman analysis, since it is able to directly alter the sample, making it mandatory to carefully tune this parameter, specifically when oxidized or, in general, covalently modified CNTs are concerned. Second, attention should be paid in analyzing samples deriving from a strong covalent modification, since the RBM zone is usually considered as a fingerprint to identify SWCNTs among the different nanocarbon-based materials. Our approach could thus be exploited to (i) quickly verify the presence of the tubes in a deeply modified sample, and (ii) establish which fraction of the original mixture of SWCNTs survived the treatment. Lastly, it is possible to envisage the development of a laserbased system to heal in situ oxidized SWCNTs. A first strong oxidation step can be performed in order to be able to properly disperse CNTs for an easier handling. Once the material has been deposited, a following repristinization could be performed in situ by means of laser treatment. Though being basically a thermal effect, the laser offers a precise spatial control, thus allowing to foresee an accurate design of specific devices. Therefore, the development of such an effect could offer an ideal solution for all those micro-scaled CNT applications where a low metal content and a better dispersibility are needed and at the same time CNT sp2 structural integrity is a requirement for the final device.

4.

Experimental part

4.1.

Material

Solvents, acid solutions and NaOH were purchased from Sigma–Aldrich and used as received. HiPCO Single-Walled Carbon Nanotubes were purchased from Carbon Nanotechnologies (lot #R0510C). For the filtration of the CNTs, hydrophilic OmniporeTM filters of the specified pore size were used (Merck Millipore).

4.2.

Instruments

TEM analyses were performed with a Philips EM 208 microscope with an accelerating voltage of 100 kV (images were acquired using an Olympus Morada CCD camera). CNTs samples were typically suspended in DMF with the help of sonication in a sonic bath, and these suspensions were drop-casted on copper or nickel grids (diameter = 3.00 mm, 200 mesh, coated with carbon film), which were subsequently dried under high vacuum overnight, prior to the TEM analysis. Thermogravimetric analyses were performed using a TGA

Please cite this article in press as: Fabbro C, Da Ros T. Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.04.054

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Q500 (TA Instruments), treating the samples placed in Pt pans with the following procedure: isotherm at 100 C for 20 min (to remove residual solvent, if any), ramp from 100 to 800 C at 10 C/min, under N2 or air (flow rate on the sample of 60 mL/min). Reported graphs are average of at least two separate measurements. TGA–MS experiments were performed on the same TGA instrument coupled with a ThermoStar Mass Spectrometer (Pfeiffer Vacuum) with the following procedure: isotherm at 100 C for 20 min, ramp from 100 to 800 C at 20 C/min, under He (flow rate on the sample of 60 mL/min). Infrared spectra were recorded on a Perkin–Elmer System 2000 FT-IR spectrometer, using the KBr pellets technique. UV–Vis–NIR spectra were recorded on a Cary 5000 spectrophotometer (Varian), using 1 cm path quartz cuvettes. Raman spectroscopy analyses were performed under ambient conditions, on solid samples deposited onto a glass coverslip, on an inVia Raman microscope (Renishaw), equipped with lasers at 532, 633 or 785 nm, using a 50· objective to focus the laser beam on the sample, at a working distance of 1–2 mm (spot size = 2 lm). All reported spectra are normalized to the G-band maximum.

4.3.

Procedures

4.3.1.

SWCNTs 1

HiPCO SWCNTs (1 g) were dispersed in oleum (500 mL), and the mixture was stirred for 12 h under Ar atmosphere. Then, a mixture of oleum and aqueous nitric acid (65%) (500 mL) was added, to a final ratio oleum/nitric acid of 3:1, allowing air in, and stirring the mixture at 65 C for 2 h. The reaction mixture was diluted in water and filtered (pore size = 5 lm). Afterwards, the material was washed several times (each washing cycle consisting in: dispersion in the specified solvent, sonication in a sonic bath for 15–20 min and vacuum filtration) with H2O, up to a neutral pH in the filtrate, and MeOH. Et2O was subsequently poured on the filtered CNTs, and vacuum was applied for 30 min after emptying the filtration flask. Finally, SWCNTs 1 were scratched from the filter and dried under high vacuum (770 mg, 77% weight yield).

4.3.2.

SWCNTs 2-t(1)

SWCNTs 1 (2 mg) were placed in a Pt TGA pan and heated with the following procedure: equilibrate at 500 C, equilibrate at 50 C, under air (flow rate on the sample of 60 mL/min), the total duration of the experiment being 28.6 min.

4.3.3.

SWCNTs 3

SWCNTs 1 (100 mg) were dispersed in aqueous NaOH 8 M (100 mL) by sonication and stirred at 100 C for 48 h under Ar atmosphere (in a PTFE round-bottom flask). Afterwards, CNTs were separated from the dissolved amorphous material by filtration (pore size = 0.45 lm), and thoroughly washed, as described above, with H2O, MeOH and Et2O to afford, after high vacuum drying, SWCNTs 2 (40 mg, 40% weight yield). In order to perform Raman analysis of the aqueous filtrate, it was neutralized by stirring the solution in 0.1 M HCl and subsequently filtered (pore size = 0.45 lm). The material collected on the filter was then washed with H2O, MeOH and Et2O.

Acknowledgement The authors wish to thank Prof. Davide Bonifazi for fruitful discussions.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2014.04.054.

R E F E R E N C E S

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Please cite this article in press as: Fabbro C, Da Ros T. Local ‘‘repristinization’’ of oxidized single-walled carbon nanotubes by laser treatment. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.04.054