Molten salt activation for synthesis of porous carbon nanostructures and carbon sheets

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Molten salt activation for synthesis of porous carbon nanostructures and carbon sheets Xiaofeng Liu *, Markus Antonietti Department of Colloid Chemistry, Max-Plank Institute of Colloids and Interfaces, Science Park Golm, D-14424 Potsdam, Germany

A R T I C L E I N F O

A B S T R A C T

Article history:

We report here a molten salt (MS) process for the synthesis of nanoporous carbon structures

Received 15 September 2013

and carbon sheets. Using glucose as the model carbon precursor, the process yields

Accepted 16 December 2013

different porous carbon structures with specific surface area up to near 2000 m2/g in molten

Available online 27 December 2013

LiCl/KCl containing different dissolved oxysalts KNmOx, where Nm are nonmetal elements of H, B, C, N, P, S, and Cl. These oxysalts dissolved in LiCl/KCl exert a dominating influence on the pore formation as well as two-dimensional growth of the carbons in MS. Based on the energetics of the redox reaction between carbon and the above oxysalts, a general mechanistic explanation for the pore formation and the activation process in MS is presented. The process reported here not only provides an easy route to convert biomass molecules to carbon-based functional nanomaterials, but also opens up a new direction towards carbonization and two-dimensional growth in high temperature ionic solvent systems. Ó 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

sp2 hybridized carbon nanostructures represent a versatile materials platform where diverse applications have been pursued in recent years. In most applications such as energy storage and sorption, their performances largely relate to the magnitude of the accessible surfaces at working conditions. The pursuit of high surface area with tailored porosity has therefore stimulated continuous effort in the development of more efficient synthetic routes for porous carbons. Up to the present, porous carbons have been mainly accessed through thermal carbonization from various sources, such as carbohydrates, and different raw biomasses [1–4]. In terms of chemical bonding, the carbons generally experience steady transition from sp3CAX (X: e.g., C, O, H) bonds to the aromatic sp2 C@C bonds during conversion from the precursors to the targeted carbonaceous matter. From an energetic point of view, in this process a porous structure is however less favorable, as it comes with high surface energies, or in another language: the sp2 carbons tend to stack, leading finally to

the thermodynamically stable, dense graphitic matter. In fact, a metastable porous structure can be generated for only certain organics by direct carbonization where rigid pores are developed due to a special molecular design [5–7]. For common carbon-containing organics like carbohydrates, biomass, and coal, the generation of porosity is often realized by either physical or chemical post-synthesis activation processes, the results of which are almost independent of the precursors [8–11]. In these activation processes, it has been generally accepted that pores are generated through selective cleavage of the weakly bonded carbons at elevated temperatures in oxidizing condition. This oxidative carbon removal is often regarded as the major mechanism behind different chemical activation processes in the presence of various chemicals, such as hydroxides or CO2 [9,10]. From a thermodynamic viewpoint, carbon itself is highly reducing and becomes more reductive at high temperatures, therefore in principle different elements at their oxidized state can be employed to ‘‘activate’’ carbon and generate porous structures.

* Corresponding author. E-mail address: [email protected] (X. Liu). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.12.049

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Fig. 1 – (top) Schematic illustration for the synthesis of nanoporous carbons in molten LiCl/KCl containing oxysalts of KNmOx. The corresponding oxysalts examined are KOH (Nm = H), NaBO2 (Nm = B), K2CO3 (Nm = C), KNO3 (Nm = N), KH2PO4 (Nm = P), K2SO4 (Nm = S) and KClO3 (Nm = Cl). (bottom) Digital images of 20 mg of carbon samples produced from glucose in each salt system of KNmOx@LiCl/ KCl. The same amount of graphite and ‘‘blank’’ (in only LiCl/ KCl) sample are shown for comparison. The images reveal large difference in the packing densities of the products derived from different salt systems. (A colour version of this figure can be viewed online.)

We described herein a general molten salt (MS) activation process for porous carbon production using the common carbohydrate glucose as the model precursor. In our process, the precursor was activated in an inert salt melt (LiCl/KCl) media containing oxysalts (KNmOx) of the non-metal elements Nm (Nm = H, B, C, N, P, S, Cl), as illustrated in Fig. 1. (It has to be noted here that the corresponding ‘‘oxysalt’’ of ‘‘H’’ is hydroxide – KOH  which behaviors similarly to other oxysalts after melting in terms of oxidizing nature and ionic conduction.) Depending on the type of activation chemicals and synthetic conditions, the process produced porous carbons with high porosity and different microstructures, covering the range from irregular hierarchical particles to two-dimensional (2D) sheets.

2.

Experimental section

2.1.

Synthesis procedures

temperature, the as-received product was crushed into powders and dispersed into sufficient amount of water. The carbon sample was then collected by filtration and finally dried in vacuum at 50 °C for over 12 h. For certain cases where borate or phosphate was employed as the oxysalt, hot HCl solution (1 M) was applied afterwards to wash away the insoluble inorganic side products. The complete removal of crystalline inorganic impurities was confirmed by X-ray diffraction (XRD). The relative carbon yield was calculated with the equation: Y% = Wx/W0, where W0 and Wx are product yields (in weight) obtained under the same thermal condition in salt systems of LiCl/KCl and KNmOx@LiCl/KCl, respectively. That is, the relative carbon yield reflects the amount of carbon removed by the added chemical activation agent. For the ‘‘blank sample’’ the mass yield is also temperature-dependent. At 900 °C, for instance, the mass yield is 28% (i.e., for 1 g precursor, 0.28 g product is received) and increases to 32% at 800 °C.

2.2.

Characterizations

The carbon samples were examined with different techniques. XRD was performed with a Bruker D8 Advance (CuKa radiation) diffractometer for powder samples. Raman spectra were recorded with a LABRAM-HR Confocal Laser Micro Raman Spectrometer using a 532 nm laser diode as the excitation. The microstructure was observed both by scanning electron microscopy (Gemini SEM, LEO 1550 system), and transmission electron microscopy (Zeiss EM 912 Omega system). Atomic force microscope (AFM) images were made with a Veeco instrument in tapping mode. Combustion elemental analysis for C, H, N and S was examined by a Vario MICRO Cube CHNS analyzer (Elementar Analysensysteme GmbH). Nitrogen physisorption measurements were performed at liquid N2 temperature (77 K) with a Quadrasorb Adsorption Instrument (Quantachrome Instruments). The specific surface area (SSA) was calculated according to a multi-point Brunauer–Emmett–Teller (BET) method using the date in the pressure range of 0.1–0.3 P/P0. A quenched-solid density functional theory model (slit pores, QSDFT equilibrium model) was applied to each physisorption curves to extract the pore volume and the pore size distributions.

3. The MS syntheses in the present investigation were carried out in an electric oven (NaberTherm) equipped with continuous N2 flow. The chemicals involved were all purchased from Sigma–Aldrich unless otherwise mentioned. In a typical process, the carbon precursor, glucose monohydrate (AppliChem, pure Ph. Eur.) was mixed with the activating oxysalt, for instance KNO3, and LiCl/KCl (45/55 in mass ratio) in a mass ratio of 1/1/10 using a ball mill. The homogeneous powder mixture was then transfer to the oven and flushed with N2 for 100 min, and then the sample was heated at 5 K/min to the reaction temperature and dwelled there for 5 h to allow complete conversion. (Caution: Heating a mixture of nitrate salt and glucose powder WITHOUT salt in a covered crucible in nitrogen can result in an explosion.) After natural cooling to ambient

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Results

The different oxysalts dissolved in the LiCl/KCl solvent regulate the progress of carbonization and the evolution of the microstructure differently. The interaction of the as-formed carbon intermediate in MS with the dissolved oxysalt is, in the first place, reflected in the drastic difference in the packing density of the resultant products (Fig. 1). Furthermore, the amount of carbon consumed (etched) by these salts varies as the oxidation power of the added oxysalt is different from each other. With the selected ratio of glucose/oxysalt = 1/1, the relative carbon yield ranges from down to 10% to near 100%. From Table 1, the products derived from the system containing KNO3 and KOH suffer the heaviest mass loss. In comparison, the salts of borate and phosphate do not seem to etch away the carbons, suggesting their chemical inertness in the examined conditions.

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Table 1 – SSA, pore volume and relative sample yield for the samples derived in different salt systems at 900 °C. Oxysalt in

Yield (%)b

SBET (m2/g)

Total pore volume (m3/g)

Micropore volume (m3/g)

Blanka KOH NaBO2 K2CO3 KNO3 KH2PO4 K2SO4 KClO3

100.0 10.3 93.0 52.7 32.6 93.7 55.4 40.2

82 997 137 1064 1912 64 1520 1252

0.141 0.492 0.314 0.672 0.930 – 0.923 0.920

0.006 0.386 0.020 0.359 0.705 – 0.495 0.440

a b

This is a reference sample synthesized in only LiCl/KCl under the same thermal conditions. Yield is calculated relative to the ‘‘blank’’ sample, which is synthesized in only LiCl/KCl at the same condition.

These MS activated carbons are in general weakly X-ray ordered, irrespective of the type of oxysalt employed (Fig. S1 in Supplementary data). Nevertheless, the stacked sp2 nature is still quite obvious from the strongly broadened (0 0 2) and (1 1 0) humps located near 21–26°, and 42°, respectively. Compared to graphite the position of the (0 0 2) diffraction is slightly shifted towards lower angle, related to looser packing between the adjacent sp2 layers as compared to its crystalline counterpart (graphite). These structural features are responsible for the co-existence of strong D and G band in the Raman spectra given in Fig. S2. For different biomass derived carbons, these are common characters associated with structural disorder and the presence of foreign atoms and functional groups trapped (or grafted) in-between the aromatic sp2 layers [12,13]. Fig. 2 shows the SEM images of carbon received from different salt systems. In the absence of an oxysalt, mostly irregularshaped carbon particles together with trace of layered structures are developed as we have shown in our previous work (see also Fig. S3) [14]. For the carbons produced from the salt containing borate (Fig. 2c), the irregular, inter-connected particle morphology is kept. This is on the whole similar to the carbons produced in the phosphate@LiCl/KCl systems (Fig. 2f), suggesting limited chemical interaction of carbon with these two types of dissolved oxysalts. With increasing oxidizing nature of the salt melts, these added salts strongly affect the microstructures of the resultant carbons.

In the presence of carbonate (K2CO3), besides the similar irregular-shaped particles, a larger fraction of layered structures on the micrometer scale with wrinkled surfaces are formed, as seen from SEM (Fig. 2d), presumably the reminiscence of an interaction between carbon intermediates and the carbonate salt. From a detailed TEM observation (Fig. S4b), these carbons structures show large domain of thin-layer morphologies similar to that of agglomerated puckered graphenes. Quite different from the carbonate systems, for KOH@LiCl/KCl system shown in Fig. 2b, the resultant carbons platelets and particles show characteristic porous structures, with pore sizes around 10–100 nm. These are however comparably big pores which are larger than the lower nanoscale pores mainly caught by nitrogen physisorption (discussed below). In accordance with the pore formation, the relative yield of carbon also decreases drastically to 10% (Table 1) as a result of partial gasification of carbon atoms by KOH. In contrast to the KOH system, in the system of KNO3@LiCl/KCl, particle/sheet morphology is developed (Fig. 2e and Fig. S4) which is very similar to the product obtained in the systems of LiNO3@LiCl/KCl as reported previously [15]. The fraction of sheet increases with the reducing in the concentration of precursors in the MS, which is interpreted in terms of a precipitation/dissolution mechanism for carbon structure growth in MS. In this sense the structures produced in the melt of K2SO4@LiCl/KCl (Fig. 2g) can be rationalized with a similar mechanism [14], as the products are

Fig. 2 – SEM images the salt-derived carbons synthesized in (a) only LiCl/KCl, and with dissolved oxysalts KNmOx, Nm = (b) H, (c) B, (d) C, (e) N, (f) P, (g) S and (h) Cl. Scale bars: 2 lm.

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also composed of thin sheets together with precipitated pristine irregular carbon networks. For the system containing KClO3, which is the most oxidizing salt employed, the carbon growth within the melt of KClO3@LiCl/KCl becomes completely 2D, such that isolated carbon sheet with typical graphene-like wrinkles are omnipresent in the product (Figs. 2h, 3a). These carbon layers however give diffused rings instead of spots in the electron diffraction pattern, suggesting these sheets are quit defectious and disordered due to the presence of numerous pores within the planes, which is similar to our previous observation. As seen by AFM (Fig. S5), the majority of the carbon sheets have thicknesses lower than 10 nm, while thinner structure of close to single-layer thickness (0.5 nm, Fig. 3b) are also present in considerable quantity. The deviation from the ideal value (0.335 nm) is possibly due to the presence of surface functional groups (considering 9 wt.% of O) and adsorbed molecules. Compared to our previous system for graphene production, the chlorate@chloride system discussed here is obviously advantageous in terms of efficiency

463

and scalability as it starts from a much higher precursor concentration (10 times higher than Ref. [14]). The porous characteristics of the salt-activated carbons were quantified by nitrogen physisorption experiments. Fig. 4 presents the physisorption curves for the examined samples discussed above. Compared to the reference sample (produced in only LiCl/KCl), there is no obvious increase in specific surface area (SSA) for samples derived from salt containing borate and phosphate (at 900 °C). In sharp contrast to that, high porosity is found for samples derived from the other high oxidizing salt systems, as it is typical of activated carbons. The highest SSA among these samples is 1912 m2/g observed for the carbon from the system of KNO3@LiCl/KCl, and the samples also shows highest pore fraction of 0.93 cm3/g. Regarding pore size distribution, these salt-activated carbons all showed peaks below 2 nm (in Fig. S6), and pore volume are contributed mainly from micropores (pore size < 2 nm). This is again a typical consequence of chemical activation for carbonaceous matter. To examine the influence of synthetic parameters on the growth of carbon structures in the MS, we here select the system of KClO3@LiCl/KCl as a case study. From SEM, the microstructure is apparently not affected by synthesis temperature, graphene-like structures are still observed as a dominating fraction at the synthesis temperature of 1000 °C (Fig. 5c). However, higher reaction temperature leads to the loss of yield as more carbon atoms are being gasified by the oxidant. This result on the other hand suggests that the oxidation of carbon by KClO3 diluted in molten LiCl/KCl can be slowed down and controlled, rather than occurring in a sudden run-away explosion (as well known for other carbon-oxidant combinations). From nitrogen physisorption results, however, higher reaction temperatures leads to larger pore volume and changes SSA (Fig. S7). Maxima SSA is observed for the sample derived at 900 °C with also maxima pore volume (Fig. 5d). From SEM and nitrogen physisorption, a steady evolution in product character is also revealed upon changing of the mass ration KClO3/glucose (K/G). Thin film structures are present, but in only small fraction for samples synthesized in low K/ G (=0.7), as given in Fig. 5a. At higher K/G ratios, more layer structure seems to be liberated and the product become dominated by thin layer carbons for K/G = 1 (Fig. 5b). However, at K/G = 2, accompanied by the severe loss of yield, porous textures appears (Fig. S5) and the SSA (Fig. 5e) also shrinks, suggesting the influence of oxysalt concentration could be complicated.

Height (nm)

1.0

4. 0.5

0.0 0.0

0.5

1.0 Distance (nm)

1.5

2.0

Fig. 3 – (a) Typical TEM and (b) AFM images for the graphenelike thin carbon layers synthesized in KClO3@LiCl/KCl at 900 °C. Scale bar: 0.5 lm. (A colour version of this figure can be viewed online.)

Discussion

It was shown in a recent report that graphite carbon can be corroded and exfoliated by the ‘‘inert’’ chloride salt with the participation of oxygen at temperatures over 1000 °C [16]. In comparison, the result here obtained in inert atmosphere demonstrate clearly that the dissolved oxysalt modulate the growth and etching of the carbon structures differently. According to our EM observation, these salts can be divided into three groups. Borate and phosphate are quite inert or weakly oxidizing, such that they do not change drastically the carbon formation in the chloride salts, generating low

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blank Nm = H Nm = B Nm = C

500

3

-1

Volume @ STP (cm g )

1000

0

Nm = N Nm = p Nm = S Nm = Cl

1000

500

0 0.0

0.2

0.4

0.6

0.8

1.0

Relave pressure (P/P0) Fig. 4 – Nitrogen physisorption isotherms for the carbons synthesized in different KNmOx@LiCl/KCl salts systems at 900 °C (except for sample Nm = P, the temperature is 1000 °C). The sample synthesized in only LiCl/KCl is shown as ‘‘blank’’ for reference. (A colour version of this figure can be viewed online.)

1500

80

80

Yield SBET

800 °C

60

60

2000

900 °C

Yield SBET

20

500

0 600

700

800

900

1000

Synthesis temperature (°C)

900 °C

1000

2

40

SBET (m /g)

Relave yield (%)

2

1000

40

SBET (m /g)

Relave yield (%)

800 °C

20

0

0

0.7

1.0

1.5 Mass rao, KClO3/glucose

2.0

Fig. 5 – Influence of synthetic conditions on the carbons obtained in KClO3@LiCl/KCl. (a–c) SEM images for carbon synthesized at 800 °C and KClO3/glucose (K/G) ratio of (a) 0.7/1 and (b) 2:1, and (c) at 1000 °C and K/G = 1:1. Scale bars: 5 lm. (d, e) SSA and relative yield as a function of (d) synthesis temperature, and (e) K/G ratio. (A colour version of this figure can be viewed online.)

porosity and similar irregular morphologies. Hydroxide, being the second group, is oxidizing after melting, such that it modifies micrometer-scale morphologies and improves remarkably porosity. The third group of salts includes carbonate, nitrate, sulfate and chlorate, all of which are not only efficient in generating pores but their presence also enhances the 2D

growth of carbon intermediate, despite that the 2D carbon structures developed in each systems seems to have their own characteristics. The formation of pores in the examined systems are apparently not based on the ‘‘salt templating’’ as in the systems of ZnCl2–ACl (A = Li, Na, K or Cs), which is interpreted

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in terms of phase separation in organic–inorganic salt melts in Ref. [17]. The generation of porosity by the MS process described here are, however, basically similar to that of the chemical activation process where weakly bonded carbon atoms are oxidized by the activation agent [10], leaving void of various size depending on the progress of activation. The major difference is evident that the MS process not only generates porosity but also allows the controllable production of doped 2D carbon sheets. We can now only speculate that driving force behind the 2D growth is that sp2 CAC bonding favors energetically a 2D assembly of carbon intermediate mediated by ionic environment. The growth of 2D carbon sheet then starts by organizing these primitive structures to large micrometer-scale sheets. The detailed mechanisms are still under investigation. Concerning the mechanism of activation in the examined salt system, it is in general accepted that pore are generated due to the partial oxidation of the as-formed carbon intermediate by the added oxysalt. Assuming CO is the product (CO2 is considered as oxidation the product in the case of only strong oxidants, nitrate and chlorate), the following reactions between carbon and the oxysalt anions are considered to occur in the different salt-carbon systems we studied: OH : C þ 2OH ! CO þ H2 " þO2

ð1Þ

2 2 CO2 3 : C þ CO3 ! 2CO þ O

ð2Þ

 2 NO 3 : 5C þ 4NO3 ! 5CO2 þ 2N2 " þ2O

ð3Þ

3 3 PO3 4 : a: 2C þ PO4 ! 2CO þ PO2

! CO þ 2P " þ3O

2 NmO2 x ! NmOx1 þ O



2 b: 2C þ SO2 3 ! 2CO þ S " þO



ClO3 : 3C þ 2ClO3 ! 3CO2 þ 2Cl

ð7Þ

For instance, carbonate and sulfate dissociate into oxide 2 ion (by, e.g.: CO2 3 ! CO2 þ O ) and the corresponding oxides (CO2 or SO3). The energetics of the reactions between carbon and these oxides then appear to be the main driving force. We calculated the temperature dependence of free energies for the oxides under discussion (Fig. 6) [20,21]. The reduction of nitrogen or chlorine oxide by carbon occurs at all temperatures (free energies are all positive, not shown in Fig. 6), while the reduction of B2O3 by carbon is not possible up to 2000 °C. The temperature where carbothermal reduction occurs exhibits the following sequence (assuming CO to be the oxidation product, bold black line in Fig. 6): P2O5 > CO2 > H2O > SO2  NOx, ClOx. This sequence can explain the low SSA for the carbons derived from the salts containing phosphate and borate. According to Fig. 6, reduction of P2O5 starts from temperatures of around 1080 K. We have indeed found that the remarkable improvement in porosity for the phosphate system (KH2PO4@LiCl/KCl) carbon at higher reaction temperatures of over 1000 °C (722 m2/g at 1000 °C, and 939 m2/g at 1100 °C). For the borate system, however, no increase in porosity could be observed under all examined conditions (T < 1300 °C), which is again in agreement with energetic interpretations. The redox reaction between carbon and K+ has also been considered as a plausible activation pathway in KOH activated carbons [10,22,23]. From thermodynamic consideration, this redox reaction does not appear to be directly thermodynamically possible at the examined temperatures (T < 1000 °C). The

ð4Þ

2 2 SO2 4 : a: C þ SO4 ! CO þ SO3



In the framework of the above reactions, the mechanisms of these reactive salts to drive the diverse structure building reactions can be interpreted in a clearer energetic manner. Similar to aqueous media, in MS a Lux-Flood acid–base equilibrium is built up [18,19], such that the salts dissociate into oxide ion (O2–) and the corresponding oxides by:

b: C þ 2PO3 2

2

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ð5Þ

ð6Þ

0

2/3S + O2 → 2/3SO3 565 K

-1

Free energy (kJ mol )

-200

1080 K -400

1170 K

S + O2→ SO2 2H2 + O2 → 2H2O 4/5P + O2 → 2/5P2O5 2K + O2 → 2K2O C + O2 → CO2 2C + O2 → 2CO 4/3B + O2 → 2/3B2O3

-600

-800 500

1000

1500

Temperature (K) Fig. 6 – Calculated temperature dependence of free energies (Ellingham diagram) for the oxidation of H and p-block elements. (A colour version of this figure can be viewed online.)

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carbothermal reduction of K+ starts at temperatures higher than that of all other systems, suggesting the redox reactions between carbon and the anion groups NmOx (Nm = H, C, N, P, S and Cl) to play the major role in the activation. On the other hand, the limited activation behavior of KH2PO4 to generate SSA also implies that carbon might not be etched by K+. The above analysis now can be generalized: to be an efficient activation salt for opening porosity, carbon should be able to reduce the corresponding oxide. Regardless of the cation, the anion groups, either inorganic or organic, should contain oxidized elements that can be reduced by carbon. This was testified further recently. We found that even acetate and oxalate have similar activation behavior and drive pore formation, which can be again energetically rationalized. The final comment concerns the incorporation of p-block foreign atoms into the sp2 carbons during the MS carbonization process. From elemental analysis, all N, P and S are incorporated into the final product. As pointed out previously [15], it is obvious that carbothermal reduction can be extended to the p-block nonmetal oxides, which the reduced elements end up to be integrated into the sp2 carbon framework. At the moment, it is not possible to conclude here with a more precise interpretation for the mechanisms based on only current observations without the aid of advanced techniques, e.g., solid state NMR techniques, while it is clear that the chemistry of this process deserves further investigation as it may have implications for synthetic organic chemistry.

5.

Conclusions

In summary, a simple MS activation process for the production of porous carbon from biomasses was generalized. The process of pore generation in the MS is interpreted in terms of a redox reaction between the carbon intermediate phases and the anion groups of the dissolved salts. Depending on the type of oxysalt employed, the resultant carbons show different morphologies, from irregular inter-connected particles to thin graphene-like carbon layers. The MS process delineated here thereby opens up a new avenue towards the production of functional carbon materials.

Acknowledgements This research was supported by the Max-Plank Society. The authors acknowledge Ms. A. Heilig for AFM and Dr. Guylhaine Clavel for the TEM measurement. X. L. thanks the Alexander von Humboldt foundation for the postdoctoral research fellowship.

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. 2013.12.049.

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