Journal of Materials Chemistry AMaterials for energy and sustainabilitywww.rsc.org/MaterialsA

ISSN 2050-7488

Volume 4 Number 25 7 July 2016 Pages 9703–10024

COMMUNICATIONYafei Zhang et al. A new strategy to prepare N-doped holey graphene for high-volumetric supercapacitors

Journal ofMaterials Chemistry A

COMMUNICATION

Publ

ishe

d on

14

Apr

il 20

16. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

12/0

3/20

17 0

1:13

:08.

View Article OnlineView Journal | View Issue

A new strategy to

Key Laboratory for Thin Film and Microfa

Department of Micro/Nano Electronics, Sh

200240, PR China. E-mail: yfzhang@sjtu.e

21 3420 5665

† Electronic supplementary informationprocess, characterization, SEM and TEMelectrochemical performance of N-HG-800

Cite this: J. Mater. Chem. A, 2016, 4,9739

Received 16th February 2016Accepted 14th April 2016

DOI: 10.1039/c6ta01406b

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

prepare N-doped holey graphenefor high-volumetric supercapacitors†

Xinwei Dong, Nantao Hu, Liangming Wei, Yanjie Su, Hao Wei, Lu Yao, Xiaolin Liand Yafei Zhang*

N-doped holey graphene (N-HG) was successfully prepared by a novel

“Bottom-up” strategy with scalable and low cost characteristics. The

as-obtained N-HG possessed amounts of in-plane holes, a high

specific surface area (1602 m2 g�1), a high nitrogen content and could

be easily stacked to form a high density carbon monolith which pre-

sented a maximum volumetric capacitance of 397 F cm�3 for

supercapacitors.

Graphene has recently received intensive interest as a newsupercapacitor electrode material because of its high intrinsicelectrical conductivity, excellent mechanical exibility, anexceptionally large theoretical surface area of 2630 m2 g�1 anda theoretical gravimetric capacitance of about 550 F g�1.1–3

However, it is usually hard for the graphene electrode to havehigh volumetric capacitance. The high volumetric capaci-tance is the major way to improve the energy density ofsupercapacitors, which usually have much lower energydensity than traditional batteries.2,3 On one hand, the porousstructures of graphene can offer high gravimetric capaci-tances due to the large specic surface area (SSA) and superiorion diffusion access, but always have a lower volumetriccapacitance due to the low packing density. On the otherhand, due to the strong p–p interaction, the graphene sheetstend to restack to form graphite-like powders or lms, whichcan severely decrease the accessible surface areas and reducethe ion diffusion rate, which greatly result in a lower gravi-metric capacitance and poor rate performance.4–6 Thus,how to achieve a densely compact graphene electrode butretain a high porosity is a challenge for high-volumetricsupercapacitors.

brication of the Ministry of Education,

anghai Jiao Tong University, Shanghai

du.cn; Fax: +86 21 3420 5665; Tel: +86

(ESI) available: Detailed experimentalimages, Raman and FTIR spectra, and. See DOI: 10.1039/c6ta01406b

hemistry 2016

Very recent studies on holey graphene (HG) have shown thatintroducing holes on the surfaces of nanosheets could be anefficient strategy to achieve an improved volumetric perfor-mance.3,7,8 These HGs could lead to excellent ion transportacross the graphene plane and ultimately access to the innerelectrode surfaces even with a densely layered morphology. Atthe same time, the introduction of holes in graphene sheetsenabled a densely layered morphology by allowing pathways forsolvent molecules to escape, resulting in a high-density carbonthat can greatly improve volumetric capacitance.3,7 On the otherhand, doping graphene with nitrogen has also been provedeffective in improving the energy storage performance ofsupercapacitors.9–14

The conventional strategy to prepare N-HG is usually appliedthrough three steps of the “Top-down” approach using GO asthe precursor, obtaining holey graphene through etching GOand doping holey graphene with nitrogen.3,15–21 However, thosemethods always suffer from complicated preparation processes,wide hole size distribution, low porosity and low specic surfaceareas (SSAs) (lower than 1000 m2 g�1), low nitrogen content andpossible aggregation and the restacking of individual graphenesheets. The low porosity and low nitrogen content can only leadto limited progress in enhancing the electrochemical perfor-mance. However, for the HG prepared from the “Top-down”approach, usually a very high pressure is applied, as high as76 000 psi, to make the HG high-density lm.3,7 It is still a greatchallenge to fabricate N-HG nanosheets with uniform poresizes, high porosity and high nitrogen content. Based on theabove considerations, it will be certainly great if such inter-esting and promising single or few-layer porous N-HG could besynthesized in an economic and scalable route.

In this work, a novel simple and scalable “Bottom-up”strategy is presented to prepare N-HG for high-volumetricsupercapacitors. The as-obtained N-HG in our work has anamount of in-plane holes, a high SSA (1602 m2 g�1), and a highnitrogen content (9.92 at%) and exhibits super-high volumetriccapacitance for supercapacitors. In this strategy, the prepara-tion of N-HG was based on a facial self-template pathway, using

J. Mater. Chem. A, 2016, 4, 9739–9743 | 9739

Journal of Materials Chemistry A Communication

Publ

ishe

d on

14

Apr

il 20

16. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

12/0

3/20

17 0

1:13

:08.

View Article Online

glucose as the carbon precursor, ZnO derived from zinc nitratehexahydrate (ZNH) as the template, as well as the porogen. TheZnO/ultrathin holey carbon sheet/ZnO sandwiched nano-architecture (ZnO/C/ZnO) was prepared rst, and then ZnO/C/ZnO was further subjected to a carbonization process at highertemperatures. Only ZNH and glucose are needed as raw mate-rials, and the preparation process is very simple and scalable. Ittherefore provides a new insight into high-volumetric N-HGbased supercapacitors.

Fig. 1a simply illustrates the preparation process of ZnO/C/ZnO. A mixed aqueous solution of glucose and ZNH was directlyput onto a stove at 220 �C under air environments (details in theESI†). In this process, ZnO/C/ZnO was fabricated rst throughthe fast reaction of glucose and ZNH aqueous solution at 220 �C.To examine the morphology of the brown foamy products,scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM) images were obtained. Fig. 1b shows that thesurfaces of products were coated all around by ZnO nano-crystals. ZnO nanoparticles uniformly coated the thin carbonsheets forming a rough continuous surface. As shown inFig. S1,† both the SEM and TEM images illustrate the generalframe structure of the ZnO/C/ZnO, and both the porous struc-ture and the at layer can be clearly observed throughout theentire products, suggesting that the foam-like structure is con-structed by thin carbon sheets coated with ZnO. From the high-resolution TEM (HRTEM) image (Fig. S2a†), we can observe thatthe specic interplanar distance of 0.28 nm (100) and 0.25 nm(101) for ZnO can be identied. As shown in Fig. 1c, the wide-angle XRD pattern of products with diffraction peaks indexedfurther conrms the existence of ZnO nanocrystals. All thecharacterizations present that the foam-like architecture con-sisted of ZnO/C/ZnO sandwiched nanoarchitectures.

Then, the ZnO/C/ZnO products were subjected to differenthigh temperature treatments under an N2 atmosphere fora period of time to make the middle holey carbon sheets via

Fig. 1 (a) Schematic illustration of the simple preparation process ofZnO/C/ZnO; high-magnification SEM images of (b) as-prepared ZnO/C/ZnO, (c) ZnO/C/ZnO annealed at 600 �C and (d) ZnO/C/ZnOannealed at 800 �C.

9740 | J. Mater. Chem. A, 2016, 4, 9739–9743

further carbonization. As shown in Fig. 1b–d, the ZnO nano-crystals grew from several nanometers to several hundrednanometers as the annealing temperature increased to 800 �C.The ultrathin holey graphene sheets were exposed completelyfor ZnO/C/ZnO annealed at 800 �C. The N-HG was nally ob-tained aer etching ZnO nanocrystals using a dilute HCl solu-tion. In this strategy to prepare N-HG, the mass ratio of ZNHand glucose is very important. Too much ZNH usually results innearly all the carbon sources etching away, and too muchglucose usually results in carbon sheets not thin enough. In thispaper, we chose the mass ratio of ZNH and glucose to be 4 : 1.For comparison, we also prepared N-HG samples by adjustingthe carbonization temperature at 600 �C and 800 �C, named asN-HG-600 and N-HG-800, respectively.

ZNH usually decomposed into solid ZnO together with somegases beyond 450 �C.22,23 In the experimental process used inthis study, the chemical reducing agent glucose reduced thedecomposition temperature of ZNH obviously in the aqueoussolution.24 As the reaction is highly exothermic, ZNH decom-posed by glucose presents an autocatalytic and short timereaction.24 In this reaction, a great quantity of bubbles wasgenerated by the accompanying gases in a short time, which arealso favourable for generating a porous foamy structure witha thin wall. With the aid of the exothermic energy, the glucosewas carbonized into carbon nanosheets which have abundantin-plane holes due to the existence of strong oxidant ZNH, at thesame time, ZNHwas mostly transformed into a ZnO nanocrystallm coating the carbon sheet to prevent further oxidation ofintermediate carbon sheets. In this process, ZnO/C/ZnO wasformed due to the high thermal stability of ZnO which acted asa ame-retardant. Beneting from that ZNH and glucose can beevenly mixed; this process is very benecial to form a uniformporous structure. The graphitization on such thin-orientedcarbon predecessors is easier than the conventional graphiti-zation of a bulk carbon resource or carbon lms/bres at themicrometre level, because the energy used for the parallelarrangement of (002) graphitic layers becomes much lower inthe case of an oriented predecessor.25–28 In addition, ZNH alsoserved as a nitrogen source for N-HG.

The TEM images of N-HG-600 (Fig. 2a) and N-HG-800(Fig. S4†) all show similar in-plane holes and porous structures.The high-resolution TEM image (Fig. 2c) of N-HG-800 furtherrevealed graphene sheets with no more than 5 layers. Inprevious reports, the intercalated water molecules play a deci-sive role in the restacking of the nal rGO because the hydrogenbonding facilitates interactions between GO sheets, resulting inaligning the graphene sheets in the same orientations duringthe drying process.29–32 On the other hand, the introduction ofholes in graphene sheets enabled a densely layered morphologyby allowing pathways for solvent molecules to escape, resultingin excellent process ability and a high-density lm.7 In thisstudy, the N-HG-600 exhibited apparent volume shrinkage aerbeing vacuum ltered through a cellulose membrane lter(220 nm) and normally dried at room temperature. In thispaper, as we all know, there are lots of oxygen-containingfunctional groups and hydrogen bonding in glucose, the surfaceoxygen concentration and hydrogen bonding contents of N-HG

This journal is © The Royal Society of Chemistry 2016

Fig. 2 (a) TEM image of N-HG-600, the inset is the picture of N-HG-600 before being dried; (b) SEM image of the N-HG-600 after beingdried, the inset in the upper left corner is the picture of N-HG-600after being dried, the inset in the lower right corner is the high-magnification SEM image of a N-HG-600 film section; (c) high-magnification TEM image of the N-HG-800; (d) SEM and high-magnification SEM image (inset) of the N-HG-800.

Fig. 3 (a) XRD pattern of the N-HG-600 film; (b) nitrogen adsorptionisotherms of N-HG-800 at 77 K, the inset shows the pore size distri-butions of N-HG-800; (c and d) high-resolution N 1s of N-HG-600 (c)and N-HG-800 (d).

Communication Journal of Materials Chemistry A

Publ

ishe

d on

14

Apr

il 20

16. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

12/0

3/20

17 0

1:13

:08.

View Article Online

rapidly decreased when the heating temperature was variedfrom 600 �C to 800 �C. The FTIR spectra (Fig. S6†) of N-HG-600and N-HG-800 show that the hydroxyl functional groups dis-appeared. Water molecules are attracted to hydroxyl groups andepoxy groups in N-HG due to hydrogen bonding.29 However, forthe N-HG prepared from the “Top -down” approach, very highpressure, as high as 76 000 psi, was usually applied to makeN-HG high-density lms.7 In this paper, it is very interesting thatN-HG can transform to very high-density carbon just throughdrying under normal conditions. As shown in Fig. 2b, whenN-HG-600 was dried, it could restack into a high-density carbonlm, the N-HG-600 has a compact stacking structure, no inter-layer porous structure existed. This was clearly distinguishedfrom the initial porous foamy structures. The density of N-HG-600 was calculated to be about 1.59 g cm�3, which is nearly 3/4that of graphite. As a striking contrast, N-HG-800 (Fig. 2d)retained the original typical stagger, wrinkle and interconnectedgraphene 3Dmacro-porous networkmorphology. As a result, thedensity of N-HG-800 is only about 0.2 g m�3.

The X-ray diffraction (XRD) patterns of N-HG-600 presentremarkable peaks at 26.2�and 42.3�, which indicates the (002)and (100) planes of the graphite structure, respectively.33,34

Raman spectroscopy was also used to characterize N-HG-600and N-HG-800. Both N-HG-600 and N-HG-800 (Fig. S5†) presenta G mode and a D mode. The mode at 1585 cm�1 is referred toas the G mode, which was assigned to the in-plane vibration ofsp2 carbon atoms, and the D mode at 1345 cm�1 is typically‘disorder-induced’.35 The ratio of the D band intensity to G bandintensity (ID/IG) is �0.83 for N-HG-600, which is similar to thatof other holey graphene. As for N-HG-800, the value of ID/IG is�0.95, indicating the increase of defects with the disappearanceof some functional groups.36 XPS was performed to evaluatethe chemical composition of N-HG sheets. XPS elemental

This journal is © The Royal Society of Chemistry 2016

quantitative analysis indicates that N-HG-600 and N-HG-800have a high nitrogen content of 9.92 at% and 6.43 at%,respectively. The high-resolution spectra of N 1s regions couldbe deconvoluted into three components, 398.5, 400.2, and 401.2eV, corresponding to the pyridinic, pyrrolic and graphiticnitrogens, respectively.16 Compared with N-HG-800 carbonizedat a higher temperature, N-HG-600 contains more pyridonic N,and no graphitic N exists. The porous structures of N-HG-800and N-HG-600 were further characterized by N2 adsorption/desorption tests. The Brunauer–Emmett–Teller (BET) methodand Barrett–Joyner–Halenda (BJH) models are separately usedto evaluate the SSA and pore size distribution. The isotherm ofN-HG-800 (Fig. 3b) is similarly composed of a linear part origi-nating from the monolayer absorption and a hysteresis loopbelonging to the capillary condensation of the pores, which isnormally obtained from layer-like materials.37 The shape of theisotherm is classied as type IV, indicating the presence ofmesopores. Due to the fact that the large amount of in-planeholes prevents those graphene sheets from stacking together, N-HG-800 possesses a high SSA of 1602 m2 g�1, which is muchlarger than that of HG (usually less than 1000 m2 g�1) preparedby etching GO, and an ultra large pore volume of �2.98 cm3 g�1

is achieved. The majority of pores are narrowly distributed from2 to 6 nm, which is also much narrower than those of HGsprepared by etching GO. In contrast, the isotherm of N-HG-600(Fig. S7a†) exhibits a combination of type I and type IVadsorption behaviors, which is pronouncedly different fromthat of N-HG-800. The increased amount of adsorption at lowpressures indicates an increase of small-sized micropores, andthe appearance of a hysteresis loop can also be associated withthe existence of mesopores. N-HG-600 (Fig. S7b†) has a majorityof pores distributed around 2 nm along with some small-sizedmicropores. This may be due to the fact that the original in-plane mesopores transformed to smaller pores duringthe shrinking processes. N-HG-600 possesses a high SSA of 374m2 g�1, and a much smaller pore volume of �0.183 cm3 g�1.

J. Mater. Chem. A, 2016, 4, 9739–9743 | 9741

Journal of Materials Chemistry A Communication

Publ

ishe

d on

14

Apr

il 20

16. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

12/0

3/20

17 0

1:13

:08.

View Article Online

The plentiful ordered accessible in-plane mesopores, highdensity, and high nitrogen contents make N-HG-600 verypromising as a high volumetric capacitance electrode materialfor supercapacitors. The capacitive behaviour of the N-HG-600electrode using 6 M KOH as the electrolyte in three-electrodesystems was studied. The cyclic voltammetry (CV) prole(Fig. 4a) of the N-HG-600 electrode shows similar quasi-rect-angular shapes at 20 mV s�1, indicating that the overallcapacitance originated from both double layer capacitance andpseudo capacitance. The galvanostatic charge/discharge curvesof the N-HG-600 electrode in Fig. 4b show symmetric shapeswithout obvious IR drop, demonstrating its small resistance,excellent reversibility and high coulombic efficiency. Notably,the maximum gravimetric and volumetric capacitances of theN-HG-600 electrode calculated from galvanostatic charge/discharge curves are 250 F g�1 and 397 F cm�3, respectively, at0.5 A g�1, which are the highest values reported for carbonbased materials as far as we know. The gravimetric capacitanceof the N-HG-600 electrode still retains 153 F g�1 at 10 A g�1. TheN-HG-600 electrode also shows excellent rate performance andcyclic stability. The cycling stability of the N-HG-600 electrodewas tested by repeating the galvanostatic charge/discharge testfor 10 000 cycles at 10 A g�1, as depicted in Fig. 4b. The speciccapacitance of the N-HG-600 electrode still retains 95.7% of theinitial capacitance aer 10 000 cycles, suggesting superb elec-trochemical stability and outstanding reversibility. The N-hG-600 electrode also shows exceptional electrochemical perfor-mance at very-high mass loadings up to 10 mg cm�2 (the insetof Fig. 4d), as tested in 6 M KOH, the gravimetric capacitance ofthe N-HG-600 electrode in 6 M KOH dropped from 235 to 193 Fg�1 at 1 A g�1. The N-HG-800 was also used as the electrodematerial for contrast. The CV proles (Fig. S9†) of the N-HG-800electrode exhibited rectangular shapes similar to that of N-HG-

Fig. 4 (a) CV curves of the N-HG-600 electrode at a scan rate of 20mV s�1; (b) galvanostatic charge/discharge curves of the N-HG-600electrode at various current densities; (c) gravimetric capacitances ofthe N-HG-600 electrode at different current densities; (d) stabilityevaluation of the N-HG-600 electrode at a charge current of 10 A g�1

for 10 000 cycles, and gravimetric capacitances at different massloadings (inset, 1 A g�1).

9742 | J. Mater. Chem. A, 2016, 4, 9739–9743

600. The gravimetric capacitance of the N-HG-800 electrodecalculated from galvanostatic charge/discharge curves is 212F g�1 at 1 A g�1. Due to its low stacking density, the volumetriccapacitance of N-HG-800 is only 42.4 F cm�3.

A symmetric supercapacitor based on N-HG-600 electrodeswas also studied using a 1 M Na2SO4 aqueous solution (detailsin Fig. S12†). Fig. S12a† exhibits the CV curves of symmetricsupercapacitors operated in different potential windows at20 mV s�1. It can be found that the anodic current has noobvious increase even when the voltage window reaches 1.6 V,meaning that the electrolyte is stable in this system. Further-more, the CV curves of the symmetrical supercapacitor exhibitrectangular-like shapes even at the potential window up to 1.6 V,demonstrating an ideal capacitive behaviour. The gravimetriccapacitance and volumetric capacitance of the symmetricsupercapacitor are 39.6 F g�1 and 63 F cm�3 at 0.5 A g�1. Owingto its wide potential window (1.6 V), the symmetrical super-capacitor delivers a high energy density of 22.4 W h L�1, whichis superior to that of many reported symmetric super-capacitors.38–40 Furthermore, a maximum volumetric powerdensity of 5.6 kW L�1 can be obtained, highlighting its greatpotential in the eld of energy storage.

Conclusions

In summary, we have developed a novel, scalable and economic“Bottom-up” route to prepare N-HG for high-volumetric super-capacitors. The as-obtained N-HG, with amounts of in-planeholes, has a high SSA, a narrow hole size distribution and a highnitrogen content (9.92 at%). At the same time, the stackingdensity of N-HG through drying under normal conditions canbeen easily controlled by changing the experimental conditions.This work denes a novel but effective “Bottom-up” pathway toprepare N-HG and its derived high density carbon has a highvolumetric capacitance (397 F cm�3) for supercapacitors.

Acknowledgements

This work was nancially supported by the National NaturalScience Foundation of China (No. 51272155 and No. 61574091).We also thank the Instrumental Analysis Center of SJTU for theanalysis support.

Notes and references

1 J. R. Miller and P. Simon, Science, 2008, 321, 651–652.2 P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854.3 Y. Xu, Z. Lin, X. Zhong, X. Huang, N. O. Weiss, Y. Huang andX. Duan, Nat. Commun., 2014, 5, 4554.

4 R. Raccichini, A. Varzi, S. Passerini and B. Scrosati, Nat.Mater., 2015, 14, 271–279.

5 X. Yang, C. Cheng, Y. Wang, L. Qiu and D. Li, Science, 2013,341, 534–537.

6 C. Zhang, W. Lv, Y. Tao and Q.-H. Yang, Energy Environ. Sci.,2015, 8, 1390–1403.

This journal is © The Royal Society of Chemistry 2016

Communication Journal of Materials Chemistry A

Publ

ishe

d on

14

Apr

il 20

16. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

12/0

3/20

17 0

1:13

:08.

View Article Online

7 X. Han, M. R. Funk, F. Shen, Y.-C. Chen, Y. Li, C. J. Campbell,J. Dai, X. Yang, J.-W. Kim and Y. Liao, ACS Nano, 2014, 8,8255–8265.

8 Y. Lin, X. Han, C. J. Campbell, J. W. Kim, B. Zhao, W. Luo,J. Dai, L. Hu and J. W. Connell, Adv. Funct. Mater., 2015,25, 2920–2927.

9 V. Sahu, S. Grover, B. Tulachan, M. Sharma, G. Srivastava,M. Roy, M. Saxena, N. Sethy, K. Bhargava and D. Philip,Electrochim. Acta, 2015, 160, 244–253.

10 B. Li, F. Dai, Q. Xiao, L. Yang, J. Shen, C. Zhang and M. Cai,Energy Environ. Sci., 2016, 9, 102–106.

11 J. Hou, C. Cao, F. Idrees and X. Ma, ACS Nano, 2015, 9, 2556–2564.

12 T. Lin, I.-W. Chen, F. Liu, C. Yang, H. Bi, F. Xu and F. Huang,Science, 2015, 350, 1508–1513.

13 Z. Li, Z. Xu, H. Wang, J. Ding, B. Zahiri, C. M. Holt, X. Tanand D. Mitlin, Energy Environ. Sci., 2014, 7, 1708–1718.

14 J. Zhou, J. Lian, L. Hou, J. Zhang, H. Gou, M. Xia, Y. Zhao,T. A. Strobel, L. Tao and F. Gao, Nat. Commun., 2015, 6, 8503.

15 M. Yu, J. Zhang, S. Li, Y. Meng and J. Liu, J. Power Sources,2016, 308, 44–51.

16 X. Wang, L. Lv, Z. Cheng, J. Gao, L. Dong, C. Hu and L. Qu,Adv. Energy Mater., 2016, 6, 6.

17 J. Sun, L. Wang, R. Song and S. Yang, RSC Adv., 2015, 5,91114–91119.

18 L. L. Zhang, X. Zhao, M. D. Stoller, Y. Zhu, H. Ji, S. Murali,Y. Wu, S. Perales, B. Clevenger and R. S. Ruoff, Nano Lett.,2012, 12, 1806–1812.

19 Y. Xu, C.-Y. Chen, Z. Zhao, Z. Lin, C. Lee, X. Xu, C. Wang,Y. Huang, M. I. Shakir and X. Duan, Nano Lett., 2015, 15,4605–4610.

20 D. Yu, L. Wei, W. Jiang, H. Wang, B. Sun, Q. Zhang, K. Goh,R. Si and Y. Chen, Nanoscale, 2013, 5, 3457–3464.

21 D. Liu, W. Lei, D. Portehault, S. Qin and Y. Chen, J. Mater.Chem. A, 2015, 3, 1682–1687.

This journal is © The Royal Society of Chemistry 2016

22 B. Małecka, R. Gajerski, A. Małecki, M. Wierzbicka andP. Olszewski, Thermochim. Acta, 2003, 404, 125–132.

23 X. Y. Chen, C. Chen, Z. J. Zhang and D. H. Xie, J. Mater. Chem.A, 2013, 1, 10903–10911.

24 J. C. Fanning, Coord. Chem. Rev., 2000, 199, 159–179.25 X. Wang, Y. Zhang, C. Zhi, X. Wang, D. Tang, Y. Xu, Q. Weng,

X. Jiang, M. Mitome and D. Golberg, Nat. Commun., 2013, 4,2905.

26 C. Renschler and A. Sylwester, Appl. Phys. Lett., 1987, 50,1420–1422.

27 J. Bailey and A. Clarke, Nature, 1971, 234, 529–531.28 W. Watt and W. Johnson, Nature, 1975, 257, 210–212.29 J. H. Lee, N. Park, B. G. Kim, D. S. Jung, K. Im, J. Hur and

J. W. Choi, ACS Nano, 2013, 7, 9366–9374.30 M. Acik, C. Mattevi, C. Gong, G. Lee, K. Cho, M. Chhowalla

and Y. J. Chabal, ACS Nano, 2010, 4, 5861–5868.31 D. R. Dreyer, S. Park, C. W. Bielawski and R. S. Ruoff, Chem.

Soc. Rev., 2010, 39, 228–240.32 Y. Yoon, K. Lee, C. Baik, H. Yoo, M. Min, Y. Park, S. M. Lee

and H. Lee, Adv. Mater., 2013, 25, 4437–4444.33 J. Aladekomo and R. Bragg, Carbon, 1990, 28, 897–906.34 A. Thess, R. Lee, P. Nikolaev and H. Dai, Science, 1996, 273,

483.35 L. Malard, M. Pimenta, G. Dresselhaus and M. Dresselhaus,

Phys. Rep., 2009, 473, 51–87.36 L. Tao, Q. Wang, S. Dou, Z. Ma, J. Huo, S. Wang and L. Dai,

Chem. Commun., 2016, 52, 2764–2767.37 Z. Zuo, T. Y. Kim, I. Kholmanov, H. Li, H. Chou and Y. Li,

Small, 2015, 11, 4922–4930.38 C. Long, X. Chen, L. Jiang, L. Zhi and Z. Fan, Nano Energy,

2015, 12, 141–151.39 Q. Wang, J. Yan, Y. Wang, T. Wei, M. Zhang, X. Jing and

Z. Fan, Carbon, 2014, 67, 119–127.40 J. Yan, Q.Wang, C. Lin, T. Wei and Z. Fan, Adv. Energy Mater.,

2014, 4(13), 1400500.

J. Mater. Chem. A, 2016, 4, 9739–9743 | 9743