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One-pot liquid-phase exfoliation from graphite to graphene with carbon quantum dots Cite this: DOI: 10.1039/x0xx00000x

Minghan Xu1, Wei Zhang1, Fan Yu3, Yujie Ma1, Nantao Hu1, Yanjie Su1, Dannong He2, Qi Liang3, Zhi Yang1,2,* and Yafei Zhang1* Carbon quantum dots (CQDs) are novel carbon nanomaterials with increasing interest due to their good characteristics, such as hydrophilicity, chemical stability, quantum yield, small particle sizes, and low

Received 00th December 2014, Accepted 00th December 2014

cytotoxicity. Herein, we used CQDs as stabilizers and exfoliation agents to exfoliate from graphite to

DOI: 10.1039/x0xx00000x

water matching with graphite and taking weak interaction (π-π conjugation, hydrophobic force, and the

www.rsc.org/

Coulomb attraction) with graphite surface. Different characterization methods were used to evaluate the

graphene in aqueous medium for the first time. The functions of CQDs are reducing surface tension of

presence of few-layer (< 5 layers) of graphene sheets with less defect and low oxidation. In the future, CQDs can also be good candidates to exfoliate other two-dimensional materials, such as WS2, BN, MoS2, and g-C3N4 to form two-dimensional heterostructures for a range of possible applications. Carbon “nanobubbles” with a diameter of 25 ~ 90 nm were used

1 Introduction

as stabilizers to disperse carbon nanotubes relying on the π-π Liquid-phase exfoliation of two-dimensional

(2D) materials

(graphene, MoS2, BN, g-C3N4, etc.) has drawn great attention due to their lack of oxidation, free-defect, easy preparation, and large-scale manufacture.1 Graphene, a widely used 2D material, is a single layer of carbon atoms like honeycomb structure and also the elementary unit of zero-dimensional (0D) fullerenes, one-dimensional (1D) nanotubes, and three-dimensional (3D) graphite.2 Graphene has been applied to field-effect devices, gas sensors, transparent electrodes, 3

interaction. The nanocomposites could be used to form conductive membranes based on cellulose filter membranes.6 Despite some researches relating to the use of bench chemistry to exfoliate from graphite to graphene,7 the liquid-phase exfoliation employing noncovalent π-π interactions with CQDs is still unexplored. CQDs due to full of hydrophilic groups are highly dissolved in water. Water opens perspective on the formation of biocompatible

and so on. Liquid-phase exfoliation from graphite to graphene

graphene nanomaterials for bio-related applications due to its non-

mainly uses surfactant molecules, polymers, proteins, and aromatic

toxicity. Nevertheless, liquid-phase exfoliation of graphite in

organic compounds as exfoliation agents in organic solvents or

aqueous medium is fairly difficult by reason of the hydrophobic

aqueous medium based on reducing surface energy matching with

property of the graphite sheets.8 Herein, we used CQDs as stabilizers

graphite or taking weak interaction (π-π conjugation, hydrophobic

and exfoliation agents to exfoliate from graphite to graphene in

force, and the Coulomb attraction) with graphite surface. 4

water for the first time. The functions of CQDs are reducing surface tension of water matching with graphite and taking weak interaction

Carbon quantum dots (CQDs) are novel carbon nanomaterials with increasing interest due to their good characteristics, such as hydrophilicity, chemical stability, quantum yield, small particle sizes and low cytotoxicity. They are competent and can be used in all sorts of applications including cell imaging, photocatalysis, chemical sensing, light-emitting diode (LED), solar cells and storage devices.5

This journal is © The Royal Society of Chemistry 2013

(π-π conjugation, hydrophobic force, and the Coulomb attraction) with graphite surface.9 We synthesized large-scale mass of CQDs under microwave irradiation with citrate acid and urea, followed by exfoliating from graphite to graphene in aqueous medium with CQDs (Fig. 1). The relation equation about liquid-phase exfoliation

J. Name., 2013, 00, 1-3 | 1

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can be mathematically expressed as: 3  0  2 . Namely, graphene

exfoliated graphene with less defect and low oxidation. In the future,

(2D material) can be produced by mixing graphite (3D material)

CQDs can also be good candidates to exfoliate other 2D materials,

with CQDs (0D material) under ultrasonic condition. Different

such as WS2, BN, MoS2, and g-C3N4 to form 2D heterostructures for

characterization methods were used to evaluate the quality of

a range of possible applications.10

Fig. 1 Schematic diagram on the transformation of exfoliating graphite (3D material) with the aid of CQDs (0D material) into graphene (2D material).

2 Experimental

2.3 Exfoliation from graphite to graphene

2.1 Chemicals

CQDs (24.97 mg) and graphite (24.13 mg) were added into 50 mL DI water, then tip-sonicated for 10 ~ 60 min (800 W) in ice-water

Citrate acid was purchased from Sigma-aldrich@, and urea was

bath. The obtained dispersion was standing to keep stable for

purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai,

overnight to allow any unstable un-exfoliated graphite to

China). High purity natural graphite (99.99%, 500 mesh) was

precipitation and then centrifuged for 0.5 h at 500 rpm (×8g). The

obtained from Qingdao Tianyuan Graphite Co., Ltd (Shandong,

supernatant of the dispersion was drawn by pipette and centrifuged

China). Deionized (DI) water with a resistivity of 18.1 MΩ cm was

for 10 min at 11000 rpm (×8410g) to obtain the sediment after

used for all experiments.

centrifugation. The sediment was redispersed and centrifuged for another twice. Finally, the sediment was exfoliated graphene for

2.2 Large scale preparation of CQDs The modified synthesized method was based on previous report.

further characterization and usage. 1

Typically, citric acid (10 g, 52.0 mmol) and urea (10 g, 166.5 mmol)

2.4 Film formation

were added into 35 mL deionized water and stirred to be dissolved

The dispersed graphene solution was obtained after tip sonication of

into a transparent solution. The solution was irradiated in a

CQDs and graphite solution and then centrifugation at 500 rpm (×8g)

commercial microwave oven (800 W) for 5 min, then the solution

for 0.5 h. The exfoliated graphene film was fabricated by the

was converted to black solid, demonstrating the production of CQDs.

vacuum filtration of dispersed graphene employing cellulose

The fore-mentioned solid was dissolved in water. The obtained

filtration membranes with 0.22 μm pores and washed with amounts

solution was filtered by the cellulose filtration membrane with 0.22

of DI water. Finally, the film was fully dried in a vacuum drying

μm pores to remove large and agglomerated particles, then dialyzed

chamber at room temperature overnight.

with 1000 Da dialysis bag for 3 days to remove unreacted molecules. Finally, the resulting black colored aqueous solution was evaporated

2.5 Characterization

into solid powders for further characterization and usage.

The morphologies of the samples were observed by transmission electron microscope (TEM, JEM-2100, JEOL, Japan) at 200 kV,

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atomic force microscopy (AFM, Multimode Nanoscope, DI,

absorption bands at 1704 and 1626 cm -1 are assigned to ν(C=O)

USA), and field emission scanning electron microscope (FE-

and δ(C=C), respectively. Hydroxyl and carboxyl groups improve

SEM, Carl Zeiss Ultra Plus, Germany) at 5 kV. Samples for FE-

the solubility and stability of CQDs in aqueous medium.

SEM and TEM were prepared by dropping a few microliters of

Ultraviolet-visible (UV-vis) spectrum (Fig. S2) of CQDs has

sample dispersion onto silicon wafer and holey carbon grids (and

absorption peaks at 233, 337, and 392 nm, which represents an

micro grid copper network), respectively. Sample for AFM was

aromatic π system of CQDs. CQDs exhibit an excitation-

prepared by pipetting a few microliters of dispersion from

wavelength-dependent photoluminescence property (EWDPP)

graphene film after sonication. The photoluminescence (PL)

with emission peaks ranging from 450 to 570 nm at excitation

spectra were performed through a fluorescent spectrophotometer

wavelengths from 320 to 500 nm (Fig. 2c). This is the most

(F-4600, Hitachi, Japan). The ultraviolet-visible (UV-vis)

characteristic phenomenon of CQDs. Raman spectrum of CQDs

absorption spectrum was recorded by a Perkin-Elmer (USA)

is shown in Fig. 2d. The relative intensity ratio of sp3 D band to

Lambda 950 UV-vis-NIR spectrophotometer. Fourier transform

sp2 G band (ID/IG) is around 0.96, which indicates that they have

infrared (FT-IR) spectrum was recorded on a VERTEX 70,

an analogous structure to graphite. None of 2D band at 2700 ~

Bruker, Germany spectrometer. Sample for FT-IR measurement

2900 cm-1 indicates that CQDs are spherical structures, rather

was prepared by dissolving graphene film after sonication.

than layer structures. XRD pattern (Fig. S3) of CQDs displays a

Raman spectroscopy was characterized on graphene film using

peak situated at 3.4 Å corresponding to highly ordered carbon

Bruker Senterra dispersive Raman microscopy with laser

atoms. CQDs solution shows green fluorescence under UV

wavelength at 633 nm. X-ray diffraction (XRD, Bruker AXS

irradiation of 365 nm and has a Tyndall effect under a green laser

Corporation, Germany) analysis was carried out on graphene film

bean irradiation of 532 nm (Fig. S4).11 Above all, the synthesized

using an Advance D8. X-ray photoelectron spectrum (XPS) was

CQDs have green fluorescence under UV irradiation of 365 nm,

acquired on graphene film using a Japan Kratos Axis UltraDLD

hydrophilicity due to the surface of CQDs which is full of hydroxyl

spectrometer with a monochromatic Al Kα source (1486.6 eV).

and carboxyl groups, and well crystalline structure which is

Sheet resistance was obtained by digital four-point probe tester

conformed by Raman and XRD characterizations.

(RTS-8, Probes Tech., China) with a separation distance of 1 mm 3.2 Liquid-phase exfoliation from graphite to graphene

between probe tips.

Much work has reported that noncovalent interaction of graphene

3 Results and discussion

with hydrophilic carboxylic acid compounds promotes the aqueous dispersions of graphene sheets.12 Carboxylic acid

3.1 The properties of CQDs

compounds positioned at the surface of graphene could stabilize The morphology of CQDs was observed by transmission electron

aqueous dispersions of the graphene sheets. CQDs are full of

microscope (TEM, Fig. 2a). TEM image and particle size

carboxylic acid groups at the surfaces of their spherical structures.

distribution (Fig. S1 of the Supporting Information) are showed

Ultrasonication

that CQDs are well dispersed without aggregation, and

nanomaterials from their bulk state due to the effect of acoustic

approximately 1.76 nm of particle size. Fourier transform

cavitation of high frequency ultrasound. 13 Therefore, sonication

infrared spectrum (FT-IR, Fig. 2b) indicates the surface

of graphite in CQDs-water mixture expedites the exfoliation of

functional groups of CQDs. Absorption band at 3438 cm-1 is

graphite. It can be explained that the formation process of

-1

is

an

effective

method

for

exfoliating

assigned to ν(O-H). Absorption bands at 2927 and 1388 cm are

microbubbles in aqueous medium induces knock waves on the

assigned to ν(C-H) and δ(CH2), respectively. Meanwhile,

surface of graphite.14

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Fig. 2 The characterizations of CQDs: (a) TEM image, (b) FT-IR spectrum, (c) photoluminescence (PL) spectra with different excitation wavelengths from 320 to 500 nm , and (d) Raman spectrum.

Photoluminescence (PL) measurements (Fig. 3a) show PL

quantum dots.17 The EWDPP phenomenon may come from the

quenching with increasing sonication time. This is the fingerprint

releasing CQDs under dispersing graphene in sonication

of weak interaction (π-π conjugation, hydrophobic force, and the

condition. Hence, the interaction force between CQDs and

15

exfoliated graphene is non-covalence. The exfoliated graphene

The obvious fluorescence quenching property of CQDs with

shows a absorption peak at 266 nm (Fig. 3c), which is equivalent

graphene manifests that the exfoliated graphene is used as an

with the prevenient values for graphene. 18 The peak position

electron acceptor. Therefore, the fluorescence quenching of

demonstrates that there is no marked change in oxidation that is

CQDs comes about by energy transferring from CQDs to

usually found in graphene oxide solution with a shifted 230 nm

graphene. The function of charge transfer is important for

absorption peak.19 The concentration of exfoliated graphene is

designing optoelectronic nanomaterials.16 After dispersion and

0.4 mg/mL based on calculating cellulose membrane mass

centrifugation for more than three times, the exfoliated graphene

difference between pre-treatment and post-treatment of filtration

solution is detected with EWDPP (Fig. 3b). It is known that

exfoliated graphene with a certain volume, This value is much

Coulomb attraction) between CQDs and exfoliated graphene.

CQDs have EWDPP based on previous reports

17

and above

higher than those (0.1 mg/mL) obtained with aromatic and

discussion (Fig. 2c). It can be distinguished them from other

nonaromatic surfactants.7c,12a-b,15c It should be noted that CQDs

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are of 1.76 nm sizes, which allows them to penetrate through

graphene is 0.55 shown in Fig. 3d, an increase compared to that

cellulose filter membranes. Inset of Fig. 3c shows that the

of graphite (I2D/IG = 0.35), further indicating that CQDs

exfoliated graphene solution after centrifugation can be stable for

successfully

from 21

graphite

to

graphene

under

several months with only a very small amount of precipitation,

ultrasonication condition.

while pristine graphite is unstable in water. Raman spectrum of

about 0.53 which shows fewer defects than those of graphene

-1

sheets that were from edge defects instead of plane defects. 22 The

exfoliated graphene displays a prominent G band at 1578 cm , 2

corresponding to ordered sp hybridization, and a D band at 1328 -1

3

cm , corresponding to disordered sp hybridization, and a 2D

The ratio of D to G band intensity is

2D line can be fitting as a single Lorentzian peak indicating that the exfoliated sheets with random stacking.23

band at 2674 cm-1.20 The ratio of 2D to G band intensity of

Fig. 3 (a) PL intensity of the exfoliated graphene solution decreases with the increasing sonication time; (b) PL spectra with different excitation wavelengths from 270 to 600 nm; (c) UV-vis spectrum of exfoliated graphene (Inset: photo of pristine graphite (left) and exfoliated graphene (right) in water); and (d) Raman spectra of pristine graphite and exfoliated graphene.

Field emission scanning electron microscopy (FE-SEM) image

flakes. The control experiment with only graphite and water

shows that the pristine graphite powders consist of irregular

under sonication condition was also carried out. From FE-SEM

flakes with a wide range of lateral sizes of several micrometers

images in Fig. S5, the irregular, thick, and bulk graphite is

(Fig. 4a). In contrast, the dispersed portion after sonication

obtained. The topography profile of atomic force microscopy

contains more uniform and smooth sheets, which are much

(AFM) is revealed in Fig. 4d, corresponding to the line in Fig. 4c,

smaller with lateral sizes measured in only several hundreds of

which is demonstrated that the thicknesses of the two sheets are

nanometers (Fig. 4b). Clearly, the sonication breaks up the bulk

approximately 0.63 nm and 0.65 nm corresponding to single

graphite crystallites and results in fragmentation of the initial

layer graphenes. The real thickness of the sheets can be even

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lower due to the environmental conditions such as relative

selected area electron diffraction (SAED) image. The SAED

humidity in AFM measurement. TEM analysis is performed for

pattern was recorded from circle-marked regions of the graphene

further characterization of dispersed graphene. Fig. 4e shows

sheet, in which spots at (0-110) and (-1010) are stronger in

typical single-layer graphene sheets, which can further affect D

intensity than spots at (1-210) and (-2100), indicating single layer

peak intensity of Raman spectrum and the thickness characterized

graphene sheet (most of the graphene flasks are < 5 layers).25

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by AFM measurement.24 Fig. 4f shows the corresponding

Fig. 4 FE-SEM images of (a) pristine graphite (inset: a random pristine graphite) and (b) the exfoliated graphene film; (c) AFM image and (d) the corresponding height profiles of the exfoliated graphenes; (e) TEM image and (f) the corresponding SAED pattern of the exfoliated graphene.

6 | J. Name., 2012, 00, 1-3

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FT-IR spectra of pristine graphite and exfoliated graphene are

The two lines are corresponding to C=O and C-O groups,

exhibited in Fig. S6. It is obvious that the peaks of pristine

respectively. It indicates that very low level of oxidation has

graphite are almost the same as exfoliated graphene. The 3430

occurred to the graphite during the exfoliation/dispersion

-1

cm absorption band is corresponding to ν(O-H) of the adsorbed

process.26 After sonication, the exfoliated graphene becomes

water. Absorption bands at 2924, 2857 and 1634 cm -1 are

thinner and the lateral sizes of the sheets become smaller. As a

corresponding to ν(CH3), ν(CH2) and δ(C=C), respectively. These

consequence, the full width at half maximum of the (002) peak

three bands are inherited from pristine graphite.

25a

A C1s

increases from ~ 0.3ºto ~ 0.6ºas a result of Scherrer broadening

spectrum of XPS measured on an exfoliated graphene film was

which was measured by XRD measurement (Fig. S8).27 The

illustrated in Fig. S7. This peak is predominated by the feature

mechanism of exfoliation is that CQDs is functionalized at the

about 284.8 eV belonging to graphite carbon. Fitting curves show

surface of the graphite due to the nanometer size as gum arabic

that two low splitting lines at 286.8 and 285.6 eV are obtained.

and HFBI protein.28

To test the quality of exfoliated graphene in the electronic applications,

the

electrical

characterizations

of

exfoliated

graphene film are studied. Common cellulose filter membranes are electrically non-conducting materials, but exfoliated graphene film could be used as an effective conductive material, as shown by connecting electrical circuit of an LED lamp (Fig. 5). The exfoliated graphene film was prepared from filtering exfoliated graphene aqueous solution. In Fig. S9, it can be seen that the graphene film with approximately 193 nm surface roughness (Table S1) is stacked with edge to edge, only a small part overlaps. In addition, the thickness of the exfoliated graphene film can be regulated by controlling the volume ratio of the CQDs/graphene solution or the centrifugation speed. The graphene film has an electrical resistance of ~ 470 Ω (~ 1 cm distance) higher than that of pristine graphite (1.53 Ω), which is measured by a hand-held multimeter,29 but observably lower than that of graphene oxide (> 2106 Ω). The sheet resistance of exfoliated graphene film (Fig. 10) is 442 Ω/ □ by four-point

Fig. 5 Photographs of (a) pristine cellulose filter membranes, and (b)

probe conductivity measurements with calculating eight different

exfoliated graphene film measured by electrical conductivity test.

30

places, which is lower than previous reports (Table S2).

The

corresponding electrical conductivity of exfoliated graphene film is 2.3×10-2 S/cm, which is substantially higher than GO film. 25b The results show the exfoliated graphene will be useful for transparent and flexible electrodes. 31

This journal is © The Royal Society of Chemistry 2013

4 Conclusions In summary, we have demonstrated a one-pot and green method for the preparation of graphene by liquid-phase exfoliation from pristine graphite with CQDs in aqueous medium.

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The microscopic (FE-SEM, AFM, TEM, and SAED) and

1 a) J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U.

spectroscopic (Raman and XRD) measurements show the

Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K.

presence of few-layer (< 5 layers) of graphene sheets with less

Arora, G. Stanton, H. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam,

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the carboxyl groups of CQDs and weak interaction (π-π

J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist, and V. Nicolosi, Science, 2011, 331, 568;

conjugation, hydrophobic force, and the Coulomb attraction) with

b) V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano, and J.

graphite surface. In the future, CQDs can also be good candidates

N.Coleman, Science, 2013, 340, 1226419; c) K. P. Paton, E. Varrla, C.

to exfoliate other two-dimensional materials to form two-

Backes, R. J. Smith, U. Khan, A. O’Neill, C. Boland, M. Lotya, O. M.

dimensional heterostructures. The exfoliated graphene may be

Istrate, P. King, T. Higgins, S. Barwich, P. May, P. Puczkarski, I. Ahmed,

useful for practical applications of electrode material and

M. Moebius, H. Pettersson, E. Long, J. Coelho, S. E. O’Brien, E. K.

optoelectronic devices.

McGuire, B. M. Sanchez, G. S. Duesberg, N. McEvoy, T. J. Pennycook, C. Downing, A. Crossley, V. Nicolosi, and J. N. Coleman, Nat. Mater., 2014, 13, 624; d) K. G. Zhou, M. Zhao, M. J. Chang, Q. Wang, X. Z. Wu,

Acknowledgements

Y. L. Song, and H. L. Zhang, Small, 2014, DOI:10.1002/smll.201400541.

The authors gratefully acknowledge financial supports by the

2 A. K. Geim, and K. S. Novoselov, Nat. Mater., 2007, 6, 183.

National Basic Research Program of China (2013CB932500), National High-Tech R & D Program of China (863 program, 2011AA050504), National Natural Science Foundation of China (51402190 and 21171117), Program for New Century Excellent Talents in University (NCET-12-0356), Shanghai Natural Science Foundation (13ZR1456600 and 15XD1525200), and the

3 a) M. J. Allen, V. C. Tung, and R. B.Kaner, Chem. Rev., 2010, 110, 132; b) E. Singh, Z. P. Chen, F. Houshmand, W. C. Ren, Y. Peles, H. M. Cheng, and N. Koratkar, Small, 2013, 9, 75; c) S. X. Wu, Q. Y. He, C. L. Tan, Y. D. Wang, and H. Zhang, Small, 2013, 9, 1160; d) J. X. Zhu, D. Yang, Z. Y. Yin, Q. Y. Yan, and H. Zhang, Small, 2014, 10, 3480; e) W. G. Xu, N. N. Mao, and J. Zhang, Small, 2013, 9, 1206.

Program for Professor of Special Appointment (Eastern Scholar) at

Shanghai

Institutions

of

Higher

Learning.

We

also

acknowledge the analysis support from Instrumental Analysis

4 W. C. Du, X. Q. Jiang, and L. H. Zhu, J. Mater. Chem. A, 2013, 1, 10592.

Center of Shanghai Jiao Tong University and the Center for

5 a) S. N. Baker, and G. A. Baker, Angew. Chem. Int. Ed., 2010, 49, 6726;

Advanced Electronic Materials and Devices of Shanghai Jiao

b) M. J. Krysmann, A. Kelarakis, P. Dallas, and E. P. Giannelis, J. Am.

Tong University.

Chem. Soc., 2012, 134, 747; c) Z. Yang, M. H. Xu, Y. Liu, F. J. He, F. Gao, Y. J. Su, H. Wei, and Y. F. Zhang, Nanoscale, 2014, 6, 1890; d) M.

Notes and references

H. Xu, G. L. He, Z. H. Li, F. J. He, F. Gao, Y. J. Su, L. Y. Zhang, Z. Yang, and Y. F. Zhang, Nanoscale, 2014, 6, 10307.

1 Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. E-mail: [email protected]; [email protected]; Tel.:+86-21-34206398; Fax: +86-21-34205665

6 D. Kuzmicz, S. Prescher, F. Polzer, S. Soll, C. Seitz, M. Antonierri, and J. Yuan, Angew. Chem. Int. Ed., 2014, 53, 1062. 7 a) J. H. Lee, D. W. Shin, V. G. Makotchenko, A. S. Nazarov, V. E.

2 National Engineering Research Center for Nanotechnology, Shanghai 200241, P. R. China

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3 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China.

and D. M. Guldi, ACS Nano, 2012, 6, 1915; c) S. Sampath, A. N. Basuray, K. J. Hartlieb, T. Aytun, S. I. Stupp, and J. F. Stoddart, Adv. Mater., 2013,

Electronic Supplementary Information (ESI) available: Particle size distribution, UV-vis spectrum, and XRD pattern of CQDs; photoluminescent and Tyndall effect photos of CQDs; FE-SEM images of controlled exfoliated experiment; FT-IR spectra, C1s core level XPS spectrum, XRD pattern, AFM images, and photos of exfoliated graphene. See DOI: 10.1039/b000000x/

25, 2740. 8 A. Bianco, Angew. Chem. Int. Ed., 2013, 52, 4986. 9 J. Wang, C. F. Wang, and S. Chen, Angew. Chem. Int. Ed., 2012, 51, 9297.

8 | J. Name., 2012, 00, 1-3

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defect and low oxidation. The exfoliation mechanism is due to

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