C A R B O N 4 9 ( 2 0 1 1 ) 3 9 2 0 – 3 9 2 7

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Solvothermal synthesis of homogeneous graphene dispersionwith high concentration

Ding Zhou, Qian-Yi Cheng, Bao-Hang Han *

Laboratory for Nanodevices and CAS Key Laboratory for Standardization and Measurement for Nanotechnology,

National Center for Nanoscience and Technology, Beijing 100190, China

State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Shanghai 200050, China

A R T I C L E I N F O

Article history:

Received 24 January 2011

Accepted 15 May 2011

Available online 23 May 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.05.030

* Corresponding author: Fax: +86 10 82545576E-mail address: hanbh@nanoctr.cn (B.-H.

A B S T R A C T

We developed a simple surfactant-free approach to graphene dispersion through a solvo-

thermal reduction of graphene oxides in N,N-dimethylformamide, and the concentration

of the as-prepared graphene dispersion can reach up to 0.3 mg mL�1. The as-prepared

graphene could be re-dispersed well in more than six kinds of solvents, such as N-methyl-

pyrrolidone, N,N-dimethylacetamide, and acetonitrile. In the solvothermal reduction pro-

cess, no extra reducing agent and/or stabilizer is needed, and the deoxygenation is

promoted by the relatively high temperature and autogenous pressure in the autoclave.

Atomic force microscopy analysis shows most of the graphene sheets in the as-prepared

dispersion are single-layered. X-ray photoelectron spectroscopy, thermal gravimetric anal-

ysis, and infrared spectroscopy confirm the efficient removal of the oxygen-containing

groups.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The synthesis of graphene has become one of the key issues

for the fabrication of graphene-based devices and materials

[1–3]. In general, methods for producing graphene sheets

can be classified into five main classes: (a) chemical vapor

deposition of graphene layers [4,5], (b) micromechanical exfo-

liation of graphite using peel-off method with Scotch-tape [6],

(c) epitaxial growth of graphene films [7], (d) bottom-up

synthesis of graphene from organic molecules [8,9], and (e)

reduction/deoxygenation of graphene oxide (GO) sheets [10–

12]. Among these methods, methods a, b, and c yield graphene

sheets with good quality [13], thus are more suitable for fabri-

cation of graphene-based electronic devices. Chemical ap-

proaches d and e are two kinds of more realistic approaches

to produce graphene in gram-scale, thus are practical in

material views [10], and reduction of GOs is the most common

method to obtain graphene with high-volume production,

er Ltd. All rights reserved

.Han).

which can be achieved through chemical [14,15], thermal

[16,17], or electrochemical reduction pathways [18].

A wide variety of reducing agents, such as hydrazine

[14,15], sodium borohydride [19], hydroquinone [20], aqueous

alkaline solution [21,22], and L-ascorbic acid [23], are widely

used for chemical reduction of GO. Besides chemical reduc-

tion, the oxygen-containing groups on the GO sheets can also

be removed with thermal treatment [16]. Thermal expansion

strips the oxide functionality through the extrusion of carbon

oxide and water molecules by heating GO in inert gases to

1050 �C, which can be lower to about 200 �C with the assistant

of vacuum [17].

Hydrothermal or solvothermal method is also a thermal

method to obtain graphene sheets. GO dispersions in differ-

ent solvents were subjected to solvothermal reduction by

Rajamathi et al., after the removal of oxygen-containing

groups of GO, the graphene sheets suffer from very limited

dispersibility and even irreversible agglomeration due to the

.

C A R B O N 4 9 ( 2 0 1 1 ) 3 9 2 0 – 3 9 2 7 3921

increase in the hydrophobicity and the p–p staking interaction

between graphene sheets [24–26]. To avoid the aggregation of

graphene sheets, covalent or noncovalent modification is

needed by introducing extra stabilizers into the reducing sys-

tems [10–12]. Solvothermal reduction of GO and less defective

GO sheets were also carried out by Dai and coworkers in N,N-

dimethylformamide (DMF) at 180 �C using hydrazine mono-

hydrate as the reducing agent, and the dispersion remained

homogeneous, and the formed graphene sheets were well-

dispersed in DMF [27]. In view of the fact that hydrazine is

toxic and dangerously unstable and the extra reductants

and stabilizers may set limitations to the further functionali-

zation and application, new strategies to produce relatively

clean graphene sheets in bulk quantity while keeping them

individually separated are required.

We demonstrate herein a simple method to obtain a stable

homogeneous graphene dispersion in DMF by solvothermal

treatment of GO dispersion in DMF without addition of any

extra reducing agent and stabilizer, and the concentration

of the dispersion could reach up to 0.3 mg mL�1. Furthermore,

the solvothermal approach produces graphene dispersion at a

high concentration in a large scale, and the solvothermal

graphene (STG) could re-disperse in several kinds of solvents,

such as N-methyl-pyrrolidone (NMP), dimethyl sulfoxide

(DMSO), N,N-dimethylacetamide (DMAC), and acetonitrile,

making it possible to be applied in various potential fields.

2. Experimental

2.1. Materials

Natural flake graphite with an average particle diameter of

20 lm (99 wt.% purity) was obtained from Yingshida graphite

Co. Ltd., Qingdao, China. Sulfuric acid (98 wt.%), hydrogen

peroxide (30 wt.%), sodium nitrate, DMF, methanol, ethanol,

tetrahydrofuran (THF), NMP, DMSO, DMAC, hexane, diethyl-

amide (DEA), toluene, diethyl ether, triethylamide (TEA),

dichloromethane (DCM), chloroform, ethyl acetate, pyridine,

acetone, 2-isopropanol, acetonitrile, and ethylene glycol were

purchased from Beijing chemical works, China. All these re-

agents were of reagent grade and used without further purifi-

cation. Ultra-pure water (18.2 MX cm) was obtained by the

Millipore–ELIX water purification system.

2.2. Preparation of aqueous GO dispersion and GOdispersions in organic solvents

Aqueous GO dispersion was prepared by chemical exfoliation

of the natural flake graphite by a modified Hummers’ method

[12,28]. GO dispersions in various organic solvents, such as

DMF, methanol, ethanol, THF, DMSO, DMAC, and ethylene

glycol, were obtained by a solvent exchange method from

the as-exfoliated aqueous GO dispersion [29,30]. Aqueous

GO dispersion was subjected to the centrifugation tube and

centrifuged at 11,000 rpm for 30 min. The supernatant liquid

was removed and the GOs were re-dispersed in an organic

solvent. The process was repeated for more than five times,

and finally the GO dispersion was diluted to 0.5 mg mL�1,

and there were trace water in the dispersion.

2.3. Solvothermal reduction of GO dispersion

After mild sonication (80 W) for 30 min, GO dispersion in DMF

(0.5 mg mL�1, 20 mL) was transferred into a Teflon-lined

stainless-steel autoclave (50 mL) and was heated to a set tem-

perature (120, 160, 180, or 200 �C) and maintained for 12 h.

After cooled to room temperature, a black colloid dispersion

was obtained and the dispersion was centrifuged at

5000 rpm for 5 min to remove any large particles.

2.4. Control experiments

2.4.1. Solvothermal reduction of GO dispersion in otherorganic solventsGO dispersion in DMAC, methanol, ethanol, THF, DMSO, or

ethylene glycol (0.5 mg mL�1, 20 mL) was transferred into a

Teflon-lined stainless-steel autoclave (50 mL) after mild

sonication (80 W) for 30 min, and was heated to 180 �C and

maintained for 12 h. The black products were defined as

STG–DMAC, STG–methanol, STG–ethanol, STG–THF, STG–

DMSO, and STG–EG, respectively.

2.4.2. Solvothermal reduction of GO dispersion in DMSO withthe assistance of DEADEA (2 mL) was added into GO dispersion in DMSO

(0.5 mg mL�1, 18 mL). After mild sonication (80 W) for

30 min, the mixture was transferred into a Teflon-lined stain-

less-steel autoclave (50 mL), and was heated to 180 �C and

maintained for 12 h. The black dispersion was defined as

STG–DMSO–DEA.

2.4.3. Reduction of GO dispersion in DMF in an open systemGO dispersion in DMF (0.5 mg mL�1, 20 mL) was loaded into a

round-bottomed flask after sonicated for 30 min (80 W). After

heated under reflux at 140 �C for about 3 h, the dispersion

turned from yellow brown into deep black and finally aggre-

gated together.

2.5. Dispersion test of STG in various solvents

Ultra-pure water (with the same volume as the STG disper-

sion) was added into the STG dispersion in DMF and the black

product was collected by filtration and dried in vacuum at

60 �C overnight. Solid STG was re-dispersed in 21 solvents

(hexane, DEA, toluene, diethyl ether, TEA, DCE, chloroform,

ethyl acetate, THF, pyridine, acetone, NMP, DMAC, 2-isopropa-

nol, acetonitrile, DMF, DMSO, ethanol, ethylene glycol, ultra-

pure water, and methanol) and sonicated for 1 h (160 W).

After kept quiescent for a night, the colors of the dispersions

were recorded.

2.6. Instrumental characterization

Transmission electron microscopy (TEM) observations were

carried out using a Tecnai G2 20 S-TWIN microscope (FEI,

USA) at an accelerating voltage of 200 kV. STG dispersion in

DMF was dropped on a copper grid and dried in vacuum at

60 �C overnight. Atomic force microscopy (AFM) images

were taken in tapping mode with a Dimension 3100 atomic

force microscope and Nanoscope IVa NS4a controller

Fig. 1 – Photographs of (a) GO in DMF (yellow brown,

0.5 mg mL�1), (b) STG-180 in DMF (deep black, 0.3 mg mL�1),

and (c) STG-180 in DMF (pale black, 0.01 mg mL�1). (For

interpretation of the references to color in this figure legend,

the reader is referred to the web version of this paper.)

3922 C A R B O N 4 9 ( 2 0 1 1 ) 3 9 2 0 – 3 9 2 7

(Veeco Instruments Inc., USA). AFM samples were prepared by

dropping GO or STG dispersion (0.001 mg mL�1 in DMF) on a

silicon wafer, and then dried in vacuum at 100 �C for 4 h. X-

ray photoelectron spectroscopy (XPS) data were obtained with

an ESCALab220i-XL electron spectrometer (VG Scientific Ltd.,

UK) using 300 W Al Ka radiation. The base pressure was about

3 · 10�9 mbar. The binding energies were referenced to the

C1s line at 284.8 eV from adventitious carbon. Thermal gravi-

metric analysis (TGA) was performed on a Pyris Diamond

thermogravimetric/differential thermal analyzer by heating

the samples at 5 �C min�1 to 800 �C in the atmosphere of

nitrogen. X-ray diffraction (XRD) patterns of the samples were

measured from 0.5� to 90� by a Philips X’Pert PRO X-ray dif-

fraction instrument.

Infrared (IR) spectra were recorded in KBr pellets using a

Spectrum One Fourier transform infrared (FTIR) spectrometer

(Perkin-Elmer Instruments Co. Ltd., USA). Raman spectra

were recorded with a Renishaw inVia Raman spectrometer

(Renishaw plc, UK). All samples were tested in powder form

on silicon wafer without using any solvent. The laser excita-

tion was provided by a regular model laser operating at

514 nm.

The electrical conductivity of STG sample was measured

without any annealing process. The STG sample was com-

pressed into a tablet by using a tablet compression machine

and then the tablet was cut into rectangular pieces for electri-

cal measurement. Platinum threads were chosen as elec-

trodes and glued on the rectangular tablet by using silver

gel. The electrical conductivities of the pressed STG tablets

were measured by four-probe electrometer method with a

Quantum Design physical property measurement system

(Quantum Design Inc., USA).

Fig. 2 – TEM images of GO (a) and STG-180 (c), and HRTEM

images of GO (b) and STG-180 (d).

3. Results and discussion

3.1. Preparation of homogeneous graphene dispersion viasolvothermal approach

GO dispersion in DMF was prepared through a solvent ex-

change method, and was then sealed in a Teflon-lined stain-

less-steel autoclave and heated to a certain temperature. A

homogenous graphene dispersion in DMF was obtained after

several hours. The as-prepared graphene dispersion was de-

fined as STG. Different temperatures were taken at 120, 160,

180, and 200 �C to prepare STG dispersions, and the homoge-

neous dispersions were named as STG-120, STG-160, STG-180,

and STG-200, respectively. As shown in Fig. 1, the color of the

dispersion changed from yellow brown into deep black after

solvothermal reduction. Furthermore, an intense odor like

amine could be smelled from the STG dispersions. The as-

prepared STG dispersion in DMF is very stable and does not

aggregate after more than 1 year, and the concentration could

reach up to 0.3 mg mL�1.

3.2. Characterizations of STG

3.2.1. TEM and AFM imagingFlexible sheets with numerous wrinkles of GO and STG-180

can be observed in the TEM images of GO and STG-180

(Fig. 2a and c). Fig. 2d shows the high-resolution TEM (HRTEM)

image of STG-180, in which the crystal structure indicates the

reduction restores the conjugated system of the carbon

sheets. Owing to the oxygen-containing groups and defects,

GO exhibits amorphous structure in the HRTEM image

(Fig. 2b).

As observed by AFM (Fig. 3), most graphene sheets in the

homogeneous dispersion are single-layered, and the thick-

ness of STG-180 is about 0.8 nm (Fig. 3d) and smaller than

those of GO sheets (Fig. 3c). This might be owing to the effi-

cient removal of the oxygen-containing groups on the GO

sheets.

3.2.2. Deoxygenation during solvothermal processThere are numerous oxygen-containing groups on GO sheets,

making GO well-dispersed in water and many other polar

solvents [31,32]. After the removal of most of the oxygen-

containing groups on the GO sheets, the hydrophobicity of

the STG sheets greatly increases. After adding some water

Fig. 3 – AFM images and section analysis of GO (a and c) and STG-180 (b and d).

C A R B O N 4 9 ( 2 0 1 1 ) 3 9 2 0 – 3 9 2 7 3923

into the STG dispersion in DMF, the STG can be separated out

and collected as solid STG. XPS, TGA, IR, and XRD measure-

ments were taken to validate the deoxygenation of GO during

the solvothermal process.

We can observe in the survey spectra (Fig. 4a) that the

intensity of O1s decrease significantly from GO to STG-180,

indicating considerable deoxygenation by the solvothermal

process. Table 1 lists the atomic and mass composition of

GO and STG-180 obtained by XPS. From the data in Table 1,

it can be found that the C/O atomic ratio increases from

2.02 for GO to 5.52 for STG-180, which is comparable to the

C/O ratio of chemical reduced GO [10,11], and is half of the va-

lue of graphene obtained by thermal expansion method [16].

The XPS spectrum of GO in the C1s region (Fig. 4b) clearly

indicates a considerable degree of oxidation of the carbon

atoms in different functional groups: the carbon atoms in

C@C (284.7 eV), the carbon atoms in C–O bonds (285.7 eV),

the epoxy carbon atoms (286.9 eV), the carbonyl carbon atoms

(287.9 eV), and the carboxyl carbon atoms (289.1 eV) [33]. After

solvothermal treatment, the STG-180 shows only one pre-

dominant peak at 284.8 eV (Fig. 4c), which can be attributed

to graphitic carbon systems, and the tail of this signal at high

binding energy indicates that there are still a small amount of

carbon atoms covalently bound with oxygen remaining in the

STG after the solvothermal treatment.

The removal of oxygen-containing groups is also con-

firmed by TGA and IR of the samples. A rapid weight loss is

observed at 150–250 �C (Fig. 5), corresponding to the decompo-

sition of oxygen-containing groups on the GO sheets [16].

Therefore, there are approximately 30 wt.% of the oxygen-

containing groups in the GO, whereas, STG-180 only shows

a slight weight loss of about 7.5 wt.% at this temperature

range.

In the IR spectrum of GO (Fig. 6, trace a), the absorbtion

peaks can be assigned to the O–H stretching vibrations at

3430 cm�1, the carbonyl stretching at 1730 cm�1, the C@C ring

stretching at 1624 cm�1, the phenol O–H deformation vibra-

tion at 1402 cm�1, the C–O stretching at 1053 cm�1, and the

C–OH stretching at 1228 cm�1 [34]. The absorption peak of

the carbonyl group decreases or even disappears after the

solvothermal treatment is another evidence for the deoxy-

genation of GO (Fig. 6, trace b and c).

It can be observed in the XRD patterns of GO and STG-180

(Fig. 7), the interlayer distance between STG smaller than GO

sheets, which decreases from 0.78 nm of GO (2h = 11.3�) to

0.37 nm of STG-180 (2h = 24.2�).

3.2.3. Cutting effects during the solvothermal reductionSolvothermal treatment shows a cutting effect on the carbon

sheets [35], thus an increase in D/G intensity ratio can be ob-

served when comparing STG with GO due to the increase in

the numbers of the carbon atoms on the edge of the sheets

(‘‘edge effect’’) [36,37]. Furthermore, the D/G intensity ratio

of the STG increases with the temperature of the solvother-

mal reaction. However, the D/G intensities of STG (lower than

1.00, Fig. 8, trace b–e) are lower than the reduced-GO with

hydrazine in our previous experiment (larger than 1.15),

[30,38] indicating the effectiveness of the solvothermal reduc-

tion in restoring the conjugated system of the graphene

sheets.

Another evidence of the cutting effect is the electrical con-

ductance. Owing to the partial restoring of the conjugated

system, the electrical conductivity of STG is significantly en-

hanced and was measured as 19 S m�1, which is much larger

than that of GO film [14]. However, the cutting effect makes

the graphene sheets cut into much smaller ones, the conduc-

tivity of STG is about one order of magnitude lower than

chemical reduced GO [14].

3.3. Solvothermal reduction mechanism

In the closed system of the sealed autoclave, the relatively

high temperature and autogenous pressure promote the

deoxygenation reaction, and the dehydrating reaction of hy-

droxyl and epoxy groups results in a recovery of p-conjuga-

tion system. The fact that the dispersion turns into black

0 200 400 600 800 1000 1200

a

STG-180

GOC1s

N1s

O1s

Binding Energy (eV)

292 290 288 286 284 282 280

b

C(O)O

C in graphite

C(epoxy)/C-OH

C(epoxy)/C-OH

C=O

Binding Energy (eV)

294 292 290 288 286 284 282 280

c

C(epoxy)/C-OH

C(O)O

C=O

C(epoxy)/C-OH

C in graphite

Binding Energy (eV)

Fig. 4 – XPS spectra of GO and STG-180: (a) survey spectra of

GO and STG-180, and C1s region of (b) GO, and (c) STG-180.

0 100 200 300 400 500 600 700 800

20

40

60

80

100

a

b

Wei

ght L

oss/

wt%

Temperature/οC

Fig. 5 – TGA analysis of GO (a) and STG-180 (b).

4000 3500 3000 2500 2000 1500 1000 5000

10

20

30

40

50

60

70

80

90

100

bc

a

Tran

smitt

ance

(%)

Wavenumber (cm-1)

Fig. 6 – IR spectra for (a) GO, (b) STG-160, and (c) STG-180.

3924 C A R B O N 4 9 ( 2 0 1 1 ) 3 9 2 0 – 3 9 2 7

color after solvothermal treatment is one of the evidence of

the partial restoration of the conjugation system within the

graphene sheets. Furthermore, DMF possesses a weak reduc-

ing ability, therefore the reduction process undergoes with

thermal and chemical reduction at the same time.

Table 1 – The atomic and mass composition of GO and STG-180

Ca Cw Oa

GO 66.45 59.82 32.94 3STG-180 82.02 77.75 14.85 1

a Atomic percentage.

w Weight percentage.

Meanwhile, solvothermal reduction of GO was also carried

out in the other solvents, such as methanol, ethanol, THF,

DMSO, and ethylene glycol, homogeneous dispersion could

not obtained. Therefore, the solvent plays an important role

in preparation of homogeneous dispersion of graphene sheets

in the solvothermal reduction process.

3.3.1. Suitable solvents from the view of Hansen solubilityparameterFrom the above results, we obtained the stable STG dispersion

in DMF, rather than in the other solvents employed, metha-

nol, ethanol, THF, DMSO, and ethylene glycol. One factor is

the physical properties of the solvent itself, i.e., the solubility

parameters and the surface tension of a solvent. ‘‘Hansen sol-

ubility parameters’’ could give valuable information for the

discussion and prediction of the solubility. There are three

parameters included in the Hansen solubility parameters:

dispersion cohesion parameter (dd), polarity cohesion

measured by XPS.

Ow Na Nw C/Oa C/Ow

9.53 0.61 0.64 2.02 1.518.77 3.14 3.47 5.52 4.14

0 10 20 30 40 50 60 70 80 90

STG-180

GO

2θ = 24.2o d = 0.37 nm

2θ = 11.3o d = 0.78 nm

Inte

nsity

(a.u

.)

2θ (ο)

Fig. 7 – XRD patterns of GO and solid STG.

1000 1200 1400 1600 1800 2000

(0.83)

(0.93)

(0.92)

(0.92)

edcb

a(0.88)

Raman shift (cm-1)

Inte

nsity

(a. u

.)

Fig. 8 – Raman spectra for (a) GO, (b) STG-120, (c) STG-160, (d)

STG-180, and (e) STG-200. Numbers in the bracket represent

for the D/G intensity ratio of the samples.

C A R B O N 4 9 ( 2 0 1 1 ) 3 9 2 0 – 3 9 2 7 3925

parameter (dp), and hydrogen bonding cohesion parameter

(dh) [39]. Ruoff and coworkers have reported that graphene

can be dissolved in the solvents that have a value of (dp + dh)

in the range of 13–29 [40]. As listed in Table 2, the values of

(dp + dh) of the solvents are all in this range except methanol,

but the values of THF and ethanol are near the top and bot-

tom limitation of the range, respectively. DMF and DMSO pos-

sess the suitable values in these solvents for dispersing

graphene.

Table 2 – Hansen solubility parameters of the solvents used in

Solvents dd

THF 16.8DMF 17.4DMSO 18.4Ethanol 15.8Methanol 15.1Water 15.5DMAC 16.8Ethylene glycol 17.0

3.3.2. Self-generated stabilizerAs mentioned, in the solvothermal reduction of GO in DMSO,

we could not obtain homogeneous dispersion (Fig. S1b). An-

other factor might be the resultant derivative from DMF dur-

ing the solvothermal process, which play a role of stabilizing

agent to some extent. Therefore, a solvothermal reduction

mechanism was proposed as shown in Fig. 9.

In the solvothermal process, DMF can be hydrolyzed into

NH(CH3)2 and HCOOH with a small amount of water in the

dispersion (reaction formula b in Fig. 9) [41,42]. This hydroly-

sis process can be confirmed by the intense odor of amine

smelled from the dispersion after the reaction. As a result

of protonation, NH(CH3)2 becomes positively charged in the

form of [NH2(CH3)2]+ (reaction formula c in Fig. 9) [41,42].

The XPS results show there remains a small amount of car-

boxyl groups on the STG (Fig. 4c), and the [NH2(CH3)2]+ could

interact with the negatively charged –COO� on the edge of

GO sheet with the driving force of electrostatic interaction

(reaction formula d in Fig. 9) [43,44]. With the alkyl groups

modified on the edges of STG sheets, the STG sheets are sta-

bilized, and its dispersibility is enhanced. The existence of the

nitrogen species is verified by the XPS analysis for the N1s

peak at 400 eV (Fig. 4a), and the increase at 285.6 eV is par-

tially because of the C–N in the [NH2(CH3)2]+ (Fig. 4c). The

in situ produced [NH2(CH3)2]+ acts as the stabilizer for the

STG, therefore, without any formed stabilizer in the disper-

sion of methanol, ethanol, THF, DMSO, and ethylene glycol,

the carbon sheets aggregated together due to the p–p interac-

tion between sheets.

3.3.3. Confirmation of the reduction mechanismTo validate the mechanism, three control experiments have

been performed. In the first control experiment, the solvo-

thermal reaction was carried out in six other kinds of organic

solvents. As predicted from the abovementioned viewpoint,

black precipitate forms in the dispersions of GO in methanol,

ethanol, THF, DMSO, and ethylene glycol, while homogeneous

dispersion in deep black color can be obtained in DMAC

(Fig. S1a). DMAC has a similar structure and solubility param-

eter to DMF, can also be hydrolyzed and produce NH(CH3)2,

which can act as stabilizer to the formed graphene.

Furthermore, in the second control experiment, in order to

validate the effect of [NH2(CH3)2]+, GO dispersion in DMF was

reflux at 140 �C, the color of dispersion gradually became

black and aggregation appeared after about 3 h. The odor of

amine of the mixture is not as strong as solvothermal treated

GO dispersion in DMF. In the last control experiment, as

the solvothermal reduction process.

dp dh dp + dh

5.7 8.0 13.713.7 11.3 25.016.4 10.2 26.68.8 19.4 28.2

12.3 22.3 34.616.0 42.3 58.311.5 10.2 21.111.0 26.0 37.0

Fig. 9 – Mechanism of the formation of STG.

3926 C A R B O N 4 9 ( 2 0 1 1 ) 3 9 2 0 – 3 9 2 7

mentioned above, DMSO has a similar dispersibility as DMF,

but the products aggregated together after solvothermal pro-

cess. After adding some DEA into the GO dispersion in DMSO,

the aggregation can be effectively prevented and homoge-

neous dispersion could be obtained after solvothermal reduc-

tion (Fig. S1b).

3.4. Dispersion test of STG in various organic solvents

As shown in Fig. 10, solid STG can be dispersed in many sol-

vents with the assistance of sonication. In the dispersion test

experiment of STG, STG shows a good dispersibility in NMP,

pyridine, DMF, DMAC, acetonitrile, and DMSO with the deep

black color of the dispersions, and the concentration of STG

in NMP, DMAC, and acetonitrile could reach up to

1.0 mg mL�1. As shown in Table S1 and Fig. S2d, all the values

of (dp + dh) of these solvents are in the range of 12.7–28.7,

which consistent well with the literature report [40]. STG is

slightly dispersible in ethyl acetate, THF, acetone, 2-isopropa-

nol, ethanol, or ethylene glycol, and is not dispersible in the

hexane, TEA, toluene, diethyl ether, DEA, DCM, chloroform,

water, or methanol. It is observed in Figs. S2b and S2c, the sol-

vents that could disperse STG, both dp and dh should be larger

than 6.0. Owing to the numerous hydrogen bonds between

the solvents and the oxygen-containing groups on GO sheets,

GO could be dispersed well in the solvents with larger dh

[31,40]. However, the value of dh should not be larger than

20 for dispersing STG due to the removal of most of the oxy-

gen-containing groups.

Fig. 10 – Dispersion test experiment of STG: solvents 1–21

represent for (1) hexane, (2) TEA, (3) toluene, (4) diethyl

ether, (5) DEA, (6) DCM, (7) chloroform, (8) ethyl acetate, (9)

THF, (10) pyridine, (11) acetone, (12) NMP, (13) DMAC, (14) 2-

isopropanol, (15) acetonitrile, (16) DMF, (17) DMSO, (18)

ethanol, (19) ethylene glycol, (20) water, and (21) methanol.

With the stable graphene dispersion in DMF, or re-dis-

persed STG in other solvents, graphene-based composites

could easily be prepared by the assembly of graphene with

nanoparticles, polymers, and other functional composites in

the STG dispersion. Moreover, construction of the graphene-

based materials could also be carried out in the solvothermal

reduction process by combining the reduction of GO with the

formation of the functional composites simultaneously.

4. Conclusion

We demonstrated herein a method to prepare a homogeneous

dispersion of graphene sheets in DMF by solvothermal reduc-

tion of GO. The concentration of STG can reach up to

0.3 mg mL�1, and the STG dispersion is stable for more than

1 year. In the solvothermal process, the high temperature

and the autogenous high pressure promote the deoxygen-

ation of GO, and DMF acts as a weak reductant and stabilizer

in keeping the graphene dispersion homogeneous and stable.

The method is simple and efficient, because graphene disper-

sion could be obtained directly by solvothermal treatment of

GO dispersion in DMF, and no extra reducing agent and/or

stabilizer is needed. Meanwhile, the solid STG could be dis-

persed in several kinds of solvents with the (dp + dh) in the

range of 13–29, such as NMP, DMAC, and acetonitrile, making

it possible for further modification and application.

Acknowledgements

The financial support of the Ministry of Science and

Technology of China (National Major Scientific Research

Program, Grant no. 2011CB932500), the National Science

Foundation of China (Grants no. 20972035 and 91023001),

and the Chinese Academy of Sciences (Knowledge Innovation

Program, Grant no. KJCX2-YW-H21) is acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.carbon.2011.05.030.

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