SYNTHESIS AND CHARACTERIZATION OF REDUCED GRAPHENE OXIDE

Minitha Cherukutty Ramakrishnana, Rajendrakumar Ramasamy Thangavelu*,b

Advanced Materials and Devices Laboratory, Department of Physics,

Bharathiar University, Coimbatore, India.

*Corresponding author, e-mail: a minitha.cr@gmail.com, b rtrkumar@buc.edu.in

Keywords: Graphene oxide, Hydrazine hydrate, Reduced graphene oxide, Spin coating, Hummer’s method.

Abstract Reduced graphene oxide (rGO) is an excellent candidate for various electronic devices

such as high performance gas sensors. In this work, graphene oxide was prepared by oxidizing

graphite to form graphite oxide (GO). From X - ray diffraction analysis, the peak around 11.5o

confirmed that oxygen was intercalated into graphite. By using hydrazine hydrate, the epoxy group

in graphite oxide was reduced and then the reduced graphite oxide (rGO) solution was exfoliated.

Raman spectrum of both GO and rGO contains G band (1580 cm-1

) and D band (1350 cm-1

)

features. The increase in intensity ratio of D band to G band (ID/IG) from 0.727 (GO) to 1.414

(rGO) indicates the successful reduction of GO into rGO. The exfoliated reduced graphite oxide

solution is spin coated on to the SiO2/Si substrates.

Introduction

Graphene oxide has attracted considerable attention due to its potential applications in

electronics as electrically insulating material, used as an adsorbent. Graphene oxide (GO) – (a

graphene sheet decorated with oxygen functional groups such as epoxides, carboxylic groups) is a

water soluble nanomaterial. It has a hydrophilic character and molecules of water can easily

intercalate between the graphite layers [1]. Three methods are commonly used to synthesize

graphite oxide: Staudenmaier [2], Brodie [3], Hummers-Offeman/(Hummers Method) [4] methods.

All of them involve the oxidation of graphite, but differ in the kind of mineral acids, oxidizing

agents, time of preparation and the type of washing and drying processes. Graphite oxide obtained

from Hummers method has larger interlayer distance and higher C/O atomic ratio than the other two

methods. Generally GO is insulator in nature; however it can become conductive by exposing it to

reducing agents such as hydrazine hydrate or NaBH4 (Sodium Boro Hydrate) through high

temperature treatment or by UV assisted photo catalysis and thermal treatment (low- temperature

annealing reduction of graphite oxide). Hydrazine reduced GO shows remarkable electrical

properties and excellent performance for detecting analytes [5]. The chemical reduction of graphite

oxide is one of the established procedures to make reduced graphene oxide films in large area. Very

few reports are available on preparation of reduced graphene oxide. Understanding and

optimization of the process is important for the large area production of rGO. Here we made an

attempt to synthesize reduced graphene oxide films by oxidation of graphite to form graphite oxide

followed by exfoliation and reduction of GO.

Materials and Methods

The preparation process of rGO consists of three steps: i) Oxidation or intercalation (ii)

exfoliation and (iii) reduction. Firstly Graphite oxide was prepared from the graphite powder by

modified Hummer’s method. A 3g of graphite powder is put into concentrated H2SO4 (69 ml).

KMnO4 (9 g) is added gradually with stirring and cooling in an ice bath, so that the temperature of

the mixture was maintained below 20°C (# Caution: sudden temperature rise could cause explosion.

Care must be taken to maintain temperature below 50oC). The mixture again stirred at 35 °C for 45

min, and distilled water (150 ml) is added which causes violent effervescence and temperature is

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increased to 100oC. After 1hour, the reaction is terminated by the addition of distilled water (500

ml) and 30% H2O2 solution (30 ml), after which the colour of the suspension changes to bright

yellow. The suspension is then washed with 1:10 HCl solution (750 ml) in order to remove metal

ions by filter paper and funnel. The paste collected from the filter paper is then dried at 60°C, until

it becomes agglomeration. The agglomeration is dispersed into distilled water in static state for 2-3

hours and slightly mixed manually using glass rod. The suspension is washed with more distilled

water for a week (7-9 times), until we get pH nearly 7. The paste so collected on the filter paper is

dispersed into water by ultrasonication. The obtained brown dispersion is then subjected to

centrifugation at 4000 rpm for 30 min, to remove any unexfoliated GO and then dried at 60 °C for 1

hour [4].

In a typical procedure of reduced graphene oxide, first graphite oxide (100 mg) is added

with distilled water in a 250 ml Beaker; this yielded an inhomogeneous yellow-brown dispersion.

This dispersion was sonicated using a TOSHCON ultrasonication bath cleaner (150 W) until it

became clear with no visible particulate matter. Hydrazine hydrate (1 ml) was then added and the

solution was heated in the oil bath at 100˚C for 24 hours over which the reduced GO gradually

precipitated into a black solid. This product was isolated by filtration and then washed several

times in distilled water (500 ml) and Methanol (500ml) and dried at room temperature. The

resultant product is the reduced graphite oxide [5]. Finally for exfoliation, reduced graphite oxide

was taken in the concentration 3.5mg/ml. First the suspension was subjected to ultrasonication for

30 min followed by centrifugation at 14500 rpm for 30 min. At the end of the centrifugation

process the supernatant solution was carefully separated and spin coated (Apex Instruments Co.,-

Programmable Spin Coater Model No. SCU-2008C) on the 300 nm thick SiO2 grown Si substrate.

The thickness of the SiO2 layer measured with a spectroscopic ellipsometer (SE 800).

Results and Discussion

The X - ray diffraction (XRD) patterns of graphite powder, graphite oxide (GO), and

reduced graphite oxide (rGO) are shown in Fig. 1 (a, b, c) respectively. The identification of the

diffraction peaks was done by comparing the XRD spectra with the JCPDS database. The obtained

diffraction spectra graphite matches well with JCPDS card number: 41-1487. The diffraction

spectra of graphite is dominated by the strong (002) reflection at 2θ = 26.52º while that of GO

shows a strong reflection at 2θ = 11.49º, which was assigned to (001) reflection. In the case of the

graphite sample, the weak peaks in the 2θ range 40-49º, are due to (100) and (101) reflections.

Fig. 1 XRD Patterns of (a) Graphite, (b) Graphite oxide (GO), and (c) Reduced Graphite Oxide

(rGO).

The (100) line of the hexagonal graphite is clearly distinguishable, however, the (101) line

is a very broad signal it is the combination of hexagonal and rhombohedral peaks, which usually

coexists with the more common hexagonal graphite. The inset in Fig. 1(a) depicts a rhombohedral

10 20 30 40 50 60 70 80 90 100

(110)

(004)

(002)

3.34 A0

a

43.6

25.2

c

2θθθθ (Degree)

(001)

7.643 A0

b

Inte

nsi

ty (

arb

.un

it)

38 40 42 44 46 48 50

(101)

(100)

Inte

nsi

ty (

arb

.un

it)

2θθθθ (Degree)

Graphite

GO

rGO

Advanced Materials Research Vol. 678 57

peak in graphite sample. The peak at 77.78º in graphite arise from the (110) reflection. In the GO

sample, only the (100) line can be clearly observed. Also the (110) reflection in GO was very weak

[6]. The reduction peak is confirmed by the previous report by Laura et al [7]. XRD spectra of

graphite and graphite oxide, indicated that the transformation of the interlayer spacing from 0.335

nm to 0.764 nm. Also the crystallite size of graphite oxide reduced compare with graphite powder.

The Fig. 2 (a, b) shows the Fourier transform infrared (FTIR) spectra of GO and rGO. The spectra

reveals the successful oxidation and reduction of both GO and rGO. There is an aromatic C = C

group around 1625 cm-1

, which is assigned to the vibrations of absorbed water molecules and also

skeletal vibrations of non-oxidized graphitic domains. A broad and intense peak around 3400 cm-1

is assigned to O-H groups. Another peak around 1067 cm-1

is found to be due to epoxy or C–O

stretching group. A peak around 650 cm-1

is due to C-C out of plane bending (benzene ring) [8].

Fig. 2 FTIR spectrum of (a) Graphite oxide and (b) Reduced Graphite oxide

The peak at 1067 cm-1

(epoxy) associated with the vibration of epoxy group indicates the

successful intercalation of oxygen and formation of graphite oxide. When as in the case of rGO the

intensity of the epoxy group peak is decreased, which confirms the reduction of oxygen functional

groups to form rGO. Here the absorption due to the C=O group (1729 cm -1

) is found to be

decreased very much in intensity which confirm the removal of carboxylic group upon reduction.

An absorption band that appears at 1583 cm-1

may be attributed to the skeletal vibration of the

reduced graphite oxide [9]. The significant structural changes occur during the chemical processing

from pristine graphite to GO, and then to the rGO, are also reflected in their Raman spectra shown

in Fig. 3.

Fig. 3 Raman spectra of a) Graphite Oxide and b) Reduced Graphite oxide.

The Raman spectrum of the graphite material, displays a prominent G peak corresponding to

the first-order scattering of the E2g mode as shown in Fig. 3. The G band is usually assigned to the

E2g (in-plane shear mode) phonon of C sp2

atoms, this is due to the weak interlayer forces. The D

band (defect induced mode) is arises due the collapse of sp2 symmetry of carbon atoms. In the

Raman spectrum of GO, the G band is broadened and shifted to 1579 cm-1

. In addition, the D band

1000 1500 2000 2500

ID/I

G = 1.4136

ID/I

G = 0.7265

rGO

GO1354

1579

1350

1580

GD

Inte

nsi

ty (

arb

.un

it)

Raman Shift (cm-1)

GO

a

E*

CX

(C-C

)

(C-O

)

(C-O

H)

(C-H

)

(C=

O)

γγ γγ (

O-H

)

65

0

10

67

12

38

137

7

16251

72

9

3400

Tra

nsm

itta

nce

(%

)

4000 3500 3000 2500 2000 1500 1000 500

b

75

0

1583

3482

W

(C=

C)

Wavenumber (cm-1)

rGO

58 Advances in Nanoscience and Nanotechnology

at 1354 cm-1

is also becomes prominent, indicating that the reduction in size of the in-plane sp2

domains, possibly due to the extensive oxidation. The Raman spectrum of the reduced GO also

contains both G and D bands at 1580 and 1350 cm-1

respectively; however, with an increased

intensity ratio of D band to G band (ID/IG) compared to that in GO. This change suggests a decrease

in the average size of the sp2 domains [10]. The defects can be related to several features: the

presence of edges in small crystals, deviations from planarity, the presence of a certain number of

Carbon atoms in the sp3 hybridization state, etc. From Raman spectrum of GO and rGO we can

predict that GO was reduced to rGO. The increased intensity ratio ID/IG that is integrated degree of

disorder which leads to reduction process. XRD, FTIR, Raman spectra show the oxidation of

graphite to form GO and then successful reduction of oxygen to form rGO.

Fig. 4(a, b) shows that the optical image of the spin coated reduced graphite oxide on the

300 nm SiO2 grown Si substrate. From this image we could find traces of few layered reduced

graphene sheet (inset in Fig. 4(a)) along with majority of bulk reduced graphite oxide layers. In

Fig. 4(b) shows that the presence of multilayer (>ten layers) of rGO is found. Fig. 4c shows the

scanning electron microscopy image of exfoliated reduced graphite oxide. From this observation

the exfoliation of graphite oxide is not obtained as expected. The presence of single or multilayer

r-GO layers is meagre and majority of the materials remain as bulk r-GO [11].

Fig. 4(a, b) shows the optical image of reduced graphene oxide layer on the 300 nm SiO2/Si

substrate by spin coating. Inset shows the enlarged image of rGO layers with 100x magnification.

The scale bar is 50 µm. (c) shows the SEM image of exfoliated reduced graphite oxide.

Conclusion

In summary, the graphite oxide was synthesized by modified Hummers method. From XRD

pattern it is confirmed that there is successful intercalation of oxygen functional groups into the

graphite layers. This can be seen from the increased inter planar distance from 0.334 nm

(2θ = 26.52˚) to 0.764 nm (2θ =11.49˚) for graphite and GO respectively. The reduction of GO

spectra also confirmed in FTIR and Raman Spectrum by the absence of epoxy, carboxylic and

hydroxyl groups in rGO and increases the intensity ratio of D band to G band (ID/IG) respectively.

For large area and defect free fabrication of rGO, optimization of both intercalation and exfoliation

processes is important. Also the development of reduction method requires minimizing the residual

oxygen functionality.

Acknowledgement

R.T. Rajendrakumar gratefully acknowledges the financial support from Department of

Science & Technology, India under SERB scheme (SR/FTP/PS-099/2011). Authors are also

grateful to Dr. J. Arokiaraj, Principal Engineer, 3M - R&D centre, for his timely help and

discussion. The authors would like to thank the DRDO-BU-CLS for permitting to utilise the

laboratory.

a) b)

>10 layers Reduced graphite oxide

c)

Advanced Materials Research Vol. 678 59

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