The reduction of graphene oxide Songfeng Pei, Hui-Ming Cheng * Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China ARTICLE INFO Article history: Received 28 September 2011 Accepted 8 November 2011 Available online xxxx ABSTRACT Graphene has attracted great interest for its excellent mechanical, electrical, thermal and optical properties. It can be produced by micro-mechanical exfoliation of highly ordered pyrolytic graphite, epitaxial growth, chemical vapor deposition, and the reduction of graph- ene oxide (GO). The first three methods can produce graphene with a relatively perfect structure and excellent properties, while in comparison, GO has two important character- istics: (1) it can be produced using inexpensive graphite as raw material by cost-effective chemical methods with a high yield, and (2) it is highly hydrophilic and can form stable aqueous colloids to facilitate the assembly of macroscopic structures by simple and cheap solution processes, both of which are important to the large-scale uses of graphene. A key topic in the research and applications of GO is the reduction, which partly restores the structure and properties of graphene. Different reduction processes result in different properties of reduced GO (rGO), which in turn affect the final performance of materials or devices composed of rGO. In this contribution, we review the state-of-art status of the reduction of GO on both techniques and mechanisms. The development in this field will speed the applications of graphene. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction A report in 2004 by Geim and Novoselov et al. of a method to prepare individual graphene sheets has initiated enormous scientific activity [1–3]. Graphene is a two dimensional (2D) crystal that is stable under ambient conditions; it has a spe- cial electronic structure, which gives it unusual electronic properties such as the anomalous quantum Hall effect [4] and astonishing high carrier mobility at relatively high charge carrier concentrations and at room temperature [1,5]. As a new material, the uses of graphene are very attractive since many interesting properties, mechanical [6], thermal [7] and electrical [8] have been reported to confirm the superiority of graphene to traditional materials [9]. Following this trend, graphite oxide, first reported over 150 years ago [10], has re-emerged as an intense research interest due to its role as a precursor for the cost-effective and mass production of graphene-based materials. Graphite oxide has a similar layered structure to graphite, but the plane of carbon atoms in graphite oxide is heavily decorated by oxygen-containing groups, which not only ex- pand the interlayer distance but also make the atomic-thick layers hydrophilic. As a result, these oxidized layers can be exfoliated in water under moderate ultrasonication. If the exfoliated sheets contain only one or few layers of carbon atoms like graphene, these sheets are named graphene oxide (GO). 1 The most attractive property of GO is that it can be (partly) reduced to graphene-like sheets by removing the oxygen-containing groups with the recovery of a conjugated structure. The reduced GO (rGO) sheets are usually considered as one kind of chemically derived graphene. Some other names have also been given to rGO, such as functionalized 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.11.010 * Corresponding author: Fax: +86 24 2390 3126. E-mail address: [email protected](H.-M. Cheng). 1 ‘GO’ in this paper refers only to graphene oxide, while graphite oxide is not abbreviated in this paper. CARBON xxx (2011) xxx – xxx Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon Please cite this article in press as: Pei S, Cheng H.-M. The reduction of graphene oxide. Carbon (2011), doi:10.1016/j.carbon.2011.11.010
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Table 1 – Comparison of the reducing effect of GO by different methods.
Ref. no. Reduction method Form C/O ratio r (S/cm)
[56] Hydrazine hydrate Powder 10.3 2[69] Hydrazine reduction in colloid state Film NAb 72[70] 150 mM NaBH4 solution, 2 h TCF 8.6 0.045[71] Hydrazine vapor Film �8.8 NG
Thermal annealing at 900 �C, UHVa �14.1 NG[55] Thermal annealing at 1100 �C, UHV TCF NA �103
[72] Thermal annealing at 1100 �C in Ar/H2 TCF NA 727[42] Multi-step treatment:
(I) NaBH4 solution(II) Concentrated H2SO4 180 �C, 12 h(III) Thermal annealing at 1100 �C in Ar/H2
Powder (I) 4.78(II) 8.57(III) >246
(I) 0.823(II) 16.6(III) 202
[73] Vitamin C Film 12.5 77Hydrazine monohydrate 12.5 99.6Pyrogallol NA 4.8KOH NA 1.910�3
[58] 55% HI reduction Film >14.9 298a UHV: ultra high vacuum.b NA: not available.
C A R B O N x x x ( 2 0 1 1 ) x x x – x x x 5
In addition to the three parameters presented above, some
other analysis techniques, such as Raman spectroscopy, so-
lid-state FT-NMR spectroscopy, transmission electron micros-
copy (TEM), and atomic force microscopy (AFM), are also used
to show the structure and property changes of GO after reduc-
tion. These analyses can give more detailed information on
the structure of GO and rGO, and be helpful to understand
the mechanisms of reduction processes, but in most cases,
these results are not as clear in showing the reducing effect
as are the three parameters mentioned earlier.
4. Reduction strategies
4.1. Thermal reduction
4.1.1. Thermal annealingGO can be reduced solely by heat treatment and the process is
named thermal annealing reduction. In the initial stages of
graphene research, rapid heating (>2000 �C/min) was usually
used to exfoliate graphite oxide to achieve graphene
[35,45,74,75]. The mechanism of exfoliation is mainly the sud-
den expansion of CO or CO2 gases evolved into the spaces be-
tween graphene sheets during rapid heating of the graphite
oxide. The rapid temperature increase makes the oxygen-
containing functional groups attached on carbon plane
decompose into gases that create huge pressure between
the stacked layers. Based on state equation, a pressure of
40 MPa is generated at 300 �C, while 130 MPa is generated at
1000 �C [74]. Evaluation of the Hamaker constant predicts that
a pressure of only 2.5 MPa is enough to separate two stacked
GO platelets [74].
The exfoliated sheets can be directly named graphene (or
chemically derived graphene) rather than GO, which means
that the rapid heating process not only exfoliates graphite
oxide but also reduces the functionalized graphene sheets
by decomposing oxygen-containing groups at elevated tem-
perature. This dual-effect makes thermal expansion of graph-
ite oxide a good strategy to produce bulk quantity graphene.
Please cite this article in press as: Pei S, Cheng H.-M. The reduction of g
However, this procedure is found only to produce small size
and wrinkled graphene sheets [45]. This is mainly because
the decomposition of oxygen-containing groups also removes
carbon atoms from the carbon plane, which splits the graph-
ene sheets into small pieces and results in the distortion of
the carbon plane, as shown in Fig. 5. A notable effect of ther-
mal exfoliation is the structural damage to graphene sheets
caused by the release of carbon dioxide [49]. Approximately
30% of the mass of the graphite oxide is lost during the exfo-
liation process, leaving behind lattice defects throughout the
sheet [45]. Defects inevitably affect the electronic properties
of the product by decreasing the ballistic transport path
length and introducing scattering centers. As a result, the
electrical conductivity of the graphene sheets has a typical
mean value of 10–23 S/cm that is much lower than that of per-
fect graphene, indicating a weak effect on reduction and res-
toration of the electronic structure of carbon plane.
An alternative way is to exfoliate graphite oxide in the li-
quid phase, which enables the exfoliation of graphene sheets
with large lateral sizes [34]. The reduction is carried out after
the formation of macroscopic materials, e.g. films or powders,
by annealing in inert or reducing atmospheres.
In this strategy, the heating temperature significantly af-
fects the effect of reduction on GO [45,55,66,71,72,76].
Schniepp et al. [45] found that if the temperature was less
than 500 �C, the C/O ratio was no more than 7, while if the
temperature reached 750 �C, the C/O ratio could be higher
than 13. Li et al. have monitored the chemical structure vari-
ation with annealing temperature, and the XPS spectrum evo-
lution shown in Fig. 6 reveals that high temperature is needed
to achieve the good reduction of GO. Wang et al. [72] annealed
GO thin films at different temperatures, and showed that the
volume electrical conductivity of the reduced GO film ob-
tained at 500 �C was only 50 S/cm, while for those at 700 �Cand 1100 �C it could be 100 S/cm and 550 S/cm (Fig. 7), respec-
tively. Wu et al. [76] used arc-discharge treatment to exfoliate
graphite oxide to prepare graphene. Since the arc-discharge
could provide temperatures above 2000 �C in a short time,
needed to achieve highly reducible GO, which can be con-
verted to graphene with high quality and good properties.
The future research on the reduction of GO should mainly
focus on two topics: (1) a much deeper understanding of the
reduction mechanism and (2) how to control the oxidation
of graphite and the reduction of GO. This is because that a
controllable functionalization that can alter the properties
of graphene to fulfill specific requirements in applications is
equally important to obtain a non-defective graphene, for
example, to change the gapless semi-metallic graphene into
a semiconductor with proper band gap. The previous research
on GO and rGO has inspired a possible way to achieve such
change that GO and rGO show obvious semiconductor-like
properties [11]. Recently, Eda et al. [140] and Pan et al. [76] re-
ported a blue photoluminescence of GO (or rGO), which
proves that a properly functionalized graphene sheet can be
a semiconductor. Then the question is how we can obtain
such functionalization of graphene by a reliable technique,
but not an occasional observation. Research on the oxidation
and reduction combined with a deep understanding of graph-
ene structure may give us the key to realize good control of
the attaching and elimination of functional groups to some
specific locations on the carbon plane. Further research on
the controllable oxidation and reduction of graphene may
facilitate the applications of graphene as semiconductors
used in transistor and photo-electronic devices.
Acknowledgements
This work was supported by the Key Research Program of
Ministry of Science and Technology, China (No.
2011CB932604), the National Natural Science Foundation of
China (Nos. 51102243 and 50921004), and by the Chinese
Academy of Sciences (KGCX2-YW-231).
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