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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
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
A R T I C L E I N F O
A B S T R A C T
Article history:
Graphene has attracted great interest for its excellent mechanical, electrical, thermal and
Received 28 September 2011
optical properties. It can be produced by micro-mechanical exfoliation of highly ordered
Accepted 8 November 2011
pyrolytic graphite, epitaxial growth, chemical vapor deposition, and the reduction of graph-
Available online 16 November 2011
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 characteristics: (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.
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 special 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 expand 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
* Corresponding author: Fax: +86 24 2390 3126.
E-mail address: cheng@imr.ac.cn (H.-M. Cheng).
1
‘GO’ in this paper refers only to graphene oxide, while graphite oxide is not abbreviated in this paper.
0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2011.11.010
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graphene, chemically modified graphene, chemically converted graphene, or reduced graphene [11]. The most straightforward goal of any reduction protocol is to produce
graphene-like materials similar to the pristine graphene obtained from direct mechanical exfoliation (i.e. the ‘‘Scotch
tape method’’) of individual layers of graphite both in structure and properties. Though numerous efforts have been
made, the final target is still a dream. Residual functional
groups and defects dramatically alter the structure of the carbon plane, therefore, it is not appropriate to refer to rGO, even
today, simply as graphene since the properties are substantially different.
Nowadays, in addition to reduction from GO, graphene can
be produced by micro-mechanical exfoliation of highly ordered pyrolytic graphite [1], epitaxial growth [12–14], and
chemical vapor deposition (CVD) [13,15,16]. These three
methods can produce graphene with a relatively perfect
structure and excellent properties. While in comparison, GO
has two important characteristics: (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. As a result, GO and rGO are still
hot topics in the research and development of graphene,
especially in regard to mass applications.
Therefore, the reduction of GO is definitely a key topic, and
different reduction processes result in different properties
that in turn affect the final performance of materials or devices composed of rGO. Though the final target to achieve perfect graphene is hard to reach, research efforts have
continuously made it closer. Here we review work on the
reduction of GO, and because there are many review papers
on synthesis methods [13,17–23], and the physical [2,3,24–
26] and chemical [9,27–31] characteristics of graphene, details
on them will not be repeated.
2.
Preparation and characteristics of GO
GO was firstly reported in 1840 by Schafhaeutl [10] and 1859 by
Brodie [32]. The history of the evolution of synthesis methods
and chemical structure of GO has been extensively reviewed
by Dreyer et al. [9] and Compton and Nguyen [19]. Currently,
GO is prepared mostly based on the method proposed by
Hummers and Offeman [33] in 1958, where the oxidation of
graphite to graphite oxide is accomplished by treating graphite with a water-free mixture of concentrated sulfuric acid, sodium nitrate and potassium permanganate. Though some
modification has been proposed [34–37], the main strategy is
unchanged. As a result, these methods are usually named
modified Hummers methods.
Though it has been developed for over a century, the precise chemical structure of GO is still not quite clear, which
contributes to the complexity of GO due to its partial amorphous character. Several early investigations have proposed
structural models of GO with a regular lattice composed of discrete repeat units [38], and the widely accepted GO model proposed by Lerf and Klinowski [39,40] is a nonstoichiometric
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model (shown in Fig. 1), wherein the carbon plane is decorated
with hydroxyl and epoxy (1,2-ether) functional groups.
Carbonyl groups are also present, most likely as carboxylic
acids along the sheet edge but also as organic carbonyl defects
within the sheet. Recent nuclear magnetic resonance (NMR)
spectroscopy studies [41,42] of GO have made slight modifications to the proposed structure including the presence of 5and 6-membered lactols on the periphery of graphitic platelets
as well as the presence of esters and tertiary alcohols on the
surface, though epoxy and alcohol groups on the plane are still
dominant. More detailed information on this evolution can be
found in the review by Dreyer et al. [9].
An ideal sheet of graphene consists of only trigonally
bonded sp2 carbon atoms and is perfectly flat [43] apart from
microscopic ripples [44]. The heavily decorated GO sheets consist partly of tetrahedrally bonded sp3 carbon atoms, which are
displaced slightly above or below the graphene plane [45]. Due
to the structure deformation and the presence of covalentlybonded functional groups, GO sheets are atomically rough
[46–48]. Mkhoyan et al. [47] examined the oxygen distribution
on a GO monolayer using high-resolution annular dark field
(ADF) imaging in a scanning transmission electron microscope (STEM), as shown in Fig. 2. The results indicate that
the degree of oxidation fluctuates at the nanometer-scale,
suggesting the presence of sp2 and sp3 carbon clusters of a
few nanometers. Several groups [46,49–51] have studied the
surface of GO with scanning tunneling microscopy (STM)
and observed highly defective regions, probably due to the
presence of oxygen and other areas are nearly intact. Surprisingly, a report shows that the graphene-like honeycomb lattice
in GO is preserved, albeit with disorder, that is, the carbon
atoms attached to functional groups are slightly displaced
but the overall size of the unit cell in GO remains similar to
that of graphene [52]. As a result, GO can be described as a random distribution of oxidized areas with oxygen-containing
functional groups, combined with non-oxidized regions where
most of the carbon atoms preserve sp2 hybridization.
The conductivity of graphene mainly relies on the longrange conjugated network of the graphitic lattice [53,54].
Functionalization breaks the conjugated structure and
localizes p-electrons, which results in a decrease of both
carrier mobility and carrier concentration. Though there are
conjugated areas in GO, long-range (>lm) conductivity is
Copyright 1998 Elsevier. 1998
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Fig. 1 – Lerf–Klinowski model of GO with the omission of
minor groups (carboxyl, carbonyl, ester, etc.) on the
periphery of the carbon plane of the graphitic platelets of GO
[39,40].
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Copyright 2009 ACS. 2009
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Fig. 2 – (a) AFM image of GO sheets. (b) STEM-ADF image of a GO film where mono-, bi- and tri-layers are labeled as a, b, and c.
The round opening in the middle is a hole through the single film. (c) High-magnification ADF image of a monolayer GO film.
(d) Simple drawing of monolayer and possible packing of bi- and tri-layers [47].
3.
Criteria used in determining the effect of
reduction
Since reduction can make a great change in the microstructure and properties of GO, some obvious changes can be directly observed or measured to judge the reducing effect of
different reduction processes.
3.1.
Visual characteristics
Optical observation is a direct way to see the changes in GO
before and after reduction. Since a reduction process can dramatically improve the electrical conductivity of GO, the increased charge carrier concentration and mobility will
improve the reflection to incident light, which makes a rGO
film have a metallic luster compared to its GO film precursor
with a brown color and semi-transparency, as shown in
Fig. 3a. The reduction in a colloid state by chemical reduction,
e.g. hydrazine reduction, usually results in a black precipitation from the original yellow–brown suspension, which is
probably a result of an increase in the hydrophobicity of the
Copyright 2010 ACS. 2010
blocked by the absence of percolating pathways between sp2
carbon clusters to allow classical carrier transport to occur.
As a result, as-synthesized GO sheets or films are typically
insulating, exhibiting a sheet resistance of about 1012 X/sq
or higher [34,55]. The attached groups and lattice defects
modify the electronic structure of graphene and serve as
strong scattering centers that affect the electrical transport.
Therefore, the reduction of GO is not only concerned with
removing the oxygen-containing groups bonded to the graphene and removing other atomic-scale lattice defects, but is
also aimed at recovering the conjugated network of the graphitic lattice. These structure changes result in the recovery
of electrical conductivity and other properties of graphene.
Fig. 3 – Typical optical images of (a) a GO film and rGO film
[58], Copyright 2011 Elsevier. (b) GO solution and rGO
solution [59], Copyright 2009 ACS. (c, d) GO and rGO sheets
on a 300 nm SiO2/Si substrate [34].
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material caused by a decrease in polar functionality on the
surface of the sheets [56]. To improve the processibility of
rGO, some strategies have been proposed to keep the colloid
state by adding surfactants or adjusting solvent properties,
while the change in color to black can be an obvious visible
characteristic of the effect of reduction, as shown in Fig. 3b.
The related change can be also observed on the microscale
by optical microscopy of GO/rGO sheets lying on a properlyselected substrate like a SiO2/Si wafer. As shown in Fig. 3c
and d, the as-prepared GO sheets are almost transparent with
a very subtle optical contrast with the substrate, which confirms the insulating nature of the GO sheets. The small blue
regions near the edges, corresponding to a larger thickness,
are attributed to the commonly-observed edge folding [57],
while after reduction, the rGO sheets show much improved
contrast with the substrate, which is the same as that of pristine graphene sheets lying on the same substrate.
3.2.
Electrical conductivity
Graphene is reported to have a high electrical conductivity. A
few-layer graphene sheet (thickness < 3 nm) has a sheet resistance (Rs) of around 400 X/sq at room temperature [1]. Recently, Bae et al. [60] have reported the production of
graphene films by CVD. After transferring them to transparent substrate, a graphene-based transparent conductive film
(TCF) composed of 4 layers has a sheet resistance of around
30 X/sq with transparency around 90% [60]. Assuming the film
thickness was 2 nm, the calculated bulk conductivity of this
film is 1.6 · 105 S/cm (107 S/m), which is much higher than
for indium tin oxide (ITO) or metal films with the same thickness [61]. Since the purpose of reduction is mainly to restore
the high conductivity of graphene, the electrical conductivity
of rGO can be a direct criterion to judge the effect of different
reduction methods. The electrical conductivity of rGO can be
described in several ways: Rs of an individual rGO sheet (Rs-is),
Rs of a thin film assembly of rGO sheets (Rs-f), powder conductivity (rp) and bulk conductivity (r) of rGO. Sheet resistance
(Rs; X/sq) is a measure of the electrical resistance of a sheet,
independent of its thickness. It is related to bulk conductivity
by Eq. (1), where r is bulk conductivity (unit: S/cm) and t is
sample thickness (unit: cm):
Rs ¼
1
rt
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reduction of hydroiodic acid (HI) [34] with a transparency of
78% at 550 nm wavelength, the calculated bulk conductivity
of the film is about 1190 S/cm. Stankovich et al. [56] has used
powder conductivity to describe the conductivity of rGO. In
their measurement, rGO powders are compressed to pellets
with different apparent densities and then measured by a
two-probe method [56].
3.3.
Carbon to oxygen atomic ratio (C/O ratio)
Depending on the preparation method, GO with chemical
compositions ranging from C8O2H3 to C8O4H5, corresponding
to a C/O ratio of 4:1–2:1, is typically produced [38,64,65]. After
reduction, the C/O ratio can be improved to approximately
12:1 in most cases [45,66], but values as large as 246:1 have
been recently reported [42].
The C/O ratio is usually obtained through elemental analysis measurements by combustion, and also by X-ray photoelectron spectrometry (XPS) analysis. It has been proved
that the data obtained by elemental analysis are reasonably
consistent with the data by XPS, considering the fact that elemental analysis gives the bulk composition while XPS is a surface analysis technique [45]. Furthermore, XPS spectra can
give more information on the chemical structures of GO and
rGO. Since it is p-electrons from the sp2 carbon that largely
determine the optical and electrical properties of carbonbased materials [67], the fraction of sp2 bonding can provide
insight into structure–property relationships. Briefly, as
shown in Fig. 4, the C1s XPS spectrum of GO clearly indicates
a considerable degree of oxidation with four components that
correspond to carbon atoms in different functional groups:
the non-oxygenated ring C (284.6 eV), the C in C–O bonds
(286.0 eV), the carbonyl C (287.8 eV), and the carboxylate
carbon (O–C = O, 289.0 eV) [68]. Although the C1s XPS spectrum of rGO also exhibits these oxygen functional groups,
their peak intensities are much weaker than those in GO.
Table 1 summarizes the electrical conductivity and C/O ratio of typical reports on the reduction of GO. The details on
each reduction method will be discussed in Section 4.
ð1Þ
Rs-is can be measured by a two-probe method or four-probe
method using an in situ fabricated microelectrode pair on an
individual rGO sheet with the assistance of delicate photo- or
electro-lithography. The lowest Rs-is was reported to be about
14 kX/sq (350 S/cm) by Lopez et al. [62], about two order higher
than that of pristine graphene [1]. The highest bulk conductivity of a rGO sheet was reported to be 1314 S/cm by Su
et al. [63]. Both values are obtained from rGO by thermal
annealing at high temperature, and the details will be discussed in Section 5. Because graphene is usually used in the
form of thin films, like TCF, Rs-f by a four-probe method on
the surface of a macroscopic film is often used to describe
its electrical conductivity when prepared using different
ways. The lowest Rs-f (0.84 kX/sq) of rGO-based TCF
(10 nm in thickness) was achieved by Zhao et al. by chemical
Copyright 2008 Elsevier. 2008
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Fig. 4 – The C1s XPS spectra of (a) GO and (b) rGO
[56].
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Table 1 – Comparison of the reducing effect of GO by different methods.
Ref. no.
[56]
[69]
[70]
[71]
[55]
[72]
[42]
[73]
[58]
a
b
Reduction method
Form
C/O ratio
r (S/cm)
Hydrazine hydrate
Hydrazine reduction in colloid state
150 mM NaBH4 solution, 2 h
Hydrazine vapor
Thermal annealing at 900 C, UHVa
Thermal annealing at 1100 C, UHV
Thermal annealing at 1100 C in Ar/H2
Multi-step treatment:
(I) NaBH4 solution
(II) Concentrated H2SO4 180 C, 12 h
(III) Thermal annealing at 1100 C in Ar/H2
Vitamin C
Hydrazine monohydrate
Pyrogallol
KOH
55% HI reduction
Powder
Film
TCF
Film
10.3
NAb
8.6
8.8
14.1
NA
NA
(I) 4.78
(II) 8.57
(III) >246
2
72
0.045
NG
NG
103
727
(I) 0.823
(II) 16.6
(III) 202
12.5
12.5
NA
NA
>14.9
77
99.6
4.8
1.9103
298
TCF
TCF
Powder
Film
Film
UHV: ultra high vacuum.
NA: not available.
In addition to the three parameters presented above, some
other analysis techniques, such as Raman spectroscopy, solid-state FT-NMR spectroscopy, transmission electron microscopy (TEM), and atomic force microscopy (AFM), are also used
to show the structure and property changes of GO after reduction. 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 annealing
GO 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 sudden expansion of CO or CO2 gases evolved into the spaces between graphene sheets during rapid heating of the graphite
oxide. The rapid temperature increase makes the oxygencontaining 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 temperature. This dual-effect makes thermal expansion of graphite oxide a good strategy to produce bulk quantity graphene.
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 graphene sheets into small pieces and results in the distortion of
the carbon plane, as shown in Fig. 5. A notable effect of thermal 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 exfoliation 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 perfect graphene, indicating a weak effect on reduction and restoration of the electronic structure of carbon plane.
An alternative way is to exfoliate graphite oxide in the liquid 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 affects 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 variation with annealing temperature, and the XPS spectrum evolution 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 obtained at 500 C was only 50 S/cm, while for those at 700 C
and 1100 C it could be 100 S/cm and 550 S/cm (Fig. 7), respectively. 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,
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Copyright 2006 ACS. 2006
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Copyright 2009 ACS. 2009
Fig. 5 – Pseudo-3D representation of a 600 nm · 600 nm AFM scan of an individual graphene sheet showing the wrinkled and
rough structure of the surface, and an atomistic model of the graphite oxide to graphene transition [45].
Copyright 2008 ACS. 2008
Fig. 6 – XPS spectra of GO sheets annealed in 2 Torr of (a) NH3/Ar (10% NH3) and (b) H2 at various temperatures [77].
Fig. 7 – Increase of the average conductivity of graphene
films from 49, 93, 383 to 550 S/cm, along with the
temperature increasing from 550 C, 700 C, 900 C to
1100 C, respectively [72].
the typical sheet electrical conductivity of graphene sheets
was about 2000 S/cm, and elemental analysis revealed that
the exfoliated graphene sheets had a C/O ratio of 15–18.
In addition to annealing temperature, annealing atmosphere is important for the thermal annealing reduction of
GO. Since the etching of oxygen will be dramatically increased
at high temperatures, oxygen gas should be excluded during
annealing. As a result, annealing reduction is usually carried
out in vacuum [55], or an inert [72] or reducing atmosphere
[35,72,75,77]. Becerril et al. [55] have reduced GO films by thermal annealing at 1000 C, and found that a quality vacuum
(<105 Torr) is key for the recovery of GO, otherwise the films
can be quickly lost through reaction with residual oxygen in
the system. The same condition should also be considered
in inert atmospheres. Therefore, a reducing gas such as H2
is added to consume the residual oxygen in the atmosphere.
Furthermore, because of the high reducing ability of hydrogen
at elevated temperatures, the reduction of GO can be realized
at a relatively low temperature in a H2 atmosphere. Wu et al.
reported that GO can be well reduced at 450 C for 2 h in an
Ar/H2 (1:1) mixture with a resulting C/O ratio of 14.9 and conductivity of 1 · 103 S/cm. Li et al. [77] reported that annealing
GO in low-pressure ammonia (2 Torr NH3/Ar (10% NH3)) can
produce simultaneous nitrogen doping and reduction of GO.
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As shown in Fig. 6, the highest doping level of 5% N is obtained at 500 C, and electrical measurements of GO sheets
demonstrate that GO annealed in NH3 exhibits a higher conductivity than that annealed in H2 and clearly shows n-type
electron doping behavior. The latter may be beneficial for
the fabrication of electronic devices. Recently, Lopez et al.
[62] demonstrated that vacancies can be ‘‘repaired’’ partially
by exposing rGO to a carbon source such as ethylene at a high
temperature (800 C), similar to the conditions used for CVD
growth of SWCNTs. With this post-reduction deposition of
carbon, the sheet resistance of individual rGO sheet can be
decreased to 28.6 kX/sq (or 350 S/cm) [78]. Su et al. reported
a similar defect healing effect for rGO sheets functionalized
with aromatic molecules during pyrolysis that results in a
highly graphitic material with a conductivity as high as
1314 S/cm [63].
Based on the above results, reduction of GO by high temperature annealing is highly effective. But the drawback of
thermal annealing is also obvious. First, high temperature
means large energy consumption and critical treatment conditions. Second, if the reduction is performed to an assembled
GO structure, e.g. a GO film, heating must be slow enough to
prevent the expansion of the structure, otherwise quick heating may explode the structure just like the exfoliation of
graphite oxide. But slow heating makes the thermal reduction
of GO a time-consuming process. Finally and importantly,
some applications need to assemble GO on substrates, e.g.
thin carbon films, but the high temperature means that this
reduction method cannot be used for GO films on substrates
with a low melting-point, such as glass and polymers.
Flash reduction [81] of free-standing GO films can be done
with a single, close-up (<1 cm) flash from a xenon lamp such
as exists on a camera. The photo energy emitted by the flash
lamp at a close distance (<2 mm: 1 J/cm2) can provide 9 times
the thermal energy needed for heating GO (thickness 1 lm)
over 100 C, which should be more than enough to induce
deoxygenating reactions, and suggests that flash irradiation
could lead to a much higher degree of reduction of GO. The
GO films typically expand tens of times after flash reduction
because of rapid degassing, and the electrical conductivity of
the expanded film is around 10 S/cm using its maximum expanded thickness in the calculation. Because the light can be
easily shielded, rGO patterns can be easily fabricated with
photomasks, which facilitates the direct fabrication of electronic devices based on rGO films, as shown in Fig. 8a.
A further improvement of the photo-reduction and patterned film fabrication was carried out with femtosecond laser irradiation as proposed by Zhang et al. [82] The focused
laser beam (laser pulse of 790 nm central wavelength, 120 fs
pulse width, 80 MHz repetition rate, focused by a ·100 objective lens) has even higher power density than a xenon lamp
flash and the heated area in a GO film is very localized with
a line width in the range of 101–101 lm. As a result, the laser
reduction can produce rGO films with a much higher conductivity of 256 S/cm, and the rGO film patterns can be drawn directly by a pre-programmed laser on the GO film to form more
complicated and delicate circuits as shown in Fig. 8b–e.
4.1.2.
Reduction by chemical reagents is based on their chemical
reactions with GO. Usually, the reduction can be realized at
room temperature or by moderate heating. As a result, the
requirement for equipment and environment is not as critical
as that of thermal annealing treatment, which makes chemical reduction a cheaper and easily available way for the mass
production of graphene compared with thermal reduction.
The reduction of graphite oxide by hydrazine was used before the discovery of graphene [83], while the use of hydrazine
to prepare chemically derived graphene was first reported by
Stankovich et al. [56,84]. These reports open an easy way for
Microwave and photo reduction
Thermal annealing is usually carried out by thermal irradiation. As an alternative, some unconventional heating resources have been tried to realize thermal reduction
including microwave irradiation (MWI) [79,80] and photo-irradiation [81,82].
The main advantage of MWI over conventional heating
methods is heating substances uniformly and rapidly. By
treating graphite oxide powders in a commercial microwave
oven, rGO can be readily obtained within 1 min in ambient
conditions [79].
4.2.
Chemical reduction
4.2.1.
Chemical reagent reduction
Fig. 8 – Patterned rGO film obtained by (a) flash reduction [81] (Copyright 2009 ACS) and (b–e) femtosecond laser reduction [82].
Scale bars, 10 lm (Copyright 2009 Elsevier). The black parts in the films are the reduced GO patterns.
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an additional dehydration process using concentrated sulfuric acid (98% H2SO4) at 180 C after reduction by NaBH4 to further improve the reduction effect on GO. The C/O ratio of rGO
by the two-step treatment is about 8.6 and the conductivity of
the rGO powder produced is about 16.6 S/cm.
Ascorbic acid (Vitamin C: VC) is a newly reported reducing
reagent for GO, which is considered to be an ideal substitute
for hydrazine [73]. Fernandez-Merino et al. revealed that GO
reduced by VC could achieve a C/O ratio of about 12.5 and a
conductivity of 77 S/cm, which are comparable to those produced by hydrazine in a parallel experiment. In addition, VC
has great advantage of its non-toxicity in contrast to hydrazine and a higher chemical stability with water than NaBH4.
Furthermore, the reduction in colloid state does not result
in the aggregation of rGO sheets as produced by hydrazine,
which is beneficial for further applications.
Recently, Pei et al. [58] and Moon et al. [97] reported another strong reducing reagent, hydroiodic acid (HI), for GO.
The two independent investigations report similar reduction
results in that the C/O ratio of rGO is around 15, and the conductivity of the rGO films is around 300 S/cm, both of which
are much better than obtained by other chemical reduction
methods. The reduction by HI can be realized using GO in
the form of a colloid, powder or film in a gas or solution environment, even at room temperature [97]. The comparison of
the reduction effects on GO films with HI, hydrazine vapor,
85% hydrazine hydrate and NaBH4 solution are shown in
Fig. 9. The GO film reduced by HI has good flexibility and even
improved tensile strength, while the hydrazine vapor-reduced
GO film becomes too rigid to be rolled and the film thickness
expanded more than 10 times. Contrarily, the GO films re-
Copyright 2010 Elsevier. 2010
the mass-production of graphene. As a result, hydrazine has
been accepted as a good chemical reagent to reduce GO
[51,66,69,73,84–94]. The reduction by hydrazine and its derivatives, e.g. hydrazine hydrate and dimethylhydrazine [95], can
be achieved by adding the liquid reagents to a GO aqueous
dispersion, which results in agglomerated graphene-based
nanosheets due to the increase of hydrophobility. When
dried, an electrically conductive black powder with C/O ratio
around 10 [56] can be obtained. The highest conductivity of
rGO films produced solely by hydrazine reduction is 99.6 S/
cm combined with a C/O ratio of around 12.5 [73]. To facilitate
the application of graphene, efforts have been made to reduce
GO while retaining the colloidal state in water by adding soluble polymers [84] as surfactant, or ammonia [69] to change
the charge state of rGO sheets. The graphene sheets suspended in colloidal solutions can be used to assemble macroscopic structures by simple solution processes like filtration
[69].
Metal hydrides, e.g. sodium hydride, sodium borohydride
(NaBH4) and lithium aluminium hydride, have been accepted
as strong reducing reagents in organic chemistry, but unfortunately, these reductants have a slight to very strong reactivity
with water, which is the main solvent for the exfoliation and
dispersion of GO. Recently, NaBH4 was demonstrated more
effective than hydrazine as a reductant of GO [70]. Although
it is also slowly hydrolyzed by water, its use is kinetically slow
enough that the freshly-formed solution functions effectively
to reduce GO. Since NaBH4 is most effective at reducing C = O
species but has low to moderate efficiency in the reduction of
epoxy groups and carboxylic acids [96], alcohol groups remain
after reduction. As an improvement, Gao et al. [42] proposed
3217
Fig. 9 – Optical photographs of GO films (a) before and (b–d) after chemical reduction by different agents: (b) HI, (c) hydrazine
vapor, (d) 85% N2H4ÆH2O (N2H4), 50 mM NaBH4 solution (NaBH4) and 55% HI after immersion for 16 h at room temperature, (e)
the stress–strain curve of the GO film and HI reduced GO film (r = stress, e = strain), and SEM images of the cross-section views
of GO films (f) before and (g, h) after reduction by (g) HI and (h) hydrazine vapor [58].
3218
CARBON
5 0 ( 2 0 1 2 ) 3 2 1 0 –3 2 2 8
duced by N2H4ÆH2O and NaBH4 solutions broke up into pieces.
These results show that HI not only has a better reducing effect than hydrazine, but is also suitable for the reduction of
GO films. As a result, reduction by HI can be used to reduce
GO thin films to high-performance TCFs [34,58].
Other reductants including hydroquinone [98], pyrogallol
[73], hot strong alkaline solutions (KOH, NaOH) [99], hydroxylamine [100], urea and thiourea have been used. However,
these reagents tend to be inferior to strong reductants, such
as hydrazine, NaBH4, and HI, based on the reported results.
4.2.2.
Photocatalyst reduction
Copyright 2008 ACS. 2008
Different from the photothermal reduction described above,
GO can also be reduced by photo-chemical reactions with
the assistance of a photocatalyst like TiO2. Recently, Williams
et al. reported the reduction of GO in a colloid state with the
assistance of TiO2 particles under ultraviolet (UV) irradiation.
As shown in Fig. 10, a change in color from light brown to dark
brown to black can be seen as the reduction of GO proceeds
[101]. This color change has previously been suggested as partial restoration of the conjugated network in the carbon plane
like that in chemical reduction processes.
The photocatalytic properties of semiconducting TiO2 particles have been thoroughly investigated [102]. According to
the formula shown in Fig. 10, upon UV-irradiation, charge
separation occurs on the surface of TiO2 particles. In the presence of ethanol the holes are scavenged to produce ethoxy
radicals, thus leaving the electrons to accumulate within
the TiO2 particles. The accumulated electrons interact with
GO sheets to reduce functional groups. The same reduction
effect has also been found in other carbon nanostructures
such as fullerene and carbon nanotubes [103,104].
Before reduction, the carboxyl groups in GO sheets can
interact with the hydroxyl groups on the TiO2 surface by
charge transfer, producing a hybrid between the TiO2 nanoparticles and the GO sheets, and this structure can be retained after reduction. The rGO sheets can work as a
current collector to facilitate the separation of electron/hole
pairs in some photovoltaic devices like a photocatalysis device [105] and a dye-sensitized solar cell [106,107]. Following
the same idea, some other materials with photocatalytic
activity, like ZnO [108] and BiVO4 [109], have also been reported to achieve the reduction of GO.
Fig. 10 – Color change of a 10 mM solution of TiO2
nanoparticles with 0.5 mg/mL GO before and after UV
irradiation for 2 h in ethanol. A suspension of 10 mM TiO2
nanoparticles is also shown for comparison
[101].
4.2.3.
Electrochemical reduction
Another method that shows promise for the reduction of GO
relies on the electrochemical removal of oxygen functionalities [110–113]. Electrochemical reduction of GO sheets or
films can be carried out in a normal electrochemical cell using
an aqueous buffer solution at room temperature. The reduction usually needs no special chemical agent, and is mainly
caused by the electron exchange between GO and electrodes.
As a merit, this could avoid the use of dangerous reductants
(e.g. hydrazine) and eliminate byproducts.
After depositing a thin film of GO on a substrate (glass,
plastic, ITO, etc.), an inert electrode is placed opposite the film
in an electrochemical cell and reducing occurs during charging of the cell. By cyclic voltammetric scanning in the range
of 0 to 0.1 V (respect to a saturated calomel electrode) to a
GO-modified electrode in a 0.1 M KNO3 solution, Ramesha
and Sampath [113] found that the reduction of GO began at
0.6 V and reached a maximum at 0.87 V. The reduction
can be achieved by only one scan and is an electrochemically
irreversible process in this scanning voltage range.
Zhou et al. [110] reported the best reduction effect using an
electrochemical method. Elemental analysis of the resultant
rGO revealed a C/O ratio of 23.9, and the conductivity of the
rGO film produced was measured to be approximately 85 S/
cm. They found that the potential needed to realize the reduction is controlled by the pH value of the buffer solution. A low
pH value is favorable to the reduction of GO, so the authors
proposed that H+ ions participate in the reaction.
An et al. [112] used electrophoretic deposition (EPD) to
make GO films. They found that GO sheets can also be reduced on the anode surface during EPD, which seems counter-intuitive to the general belief that oxidation occurs at
the anode in an electrolysis cell. Though the reduction mechanism is not clear, the simultaneous film assembly and reduction might be favorable to some electrochemical applications.
4.2.4.
Solvothermal reduction
Another emerging chemical reduction method is solvothermal reduction [59,114,115]. A solvothermal process is performed in a sealed container, so that the solvent can be
brought to a temperature well above its boiling point by the
increase of pressure resulting from heating [116]. In a hydrothermal process, overheated supercritical (SC) water can play
the role of reducing agent and offers a green chemistry alternative to organic solvents. In addition, its physiochemical
properties can be widely changed with changes in pressure
and temperature, which allows the catalysis of a variety of
heterolytic (ionic) bond cleavage reactions in water. Hydrothermal routes have been used for remarkable transformation of carbohydrate molecules to form homogeneous
carbon nanospheres [117,118] and nanotubes [119].
Zhou et al. [59] proposed a ‘‘water-only’’ route by hydrothermal treatment of GO solutions. The results show that
the SC water not only partly removes the functional groups
on GO, but also recovers the aromatic structures in the carbon
lattice. The investigation of the pH dependence of the hydrothermal reaction found that a basic solution (pH = 11) yields a
stable rGO solution while an acidic solution (pH = 3) results in
aggregation of rGO sheets, which cannot be re-dispersed even
in a concentrated ammonia solution. This reduction process
CARBON
5 0 ( 20 1 2 ) 3 2 1 0–32 2 8
is believed to be analogous to the H+-catalyzed dehydration of
alcohol, where water acts as a source of H+ for the protonation of hydroxyl groups.
Wang et al. [114] reported the reduction of GO by solvothermal reduction using N,N-dimethylformamide as the solvent.
This is different from the hydrothermal reduction in that a
small amount of hydrazine was added as the reduction agent.
After solvothermal treatment at 180 C for 12 h, the C/O ratio
of rGO (detected by Auger spectroscopy) reached 14.3, which
is much higher than that produced by hydrazine reduction
at normal pressure. However, the sheet resistance of the
rGO is in the range of 105–106 X/sq, and the poor conductivity
may result from nitrogen-doping caused by the hydrazine
reduction.
Dubin et al. [115] proposed a modified solvothermal reduction method using N-methyl-2-purrolidinone (NMP) as solvent. This treatment is different from the traditional way in
that the reduction is not performed in a sealed container
and the heating temperature (200 C) is lower than the boiling
point of NMP (202 C, 1 atm). As a result, there is no additional
pressure present in the reduction process. The deoxygenation
of GO was proposed to be realized in combination with the
moderate thermal annealing and the oxygen-scavenging
properties of NMP at high temperature. The electrical conductivity of the rGO films achieved by this solvothermal reduction
and subsequent vacuum filtration is 3.74 S/cm, which is one
order of magnitude smaller than that produced by hydrazine
reduction (82.8 S/cm). The C/O ratio of the solvothermal reduced GO is only 5.15, much lower than the other results
shown above. Therefore, this moderate temperature NMPonly solvothermal reduction method can only achieve a moderate reduction of GO.
Besides the above features of each method, these solvothermal reduction methods have the common merit that all
can produce a stable dispersion of rGO sheets, which is valuable for applications.
4.3.
Multi-step reduction
The reduction strategies introduced above are mostly realized
in one step. To further improve or optimize the reduction effect for some special purposes, multi-step reduction has been
proposed [42,88,120,121]. For example, Eda et al. [121] found
that pre-reduction by hydrazine vapor could effectively decrease the annealing temperature needed to obtain the good
reduction of a GO film. The combination of hydrazine reduction and low temperature thermal annealing at 200 C could
produce an rGO film with better conductivity than that produced by only thermal annealing at 550 C, which is important for flexible devices attached to polymer substrates.
In most chemical reactions, the effect of reagents is selective. As a result, one reducing reagent usually cannot eliminate all oxygen-containing functional groups. On the basis
of the chemical composition of GO, multi-step reduction is
proposed to be much effective. Gao et al. [42] proposed a
three-step reduction process: deoxygenation with NaBH4,
dehydration with concentrated sulfuric acid and thermal
annealing. The treatment by NaBH4 can eliminate ketone, lactol, ester and most alcohol groups. Treatment in concentrated
H2SO4 at 180 C can then dehydrate the remaining alcohol
3219
groups to form alkene bonds that are part of a conjugated
sp2 carbon network. Subsequent annealing in Ar/H2 at
1100 C for 15 min reduces the oxygen content to less than
0.5 wt.%, which is close to the value in graphite powder. This
treatment gives rGO with a C/O ratio higher than 246, which is
the highest value reported until now, but the electrical conductivity of the rGO film is only 202 S/cm, much lower than
that achieved by direct annealing in an Ar atmosphere at
the same temperature reported by Wang et al. [72].
5.
Reduction mechanism
Though numerous strategies have been proposed to reduce
GO, there are still many questions without clear answers.
For example, can the functional groups of a GO sheet be fully
eliminated? Can the lattice defects formed during oxidation
be restored during reduction? Does a reduction process decrease or increase the defect density in a graphene sheet?
The answers and further improvements in GO reduction will
rely on an improved understanding of reduction mechanisms.
But only limited work has focused on the reduction mechanisms of GO, which may be caused by the amorphous nature
of rGO, the complexity of chemical reactions and the lack of
means to directly monitor reduction processes. As a result,
most of the research was performed fully or mostly through
computer simulation at a molecular level.
As proposed in Section 2, the difference in structure of GO
and graphene lies in a large amount of chemical functional
groups attached to the carbon plane and structural defects
within the plane, both of which can severely decrease the
electrical conductivity. As a result, the reduction of GO can
be considered to be aimed at achieving two targets: the elimination of functional groups and the healing of structural defects. For the elimination of functional groups, there are also
two effects that should be considered: whether the oxygencontaining groups can be removed and whether the areas
after removal can be restored to a long-range conjugated
structure, so that there are pathways for carrier transport
within the rGO sheet. For the healing of structural defects,
there are two possibilities, graphitization at high temperature
and epitaxial growth or CVD in the defective area with an extra carbon supply.
5.1.
Elimination of functional groups
The conductivity of monolayer graphene mainly relies on carrier transport within the carbon plane, as a result, functional
groups attached to the plane are the main influencing factor
on its conductivity, while functional groups attached to the
edge have less influence. Consequently, the reduction of GO
must be mainly aimed at eliminating epoxy and hydroxyl
groups on the plane, while other groups, e.g. carboxyl, carbonyl and ester groups, present at the edges or defective areas
only have a limited influence on the conductivity of an rGO
sheet. As proof, Li et al. [69] reduced GO using hydrazine in
a solvent, and the carboxyl groups attached to the GO are preserved after reduction. This can be used to disperse rGO
sheets in a basic solution but has little influence on the conductivity of rGO sheets and assembled films.
3220
5.1.1.
CARBON
5 0 ( 2 0 1 2 ) 3 2 1 0 –3 2 2 8
Thermal deoxygenation
Copyright 2009 ACS. 2009
Though reduction methods are different from each other, the
nature of deoxygenation is common for the removal of oxygen-containing groups from the graphene. The binding energy (or dissociation energy) between graphene and
different oxygen-containing functional groups can be an
important index to evaluate the reducibility of each group attached to the carbon plane, especially during the thermal
deoxygenation processes.
By using density functional theory (DFT) calculation, Kim
et al. [122] obtained the binding energy of an epoxy group
(62 kcal/mol) and a hydroxyl group (15.4 kcal/mol) to a 32-carbon-atom graphene unit, which indicates that epoxy groups
are much more stable than hydroxyl groups in GO. In a calculation by Gao et al. [123], the epoxy and hydroxyl groups in GO
are divided into two types for their different locations at the
interior of an aromatic domain of GO (A, B) and at the edge
of an aromatic domain (A 0 , B 0 ), as shown in Fig. 11. Because
of the low binding energy, a single hydroxyl group attached
to the interior aromatic domain is not stable and is subject
to dissociation at room temperature, while a hydroxyl group
attached to the edge is stable at room temperature. As a result, hydroxyl groups attached to the inner aromatic domains
Fig. 11 – Schematic of oxygen-containing groups in GO: A,
epoxy groups located at the interior of an aromatic domain
of GO; A 0 , epoxy groups located at the edge of an aromatic
domain; B, hydroxyl located at the interior of an aromatic
domain; B 0 , hydroxyl at the edge of an aromatic domain; C,
carbonyl at the edge of an aromatic domain; and D, carboxyl
at the edge of an aromatic domain [123].
of GO are expected to dissociate or migrate to the edges of
aromatic domains.
An increase in temperature can facilitate the thermal
deoxygenation of GO. According to Gao’s calculations, the
critical dissociation temperature (Tc) of hydroxyl groups attached to the edges of GO is 650 C, and only above this temperature can hydroxyl groups be fully removed. For
dehydroxylation, a hydroxyl group is believed to more favorably leave the graphene sheet directly, producing an OH radical and a graphene radical [123,124], which does not result
in the formation of a lattice defect within the plane.
There is no exact Tc for epoxy groups given in Gao’s paper
[123], but, as shown in Table 2, after thermal annealing at
temperatures of 700–1200 C in vacuum, the hydroxyl groups
can be fully eliminated, while the epoxy groups are retained.
In comparison, carboxyl groups are expected to be slowly reduced at 100–150 C, while carbonyl groups are much more
stable, and can only be removed above a Tc as high as 1730 C.
According to these simulations, many functional groups
are hard to remove even after thermal annealing above
1000 C, while in some experimental work, the deoxygenation
processes are not as difficult as predicted.
Jeong et al. [125] has investigated the thermal stability of
graphite oxide. According to their results, most of the oxygen-containing groups can be removed by annealing at
200 C in low-pressure argon (550 mTorr). After annealing for
6 h, according to the results of Fourier-transformed infrared
spectroscopy (FTIR), the peaks representing epoxy and carboxyl groups obviously decrease, and the peak for hydroxyl
groups totally disappears. These phenomena become even
more pronounced after annealing for 10 h, and the reduced
graphite oxide can have a C/O ratio of 10. Yang et al. [71]
and Mattevi et al. [66] evaluated the chemical structure
change of GO films on substrates by thermal annealing at different temperatures as well as atmospheres. Annealing at
certain temperatures takes a relatively short time (15 min or
30 min) compared with Jeong’s work. The increase of annealing temperature shows obvious improvement in the deoxygenation of GO. In Yang’s work, the highest temperature
used was only 1000 C, and the highest C/O ratio achieved is
around 14 after annealing at 900 C in ultra high vacuum for
15 min. The good reduction effect by thermal annealing at
around 1000 C was also proved by the high conductivity reported by Becerril et al. [55] and Wang et al. [72].
The results of theoretical simulations and experiments do
not seem to agree with each other. One difference that should
be noted is that the simulations are usually carried out using
Table 2 – Status of oxygen-containing functional groups upon treatment with hydrazine and thermal annealing [123].
Copyright 2009 ACS.
Groups in Fig. 11
A
A0
B
B0
C
D
Hydrazine reduction in room
temperature
Removed
Converted to hydrazino alcohol
Removed
Not removed
Not removed
Partly removed
Thermal annealing at
700–1200 C
Not removed
Not removed
Removed
Removed
Not removed
Removed
Hydrazine reduction
plus thermal annealing
Removed
Not removed
Removed
Removed
Not removed
Removed
CARBON
a model with a low functional group density on a graphene,
but the functional groups in a real GO sheet are more crowded
according to the low C/O ratio (2:1–4:1) detected by elemental
analysis. Boukhvalov and Katsnelson [126] have calculated
the chemisorption energy of oxygen atoms (epoxy groups)
and hydroxyl groups on graphene with different coverages.
Their results indicate that the interaction among adjacent
groups and the lattice distortion caused by the attaching of
functional groups with high coverage to the carbon plane
can make desorption of them much easier. As a result, a
reduction of GO from 75% to 6.25% (C/O ratio 16:1) coverage
is relatively easy, but further reduction seems to be more difficult. Recently, Bagri et al. [127] used molecular dynamics
(MD) simulations to study the atomistic structure of progressively reduced GO. Their results confirm that hydroxyl groups
desorb at low temperatures without altering the graphene
plane. Isolated epoxy groups are more stable, but substantially distort the graphene lattice on desorption. The removal
of carbon from the graphene is more likely to occur when the
initial hydroxyl and epoxy groups are in close proximity to
each other. The reaction pathway between two nearby
functional groups during thermal annealing leads to the formation of carbonyl and ether groups, which are thermodynamically very stable. The chemical changes of oxygencontaining functional groups on the annealing of GO were
elucidated and the simulations reveal the formation of highly
stable carbonyl and ether groups that hinder its complete
reduction to graphene.
As a result, as the experimental work has discovered, a
large number of functional groups can be removed by moderate heating above 200 C with enough time, but the full deoxygenation of GO solely by thermal annealing is rather difficult
even at temperatures as high as 1200 C.
the formation of an aminoaziridine moiety which then undergoes thermal elimination of di-imide to form a double bond
[128,129], resulting in the re-establishment of the conjugated
graphene network.
Kim et al. [122] considered the epoxide reduction with
hydrazine on a graphene monolayer using DFT calculations.
Their results proved that the reduction reaction is mainly
governed by epoxide ring opening which is initiated by hydrogen transfer from hydrazine, and the formation of derivatives
such as NHNH2 during the reduction can facilitate the deepoxidation by lowering the barrier height of the ring-opening
reaction. Gao et al. [123] further elucidated the effect of hydrazine treatment on different functional groups by DFT simulation. Their results show that the hydrazine reduction can only
result in reducing epoxy groups, while no reaction path was
found for the reduction of the hydroxyl, carbonyl and carboxyl groups of GO. They designed several reduction routes
for de-epoxidation by hydrazine, and all the routes start from
the ring-opening of epoxy groups and form hydroxyl groups
on the original sites. According to their calculations (as
shown in former section), hydroxyl groups attached within
an aromatic domain are not stable even at moderate temperatures, and can be removed or migrate to the edges of aromatic domains and restore the conjugated structure after
dehydroxylation.
As a result, a much simple reduction pathway can be expected, in which the reduction of GO is simply the combination of a ring-opening of epoxy groups to form hydroxyl
groups and dehydroxylation by moderate heat treatment.
After this process, the carbon plane of GO can be as clean
as that of pure graphene. This proposal is supported by the
reduction of GO by hot alkaline solutions [99] and hydrohalic
acids [58,97] since the ring-opening reactions can be catalyzed
by both alkalis and acids [130].
Chemical deoxygenation
As is true for thermal reduction, chemical reduction can also
not fully remove the functional groups in GO since the highest C/O ratio reported is no more than 15 [58]. One important
feature of the chemical reduction of GO is that the deoxygenation can happen at low or moderate temperatures with the
help of reducing reagents. Since chemical reduction processes rely on chemical reactions, the chemical deoxygenation may be selective to certain groups depending on the
reducing reagent. But because of the complexity of chemical
reactions, the mechanisms of the chemical reduction of GO
are mostly proposed, and only a few papers have dealt with
the reduction by hydrazine using molecular simulations.
The reduction mechanism by hydrazine was firstly proposed by Stankovich et al. [56] as shown in Fig. 12. This reduction process starts from the ring-opening of epoxy groups by
hydrazine to form hydrazino alcohols, and the initial derivative produced by the epoxide opening reacts further with
5.1.3.
Restoration of long-range conjugated structures
A final target of GO reduction is to achieve as high an electrical conductivity as that of graphene. In ideal graphene, electrons can transport without scattering within a graphene
sheet with a lateral size more than sub-micrometers. This is
called the long-range ballistic transport of graphene, which
relies on its perfect long-range conjugated structure. After
oxidation, this perfect structure is destroyed by functional
groups and defects, so the recovery of conductivity depends
on the restoration of the long-range conjugated structure.
Mattevi et al. [66] proposed a structure evolution of GO during thermal annealing as shown in Fig. 13a–d. Initially, the sp2
clusters in GO are isolated by functionalized and defective
areas (indicated by light gray dots). As the material is progressively reduced, interactions (hopping and tunneling) among
the clusters increase (Fig. 13b). Further reduction by the removal of oxygen leads to greater connectivity among the ori-
Fig. 12 – Proposed reaction pathway for epoxide reduction with hydrazine [56].
Copyr ight
2007 Elsevier.
5.1.2.
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5 0 ( 20 1 2 ) 3 2 1 0–32 2 8
CARBON
5 0 ( 2 0 1 2 ) 3 2 1 0 –3 2 2 8
Copyright 2011 NPG. 2011
3222
Fig. 13 – (a–d) Structural model of GO at different stages of reduction by thermal annealing [66]. (a) Room temperature, (b)
100 C, (c) 220 C, (d) 500 C. The dark grey areas represent sp2 carbon clusters and the light grey areas represent sp3
carbon bonded to oxygen groups (represented by small dots). At 220 C, percolation among the sp2 clusters is initiated
(corresponding to sp2 fraction of 0.6). Copyright 2009 Wiley-VCH. (e–j) Simulated morphology of (e, h) GO and (f, g, i, j) rGO
sheets with an initial oxygen concentration of 20% (e–g) and 33% (h–j) in the form of hydroxyl and epoxy groups in the ratio of
3/2 after annealing at 1500 K in (f, i) vacuum and (g, j) H2 atmosphere [127].
ginal graphitic domains by the formation of new sp2 clusters.
This phenomenon is the restoration of the long-range conjugated structure of GO. According to their results, if the
amount of sp2 structure reaches 60%, the conductivity of GO
meets a threshold of percolation, which is in agreement with
theoretical threshold values for conduction in two dimensional disks [131]. Boukhvalov et al. [126] also predicted that
GO becomes conducting at 25% coverage by functional
groups. That is, if a reduction process can make the C/O ratio
of GO more than 4, an insulating GO sheet can become conductive even though the conductivity is low.
The improvement of conductivity relies on the presence of
more conductive pathways in carbon plane, but not all reduction methods can restore these pathways. Bagri et al. [127]
studied the atomistic structure change of progressively reduced GO using molecular dynamics (MD) simulations, as
shown in Fig. 13e, f, h and i, GO sheets with different oxygen
concentrations (oxygen present in the form of hydroxyl and
epoxy groups in the ratio of 3/2) become more defective and
disordered after thermal annealing at 1500 K. In comparison,
GO with a higher initial oxygen-content is more defective
(Fig. 13f and i), and an increase in the number of vacancy defects results from the desorption of epoxy groups by forming
CO2 and CO gases. The disordered lattice structure is caused
by the re-arrangement of carbon atoms to release the stress
caused by new defects.
More efficient reduction along with healing of rGO was proposed by annealing in the presence of hydrogen [127]. The
structures of rGO with different concentrations of oxygen
atoms annealed in a hydrogen atmosphere are shown in
Fig. 13g and j. Three mechanisms are proposed for the increase
in reduction using hydrogen. The first is the evolution of residual carbonyl pairs through the formation of water molecules
and hydroxyl groups and re-arrangement of the carbon atoms
in the graphene sp2 configuration, which leads to the healing of
holes formed by a carbonyl pair. The second is the formation of
hydroxyl groups with residual ether and epoxy groups in the
presence of a hydrogen atmosphere and the subsequent evolution of the hydroxyl functional groups by thermal annealing
without introducing additional defects. Finally, residual hydroxyl groups are released from the carbon plane by the formation of water molecules. The participation of hydrogen
atoms makes deoxygenation become a series of chemical reactions with a relatively low energy barrier compared with the direct rupture of the C–O bond, which usually needs more energy
than that of breaking C–C bonds in graphene [58].
Consequently, the reduction of GO by chemical reactions
has the advantage of maintaining the structure of the carbon
plane, and thermal annealing at high temperature can facilitate the desorption of various functional groups. As a result, a
combination of chemical reactions and thermal annealing is
more efficient for deoxygenation compared with any one-step
processes by thermal or chemical reduction alone. Experimentally, an astonishingly high C/O ratio of 246 and a relatively high conductivity were obtained by a designed multistep reduction by Gao et al. [42].
CARBON
5.2.
5 0 ( 20 1 2 ) 3 2 1 0–32 2 8
Healing of defects
sequential exposure of GO to CO and NO molecules, but no
experimental result is given to prove this prediction. Dai
et al. [135] presented a strategy for the real-time repair of
the newborn vacancies with carbon radicals produced by
the thermal decomposition of precursors. The sheet conductivity of the monolayer graphene obtained was raised more
than sixfold to 350–410 S/cm with a transparency more than
96%.
Except for the improved conductivity, a very common phenomenon in these reported results is the increase of the
intensity ratio of the D and G bands (ID/IG) in Raman spectra
after the reduction. Lopez et al. [62] even found that the
CVD-GO exhibits an approximately linear rise of electrical
conductivity with increasing ID/IG. Usually, ID/IG is a measure
of disordered carbon, as expressed by the sp3/sp2 carbon ratio
[136] and an increase of ID/IG means the degradation of crystallinity of graphitic materials. The increase of ID/IG is usually
explained as a decrease in the average size but an increase in
the number of sp2 domains upon reduction [137], but this effect obviously cannot be considered as the healing or repairing of defects in GO.
Lucchese et al. [138] studied the evolution of the Raman
spectrum of monolayer graphene by consecutive Ar+ ion bombardment to the sample. The evolution of the resulting ID/IG
data as a function of the average distance between defects
(LD) is shown in Fig. 15b. The ID/IG ratio has a non-monotonic
dependence on LD, increasing with increasing LD up to
LD 4 nm where ID/IG has a peak value, and then decreasing
for LD > 4 nm. Such behavior is explained by the existence of
two disorder-induced competing mechanisms contributing
Copyright 2011 ACS. 2011
Since oxygen-containing functional groups can be well removed by a proper reduction route, what is the main reason
for the low conductivity of rGO compared with that of graphene? As shown in Fig. 14, Gomez-Navarro et al. [50] identified
the atomic scale features of an rGO monolayer that was reduced by hydrogen-plasma [51]. The layers are found to comprise defect-free graphene areas with sizes of a few
nanometers interspersed with defective areas dominated by
clustered pentagons and heptagons. Similar to other lowdimensional carbon nanostructures like carbon nanotubes
[132] and fullerenes [133], disorder and defects in graphene
strongly affect its electronic properties, and thus account
for the low conductivity of as-reduced rGO. Thus, if these lattice defects can be healed during reduction, the GO could possibly behave as perfect graphene. Several studies in this
direction have been tried with the expectation of achieving
much improved conductivity from GO.
Lopez et al. [62] proposed a strategy to repair GO by CVD.
The CVD was carried out using ethylene as a carbon source,
under conditions that are very similar to those in the CVD
synthesis of single-wall carbon nanotubes on SiO2 substrates,
except for the presence of metal catalysts in the latter case.
After the CVD, the CVD-GO has a more than 50-fold increase
in electrical conductivity over the rGO prepared by traditional
reduction methods. Unfortunately, the authors did not give
any direct evidence of the restoration of the graphene structures. Recently, by MD simulation, Wang et al. [134] proposed
a possible way to heal defects as well as doping graphene by
3223
Fig. 14 – Atomic resolution, aberration-corrected TEM image of a single layer H-plasma-reduced-GO membrane [50]. (a)
Original image and (b) with color added to highlight the different features, (c) atomic resolution TEM image of a nonperiodic
defect configuration, (d) partial assignment of the configurations in defective areas, the inset shows a structural model
showing clearly the strong local deformations associated with defects. All scale bar 1 nm.
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5 0 ( 2 0 1 2 ) 3 2 1 0 –3 2 2 8
Copyright 2010 Elsevier. 2010
3224
Fig. 15 – (a) Evolution of the first-order Raman spectra of a monolayer graphene sample deposited on an SiO2 substrate,
subjected to ion bombardment with different doses (indicated next to the respective spectrum in units of Ar+/cm2), (b) the ID/
IG data points from the evolution as a function of the average distance LD between defects. The solid line is the result of a
modeling calculation. The inset to (b) shows ID/IG vs LD plotted on a log scale for the LD axis for two ion-implanted graphite
samples [138].
to the Raman D-band. As a result, the increase of ID/IG ratio
might be caused by the rather small size of sp2 domains within the initial GO sheets. The increase in the size of sp2 domains results in the increase of ID/IG. But one condition
should be confirmed that the area of sp2 domains is very
small. As a result, according to the reported results, the healing effect, even if it exists, is rather weak and is far from the
target to ‘repair’ rGO to form graphene.
5.3.
ported results and the results on GO obtained by the same
reduction treatment but with more severe degrees of
oxidation.
As a result, research on the production of high-quality
graphene through oxidation-and-reduction should be a comprehensive study of both the control of the oxidation of the
raw graphite and choosing the methods to reduce the GO.
The former may be more important to determine the quality
of rGO.
Towards the synthesis of highly reducible GO
6.
A brief conclusion can be given to the effects and mechanism
of GO reduction as follows. Both functional groups and defects affect the conductivity of GO. Functional groups are relatively easy to remove, while defects, whether formed during
oxidation or reduction, are difficult to heal by post-treatment.
Furthermore, functional groups attached to edges and defects
are more difficult to remove than those attached to graphitic
areas. Thus, the concentration of lattice defects in the carbon
plane is the key to determine whether a GO sheet can be well
reduced.
Where do defects come from? According to the simulation
results proposed by Bagri et al. [127], if the carbon plane of GO
is only covered by functional groups with no lattice defects,
reduction can be realized by choosing a proper reduction
method. Lattice defects in the carbon plane that remain after
reduction are more likely formed during oxidation. Recently,
Zhao et al. [34] and Xu et al. [139] reported mildly-oxidized
GO (MOGO) produced by a modified Hummers method with
a low oxidation degree of the graphite. Though the MOGO
sheets are also highly functionalized according to their low
C/O ratio, they preserve the structure of the conjugated carbon framework with relatively low defect concentrations.
Thus, the MOGO can be reduced to become highly conductive
rGO by hydrazine or HI reduction, both of which show much
improved reduction effects compared with most of the re-
Summary and prospects
We have reviewed the reduction of GO to prepare graphenelike rGO. This is an attractive route for the mass-scale production and applications of graphene. Though the full reduction
of GO to graphene is still hard to achieve, partial reduction of
GO is rather easy and tens of reduction methods have been
proposed. The accumulation of experimental phenomena
and theoretical simulation results has provided clearer views
of the structure and chemistry of graphene, GO and rGO, and
this may be helpful in promoting the uses as well as the scientific understanding of the nature of graphene.
Different functional groups in a GO sheet have different
binding energies to the carbon plane according to the type
and location of each group. Epoxy and hydroxyl groups located within a graphitic domain without lattice defects are
relatively easy to remove, while those located on the defective
sites and edges are hard to fully remove. A well-designed
reduction procedure with a combination of chemical reduction and thermal annealing is possible to remove most of
the functionalizations in a GO sheet with low defect concentrations. However, GO sheets with a high concentration of lattice defects are difficult to fully deoxygenate and the defects
themselves are difficult to heal by post-treatment. As a result,
a controllable oxidation during the production of GO is
CARBON
5 0 ( 20 1 2 ) 3 2 1 0–32 2 8
needed to achieve highly reducible GO, which can be converted 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] reported 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 graphene 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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