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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

Article history:

Received 28 September 2011

Accepted 8 November 2011

Available online xxxx

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.11.010

* Corresponding author: Fax: +86 24 2390 312E-mail address: [email protected] (H.-M. C

1 ‘GO’ in this paper refers only to graphene

Please cite this article in press as: Pei S, Cheng

A B S T R A C T

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.

� 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

er Ltd. All rights reserved

6.heng).

oxide, while graphite ox

H.-M. The reduction of g

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

.ide is not abbreviated in this paper.

raphene oxide. Carbon (2011), doi:10.1016/j.carbon.2011.11.010

http://dx.doi.org/10.1016/j.carbon.2011.11.010

mailto:[email protected]




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2 C A R B O N x x x ( 2 0 1 1 ) x x x – x x x

graphene, chemically modified graphene, chemically con-

verted graphene, or reduced graphene [11]. The most straight-

forward goal of any reduction protocol is to produce

graphene-like materials similar to the pristine graphene ob-

tained from direct mechanical exfoliation (i.e. the ‘‘Scotch

tape method’’) of individual layers of graphite both in struc-

ture 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 car-

bon plane, therefore, it is not appropriate to refer to rGO, even

today, simply as graphene since the properties are substan-

tially different.

Nowadays, in addition to reduction from GO, graphene can

be produced by micro-mechanical exfoliation of highly or-

dered 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 hydro-

philic 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 de-

vices composed of rGO. Though the final target to achieve per-

fect 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.

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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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 graph-

ite with a water-free mixture of concentrated sulfuric acid, so-

dium 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 pre-

cise chemical structure of GO is still not quite clear, which

contributes to the complexity of GO due to its partial amor-

phous character. Several early investigations have proposed

structural models of GO with a regular lattice composed of dis-

crete repeat units [38], and the widely accepted GO model pro-

posed by Lerf and Klinowski [39,40] is a nonstoichiometric

Please cite this article in press as: Pei S, Cheng H.-M. The reduction of g

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 modifica-

tions to the proposed structure including the presence of 5-

and 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 con-

sist 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 covalently-

bonded 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 micro-

scope (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. Surpris-

ingly, 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 ran-

dom 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 long-

range 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



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].

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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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C A R B O N x x x ( 2 0 1 1 ) x x x – x x x 3

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 graph-

ene and removing other atomic-scale lattice defects, but is

also aimed at recovering the conjugated network of the gra-

phitic lattice. These structure changes result in the recovery

of electrical conductivity and other properties of graphene.

3. Criteria used in determining the effect ofreduction

Since reduction can make a great change in the microstruc-

ture and properties of GO, some obvious changes can be di-

rectly 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 dra-

matically improve the electrical conductivity of GO, the in-

creased 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 precipita-

tion from the original yellow–brown suspension, which is

probably a result of an increase in the hydrophobicity of the




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 properly-

selected 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 con-

firms 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 pris-

tine graphene sheets lying on the same substrate.

Fig. 4 – The C1s XPS spectra of (a) GO and (b) rGO

[56].

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3.2. Electrical conductivity

Graphene is reported to have a high electrical conductivity. A

few-layer graphene sheet (thickness < 3 nm) has a sheet resis-

tance (Rs) of around 400 X/sq at room temperature [1]. Re-

cently, Bae et al. [60] have reported the production of

graphene films by CVD. After transferring them to transpar-

ent 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 thick-

ness [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 conduc-

tivity (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 ¼1rt

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 conduc-

tivity 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 dis-

cussed 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


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 anal-

ysis measurements by combustion, and also by X-ray photo-

electron 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 ele-

mental analysis gives the bulk composition while XPS is a sur-

face 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 carbon-

based 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 spec-

trum 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 ra-

tio of typical reports on the reduction of GO. The details on

each reduction method will be discussed in Section 4.



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.


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.


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,



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].

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Fig. 6 – XPS spectra of GO sheets annealed in 2 Torr of (a) NH3/Ar (10% NH3) and (b) H2 at various temperatures [77].

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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].

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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 atmo-

sphere 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 ther-

mal annealing at 1000 �C, and found that a quality vacuum

(<10�5 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 con-

ductivity 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.




As shown in Fig. 6, the highest doping level of �5% N is ob-

tained at 500 �C, and electrical measurements of GO sheets

demonstrate that GO annealed in NH3 exhibits a higher con-

ductivity 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 tem-

perature annealing is highly effective. But the drawback of

thermal annealing is also obvious. First, high temperature

means large energy consumption and critical treatment con-

ditions. 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 heat-

ing 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.

4.1.2. Microwave and photo reductionThermal annealing is usually carried out by thermal irradia-

tion. As an alternative, some unconventional heating re-

sources have been tried to realize thermal reduction

including microwave irradiation (MWI) [79,80] and photo-irra-

diation [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].

Fig. 8 – Patterned rGO film obtained by (a) flash reduction [81] (Co

Scale bars, 10 lm (Copyright 2009 Elsevier). The black parts in t


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 ex-

panded 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 elec-

tronic devices based on rGO films, as shown in Fig. 8a.

A further improvement of the photo-reduction and pat-

terned film fabrication was carried out with femtosecond la-

ser 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 objec-

tive 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 10�1–101 lm. As a result, the laser

reduction can produce rGO films with a much higher conduc-

tivity of 256 S/cm, and the rGO film patterns can be drawn di-

rectly by a pre-programmed laser on the GO film to form more

complicated and delicate circuits as shown in Fig. 8b–e.

4.2. Chemical reduction

4.2.1. Chemical reagent reductionReduction 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 chem-

ical 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 be-

fore 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

pyright 2009 ACS) and (b–e) femtosecond laser reduction [82].

he films are the reduced GO patterns.




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 deriv-

atives, 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 sol-

uble polymers [84] as surfactant, or ammonia [69] to change

the charge state of rGO sheets. The graphene sheets sus-

pended in colloidal solutions can be used to assemble macro-

scopic 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 unfortu-

nately, 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

Fig. 9 – Optical photographs of GO films (a) before and (b–d) afte

vapor, (d) 85% N2H4ÆH2O (N2H4), 50 mM NaBH4 solution (NaBH4) a

the stress–strain curve of the GO film and HI reduced GO film (r =

of GO films (f) before and (g, h) after reduction by (g) HI and (h) hy


an additional dehydration process using concentrated sulfu-

ric acid (98% H2SO4) at 180 �C after reduction by NaBH4 to fur-

ther 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 pro-

duced by hydrazine in a parallel experiment. In addition, VC

has great advantage of its non-toxicity in contrast to hydra-

zine 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 an-

other 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 con-

ductivity 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 envi-

ronment, 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-

r chemical reduction by different agents: (b) HI, (c) hydrazine

nd 55% HI after immersion for 16 h at room temperature, (e)

stress, e = strain), and SEM images of the cross-section views

drazine vapor [58].

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duced by N2H4ÆH2O and NaBH4 solutions broke up into pieces.

These results show that HI not only has a better reducing ef-

fect 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], hydroxyl-

amine [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 reductionDifferent 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 par-

tial restoration of the conjugated network in the carbon plane

like that in chemical reduction processes.

The photocatalytic properties of semiconducting TiO2 par-

ticles 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 pres-

ence 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 nano-

particles and the GO sheets, and this structure can be re-

tained 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 de-

vice [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 re-

ported 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].

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4.2.3. Electrochemical reductionAnother method that shows promise for the reduction of GO

relies on the electrochemical removal of oxygen functional-

ities [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 reduc-

tion 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 charg-

ing 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 reduc-

tion 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 re-

duced on the anode surface during EPD, which seems coun-

ter-intuitive to the general belief that oxidation occurs at

the anode in an electrolysis cell. Though the reduction mech-

anism is not clear, the simultaneous film assembly and reduc-

tion might be favorable to some electrochemical applications.

4.2.4. Solvothermal reductionAnother emerging chemical reduction method is solvother-

mal reduction [59,114,115]. A solvothermal process is per-

formed 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 hydro-

thermal process, overheated supercritical (SC) water can play

the role of reducing agent and offers a green chemistry alter-

native 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. Hydro-

thermal routes have been used for remarkable transforma-

tion of carbohydrate molecules to form homogeneous

carbon nanospheres [117,118] and nanotubes [119].

Zhou et al. [59] proposed a ‘‘water-only’’ route by hydro-

thermal 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 hydro-

thermal 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




is believed to be analogous to the H+-catalyzed dehydration of

alcohol, where water acts as a source of H+ for the proton-

ation of hydroxyl groups.

Wang et al. [114] reported the reduction of GO by solvother-

mal 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 reduc-

tion method using N-methyl-2-purrolidinone (NMP) as sol-

vent. 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 conduc-

tivity 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 re-

duced GO is only 5.15, much lower than the other results

shown above. Therefore, this moderate temperature NMP-

only solvothermal reduction method can only achieve a mod-

erate reduction of GO.

Besides the above features of each method, these solvo-

thermal reduction methods have the common merit that all

can produce a stable dispersion of rGO sheets, which is valu-

able 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 ef-

fect 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 de-

crease the annealing temperature needed to obtain the good

reduction of a GO film. The combination of hydrazine reduc-

tion and low temperature thermal annealing at 200 �C could

produce an rGO film with better conductivity than that pro-

duced by only thermal annealing at 550 �C, which is impor-

tant for flexible devices attached to polymer substrates.

In most chemical reactions, the effect of reagents is selec-

tive. As a result, one reducing reagent usually cannot elimi-

nate 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, lac-

tol, ester and most alcohol groups. Treatment in concentrated

H2SO4 at 180 �C can then dehydrate the remaining alcohol


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 con-

ductivity 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 de-

crease 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 mecha-

nisms 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 elim-

ination of functional groups and the healing of structural de-

fects. For the elimination of functional groups, there are also

two effects that should be considered: whether the oxygen-

containing 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 ex-

tra carbon supply.

5.1. Elimination of functional groups

The conductivity of monolayer graphene mainly relies on car-

rier 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, car-

bonyl 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 pre-

served after reduction. This can be used to disperse rGO

sheets in a basic solution but has little influence on the con-

ductivity of rGO sheets and assembled films.




5.1.1. Thermal deoxygenationThough reduction methods are different from each other, the

nature of deoxygenation is common for the removal of oxy-

gen-containing groups from the graphene. The binding en-

ergy (or dissociation energy) between graphene and

different oxygen-containing functional groups can be an

important index to evaluate the reducibility of each group at-

tached 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-car-

bon-atom graphene unit, which indicates that epoxy groups

are much more stable than hydroxyl groups in GO. In a calcu-

lation 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 re-

sult, 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].

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Table 2 – Status of oxygen-containing functional groups upon tCopyright 2009 ACS.

Groups in Fig. 11 Hydrazine reduction in roomtemperature

A RemovedA 0 Converted to hydrazino alcoholB RemovedB 0 Not removedC Not removedD Partly removed


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 at-

tached to the edges of GO is 650 �C, and only above this tem-

perature can hydroxyl groups be fully removed. For

dehydroxylation, a hydroxyl group is believed to more favor-

ably leave the graphene sheet directly, producing an OH rad-

ical 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 re-

duced 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 oxy-

gen-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 car-

boxyl 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 dif-

ferent 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 anneal-

ing temperature shows obvious improvement in the deoxy-

genation 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 re-

ported 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

reatment with hydrazine and thermal annealing [123].

Thermal annealing at700–1200 �C

Hydrazine reductionplus thermal annealing

Not removed RemovedNot removed Not removedRemoved RemovedRemoved RemovedNot removed Not removedRemoved Removed




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 dif-

ficult. Recently, Bagri et al. [127] used molecular dynamics

(MD) simulations to study the atomistic structure of progres-

sively reduced GO. Their results confirm that hydroxyl groups

desorb at low temperatures without altering the graphene

plane. Isolated epoxy groups are more stable, but substan-

tially 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 for-

mation of carbonyl and ether groups, which are thermody-

namically very stable. The chemical changes of oxygen-

containing 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 moder-

ate heating above 200 �C with enough time, but the full deox-

ygenation of GO solely by thermal annealing is rather difficult

even at temperatures as high as 1200 �C.

5.1.2. Chemical deoxygenationAs is true for thermal reduction, chemical reduction can also

not fully remove the functional groups in GO since the high-

est C/O ratio reported is no more than 15 [58]. One important

feature of the chemical reduction of GO is that the deoxygen-

ation can happen at low or moderate temperatures with the

help of reducing reagents. Since chemical reduction pro-

cesses rely on chemical reactions, the chemical deoxygen-

ation 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 pro-

posed by Stankovich et al. [56] as shown in Fig. 12. This reduc-

tion process starts from the ring-opening of epoxy groups by

hydrazine to form hydrazino alcohols, and the initial deriva-

tive produced by the epoxide opening reacts further with

Fig. 12 – Proposed reaction pathway for e


the formation of an aminoaziridine moiety which then under-

goes 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 hydro-

gen transfer from hydrazine, and the formation of derivatives

such as NHNH2 during the reduction can facilitate the de-

epoxidation by lowering the barrier height of the ring-opening

reaction. Gao et al. [123] further elucidated the effect of hydra-

zine treatment on different functional groups by DFT simula-

tion. 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 car-

boxyl 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 temper-

atures, and can be removed or migrate to the edges of aro-

matic domains and restore the conjugated structure after

dehydroxylation.

As a result, a much simple reduction pathway can be ex-

pected, in which the reduction of GO is simply the combina-

tion 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].

5.1.3. Restoration of long-range conjugated structuresA final target of GO reduction is to achieve as high an electri-

cal conductivity as that of graphene. In ideal graphene, elec-

trons 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 dur-

ing 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 progres-

sively reduced, interactions (hopping and tunneling) among

the clusters increase (Fig. 13b). Further reduction by the re-

moval of oxygen leads to greater connectivity among the ori-

poxide reduction with hydrazine [56]. �C

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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].

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ginal graphitic domains by the formation of new sp2 clusters.

This phenomenon is the restoration of the long-range conju-

gated 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 dimen-

sional 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 con-

ductive even though the conductivity is low.

The improvement of conductivity relies on the presence of

more conductive pathways in carbon plane, but not all reduc-

tion methods can restore these pathways. Bagri et al. [127]

studied the atomistic structure change of progressively re-

duced 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 de-

fects 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 pro-

posed 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 resid-

ual 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 evolu-

tion of the hydroxyl functional groups by thermal annealing

without introducing additional defects. Finally, residual hy-

droxyl groups are released from the carbon plane by the for-

mation of water molecules. The participation of hydrogen

atoms makes deoxygenation become a series of chemical reac-

tions with a relatively low energy barrier compared with the di-

rect 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 facili-

tate 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. Experi-

mentally, an astonishingly high C/O ratio of 246 and a rela-

tively high conductivity were obtained by a designed multi-

step reduction by Gao et al. [42].




5.2. Healing of defects

Since oxygen-containing functional groups can be well re-

moved by a proper reduction route, what is the main reason

for the low conductivity of rGO compared with that of graph-

ene? As shown in Fig. 14, Gomez-Navarro et al. [50] identified

the atomic scale features of an rGO monolayer that was re-

duced by hydrogen-plasma [51]. The layers are found to com-

prise defect-free graphene areas with sizes of a few

nanometers interspersed with defective areas dominated by

clustered pentagons and heptagons. Similar to other low-

dimensional 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 lat-

tice defects can be healed during reduction, the GO could pos-

sibly 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 struc-

tures. Recently, by MD simulation, Wang et al. [134] proposed

a possible way to heal defects as well as doping graphene by

Fig. 14 – Atomic resolution, aberration-corrected TEM image of

Original image and (b) with color added to highlight the differen

defect configuration, (d) partial assignment of the configuration

showing clearly the strong local deformations associated with d


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 conduc-

tivity 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 phe-

nomenon 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 crys-

tallinity 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 ef-

fect obviously cannot be considered as the healing or repair-

ing of defects in GO.

Lucchese et al. [138] studied the evolution of the Raman

spectrum of monolayer graphene by consecutive Ar+ ion bom-

bardment to the sample. The evolution of the resulting ID/IGdata 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

a single layer H-plasma-reduced-GO membrane [50]. (a)

t features, (c) atomic resolution TEM image of a nonperiodic

s in defective areas, the inset shows a structural model

efects. All scale bar 1 nm.

�C

op

yri

gh

t2

01

1A

CS

.2

01

1



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].

�C

op

yri

gh

t2

01

0E

lsev

ier.

20

10


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 with-

in the initial GO sheets. The increase in the size of sp2 do-

mains 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 heal-

ing effect, even if it exists, is rather weak and is far from the

target to ‘repair’ rGO to form graphene.

5.3. Towards the synthesis of highly reducible GO

A brief conclusion can be given to the effects and mechanism

of GO reduction as follows. Both functional groups and de-

fects affect the conductivity of GO. Functional groups are rel-

atively 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 car-

bon 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-


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 com-

prehensive 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.

6. Summary and prospects

We have reviewed the reduction of GO to prepare graphene-

like rGO. This is an attractive route for the mass-scale produc-

tion 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 sci-

entific 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 lo-

cated 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 reduc-

tion and thermal annealing is possible to remove most of

the functionalizations in a GO sheet with low defect concen-

trations. However, GO sheets with a high concentration of lat-

tice 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




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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