Synthesis of graphene-based nanomaterials and their application in energy-related and environmental-related areas

Guixia Zhao, Tao Wen, Changlun Chen and Xiangke Wang*

Received 20th May 2012, Accepted 23rd July 2012

DOI: 10.1039/c2ra20990j

As a fascinating two-dimensional carbon allotrope, graphene has triggered a gold rush all over

scientific research areas especially since the Nobel Prize for Physics in 2010. To exploit the prominent

properties of graphene-based nanomaterials, two important problems are focused in this review: one

is the synthesis of these graphene-based nanomaterials with different kinds of well-defined structures,

and the other is the effective application of them as active nanomaterials in functional devices or

processes. In this critical review, from the viewpoint of chemistry and materials, we give a brief

overview of the recent significant advances in the synthesis of graphene-based nanomaterials and their

applications in energy-related areas and environmental pollution remediation areas, including

supercapacitors, lithium ion batteries, solar cells, adsorption, and degradation of organic/inorganic

pollutants from large volumes of aqueous solutions in environmental pollution cleanup. The main

challenges and perspectives of the materials for future research are also discussed.

1. Introduction

Owing to its extraordinary structure and properties, graphene, a

honeycomb network of sp2 carbon atoms, with single-atom

thickness, has attained a central position as a miraculous

material in the beginning of the 21st century. Graphene exhibits

excellent electronic, optical, mechanical and thermal pro-

perties, such as the high values of its mobility of charge carriers

(200 000 cm2 V21 s21),1 fracture strength (125 GPa),2 Youngs mo-

dulus (y1100 GPa),2 thermal conductivity (y5000 W m21 K21),3

specific surface area (theoretical value of 2630 m2 g21),4 and high

optical transmittance.4 These attractive properties have triggered

huge interest from different research fields concerned with energy

conversion/storage and environmental pollution remediation,

both of which are the most pressing and hottest issues in modern

society. However, for those practical applications in super-

capacitors, lithium ion batteries, solar cells, adsorption and

degradation of different kinds of pollutants from large volumes of

aqueous solutions, graphene needs to be available and proces-

sable in large quantity and good quality. To fully utilise the

superior properties of graphene, fabrication of graphene compo-

sites with other functional materials is another important

academic and technological endeavor, for which extraordinary

performances have been witnessed in many scientific research

areas. For instance, in energy-related areas, modified graphene

materials have been used in solar cells5 while metallic/metal

oxides combined with graphene have been used in lithium ion

batteries,6 supercapacitors7 and in fuel cells as catalysts.8 In the

environmental pollution remediation area, many graphene and

magnetic graphene nanomaterials have been used as adsorbents

for heavy metal ions and organic pollutants9 while several

transition-metal oxide graphene hybrids have been used for the

degradation of toxic organic pollutants.10 Some graphene-based

materials have been fabricated as sensor devices for pollutant

analysis.11 These treatments toward graphene not only im-

prove its pristine properties, but also introduce some specific

functional groups or other functional nanomaterials on the

surfaces of graphene layers. Thus, these studies have extended

the research and application of graphene-based nanomaterials

in multidisciplinary areas. Although some researchers have

studied the toxicity of graphene nanosheets,12 we believe that

their intriguing properties outweigh possible biohazards and it

is hoped that any possible toxicity can be reduced or avoided in

the near future.

Herein, the recent significant advance in the synthesis of

graphene-based nanomaterials and their applications in energy-

related areas and environmental-related areas, including super-

capacitors, lithium ion batteries, solar cells and organic/

inorganic pollutant management, are focused and reviewed.

We believe that this comprehensive review will provide a general

understanding to the present research about graphene-based

nanomaterials and their perspective in future nanomaterials

research, although clearly it can not include all of the published

work, due to the large activity of this field.

2. Synthesis of graphene-based nanomaterials

Since the micromechanical exfoliation of graphene in 1999, a

wide variety of studies have been focused on its fabrication. To

realize their various applications, graphene-based nanomaterials

Key Laboratory of Novel Thin Film Solar Cells, Institute of PlasmaPhysics, Chinese Academy of Sciences, Hefei, 230031, China.E-mail: xkwang@ipp.ac.cn; Fax: +86-0551-5591310;Tel: +86-0551-5592788

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are firstly needed to be synthesized with controlled size, thickness

and morphology. Therefore, the development of effective

approaches for their synthesis is of crucial importance. Up to

now, there are two main strategies for the synthesis of graphene

originated from different carbon sources, the top-down and

bottom-up approaches.

2.1. The top-down approach

The graphitic materials, such as 0D fullerenes, 1D nanotubes and

3D graphite, can be chosen as carbon source for the synthesis of

graphene. The most popular top-down method is via the

oxidative exfoliation of graphite through the Staudemaier,

Brodie and Hummers methods.13 The prepared graphite oxides

are further expanded into one- or few-layered graphenes by fast

heating-treatment or exfoliated into individual graphene oxide

nanosheets by ultrasonic treatment in aqueous solution. In

Chens group, a lot of research on the controlled synthesis of

graphene oxides has been carried out.14 Fig. 1 shows the basic

process of the oxidative exfoliation. The graphene oxides are

then reduced into graphene by means of thermal annealing,15

solvothermal reduction,16 electrochemical reduction,17 hydrogen

plasma treatment,18 or radiation-induced reduction,19 etc.

Among these methods, the most frequently used method is

chemical reduction using reductants such as hydrazine, sodium

borohydride, metal Fe, Vitamin C, dimethylhydrazine, alcohols

and hydroquinone.20 By using sulfur-containing compounds

such as NaHSO3, Na2SO3, Na2S2O3, Na2S, Chen et al.21 found

that the reducing ability of NaHSO3 is comparable to that of

hydrazine, with the advantages of low toxicity and non-

volatility. However, these reductions are processed at high

reduction temperature. Recently, great efforts have been devoted

to develop mild reduction processes. For example, Kaminska

et al.22 proposed an easy and environmental friendly method for

the reduction and simultaneously noncovalent functionalization

of graphene oxide by using dopamine at room temperature.

Under a similar condition, Tung et al.23 presented a solution-

based approach to synthesize large-scale and single-layer

chemically converted graphene by directly dispersing graphene

oxide paper in pure hydrazine. At a lower temperature (i.e.,

subzero temperature), a new reducing agent system, hydriodic

acid with trifluoroacetic acid can chemically reduce graphene

oxide into graphene according to Cui et al.24 The above

mentioned methods are processed in solution, while Liang

et al.25 put forward a facile green approach for flexible graphene

film by the reduction of graphene oxide in the gas phase, in

which a solid graphene oxide film can be efficiently reduced by

hydrogen at room temperature with a small amount of Pd as

catalyst. Thermal expansion of graphite oxide is a useful method

for the synthesis of functionalized single-layered graphene.15a,26

McAllister and co-workers26 provided a detailed analysis on the

thermal expansion mechanism. It is considered that the

exfoliation takes place when the decomposition rate of the

epoxy and hydroxyl sites of graphite oxide exceeds the diffusion

rate of the evolved gases.26 Peng et al.17 introduced an

electrochemical method to reduce graphene oxide under constant

potential. By control of the electrical current, applied voltage,

reduction time and the amount of precursor (graphene oxide),

high quality electrochemically reduced graphene oxide film with

controllable size and thickness was obtained. More conveniently,

Kumar et al.19 reported the radiation-induced reduction of

graphene oxide by using sunlight, UV light and KrF excimer

laser. It has been found that after prolonged irradiation under

sunlight or ultraviolet light, graphene oxide can be well reduced.

Within a short time under laser irradiation, graphene with

negligible oxygen functionalities can be obtained.19

It is obvious that the exfoliation of the graphite oxide and the

subsequent chemical reduction are convenient for the mass

production, functionalization and solution processing of gra-

phene. However, this method inevitably introduces large

amounts of defects into the graphene framework because of

the formation of oxygen-containing groups, the ultrasonic

treatment and the incomplete reduction. These defects lead to

deterioration of the electronic properties of the reduced graphene

oxide. Several groups reported that pristine graphite could be

exfoliated to defect-free graphene monolayers in different

solvents such as N,N-dimethylformamide (DMF), ortho-dichlor-

obenzene, N-methylpyrrolidone (NMP), benzylamine, ionic

liquid, or in some surfactantwater solutions. For example, as

shown in Fig. 2, Hernandez et al.27 demonstrated that through

dispersion and exfoliation of graphite in organic solvents such as

Fig. 1 (A) Outlined oxidization/intercalation process for the preparation of FGO and GO; (B) XRD patterns of FGO and GO.14b

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N-methylpyrrolidone, whose surface energy matches that of

graphene, the graphite layers can be exfoliated because the

required energy for the exfoliation is balanced by the solvent

graphene interaction. One of the problems for this method is that

the effective solvents often have high boiling points, which makes

it difficult to remove the solvents. According to ONeill et al.,28

the exfoliation of graphene at higher concentration in low-

boiling point solvents such as chloroform and isopropanol can

be achieved, with a thickness of less than 10 layers. Apart from

these exfoliation methods, high-yield graphene with good quality

was also prepared via the exfoliationreinteractionexpansion of

graphite, where graphite was treated with oleum without

excessive chemical functionalization.29 Through fluorination of

graphite, thermal expansion process and the followed ultrasonic

treatment, the obtained graphene can be dispersed well in

organic solvents or in aqueous solutions containing sodium

dodecylbenzene sulfonate as surfactant according to Lee et al.30

There is also a more facile method for the exfoliation of

graphite oxide by using laser excitation other than using any

chemical reducing agent.31 Abdelsayed et al.31a reported a

solution processable synthesis of individual graphene nanosheets

in water without reductants. According to their report, the

authors confirmed the high performance of graphite oxide and

laser converted graphene in the conversion of the laser radiation

into usable heat. More specifically, Sokolov et al.31b found

that graphene features could be produced via continuous-wave

(532 nm) or pulsed (532 and 355 nm) laser excitation of graphite

oxide. According to the Sokolov et al.,31b initial excitation results

in electronhole plasmas within the material. Then the strain

associated with the oxygen-containing groups leads to the

trapping of excitons and holes. The phonon coupling results in

efficient heating and material removal, which lead to an

expanding plasma plume. Finally, the formation and growth of

graphene nanoparticles/sheets in the gas phase and on the

surface are realized.31b

Another interesting carbon source for top-down synthesis of

graphene is carbon nanotubes (CNTs), especially for the

synthesis of graphene nanoribbons (GNRs). The chemical

unzipping of CNTs resulted in elongated graphene strips by

using acid reactions,32 plasma treatment,33 liquid NH3 and Li

intercalationexfoliation34 and catalytic approaches.35 Accord-

ing to Kosynkin and co-workers,32 a high-yielding procedure for

the fabrication of single- and few-layered GNRs through

oxidative longitudinal unzipping of multiwalled carbon nano-

tubes (MWCNTs) in sulfuric acid was realized. However, due to

the excessive oxidation, those narrow ribbons derived from the

inner tubes are readily destroyed while the contiguous regions of

the basal plane from the wider ribbons are often disrupted,

resulting in holes of various shapes and sizes; as a consequence

the electronic performance is detrimentally affected.32 To over-

come this disadvantage an improved method was further

proposed and changes in the reaction conditions such as acid

ratio, time and temperature were investigated (see Fig. 3). It was

found that sufficiently concentrated H2SO4 (y90 vol%) and theelevation of the reaction temperature to 60 uC were importantfor the formation and exfoliation of GONRs, and the addition of

10 vol% of a second acid (H3PO4 or trifluoroacetic acid) can

greatly enhance the quality of the GONRs.36 In Dais group,37

the atomic structures, Raman spectroscopic and the electrical

transport properties of individual graphene nanoribbons

obtained by sonochemical unzipping of MWCNTs were clearly

investigated, and they found that a large fraction of GNRs were

bent with smooth edges, and most of them were two-layered.

To better control the quality of graphene layers along with the

high-field preparation, Shinde et al.38 used electrochemical

oxidation to accurately control the degree and sites of oxidation

under ambient conditions with the unique advantage of thickness

and orientation control. In their method, an interfacial electric

field is able to orient the CNTs and longitudinal unzipping is

more favorable instead of a random breakdown in chemical

methods because possible CC cleavage initiated at topological

defects has sufficient strain which makes the CC cleavage is

easy to realize.

In 2011, Perdigao and co-workers39 reported graphene

formation by the thermal induced decomposition of C60 in

combination with a Ni thin film. The formation of graphene was

verified by releasing the resulting layers by etching the metal

substrate. The main processes are illustrated in Fig. 4. They also

found that the carbon present at a buried metal/SiO2 layer can

diffuse and segregate at the surface, and graphene can grow from

the adsorbed carbon.39 However, from the high D/G intensity

ratio in the Raman spectra, it can be concluded that graphene

produced by this method appears to be highly defective.

There are some new methods for graphene synthesis.40

Theoretically, Miyamato et al.40a proposed to use ultrashort

laser pulses to detach graphene monolayers from graphite, one at

a time. After calculations by an ab initio study, the authors

inferred that photo-exfoliation should be able to produce intact

graphene monolayers free of contaminants and defects at a high

rate. Laser-induced melting of graphite under different pressure

conditions has been investigated by Garcia and Jeschke.40b Their

results showed that two steps were involved in the laser-induced

melting process: the destruction of the graphite sheets via bond

breaking and merging of the melted layers. Under the external

pressure (10 GPa), the separation of the two steps is more

evident for graphite, although it is also present in graphite films

at normal pressure. Experimentally, Carey et al.40c investigated

the interaction between the laser and graphite from various laser

Fig. 2 Electron microscopy of graphite and graphene: (a) SEM image

of sieved, pristine graphite (scale bar: 500 mm). (b) SEM image of

sediment after centrifugation (scale bar: 25 mm). (ce): Bright-field TEM

images of monolayer graphene flakes deposited from c-butyrolactone

(GBL) (c), 1,3-dimethyl-2-imidazolidinone (DMEU) (d) and NMP (e),

respectively (scale bar: 500 nm). (f), (g), Bright-field TEM images of a

folded graphene sheet and multilayer graphene, both deposited from

NMP (scale bar: 500 nm). (h), Histogram of the number of visual

observations of flakes as a function of the number of monolayers per

flake for NMP dispersions.27

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Fig. 3 a Previously published conditions and results.32 b Weight equivalents of KMnO4 relative to MWCNT weight in the starting material.c Weight

gain (%) upon isolation of the product relative to the starting weight of MWCNTs. d All scale bars are 100 nm. e Weight percent remaining at 250 uC byTGA at 10 uC min21 under Ar. f Weight percent remaining at 950 uC. g Recorded in water. h The darker portion in the TEM micrograph is part of thelacey carbon grid. i Preferred conditions for optimized GONRs.36

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and graphite parameters, which supports well the formal

theoretical researches. Recently, Zhao et al.40d attempted to

synthesize graphene by etching of graphite using a low-

temperature plasma technique. The firstly introduced H2O2plasma technique is an easy and environmental friendly method

although the yield, as yet, is not high enough. The oxidation and

etching of reduced graphene oxide by thermal oxidation in air,

ultraviolet-generated ozone, microwave oxygen plasma and

scanning tunneling microscopy lithography have been investi-

gated by Sols-Fernandez et al.41 It was found that reduced

graphene oxide exhibited a higher reactivity toward oxidation

than the pristine graphite. Another new wet chemical bulk

functionalization route using pristine graphite was achieved by

coupling reductive graphite activation with oxidative arylation

by organic aryldiazonium salts in a single-step procedure,

resulting in single-layered graphene without reaggregation and

substrate-induced doping.42

Graphene obtained through the top-down approach has been

used to blend with other components to synthesize functional

composites because of the intriguing properties derived from the

synergistic effects of the components and graphene. For example,

Chidembo et al.43 employed an efficient and versatile in situ spray

pyrolysis method to synthesize globular metal oxidegraphene

composites (rGO-Co3O4 or rGO-NiO) with highly porous

morphologies. Zhao et al.44 mixed a graphene oxide solution

with a Ni(NO3)2 solution, then added NH4HCO3 to produce basic

nickel carbonate depositing on both sides of graphene layers. The

precursor was heated at 400 uC to obtain graphene/NiOnanocomposites. Liang et al.45 reported well-distributed TiO2nanocrystals grown on graphene as shown in Fig. 5. This synthesis

process has been applied to obtain other transition-metal oxides/

graphene composites, such as MnO2/GNS,46 Fe3O4/GNS,

47 TiO2/

GNS,48 SnO2/GNS,49 Co3O4/GNS.

50

Apart from complexation with inorganic compounds, graphene

also has been utilized in the synthesis of polymergraphene

composites. For instance, in situ reduction of graphite oxide in

polymer powder has been carried out using focused solar

electromagnetic radiation, where graphene oxide is completely

reduced in polyvinylidene fluoride (PVDF) through photo-

chemical reduction, resulting in highly conducting graphene

PVDF composites.51 Tang et al.52 reported a simple casting of a

polymer solution which contains dispersed graphene oxide,

followed by thermal reduction, for producing nanocomposites

containing well-isolated monolayer reduced-graphene oxide

nanosheets. Potts et al.53 firstly reported polymer composites

using microwave-exfoliated graphite oxide (MEGO) to produce

MEGO/polycarbonate composites at various loadings, which

resulted in improvements of multifunctional properties as

compared with neat polycarbonate. In these composites, graphene

and polymer are noncovalently mixed together. By chemically

grafting an organosilane, 3-aminopropyl triethoxysilane, onto the

graphene skeleton, a covalent functionalization of graphene

nanosheets (f-GNS) was realized by Wang and co-workers.54

The top-down approach is widely used for graphene synthesis

in large quantity, and the resulting graphene nanosheets can be

Fig. 4 Graphene synthesized from C60 buried under a nickel film on a

SiO2 surface. (a) The preparation procedure performed in vacuum: (I)

y1.6 nm of C60 was deposited on a predegassed SiO2 surface, (II) nickelfilm was grown by evaporation, and then (III) the whole assembly

was annealed at a chosen temperature for 15 min. (bd) tapping mode

2 6 2 mm AFM images of samples taken out after stages IIII,respectively. (e) Raman spectra of transferred graphene prepared at

different annealing temperatures, with the peak assignments D, G, D9,

2D and D + G shown by dashed lines.39

Fig. 5 (a) SEM image, (b) low magnification and (c) high magnification

TEM images of TiO2 nanocrystals grown on GO sheets. The scale bar is

400 nm for the SEM image in (a) and 20 nm for the TEM image in (b).

(d) An XRD pattern of the graphene/TiO2 nanocrystals hybrid.45

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conveniently complexed with other functional components.

However, this method has several disadvantages such as: (i)

use of hazardous and toxic reagents (i.e., concentrated sulfuric

acid, organic solvents) in the synthesis processes; and (ii) the as-

prepared graphene frameworks contain a large amounts of

defects, which limit the application of synthesized graphene in

many research areas.

2.2. The bottom-up approach

Different from the top-down chemical approach, which starts

from graphitic materials and provides a feasible way for the mass

production of graphene, the bottom-up approach starting from

small organic molecules is another important method for precise

control over the morphology and structure of graphene. It has

been well documented that the decomposition of hydrocarbons

into graphitic materials can be catalyzed by metal surfaces

through chemical vapor deposition (CVD).55 Hydrocarbon gases

such as methane were chosen as carbon source and large

domains of single-crystalline graphene were successfully grown via

different CVD processes on various metal substrates. One of the

best templates for the deposition is the Ni(111) surface due to the

small lattice mismatch of this surface with the graphene surface.

It has been reported that graphene films with resistances of

280 V sq21 (80% transparent) and 770 V sq21 (90% transparent)

were realized through the growth of graphene on Ni film.56

Furthermore, the control in graphene scale makes CVD the most

attractive method for devices fabrication. However, precise

controlling the edge structure and topology of graphene is still a

great challenge for the CVD method. Xu et al.57 produced

extended single-layer and centimeter-scale graphene on a nickel

surface deposited on a highly oriented pyrolytic graphite substrate

by the diffusion of carbon atoms through the nickel template.

Their results demonstrate the optimization of the relevant

parameters for graphene growth (annealing time and temperature)

to yield fine control of thickness and structure of the graphene

layers. Fig. 6 shows the synthetic process for patterned graphene

film growth on the thin nickel layers.

In Ruoffs group and Kaners group, more researches were

focused on CVD growth of graphene on Cu substrates.55,58

Bhaviripudi et al.59 presented the role of kinetic factors in CVD

synthesis of uniform large area graphene using Cu catalyst. It

was found out that the growth of graphene varied from a

monolayer at low methane concentration (ppm) to multilayers at

higher methane concentration (510% by volume) under atmo-

spheric pressure. One difficulty in the CVD synthesis on Cu

substrates is that the domain size of the resulting graphene is not

large enough.55a Li et al.55c studied the effect of growth

parameters and developed a two-step CVD process to synthesize

graphene films with an area of hundreds of square micrometers.

In Ahn and Hongs group,60 a new route to large-scale synthesis

of high-quality graphene films was demonstrated by using

centimeter-scale Cu substrates. This cost- and time-effective

roll-to-roll production method resulted in 30-inch graphene films

and simultaneously provided an efficient synthesis of graphene

with large scale and good quality for practical application.60

From the work in Kongs group61 and Ruoffs group,62 the

transfer of these deposited graphene onto arbitrary substrates

was also been launched.

Another common method for graphene synthesis is the

epitaxial growth of graphene on SiC wafer surfaces through

the decomposition of SiC followed by the desorption of Si from

the surface.63 By using in situ low-temperature scanning

tunneling microscopy to study the epitaxial growth, Huang

et al.63d inferred the bottom-up growth mechanism. According to

Heer et al.,63a the quantum Hall effect was suppressed due to the

absence of localized states in the bulk of the as-prepared

material. The advantages of the epitaxial growth can be

summarized by: (i) the resulting material does not need to be

transferred from the metal to another dielectric substrate; (ii)

there are no trapped impurities under the graphene; and (iii) the

growth can be directed by proper tailoring of the substrate.63c

Interestingly, an organic synthesis method for graphene has

been put forward, in which precursors are those discotic

aromatic hydrocarbons with precise chemical structures and

functional groups.64 In cyclodehydrogenation of well-defined

polyphenylene precursors, dendritic polyphenylene is easier to

process in solution due to its good solubility, which decreases p

p stacking significantly.65 By designing the structure of the

precursor monomers, a wide range of nanographenes with

various topologies and widths can be produced.65b,66 In the

organic synthesis field, there are some researches on nanogra-

phene synthesis, where the crucial problem is the practical

processability of large polycyclic aromatic hydrocarbons

(PAHs). It is found that PAHs which are larger than

hexabenzocoronene (C42H18), are not soluble in organic solvents

or not sublimable without decomposition because of strong

intramolecular pp interactions.67 One strategy to solubilize the

nanographenes is to introduce the long, flexible aliphatic chains

to the edge of the planar nanographene.68 However, with the size

of graphene increasing, such a strategy makes less difference.

Fig. 6 Novel Ni(111)/HOPG(0001) system for production of high-

quality single-atomic graphene layers: (a) schematic diagram of graphene

growth system and formation mechanism; (b) graphene on Ni(111)/

HOPG (0001) (size: 2 cm6 2 cm); (c) AFM image of our graphene sheet;(d) constant current STM image of the as-prepared graphene sheet. The

top-right and bottom-left insets show a two-dimensional fast Fourier

transform taken from the STM image and STM image superimposed by

the honeycomb lattice of graphene, respectively.57

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Thus, it is seriously needed to develop new methods to solve this

problem. Without using aromatic polymers, Sun et al.69 carried

out the deposition of large area and high-quality graphene with

controllable thickness using different solid carbon sources, such

as polymer film (polymethyl methacrylate) molecules, where a

metal catalyst substrate and a temperature as low as 800 uC wasused. This method stands as a complementary method to CVD

growth with the advantage of permitting growth at low

temperature and overcoming the limitation in the use of gaseous

materials in the CVD method.

Although the bottom-up approach introduces much less

defects on the graphene surfaces as compared the top-down

approach, the operation is relatively more difficult and costs

more. In summary of the two approaches, both of which have

their advantages and disadvantages, it is still needed to develop

new methods or make some improvements on these known

methods for the preparation of high-quantity and high-quality

graphene nanomaterials. To clearly understand the synthesis

methods for graphene, the mature methods mentioned above are

summarized and listed in Table 1, including the used chemicals,

and the structures and properties of the resulting graphene

nanomaterials.

3. Application of graphene-based nanomaterials inenergy-related areas

Due to the special one-atom thick layer 2D structure and its

superb characteristics of large surface area, high electrical

conductivity and chemical stability, graphene is emerging as a

particular carbon material for application in energy storage

device such as supercapacitors, solar cells and lithium ion

batteries.

3.1. Application as supercapacitor materials

Usually, supercapacitors can be divided into two types, denoted

electrochemical double-layer capacitors (EDLCs) and pseudo-

capacitors due to the different mechanism of charge storage.

Different from those conventional high-surface-area materials

such as activated carbons and CNTs, the available surface area

of graphene nanomaterials as capacitor electrodes is independent

of the pore distribution in the solid state.70 Vivekchand et al.71

reported graphene materials prepared from the thermal treat-

ment of graphite oxide and used them as electrodes for EDLCs.

At a scan rate of 100 and 1000 mV s21 with H2SO4 aqueous

solution as the electrolyte, the capacitance of graphene was 117

and 100 F g21, respectively. In Chens group,72 graphene sheets

synthesized by simultaneously exfoliating and reducing gra-

phite oxide under low-temperature plasma also showed high

specific capacitance and good electrochemical stability. It is

obvious that graphene with less agglomeration, fewer layers,

higher effective surface area should be expected to exhibit a

better supercapacitor performance.73 Therefore, the chemical

modifications of graphene are focused in many research

groups.70a,74 According to Zhang et al.,75 the performance of

supercapacitors using chemically modified graphene as electrode

depends on the flexible graphene sheets, not on the rigid, porous

structure of activated carbon to provide its large surface area.75

Du et al.76 synthesized two kinds of functionalized graphene

nanosheets through thermal exfoliation of graphite oxide at low

temperature in air and at higher temperature in N2 conditions. It

was found that the former functionalized graphene showed much

higher specific capacitance than the latter. Similar to Du et al.,

Lv et al.77 reported a high capacitance (up to 264 F g21) of a

functionalized graphene by vacuum-promoted and low-tempera-

ture exfoliation of graphite oxide. The high capacitance of this

kind of functionalized graphene is due to the oxygen-containing

groups on the surface such as hydroxyl, carboxyl and epoxy

groups, which can bring in the fast redox processes for the

pseudocapacitance. Yang et al.78 reported a multilayered

graphene film with highly open pore morphology, which allowed

the electrolyte solution to access the individual sheet surfaces

using water as an effective spacer to prevent the restacking of

graphene layers. The as-prepared film gave a specific capacitance

as high as 215 F g21 in aqueous solution. Another porous 3D

graphene network was reported by using Ni foam as a sacrificial

template in a facile CVD process with ethanol as the carbon

source (Fig. 7). The obtained NiO/graphene composites exhib-

ited a high specific capacitance of y816 F g21 at a scan rateof 5 mV s21, with a stable cycling performance even after

2000 cycles.79

Table 1 The approaches, methods and used chemicals for the synthesis of graphene nanomaterials, and the structures and properties of the resultinggraphene nanomaterials

Approach Method Chemicals used Structure(s) Properties or applications

Top-downapproach

Exfoliation Graphite, concentratedacid (H2SO4 ,HNO3, H3PO4),KMnO4

One- or few-layeredgraphene oxide nanosheets

1. to be reduced by reductants toform reduced graphene oxide2. to be blended with other componentsto synthesize functional composites3. to remove pollutants from aqueous solutions

Unzipping Carbon nanotubes,concentrated acid (H2SO4,HNO3, H3PO4), KMnO4

Narrow graphenenanoribbons

1. to be blended with other componentsto synthesize functional composites2. to be applied as biosensors

Bottom-upapproach

Chemical vapordeposition

Hydrocarbon gases,metal substrates

Patterned graphene films 1. to be used as transparent conductive filmsfor device fabrication

Epitaxial growth SiC substrates Graphene films 1. to be used in electronic devices such asfield effect transistors

Organic synthesis Large polycyclicaromatic hydrocarbonsor solid polymer film anda metal catalyst substrate

Nanographene 1. to be used as transparent conductive filmsfor device fabrication2. to be constructed with other componentsfor application in lithium ion batteries

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One feasible strategy to prevent the aggregation of graphene in

the solution and functionalization process is to use other

materials such as carbon black,80 CNTs,81 or metal oxide

nanoparticles 49,82 to separate 2D graphene nanosheets and

further to obtain high surface areas. For instance, Fan et al.83

constructed a novel 3D CNT/graphene sandwich (CGS) and

used it as an electrode in supercapacitors, where CNT pillars

were grown in the graphene layers. A maximum specific

capacitance of 385 F g21 was obtained with a scan rate of

10 mV s21 in 6 M KOH solution owing to the comprehensive

utilization of pseudocapacitance from a catalyst and the double-

layer capacitance from graphene, adding to the rapid transport

of the electrolyte ions or electrons throughout the sandwich

structure. Wang et al.84 did similar research by using a simple

green hydrothermal route to prevent restacking of individual

graphene sheets. Guo and Li85 reported a self-assembly

approach to synthesize a hierarchical nanostructure comprised

of carbon spheres and graphene nanosheets. The carbon spheres

act as a nanospacer to separate graphene nanosheets, resulting in

a high power density of 15.4 kW kg21 and long cycle life of the

supercapacitor.

The most popular pseudocapacitive materials are transition-

metal oxides. Recently, there have been many reports on

graphenemetal oxide/hydroxide composites, such as SnO2,49,86

Mn3O4,87 Fe3O4,

88 Co3O4,89 Co(OH)2,

90 and ZnO.49,82a Chen

et al.91 synthesized grapheneCo(OH)2 nanocomposites and

found that the specific capacitance of the composites reached a

high value of 972.5 F g21, with a significant improvement

relative to the individual components. A spherical a-Ni(OH)2nanoarchitecture grown on graphene was realized and used as an

advanced electrochemical pseudocapacitor material, whose

maximum specific capacitance was found to be high up to

1760.72 F g21 at a scan rate of 5 mV s21 according to Yang

et al.92 Similarly, Zhao et al.44 applied monolayer graphene/NiO

nanosheets with two-dimensional structure for supercapacitors.

It was found that the NiO nanoparticles (57 nm) were

uniformly dispersed on the graphene surface, and the two-

dimensional structure enhanced the supercapacitive perfor-

mance, with a high specific capacitance of 525 F g21 at a

current density of 200 mA g21 (Fig. 8). Other reduced graphene

oxidemetal oxide materials have also been applied in super-

capacitors such as rGOCo3O4,93 rGOZnO,49 and rGO

SnO2.49,94 In Dais group, an advanced asymmetrical super-

capacitor obtained by coupling Ni(OH)2/graphene and RuO2/

graphene hybrids were firstly fabricated, which showed high

specific capacitances and high energy and power densities.95 For

the first time, Zhao et al.96 synthesized few-layered MnO2nanosheets using graphene oxide nanosheets as templates. The in

situ replacement of carbon atoms on graphene oxide framework

by the edge-shared [MnO6] octahedron resulted in this special

graphene-based nanomaterial, which showed a notable capaci-

tance of y1017 F g21 at a scan rate of 3 mV s21, and y1183 F g21

at a current density of 5 A g21.

As important synthetic organic capacitor materials, polymers

are often complexed with graphene to form graphene/polymer

nanocomposites, which have turned out to show excellent

performance in supercapacitors.74,97 Graphene oxide and gra-

phene nanosheets were often doped into a polyaniline

matrix.7,74c,98 For example, Wang et al.7 prepared a graphene/

polyaniline composite paper (GPCP) by in situ anodic electro-

polymerization of aniline on graphene paper, and its electro-

chemical capacitance was as high as 233 F g21. Mini et al.97 have

developed high performance supercapacitor electrodes based on

graphene/poly(pyrrole). The composites were synthesized by

Fig. 7 Photographs of (a) Ni foam before and after the growth of graphene, and (b) y0.1 g 3D graphene networks obtained in a single CVD processafter removal of the Ni foam. SEM images of (c) 3D graphene networks grown on Ni foam after CVD, and (d) 3D graphene networks after removal of

Ni foam.79

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using electrophoretic deposition of graphene and then the

poly(pyrrole) layer was electropolymerized upon the graphene.

The specific capacitance was found out to be 1510 F g21, with

area capacitance of 151 mF cm22 and volume capacitance

of 151 F cm23 at 10 mV s21.97 Another nanocomposite,

poly(sodium 4-styrensulfonate) (PSS)-graphene, proved to have

very high cycle stability, the specific capacitance (190 F g21)

decreasing by only by 12% after 14 860 cycles.99 It is commonly

considered that the excellent capacitive performance of these

graphenepolymer composites is due to the good EDL capaci-

tance and pseudocapacitance, besides their good electrical con-

ductivity. In Ruoffs group, a high-performance supercapacitor

comprising a poly(ionic liquid)-modified reduced graphene oxide

(PIL:RG-O) electrode and an ionic liquid (IL) electrolyte showed

a stable electrochemical response up to 3.5 V operating voltage, a

maximum energy density of 6.5 W h kg21 and a power density of

2.4 kW kg21.74a

In summarizing the research on transition-metal oxide/

graphene hybrids, the excellent specific capacitance and cycle

performance at high charge/discharge current can be attributed

to three factors: (i) enhanced electronic conductivity of the

composite because of graphene matrix; (ii) uniform dispersion of

the transition-metal oxide nanoparticles among the graphene

nanosheets ensure good interconnection of transition-metal

oxide nanoparticles at the surface and interior of the electrodes;

and (iii) graphene can act as a buffer for the volume change

between the oxidized form and reduced form in the whole

process, which ensures a good cycling performance of the

composites. As promising electrode materials for supercapacitors

the development of graphenemetal oxide materials is still in its

early stage and requires further research.

3.2. Application in lithium ion batteries

In the modern society, rechargeable lithium ion batteries (LIBs)

are widely used in portable devices. Thus, a great effort is being

made to improve the performance of LIBs, especially their

energy density, rate capability and cycling stability, which are

largely dependent on the electrode materials since the recharge of

LIBs is completed with the lithium ion insertion/extraction

process in the electrodes. Because the commonly used graphite

has low specific capacity many workers have turned their

attention to alternative anode materials. It has been discovered

that disordered carbon shows a higher capacitance than the

ordered graphitic carbons.100 Thus it is reasonable for randomly

organized graphene nanosheets to show excellent lithium storage

capacity, due to the high specific surface areas, especially though

the facile functionalization of oxygen-containing functional

groups in the graphene oxide precursor through the chemical

route.101 In order to prevent restacking of graphene nanosheets,

CNTs or C60 molecules were introduced as spacers, which led to

an increase in capacity of LIBs from 540 mA h g21, to 730 and

784 mA h g21, respectively.102

Fig. 8 The first three cycles of CV curves of (a) bare NiO and (b) graphene/NiO. (c) Galvanostatic charge/discharge curves of NiO and graphene/NiO

at 200 mA g21. (d) Rate capacitance of NiO and graphene/NiO with increasing current density. (e) Galvanostatic charge/discharge curves of graphene/

NiO at 0.2, 2, 4 and 6 A g21. (f) Cycle life of NiO and graphene/NiO at 200 mA g21 in 6 M KOH solution.44

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Metal oxide nanoparticles are attractive anode materials

owing to their high specific capacities.6,103 However, the huge

volume variation during the charge/discharge processes causes

the pulverization of the electrode, which results in poor

reversibility, although by reducing the size of these materials to

nanoscale this drawback can be partly improved. Recently, high

specific capacity and good cycling performance of graphene

metal oxide hybrids as active materials in LIBs have been

reported. The metal oxides mainly include Co3O4,104 NiO,105

Mn3O4,106 CuO,107 TiO2,

108 SnO2105,109 and Co(OH)2.

110 These

nanoparticles not only play an important role as spacers between

graphene layers, but also provide as active materials to interact

reversibly with lithium ions. Taking SnO2 for example, it has

high specific capacity (782 mA h g21) theoretically as an anode in

LIBs. However, its cycling performance is not satisfactory

because of pulverization. By incorporating SnO2 with graphene

to form a delaminated, 3D, flexible structure, the lithium storage

capacity and cycling performance have been improved signifi-

cantly according to the work from Paek et al.111 In these hybrid

materials, graphene nanosheets act as not only lithium storage

electrodes, but also electronically conductive channels. At the

same time, the volume expansion upon lithium insertion is

limited due to the dimensional confinement of SnO2 nanopar-

ticles by the surrounding graphene. Furthermore, the developed

pores between graphene and SnO2 act as buffer spaces during

charge/discharge processes. However, in fact, this kind of

structure is difficult to operate in a controlled fashion. Xu

et al.112 synthesized MoO2/graphene oxide composites by a

simple solvothermal method. It was found that with the addition

of graphene oxide and the increase of graphene oxide content in

the precursor solutions, MoO3 rods changed to MoO2 nanorods

and further to MoO2 nanoparticles, both of which were

uniformly distributed on the graphene surfaces. As shown in

Fig. 9, the MoO2/graphene oxide composite with 10 wt%

graphene oxide shows a reversible capacity of 720 mA h g21

with a current density of 100 mA g21 and 560 mA h g21 at a high

current density of 800 mA g21 after 30 cycles. A two-step

solution-phase reactions to fabricate Mn3O4/graphene hybrids

was reported by Dais group.113 The intimate interaction

between the Mn3O4 nanoparticles and graphene gave the hybrid

a high specific capacity (y900 mA h g21) as an anode materialfor LIBs. Another kind of nitrogen-doped MnO/graphene

nanosheets (N-MnO/GNS) hybrid material was obtained

through a hydrothermal method and subsequent annealing by

ammonia. The nanostructured hybrid material exhibited a

reversible electrochemical lithium storage capacity of 772 mA h g21

at 100 mA g21 after 90 cycles and even a high capacity of

202 mA h g21 at a high current density of 5 A g21.114 In the work

of Peng et al.,115 a facile ultrasonic synthesis of CoO quantum dot/

graphene nanosheet composites at room temperature using

Co4(CO)12 as cobalt precursor was proposed. The nanosized

CoO quantum dots with high dispersity on conductive graphene

provide not only large amounts of accessible active sites for

lithium-ion insertion but also good conductivity and short

diffusion length for lithium ions, resulting in high capacity and

rate capability.

Interestingly, a hierarchically nanostructured composite of

MnO2/conjugated polymer/graphene was fabricated by self-

assembly.116 The enhanced performance of the composite can

be attributed to: (i) the interlayer spacing of y0.72 nm in MnO2promotes efficient lithium intercalation/deintercalation; (ii) the

polymer coating directs the ordered growth of MnO2 and

prevents the aggregation, thus large volume expansion can be

avoided; and (iii) the highly conductive 3-D graphene material

Fig. 9 Electrochemical performance of MoO2/GO composites and MoO2. (a) Cycling performance of MoO2/GO composites prepared by adding 3, 5

and 9 mL of GO suspension, respectively, at a current density of 800 mA g21. (b) Discharge/charge voltage profiles of MoO2/GO composite (with 5 mL

of GO suspension) at a current density of 100 mA g21. (d) Rate performance of MoO2 and MoO2/GO composite (with 5 mL of GO suspension)

between 0.005 and 3.00 V with increasing current density from 100 to 800 mA g21.112

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ensures a fast charge/discharge rate. This work provides a

universal method to fabricate hierarchically nanostructured

composites for energy conversion/storage systems.

Most of the nanomaterials mentioned above are derived from

graphene oxide obtained by the chemical top-down approach.

There are also some graphene-encapsulated metal or metal oxide

nanoparticles synthesized by the in situ, bottom-up method, in

which the particle formation and encapsulation process are

conducted simultaneously.117 For instance, the carbon/Co3O4composites were synthesized by solid-state pyrolysis of organic

cobalt precursors.117a The resulting graphene-coated Co3O4showed a high capacity of 940 mA h g21 even after 20 cycles.

It is noted that in these LIBs, after a loss in the initial capacity

the subsequent electrochemical lithium storage performance is

much better, which may be due to the formation of a solid

electrolyte interphase (SEI) layer.50,118 Considering the large

surface area of graphene-based materials, the effect of the SEI

layer may be larger than that of other anode materials.

In brief, graphene-based nanomaterials can largely decrease

the volume variation of the active anode materials during the

charge/discharge process, thus the high specific capacities of

metal oxide nanoparticles (or other active components) can be

maintained. As for the graphitic material itself, the high specific

surface area compared with graphite and the good electrical

conductivity ensure graphene-based materials are good alter-

native anode materials.

3.3. Application in solar cells

Due to the great potential for transparent and conductive

electrodes in solar cells, graphene has been widely studied as

different parts of solar cells in terms of their low cost, flexibility

and high efficiency. As an ideal 2D material that can be

assembled into film electrodes with low roughness and high

conductivity, large surface area, thin graphene films are often

used as window or/and counter electrodes, electron and hole

transport materials, and buffer layers in solar cells.119 Wang

et al.120 first reported a 10 nm-thick graphene film with a trans-

parency of more than 70% and a conductivity of 550 S cm21 by

the thermal reduction of a graphene oxide film. This graphene

was then used as a window electrode in dye-sensitized solar cells

(DSSC), in which the currentvoltage characteristics showed a

short-circuit photocurrent density (Isc) of 1.01 mA cm21, with an

open-circuit voltage (Voc) of 0.7 V, a calculated fill factor (FF) of

0.36, and an power-conversion efficiency of 0.26%. The excellent

performance is considered due to the low transmittance of the

electrode and the series resistance of the device. Up to now,

many efforts have been made to enhance the transparency

and conductivity of such graphene thin films for application

as electrodes in solar cells.121 However, these graphene films

produced from solution often have many grain boundaries and

defects which significantly increases the resistance. One stra-

tegy to tackle this problem is to incorporate other conductive

filler materials such as silica,122 polymer123 or CNTs124 into the

graphene matrix. For example, a nanocomposite of chemical con-

verted graphene and CNTs gave a film resistance of 240 V sq21 at

86% transmittance.124a Su et al.125 used large aromatic mole-

cules such as pyrene-1-sulfonic acid sodium salt (PyS) and

the disodium salt of 3,4,9,10-perylene-tetracarboxylic diimide

bis-benzenesulfonic acid (PDI) to functionalize graphene,

obtaining the enhancement in both the conductivity and

electronic structure of graphene. The subsequent thermal

reduction of graphene nanosheets led to a great increase of

the conductivity and the overall power efficiency of 1.12% of

the solar cells.

Besides the solution process, a more commonly used route for

high-quality graphene is CVD process, especially for their

application as transparent conductive electrodes in photovoltaic

devices.126 Yao et al.126b synthesized graphene film on Ni film-

coated SiO2/Si wafers by the CVD method, and further

transferred it to other substrates directly by dry-transfer

technology. It was found that the average resistance of the 6

30 nm-thick graphene films varied from 1350 to 210 V sq21 with

an optical transparency from 91 to 72% in the visible light

wavelength range, which was much lower than that of graphene

oxide films processed from solution oxidation. In the report of

Yan et al.,127 soluble graphene quantum dots were employed as a

sensitizer for dye-sensitized solar cells (DSSC), in which an open-

circuit voltage of 0.48 V and a short-circuit current density of

200 mA cm22 were obtained, with a fill factor of 0.58.

Apart from DSSC, graphene has also been used in organic

solar cells.5b,128 Compared with indium tin oxide (ITO), which

suffers from rising cost and brittleness, graphene has many

advantages in terms of its low cost, transparency, chemical

robustness, flexibility and high electrical conductivity. Wang

et al.129 reported high-performance organic solar cells by

interface engineering of layer-by-layer stacked graphene anodes.

The key for the further improvement in PCE is to reduce the

sheet resistance of graphene, which can be achieved by increasing

the carrier mobility via interface control or by more effective

doping.

Differently, another graphene-based film from the nanogra-

phene molecules of large polycyclic aromatic hydrocarbons

(PAHs) were also employed as a window electrode in organic

solar cells.5b The 4 nm-thick films hold a transparency of 90% at

a wavelength of 500 nm. Furthermore, the highest external

quantum efficiency (EQE) of the solar cell was found to be 43%

at a monochromatic light of 520 nm, suggesting that the output

voltage of this graphene-based cell is comparable to that of ITO-

based solar cells.5b Many applications of graphene-based

materials as a transparent conductive electrode in solar cells

have been reported in recent years,130 which are not mentioned

specifically here. In conclusion, the present research of graphene

on solar cells is mainly focused on three aspects: (i) the

development of advanced techniques for graphene-based elec-

trodes with high transparency and excellent conductivity; (ii) the

synthesis of graphene nanomaterials as efficient acceptors to

favor more efficient electronhole separation; and (iii) the use of

graphene-based films as an efficient hole transport layer.

Besides acting as a transparent conductive electrode, graphene

has other potential uses for photovoltaic devices. For instance,

through incorporation into conjugated polymers, graphene has

largely improved the exciton dissociation and charge-transport

properties.131 In Chens group, organic solution-processed

graphene material has been applied as an electron acceptor in

organic bulk heterojunction (BHJ) photovoltaic devices with

P3HT and P3OT as electron donor.131,132 By controlling

annealing, the device performance is improved considerably,

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and under simulated 100 mW cm22 AM 1.5 G light illumination

a best power conversion efficiency of 1.4% was obtained.

Solution-processed graphene was further used as a transparent

anode to fabricate bulk heterojunction polymer organic photo-

voltaic devices, which displayed a power-conversion efficiency of

0.13%.133 Wu et al.5a also demonstrated organic light-emitting

diodes with these solution-processed graphene thin films as

transparent conductive anodes. Guo et al.130c used a simple

bottom-up approach to create a novel electron transfer system

based on layered graphene/CdS quantum dots, which showed a

significantly improved photoresponse and offering a promising

path for the development of photovoltaic devices. Organic

photovoltaic devices were also fabricated by using graphene-

based materials by Yin et al.,121b Park et al.134 and Zhu et al.135

In summary for the published results, graphene-based

materials are appropriate for photoconversion due to their

strong light absorption, high charge mobility, good stability and

good HOMO/LUMO matching.136

4. Application of graphene-based nanomaterials in

environmental-related areas

Environmental pollution remediation is another hot issue that

needs serious concern in scientific research. Due to the develop-

ment of economies and the expansion of industrial activities,

contamination of heavy metal ions and toxic organic compounds

in water has been increased over the past decades. Considering

their high specific surface area, which would give the sufficient

contact area for pollutants, various studies have been focused on

the application of graphene-based nanomaterials for the high-

performance removal of different organic and inorganic pollu-

tants from aqueous solutions, such as through adsorption to

reduce concentration, decomposition to less toxic molecules, and

reduction to low-valent species. In other words, graphene-based

materials have been applied as adsorbents, photodegradants and

photoreductants in environmental pollution cleanup.

4.1. Application as adsorbents for pollutant removal from aqueous

solutions

Due to its large surface area, graphene is believed to be a good

adsorbent for many pollutants. For example, Li et al.137

obtained monolayer or few-layered graphene nanosheets by

ultrasonication and centrifugation of a 1-methyl-2-pyrrolidinone

(NMP) suspension, and further removed NMP for the adsorp-

tion of fluoride from aqueous solutions, resulting an adsorption

capacity of up to 35 mg g21. To improve the dispersion of

graphene in the aqueous solutions, many researches were carried

out using the modified graphene as adsorbents for pollutant

removal, among which, graphene oxide nanosheets are com-

monly used as adsorbents for heavy metal ions owing to the

abundant functional groups such as hydroxyl, epoxide, carboxyl

and carbonyl on the surfaces, which are expected to form strong

surface complexes with toxic metal ions. According to the

reports from Wangs group, the synthesized few-layered gra-

phene oxide showed a high adsorption ability toward many

heavy metal ions such as Pb(II),138 Cd(II),139 Co(II)139 and

U(VI).140 In their reports, the maximum adsorption capabilities

were 842 mg g21 for Pb(II), 106.3 mg g21 for Cd(II), 68.2 mg g21

for Co(II) and 97.5 mg g21 for U(VI), which are all much higher

than that of other adsorbents. Fig. 10 shows the sorption

isotherms of Cd(II) and Co(II) on graphene oxide nanosheets,

and from their report, one can see that the sorption of Cd(II) and

Co(II) on graphene oxide nanosheets is far from saturation.

Deng et al.141 also reported the removal of Pb(II) and Cd(II) ions

from wastewater using one-step synthesized ionic-liquid-functio-

nalized graphene nanosheets directly from flake graphite, with

adsorption capacities up to 406 mg g21 for Pb(II) and 73 mg g21

for Cd(II), respectively.

Besides metal ions, there are also many reports on the

adsorption of organic pollutants. Reduced graphene oxide was

used to remove anionic dyes by Ramesha et al.,142 which turned

out to be a good adsorbent because of the high surface area and

the lack of negative surface charge. Zhao et al.9 used sulfonated

graphene for the removal of naphthalene and 1-naphthol from

aqueous solutions, and obtained maximum adsorption capacities

of 2.33 mmol g21 for naphthalene and 2.41 mmol g21 for

1-naphthol (Fig. 11 and 12), which are much higher than for

other reported materials.

For the full utilization of graphene building blocks in

environment remediation, some functional nanoparticles are

combined with graphene in composites to give some additional

properties to graphene. Typically for the adsorption of

pollutants, magnetic Fe3O4 nanoparticles are incorporated to

Fig. 10 Sorption isotherms of Cd(II) (A) and Co(II) (B) on graphene

oxide nanosheets (0.1 g L21) at different temperatures: pH = 6.0 0.1, I

= 0.01 M NaClO4. The solid and dashed lines correspond to Langmuir

and Freundlich model simulations.139

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facilitate separation in the pollutant treatment. For example,

Shen et al.143 synthesized graphene oxidemagnetic nanocom-

posites through a high temperature reaction of ferric triacetyla-

cetonate with graphene oxide in 1-methyl-2-pyrrolidone. Many

similar magnetic graphene nanocomposites have been proposed

through solvothermal method or in situ chemical coprecipita-

tion.144 Wang et al.144 applied graphene/Fe3O4 composites for

the removal of organic dyes from aqueous solutions, with the

adsorption capacity for fuchsine as high as 89 mg g21, and He

et al.145 obtained an adsorption capacity of 190 mg g21 for

methylene blue and 140 mg g21 for neutral red cationic dyes

using as-prepared magnetic graphene hybrids. Very similar to

He et al., Xie et al.146 reported the adsorption capacities

of superparamagnetic graphene oxideFe3O4 composites for

methylene blue and neutral red were 167.2 and 171.3 mg g21,

respectively. The magnetic graphene hybrids were also used to

adsorb As(III) and As(V), resulting in arsenic removal to as low

as 1 ppb in aqueous solutions.147 These hybrids also showed an

endothermic and spontaneous adsorption process toward Co(II)

from aqueous solutions, with the maximum adsorption capacity

up to 12.98 mg g21 at room temperature according Liu et al.148

The growth of noble metal nanoparticles such as Au, Pt and Pd

on graphene showed enhanced electronic properties due to the

spatial confinement and synergetic electric interactions between

the metal and graphene. These kinds of composites are mainly

applied as catalysts in organic synthesis and transformation and

less so for water purification.149 Only Sreeprasad et al.150

reported the use of graphene-Au composites supported on river

sand for the excellent uptake of Hg(II) from aqueous solutions.

More research about the application of graphenemetal compo-

sites in pollutant removal is expected. Besides the usual

composites mentioned above, complex composite components

such as CoFe2O4 and ZnFe2O4, also play an important role in

the application of nanomaterials in environmental pollution

cleanup. For example, magnetic CoFe2O4-functionalized gra-

phene nanosheets were used to adsorb methyl orange from

aqueous solution with an adsorption capability of 71 mg g21.79

In conclusion, the excellent adsorption capabilities of graphene-

based nanomaterials toward pollutants are due to: (i) the

inherent high specific surface area of graphene; (ii) the decreased

restacking and agglomeration of graphene layers; and (iii) the

strong interaction between the adsorbates and the surface of the

modified or functionalized graphene nanosheets. It should be

noted that the interaction mechanism of organic pollutants with

graphene-based materials is different to that of inorganic

pollutants with graphene-based materials. Thereby, in the

application of graphene-based materials in the removal of

organic or inorganic pollutants from aqueous solutions, different

types of graphene-based materials should be selected, and this

will lead to much future research.

4.2. Application as degradants for pollutants in aqueous solutions

Except for the adsorption of the pollutants in water, many toxic

compounds can be eliminated by photocatalytic degradation

with the aid of photocatalysts such as ZnO, TiO2 and CdS

incorporated with graphene, which shows prominent activity in

photocatalytic application due to the novel electronic property of

graphene and its zero-band gap. Furthermore, the nanoparticles

on the graphene surface prevent the aggregation between the

Fig. 11 (A) The synthesis processes of sulfonated graphene from graphite. (B) SEM image of sulfonated graphene on Si substrates. (C) AFM image of

sulfonated graphene on Si/SiO2 substrates (SiO2 ca. 300 nm). The SEM and AFM images show that few-layered sulfonated graphene sheets are

formed.9

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graphene layers in some extent, so that it is understandable to

expect efficient properties in the pollutant treatment. For

instance, Liang et al.45 synthesized graphene/TiO2 nanocrystal

hybrids through the growth of TiO2 on graphene oxide

nanosheets by hydrolysis and further hydrothermal treatment,

and studied their photocatalytic activity on the degradation of

rhodamine B. It is demonstrated that the as-prepared graphene/

TiO2 nanocrystal hybrids show an impressive three-fold photo-

catalytic enhancement relative to P25 particles (Fig. 13). TiO2

graphene nanocomposites were also used for the efficient

photocatalytic degradation of butane in the gas phase by

Stengl et al.48 Zhao et al.10 also synthesized graphene/TiO2composites and the results of photocatalytic degradation of

methylene blue over UV and visible-light spectrum regions

indicated that the enhanced photocatalytic activity was due to e2

adsorption and transportation in the presence of graphene, as a

result of which, graphene/TiO2 was able to absorb a high amount

of photo-energy in the visible-light region and drive efficient

photochemical degradation reactions. According to Zhang

et al.,151 the enhanced degradation performance was originated

from three attributes: (i) the increasing adsorption of pollutants

on graphene-based nanomaterials; (ii) the extended light

absorption range; (iii) facile charge transportation and separa-

tion. Li and Cao152 synthesized ZnO/graphene composites via a

chemical deposition route and demonstrated their efficient

photosensitized electron injection, slow electron recombination

and enhanced photocatalytic activity under UV and visible light.

Similar research was reported by Xu et al.153 As a well-known

IIVI semiconductor, CdS has attracted extensive attention in

photocatalytic research because of its suitable band gap (2.4 eV),

which corresponds well with the spectrum of sunlight, and

extensive attention has been paid to CdSgraphene compo-

sites.154 Liu et al.154d utilized as-prepared CdS-reduced graphene

oxide composites for photocatalytic reduction of Cr(VI), and

they found that: (i) CdS-graphene composites exhibited a better

photocatalytic performance than CdS; (ii) the performance was

dependent on the proportion of graphene in the composites, with

the composites containing 1.5 wt% graphene achieving the

highest Cr(VI) removal rate of 92%. The more complex

composites, such as CoFe2O4 and ZnFe2O4, have also been

studied for the degradation of organic pollutants. According to

Wangs research group, the combination of CoFe2O4 or

ZnFe2O4 nanoparticles with graphene leads to high activity for

organic pollutant degradation.155 They noticed that CoFe2O4showed an excellent degradation ability toward methylene blue,

rhodamine B, methyl orange, active black BL-G and active red

Fig. 12 (A) Effect of sulfonated graphene content on the adsorption of

naphthalene and 1-naphthol on sulfonated graphene. C(naphthalene/1-

naphthol)initial = 0.2 mmol L21, pH = 7.0 0.1, I = 0.01 M NaClO4, T =

293 K. (B) Adsorption isotherms of naphthalene and 1-naphthol on

sulfonated graphene (0.04 g L21 for naphthalene; 0.08 g L21 for

1-naphthol); pH = 7.0 0.1, I = 0.01 M NaClO4, T = 293 K,

C(naphthalene)initial = 0.100.39 mmol L21, C(1-naphthol)initial = 0.17

0.70 mmol L21.9

Fig. 13 (a) Schematic illustration of the photodegradation of rhoda-

mine B molecules by graphene/TiO2 nanocrystals hybrid under irradia-

tion by a mercury lamp. The inset shows the solution of the graphene/

TiO2 nanocrystals hybrid. (b) Photocatalytic degradation of rhodamine

B monitored as normalized concentration change vs. irradiation time in

the presence of free TiO2, P25, graphene/TiO2 nanocrystals hybrid and a

graphene/TiO2 mixture. (c) Average reaction rate constant (min21) for

the photodegradation of rhodamine B with free TiO2, P25, graphene/

TiO2 nanocrystals hybrid, and graphene/TiO2 mixture. The error bars are

based on measurements on at least four different samples.45

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RGB without the presence of H2O2 under visible light

irradiation.155a For the magnetic ZnFe2O4-graphene nanocom-

posites, the experimental results showed that these composites

can serve as both photoelectrochemical degrader for organic

molecules and the generator for hydroxyl radicals via photo-

electrochemical decomposition of H2O2 under visible light

irradiation with no noticeable change in the structure and

composition.155b

The photocatalytic degradation of organic pollutants is an

environmental friendly method to eliminate organic pollutants

from aqueous solutions in environmental pollution cleanup.

First, the high surface area assures the high adsorption of

organic pollutants on the surface of graphene-based nanomater-

ials. Furthermore, the high dispersion of metal oxides on

graphene nanosheets and the graphene as electron transfer

channels make the degradation process occur efficiently on these

graphene-based nanomaterials. Therefore, the synthesis of

graphene-based composites with high photocatalytic capacity is

important for the degradation of organic pollutants.

5. Summary and outlook

As a unique 2D carbon material, graphene has shown a

continuously growing research upsurge since its appearance.

Very soon, more researches will be focused on the modification of

graphene and the incorporation of graphene with other functional

nanoparticles and their further applications in multidisciplinary

areas. In this critical review, we have covered recent researches on

the synthesis of graphene-based materials and their applications in

energy-related and environmental pollution remediation areas,

both of which are the issues of most concern. Specifically, the

utilization of graphene-based materials in supercapacitors, lithium

ion batteries, solar cells, adsorption and degradation for the toxic

pollutants from large volumes of aqueous solutions are covered. It

is concluded that graphene-based materials, synthesized through

top-down or bottom-up methods, show prominent properties

in the energy-related and environmental-related areas, compared

with other nanomaterials.

However, there are still many difficulties waiting for efficient

solutions. Firstly, the top-down synthesis often results in large

amounts of defects on the graphene framework, while the

bottom-up method is difficult to realize for mass production in

practical applications. New approaches for the synthesis of high-

quality graphene in large scale at low price are thus required.

Secondly, although graphene-based materials are used as active

materials for charge collection and transportation in electronic

devices, there are still numerous critical problems requiring

further study for the improvement in quality and function, which

include the morphology of graphene, defects, functional groups,

assembly behavior between graphene and other functional

particles. Thirdly, for the application in environmental pollutant

remediation, although the functionalized graphene-based nano-

materials show prominent ability in the removal of many

pollutants from aqueous solutions, the selective adsorption for

specific pollutants is rarely reported; and degradation reactions

mainly focus on organic dyes, and relatively less on other toxic

organic pollutants or high-valent metal ions. Furthermore, the

research for the understanding of the relationship between

graphene-based nanomaterials and improved performance is still

at its early stage, which also hinders the further design for the

effective functional graphene-based materials. It is necessary to

point out that the synthesis of graphene-based materials is still

expensive and in low scale at present, which limits the

application of graphene-based nanomaterials in practical envir-

onmental pollution cleanup. With the development of technol-

ogy, no doubt that these problems will be solved and more

functional graphene-based materials will find extensive use in the

near future.

Acknowledgements

Financial support from 973 projects of MOST (2011CB933700),

NSFC (20971126, 21071147, 21071107, 91126020), and open

foundation of State Key Lab of Pollution Control and Resource

Reuse are acknowledged.

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