(22)Review Graphene Based Materials

54 Catal. Sci. Technol., 2012, 2, 54–75 This journal is c The Royal Society of Chemistry 2012

Cite this: Catal. Sci. Technol., 2012, 2, 54–75

Graphene-based materials for catalysis

Bruno F. Machadoab

and Philippe Serp*a

Received 9th September 2011, Accepted 13th October 2011

DOI: 10.1039/c1cy00361e

Graphene is one of the most promising materials in nanotechnology. From a theoretical point of

view, it provides the ultimate two-dimensional model of a catalytic support. Its unique physical,

chemical and mechanical properties are outstanding, and could allow the preparation of

composite-materials with unprecedented characteristics. Even though the use of a single graphene

sheet as a catalytic support has not yet been reported, some promising results have already been

obtained with few-layer graphene. In this review, we will briefly discuss the most relevant

synthetic routes to obtain graphene. Then, we will focus our attention on the properties and

characterization techniques of graphene that are of relevance to catalysis, with emphasis on

adsorption. After presenting an overview of the most common and effective preparation methods,

we will discuss the catalytic application of graphene and graphene-based composites, with

particular attention on energy conversion and photocatalysis.

1. Introduction

The use of carbon nanomaterials in catalysis has grown in

importance and has dominated advances in nanoscience and

nanotechnology for the last 25 years. They are nowadays one

of the most commonly used materials and can be used either as

supports for immobilizing active species or as metal-free

catalysts.1 This is mainly due to their unique structure and

intrinsic properties including high specific surface areas,

chemical and electrochemical inertness and easy surface

modification.2 Carbon nanotubes (CNTs) were discovered

soon after the successful laboratory synthesis of fullerenes.3

Since their first observation using high resolution electron

microscopy in 1991 by Iijima,4 CNTs have been the focus of

materials research mainly because of their unique structural,

electronic and mechanical properties. However, following

the report by Novoselov et al.5 on the direct observation

and characterization of mechanically exfoliated graphene, a

a Laboratoire de Chimie de Coordination, UPR CNRS 8241,Composante ENSIACET, Universite de Toulouse UPS-INP-LCC,4 allee Emile Monso, BP 44362, 31432 Toulouse Cedex 4, France.E-mail: [email protected]; Fax: +33 05 34 32 35 96;Tel: +33 05 34 32 35 72

b Laboratorio de Catalise e Materiais (LCM), Laboratorio AssociadoLSRE/LCM, Departamento de Engenharia Quımica, Faculdade deEngenharia, Universidade do Porto, Rua Dr. Roberto Frias,4200-465 Porto, Portugal

Bruno F. Machado

Bruno F. Machado received isPhD in chemical and biologicalengineering from the Universityof Porto in 2009. He is currentlycarrying out his postdoctoralwork at Laboratory of Coordi-nation Chemistry (University ofToulouse, France) in the field ofnanotechnology and heteroge-neous catalysis. His research in-terests include the developmentand preparation of carbon-basednanocomposites with semicon-ductor nanostructures, as wellas their applications in catalysis. Philippe Serp

Philippe SERP is full Professorof Inorganic Chemistry at EcoleNationale superieure desIngenieurs en Arts ChimiquesEt Technologique ToulouseUniversity. He was the recipientof the Catalysis Division ofthe French Chemical SocietyAward in 2004, and the APDF‘‘Celestino da Costa/Jean Perrin’’award in 2005. His currentresearch interests in Laboratoirede Chimie de Coordination(UPR 8241 CNRS) includenanocatalysis, gas phase prepara-tion of nanostructured catalytic

materials and the understanding of homogeneous catalytic reactions,fields in which with co-workers he has published over 120 papers,among them 5 review articles, 12 book chapters and 13 patents.

CatalysisScience & Technology

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This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 54–75 55

single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon

atoms, there has been an exponential growth in graphene

research among both the scientific and engineering communities.

Not surprisingly, the importance regarding the discovery of

these nanostructured carbons was recognized by the Nobel

Prize committee with the award of two Nobel Prizes. In 1996,

Robert F. Curl Jr., Sir Harold Kroto and Richard E. Smalley

shared the Nobel Prize in Chemistry ‘‘for their discovery of

fullerenes’’, and in 2010, Andre Geim and Konstantin Novoselov

received the Nobel Prize in Physics ‘‘for groundbreaking experi-

ments regarding the two-dimensional material graphene’’.

Ideally, graphene is a single-layer material, but graphene

samples with two or more layers are being investigated with

equal interest. Graphene possesses unique electronic, optical,

thermal, and mechanical properties. In addition, given its large

specific surface area, good biocompatibility and high adsorption

capacity, graphene and its derivatives can be used as valuable

substrates to interact with various species. These composites

can then be used over a wide range of applications, including

memory devices,6–8 energy storage,9–11 catalysis,12–21 photo-

catalysis,22–27 solar cells,28–32 sensing platforms,33–36 Raman

enhancement,37–39 molecular imaging,40,41 and even drug

delivery.42

Given the massive attention inspired by the properties and

potential applications of graphene-based materials, the number

of publications has increased exponentially in the last few

years. Despite the existence of several reviews highlighting the

unique physical, chemical and mechanical properties of

graphene,43–55 only a very limited number deal with the

application of these materials in catalysis.56–58 Because the

success of a catalytic application begins with catalyst design, it

is fundamental to understand all the key aspects involved.

Hence, we will briefly discuss the most common synthetic routes

to obtain graphene. Afterwards, we will focus our attention on

the properties and characterization techniques of graphene that

are of relevance to catalysis, emphasis being given to adsorption.

After presenting an overview of the most common and effective

preparation methods, we will discuss the most recent advances

in the catalytic application of graphene and graphene-based

composites. Finally, we will present a brief summary and an

outlook on what to expect regarding future applications of

graphene-based composites in the field of catalysis.

2. Synthesis and properties of graphene

2.1 Synthesis

Graphene is a two-dimensional crystal that can be considered

as the basic building block for carbon materials of different

dimensionalities: fullerenes (0D), nanotubes (1D) or graphite

(3D).49 This unique nanostructure holds great promise for

potential applications in technological fields such as optical

electronics, sensors, energy conversion and storage, catalysis,

among many others. In that context, to realize this potential,

reliable methods for producing large-area single-crystalline

graphene domains are required. However, as was the case in

the early days of nanotube and nanowire research, graphene

faces a problem that is common to many novel materials: the

absence of process for production in high yields. In order to

overcome this deficiency, there are currently several methods

that can be used for the production of single or few-layered

graphene samples, evidencing variable degrees of success. We

present here a brief overview of the most common.

The existence of single-layer graphene was not considered

possible until the recent achievement of the micromechanical

cleavage of highly ordered pyrolytic graphite (HOPG).5,59

Unfortunately, the single layers obtained by repeated peeling

are only a small portion amongst the large quantities of thin

graphite flakes (few- to multi-layer graphene), and, thus is not

suitable for large-scale fabrication processes.

High-quality large-area graphene sheets can be prepared by

epitaxial growth on single-crystal silicon carbide (SiC). This is

commonly achieved through ultrahigh vacuum annealing of

the SiC surface.60–63 Since the sublimation rate of silicon is

higher than that of carbon, excess carbon is left behind on the

surface, which rearranges to form graphene nanosheets. An

important issue within this technique is related to the interface

between the graphene layer and the substrate, since it is

recognized that both the structure and electronic properties

of graphene are affected.64 Once there is an improved control

over the growth mechanism (leading to a control in the number

of layers) and interface effects, this method is set to be used

industrially to produce wafer scale graphene. On the other hand,

conditions for graphene growth such as high-temperature, ultra-

high vacuum and single-crystal substrate will likely hinder the

use of this technique for large-scale applications.65

One attractive alternative to the production of individual

graphene sheets is the epitaxial growth of graphene films on

metal surfaces such as Ni, Cu, Co, Pt, Ir and Ru, using

chemical vapor deposition (CVD).62,66–70 This method uses

the atomic structure of the metal substrate to seed the growth

of graphene. The nucleation and growth of graphene usually

occurs by exposure of the transition metal surface to a hydro-

carbon gas under low pressure or ultra-high vacuum conditions.

Large area epitaxial graphene films up to a few micrometres in

size can be subsequently transferred to other substrates after

etching off the metals. Epitaxial growth of graphene offers

probably the only viable route towards electronic applications,

and so a rapid progress in this direction is expected over the

next few years.

One of the most developed methods to obtain higher yields

of single-layered graphene consists of the initial oxidation of

graphite to graphite oxide (GO), followed by the subsequent

mechanical/chemical or thermal exfoliation of graphite oxide

to graphene oxide sheets, and their eventual reduction to

graphene (Fig. 1).71–73 Although the exact structure of GO is

still subject to intense debate, it is believed that for GO, the

aromatic lattice of graphene is interrupted by epoxide, hydroxyl,

carbonyl and carboxylic groups.74 The most accepted model is

the one by Lerf and Klinowski,75 where it is assumed that the

heavily oxygenated graphite oxide contains hydroxyl and

epoxide functional groups on the basal planes, in addition to

carbonyl and carboxyl groups located at the edges. These

oxygen functionalities render the graphene oxide layers of

hydrophilic GO and water molecules can readily intercalate

between the layers. This results in an increase of the inter-layer

distance (d-spacing, d002 in Fig. 1) of GO as well as a change of

hybridization of the oxidized carbon atoms from planar sp2 to

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tetrahedral sp3.72 Rapid heating of GO causes a rapid

evaporation of the intercalated water resulting in its expansion

and delamination. Even though this simple method has been

applied on a large scale, if the oxidation is not sufficient it

could result in incomplete exfoliation of graphite to the level of

individual graphene sheets. In addition, the functionalization

disrupts the electronic structure of graphene by several orders

of magnitude, compared to pristine graphene. Chemical,

electrochemical or thermal reduction of graphene oxide

(removal of the functional groups) into graphene can partly

restore its graphitic structure as well as conductivity. Although

reduced graphene oxide (rGO, also called chemically converted

graphene, chemically modified graphene, or simply graphene)

presents considerable amount of defects, which continue to

disrupt the electronic properties, it is one of the most widely

used methods due to its cost, facile preparation process, large

productivity and potential for functionalization.

In this context, the preparation of high-quality 2D graphene

sheets is the first and most crucial step, since the existence of

residual defects (oxygenated species that cannot be fully

removed by chemical treatments) will severely influence the

properties (mainly electronic) of graphene and limit its applica-

tions. Thus, an efficient process for the large scale production of

high quality few-layer graphene should be modelled.

2.2 Properties

Not all physicochemical properties determined for graphene

are of interest for catalyst design and application. Hence, only

those potentially interesting for catalysis are discussed in this

section. For more details regarding other properties of graphene,

the reader can refer to specific review articles.50,51,65,76,77

Based on theoretical calculations for graphene as the parent

material of carbon nanotubes, its properties were expected to

be outstanding. With the development of new methodologies

to increase both the yield and the quality of graphene samples,

these estimates could be finally experimentally assessed.

Unfortunately, some of the properties can only be observed

at an extremely low defect concentration. Nevertheless, like in

any other real material, structural defects do exist in graphene

and can radically alter its properties.

In graphene, carbon atoms are arranged in a hexagonal

manner, forming a 2D honeycomb structure. While the strong

s bonds work as the rigid backbone of the hexagonal structure,

the out-of plane p bonds control interaction between different

graphene layers. This allows delocalized p electrons to be

easily conducted through the basal plane, i.e., the plane of

the graphene sheets normal to the c-axis of graphite (Fig. 1).

For this reason, graphene is considered a zero-bandgap semi-

conductor, possessing a small overlap between the valence and

conduction bands.49 Furthermore, graphene sheets exhibit a

highly anisotropic behavior as the electron conduction along

the c-axis is much lower (around three orders of magnitude)

than that observed through the basal planes.9 The electronic

properties of graphene vary both with the number of layers

and the relative position of atoms in adjacent layers (stacking

order). For double-layer graphene, the stacking order can be

either AA (each atom on top of another atom) or AB (a set of

atoms in the second layer sits on top of the empty center of a

hexagon of the first layer). Other properties, such as thermal

Fig. 1 Illustration on the preparation of reduced graphene oxide. Reprinted with permission from ref. 56. Copyright 2011 Wiley-VCH.

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conductivity and thermal expansion also present a similar

variation. Upon comparing graphene with graphene oxide, it

can be observed that the latter exhibits a significant loss of

conductivity (up to several orders of magnitude) due to the

presence of oxygenated surface groups and defects in the basal

plane. The GO sheets need to be reduced in order to restore the

sp2 hybrid network and, thus, reintroduce the conductivity.

Depending on the level of reduction the surface of the graphene

sample can be fine-tuned to achieve different electronic and

optoelectronic properties.28,78

In addition, graphene is found to possess a high optical

transparency,79 due to its one-atom thickness, rendering it

extremely useful in transparent conducting electrodes, used for

example in touch-screens, liquid crystal displays and solar cells.

Furthermore, it is noteworthy to highlight its excellent

chemical stability and mechanical strength. The mechanical

properties of a defect-free monolayer of graphene were measured

using a nano-indentation technique and have shown it to be

one of the strongest materials ever investigated.80

Among other properties that have received considerable

interest, one of the most important is adsorption. Under-

standing the adsorption mechanism and interaction between

adsorbed species and the carbon surface is essential to fabricate

graphene-based materials. In order to develop this knowledge,

theoretical studies are of great interest and allow an optimization

regarding the characteristics of novel materials. Regarding

metal deposition, several theoretical calculations have been

performed in order to provide an atomic level understanding

of the interactions between adatoms (adsorbed atoms) and

graphene. These investigations focused on the stable config-

urations of metal adatoms on graphene, embedding transition-

metal atoms in graphene, charge transfer between graphene

and metal adatoms, and magnetism. Commonly, authors

consider the binding of the adatom over graphene on three

sites of high symmetry: hollow (H) at the center of a hexagon,

bridge (B) at the midpoint of a carbon–carbon bond, and top

(T) directly above a carbon atom (Fig. 2). Hu et al.81 studied

the adsorption of various adatoms over graphene using first-

principles density-functional theory with the generalized gradient

approximation. They found that H sites are the most favorable

for Sc, Ti, V, Fe, Co and Ni adsorbed on graphene, while B or

T sites are the most stable for transition metals that have a

filled or near filled d-shell (Cu, Pd and Pt). Half-filled d-shell

transition metal atoms and Au, Ag, Zn have small adsorption

energies. Depending on the intensity of the adsorption energy,

the favored adsorption site can indicate the nature of the

chemical bond between adatoms and graphene. Hence, the

adsorption of Au, Ag and Cu was considered as physisorption,

whereas Co, Ni, Pt and Pd covalently bonded to graphene

(chemisorption). Results obtained for Pt, Ag and Au were

supported by Tang et al. in a similar work.82

Recently, Nakada and Ishii83 studied the adsorption and

migration energies for different atomic species from hydrogen

to bismuth (except lanthanides and noble gases) over a

graphene sheet using a first-principles band calculation technique

based on density functional theory. Table 1 shows the most

stable sites for each adatom (the orange, blue and white boxes

represent the most stable sites T, B or H, respectively) accom-

panied by the corresponding adsorption energy. For transition

metal elements the most commonly stable site is H; for the

non-metallic elements, B site is the most stable; for H, F, Cl,

Br and I, the most stable adsorption site is T. When the

adsorption energy of the adatom is small, there is almost no

difference between the three adsorption sites. Marked in bold

in Table 1 are adatoms that cumulatively have bond distances

(between graphene and adatom) smaller than 2 A and migration

energy (for the most stable sites) above 0.5 eV. For large bond

distances, the adatom shows physisorption bonding; when the

bond distance is short, the bond energy tends to increase and

the adatom shows chemisorption bonding. The migration

energy gives an indication of the ease of mobility of the adatom

on the surface. Hence, the closer the adatom to the graphene

sheet, the stronger it bonds to graphene and thus the highest the

resistance to the adatom movement through the basal plane.

Computation results show that chemisorption of transition

metals involves hybridization of adatom d-orbitals with the

orbitals of graphene. Despite the presence of a different set of

orbitals, a similar observation can also be made for hydrogen,

nitrogen and oxygen adatoms. This induces a strong distortion

on the carbon atom beneath the adsorbed atom, which is likely

to change some of the sp2-like orbital character to a more

covalently reactive sp3-like character, given the re-hybridization

of the valence carbon orbitals required for the bond formation.

From the adatom point of view, there is a reduction in the

magnetic moment from the isolated to the adsorbed metal

atom, which is thought to be related with an electronic charge

transfer between adatom and graphene coupled with an

electron shift between different orbitals within the adatom.81

Many researchers have also investigated the interactions

between chemical or biological molecules and graphene or

metal-doped graphene.84–95 The adsorption of gas-phase molecules

(H2O, NH3, CO, N2O and NO) on graphene was reported by

Leenaerts et al.84 They observed that the charge transfer

between the adsorbates and graphene was found to be almost

independent of the adsorption site, but it did depend strongly

on the orientation of the adsorbate with respect to the graphene

surface. Ma et al.85 investigated the adsorption of cysteine on

Pt-doped graphene. Compared with graphene, Pt-doped graphene

had higher binding energy value and shorter binding distance

between the cysteine molecule and the graphene surface, which

was caused by strong adsorption of the cysteine molecule

(DOS results showed significant orbital hybridization between

cysteine and Pt-doped graphene sheet). Majumder et al.86

investigated the adsorption of aromatic amino acids on graphene,Fig. 2 Three different high symmetry adsorption sites: hollow (H),

bridge (B) and top (T).

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and found that phenolic rings oriented preferentially parallel

to the plane of graphene. This behavior is typical of low

adsorbate coverage; when the coverage is higher, the molecules

often tilt to the vertical position, given that it requires less

space, in order to accommodate a higher number of molecules.

Voloshina et al.93 focused their study on the bonding of a

single pyridine molecule adsorbed on a graphene surface. They

demonstrated that a H adsorption site was preferred by

pyridine and that the parallel orientation was more favorable

than the perpendicular one. Interaction energy between pyridine

and metal substrates was found to be at least 30% stronger

than that obtained between pyridine and graphene. Similar

conclusions, using nitrated tyrosine, were also observed by

Ding et al.94 Zhou et al.95 reported an investigation on CO

oxidation catalyzed by Au8 or Pt4 clusters on defective graphene.

They found that a defect greatly enhanced the reactivity of Au8and Pt4 clusters, and reduced the reaction barrier of catalyzed

CO oxidation from around 3.0 eV for the case of Au8 (0.5 eV for

the case of Pt4) to less than 0.2 eV (0.13 eV for Pt4).

Hence, theoretical calculations of graphene interactions

with different species can provide a huge help in establishing

structures and reaction mechanisms for the chemical modifi-

cations of graphene.96–98 This could lead to the preparation of

higher quality composite materials for a wide variety of

applications, among which are gas sensing and storage. Gas

sensing by graphene generally involves the adsorption and

desorption of gaseous molecules. These act as electron donors

(e.g. CO, ethanol and NH3) or acceptors (e.g. NO2, H2O and

I2) on the graphene surface, which lead to a change in local

carrier concentration, allowing the resistivity to be used as

a convenient means of measurement. Experiments by

Schedin et al.36 show that graphene-based sensors are capable

of detecting individual gas molecules due to their high sensi-

tivity to chemical doping. They found that graphene is highly

sensitive to NH3, CO, H2O and especially NO2. Theoretical

studies aided by DFT calculations were conducted to explain

the interactions of nitrogen oxides NOX (X = 1, 2, 3) and

N2O4 with graphene and graphene oxides by Tang and Cao.99

They observed that the adsorption of NOX on GO was

generally stronger than that on graphene due to the presence

of the active defect sites, which increase the binding energies

and enhance charge transfers from NOX to GO, eventually

inducing the chemisorption of gas molecules. In another case,

Fowler et al.100 reported the development of practical chemical

sensors from chemically converted graphene for the detection

of NO2, NH3, and 2,4-dinitrotoluene. They found that the

primary mechanism of the chemical response in sensors is

charge transfer between the analyte and graphene, while the

electrical contacts play only a limited role. Graphene-based

biosensors and devices have also exhibited good sensitivity and

selectivity towards the detection of glucose, hemoglobin,

cholesterol, H2O2, small biomolecules, DNA, heavy metal

ions, poisonous gaseous molecules, among others.101,102

The search for new hydrogen storage materials has attracted

a great deal of interest due to their important role in clean

energy alternatives. The ability of graphene to adsorb hydrogen

makes it an excellent candidate for hydrogen storage. In a

work published by Ghosh et al.,103 theoretical calculations are

directly compared against experimental results for the adsorption

of both H2 and CO2. Hydrogen storage reached 3.1 wt% at

100 bar and 298 K, and the uptake varied linearly with the

surface area. Theoretical calculations showed that single-layer

graphene could accommodate up to 7.7 wt% of hydrogen,

while double- and triple-layer graphene can have an uptake of

Table 1 The most stable site when an adatom is adsorbed on graphene together with bond energy (top position, eV); distance between the adatomand graphene (middle position, A); and migration energy for the most stable site (bottom position, eV). In bold cases the most stable elements ongraphene are defined by a bond distanceo2 A and migration energy >0.5 eV. Adapted with permission from ref. 83. Copyright 2010 Elsevier Ltd

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ca. 2.7 wt%. CO2 uptake of few-layer graphene at 1 atm and

195 K was around 35 wt% (theoretical calculations show that

graphene could have a maximum uptake of 37.9 wt% of CO2).

Ruoff et al.104 prepared a graphene-based powder sample by

chemical reduction of a colloidal suspension of exfoliated GO

with a BET surface area of 640 m2 g�1. The hydrogen

adsorption capacity of the obtained graphene was 1.2 wt%

at 77 K and 10 bar and 0.72 wt% at 100 bar and room

temperature.

The properties described in this section show that graphene

has an enormous potential in the catalysis field. Unfortunately,

the physical properties of graphene in its powder form do not

allow this material to be used in industrial reactors (e.g. fixed-

and fluidized-bed). The problem arises mostly due to the low

bulk density of graphene, which induces either a low catalyst

mass/reactor volume ratio (fixed-bed reactor) or a difficult

fluidization process (fluidized-bed reactor). However, this

limitation should depend on the number of layers. For example,

we have measured an apparent density of ca. 1.2 g cm�3 for

few-layer graphene of low surface area (40 m2 g�1) which can

be compared to the value reported forMWCNT (0.02–0.3 g cm�3)

and CNF (0.3–1.4 g cm�3).1 In an attempt to overcome these

difficulties, several strategies can be envisaged. One of the most

relevant consists in the macroscopic shaping of graphene to form

pellets. This pelletization process consists in the production of rigid

porous granules by extrusion, using binders to assist the aggrega-

tion. The process was already successfully performed to pelletize

CNT, and it was observed that the resulting material possessed

very similar textural properties compared to those observed for

CNTs in the powder form.105

In addition to the dependency regarding the number of defects,

most of these properties are also dependent on the number of

graphene layers present on the sample. In order to normalize

the terminology used, and based on their electronic spectra, it

is widely accepted that three different types of graphene can be

distinguished: single-, double- and few- (3 to 10) layer graphene.49

Thicker structures should be considered as thin graphite films

or flakes. In order to differentiate between these types of

graphene, several characterization techniques are presented

in the next section, special emphasis being given on how the

analysis results are affected by the number of layers.

2.3 Characterization

The identification and counting of graphene layers is one of

the major difficulties encountered during the characterization

of these materials. This is mainly due to the fact that mono-

layers are often in great minority among ensembles of thicker

crystals. Accordingly, a variety of techniques can be envisaged

for the analysis of graphene and its derivative materials.

Among the more frequently applied techniques are high

resolution transmission electron microscopy (HRTEM), Raman

spectroscopy, X-ray diffraction (XRD), atomic force microscopy

(AFM), scanning tunneling microscopy (STM) and nitrogen

adsorption–desorption at 77 K. Given that no single technique

is able to provide all the necessary information, it is necessary to

couple two, and sometimes more, of these tools to accurately

characterize the morphology, texture, crystal structure and in-

trinsic properties of graphene-based materials.65,43–45,106 Specific

information about the most relevant techniques for the charac-

terizations of graphene-based materials is summarized below.

The atomic structure of single-layer graphene can be studied

by HRTEM (Fig. 3). This technique is especially important

because it allows the evaluation of the crystalline character for

graphene flakes based on their electron diffraction patterns.

The central part of the sheets usually appears on TEM images

as uniform and spotless areas (sheets often present wrinkles,

identifiable as dark marks), whereas near the edges the sheets

tend to roll (Fig. 3a). Such folds provide a clear TEM signature

for the number of graphene layers. A folded graphene sheet is

locally parallel to the electron beam and for monolayer

graphene a fold exhibits a dark line. Under favorable conditions

(namely sample orientation), these folds situated at the edges

or within the free hanging sheets could be used to estimate the

number of layers present in a sample by direct visualization.

Nevertheless, this counting technique should be done very

carefully, since multiple folds can give rise to several dark lines

(even for monolayer graphene), as evidenced experimentally.107

Fig. 3 (a) TEM with the corresponding selected area diffraction

pattern in inset, and (b) HRTEM micrograph of graphene.

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Electron diffraction patterns show an expected hexagonal

lattice of graphene (inset, Fig. 3a). Suspended graphene also

evidences ‘‘rippling’’ of the flat sheet. Fig. 3b evidences regions

where fringes are observed and regions where they are not,

which indicates that there is a curvature in the sheets. These

ripples may be intrinsic to graphene as a result of the instability

of two-dimensional crystals, or may be extrinsic, originating

from the presence of contaminations.108

Raman spectroscopy is a powerful non-destructive tool

to characterize carbonaceous materials, particularly for

distinguishing ordered and disordered crystal structures of

carbon.109,110 Accordingly, it provides a quick and facile way

to characterize the structure and quality of graphene. The two

most intense features are the G (ca. 1580 cm�1) and the G0

band (ca. 2700 cm�1), the second most prominent peak always

observed. When a certain amount of disorder or edges appear

within the structure, a disorder-induced band (D-band, ca.

1350 cm�1) appears. If no D-band is observed for graphene,

this indicates the absence of a significant number of defects.

On the other hand, a D-band is often observed when symmetry is

broken by edges or in samples with a high density of defects.

Similarly to TEM analysis, Raman spectroscopy can also be

used to determine the number of graphene sheets present. This

technique is particularly sensitive for low number of layers.

Hence, one can clearly distinguish a single layer, from a

bilayer, from few layers (less than 5). For more than 5 layers

the Raman spectra become hardly distinguishable from that of

bulk graphite (Fig. 4).111 In bulk graphite, the G0 band

includes two contributions, the intensities of which are roughly14and 1

2that of the G peak for the low and high shifts,

respectively. For single layer graphene, the G0 band is a single

sharp peak at the lower shift, with intensity roughly 4 times

that of the G peak.44 As a consequence from varying number

of layers, the G0 band changes its shape, width and position

with increasing number of layers, whereas the G peak position

shows a down-shift with number of layers (Fig. 4).69 The

in-plane crystallite sizes (La) can be calculated from the Raman

spectra of the graphene samples by employing the relation

La = 4.4 (IG/ID).112

The crystalline structures of pristine graphite and graphite

oxides can also be evaluated by X-ray diffraction (XRD). The

feature diffraction peak for both graphene and exfoliated

graphene oxide is related to the A–B stacking order, corres-

ponding to the (002) reflection. This peak appears at 2y E 261

for pristine graphite, whereas the same peak is shifted to 2yE 111

after the oxidation of the layers (Fig. 5). Using the Scherrer

equation, the number of layers in graphene samples can be

obtained from the corresponding line broadening by Lorentzian

fitting of the (002) reflection.45 The distance between layers

(d-spacing) is typically 0.335 nm for graphite. The oxidation of

graphite is accompanied by the increase of the d-spacing,

indicating the presence of intercalated species between the

graphene layers. A sharp reflection (low full width at half

maximum) in the XRD pattern indicates that the sample

contains a large number of layers.

Atomic force microscopy (AFM) is currently the leading

method allowing the identification of single- and few-layer

crystals.59 As the tip scans across the surface, it is possible to

analyze the topography of the sample, and thus, count the

number of layers present by differential height measurements

at the edge. The use of different AFM techniques allows the

study of mechanical, electrical, magnetic, and even elastic

properties of graphene flakes.51,80,114

While with AFM one can directly obtain the number of

layers, scanning tunneling microscopy (STM) images are

useful in determining the morphology and presence of defects

Fig. 4 Both the G and G0 bands undergo significant changes due to

the number of layers. Reprinted with permission from ref. 111.

Copyright 2006 American Physical Society.

Fig. 5 X-Ray diffraction patterns of (a) pristine graphite, (b) exfoliated

GO, (c) electrochemically reduced GO and (d) chemically reduced GO.

Reprinted with permission from ref. 113. Copyright 2009 American

Chemical Society.

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on graphene. This technique allows one to obtain atomically

resolved images of single-layered graphene. The characteristic

features of STM images are readily interpreted in terms of the

A–B stacking of the graphene planes in graphite. In bulk

graphite, the carbon atoms on the surface are not equivalent to

those directly beneath (Fig. 6b). Hence, half of the carbon

atoms in the surface layer are located above carbon atoms in

the adjacent lower layer (A-type atoms); the other half is

placed over a void (B-type). This asymmetry in the surface

atom electronic environment results in a threefold symmetry

(‘‘three-for-six’’) pattern in which three bright or dark features

can be observed for each set of six carbon atoms. This

behavior also is present for graphene flakes that are two or

more atomic layers thick. For single-layer sheets of graphene,

this asymmetry disappears (Fig. 6a).115

With the exception of STM, all other previously described

local analysis techniques can, to some extent, enable an

approximate calculation of the number of layers present in a

graphene sample. Even though this is not the case for gas

adsorption measurements (commonly N2 adsorption-desorption

at 77K), these can provide important data regarding the textural

properties of bulk samples. Despite the possibility to determine

the specific surface area, total pore volume, and pore size

distribution for most solids, for graphene only the first is of

significant importance, due to the nature of its texture. Theoretical

calculations have shown that the highest surface area possible for

a single layer graphene is 2630 m2 g�1.116 Unfortunately, this

value would only be observed in a hypothetical case where no

overlap of sheets existed. In a real system, a significant amount of

surface area is not available for N2 adsorption because of the

overlap of exfoliated graphene sheets. From an experimental

point of view, the calculation of specific surface areas can be

severely affected by the random agglomeration state of the dry

powders. This often results in large surface area variations, even

within the same batch. Table 2 compares the specific BET surface

areas as a function of the preparation method and corresponding

number of layers. As shown in Table 2, most of the surface areas

published in the literature are obtained through the exfoliation of

graphitic oxide. This is probably due to the ease of this technique

to provide larger productivities, necessary for reliable measure-

ments. Given the absence of textural data for graphene samples

synthesized by micromechanical cleavage, epitaxial growth on

single-crystal silicon carbide and chemical vapor deposition a full

comparison between most common methods cannot be made.

3. Decoration of graphene-based materials

As mentioned in the previous section, graphene possesses a

plate-like structure with a large specific surface area. This

textural property, coupled with its excellent thermal, electronic

and mechanical features, makes graphene an attractive substrate

for the deposition of inorganic nanoparticles to produce highly

dispersed composites. In addition, aggregation of graphene

sheets can be partly prevented by intercalating particles within

the graphene layers.

Graphene sheets can be blended with various components to

form functional composites. Most of the graphene-based

materials have two components, although materials containing

Fig. 6 (a) STM image from a single layer of graphene, where the honeycomb structure is observed; (b) STM image of graphite showing a

threefold symmetry pattern in which three bright (or dark) features can be observed for each set of six carbon atoms; reprinted with permission

from ref. 115. Copyright 2007 by National Academy of Sciences of the USA.

Table 2 Comparison between specific BET surface areas obtained for different graphene samples with the corresponding number of layers andpreparation method

Preparation method Number of layers SBET/m2 g�1 Reference

Thermal exfoliation of graphitic oxide 1–3 700–1500 73Thermal exfoliation of graphitic oxide 3–4 270–1550 103Thermal exfoliation of graphitic oxide 3–6 925 11Thermal exfoliation of graphitic oxide Ultrathin sheets 737 117Chemical exfoliation of graphitic oxide 1 705 118Chemical exfoliation of graphitic oxide 3–7 640 104Microwave exfoliation of graphite oxide Few-layered 463 119SiC-derived — 300–950 120

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more than two components can also be produced to achieve

the requirements of specific applications. The incorporation of

graphene into the composites can provide unique properties

and possibly induce new functions based on synergetic effects,

providing a new opportunity for designing and developing

next-generation catalysts. Usually, the second component can

be a metal,14–15,17,121–127 metal oxide,128–134 polymer (insulating

and conducting),135–137 small organic compound,138,139 bio-

material,34,140,141 metal–organic framework,142–144 or even other

carbon nanomaterials (carbon nanotubes or fullerenes).145–149 In

this section, we will mainly focus on recent achievements dealing

with the development of effective strategies for synthesizing high-

quality graphene–metal and graphene–metal oxide composites.

3.1 Graphene–metal nanoparticles

Graphene sheets decorated with metal nanoparticles are an

example of emerging metal–carbon composites that currently

attract special research efforts due to their enhanced potential for

several applications. Most types of composites reported in the

literature consist of noble metal nanoparticles, including

Au,14–16,88,121,122,150–169 Pt,15,122,125,126,155,166–168,170–191

Pd,12,13,15,17,127,155,167,168,170,181,192–202 Ag,15,123,124,167–169,203–212

Ru,125,167,172,180,182 Rh,167 and Ir.167 In addition, metal nano-

particles of Fe,213,214 Cu,201,215 Ni,188 Co,216 Ge,217 and Sn218

were also used to produce metal–graphene composites.

There are several strategies commonly used to synthesize well

dispersed metal nanoparticles on the basal plane of graphene.

Among those, solution-based techniques, where the liquid wets

the entire surface area of graphene, are generally preferred. To

synthesize nanoparticles using this approach, several factors need

to be carefully controlled to obtain a narrow particle size

distribution, namely, type of solvent, nature and concentration

of metal precursor, the presence of a dispersing and/or reducing

agent, and finally the deposition time and temperature. One

common technique to produce nanostructured graphene compo-

sites, already used to cover other carbonaceous materials,

consists of the chemical functionalization of the graphitic surface

in order to induce anchoring sites for the metal precursor

nucleation. This allows a covalent bond of the metal to the basal

plane of graphene, yielding high dispersions (Fig. 7 and Table 3).

Depending on the preparation method, graphene sheets can

already possess a high amount of functional groups,

e.g. graphene oxide (as a result of the exfoliation of graphite

oxide). Then, graphene oxide and the metal precursor can be

chemically reduced to form the corresponding graphene–metal

composite.122,124,125,152–155,170,172–176,196–198,208,211,219 It has been

found that graphene oxide is better than its reduced counterpart

for in situ growth of nanoparticles.152,163,204 An alternate

reduction method involves the simultaneous reduction of both

metal nanoparticles and the graphene oxide by means of micro-

wave-irradiation.13,121,150,171,179,187,199–201,220,221 The main advan-

tage of this method over other conventional heating methods is

that the reactionmixture is heated uniformly and rapidly, allowing

for large-scale and highly efficient production of graphene–metal

composites.

The downside to the covalent attachment of metal particles

on the graphene is the disruption of the sp2 bonded carbon

atoms in the basal plane which leads to reduced transport

properties of graphene because of additional scattering

sites.150 A possible way to circumvent the disruption of the

sp2 carbon atoms involves the non-covalent wrapping of the

graphitic surface with a surfactant or a polymer. By this way,

the nanoparticles can grow on the surfactant with a minimal

chemical perturbation of the basal planes. Furthermore, the

use of surfactants also allows the control of the morphology

(size and shape) of the metal nanoparticles.161,174,198,222

Unfortunately, since the metal is not covalently bonded to

the graphene, the liaison is much weaker and thus can be easily

broken and induce a leaching effect.

Other techniques for metal nanoparticle decoration on the

graphitic nanostructure include electro-deposition,186,192 thermal

evaporation,223,224 photochemical,162,225 and solventless bulk

synthesis.226 Although these methods present some processing

advantages over solution-based techniques, they can be some-

what expensive and energy consuming. Table 3 shows a

comparison between common preparation methods for metal-

graphene composites.

3.2 Graphene–metal oxide composites

There is currently great demand for the synthesis of graphene–

semiconductor composites. The development of graphene–metal

oxide composites (havingmore versatile and tailor-made properties

with performances superior to those of the individual

materials) provides an important milestone to improve the

application of oxide nanomaterials in different fields such as

energy harvesting, conversion and storage devices, nano-

electronics, nano-optics and conductors, among others. In addi-

tion, the application of these materials in the catalysis field has

grown considerably over the last few years with the use of

graphene-based composites in photocatalysis (see Section 4.2.3).

To date, various kinds of inorganic nanomaterials have been

synthesized and supported on graphene-based templates including

TiO2,10,24–27,128,228–252 ZnO,129,253–266 SnO2,

235,267–278

MnO2,130,268,279–287 Fe3O4,

10,131,288–301 Fe2O3,291,302–306

Co3O4,134,216,307–312 NiO,6,268,313–316 ZrO2,

317 SiO2,318

Cu2O,253,319–321 RuO2,

10,132,322,323 Al2O3,133,324–327 MoO3,

328,329

ZnFeO4,330 BiWO6

331 and LiFePO4.332,333 Additionally, other

materials like CdS,29–31,334–339 CdSe,340–344 and ZnS31,334 have

also been used to fabricate graphene-based composites.

Fig. 7 TEM micrograph of a Pt–Ru/G catalyst.

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One important obstacle in producing graphene–metal oxide

nanomaterials is the difficulty of homogeneously dispersing

the oxide over the graphene, since aggregation reduces the

electrical, optical and magnetic properties of the resulting

composite. In order to overcome this limitation, several de-

position techniques have been developed with varying

degrees of success. The synthetic methods for preparation of

these graphene–semiconductor nanomaterial composites include

sol–gel, hydrothermal/solvothermal process, electrochemical

deposition, microwave-assisted growth, among others.

The direct growth approach is the most commonly used to

prepare graphene–metal oxide composites. Usually, the metal

Table 3 Comparison of typical preparation methods for metal/graphene composites, accompanied by the corresponding metal precursor,loading, particle size, and related applications

Composite Metal precursor Preparation methodAmount ofmetal/wt%

Particlesize/nm Application Reference

Au/G Chloroauric acid Reduction with sodiumcitrate

8.4 20 Surface-enhancedRaman spectroscopy

152

Au/G Chloroauric acid Reduction with sodiumdodecyl sulfate

8 2–3 Suzuki reaction 16

21 7.5Au/G Chloroauric acid Photochemical reduction 1.2 o1 — 162Pt–Au/G Hexachloroplatinic acid and

chloroauric acidReduction with sodiumborohydride

18.2 3.3 Formic acid oxidation 122

Pt–Ru/G Pt(CH3)2(COD) andRu(COD)(COT)*

Gas-phase reduction 5 2–3 — This work

Pt/G Hexachloroplatinic acid Reduction with ethyleneglycol

11 2–5.5 Methanol oxidation 227

Pt/N–G Hexachloroplatinic acid Reduction with ethyleneglycol

14 2–3 Methanol oxidation 227

Pt/G Diammine dinitritoplatinum(II) Gas-phase reduction 20 0.5–2 Hydrogen oxidation 189Pt/G Potassium tetrachloroplatinate(II) Electrodeposition — As small as 2 Methanol oxidation 186Pt/G Hexachloroplatinic acid Gas-phase reduction 20 2 Oxygen reduction 126Pt/G Potassium hexachloroplatinate(IV) Microwave synthesis 30 3.31 Methanol oxidation 187

47 4.8967 5.81

Pd/G Tetraamminepalladium(II) nitrate Gas-phase reduction 0.18 1–6 Hydrogenation of alkynes 17Pd/G Potassium tetrachloropalladate(II) Reduction with sodium

borohydride0.19 2.5 Hydrogenation of alkynes 194

Pd/G Palladium(II) chloride Microwave assistedreduction

20 2.5 Oxidation of methanol andethanol

13

Pd/G Palladium(II) acetate Reduction with hydrazine 7.5 2.0–5.6 — 196Pd/G Palladium nitrate Microwave assisted

reduction7.9 7–9 Suzuki and Heck reactions 200

Pd/GO 6.4 12–15Ag/G Silver nitrate Reduction with PVP 65–88 10–30 Surface-enhanced Raman

spectroscopy209

Ag/G Silver nitrate Hydrothermal synthesis 56 10 Surface-enhanced Ramanspectroscopy

207

Ag/G Silver nitrate Reduction with sodiumborohydride

— 5–10 SERS and antibacterialactivity

124

*COD: 1,5-cyclooctadiene; COT: 1,3,5-cyclooctatriene.

Table 4 Comparison of typical preparation methods for graphene–metal oxide composites, accompanied by the corresponding metal precursor,loading, and particle size

Composite Metal precursor Preparation methodAmount ofmetal oxide/wt% Particle size/nm Reference

G-TiO2 nanorod Titanium isopropoxide Ex situ synthesis 55 2–4 diameter 20–30 length 24G-TiO2 Titanium trichloride Direct growth 85 o15 235G-TiO2 anatase Titanium trichloride Sol–gel 97.5 5 243G-TiO2 rutile Titanium trichloride Sol–gel 90–99.5 6 243G-TiO2 Tetrabutyl titanate Hydrothermal synthesis 67 9 238G-ZnO Zinc acetate Ex situ synthesis Not specified 4.5 261G-SnO2 Tin(II) chloride dihydrate Direct growth 85 B5–10 235G-MnO2 Potassium permanganate Microwave irradiation 78 5–10 282G-Mn3O4 Manganese(II) acetate Direct growth and hydrothermal 90 10–20 347G-Fe3O4 Iron(III) chloride Direct growth 5.3–57.9 1.2–6.3 298G-Fe2O3 Iron(III) chloride Direct growth 80 60 304G-Co3O4 Cobalt nitrate Direct growth 75.4 10–30 307G-RuO2 Ruthenium(III) chloride Sol–gel 38.3 5–20 132G-Al2O3 a-Aluminium oxide Mechanical mixture 85–98 vol. % 2.5–20 325

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precursor is mixed with GO and then converted to the

corresponding oxide. After reduction of GO, graphene–metal

oxide composites are finally obtained.25,229,235,242,243,265,335,345,346

When reducing GO to rGO, in order to avoid the simultaneous

reduction of the as-prepared oxide, experimental conditions

should be carefully chosen (strength and concentration of the

agent, temperature, pressure, duration, etc.). The sol–gel method

is a popular approach for preparation of metal oxide structures

and film coatings. One of the greatest advantages of this process

is the fact that surface hydroxyl groups of the GO/rGO sheets

can act as nucleation sites for the hydrolysis step. Hence, the

resulting metal oxide nanostructures are chemically bonded to

the GO/rGO surfaces.25,243,245,252,268,288,318 The hydrothermal/

solvothermal process is another effective method for the

preparation of semiconductor composites with graphene. This

route is a powerful tool for the synthesis of inorganic nanocrystals.

The one-pot process can give rise to nanostructures with high

crystallinity without post-synthetic annealing or calcination, and

at the same time reduce GO to rGO.26,31,234,238–240,330,331,335,341

An interesting approach that has been mainly developed for

thin film-based applications is electrochemical deposition. This

allows the decoration of inorganic crystals on graphene-based

substrates without the requirement for post-synthetic transfer of

the composite materials.253–254,319,342 Microwave irradiation has

also been used to prepare metal oxide–rGO composites, such as

rGO–MnO2282 and rGO–Co3O4.

310 In spite of the ease of process

and scalable production, microwave-assisted synthesis does not

display a fine control over the size uniformity and surface

distribution of NPs on rGO surfaces.

Another preparation method consists in the addition of

pre-synthesized nanoparticles to the GO suspension, followed

by chemical and/or thermal reduction to yield the final composite.

This ex situ synthesis allows a more precise control over the

particle size and surface properties of nanoparticles as there is no

interference from the GO/rGO and respective reducing agents, as

observed in the in situ case. However, the synthesis process

involves a chemical/thermal reduction to obtain the NPs/rGO

composite which may change the NPs surface properties and

damage graphene lattice. Table 4 shows several preparation

methods commonly used to obtain different graphene–metal oxide

composites.

4. Application of graphene-based nanomaterials in

catalysis

Ideally, graphene is a single-layer material, but samples with

few layers are also subject to increasing interest. This is mainly

due to the fact that single crystals can be obtained on top of

non-crystalline substrates, in liquid stabilized suspensions or

as suspended membranes.49 When a dispersion of isolated

graphene sheets is dried, graphene sheets tend to couple with

one another to stabilize into thicker layers, due to their

thermodynamical instability,348 forming aggregates with very

small interlayer spacing. This makes the application of atomic

monolayers in catalysis challenging, for which reason, most

catalytic applications have used few-layered graphene instead.

This limitation can be partially overcome by intercalating

nanoparticles within the graphene layers. This decreases the

chances of formation of a stacked graphitic structure, by working

as a ‘‘spacer’’ (the nanoparticles increase the distance between the

graphene sheets to several nanometres).174 Additionally, incor-

poration of a second component onto an individual graphene or

reduced graphene oxide sheet with good distribution aims to

achieve unique properties from their interaction, targeting at

catalytic, electrocatalytic and photocatalytic applications.

Recent progresses have shown that graphene can have a

deep impact on electronic and optoelectronic devices, chemical

sensors, nanocomposites, energy conversion and storage, and

catalysis. In the following sections, we will only focus on the

properties of graphene and graphene-based composites for

energy conversion and catalysis. For a detailed analysis

regarding other applications of graphene-based materials,

the reader can refer to several review articles recently

published.9,50,54,56,57,65,102,106,118,349–354

4.1 Metal-free graphene-based materials as catalysts

The use of metal-free carbons instead of metal-supported

catalysts in synthetic chemistry has largely progressed over

the last decade. This results mainly from diminishing supplies

of metals used in common industrial processes, and the

discovery and development of novel carbon forms as full-

erenes, nanotubes, nanofibers, graphene, among others.

The performance of a catalyst is influenced by the nature,

concentration and accessibility of the active sites that are

capable of chemisorbing the reactants and form surface inter-

mediates. It has been readily observed that functionalized

carbons (containing oxygen, nitrogen, or other surface groups)

are more efficient materials for catalysis than unfunctionalized

ones. While functional group-rich materials, such as GO,

exhibit high reactivity under mild conditions (their structure

is not fully understood rendering mechanistic elucidation

challenging), unfunctionalized graphene may not have a sufficient

number of reactive sites to be a viable catalyst for many

reactions. In the absence of defects, the basal planes are not

very reactive, with the only active sites being present at the

edges of the graphene layers as unsaturated carbon atoms.

Nevertheless, pristine graphene can still find catalytic applications

that make use of its delocalized p-electron system, such as

complexation reactions.355 On the other hand, cracking and

dehydrogenation of hydrocarbons have been reported using

unfunctionalized carbon nanomaterials like nanotubes and

fullerenes,2,356 which could be advantageously replaced by

graphene in the future.

In chemistry, graphitic forms of carbon have an intriguing

potential for catalysis. Early studies focused on simple redox

processes, but the field has progressed to demonstrate that

carbons enable reactions that are more sophisticated. These

include complex functional group transformations and

carbon–carbon or carbon–heteroatom bond formations. For

example, C60 has been reported to catalyze the hydrogenation

of nitrobenzene to aniline at room temperature under UV

irradiation.357 Carbon nanotubes have been used as catalysts

for methane decomposition,358 oxidation of p-toluidine,359

and conversion of aniline to azobenzene.360 However, the

most prominent example of heterogeneous gas-phase catalysis

by oxidized carbon materials is the selective oxidative dehydro-

genation of hydrocarbons (ODH).361–363 Ethylbenzene is

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predominantly used as the substrate, as the stable conjugated

system of styrene allows for high yields. Earlier studies performed

over activated carbons found that quinone surface groups were

the active sites for this reaction.364 Since then, carbon nano-

structures have also been reported to be active in the ODH of

ethylbenzene to styrene.361,365–367 Furthermore, due to their re-

markable stability and coke-resistance for this reaction, low-

dimensional nanocarbons with well-defined microstructures

often present higher yields when compared to other carbon

forms. In contrast, light alkanes are much less reactive than the

corresponding alkenes.363 ODH is thought to proceed by nucleo-

philic oxygen atoms located at the prismatic edges of stacked

graphene sheets or at surface defects in the (0001) graphitic

surface.368 Unfortunately, ODH of light alkanes suffers from

lower selectivities because the C–H bond in the product molecule

is weaker than in the substrate.

Trunschke et al.368 recently demonstrated that graphene

could also be a selective catalyst for model chemical reactions

involving the insertion of oxygen into organic molecules

(Fig. 8). They observed a 26.5 mmol g�1 h�1 productivity

for the selective oxidation of acrolein to acrylic acid, almost

half as high as that obtained with the industrial doped MoV

mixed oxide (ca. 60 mmol g�1 h�1). Liu et al.19 studied the

catalytic oxidation of SO2 gas to SO3 over porous graphene

oxide foams. According to the authors, GO not only acted as

catalyst to promote the reaction of SO2 and O2 to form SO3,

but also as oxidant in the reaction. Upon prolonged exposure,

the GO foams color gradually changed from brown to black.

This implied that the GO was partially reduced and some of

the oxygen-containing groups were lost (the GO turned from

hydrophilic to hydrophobic and precipitated). Since the reaction

takes place at room temperature and does not need noble

metal catalysts, it could be a green and inexpensive method for

the treatment of SO2 gas.

Given its structural features, GO also finds application as a

photocatalyst for H2 generation from water,369 providing a

suitable alternative to metal-containing photocatalysts. An

application that has received growing attention is the oxygen

reduction reaction (ORR) in fuel cells. This reaction is commonly

carried out using metal nanoparticles (particularly Pt). The major

drawback of metal-based cathode materials is that they tend to

be deactivated by CO poisoning and by sintering. Hence,

preparation of metal-free catalysts is one of the most effective

approaches for overcoming these problems with supported metal

catalysts. Recently, nitrogen-containing graphene (N-graphene)

has emerged as a promising candidate for the cathode catalyst

due to the excellent oxygen reduction reaction activity without

using any metal and the simple preparation procedure.370–374

Although a full understanding of the active sites in the catalysts is

not yet completely understood, nitrogen atoms present in graphitic

carbon are considered to play an essential role in high activity.

The use of metal-free carbon materials has been recently

reviewed by Bielawski et al., who investigated the catalytic

activity of various synthetic reactions under mild conditions in

the liquid-phase.18,21,375–379 Their results highlight the unique

role that large-area, functionalized carbon materials (GO) may

find in the activation of small molecules, such as O2, for

catalysis. Exploiting the reactivity intrinsic to graphite oxide

(GO tends to be highly acidic and strongly oxidizing),

Bielawski et al.375 have identified this material as a powerful

catalyst to be used in the generation of aldehydes or ketones

from various alcohols, alkenes and alkynes (Table 5). The

authors demonstrated the efficient oxidation of benzyl alcohol

to benzaldehyde (conversion >98%) in the presence of GO as

a heterogeneous catalyst. Further oxidation to benzoic acid

was observed in only minimal amounts and only under certain

conditions (e.g. at elevated temperatures). Interestingly, this

and other oxidation reactions of alcohols were performed

under ambient conditions and did not proceed under a nitrogen

atmosphere, suggesting that oxygen could be functioning as

the terminal oxidant. In the same study, they also demon-

strated that the scope of GO catalysis extends beyond simple

oxidation reactions of alcohols. The successful oxidation of

cis-stilbene to benzyl and hydration of various alkynes indicate

that the scope of the reactivity of GO may be quite broad. The

ability of GO to function as a carbocatalyst (metal-free carbon

material as catalyst) was further confirmed in another work by

Bielawski et al.376 where they produced chalcones in a single

reaction vessel (>60% isolated yields). GO was found to

function as an auto-tandem oxidation–hydration–aldol coupling

catalyst, as various alkynes were hydrated in situ to their

corresponding methyl ketones or alcohols were oxidized in situ

to their corresponding aldehydes. The condensation of the

methyl ketones with the aldehydes was believed to proceed via

a Claisen–Schmidt-type process, where the GO acted as an acid

catalyst. When an alkyne was substituted for the methyl ketone,

however, the condensation was likely preceded by hydration of

the alkyne. Similarly, when an alcohol was substituted for the

aldehyde, the condensation was preceded by oxidation of

the alcohol.

In yet another work by the same authors, GO was found to

be an effective oxidant for use in a broad range of reactions,

including the oxidation of olefins to their respective diones,

methylbenzenes to their respective aldehydes, diarylmethanes

to their respective ketones, and dehydrogenation of various

hydrocarbons.21 In this case, GO was found to be capable of

oxidizing cis-stilbene to benzyl, optimizing the yield with

regard to GO loading and reaction temperature. The authors

then explored the ability of GO to oxidize more challenging

substrates, including hydrocarbons possessing sp3-hybridized

C–H bonds. Various substrates with benzylic methylene

Fig. 8 Suggested reaction pathway for the oxidation of acrolein to

acrylic acid at the graphitic carbon surface. Reprinted with permission

from ref. 368. Copyright 2011 Wiley-VCH.

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groups were successfully converted to their corresponding

ketone and unsaturated products.

Since no reactivity was observed when hydrazine-reduced

graphene oxide or natural flake graphite were used, as an

explanation for the results obtained with GO, the authors

suggested that the presence of the surface bound oxygen-

containing functionalities played an important role in the

observed reactivities and product formation.

The majority of the reactions discussed in this section were

performed under relatively mild conditions and produced the

desired product (aldehyde, acid or ketone) in high yields,

without the need for additional oxidants or metals as co-catalysts.

However, reactivity results should be carefully evaluated, as to

exclude the possibility of metal-mediated catalysis, due to

potential contaminations (even in trace levels). Several advantages

can be pointed out in these reactions using GO: use of a simple

and inexpensive catalyst, metal-free reactivity, and facile recovery

of the GO from the reaction media by simple filtration.

4.2 Metal supported graphene-based materials as catalysts

4.2.1 Cross-coupling reactions. One of the catalytic appli-

cations in which graphene support may provide some signifi-

cant advantages is in the area of C–C cross-coupling

chemistry. Cross-coupling reactions, such as Mizoroki–Heck

and Suzuki–Miyaura, have been typically performed under

homogeneous conditions using Pd catalysts. Unfortunately,

catalyst recovery and recyclability remains a challenge, with

several attempts being made to overcome this difficulty. One of

the most obvious is the heterogenization of the catalysts.

Although heterogeneous supports do allow a more efficient

recovery, the activity of the immobilized catalysts frequently

decreases. Therefore, the development of Pd nanocatalysts

that combine high activity, stability, and recyclability is an

important goal in nanomaterials research. It was only recently

that graphene and graphite oxide have been considered as

potential supports for Pd-catalyzed C–C coupling reactions.

Scheuermann et al.12 investigated the immobilization and

intercalation of palladium nanoparticles using oxygen func-

tional groups present in graphite oxide. In contrast to con-

ventional Pd/activated carbon catalysts, graphite oxide and

graphene-based catalysts showed much higher activities very

low palladium leaching (o1 ppm) in the Suzuki–Miyaura

coupling reaction. Reuse of the catalysts could be achieved

despite some activity loss, depending on the recycling procedure.

In another work, a Pd/G catalyst demonstrated excellent

catalytic activity for both the Suzuki and Heck carbon–carbon

cross-coupling reactions.200 The Pd/G catalyst was recycled

eight times with a quantitative reaction yield. The remarkable

reactivity of Pd/G toward Suzuki cross-coupling reactions

(TOF = 108 000 h�1) was attributed to the high degree of

Table 5 Conversion of benzylic and aliphatic alcohols to their respective ketones or aldehyde products, and conversion of aryl and aliphaticalkynes to their respective methyl ketone products when reacted with graphene oxide. Adapted from ref. 375. Copyright 2010 Wiley-VCH

Alcohol Product Conv. (%) Alkyne Product Conv. (%)

>98 >98

96 52

26 41

>98 26

18 27

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the dispersion and loading of Pd(0) supported on graphene

sheets (particle size of 7–9 nm). Li et al.198 prepared a Pd/G

catalyst using sodium dodecyl sulfate as both surfactant and

reducing agent. They obtained Pd nanoparticles with an average

particle size of 4 nm, which demonstrated a good efficiency for

the Suzuki reaction under aqueous and aerobic conditions.

Zhang et al.14 reported the use of Au/GO nanocomposites,

which showed an unusually high activity for the Suzuki–Miyaura

coupling reaction of chlorobenzene with arylboronic acid (yield

as high as 98%). Graphene modified with Au nanoparticles was

also used as an efficient catalyst for the Suzuki reaction in water

under aerobic conditions.16 The catalytic activity of Au/G

hybrids was related to the Au loading and particle size.

4.2.2 Energy conversion. Graphene and graphene-based

materials have been considered as one of the most promising

alternatives as electrode materials in energy-related devices.

The reason is related to the high surface area, high conductivity,

unique graphitized basal plane structure, chemical tolerance and

a potentially low manufacturing cost. The advantages of graphene-

based composites have been demonstrated in the oxidation of

methanol,13,155,170,173,175,178,179,185,191,380 ethanol,13,127 formic acid,127

in the reduction of oxygen,126,156,176,380,381 and in hydrogen fuel

cells.183

Metal-decorated graphene has provided enhanced electro-

catalytic activity in alcohol oxidation reactions (energy

production) more efficiently than any other commercially

available material. Li et al.175 prepared a graphene-supported

Pt catalyst (dPt = 5–6 nm) with higher electrochemically active

surface area (ECSA) and electrocatalytic activity for methanol

oxidation than a commercial Pt/C catalyst. The lower oxidation

potential and higher current density over the Pt/G indicated a

higher catalytic activity for the methanol oxidation, which the

authors attributed to the higher ECSA and good Pt dispersion.

In addition, the stability of Pt/G was also found to be better

than that of Pt/C catalysts. Yoo et al.191 also reported an

enhanced electrocatalytic activity for the same reaction using a

Pt/G with metal particles smaller than 0.5 nm. The presence of

extremely small Pt clusters suggests a strong interaction

between graphene and Pt atoms. This interaction between Pt

and graphene was thought to induce some modulation in the

electronic structure of the Pt clusters. The Pt/G electrocatalyst

revealed an unusually high activity for methanol oxidation

reaction, and also exhibited quite a different behavior for CO

Fig. 9 Formation route to anchor platinum nanoparticles onto chemically converted graphene (a); TEM images of (b) Pt/G and (c) Pt/CNTs

hybrids. Reprinted with permission from ref. 173. Copyright 2009 Elsevier Ltd.

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oxidation compared to a Pt/carbon-black catalyst. In a work

by Wang et al.173 the performance of Pt/G catalysts was

directly compared to that of Pt/CNTs (dPt = ca. 3 nm for

both supports, Fig. 9). Measurements showed that Pt/G

catalyzed the methanol oxidation more efficiently (higher

oxidation current density), which the authors attributed to a

larger ECSA.

Kundu et al.179 used ethylene glycol andmicrowave irradiation

conditions to prepare the graphene-supported Pt catalyst

(dPt = 2–3 nm) by the co-reduction of graphene oxide and

Pt salt. The catalyst exhibited an excellent catalytic activity

coupled with long-term stability for methanol oxidation.

Similarly to other works, Pt/G was found to be a better

catalyst than Pt/C in terms of both current density and CO

tolerance, although morphologically similar.

On the other hand, Pt/G has also been used to catalyze the

oxygen reduction reaction for use in fuel cells. Similar to the

catalyzed alcohol oxidation reactions, Pt/G with large ECSA

also promotes an efficient ORR catalysis. Xin et al.380 have

recently demonstrated the use of a Pt/G catalyst for high

catalytic activity of both methanol oxidation and oxygen

reduction, when compared to Pt supported on carbon-black.

The performance of Pt/G was further improved (ca. 3.5 times

higher than Pt/C) after heat treatment in a N2 atmosphere at

300 1C. Shao et al.176 reported that the ORR performance of

Pt/G was comparable to that of Pt/CNT catalysts. However,

Pt/G showed an enhanced stability compared to Pt/CNTs and

commercially available Pt/C materials. Kou et al.126 observed

similar electrochemical results using Pt nanoparticles with an

average diameter of 2 nm dispersed over GO. Pt/G showed not

only larger specific surface area and higher ORR activity, but

also excellent stability after 5000 cyclic voltammetry cycles.

These improved properties were attributed to the smaller

aggregation of Pt particles immobilized on graphene. In an

attempt to further improve the ORR activity, carbon nitride

was also incorporated into graphene to produce an electro-

catalyst composite.382,383 In a work published by Shi et al.,382

this composite exhibited an electrocatalytic activity for ORR

comparable to that of a rGO composite with ca. 23 wt% Pt

nanoparticles. Metal-free graphene–carbon nitride composites

also showed high visible-light photocatalytic activity (making

them promising nanomaterials for applications in water treatment

and dye-sensitized solar cells),384 and the ability to activate O2

for the selective oxidation of secondary C–H bonds of cyclo-

hexane (good conversion and high selectivity to the corres-

ponding ketones).385

A common problem when using Pt catalysts in fuel cells is its

poisoning by carbon monoxide. One possible solution to this

problem is the use of Pt based alloys as catalysts. Dong et al.172

studied the electrocatalytic activity of graphene supported Pt–Ru

nanoparticles for methanol and ethanol oxidation. Compared to

the widely used Vulcan XC-72R carbon-black, graphene strongly

enhanced the oxidation efficiencies of both methanol and ethanol.

Furthermore, the introduction of Ru greatly reduced the adsorption

of CO-like carbonaceous species on the surfaces of Pt particles.

Another similar work has demonstrated that using a graphene-

supported Pt–Ru nanocomposite, an improvement in catalytic

activity is achievable towards the oxidation of methanol when

Fig. 10 TEM images of (a) Pt–Au/G and (b) Pt–Au/carbon-black; (c) Pt–Au/G show the highest electrocatalytic activity and stability toward

formic acid oxidation compared to Pt–Au/carbon-black and E-TEK Pt/C. Reprinted with permission from ref. 122. Copyright 2011 American

Chemical Society.

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contrasted to Pt–Ru/C.180 Still in the field of bimetallic catalysts,

Zhang et al.122 prepared a Pt–Au/G catalyst (Pt :Au= 1 : 1) via a

polyelectrolyte-assisted process. The electrocatalytic activity of

this bimetallic catalyst was compared against that obtained with

Pt–Au/carbon-black in the formic acid oxidation. The Pt–Au/G

catalyst displayed a 37% higher electrocatalytic activity toward

formic acid oxidation than Pt–Au/carbon-black (Fig. 10). The

higher electrocatalytic activity of Pt–Au/G was attributed to a

strong electronic interaction between graphene and Pt–Au alloy

nanoparticles, which suppressed the CO poisoning and facilitated

the direct oxidation process of formic acid on the Pt–Au surface.

A composite containing Pt-on-Pd bimetallic nanodendrites

and graphene was synthesized and its catalytic activity investi-

gated using the oxidation of methanol.170 The electrochemical

data indicated that the as-prepared Pt–Pd/G composite

exhibited much higher electrocatalytic activity toward methanol

oxidation reaction than the platinum black and commercial

E-TEK Pt/C catalysts. The current density of methanol oxidation

catalyzed by this composite was about 3.0 and 9.5 times higher

than those of E-TEK and platinum black catalysts, respectively.

Nitrogen doped carbon materials are recognized as good

supports for Pt catalysts. The nitrogen atoms not only provide

anchoring sites for the metal particles, but also act as chemically

active sites for some catalytic reactions. Wu et al.227 prepared

N-doped graphene by heating graphene oxide in an ammonia flow

at different temperatures. The methanol oxidation current for

Pt/N–graphene heated at 800 1C was found to be 3 times higher

than that of those treated at lower temperatures (300, 500 or

700 1C). Moreover, it was much higher than that of the Pt/C

commercial catalyst. This behavior was attributed to a higher

conductivity and more uniformly dispersed Pt nanoparticles over

the surface of the composite treated at 800 1C. Ramaprabhu

et al.381 used graphene and nitrogen doped-graphene as catalytic

support of Pt nanoparticles for ORR. The authors attributed the

enhanced performance to an improved metal–carbon interaction

and increased electrical conductivity induced by nitrogen doping.

One other possible application of graphene-based composites

consists of the conversion of solar energy to electrical power,

i.e. solar cells. These cells with transparent and conductive

graphene film as window electrode have exhibited considerable

power conversion efficiency. Among the most commonly studied

are dye-sensitized and heterojunction solar cells.57,353,386–390

Graphene films with excellent conductance, good transparency

in both the visible and near-infrared regions, ultrasmooth surface

with tunable wettability, high chemical and thermal stabilities

and flexibility for transfer between alternative substrates, can be

used not only in solar cells as electrode but also in many other

optoelectronic devices.353

4.2.3 Photocatalytic applications. In a photocatalytic

reaction, photo-generated electron–hole pairs are formed on

the catalyst surface (e.g. TiO2). As opposed to an energy

generation device where the charge carriers are collected by an

electrode, in a photocatalytic reaction they are directly scavenged

by different species present in solution. However, the photo-

generated electrons and holes in the excited states are very

unstable and can easily recombine, dissipating the input energy

as heat, which results in low process efficiency. Owing to its

superior electron mobility and high specific surface area, graphene

is considered a high performance support for photocatalysis.

Lightcap et al.225 demonstrated the viability of using a graphene

as an electron-transfer medium. They showed that graphene was

able to store and transport electrons through a stepwise electron

transfer process: electrons were photogenerated in TiO2 and then

transferred to GO; then, part of these electrons were involved in

the reduction of GO, whereas the remaining were stored in the

rGO sheets; finally, upon introduction of silver nitrate, the stored

electrons were used to reduce Ag+ to Ag0 (Fig. 11). Hence,

graphene could be regarded as an effective tool to be used in the

prevention of electron–hole recombination by accepting and

transporting photoelectrons.

Possible applications of graphene-basedmaterials in photocatalysis

involve mainly the degradation of pollutants,24–26,234,249,266,391,392

and water splitting for hydrogen generation.27,369

A graphene–P25 TiO2 composite was applied to the photo-

catalytic degradation of organic compounds and compared

against bare P25 TiO2 and a CNT–P25 TiO2 composite

(Fig. 12).26 In their work, the graphene composite was found

to have high dye absorptivity, extended light absorption range,

and enhanced charge separation and transportation properties.

The authors attributed the enhanced photocatalytic activity in

the degradation of methylene blue dye under both UV and

visible lights to the two-dimensional conjugated structure of

graphene, which facilitated a better platform for dye adsorption

and charge transportation. Liu et al.24 reported the application

of a TiO2 nanorod–graphene composite in the degradation of

methylene blue under UV light irradiation. They observed an

effective reduction in charge recombination because of improved

contact between graphene and TiO2 nanorods, increasing the

photocatalytic activity. This result could open important perspec-

tives for improving the photocatalytic activity of graphene–TiO2

composites by optimizing the morphology and distribution of

TiO2 nanoparticles on graphene sheets. In an attempt to improve

the visible-light response of graphene–TiO2 photocatalysts,

Chen et al.231 reported a GO–TiO2 composite with p/n hetero-

junction in the degradation of methyl orange. In addition to

graphene–TiO2 composites, other materials have also been used

as efficient photocatalysts for decomposition of different pollu-

tants in water, namely graphene–SnO2,235 and graphene–ZnO.265

Photocatalytic water splitting into hydrogen and oxygen

using semiconductor photocatalysts has been considered as a

promising and attractive approach to produce hydrogen energy.

Unfortunately, due to the rapid recombination of photogenerated

electrons and holes practical applications are quite limited.

Fig. 11 Illustration of the three-step electron transfer process involved

in making a two-dimensional conducting support. Reprinted with

permission from ref. 225. Copyright 2010 American Chemical Society.

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A possible way of improving photocatalytic hydrogen production

using graphene-based materials requires the presence of a sacrificial

agent. Using graphene–TiO2 composites with different graphene

loadings (prepared by a sol–gel method), Zhang et al.27 studied the

H2 evolution from aqueous solution containing Na2S and Na2SO3

as sacrificial agents under UV-Vis irradiation. The optimal

graphene content was found to be 5 wt%, yielding aH2 production

rate which exceeded that of pure P25 TiO2 over 2 times. In a similar

work, Fan et al.393 prepared a graphene–P25 TiO2 (0.2 : 1 optimum

ratio) composite that improved the H2 production rate by more

than 10 times, when compared to that of pure P25 TiO2.

Despite some very promising results, the mechanism of

photocatalytic enhancement by graphene-based composites

is relatively uncertain. Some questions have raised the discus-

sion on whether or not the graphene composites are truly

different from other carbonaceous (activated carbon, fullerenes

or carbon nanotubes) composite materials. In a work by

Zhang et al.,234 a graphene–TiO2 composite was observed to

be essentially the same as other carbon–TiO2 (activated carbon,

fullerenes and carbon nanotubes) composite materials, regarding

the enhancement of photocatalytic activity of TiO2.

5. Summary and outlook

Graphene has come a long way since it was first reported in

2004 by Novoselov and co-workers. The rise of graphene

nanosheets has opened a new route for the use of two-

dimensional carbon materials as catalytic supports, due to

their high electrical and thermal conductivities, great mechanical

strength and huge specific surface area and adsorption capacities.

This has allowed researchers to design and develop countless

combinations of graphene-based materials, some rather simple

while others considerably more sophisticated. Graphene and

its composites have been used as catalysts for chemical, electro-

chemical and photochemical reactions, showing promising

results, especially when compared to conventional catalysts.

One of the most attractive areas in catalysis involves the use of

metal-free graphene. As we described in this review, this field has

grown in complexity over the last few years and we expect it to

know further developments, as theoretical calculations can be

very useful to fine-tune the surface properties of graphene.

Unfortunately, despite all of the promising results obtained

so far, the generalization of graphene-based materials is

dependent on one very important premise: the large-scale

availability of high-quality graphene with controllable layer

thickness, at relatively low cost. In addition to this issue,

physical properties such as low bulk density can severely

hinder the catalytic application of graphene at the industrial

scale. Thus, there is still a long road ahead before graphene-

based catalysts can effectively find a commercial application.

Acknowledgements

The authors gratefully acknowledge Dr Revathi Bacsa for the

selected area diffraction pattern, TEM and HRTEM micro-

graphs used in this work. B.F.M. acknowledges Fundacao

para a Ciencia e a Tecnologia for the grant SFRH/BPD/

70299/2010.

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(22)Review Graphene Based Materials

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