Graphene-based materials for catalysis

Abstract

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.

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Bruno F. Machado

Bruno F. Machado received is PhD in chemical and biological engineering from the University of Porto in 2009. He is currently carrying out his postdoctoral work at Laboratory of Coordination Chemistry (University of Toulouse, France) in the field of nanotechnology and heterogeneous catalysis. His research interests include the development and preparation of carbon-based nanocomposites with semiconductor nanostructures, as well as their applications in catalysis.

Philippe Serp

Philippe SERP is full Professor of Inorganic Chemistry at Ecole Nationale supérieure des Ingénieurs en Arts Chimiques Et Technologique Toulouse University. He was the recipient of the Catalysis Division of the French Chemical Society Award in 2004, and the APDF “Celestino da Costa/Jean Perrin” award in 2005. His current research interests in Laboratoire de Chimie de Coordination (UPR 8241 CNRS) include nanocatalysis, gas phase preparation 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.

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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.²Carbon 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 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 experiments 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 photocatalysis,^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 hydrocarbon 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 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.

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

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 applications. 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 σ bonds work as the rigid backbone of the hexagonal structure, the out-of plane π bonds control interaction between different graphene layers. This allows delocalized π 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 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. Understanding 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 configurations 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

Fig. 2 Three different high symmetry adsorption sites: hollow (H), bridge (B) and top (T).

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) accompanied 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 Å 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.

Table 1 The most stable site when an adatom is adsorbed on graphene together with bond energy (top position, eV); distance between the adatom and graphene (middle position, Å); and migration energy for the most stable site (bottom position, eV). In bold cases the most stable elements on graphene are defined by a bond distance <2 Å and migration energy >0.5 eV. Adapted with permission from ref. 83. Copyright 2010 Elsevier Ltd

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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, 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 Au₈ and 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 modifications 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 sensitivity 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 NO_X (X = 1, 2, 3) and N₂O₄ with graphene and graphene oxides by Tang and Cao.⁹⁹ They observed that the adsorption of NO_X 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 NO_X 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 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 for MWCNT (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 aggregation. 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 monolayers 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 intrinsic properties of graphene-based materials.^{65,43–45,106} Specific information about the most relevant techniques for the characterizations 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.¹⁰⁷Electron 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

Fig. 3 (a) TEM with the corresponding selected area diffraction pattern in inset, and (b) HRTEM micrograph of graphene.

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 G′ 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 G′ band includes two contributions, the intensities of which are roughly ¼ and ½ that of the G peak for the low and high shifts, respectively. For single layer graphene, the G′ 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 G′ 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 (L_a) can be calculated from the Raman spectra of the graphene samples by employing the relation L_a = 4.4 (I_G/I_D).¹¹²

Fig. 4 Both the G and G′ bands undergo significant changes due to the number of layers. Reprinted with permission from ref. 111. Copyright 2006 American Physical Society.

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, corresponding to the (002) reflection. This peak appears at 2θ ≈ 26° for pristine graphite, whereas the same peak is shifted to 2θ ≈ 11° 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.

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.

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 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

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.

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 N₂adsorption-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 measurements. 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.

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

Preparation method	Number of layers	S BET/m^2 g^−1	Reference
Thermal exfoliation of graphitic oxide	1–3	700–1500	73
Thermal exfoliation of graphitic oxide	3–4	270–1550	103
Thermal exfoliation of graphitic oxide	3–6	925	11
Thermal exfoliation of graphitic oxide	Ultrathin sheets	737	117
Chemical exfoliation of graphitic oxide	1	705	118
Chemical exfoliation of graphitic oxide	3–7	640	104
Microwave exfoliation of graphite oxide	Few-layered	463	119
SiC-derived	—	300–950	120

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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 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,^128–134polymer (insulating and conducting),^135–137 small organic compound,^138,139 biomaterial,^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 nanoparticles of Fe,^213,214Cu,^201,215Ni,¹⁸⁸Co,²¹⁶Ge,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 composites, 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 situgrowth of nanoparticles.^152,163,204 An alternate reduction method involves the simultaneous reduction of both metal nanoparticles and the graphene oxide by means of microwave-irradiation.^{13,121,150,171,179,187,199–201,220,221} The main advantage of this method over other conventional heating methods is that the reaction mixture is heated uniformly and rapidly, allowing for large-scale and highly efficient production of graphene–metal composites.

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

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 method	Amount of metal/wt%	Particle size/nm	Application	Reference
*COD: 1,5-cyclooctadiene; COT: 1,3,5-cyclooctatriene.
Au/G	Chloroauric acid	Reduction with sodium citrate	8.4	20	Surface-enhanced Raman spectroscopy	152
Au/G	Chloroauric acid	Reduction with sodium dodecyl sulfate	8	2–3	Suzuki reaction	16
			21	7.5
Au/G	Chloroauric acid	Photochemical reduction	1.2	<1	—	162
Pt–Au/G	Hexachloroplatinic acid and chloroauric acid	Reduction with sodium borohydride	18.2	3.3	Formic acid oxidation	122
Pt–Ru/G	Pt(CH3)2(COD) and Ru(COD)(COT)*	Gas-phase reduction	5	2–3	—	This work
Pt/G	Hexachloroplatinic acid	Reduction with ethylene glycol	11	2–5.5	Methanol oxidation	227
Pt/N–G	Hexachloroplatinic acid	Reduction with ethylene glycol	14	2–3	Methanol oxidation	227
Pt/G	Diammine dinitritoplatinum(II)	Gas-phase reduction	20	0.5–2	Hydrogen oxidation	189
Pt/G	Potassium tetrachloroplatinate(II)	Electrodeposition	—	As small as 2	Methanol oxidation	186
Pt/G	Hexachloroplatinic acid	Gas-phase reduction	20	2	Oxygen reduction	126
Pt/G	Potassium hexachloroplatinate(IV)	Microwave synthesis	30	3.31	Methanol oxidation	187
			47	4.89
			67	5.81
Pd/G	Tetraamminepalladium(II) nitrate	Gas-phase reduction	0.18	1–6	Hydrogenation of alkynes	17
Pd/G	Potassium tetrachloropalladate(II)	Reduction with sodium borohydride	0.19	2.5	Hydrogenation of alkynes	194
Pd/G	Palladium(II) chloride	Microwave assisted reduction	20	2.5	Oxidation of methanol and ethanol	13
Pd/G	Palladium(II) acetate	Reduction with hydrazine	7.5	2.0–5.6	—	196
Pd/G	Palladium nitrate	Microwave assisted reduction	7.9	7–9	Suzuki and Heck reactions	200
Pd/GO			6.4	12–15
Ag/G	Silver nitrate	Reduction with PVP	65–88	10–30	Surface-enhanced Raman spectroscopy	209
Ag/G	Silver nitrate	Hydrothermal synthesis	56	10	Surface-enhanced Raman spectroscopy	207
Ag/G	Silver nitrate	Reduction with sodium borohydride	—	5–10	SERS and antibacterial activity	124

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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 sp²carbon 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,^186,192thermal 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 somewhat 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 (having more 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 addition, 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}ZrO₂,³¹⁷SiO₂,³¹⁸Cu₂O,^{253,319–321}RuO₂,^{10,132,322,323}Al₂O₃,^{133,324–327}MoO₃,^328,329ZnFeO₄,³³⁰BiWO₆³³¹ and LiFePO₄.^332,333 Additionally, other materials like CdS,^{29–31,334–339}CdSe,^340–344 and ZnS^31,334 have also been used to fabricate graphene-based composites.

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 deposition 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 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.

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 method	Amount of metal oxide/wt%	Particle size/nm	Reference
G-TiO2 nanorod	Titanium isopropoxide	Ex situ synthesis	55	2–4 diameter 20–30 length	24
G-TiO2	Titanium trichloride	Direct growth	85	<15	235
G-TiO2 anatase	Titanium trichloride	Sol–gel	97.5	5	243
G-TiO2 rutile	Titanium trichloride	Sol–gel	90–99.5	6	243
G-TiO2	Tetrabutyl titanate	Hydrothermal synthesis	67	9	238
G-ZnO	Zinc acetate	Ex situ synthesis	Not specified	4.5	261
G-SnO2	Tin(II) chloride dihydrate	Direct growth	85	∼5–10	235
G-MnO2	Potassium permanganate	Microwave irradiation	78	5–10	282
G-Mn3O4	Manganese(II) acetate	Direct growth and hydrothermal	90	10–20	347
G-Fe3O4	Iron(III) chloride	Direct growth	5.3–57.9	1.2–6.3	298
G-Fe2O3	Iron(III) chloride	Direct growth	80	60	304
G-Co3O4	Cobalt nitrate	Direct growth	75.4	10–30	307
G-RuO2	Ruthenium(III) chloride	Sol–gel	38.3	5–20	132
G-Al2O3	α-Aluminium oxide	Mechanical mixture	85–98 vol. %	2.5–20	325

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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, incorporation 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 fullerenes, 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 intermediates. 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 π-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.³⁵⁷Carbon nanotubes have been used as catalysts for methane decomposition,³⁵⁸oxidation 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 dehydrogenation of hydrocarbons (ODH).^361–363Ethylbenzene is 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 nanostructures have also been reported to be active in the ODH of ethylbenzene to styrene.^{361,365–367} Furthermore, due to their remarkable 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.³⁶³ODH is thought to proceed by nucleophilic 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.

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.

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 demonstrated 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.

Table 5 Conversion of benzylic and aliphatic alcohols to their respective ketones or aldehyde products, and conversion of aryl and aliphatic alkynes 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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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 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 applications in which graphene support may provide some significant 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 functional groups present in graphite oxide. In contrast to conventional Pd/activated carbon catalysts, graphite oxide and graphene-based catalysts showed much higher activities with turnover frequencies exceeding 39 000 h⁻¹, accompanied by very low palladium leaching (<1 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 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,^13,127formic acid,¹²⁷ in the reduction of oxygen,^{126,156,176,380,381} and in hydrogen fuel cells.¹⁸³

Metal-decorated graphene has provided enhanced electrocatalytic 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 (d_Pt = 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 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 (d_Pt = 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.

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.

Kundu et al.¹⁷⁹ used ethylene glycol and microwave irradiation conditions to prepare the graphene-supported Pt catalyst (d_Pt = 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. Xinet 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 °C. 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 electrocatalyst 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 cyclohexane (good conversion and high selectivity to the corresponding 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 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.

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.

A composite containing Pt-on-Pd bimetallic nanodendrites and graphene was synthesized and its catalytic activity investigated 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 °C was found to be 3 times higher than that of those treated at lower temperatures (300, 500 or 700 °C). 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 °C. 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.

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.

Possible applications of graphene-based materials 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 photocatalytic 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 perspectives 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 heterojunction 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 pollutants in water, namely graphene–SnO₂,²³⁵ and graphene–ZnO.²⁶⁵

Fig. 12 (a) TEM image of P25–graphene composite, and photocatalytic degradation of methylene blue under (b) UV and (c) visible light (λ > 400 nm) with P25 TiO₂, CNT–P25 TiO₂ and graphene–P25 TiO₂. Reprinted with permission from ref. 26. Copyright 2010 American Chemical Society.

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. 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 a H₂ 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 discussion 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, electrochemical 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 micrographs used in this work. B.F.M. acknowledges Fundação para a Ciência e a Tecnologia for the grant SFRH/BPD/70299/2010.

References

1. P. Serp and J. L. Figueiredo, Carbon Materials for Catalysis, John Wiley & Sons, 2009.
2. P. Serp, M. Corrias and P. Kalck, Appl. Catal., A, 2003, 253, 337.
3. H. W. Kroto, J. R. Heath, S. C. Obrien, R. F. Curl and R. E. Smalley, Nature, 1985, 318, 162.
4. S. Iijima, Nature, 1991, 354, 56.
5. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666.
6. J. Y. Son, Y.-H. Shin, H. Kim and H. M. Jang, ACS Nano, 2010, 4, 2655.
7. X.-D. Zhuang, Y. Chen, G. Liu, P.-P. Li, C.-X. Zhu, E.-T. Kang, K.-G. Noeh, B. Zhang, J.-H. Zhu and Y.-X. Li, Adv. Mater., 2010, 22, 1731.
8. J. Liu, Z. Lin, T. Liu, Z. Yin, X. Zhou, S. Chen, L. Xie, F. Boey, H. Zhang and W. Huang, Small, 2010, 6, 1536.
9. L. L. Zhang, R. Zhou and X. S. Zhao, J. Mater. Chem., 2010, 20, 5983.
10. A. K. Mishra and S. Ramaprabhu, J. Phys. Chem. C, 2011, 115, 14006.
11. S. Vivekchand, C. Rout, K. Subrahmanyam, A. Govindaraj and C. Rao, J. Chem. Sci., 2008, 120, 9.
12. G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth and R. Mülhaupt, J. Am. Chem. Soc., 2009, 131, 8262.
13. R. N. Singh and R. Awasthi, Catal.: Sci. Technol., 2011, 1, 778.
14. N. Zhang, H. Qiu, Y. Liu, W. Wang, Y. Li, X. Wang and J. Gao, J. Mater. Chem., 2011, 21, 11080.
15. H. He and C. Gao, Sci. China: Chem., 2011, 54, 397.
16. Y. Li, X. Fan, J. Qi, J. Ji, S. Wang, G. Zhang and F. Zhang, Mater. Res. Bull., 2010, 45, 1413.
17. Á. Mastalir, Z. Király, Á. Patzkó, I. Dékány and P. L'Argentiere, Carbon, 2008, 46, 1631.
18. D. R. Dreyer and C. W. Bielawski, Chem. Sci., 2011, 2, 1233.
19. Y. Long, C. Zhang, X. Wang, J. Gao, W. Wang and Y. Liu, J. Mater. Chem., 2011, 21, 13934.
20. J. Pyun, Angew. Chem., Int. Ed., 2011, 50, 46.
21. H.-P. Jia, D. R. Dreyer and C. W. Bielawski, Tetrahedron, 2011, 67, 4431.
22. Y. Zhang and C. Pan, J. Mater. Sci., 2011, 46, 2622.
23. H. Liu, S. Ryu, Z. Chen, M. L. Steigerwald, C. Nuckolls and L. E. Brus, J. Am. Chem. Soc., 2009, 131, 17099.
24. J. Liu, H. Bai, Y. Wang, Z. Liu, X. Zhang and D. D. Sun, Adv. Funct. Mater., 2010, 20, 4175.
25. J. Du, X. Lai, N. Yang, J. Zhai, D. Kisailus, F. Su, D. Wang and L. Jiang, ACS Nano, 2010, 5, 590.
26. H. Zhang, X. Lv, Y. Li, Y. Wang and J. Li, ACS Nano, 2009, 4, 380.
27. X.-Y. Zhang, H.-P. Li, X.-L. Cui and Y. Lin, J. Mater. Chem., 2010, 20, 2801.
28. X. Wang, L. Zhi and K. Mullen, Nano Lett., 2007, 8, 323.
29. C. X. Guo, H. B. Yang, Z. M. Sheng, Z. S. Lu, Q. L. Song and C. M. Li, Angew. Chem., Int. Ed., 2010, 49, 3014.
30. H. Chang, X. Lv, H. Zhang and J. Li, Electrochem. Commun., 2010, 12, 483.
31. P. Wang, T. Jiang, C. Zhu, Y. Zhai, D. Wang and S. Dong, Nano Res., 2010, 3, 794.
32. Z. Liu, D. He, Y. Wang, H. Wu and J. Wang, Sol. Energy Mater. Sol. Cells, 2010, 94, 1196.
33. S. Park, N. Mohanty, J. W. Suk, A. Nagaraja, J. An, R. D. Piner, W. Cai, D. R. Dreyer, V. Berry and R. S. Ruoff, Adv. Mater., 2010, 22, 1736.
34. H. Chang, L. Tang, Y. Wang, J. Jiang and J. Li, Anal. Chem., 2010, 82, 2341.
35. Z. Wang, J. Zhang, P. Chen, X. Zhou, Y. Yang, S. Wu, L. Niu, Y. Han, L. Wang, F. Boey, Q. Zhang, B. Liedberg and H. Zhang, Biosens. Bioelectron., 2011, 26, 3881.
36. F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov, Nat. Mater., 2007, 6, 652.
37. L. Xie, X. Ling, Y. Fang, J. Zhang and Z. Liu, J. Am. Chem. Soc., 2009, 131, 9890.
38. X. Ling, L. Xie, Y. Fang, H. Xu, H. Zhang, J. Kong, M. S. Dresselhaus, J. Zhang and Z. Liu, Nano Lett., 2009, 10, 553.
39. X. Ling and J. Zhang, Small, 2010, 6, 2020.
40. W. Cai and X. Chen, Small, 2007, 3, 1840.
41. C. Peng, W. Hu, Y. Zhou, C. Fan and Q. Huang, Small, 2010, 6, 1686.
42. O. Akhavan, E. Ghaderi and A. Esfandiar, J. Phys. Chem. B, 2011, 115, 6279.
43. C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam and A. Govindaraj, Angew. Chem., Int. Ed., 2009, 48, 7752.
44. M. J. Allen, V. C. Tung and R. B. Kaner, Chem. Rev., 2009, 110, 132.
45. C. N. R. Rao, K. Biswas, K. S. Subrahmanyam and A. Govindaraj, J. Mater. Chem., 2009, 19, 2457.
46. M. Terrones, A. R. Botello-Méndez, J. Campos-Delgado, F. López-Urías, Y. I. Vega-Cantú, F. J. Rodríguez-Macías, A. L. Elías, E. Muñoz-Sandoval, A. G. Cano-Márquez, J.-C. Charlier and H. Terrones, Nano Today, 2010, 5, 351.
47. C. N. R. Rao, A. K. Sood, R. Voggu and K. S. Subrahmanyam, J. Phys. Chem. Lett., 2010, 1, 572.
48. F. Molitor, et al. , J. Phys.: Condens. Matter, 2011, 23, 243201.
49. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183.
50. X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey and H. Zhang, Small, 2011, 7, 1876.
51. V. Singh, D. Joung, L. Zhai, S. Das, S. I. Khondaker and S. Seal, Prog. Mater. Sci., 2011, 56, 1178.
52. A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov and A. K. Geim, Rev. Mod. Phys., 2009, 81, 109.
53. C. N. R. Rao, et al. , Sci. Technol. Adv. Mater., 2010, 11, 054502.
54. J. Wu, W. Pisula and K. Müllen, Chem. Rev., 2007, 107, 718.
55. D. R. Dreyer, R. S. Ruoff and C. W. Bielawski, Angew. Chem., Int. Ed., 2010, 49, 9336.
56. H. Bai, C. Li and G. Shi, Adv. Mater., 2011, 23, 1089.
57. Y. Sun, Q. Wu and G. Shi, Energy Environ. Sci., 2011, 4, 1113.
58. P. V. Kamat, J. Phys. Chem. Lett., 2009, 1, 520.
59. K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov and A. K. Geim, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10451.
60. J. Hass, et al. , J. Phys.: Condens. Matter, 2008, 20, 323202.
61. C. Berger, Z. Song, T. Li, X. Li, A. Y. Ogbazghi, R. Feng, Z. Dai, A. N. Marchenkov, E. H. Conrad, P. N. First and W. A. de Heer, J. Phys. Chem. B, 2004, 108, 19912.
62. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First and W. A. de Heer, Science, 2006, 312, 1191.
63. W. A. de Heer, C. Berger, X. Wu, P. N. First, E. H. Conrad, X. Li, T. Li, M. Sprinkle, J. Hass, M. L. Sadowski, M. Potemski and G. Martinez, Solid State Commun., 2007, 143, 92.
64. W. Choi, I. Lahiri, R. Seelaboyina and Y. S. Kang, Crit. Rev. Solid State Mater. Sci., 2010, 35, 52.
65. S. Guo and S. Dong, Chem. Soc. Rev., 2011, 40, 2644.
66. A. Srivastava, C. Galande, L. Ci, L. Song, C. Rai, D. Jariwala, K. F. Kelly and P. M. Ajayan, Chem. Mater., 2010, 22, 3457.
67. X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo and R. S. Ruoff, Science, 2009, 324, 1312.
68. P. W. Sutter, J.-I. Flege and E. A. Sutter, Nat. Mater., 2008, 7, 406.
69. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi and B. H. Hong, Nature, 2009, 457, 706.
70. A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. S. Dresselhaus and J. Kong, Nano Lett., 2008, 9, 30.
71. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558.
72. M. J. McAllister, J.-L. Li, D. H. Adamson, H. C. Schniepp, A. A. Abdala, J. Liu, M. Herrera-Alonso, D. L. Milius, R. Car, R. K. Prud'homme and I. A. Aksay, Chem. Mater., 2007, 19, 4396.
73. H. C. Schniepp, J.-L. Li, M. J. McAllister, H. Sai, M. Herrera-Alonso, D. H. Adamson, R. K. Prud'homme, R. Car, D. A. Saville and I. A. Aksay, J. Phys. Chem. B, 2006, 110, 8535.
74. D. R. Dreyer, S. Park, C. W. Bielawski and R. S. Ruoff, Chem. Soc. Rev., 2010, 39, 228.
75. A. Lerf, H. He, M. Forster and J. Klinowski, J. Phys. Chem. B, 1998, 102, 4477.
76. S. Park and R. S. Ruoff, Nat. Nanotechnol., 2009, 4, 217.
77. A. K. Geim, Science, 2009, 324, 1530.
78. H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao and Y. Chen, ACS Nano, 2008, 2, 463.
79. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres and A. K. Geim, Science, 2008, 320, 1308.
80. C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385.
81. L. Hu, X. Hu, X. Wu, C. Du, Y. Dai and J. Deng, Physica B (Amsterdam), 2010, 405, 3337.
82. Y. Tang, Z. Yang and X. Dai, J. Magn. Magn. Mater., 2011, 323, 2441.
83. K. Nakada and A. Ishii, Solid State Commun., 2011, 151, 13.
84. O. Leenaerts, B. Partoens and F. M. Peeters, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77, 125416.
85. F. Ma, Z. Zhang, H. Jia, X. Liu, Y. Hao and B. Xu, THEOCHEM, 2010, 955, 134.
86. C. Rajesh, C. Majumder, H. Mizuseki and Y. Kawazoe, J. Chem. Phys., 2009, 130, 124911.
87. M. Chi and Y.-P. Zhao, Comput. Mater. Sci., 2009, 46, 1085.
88. Y.-H. Lu, M. Zhou, C. Zhang and Y.-P. Feng, J. Phys. Chem. C, 2009, 113, 20156.
89. Y. Li, Z. Zhou, G. Yu, W. Chen and Z. Chen, J. Phys. Chem. C, 2010, 114, 6250.
90. Z. M. Ao, J. Yang, S. Li and Q. Jiang, Chem. Phys. Lett., 2008, 461, 276.
91. J. Dai and J. Yuan, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 81, 165414.
92. X. Qin, Q. Meng and W. Zhao, Surf. Sci., 2011, 605, 930.
93. E. N. Voloshina, D. Mollenhauer, L. Chiappisi and B. Paulus, Chem. Phys. Lett., 2011, 510, 220.
94. N. Ding, X. Lu and C.-M. L. Wu, Comput. Mater. Sci., 2012, 51, 141.
95. M. Zhou, A. Zhang, Z. Dai, C. Zhang and Y. P. Feng, J. Chem. Phys., 2010, 132, 194704.
96. G. Lee, B. Lee, J. Kim and K. Cho, J. Phys. Chem. C, 2009, 113, 14225.
97. P. A. Denis, J. Phys. Chem. C, 2009, 113, 5612.
98. J. T. Paci, T. Belytschko and G. C. Schatz, J. Phys. Chem. C, 2007, 111, 18099.
99. S. Tang and Z. Cao, J. Chem. Phys., 2011, 134, 044710.
100. J. D. Fowler, M. J. Allen, V. C. Tung, Y. Yang, R. B. Kaner and B. H. Weiller, ACS Nano, 2009, 3, 301.
101. T. Kuila, S. Bose, P. Khanra, A. K. Mishra, N. H. Kim and J. H. Lee, Biosens. Bioelectron., 2011, 26, 4637.
102. Y. Shao, J. Wang, H. Wu, J. Liu, I. A. Aksay and Y. Lin, Electroanalysis, 2010, 22, 1027.
103. A. Ghosh, K. S. Subrahmanyam, K. S. Krishna, S. Datta, A. Govindaraj, S. K. Pati and C. N. R. Rao, J. Phys. Chem. C, 2008, 112, 15704.
104. G. Srinivas, Y. Zhu, R. Piner, N. Skipper, M. Ellerby and R. Ruoff, Carbon, 2010, 48, 630.
105. Hyperion Catalysis International, Inc, http://hyperioncatalysis.com/.
106. D. Chen, L. Tang and J. Li, Chem. Soc. Rev., 2010, 39, 3157.
107. J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, T. J. Booth and S. Roth, Nature, 2007, 446, 60.
108. A. Fasolino, J. H. Los and M. I. Katsnelson, Nat. Mater., 2007, 6, 858.
109. G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu and J. Yao, J. Phys. Chem. C, 2008, 112, 8192.
110. L. M. Malard, M. A. Pimenta, G. Dresselhaus and M. S. Dresselhaus, Phys. Rep., 2009, 473, 51.
111. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth and A. K. Geim, Phys. Rev. Lett., 2006, 97, 187401.
112. M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, L. G. Cancado, A. Jorio and R. Saito, Phys. Chem. Chem. Phys., 2007, 9, 1276.
113. H.-L. Guo, X.-F. Wang, Q.-Y. Qian, F.-B. Wang and X.-H. Xia, ACS Nano, 2009, 3, 2653.
114. H. C. Schniepp, K. N. Kudin, J.-L. Li, R. K. Prud'homme, R. Car, D. A. Saville and I. A. Aksay, ACS Nano, 2008, 2, 2577.
115. E. Stolyarova, K. T. Rim, S. Ryu, J. Maultzsch, P. Kim, L. E. Brus, T. F. Heinz, M. S. Hybertsen and G. W. Flynn, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 9209.
116. A. Peigney, C. Laurent, E. Flahaut, R. R. Bacsa and A. Rousset, Carbon, 2001, 39, 507.
117. Q. Du, M. Zheng, L. Zhang, Y. Wang, J. Chen, L. Xue, W. Dai, G. Ji and J. Cao, Electrochim. Acta, 2010, 55, 3897.
118. M. D. Stoller, S. Park, Y. Zhu, J. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498.
119. Y. Zhu, S. Murali, M. D. Stoller, A. Velamakanni, R. D. Piner and R. S. Ruoff, Carbon, 2010, 48, 2118.
120. M. Käärik, M. Arulepp, M. Karelson and J. Leis, Carbon, 2008, 46, 1579.
121. K.-J. Jeon and Z. Lee, Chem. Commun., 2011, 47, 3610.
122. S. Zhang, Y. Shao, H.-g. Liao, J. Liu, I. A. Aksay, G. Yin and Y. Lin, Chem. Mater., 2011, 23, 1079.
123. R. Pasricha, S. Gupta and A. K. Srivastava, Small, 2009, 5, 2253.
124. J. Shen, M. Shi, N. Li, B. Yan, H. Ma, Y. Hu and M. Ye, Nano Res., 2010, 3, 339.
125. C. Nethravathi, E. A. Anumol, M. Rajamathi and N. Ravishankar, Nanoscale, 2011, 3, 569.
126. R. Kou, Y. Shao, D. Wang, M. H. Engelhard, J. H. Kwak, J. Wang, V. V. Viswanathan, C. Wang, Y. Lin, Y. Wang, I. A. Aksay and J. Liu, Electrochem. Commun., 2009, 11, 954.
127. X. Chen, G. Wu, J. Chen, X. Chen, Z. Xie and X. Wang, J. Am. Chem. Soc., 2011, 133, 3693.
128. G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 1487.
129. Y.-L. Chen, Z.-A. Hu, Y.-Q. Chang, H.-W. Wang, Z.-Y. Zhang, Y.-Y. Yang and H.-Y. Wu, J. Phys. Chem. C, 2011, 115, 2563.
130. S. Chen, J. Zhu, X. Wu, Q. Han and X. Wang, ACS Nano, 2010, 4, 2822.
131. K. Zhou, Y. Zhu, X. Yang and C. Li, New J. Chem., 2010, 34, 2950.
132. Z.-S. Wu, D.-W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li and H.-M. Cheng, Adv. Funct. Mater., 2010, 20, 3595.
133. Z. Jiang, J. Wang, L. Meng, Y. Huang and L. Liu, Chem. Commun., 2011, 47, 6350.
134. S. Q. Chen and Y. Wang, J. Mater. Chem., 2010, 20, 9735.
135. D. Cai and M. Song, J. Mater. Chem., 2010, 20, 7906.
136. H. Kim, A. A. Abdala and C. W. Macosko, Macromolecules, 2010, 43, 6515.
137. T. Kuilla, S. Bhadra, D. Yao, N. H. Kim, S. Bose and J. H. Lee, Prog. Polym. Sci., 2010, 35, 1350.
138. S. Wang, B. M. Goh, K. K. Manga, Q. Bao, P. Yang and K. P. Loh, ACS Nano, 2010, 4, 6180.
139. T. H. Han, W. J. Lee, D. H. Lee, J. E. Kim, E.-Y. Choi and S. O. Kim, Adv. Mater., 2010, 22, 2060.
140. C.-H. Lu, H.-H. Yang, C.-L. Zhu, X. Chen and G.-N. Chen, Angew. Chem., 2009, 121, 4879.
141. Y. Wang, Z. Li, D. Hu, C.-T. Lin, J. Li and Y. Lin, J. Am. Chem. Soc., 2010, 132, 9274.
142. M. Jahan, Q. Bao, J.-X. Yang and K. P. Loh, J. Am. Chem. Soc., 2010, 132, 14487.
143. C. Petit and T. J. Bandosz, Adv. Funct. Mater., 2010, 20, 111.
144. C. Petit, J. Burress and T. J. Bandosz, Carbon, 2011, 49, 563.
145. V. C. Tung, L.-M. Chen, M. J. Allen, J. K. Wassei, K. Nelson, R. B. Kaner and Y. Yang, Nano Lett., 2009, 9, 1949.
146. E. Yoo, J. Kim, E. Hosono, H.-s. Zhou, T. Kudo and I. Honma, Nano Lett., 2008, 8, 2277.
147. D. Yu and L. Dai, J. Phys. Chem. Lett., 2009, 1, 467.
148. Z. Fan, J. Yan, L. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian and F. Wei, Adv. Mater., 2010, 22, 3723.
149. Z.-B. Liu, Y.-F. Xu, X.-Y. Zhang, X.-L. Zhang, Y.-S. Chen and J.-G. Tian, J. Phys. Chem. B, 2009, 113, 9681.
150. J. Xiang and L. T. Drzal, ACS Appl. Mater. Interfaces, 2011, 3, 1325.
151. Z. Xiong, L. L. Zhang, J. Ma and X. S. Zhao, Chem. Commun., 2010, 46, 6099.
152. G. Gonçalves, P. A. A. P. Marques, C. M. Granadeiro, H. I. S. Nogueira, M. K. Singh and J. Grácio, Chem. Mater., 2009, 21, 4796.
153. R. Muszynski, B. Seger and P. V. Kamat, J. Phys. Chem. C, 2008, 112, 5263.
154. Z. Zhang, H. Chen, C. Xing, M. Guo, F. Xu, X. Wang, H. Gruber, B. Zhang and J. Tang, Nano Res., 2011, 4, 599.
155. C. Xu, X. Wang and J. Zhu, J. Phys. Chem. C, 2008, 112, 19841.
156. F. Li, H. Yang, C. Shan, Q. Zhang, D. Han, A. Ivaska and L. Niu, J. Mater. Chem., 2009, 19, 4022.
157. J. Liu, Y. Li, Y. Li, J. Li and Z. Deng, J. Mater. Chem., 2010, 20, 900.
158. Y. Fang, S. Guo, C. Zhu, Y. Zhai and E. Wang, Langmuir, 2010, 26, 11277.
159. J. Gong, T. Zhou, D. Song and L. Zhang, Sens. Actuators, B, 2010, 150, 491.
160. W. Hong, H. Bai, Y. Xu, Z. Yao, Z. Gu and G. Shi, J. Phys. Chem. C, 2010, 114, 1822.
161. Y.-K. Kim, H.-K. Na and D.-H. Min, Langmuir, 2010, 26, 13065.
162. X. Huang, X. Zhou, S. Wu, Y. Wei, X. Qi, J. Zhang, F. Boey and H. Zhang, Small, 2010, 6, 513.
163. K. Jasuja and V. Berry, ACS Nano, 2009, 3, 2358.
164. B.-S. Kong, J. Geng and H.-T. Jung, Chem. Commun., 2009, 2174.
165. C. Fu, Y. Kuang, Z. Huang, X. Wang, N. Du, J. Chen and H. Zhou, Chem. Phys. Lett., 2010, 499, 250.
166. Y. Hu, H. Zhang, P. Wu, H. Zhang, B. Zhou and C. Cai, Phys. Chem. Chem. Phys., 2011, 13, 4083.
167. Z. Tang, S. Shen, J. Zhuang and X. Wang, Angew. Chem., Int. Ed., 2010, 49, 4603.
168. J. Liu, S. Fu, B. Yuan, Y. Li and Z. Deng, J. Am. Chem. Soc., 2010, 132, 7279.
169. G. Lu, H. Li, C. Liusman, Z. Yin, S. Wu and H. Zhang, Chem. Sci., 2011, 2, 1817.
170. S. Guo, S. Dong and E. Wang, ACS Nano, 2009, 4, 547.
171. S. Guo, D. Wen, Y. Zhai, S. Dong and E. Wang, ACS Nano, 2010, 4, 3959.
172. L. Dong, R. R. S. Gari, Z. Li, M. M. Craig and S. Hou, Carbon, 2010, 48, 781.
173. Y. Li, W. Gao, L. Ci, C. Wang and P. M. Ajayan, Carbon, 2010, 48, 1124.
174. Y. Si and E. T. Samulski, Chem. Mater., 2008, 20, 6792.
175. Y. Li, L. Tang and J. Li, Electrochem. Commun., 2009, 11, 846.
176. Y. Shao, S. Zhang, C. Wang, Z. Nie, J. Liu, Y. Wang and Y. Lin, J. Power Sources, 2010, 195, 4600.
177. S. M. Choi, M. H. Seo, H. J. Kim and W. B. Kim, Carbon, 2011, 49, 904.
178. Y.-G. Zhou, J.-J. Chen, F.-b. Wang, Z.-H. Sheng and X.-H. Xia, Chem. Commun., 2010, 46, 5951.
179. P. Kundu, C. Nethravathi, P. A. Deshpande, M. Rajamathi, G. Madras and N. Ravishankar, Chem. Mater., 2011, 23, 2772.
180. S. Bong, Y.-R. Kim, I. Kim, S. Woo, S. Uhm, J. Lee and H. Kim, Electrochem. Commun., 2010, 12, 129.
181. M. H. Seo, S. M. Choi, H. J. Kim and W. B. Kim, Electrochem. Commun., 2011, 13, 182.
182. H. J. Kim, S. M. Choi, M. H. Seo, S. Green, G. W. Huber and W. B. Kim, Electrochem. Commun., 2011, 13, 890.
183. B. Seger and P. V. Kamat, J. Phys. Chem. C, 2009, 113, 7990.
184. G. Gupta, D. A. Slanac, P. Kumar, J. D. Wiggins-Camacho, J. Kim, R. Ryoo, K. J. Stevenson and K. P. Johnston, J. Phys. Chem. C, 2010, 114, 10796.
185. N. Shang, P. Papakonstantinou, P. Wang and S. R. P. Silva, J. Phys. Chem. C, 2010, 114, 15837.
186. M. Bayati, J. M. Abad, R. J. Nichols and D. J. Schiffrin, J. Phys. Chem. C, 2010, 114, 18439.
187. S. Sharma, A. Ganguly, P. Papakonstantinou, X. Miao, M. Li, J. L. Hutchison, M. Delichatsios and S. Ukleja, J. Phys. Chem. C, 2010, 114, 19459.
188. K. Zhang, Q. Yue, G. Chen, Y. Zhai, L. Wang, H. Wang, J. Zhao, J. Liu, J. Jia and H. Li, J. Phys. Chem. C, 2010, 115, 379.
189. E. Yoo, T. Okada, T. Akita, M. Kohyama, I. Honma and J. Nakamura, J. Power Sources, 2011, 196, 110.
190. C. Zhu, S. Guo, Y. Zhai and S. Dong, Langmuir, 2010, 26, 7614.
191. E. Yoo, T. Okata, T. Akita, M. Kohyama, J. Nakamura and I. Honma, Nano Lett., 2009, 9, 2255.
192. R. S. Sundaram, C. Gómez-Navarro, K. Balasubramanian, M. Burghard and K. Kern, Adv. Mater., 2008, 20, 3050.
193. J. L. Johnson, A. Behnam, S. J. Pearton and A. Ural, Adv. Mater., 2010, 22, 4877.
194. Á. Mastalir, Z. Király, M. Benkő and I. Dékány, Catal. Lett., 2008, 124, 34.
195. H. Zhao, J. Yang, L. Wang, C. Tian, B. Jiang and H. Fu, Chem. Commun., 2011, 47, 2014.
196. Z. Jin, D. Nackashi, W. Lu, C. Kittrell and J. M. Tour, Chem. Mater., 2010, 22, 5695.
197. J. Yang, C. Tian, L. Wang and H. Fu, J. Mater. Chem., 2011, 21, 3384.
198. Y. Li, X. Fan, J. Qi, J. Ji, S. Wang, G. Zhang and F. Zhang, Nano Res., 2010, 3, 429.
199. Z. Wen, S. Yang, Y. Liang, W. He, H. Tong, L. Hao, X. Zhang and Q. Song, Electrochim. Acta, 2010, 56, 139.
200. A. R. Siamaki, A. E. R. S. Khder, V. Abdelsayed, M. S. El-Shall and B. F. Gupton, J. Catal., 2011, 279, 1.
201. H. M. A. Hassan, V. Abdelsayed, A. E. R. S. Khder, K. M. AbouZeid, J. Terner, M. S. El-Shall, S. I. Al-Resayes and A. A. El-Azhary, J. Mater. Chem., 2009, 19, 3832.
202. Z.-L. Hu, M. Aizawa, Z.-M. Wang, N. Yoshizawa and H. Hatori, Langmuir, 2010, 26, 6681.
203. J. Lee, K. S. Novoselov and H. S. Shin, ACS Nano, 2010, 5, 608.
204. X. Zhou, X. Huang, X. Qi, S. Wu, C. Xue, F. Y. C. Boey, Q. Yan, P. Chen and H. Zhang, J. Phys. Chem. C, 2009, 113, 10842.
205. G. Lu, S. Mao, S. Park, R. Ruoff and J. Chen, Nano Res., 2009, 2, 192.
206. C. Xu and X. Wang, Small, 2009, 5, 2212.
207. J. Shen, M. Shi, B. Yan, H. Ma, N. Li and M. Ye, J. Mater. Chem., 2011, 21, 7795.
208. J. Li and C.-y. Liu, Eur. J. Inorg. Chem., 2010, 2010, 1244.
209. Z. Zhang, F. Xu, W. Yang, M. Guo, X. Wang, B. Zhang and J. Tang, Chem. Commun., 2011, 47, 6440.
210. Z. Xu, H. Gao and H. Guoxin, Carbon, 2011, 49, 4731.
211. T. T. Baby and S. Ramaprabhu, J. Mater. Chem., 2011, 21, 9702.
212. J. Yang, C. Zang, L. Sun, N. Zhao and X. Cheng, Mater. Chem. Phys., 2011, 129, 270.
213. Y.-H. Zou, H.-B. Liu, L. Yang and Z.-Z. Chen, J. Magn. Magn. Mater., 2006, 302, 343.
214. M. Stein, J. Wieland, P. Steurer, F. Tölle, R. Mülhaupt and B. Breit, Adv. Synth. Catal., 2011, 353, 523.
215. X. Bin, J. Chen, H. Cao, L. Chen and J. Yuan, J. Phys. Chem. Solids, 2009, 70, 1.
216. S. Yang, G. Cui, S. Pang, Q. Cao, U. Kolb, X. Feng, J. Maier and K. Müllen, ChemSusChem, 2010, 3, 236.
217. H. Paul and D. Mohanta, Appl. Phys. A: Mater. Sci. Process., 2011, 103, 395.
218. G. Wang, B. Wang, X. Wang, J. Park, S. Dou, H. Ahn and K. Kim, J. Mater. Chem., 2009, 19, 8378.
219. K. Zhou, Y. Zhu, X. Yang and C. Li, Electroanalysis, 2010, 22, 259.
220. Z.-J. Fan, J. Yan, T. Wei, G.-Q. Ning, L.-J. Zhi, J.-C. Liu, D.-X. Cao, G.-L. Wang and F. Wei, ACS Nano, 2011, 5, 2787.
221. Y. Lin, D. W. Baggett, J.-W. Kim, E. J. Siochi and J. W. Connell, ACS Appl. Mater. Interfaces, 2011, 3, 1652.
222. Y.-K. Kim, H.-K. Na, Y. W. Lee, H. Jang, S. W. Han and D.-H. Min, Chem. Commun., 2010, 46, 3185.
223. R. Guoqiang and X. Yangchuan, Nanotechnology, 2006, 17, 5596.
224. H. Zhou, C. Qiu, Z. Liu, H. Yang, L. Hu, J. Liu, H. Yang, C. Gu and L. Sun, J. Am. Chem. Soc., 2009, 132, 944.
225. I. V. Lightcap, T. H. Kosel and P. V. Kamat, Nano Lett., 2010, 10, 577.
226. Y. Lin, K. A. Watson, M. J. Fallbach, S. Ghose, J. G. Smith, D. M. Delozier, W. Cao, R. E. Crooks and J. W. Connell, ACS Nano, 2009, 3, 871.
227. L.-S. Zhang, X.-Q. Liang, W.-G. Song and Z.-Y. Wu, Phys. Chem. Chem. Phys., 2010, 12, 12055.
228. K. K. Manga, Y. Zhou, Y. Yan and K. P. Loh, Adv. Funct. Mater., 2009, 19, 3638.
229. C. Zhu, S. Guo, P. Wang, L. Xing, Y. Fang, Y. Zhai and S. Dong, Chem. Commun., 2010, 46, 7148.
230. B. Li, X. Zhang, X. Li, L. Wang, R. Han, B. Liu, W. Zheng, X. Li and Y. Liu, Chem. Commun., 2010, 46, 3499.
231. C. Chen, W. Cai, M. Long, B. Zhou, Y. Wu, D. Wu and Y. Feng, ACS Nano, 2010, 4, 6425.
232. T. Kamegawa, D. Yamahana and H. Yamashita, J. Phys. Chem. C, 2010, 114, 15049.
233. Y. Liang, H. Wang, H. Sanchez Casalongue, Z. Chen and H. Dai, Nano Res., 2010, 3, 701.
234. Y. Zhang, Z.-R. Tang, X. Fu and Y.-J. Xu, ACS Nano, 2010, 4, 7303.
235. J. Zhang, Z. Xiong and X. S. Zhao, J. Mater. Chem., 2011, 21, 3634.
236. F. Zou, Y. Yu, N. Cao, L. Wu and J. Zhi, Scr. Mater., 2011, 64, 621.
237. Q. Wang, X. Guo, L. Cai, Y. Cao, L. Gan, S. Liu, Z. Wang, H. Zhang and L. Li, Chem. Sci., 2011, 2, 1860.
238. J. Shen, B. Yan, M. Shi, H. Ma, N. Li and M. Ye, J. Mater. Chem., 2011, 21, 3415.
239. S. Ding, J. S. Chen, D. Luan, F. Y. C. Boey, S. Madhavi and X. W. Lou, Chem. Commun., 2011, 47, 5780.
240. P. Wang, Y. Zhai, D. Wang and S. Dong, Nanoscale, 2011, 3, 1640.
241. R. Leary and A. Westwood, Carbon, 2011, 49, 741.
242. T. N. Lambert, C. A. Chavez, B. Hernandez-Sanchez, P. Lu, N. S. Bell, A. Ambrosini, T. Friedman, T. J. Boyle, D. R. Wheeler and D. L. Huber, J. Phys. Chem. C, 2009, 113, 19812.
243. D. Wang, D. Choi, J. Li, Z. Yang, Z. Nie, R. Kou, D. Hu, C. Wang, L. V. Saraf, J. Zhang, I. A. Aksay and J. Liu, ACS Nano, 2009, 3, 907.
244. O. Akhavan, M. Abdolahad, A. Esfandiar and M. Mohatashamifar, J. Phys. Chem. C, 2010, 114, 12955.
245. Y.-B. Tang, C.-S. Lee, J. Xu, Z.-T. Liu, Z.-H. Chen, Z. He, Y.-L. Cao, G. Yuan, H. Song, L. Chen, L. Luo, H.-M. Cheng, W.-J. Zhang, I. Bello and S.-T. Lee, ACS Nano, 2010, 4, 3482.
246. S. Sun, L. Gao and Y. Liu, Appl. Phys. Lett., 2010, 96, 083113.
247. Y. Wang, R. Shi, J. Lin and Y. Zhu, Appl. Catal., B, 2010, 100, 179.
248. Y. Qiu, K. Yan, S. Yang, L. Jin, H. Deng and W. Li, ACS Nano, 2010, 4, 6515.
249. G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei and H. Tang, Carbon, 2011, 49, 2693.
250. L. Zhang, S. Diao, Y. Nie, K. Yan, N. Liu, B. Dai, Q. Xie, A. Reina, J. Kong and Z. Liu, J. Am. Chem. Soc., 2011, 133, 2706.
251. X. Zhang, Y. Sun, X. Cui and Z. Jiang, Int. J. Hydrogen Energy DOI:10.1016/j.ijhydene.2011.04.053.
252. K. K. Manga, S. Wang, M. Jaiswal, Q. Bao and K. P. Loh, Adv. Mater., 2010, 22, 5265.
253. S. Wu, Z. Yin, Q. He, X. Huang, X. Zhou and H. Zhang, J. Phys. Chem. C, 2010, 114, 11816.
254. Z. Yin, S. Wu, X. Zhou, X. Huang, Q. Zhang, F. Boey and H. Zhang, Small, 2010, 6, 307.
255. J. M. Lee, Y. B. Pyun, J. Yi, J. W. Choung and W. I. Park, J. Phys. Chem. C, 2009, 113, 19134.
256. J. Lin, M. Penchev, G. Wang, R. K. Paul, J. Zhong, X. Jing, M. Ozkan and C. S. Ozkan, Small, 2010, 6, 2448.
257. Y. Zhang, H. Li, L. Pan, T. Lu and Z. Sun, J. Electroanal. Chem., 2009, 634, 68.
258. W. T. Zheng, Y. M. Ho, H. W. Tian, M. Wen, J. L. Qi and Y. A. Li, J. Phys. Chem. C, 2009, 113, 9164.
259. J. O. Hwang, D. H. Lee, J. Y. Kim, T. H. Han, B. H. Kim, M. Park, K. No and S. O. Kim, J. Mater. Chem., 2011, 21, 3432.
260. T. V. Cuong, V. H. Pham, J. S. Chung, E. W. Shin, D. H. Yoo, S. H. Hahn, J. S. Huh, G. H. Rue, E. J. Kim, S. H. Hur and P. A. Kohl, Mater. Lett., 2010, 64, 2479.
261. K. Prashant, et al. , Nanotechnology, 2010, 21, 385701.
262. X. Liu, L. Pan, T. Lv, T. Lu, G. Zhu, Z. Sun and C. Sun, Catal.: Sci. Technol., 2011, 1, 1189.
263. G. Williams and P. V. Kamat, Langmuir, 2009, 25, 13869.
264. Y. Yang and T. Liu, Appl. Surf. Sci., 2011, 257, 8950.
265. B. Li and H. Cao, J. Mater. Chem., 2011, 21, 3346.
266. T. Xu, L. Zhang, H. Cheng and Y. Zhu, Appl. Catal., B, 2011, 101, 382.
267. S.-M. Paek, E. Yoo and I. Honma, Nano Lett., 2008, 9, 72.
268. D. Wang, R. Kou, D. Choi, Z. Yang, Z. Nie, J. Li, L. V. Saraf, D. Hu, J. Zhang, G. L. Graff, J. Liu, M. A. Pope and I. A. Aksay, ACS Nano, 2010, 4, 1587.
269. L.-S. Zhang, L.-Y. Jiang, H.-J. Yan, W. D. Wang, W. Wang, W.-G. Song, Y.-G. Guo and L.-J. Wan, J. Mater. Chem., 2010, 20, 5462.
270. L. Fenghua, et al. , Nanotechnology, 2009, 20, 455602.
271. H. Song, L. Zhang, C. He, Y. Qu, Y. Tian and Y. Lv, J. Mater. Chem., 2011, 21, 5972.
272. Y. Li, X. Lv, J. Lu and J. Li, J. Phys. Chem. C, 2010, 114, 21770.
273. H. Kim, S.-W. Kim, Y.-U. Park, H. Gwon, D.-H. Seo, Y. Kim and K. Kang, Nano Res., 2010, 3, 813.
274. Z. Wang, H. Zhang, N. Li, Z. Shi, Z. Gu and G. Cao, Nano Res., 2010, 3, 748.
275. J. Yao, X. Shen, B. Wang, H. Liu and G. Wang, Electrochem. Commun., 2009, 11, 1849.
276. P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang and H. Wang, Electrochim. Acta, 2011, 56, 4532.
277. X. Wang, X. Zhou, K. Yao, J. Zhang and Z. Liu, Carbon, 2011, 49, 133.
278. M. Zhang, D. Lei, Z. Du, X. Yin, L. Chen, Q. Li, Y. Wang and T. Wang, J. Mater. Chem., 2011, 21, 1673.
279. S. Chen, J. Zhu and X. Wang, ACS Nano, 2010, 4, 6212.
280. L. Li, Z. Du, S. Liu, Q. Hao, Y. Wang, Q. Li and T. Wang, Talanta, 2010, 82, 1637.
281. Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li and F. Wei, Adv. Funct. Mater., 2011, 21, 2366.
282. J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang and F. Wei, Carbon, 2010, 48, 3825.
283. C. Zhu, S. Guo, Y. Fang, L. Han, E. Wang and S. Dong, Nano Res., 2011, 4, 648.
284. Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya and L.-C. Qin, Carbon, 2011, 49, 2917.
285. Z. Li, J. Wang, S. Liu, X. Liu and S. Yang, J. Power Sources, 2011, 196, 8160.
286. G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J. R. McDonough, X. Cui, Y. Cui and Z. Bao, Nano Lett., 2011, 11, 2905.
287. L. Xing, C. Cui, C. Ma and X. Xue, Mater. Lett., 2011, 65, 2104.
288. G. Zhou, D.-W. Wang, F. Li, L. Zhang, N. Li, Z.-S. Wu, L. Wen, G. Q. Lu and H.-M. Cheng, Chem. Mater., 2010, 22, 5306.
289. P. Lian, X. Zhu, H. Xiang, Z. Li, W. Yang and H. Wang, Electrochim. Acta, 2010, 56, 834.
290. F. He, J. Fan, D. Ma, L. Zhang, C. Leung and H. L. Chan, Carbon, 2010, 48, 3139.
291. K. Morishige and T. Hamada, Langmuir, 2005, 21, 6277.
292. X. Yang, X. Zhang, Y. Ma, Y. Huang, Y. Wang and Y. Chen, J. Mater. Chem., 2009, 19, 2710.
293. V. Chandra, J. Park, Y. Chun, J. W. Lee, I.-C. Hwang and K. S. Kim, ACS Nano, 2010, 4, 3979.
294. M. Zhang, D. Lei, X. Yin, L. Chen, Q. Li, Y. Wang and T. Wang, J. Mater. Chem., 2010, 20, 5538.
295. J. Shen, Y. Hu, M. Shi, N. Li, H. Ma and M. Ye, J. Phys. Chem. C, 2010, 114, 1498.
296. J. Liang, Y. Xu, D. Sui, L. Zhang, Y. Huang, Y. Ma, F. Li and Y. Chen, J. Phys. Chem. C, 2010, 114, 17465.
297. H. Yin, Y. Zhou, Q. Ma, S. Ai, Q. Chen and L. Zhu, Talanta, 2010, 82, 1193.
298. H. He and C. Gao, ACS Appl. Mater. Interfaces, 2010, 2, 3201.
299. W. Shi, J. Zhu, D. H. Sim, Y. Y. Tay, Z. Lu, X. Zhang, Y. Sharma, M. Srinivasan, H. Zhang, H. H. Hng and Q. Yan, J. Mater. Chem., 2011, 21, 3422.
300. B. Li, H. Cao, J. Shao, M. Qu and J. H. Warner, J. Mater. Chem., 2011, 21, 5069.
301. G. Wang, T. Liu, X. Xie, Z. Ren, J. Bai and H. Wang, Mater. Chem. Phys., 2011, 128, 336.
302. J. Zhu, T. Zhu, X. Zhou, Y. Zhang, X. W. Lou, X. Chen, H. Zhang, H. H. Hng and Q. Yan, Nanoscale, 2011, 3, 1084.
303. V. K. Singh, M. K. Patra, M. Manoth, G. S. Gowd, S. R. Vadera and N. Kumar, New Carbon Mater., 2009, 24, 147.
304. X. Zhu, Y. Zhu, S. Murali, M. D. Stoller and R. S. Ruoff, ACS Nano, 2011, 5, 3333.
305. S. Bashkova and T. J. Bandosz, Ind. Eng. Chem. Res., 2009, 48, 10884.
306. G. Wang, T. Liu, Y. Luo, Y. Zhao, Z. Ren, J. Bai and H. Wang, J. Alloys Compd., 2011, 509, L216.
307. Z.-S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li and H.-M. Cheng, ACS Nano, 2010, 4, 3187.
308. S. Yang, X. Feng, S. Ivanovici and K. Müllen, Angew. Chem., Int. Ed., 2010, 49, 8408.
309. H. Kim, D.-H. Seo, S.-W. Kim, J. Kim and K. Kang, Carbon, 2011, 49, 326.
310. J. Yan, T. Wei, W. Qiao, B. Shao, Q. Zhao, L. Zhang and Z. Fan, Electrochim. Acta, 2010, 55, 6973.
311. S. Yang, X. Feng, L. Wang, K. Tang, J. Maier and K. Müllen, Angew. Chem., Int. Ed., 2010, 49, 4795.
312. B. Wang, Y. Wang, J. Park, H. Ahn and G. Wang, J. Alloys Compd., 2011, 509, 7778.
313. Y. Zou and Y. Wang, Nanoscale, 2011, 3, 2615.
314. I. R. M. Kottegoda, N. H. Idris, L. Lu, J.-Z. Wang and H.-K. Liu, Electrochim. Acta, 2011, 56, 5815.
315. Z. Ji, J. Wu, X. Shen, H. Zhou and H. Xi, J. Mater. Sci., 2011, 46, 1190.
316. H. Yang, G. H. Guai, C. Guo, Q. Song, S. P. Jiang, Y. Wang, W. Zhang and C. M. Li, J. Phys. Chem. C, 2011, 115, 12209.
317. D. Du, J. Liu, X. Zhang, X. Cui and Y. Lin, J. Mater. Chem., 2011, 21, 8032.
318. S. Watcharotone, D. A. Dikin, S. Stankovich, R. Piner, I. Jung, G. H. B. Dommett, G. Evmenenko, S.-E. Wu, S.-F. Chen, C.-P. Liu, S. T. Nguyen and R. S. Ruoff, Nano Lett., 2007, 7, 1888.
319. S. Wu, Z. Yin, Q. He, G. Lu, X. Zhou and H. Zhang, J. Mater. Chem., 2011, 21, 3467.
320. C. Xu, X. Wang, L. Yang and Y. Wu, J. Solid State Chem., 2009, 182, 2486.
321. B. Li, H. Cao, G. Yin, Y. Lu and J. Yin, J. Mater. Chem., 2011, 21, 10645.
322. F. Kim, J. Luo, R. Cruz-Silva, L. J. Cote, K. Sohn and J. Huang, Adv. Funct. Mater., 2010, 20, 2867.
323. H. Wang, Y. Liang, T. Mirfakhrai, Z. Chen, H. Casalongue and H. Dai, Nano Res., 2011, 4, 729.
324. X. Wang, S. M. Tabakman and H. Dai, J. Am. Chem. Soc., 2008, 130, 8152.
325. Y. Fan, L. Wang, J. Li, J. Li, S. Sun, F. Chen, L. Chen and W. Jiang, Carbon, 2010, 48, 1743.
326. K. Wang, Y. Wang, Z. Fan, J. Yan and T. Wei, Mater. Res. Bull., 2011, 46, 315.
327. J.-H. Park, S.-W. Seo, J.-H. Kim, C.-J. Choi, H. Kim, D. K. Lee, W.-S. Jung and K.-S. Ahn, Mol. Cryst. Liq. Cryst., 2011, 538, 285.
328. X. Yang, C. Lu, J. Qin, R. Zhang, H. Tang and H. Song, Mater. Lett., 2011, 65, 2341.
329. L. Xie, X. Wang, H. Mao, R. Wang, M. Ding, Y. Wang, B. Özyilmaz, K. P. Loh, A. T. S. Wee, Ariando and W. Chen, Appl. Phys. Lett., 2011, 99, 012112.
330. Y. Fu and X. Wang, Ind. Eng. Chem. Res., 2011, 50, 7210.
331. E. Gao, W. Wang, M. Shang and J. Xu, Phys. Chem. Chem. Phys., 2011, 13, 2887.
332. Y. Ding, Y. Jiang, F. Xu, J. Yin, H. Ren, Q. Zhuo, Z. Long and P. Zhang, Electrochem. Commun., 2010, 12, 10.
333. L. Wang, H. Wang, Z. Liu, C. Xiao, S. Dong, P. Han, Z. Zhang, X. Zhang, C. Bi and G. Cui, Solid State Ionics, 2010, 181, 1685.
334. C. Nethravathi, T. Nisha, N. Ravishankar, C. Shivakumara and M. Rajamathi, Carbon, 2009, 47, 2054.
335. A. Cao, Z. Liu, S. Chu, M. Wu, Z. Ye, Z. Cai, Y. Chang, S. Wang, Q. Gong and Y. Liu, Adv. Mater., 2010, 22, 103.
336. J. Wu, S. Bai, X. Shen and L. Jiang, Appl. Surf. Sci., 2010, 257, 747.
337. T. Dufaux, J. Boettcher, M. Burghard and K. Kern, Small, 2010, 6, 1868.
338. F. Miao, et al , Nanotechnology, 2010, 21, 075601.
339. K. Wang, Q. Liu, X.-Y. Wu, Q.-M. Guan and H.-N. Li, Talanta, 2010, 82, 372.
340. X. Geng, L. Niu, Z. Xing, R. Song, G. Liu, M. Sun, G. Cheng, H. Zhong, Z. Liu, Z. Zhang, L. Sun, H. Xu, L. Lu and L. Liu, Adv. Mater., 2010, 22, 638.
341. Y. Lin, K. Zhang, W. Chen, Y. Liu, Z. Geng, J. Zeng, N. Pan, L. Yan, X. Wang and J. G. Hou, ACS Nano, 2010, 4, 3033.
342. Y.-T. Kim, J. H. Han, B. H. Hong and Y.-U. Kwon, Adv. Mater., 2010, 22, 515.
343. A. F. Zedan, S. Sappal, S. Moussa and M. S. El-Shall, J. Phys. Chem. C, 2010, 114, 19920.
344. L.-L. Li, K.-P. Liu, G.-H. Yang, C.-M. Wang, J.-R. Zhang and J.-J. Zhu, Adv. Funct. Mater., 2011, 21, 869.
345. N. Li, G. Liu, C. Zhen, F. Li, L. Zhang and H.-M. Cheng, Adv. Funct. Mater., 2011, 21, 1717.
346. Y. Park, S.-H. Kang and W. Choi, Phys. Chem. Chem. Phys., 2011, 13, 9425.
347. H. Wang, L.-F. Cui, Y. Yang, H. Sanchez Casalongue, J. T. Robinson, Y. Liang, Y. Cui and H. Dai, J. Am. Chem. Soc., 2010, 132, 13978.
348. N. D. Mermin, Phys. Rev., 1968, 176, 250.
349. D. A. C. Brownson, D. K. Kampouris and C. E. Banks, J. Power Sources, 2011, 196, 4873.
350. Y. Wang, Z. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen and Y. Chen, J. Phys. Chem. C, 2009, 113, 13103.
351. M. Pumera, Chem. Rec., 2009, 9, 211.
352. P. V. Kamat, J. Phys. Chem. Lett., 2011, 2, 242.
353. Y. H. Hu, H. Wang and B. Hu, ChemSusChem, 2010, 3, 782.
354. X. Huang, X. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012 10.1039/C1CS15078B.
355. S. Sarkar, S. Niyogi, E. Bekyarova and R. C. Haddon, Chem. Sci., 2011, 2, 1326.
356. N. F. Goldshleger, Fullerene Sci. Technol., 2001, 9, 255.
357. B. Li and Z. Xu, J. Am. Chem. Soc., 2009, 131, 16380.
358. N. Muradov, Catal. Commun., 2001, 2, 89.
359. M. Croston, J. Langston, R. Sangoi and K. S. V. Santhanam, Int. J. Nanosci., 2002, 1, 277.
360. M. Croston, J. Langston, G. Takacs, T. C. Morrill, M. Miri, K. S. V. Santhanam and P. Ajayan, Int. J. Nanosci., 2002, 1, 285.
361. D. S. Su, N. Maksimova, J. J. Delgado, N. Keller, G. Mestl, M. J. Ledoux and R. Schlögl, Catal. Today, 2005, 102–103, 110.
362. J. Zhang, X. Liu, R. Blume, A. Zhang, R. Schlögl and D. S. Su, Science, 2008, 322, 73.
363. B. Frank, J. Zhang, R. Blume, R. Schlögl and D. S. Su, Angew. Chem., Int. Ed., 2009, 48, 6913.
364. M. F. R. Pereira, J. J. M. Orfão and J. L. Figueiredo, Appl. Catal., A, 2000, 196, 43.
365. G. Mestl, N. I. Maksimova, N. Keller, V. V. Roddatis and R. Schlögl, Angew. Chem., Int. Ed., 2001, 40, 2066.
366. N. Keller, N. I. Maksimova, V. V. Roddatis, M. Schur, G. Mestl, Y. V. Butenko, V. L. Kuznetsov and R. Schlögl, Angew. Chem., Int. Ed., 2002, 41, 1885.
367. M. F. R. Pereira, J. L. Figueiredo, J. J. M. Órfão, P. Serp, P. Kalck and Y. Kihn, Carbon, 2004, 42, 2807.
368. B. Frank, R. Blume, A. Rinaldi, A. Trunschke and R. Schlögl, Angew. Chem., Int. Ed., 2011, 50, 10226.
369. T.-F. Yeh, J.-M. Syu, C. Cheng, T.-H. Chang and H. Teng, Adv. Funct. Mater., 2010, 20, 2255.
370. D. Geng, Y. Chen, Y. Chen, Y. Li, R. Li, X. Sun, S. Ye and S. Knights, Energy Environ. Sci., 2011, 4, 760.
371. L. Qu, Y. Liu, J.-B. Baek and L. Dai, ACS Nano, 2010, 4, 1321.
372. Y. Shao, S. Zhang, M. H. Engelhard, G. Li, G. Shao, Y. Wang, J. Liu, I. A. Aksay and Y. Lin, J. Mater. Chem., 2010, 20, 7491.
373. L. Tang, Y. Wang, Y. Li, H. Feng, J. Lu and J. Li, Adv. Funct. Mater., 2009, 19, 2782.
374. K. R. Lee, K. U. Lee, J. W. Lee, B. T. Ahn and S. I. Woo, Electrochem. Commun., 2010, 12, 1052.
375. D. R. Dreyer, H.-P. Jia and C. W. Bielawski, Angew. Chem., Int. Ed., 2010, 49, 6813.
376. H.-P. Jia, D. R. Dreyer and C. W. Bielawski, Adv. Synth. Catal., 2011, 353, 528.
377. D. R. Dreyer, S. Murali, Y. Zhu, R. S. Ruoff and C. W. Bielawski, J. Mater. Chem., 2011, 21, 3443.
378. D. R. Dreyer, K. A. Jarvis, P. J. Ferreira and C. W. Bielawski, Macromolecules, 2011, 44, 7659.
379. D. R. Dreyer, H.-P. Jia, A. D. Todd, J. Geng and C. W. Bielawski, Org. Biomol. Chem., 2011, 9, 7292.
380. Y. Xin, J.-g. Liu, Y. Zhou, W. Liu, J. Gao, Y. Xie, Y. Yin and Z. Zou, J. Power Sources, 2011, 196, 1012.
381. R. Imran Jafri, N. Rajalakshmi and S. Ramaprabhu, J. Mater. Chem., 2010, 20, 7114.
382. Y. Sun, C. Li, Y. Xu, H. Bai, Z. Yao and G. Shi, Chem. Commun., 2010, 46, 4740.
383. S. Yang, X. Feng, X. Wang and K. Müllen, Angew. Chem., Int. Ed., 2011, 50, 5339.
384. Q. Xiang, J. Yu and M. Jaroniec, J. Phys. Chem. C, 2011, 115, 7355.
385. X.-H. Li, J.-S. Chen, X. Wang, J. Sun and M. Antonietti, J. Am. Chem. Soc., 2011, 133, 8074.
386. F. Bonaccorso, Z. Sun, T. Hasan and A. C. Ferrari, Nat. Photonics, 2010, 4, 611.
387. G. Eda and M. Chhowalla, Adv. Mater., 2010, 22, 2392.
388. M. Liang, B. Luo and L. Zhi, Int. J. Energy Res., 2009, 33, 1161.
389. J. K. Wassei and R. B. Kaner, Mater. Today, 2010, 13, 52.
390. H. Chang, Y. Liu, H. Zhang and J. Li, J. Electroanal. Chem., 2011, 656, 269.
391. K. Zhou, Y. Zhu, X. Yang, X. Jiang and C. Li, New J. Chem., 2011, 35, 353.
392. D.-H. Yoo, T. V. Cuong, V. H. Pham, J. S. Chung, N. T. Khoa, E. J. Kim and S. H. Hahn, Curr. Appl. Phys., 2011, 11, 805.
393. W. Fan, Q. Lai, Q. Zhang and Y. Wang, J. Phys. Chem. C, 2011, 115, 10694.

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