Chemically derived graphene

3 Chemically derived graphene R. S. S U N DA R A M, Max Planck Institute for Solid State Research, Germany and University of Cambridge, UK DOI: 10.1533/9780857099334.1.50 Abstract: The synthesis of graphene via chemical functionalisation of graphite is reviewed, including investigations that unravelled the atomic structure of resulting graphene oxide (GO) sheets in the suspension. The fundamental properties of GO are discussed and a summary of recent advances in device applications is provided. Key words: graphene, graphene oxide, chemically derived graphene.

3.1

Introduction

Owing to the surge of interest in graphene, research into various methods of production of this material has captivated scientists all over the world. Pioneering efforts of obtaining monolayers of graphene involved the mechanical exfoliation of natural graphite using an adhesive tape. Although, this method provides very high quality monolayers on surfaces, they have to be isolated by time-expensive manual processes. One possible solution to circumvent these problems is the use of solution-based techniques to separate the layers of graphite thus yielding a suspension of graphene. There have been numerous approaches explored in this regard, all of which follow the same underlying principle of liquid-phase exfoliation by weakening the van der Waals interaction between the layers of graphite by either intercalation or functionalisation of the individual layers. This approach is both scalable, affording the possibility of high-volume production, and versatile, making it well-suited to chemical functionalisation. It is hence promising for a wide range of applications. Graphite intercalation compounds or expandable graphites are interesting starting materials to obtain colloidal dispersions of single-layer graphene sheets. Ideally, this approach should allow the production of high-quality single-layer sheets of graphene. Colloidal suspensions of graphene sheets in organic solvents such as N-methylpyrrolidone (NMP) were obtained by sonication of graphite powder, but only with lateral sizes of a few hundreds of nanometers and quite low yield.1 Electrical sheet resistivity measurements on very thin films made from these sheets yielded, for example, an electrical conductivity of 6500 S m−1 at 42% of optical transparency. In addition, 50 © 2014 Woodhead Publishing Limited

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even after the films had been dried at 400 °C, residual NMP had not been completely removed and was estimated to be 7% by weight. A milder dissolution route has also been reported wherein starting from neutral graphite, and avoiding any kind of sonication, one could obtain large size graphene flakes. This was achieved by stirring the ternary potassium salt, K(THF)xC24 (a graphite intercalation compound), in NMP.2 Polymer-coated graphene derivatives were synthesised from thermally treated, commercial expandable graphite at high temperature by sonication in a dichloroethane solution of poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV).3 Another approach (illustrated in Fig. 3.1) was intercalation with oleum and expansion with tetrabutylammonium hydroxide (TBA). The final suspension of graphene sheets, coated with 1,2-distearoyl-snglycero-3-phosphoethanolamine-N-methoxy(polyethyleneglycol)-5000 (DSPE-mPEG), was produced by sonication in dimethyl formamide (DMF), with ∼90% of the sheets reported as individual modified graphene sheets.4 There have been numerous other efforts to produce stable colloidal suspensions of graphene by exfoliation in liquids.5–7 However, one of the most promising, low-cost, up-scalable and widely studied synthetic approaches so far has been the reduction of oxidised layers of graphite which can be deposited with controllable density onto a wide range of substrates. A major advantage of the graphite oxide approach to graphene is that it is straightforward to synthesise, process, and integrate into devices using existing top down approaches of thin film electronics technology. Furthermore, this approach offers potential for the production of chemically

x

x

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x

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x

Colloidal suspension of polymer-coated graphene

TBA

x

x

x

x

x

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DMF

Sonication of PEG–phospholipid

3.1 Schematic depiction of the exfoliation of graphite intercalated with sulfuric acid (oleum) (small circles) followed by the insertion of TBA (large circles) intercalate. Upon sonication in the phospholipid polymer, the intercalated material yields a dark colloidal suspension of separated graphene sheets. (Adapted from reference 4.)

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derived graphene on an industrial scale.8 Stankovich et al.9–11 pioneered this technique of the chemical synthesis of graphite oxide, followed by its exfoliation into individual graphene oxide (GO) sheets, and their subsequent reduction. In the following section, these steps are described in detail in order to elucidate this novel approach.

3.2

Synthesis of graphene oxide (GO)

Despite the relative novelty of the GO route to graphene, it has a history that extends back to the early nineteenth century.12–14 Since then, graphite oxide has been mainly produced by the Brodie,12 Staudenmaier13 and Hummers14 methods. One of the earliest investigations was reported by the British chemist B. C. Brodie who was exploring the structure of graphite by investigating the reactivity of flake graphite. He determined that by adding potassium chlorate (KClO3) to a slurry of graphite in fuming nitric acid (HNO3), the resulting material was composed of carbon, hydrogen, and oxygen, resulting in an increase in the overall mass of the flake graphite. Successive oxidative treatments resulted in a further increase in the oxygen content, reaching a limit after four reactions. The composition was determined to be 61.04% carbon, 1.85% hydrogen and 37.11% oxygen. Furthermore, Brodie was able to disperse the resulting material in water or alkaline solutions, but not in acidic media. Heating to a temperature of 220 °C resulted in an increase in carbon percentage to 80.13%. Although Brodie was unable to accurately determine the molecular weight of graphite through his studies, what he had unknowingly discovered a method to oxidise graphite. The German chemist Staudenmaier improved on the seminal effort of Brodie by, firstly, dividing the addition of KClO3 in multiple parts over the course of the reaction and, secondly, increasing the acidity of the mixture by addition of concentrated sulfuric acid.13 Nearly 60 years later, Hummers and Offeman developed an alternative oxidation method by reacting graphite with a mixture of potassium permanganate (KMnO4) and concentrated sulfuric acid (H2SO4), again achieving similar levels of oxidation.14 Over the years there have been numerous efforts to oxidise graphite through various modifications of this method but the underlying principle remains the same even today. The Hummers method uses a combination of potassium permanganate (KMnO4) and sulfuric acid (H2SO4). Although permanganate is a well established oxidising agent, the active species in the oxidation of graphite is dimanganese heptoxide (Mn2O7), which appears as brownish red oil formed from the reaction of KMnO4 with H2SO4 (Equation 3.1). The Mn2O7 is far more reactive than MnO−4 , and is known to undergo an explosive reaction when heated to temperatures greater than 55 °C or when placed

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in contact with organic compounds.15,16 Trömel and Russ demonstrated the ability of Mn2O7 to selectively oxidise unsaturated aliphatic double bonds over aromatic double bonds, which may have important implications for the structure of graphite and reaction pathway(s) occurring during the oxidation.17 KMnO4 + 3 H 2 SO4 → K + + MnO+3 + H 2 O+ + 3HSO4−

[3.1]

MnO+3 + MnO4− → Mn 2 O7 Oxidation of graphite via this method can be done starting from various commercially available sources. However, flake graphite is most commonly used. This is a naturally occurring mineral that is purified to remove heteroatomic contamination. The exact mechanism of oxidation of graphite is unfortunately very challenging to ascertain owing to the complexity of flake graphite and the presence of inherent defects in its structure. Graphite oxide obtained by this method exists in as a brown viscous slurry, which contains not only graphite oxide but also non-oxidised heavy graphitic particles and the residue of the reaction by-products. Pure GO suspensions are achieved by centrifugation, sedimentation, or dialysis which removes salts and ions from the oxidation process.18–21 Further centrifugation is used to achieve a suspension of monolayer GO by precipitating unoxidised graphitic particles and heavy graphite oxide platelets. Monodispersed suspensions of GO flakes (separated according to their lateral size) can also be obtained by density-gradient centrifugation.22,23

3.3

Reduction of graphene oxide (GO)

Although GO itself is insulating, its carbon framework can be substantially restored by thermal annealing or treatment with chemical reducing agents resulting in reduced graphene oxide (RGO). Earlier efforts mainly involved the use of hydrazine vapour for this purpose.10 The use of hydrazine, however, requires great care because it is both highly toxic and potentially explosive. Therefore, a number of alternative techniques have been explored. Sodium borohydride (NaBH4) was recently demonstrated to function more effectively than hydrazine as a reductant of GO.24 Though NaBH4 is slowly hydrolysed by water, this process is kinetically slow enough that freshly prepared solutions, still function effectively as reductants of GO. Several other reductants have also been used for the reduction of GO including hydroquinone,25 gaseous hydrogen,26 vitamin C (ascorbic acid)27 and strongly alkaline solutions.28 A simpler yet equally effective route for reduction of GO is via the exposure to mild hydrogen plasma which achieved results similar to other chemical reduction methods.29 Further improvements in the reduction of

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GO were obtained by thermally annealing the sheets in vacuum or an inert atmosphere such as argon. A recent study of the effects of this method revealed a strong effect of temperature.30 As an alternative to chemical methods, there has been extensive research into methods such as electrochemical reduction,31–33 photocatalytic reduction34 and reduction using photographic flash.35 It has been recently demonstrated that defects in RGO can be further ‘repaired’ by introducing a carbon source such as ethylene at elevated temperatures (800 °C), similar to conditions used for CVD growth of singlewall nanotubes (SWNTs). This incorporation of carbon decreases the sheet resistance of individual RGO sheets to ∼28.6 kΩ sq−1 (conductivity ∼350 S cm−1).36

3.4

Physicochemical structure of graphene oxide (GO)

Although extensive research has been done to determine the chemical structure of graphite oxide, several models (Fig. 3.2) have been proposed. An early interpretation of the structure proposed that the oxygen is bound to the carbon atoms of the hexagonal layered planes by epoxide (C2O) linkages (Hofmann).37 The planarity of the graphite layers was thought to be largely conserved in this case, but this was challenged in a later model where a distorted carbon sheet composed of linked cyclohexane chairs is assumed and saturation of the carbon valencies is achieved by bonding to axial –(OH) groups and ether oxygens in 1,3 positions (Ruess).38 This model was the first to account for the presence of hydrogen in GO which was later modified with C=C double bonds, and ketonic and enolic groups.39 In addition, observation of acidity in this material unravelled the presence of in-plane enolic –(OH) species and carboxylic groups. This model was then revised in accordance with stereochemical considerations by Scholz and Boehm.40 Another school of thought proposed a structure analogous to polyfluorocarbons.41 Most recent models have focused on a nonstoichiometric, amorphous alternative instead of the lattice-based model. For instance, the model proposed by Lerf et al.,43 based on NMR studies, describes GO as randomly distributed regions of unoxidised benzene rings and distorted hexagonal rings bearing (C=C), 1,2-ethers and –(OH) groups. A recent study of the evolution of the surface functional groups upon oxidation led to the model of Scholz et al., involving a few stereochemical modifications.42 Initial studies done by Lerf and coworkers used solid state nuclear magnetic resonance (NMR) spectroscopy to characterise the material. Studies using 13C labelled graphite oxide based NMR indicated that the sp2-bonded carbon network of graphite is strongly disrupted and a significant fraction of this carbon

Chemically derived graphene Scholz–Boehm

Hofmann O

O

HO O O

O O

O O

O

O

O O

O OH O O OH O

O O OH

OH

OH O–

O

OH

O HO O O O OH

O

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OH

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OH OH

OH O

OH

O– O

Ruess

OH OH

OH

OH

OH

55

O OH

O–

OH O–

O–

OH OH

OH

Nakajima–Matsuo

3.2 Older structural models of graphene oxide. (Reproduced with permission from reference 42, copyright 2006, American Chemical Society.)

network is bonded to hydroxyl groups or participates in epoxide groups. Minor components of carboxylic or carbonyl groups are thought to populate the edges of the layers in graphite oxide.44 The Dékány model42 (Fig. 3.3) deviated from the popular Lerf and Klinowski model by suggesting a regular, corrugated quinoidal structure interrupted by trans-linked cyclohexyl regions, functionalised by tertiary alcohols and 1,3-ethers. The Dékány model is composed of two distinct domains: trans-linked cyclohexyl species interspersed with tertiary alcohols and 1,3-ethers, and a corrugated network of keto/quinoidal species. No carboxylic acids are believed to be present in this description of GO. Further oxidation destroys the alkenes of the quinones through formation of 1,2ethers, as well as any pockets of aromaticity that may have persisted during the initial oxidative conditions used for its synthesis. It is also hypothesised that the quinones introduce rigidity and plane boundaries, and are a possible source of the macroscopic wrinkling of the platelets commonly seen in TEM images.42 In recent years, Raman spectroscopy has become an important tool in the study of carbon based nanostructures,45 as it is nondestructive, fast, with high resolution and gives great structural and electronic information. Highly ordered graphite displays only a few discernible Raman-active bands, namely the in-phase C–C stretch vibration of the graphite lattice (G band) observed at 1600 cm−1 and the weak disorder band caused by the graphite

56

Graphene O

HO

O HO O O

O HO

O

O OH

O O

HO

O

OH

HO

O O

HO O

OH

3.3 Structure of GO proposed by Dékány and coworkers. (Reproduced with permission from reference 42, copyright 2006, American Chemical Society.)

edges (D band) at approximately 1355 cm−1. Figure 3.4 shows representative Raman spectra of monolayers of GO and RGO obtained at an excitation wavelength of 532 nm. The transformation of graphite to GO leads to the broadening of the G as well as the D band, owing to disorder in the graphite lattice introduced upon oxidation. In addition, a shift of the G band towards higher frequencies was observed. This upshift can be attributed to the isolated double bonds which resonated at higher frequencies than graphite does.45 The reduction of GO restores the postition of the G band to almost the same position as that of graphene indicating a considerable restoration of the graphitic lattice.46 A notable feature in the spectra for mechanically exfoliated graphene is the sharp 2D with a full width at half maximum (FWHM) of 30 cm−1 compared with ∼200 cm−1 for GO. Furthermore, the defect-activated D + D′ peak manifests at ∼2950 cm−1.47 The overall Raman peak intensities are diminished after reduction treatment, suggesting loss of carbon during reduction.48 Reduction of GO induces changes in its structure owing to the removal of oxygen and carbon atoms.49,50 The area ratio of the Raman D and G peaks is a measure of the size of sp2 clusters in a network of sp2 and sp3 carbon. The Tuinstra–König relation51 relates the D/G intensity ratio to the crystallite size of graphitic samples which reported that this ratio varied inversely with the average in-plane crystallite size. Based on this relation, the average graphitic domain size GO has been calculated and values ranging from 2.5 to 6 nm have been reported.29,49 The effects of reduction on the sp2 domain size in GO reported in the literature have been equivocal with respect to

Chemically derived graphene D-band

GO RGO Graphene

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2D-band

Intensity (a.u.)

G-band

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1400

2400 2600 1600 Raman shift (cm–1)

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3.4 Raman spectra of monolayers of GO, RGO and mechanically exfoliated graphene on SiO2/Si substrates. (Reprinted with permission from reference 30, copyright 2008, American Chemical Society.)

the change in D/G ratio.10,29,49,52–55 A common observation however is the presence of a considerable D-peak signal, indicating that significant disorder remains in the reduced sample. The Tuinstra–Koenig relation however, is not valid above a critical defect density. In highly disordered materials, if the sp2 domain size is smaller than ∼3 nm, the D/G ratio increases with the number of aromatic rings, deviating from the Tuinstra–König relation.45 In order to discern the exact order of the sp2 domains in GO/RGO, several studies resorted to direct imaging using microscopic techniques. The surface of GO was probed via scanning tunnelling microscopy (STM) and it was found that the structure consisted of highly defective regions, probably owing to the presence of oxygen surrounded by areas that were nearly intact, Fig. 3.5(a),(b).29,46,57 Fourier transformation of the STM images reveals long-range crystalline order.46 In order to unravel the atomic structure of GO/RGO many studies used transmission electron microscopy (TEM).58,59 The inherent transparency of GO monolayers to the electron beam compared with the amorphous carbon support facilitates this approach to not only image its lattice using highresolution TEM but GO to be applied as TEM supports.60 Gomez-Navarro and Meyer et al. prepared monolayers of RGO on carbon grids.59 Diffraction analysis revealed that single layers exhibited only one hexagonal pattern, Fig. 3.5 (d), which implies a long-range hexagonal order in the sheets.61 Few-layer regions, however, exhibited multiple hexagonal patterns, Fig. 3.5(c), implying that the multilayers are stacked in a turbostratic manner,

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Graphene 2 nm (a)

(e)

(b)

2 nm 1.7 nm

(c)

0 nm (d)

1 nm (f)

3.5 (a) STM image of a GO monolayer on a highly ordered pyrolytic graphite (HOPG) substrate. Regions enclosed by contours are populated with oxygen functional groups. (Reproduced with permission from reference 29, copyright 2007, American Chemical Society.) (b) STM image of a RGO monolayer on a HOPG substrate and its Fourier transform (upper-right inset). The lower-left inset shows a STM image of a HOPG surface obtained under identical conditions. (Reproduced with permission from reference 46, copyright 2008, American Chemical Society.) (c) Electron diffraction pattern of a bilayer area, showing the stacking with orientational mismatch of the sheets. (d) Diffraction pattern from a single layer, atomic resolution, aberration-corrected TEM image of a single-layer RGO membrane. (e) Original image and (f) shaded to highlight the different features. [(c), (d), (e) and (f) reproduced with permission from reference 56, copyright 2010, American Chemical Society.]

unlike the AB Bernal stacking in graphite and mechanically exfoliated few-layer graphene which is characteristic of low interacting sheets.61 This is not surprising as the functional groups protruding from the GO planes decouple the interactions between the graphitic framework of stacked layers.58–60 Gomez-Navarro and Meyer et al. performed aberration-corrected highresolution imaging of single layers to gain insight into the exact atomic structure of the RGO. Figure 3.5(f) shows different regions of the TEM image, Fig. 3.5(e), marked by colours. These measurements lucidly reveal that in the majority of the layer area the hexagonal lattice of graphene is clearly preserved (light grey colour). The size of the visible well-crystallised areas varies from 3 to 6 nm, and statistics reveal that they cover ∼60% of the surface. However, a significant proportion of the regions were covered

Chemically derived graphene

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with carbonaceous adsorbates and also trapped heavier atoms62,63 (dark grey). Another feature was the considerable number of topological defects observed within the clean areas in contrast to the flawless mechanically exfoliated graphene. A closer investigation unearthed isolated topological defects (pentagon–heptagon pairs, black), and extended (clustered) topological defects that appear as quasi-amorphous single layer carbon structures (cross-hatched) within these areas. The extended topological defects cover about 5% of the surface and exhibit typical sizes of 1–2 nm in diameter. In spite of the presence of such a significant amount of defects, the long-range orientational order is maintained.58,59 TEM studies further revealed that the structure of RGO contain topological defects that remain after removing the functional groups59 in contrast to GO samples, which show much stronger coverage with adsorbates and do not display the characteristic extended defects over its surface.64 Owing to the synthesis route of GO, it contains a major proportion of its oxygen in the form of covalently bonded functional groups. This results in a significant fraction of the carbon atoms being sp3 hybridised (∼60%) and reduction results in bringing this fraction down to 20%.49 A monolayer of GO exhibits an atomic force microscopy (AFM) thickness of ∼1 nm, which is significantly larger than that of its flawless counterpart owing to the presence of these functional groups above and below the carbon basal plane. This observation was further corroborated by Pacile et al.,64 who found by near-edge x-ray and fine structure (NEXAFS) measurements that the multilayers of GO are strongly decoupled from each other. This study also takes advantage of the polarisation dependence of such a measurement and, by observing the oxygen K-edge spectra, shows that carbonyl groups, together with epoxide and hydroxyl groups are attached to aromatic rings and carboxyl groups probably attached to the edges of the membranes.64 There have been contrasting reports, however, on the ordering of inherent functional groups in GO. Using high-resolution STM imaging Pandey et al.65 showed that oxygen atoms were ordered in a rectangular lattice which pointed towards epoxy groups arranged in strips. This observation was proven to be energetically favourable by the density functional theory (DFT) calculations.66 Similar observations were also reported using NEXAFS wherein the fine structure of the oxygen edge spectra exhibits remarkably sharp, distinct features, affected by the polarisation of the light, suggesting a locally ordered arrangement of oxygen atoms.64 However, the absence of diffraction spots, for example from electron diffraction as shown in Fig. 3.5(c) and (d) other than those corresponding to the graphitic lattice shows that any oxygen-containing functional groups present lack structural order.60 The structure of GO and its reduced form indeed forms an interesting structural study, as borne out by the numerous published reports. From

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these reports one can confidently conclude that the physical structure of GO/RGO comprises regions of pristine graphitic symmetry surrounded by regions of clustered and individual topological defects. One can also agree that the major functional groups that are present in GO owing to the oxidation of graphite are hydroxyl, epoxides and carbonyls, the amount of which is considerably reduced upon reduction.

3.5

Electrical transport in graphene oxide (GO)

In the previous section we established that, although graphite oxide can generate homogeneous colloidal suspensions, the resulting sheets are electrically insulating. RGO sheets however, measured by two-probe transport measurements using lithographically defined electrodes clearly show a conductivity improvement by around three orders of magnitude (Fig. 3.6(a)) upon reduction. Room-temperature measurements yielded conductivity values of 0.1–3 S cm−1 in accordance with previous studies.29 This range is about three orders of magnitude below the values reported for pristine graphene.67 The measured resistance in a two-probe configuration can be represented as a combination of contributions from the graphene sheet and the contacts. For mechanically exfoliated graphene, this presents a significant problem because the metal contacts possess an invasive nature.68,69 The situation is different for RGO devices. Here, it was observed that the contribution of contacts is minimal and, hence, the major voltage drop occurs along the channel as shown in Fig. 3.6(b) shows the two-probe resistance measurements measured at 100 mV drain-source bias from devices with varying channel lengths. The linearly increasing resistance with the abscissa passing through the origin confirms that the resistance measured originates solely from the RGO channel. This observation has been further corroborated by photocurrent microscopy measurements which showed little or no interaction between the RGO sheet70 and metal contacts in contrast to its micromechanically exfoliated counterpart.68 Field effect transistor (FET) devices were first reported using single-layer RGO with hole and electron mobilities of 2 and 0.2 cm−2 V s.29,36,55,71–74 To fabricate single-RGO devices, individual sheets of GO are deposited onto substrates with prepatterned alignment marks, located using an optical microscope or an AFM, and contacted by metal electrodes following standard lithographic procedures. Measurements carried out at room temperature in ambient conditions showed prominent p-type behaviour. However, the device showed an ambipolar field effect characteristic of graphene when measured under inert surroundings. Jung et al. demonstrated high sensitivity of the GO to air by measuring FET properties.30 When the sample was exposed to air, the FETs, Fig. 3.6(c), exhibited high hysteresis

Chemically derived graphene (a)

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3.6 (a) Plot of current vs. voltage for graphene oxide (continuous line) and reduced graphene oxide (dashed line) sheets at room temperature. After 5 s in H2 plasma, insulating GO is converted to its conductive reduced form. (b) Room-temperature resistance (measured at 100 mV) increasing linearly with channel length, showing that it is composed primarily of the intrinsic resistance of the GO sheet with an insignificant contribution from contact resistance. (c) Schematic illustration of a field effect transistor based on RGO. (d) Roomtemperature gate dependence of resistance of a RGO sheet measured in air and under vacuum.

during gate voltage sweeps which could be remedied by measuring in a vacuum as shown in Fig. 3.6(d). Similar shifts have been reported for mechanically exfoliated graphene and single-walled carbon nanotubes (SWNTs), which are explained by doping owing to oxygen and/or water absorption.75 The adsorption of water molecules at the defect sites and dangling bonds on carbon nanotubes has been well documented.76 Water also acts as an electron acceptor when adsorbed on the graphene surface as verified by recent experimental77 and theoretical investigations.75 It has been reported that the adsorption of water on the surface of graphene is accompanied by hole injection. A p-type doping is thus expected when the electron affinity of the adsorbate molecule (water) is greater than the work function of the substrate (graphene). This is possible only if the adsorbate possesses unoccupied electronic states

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lower in energy than the highest occupied state of the substrate. Theoretical calculations show that this condition is satisfied for this system.75 RGO as elucidated in the previous section is made up of defective regions coming from the oxidation process. The morphology, thus consists of interspersed highly conducting and disordered regions (Fig. 3.5). Mott described that at very low temperatures, when there is not enough thermal energy for the charge to go from valance band to conduction band, the charge conduction occurs via hopping among the localised states.78 When the energy is low, the carrier hops longer distances. This is called variable range hopping. According to Mott, the conductivity of such a system can be described as, T σ = A exp ⎛⎜ − 1/( d0 + 1) ⎞⎟ ⎝ T ⎠

[3.1]

where A is a constant, T0 is critical temperature and d is the dimensionality of the system. Thus, for a 2D system, σ ∼ T−1/3. However, it should be noted that in Mott variable range hopping (VRH), the coulomb interaction among localised states is neglected and there is no minimum energy needed for the carriers to hop. Because RGO has many defects which localise charge carriers, the temperature dependence follows a modified variable-range hopping model with a metal-like temperature-independent term. This contribution can be discerned at low temperatures (Fig. 3.7) and is similar to that shown by single-walled carbon nanotube networks.79 Kaiser and collaborators80 reported, Fig. 3.7(a), that this behaviour can be described by a heterogeneous model of conduction with 2D VRH in disordered barrier regions in parallel with an additional constant term (e.g. from tunnelling through thinner barriers between high conductivity regions). In this model, the temperature dependence of the conductivity σ can be described by: B σ = σ 1 exp ⎛⎜ − 1/ 3 ⎞⎟ + σ 2 ⎝ T ⎠

[3.2]

where the first term represents the usual 2D VRH conduction expression and the second term represents purely field-driven conduction without thermal activation. The hopping parameter B depends on the density of states N(EF) near the Fermi level and the localisation length Ll of the electronic wave functions involved; for 2D VRH its value is given by: 3 ⎛ ⎞ B=⎜ ⎝ kB N (EF )L2l ⎟⎠

1/ 3

[3.3]

where kB is the Boltzmann constant. The temperature dependence of the two-terminal current follows this model as shown in Fig. 3.7. Applying negative Vg leads to the lower value

Chemically derived graphene

In I (A)

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Vg = –20 V Vg = –15 V Vg = –10 V Vg = –5 V Vg = 0 Vg = 10 V Vg = 20 V

–12

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40 30 Vds = 2 V

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15 m 0.16 0.20 T–1/3 (K–1/3)

30 m 0.24

3.7 (a) Natural logarithm of the measured current I vs. T −1/3 fitted to equation [3.2] representing 2D variable-range hopping in parallel with a temperature-independent term, for different values of the gate voltage Vg. Reproduced with permission from reference 80, copyright 2009, American Chemical Society. (b) Hopping parameter B for the fits in (a), plotted as a function of gate voltage Vg for various values of bias voltage Vds with permission from reference 80, copyright 2009, American Chemical Society. (c) Temperature dependence of minimum conductivity σmin of RGO as a function of T −1/3. The linear fits show agreement with the VRH transport. For samples 16 h, HG-A, and HG-B (where HG-A and HG-B are highly reduced GO using anhydrous hydrazine followed by a 150 °C annealing step), deviation to thermally activated transport is observed at temperatures indicated by the arrows. (Reproduced with permission from reference 73, copyright 2009, American Chemical Society.)

of B pointing towards an increase in localisation lengths, Fig. 3.7(b). The hopping parameter changes with Vg, indicating that the gate bias alters the hopping condition which attains a maximum at the charge neutrality point.73,80,81 The evolution of electrical transport through RGO, studied at various degrees of reduction indicated that GO undergoes an insulator– semiconductor–semimetal transition with increasing reduction.73 Eda et al.

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report that the apparent transport gap between the tail states of valence and conduction bands ranges from 10 to 50 meV and approaches zero with extensive reduction. Furthermore, it was found that reduction of GO does not lead to reduction in localised states. In fact, Ll remains nearly constant whereas N(EF) increases (also supported by the insensitivity with reduction of coherence lengths calculated using Raman spectroscopy29). Thus, the increase in conductivity upon reduction is a result of the increase in the number of localised states rather than the increase in delocalisation of charge carriers. Formation of such mid gap states is consistent with induction of local disorder in graphene.82 This explanation of electronic transport through RGO breaks down when GO is lightly reduced when it showed a linear trend over the whole temperature range, whereas well reduced GO devices showed transition from Mott VRH (T−1/3) to Arrhenius-type (T−1) hopping above 240 K, Fig. 3.7(c). Experimental observations of electrical transport on RGO films provided further insights into transport through RGO at different stages of reduction.49 In unreduced sheets, sp2 clusters are separated by regions of oxygenated functionalities, rendering GO insulating. Reduction restores the amount sp2 carbon (although, as described earlier, an increase in sp2 ratio does not indicate a proportional increase in graphitic carbon) in GO and, as a result, there is a reduction in the transport barrier between the clusters. This reduction in energy barrier facilitates carrier hopping or tunnelling between sp2 sites. At high reduction values transport is dominated by percolation among the sp2 clusters.

3.6

Applications of graphene oxide/reduced graphene oxide (GO/RGO)

3.6.1 Transparent conductors RGO is an atomically thin sheet of carbon atoms. This property of RGO renders a thin film (<30 nm) of RGO semitransparent to visible and NIR regions whereas thick films are opaque. The transmittance and conductivity of the GO/RGO film can be tuned by tuning the thickness of the film and the degree of reduction.83 Therefore, RGO has been widely investigated for transparent conductor applications as a possible replacement for indium tin oxide (ITO)84 in devices such as organic solar cells85,86 and organic lightemitting diodes.87 There is also a potentially wide market in more common day-to-day applications such as antistatic or antiglare coatings,88 and lowemissivity windows.89 An ideal sheet of graphene exhibits sheet resistance of 6 kΩ sq−1 with nearly constant optical transparency of 98% in the visible–IR range.90 This value, although an order of magnitude higher than ITO (∼100 Ω sq−1), can

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Table 3.1 Resistivity or conductivity and transparency of RGO and exfoliated graphite at a wavelength of 550 nm. (Reprinted from reference 103, copyright 2010, with permission from Elsevier.) Material

Electronic conductivity/resistivity

Transparency (%)

Reference number

Exfoliated graphene RGO

5 kΩ sq−1 1.8 kΩ sq−1 1 kΩ sq−1 5 kΩ sq−1 70 kΩ sq−1 11 kΩ sq−1 1425 S cm−1 0.45 S cm−1 240 Ω sq−1 151 kΩ sq−1

90 70 80 80 65 96 70 94 85 93

97 98 83 86 85 99 100 93 101 102

RGO–silica RGO–CNT

be improved by doping graphene.91,92 RGO–silica composite thin films were initially considered for transparent conductors,93 but they gave moderate results owing to inefficient percolation of charge carriers through the insulating matrix. Since then, there have been several studies on utilising RGO in the form of thin films1,4,49,52,83,94–96 (Table 3.1).103 A hindrance in the application of RGO thin films as transparent conductors is that most reduction procedures require quite high temperatures or strong chemicals. To address this, several groups have investigated less hazardous routes towards reducing and doping GO at room tempartures.24,28,31,53,85 An alternative approach would be the use of GO flakes with very large lateral dimensions (on average more than 25 μm) to minimise the impact of sheet-to-sheet junctions. Mobilities of 365 cm2 V−1 s−1 for holes and 281 cm2 V−1 s−1 for electrons have been reported for lateral flake dimensions of ∼50 μm, suggesting that the extended π-bonded network can be sufficiently recovered after reduction to allow efficient carrier transport in RGO thin films.

3.6.2 Photovoltaics A direct application of RGO based transparent conductors has been for transparent electrodes in photovoltaic devices.83,85,94,96,104 An important criterion for replacing ITO as a transparent electrode in such devices is the work function of the material.81 The calculated work function of graphene is 4.42 eV105 whereas that of ITO is 4.4–4.5 eV.106 This coincidence allows for RGO to be incorporated into standard photovoltaic cells without changing the device architecture. Wang et al.94 demonstrated the fabrication and

66

Graphene

Current density (mA cm–2)

characterisation of a dye-sensitised solar cell which used an RGO transparent electrode (Fig. 3.8). These solar cells function by excitation of electrons by absorption of sunlight which are injected into the conduction band of titania and transported to the RGO electrode. The excited holes then follow a similar path through the hole-transport layer and are collected at the cathode. The device architecture is schematically illustrated in Fig. 3.8(a). RGO devices, owing to their higher sheet resistance and lower transmittance than those of FTO, show lower short circuit current. The power conversion efficiency is therefore found to be correspondingly moderate. In addition to favourable electronic and optical properties, RGO exhibits excellent electrochemical properties which make it a contender even as a counter electrode Recent efforts utilising this property of RGO

(a) I

3.0 (c)

1.5 0.0 –1.5 –3.0 –4.5

–0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Vacuum level (b) –1 Excited LUMO –2 Conduction state S* –3 band Ground HOMO –4 0 Graphene –5 Valence state S Au Spiro–6 anode band Dye OMeTAD cathod –7 TiO2

Current density (mA cm–2)

E (eV)

14 12 (d) 10 8 1 sun (AM 1.5)

6

GNP cathode Pt cathode

4 2 0 0.0

0.2

0.4 0.6 0.8 Potential (V)

1.0

3.8 (a) Schematic illustration of dye-sensitised solar cells (DSSC) using RGO as the window electrode. (b) Energy level diagram of DSSC consisting of RGO/TiO2/dye/spiro-OMeTAD/Au layers. (c) Current vs. voltage plot for RGO-based (continuous line) and FTO-based (dashed line) DSSC under simulated solar illumination. (Reproduced with permission from reference 94, copyright 2007, American Chemical Society.) (d) Current vs. voltage plot for DSSC with Pt–fluorine-doped tin oxide (FTO) counterelectrode (dotted); continuous DSSC with gold nanoparticles (GNP) counterelectrode (continuous line). (Reproduced with permission from reference 107, copyright 2007, American Chemical Society.)

Chemically derived graphene

67

as counter electrodes in these devices have shown promising results, Fig. 3.8(d),108 outperforming even the well established Pt–fluorine-doped tin oxide (FTO) counter electrodes.107 Organic photovoltaic devices (OPVs) employ conductive organic polymers or small organic molecules for light absorption and charge transport. Studies based on OPVs which use RGO as a conductive and transparent electrode have reported performances similar to that of their dye sensitised counterparts.83,85,109–111 Bulk heterojunction solar cells are a class of OPVs where the electron donor and acceptor materials are blended together and cast as a mixture. The phases of this mixture then separate to form junctions. This allows for the low diffusion lengths required for charge carriers to reach the contacts, thus giving better charge collection and higher efficiencies. RGO provides the advantage of a large surface-to-volume ratio and relatively high carrier mobility, thus rendering it an alternative electron acceptor and transport material for OPV applications. Incorporation of RGO into poly(3-hexylthiophene) (P3HT) or poly(3-octylthiophene) (P3OT) can be achieved using functionalisation schemes similar to those utilised for RGO–plasticised starch (PS) composites. Liu et al.112 have reported such devices by incorporation of RGO into P3HT. Such phovoltaic systems exhibit power conversion efficiencies of ∼1.4%, significantly better than SWNT/P3HT devices.113 RGO-based bulk heterojunction devices show tremendous promise owing to their easy and inexpensive synthesis and easy incorporation into devices. However, further optimisation is required towards efficient dispersion of GO in the polymer. In addition, reduction procedures have to be developed to prevent degradation (without exposure to strong chemicals or high temperatures) of host matrix.

3.6.3 Lithium ion batteries Increasing energy demands have not only provided the motivation to look for alternative sources but they have also driven technological improvements in storage devices. Electrical energy can be stored in batteries that consist of several electrochemical cells that are connected to provide the required voltage and capacity. Each cell is composed of a positive and a negative electrode separated by an electrolyte solution containing dissociated salts. In recent years, considerable attention has been given to lithium ion batteries (LIBs) owing to their superior characteristics in terms of rechargeability, specific energy and durability. Li-based batteries account for 63% of worldwide sales values in portable batteries.114 The LIB works on the principle of the mobility of lithium ions from the positive electrode to the negative electrode during charging and in the reverse direction during discharging. Lithium ion batteries use an intercalated lithium compound as the electrode material. To date, several

68

Graphene

electrode materials have been studied. Transition metal oxides such as SnO2, Co3O4, Fe3O4, TiO2 and Mn3O4 have been proposed for LIBs to achieve higher specific capacities than the currently used graphite. Although these materials have very high specific capacities, their application is restricted owing to their low electrical conductivity. In addition, electrochemical stability is important over long cycle durations and instability limits the use of these materials.115 One strategy to increase specific capacity is the addition of conductive materials such as carbon nanotubes.116–118 This allows engineering of advanced batteries to make them flexible, thus, potentially, leading to important applications such bendable electronics. GO and RGO also present promising building units for paper-like assemblies. Flexible, robust and conductive graphene paper can be used as the sole component of anode in the lithium batteries, thus leading to important applications such as flexible lightweight power sources. Graphene paper can be assembled by vacuum filtration of water-soluble graphene dispersion, which was reduced from GO with hydrazine.119 This graphene paper was found to be an ideal battery system and was prepared from reduction of prefabricated wet GO with hydrazine vapour. The nonannealed paper was found to exhibit excellent characteristics and electrochemical activity similar to that of the anode based on polymer-bound graphene powder.120 The reversible specific capacity of the binder-free anode was 84 mA h g−1 under a charging/discharging current of 50 mA g−1. The addition of graphene-based materials to transition metal oxide has been found to enhance the specific capacity of the electrodes at high discharge rate and improve the electrochemical stability for longer cycles. The high surface area of graphene facilitates intercalation of the Li ions and significant efforts have been made towards the development of high cyclic performance of LIBs, Table 3.2.

3.6.4 Sensors A unique property of graphene-based two-dimensional materials is its high surface area-to-volume ratio. This property can be harnessed in extremely sensitive devices that depend on the change of its electrical characteristics owing to adsorption of molecules on its surface. RGO is highly promising for electrochemical and biological sensors owing to their different functionalities131–134 which are very sensitive to the change in the chemical and biological environment. These responses can be analysed by changes not only in its conductivity but also changes in capacitances, and doping effects on FETs made with RGO can be utilised. For example, flexible RGO chemical sensors fabricated, using inkjet-printed films of poly(ethylene

Chemically derived graphene

69

Table 3.2 Graphene-based lithium-ion-battery materials and properties (Reprinted from reference 115, copyright 2011, with permission from Elsevier.) Material

Energy density (mA h g−1)

GO-encapsulated Co3O4 Grapheneanchored Co3O4 RGO–Mn3O4 Free Mn3O4 Pure Co3O4 SnO2–GO RGO-wrapped Fe3O4 Anatase TiO2–RGO

1000–1100 935

GO paper–Si Graphene nanosheets RGO

730–780 115–300 900 625 1026

Current density (mA g−1)

Number of cycles

Reference number

74

∼130

121

50

∼30

122

400 40 – 10 35

∼50 ∼10 – ∼10 ∼30

123 123 124 125 126

∼100 (at 1 C charge/discharge rates) ∼50 ∼20

127

160

2200 290 794–1054

50

128 129 130

terephthalate) (PET) decorated RGO sheets reversibly detect NO2 and Cl2 vapours at sensitivities down to parts per billion. Hydrogen sensing is critical for safety and other practical concerns in the proposed hydrogen-based economy. For example, hydrogen sensors detect leaks from hydrogen-powered cars and fuelling stations long before the gas becomes an explosive hazard. Sundaram et al. demonstrated that insensitivity of RGO towards molecular hydrogen can be alleviated by functionalisation with Pd nanoparticles (NP)131 (Fig. 3.9(a) and (b)). Since then, there have been many attempts to build hydrogen sensors that detect H2 electrically through interaction with RGO.135,136 Point-of-care diagnostics is another field where the need for cost-effective solutions has rapidly increased and is estimated to be a billion dollar market. Bypassing pathology laboratories could redirect resources and, thus, allow comparatively rapid response and action for the patient. To this end, RGObased biosensors have been demonstrated to detect a variety of biological species for example bacteria and nucleic acids,137 Fig. 3.9(c) and (d). Other efforts have been targeted towards sensing glucose. Shan et al.139 employed RGO and Kang et al.140 employed thermally treated GO, both of which allowed similar direct electrochemical sensing of glucose oxidase.141 (These biosensors detect levels of glucose by keeping track of the number of

70

Graphene 25 nm 1.0

(a)

H2O

(b)

H2 Vacuum

0.8

H2

Vacuum H2O

0.8

Vacuum Bare graphene

0.6 0.0

Bacterium

Current (μA)

1.2 w 1.0 Electrode D 0.8

22

3

0.4

VGate = 0 V

gold

600

A

GA GA

t

Dgold

A

m

a

GA

riu

cte

Ba

4

0.2

400

en

m

ch tta

A

0.6

(d) 800

D

D

A

D

0

2

4 Voltage (V)

6

1 W

3

–200

–4

GO

GO G-ssDNA G-dsDNA (Hybridised) G-ssDNA (Dehybridised) G-dsDNA (Rehybridised) –2 0 2 4 Voltage (V)

Au NP Source

Au

Drain

TRGO sheet

SiO2 Si Gate (f) Au NP-anti lgG conjugates

TRGO sheet

Au SiO2 Si

Blocking buffer FET and direct current measurements

lgG

3.9 For caption see facing page.

NA

-D

ss

0

–800

20.0

lgG

Anti-lgG

(e)

W

200

–600 8

15.0

W W 2W

–400

D D

A

0.0

Current (nA)

1

(c)

10.0 Time (min)

A

0 nm

5.0

DN

500 nm

ds -

DR/R

Pd–graphene

0.6 1.0

Chemically derived graphene

71

3.9 Continued. (a) Atomic force microscopy images of an individual graphene sheet contacted by two electrodes at various stages after electrodeposition of Pd at −0.85 V versus Pt. (b) Room temperature dynamic response of a bare (lower panel) and Pd-functionalised graphene sheet (upper panel) towards changes in gas atmosphere. (Reproduced with permission from reference 131, copyright 2008, Wiley VCH.) (c) Conductivity of p-type graphene-amine (GA) device increases upon attachment of a single bacterial cell on the surface of GA (inset 1). LIVE/DEAD confocal microscopy test on the bacteria deposited on GA confirmed that most of the bacteria were alive after the electrostatic deposition (inset 3; A = alive and D = dead). The LIVE/ DEAD test conducted immediately after the electrical measurements on the GA−gold−bacteria device (inset 2 and inset 4) showed that the bacterial cells on GA atop silica remain alive, whereas the bacteria deposited on the GA atop gold electrodes die after electrical measurements (inset 4 (right)). (d) DNA transistor: ss-DNA tethering on GO increases the conductivity of the device. Successive hybridisation and dehybridisation of DNA on the G–DNA device results in a completely reversible increase and restoration of conductivity. Inset shows a G–dsDNA sheet with wrinkles and folds clearly visible. (Reproduced with permission from reference 137, copyright 2008, American Chemical Society.) (e) Schematic of a thermally reduced graphene oxide (TRGO) FET. Anti-IgG is anchored to the TRGO sheet surface through Au NPs and functions as a specific recognition group for the IgG binding. The electrical detection of protein binding (IgG to anti-IgG) is accomplished by FET and direct current measurements. (f) Schematic illustration of the TRGO FET biosensor fabrication process. TRGO sheets were firstly dispersed on the electrodes and then decorated with Au NP-antibody conjugates through noncovalent attachment. (Reproduced with permission from reference 138, copyright 2008, Wiley VCH.)

electrons passed through the enzyme by connecting it to an electrode and measuring the resulting charge). Label-free nucleic acids sensors are important for detection of selected DNA sequences or mutated genes associated with human disease. These electrochemical DNA sensors use carbon nanotubes, which allow device miniaturisation for samples with a very small volume (down to atto litres).142 Such DNA sensors have also been fabricated using RGO143 which efficiently separated the signals obtained from four different bases and demonstrated specificity along with sensitivity. RGO FETs were also able to label-freely detect hormonal catecholamine molecules and their dynamic secretion from living cells.144

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Graphene

Protein sensing devices with high sensitivity and selectivity could be vital to the quick and inexpensive detection of diseases. To this end a FET biosensor using Au NP-antibody conjugates decorated with GO sheets has been developed. This study demonstrates a GO-based biosensor using immunoglobulin G for detecting a rotavirus as a pathogen model. The presence of remnant defects and oxygenated functionalities present useful sites for functionalisation with chemical species, thus allowing detection not only with extremely high sensitivity but also with good selectivity.

3.7

Conclusion

Chemically derived graphene from the liquid phase exfoliation of graphite oxide presents an easy, cost-effective and large-scale technique for the production of graphene. In addition to its simplicity, the versatility of this material renders it useful in a myriad of applications by chemically tailoring its properties. A major bottleneck towards progress of GO and RGO based materials, i.e., its physical, chemical and electronic structure, has now been resolved with the aid of various high-resolution microscopic and spectroscopic techniques. RGO-based transparent conductors, although quite promising, require further development to compete with existing technologies. However, their inexpensive production costs make them attractive for some niche markets. Studies on fine-tuning their electrical and optical properties could lead to some of the first large-scale applications based on graphene.

3.8

Acknowledgements

Dr Ravi S. Sundaram would like to acknowledge Dr Cristina GomezNavarro for helpful discussions during the preparation of this chapter.

3.9

References

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