Graphene production via electrochemical reduction of ...

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Apr 18, 2014 - Nevertheless, GO needs to be reduced in order to recover the unique ..... $2 hd. N/Ac. [21]. 2. HMa. $30 lm. NaNO3, KMnO4, H2SO4. 3:1. $5 hd.
Chemical Engineering Journal 251 (2014) 422–434

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Review

Graphene production via electrochemical reduction of graphene oxide: Synthesis and characterisation Shaw Yong Toh a, Kee Shyuan Loh a,⇑, Siti Kartom Kamarudin b, Wan Ramli Wan Daud a a b

Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 Bangi UKM, Selangor, Malaysia Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi UKM, Selangor, Malaysia

h i g h l i g h t s  We highlight the graphene production via electrochemical reduction of GO.  We review two different routes for electrochemical reduction of GO.  Experiment setup and conditions for both electrochemical routes were reported.  Highlight of several spectroscopy characteristic properties of the graphene.  The graphene produced from both routes showed similar characteristic properties.

a r t i c l e

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Article history: Received 11 December 2013 Received in revised form 27 March 2014 Accepted 2 April 2014 Available online 18 April 2014 Keywords: Graphene Graphene oxide (GO) Electrochemically reduced graphene oxide (ERGO) Electrochemical reduction

a b s t r a c t A considerable amount of research has been devoted to the synthesis of graphene materials via graphene oxide (GO) precursor during recent years due to the fact that it is ease in processing, versatile, and scalable for mass production. Nevertheless, GO needs to be reduced in order to recover the unique properties of pristine graphene. Of the various reduction approaches, the electrochemical method provides a facile, fast, scalable, economic and environmentally benign pathway to the production of desirable quality graphene materials. The electrochemical approach can be undertaken via two different routes: the one-step route which involves direct electrochemical reduction of GO in suspension onto the substrate electrode whereas the two-step route requires pre-deposition of GO onto the substrate electrode prior to electrochemical reduction process. This paper first reviews the preparation methods and various properties of graphene oxide. This is followed by a discussion on the working parameters of the two different electrochemical routes and the associated electrochemical techniques used to produce graphene. This paper also provides reviews on the characteristic properties of the electrochemically reduced graphene through the analysis of various spectroscopic techniques, such as X-ray photoelectron spectroscopy, Raman spectroscopy, infrared spectroscopy, X-ray diffraction and electron microscopic. Ó 2014 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GO precursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of ERGO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. One-step electrochemical reduction approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Two-step electrochemical reduction approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Advantages of ERGO compared with chemically reduced graphene oxide (CRGO). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterisation techniques for studying oxygenated graphenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. X-ray photoelectron spectroscopy (XPS) characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Raman spectroscopy characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Fourier Transform Infrared (FTIR) spectroscopy characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +60 3 89216050. E-mail addresses: [email protected] (S.Y. Toh), [email protected] (K.S. Loh), [email protected] (S.K. Kamarudin), [email protected] (W.R.W. Daud). http://dx.doi.org/10.1016/j.cej.2014.04.004 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

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4.4. X-ray diffraction (XRD) characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Electron microscopy characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Graphene is a carbon allotrope that consists of a flat monolayer of sp2-carbon atoms bonded and arranged in a honeycomb lattice. Since the discovery of graphene in 2004, this single atomic layer carbon material has garnered tremendous attention from researchers around the world because of its remarkable properties, such as high surface area [1,97], strong Young’s modulus [2,3], good thermal conductivity [4,98], outstanding electrical conductivity [5,6], and optical transparency [7,99]. Because of these fascinating properties, graphene has found its way into various applications, including energy conversion and storage (e.g., fuel cells [8] and capacitors [9]), sensors [10], electrocatalysis [11,102,103] and electronic devices [12]. Several approaches have been developed for the synthesis of graphene, such as mechanical cleavage [13,14], epitaxial growth [15,16], chemical vapour deposition [17,18], electrochemical exfoliation of graphite [111,113,114,117] and reduction of graphene oxide (GO) that derived from chemical exfoliation of graphite [19]. Recently, non-covalent exfoliation of graphite by sonication in liquid phase has also been reported [115,116]. Of all these approaches, the reduction of GO is regarded as one of the most promising routes for the mass production of graphene at a low cost and high yield, albeit only partially restore the properties of pristine graphene. Thus, the product obtained from this approach has been given various names and it is more frequently known as reduced graphene oxide (RGO), as it possesses properties that are different from pristine graphene. The graphene oxide is hydrophilic [20] and electrically insulating [21] because of the disruption of the sp2 bonding network in its carbon basal plane whereby a significant fraction of the sp2 carbon network is bonded with oxygencontaining functional groups during chemical exfoliation of the graphite. Thus, the graphene oxide has to be reduced to restore the unique properties found in the pristine graphene. Despite the difference in the quality of the reduced graphene oxide from that of pristine graphene, the product from the reduction process could be further modified and used for a wide range of applications. There are a number of routes for the reduction of GO, such as chemical reduction [22,23], thermal reduction [24,25], photocatalytic reduction [26,27] and electrochemical reduction. Typically, the chemical reduction of GO route involves the use of reducing agents, such as hydrazine [71,99], dimethylhydrazine [106], metal hydrides [104,105], and hydroquinone [22]. The excessive use of reducing agents could contaminate the resulting product [28] and even be harmful to human health and the environment [29]. Moreover, some oxygen functionalities in GO are selective and could not be removed completely with only one reductant treatment [30,31]. On the other hand, the thermal reduction route involves the use of high temperature to remove the oxygen functionalities, which would result in high production cost in addition to tedious control of experimental conditions. Meanwhile, the photocatalytic reduction of graphene oxide depends heavily on the presence of photoactive materials under ultraviolet (UV) irradiation. In contrast, the electrochemical reduction of GO is a relatively simple, economic, fast and environmentally benign method [28,32] to reduce GO to the graphene material on a large scale compared to the aforementioned methods. This approach is often compared with electrochemical exfoliation approach due to the similarity

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of exploiting the external power source to yield graphene in solid form and occurring in solution phase. However, it should be noted that the electrochemical reduction of GO approach is aimed at restoring some of the original properties of pristine graphene and exploit new functionalities of the RGO, along with other nanoparticles or compounds whereas the electrochemical exfoliation approach is focused on preservation of the properties of pristine graphene. The graphene produced from this approach is more appropriately known as electrochemically reduced graphene oxide (ERGO) as its properties differ from that of pristine graphene because of several residual oxygen functionalities on the carbon basal plane, while it retains some of the graphene structures [33]. Typically, the electrochemical reduction of GO can be carried out using a standard electrochemical cell in the presence of a nonhazardous aqueous buffer solution at room temperature. An external power source (applied potential) is used to drive the reduction process, and the oxygen functionalities in GO are removed with concomitant deposition of conductive solid films onto a working electrode surface. The properties of the ERGO can be tuned by controlling the electrolysis parameters and electrolyte [34]. In this review, we discuss the graphene, ERGO, which is produced from a GO colloid suspension precursor via the electrochemical approach. The focus of this review will be on the different electrochemical synthetic routes used to convert GO to the desirable properties of the ERGO. This review will also give an overview of the GO precursor used in the electrochemical reduction process. A summary of the characteristic properties of ERGO is provided through the analysis of several spectroscopic techniques, such as X-ray photoelectron spectroscopy, Raman spectroscopy, infrared spectroscopy, X-ray diffraction, and electron microscopy.

2. GO precursor GO is typically derived from the chemical exfoliation of graphite oxide. GO is generally similar to graphite oxide [35] in terms of its chemical structure, which contains plenty of oxygen functionalities on its carbon basal plane. However, the physical structure of GO is different from graphite oxide as the latter retains a stacked structure [36] similar to that in graphite. In general, the GO is exfoliated into a single-layer or few-layered carbon sheets. The precise structure of GO is still elusive and remains under debate as the coverage of oxygen functionalities that exist on the GO varies widely with the different synthetic procedures [37,38]. However, the generally accepted structural model of GO, which was proposed by Lerf et al. [39,40], depicts the hydroxyl and epoxy groups as dominant functional groups residing mainly on the basal plane of the GO sheets while the carbonyl and carboxyl groups accommodate the edges of the GO sheets (Fig. 1). The first step in the synthesis of GO in a stable colloidal suspension begins with the oxidation of graphite to graphite oxide. Subsequently, graphite oxide is exfoliated in the solution phase into individual GO sheets to form an aqueous GO colloidal suspension. Presently, the Hummers [41] and the modified version of this method [42,43] are the most commonly used methods for the oxidation of graphite. All of these methods involve the oxidation of graphite in the presence of strong oxidants in acidic media. In Hummers’ method, the graphite is oxidised using KMnO4 and

S.Y. Toh et al. / Chemical Engineering Journal 251 (2014) 422–434

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Fig. 1. Chemical structure model of graphene oxide. Reproduced with permission [37].

NaNO3 in concentrated H2SO4 to form graphite oxide. Kovtyukhova et al. [42] showed a modified version of Hummers’ method to produce more heavily oxidised graphite, which involves pre-oxidation of graphite with K2S2O8 and P2O5 in H2SO4. In another study, Marcano et al. [43] reported an improved method to synthesising graphite oxide by the use of KMnO4 with a 9:1 ratio mixture of concentrated H2SO4 and H3PO4. The extent of graphite oxidation, which is typically quantified by the C/O atomic ratio, varies on the basis of the procedures used, reaction conditions and graphite precursor [35,36]. Shao et al. [44] reported that the oxidation degree of graphite should be as low as possible because the single layer GO sheets are attainable using subsequent reduction processes. They demonstrated that the distance between the GO sheets did not increase when oxidation proceeded to a certain extent. Xu et al. [21] demonstrated that the mildly oxidised GO could be produced by lowering the ratio of KMnO4 from 3:1 (Hummers method) to 1:1 during oxidation. The mildly oxidised GO has been reported to exhibit low defects, fewer oxygenated functionalities and a larger p-conjugated structure domain than those of the GO prepared by the typical Hummers’ method [21,44,45]. Table 1 gives a summary of the Hummers’ method and the modified versions of this method used to synthesise graphite oxide, which in turn produces GO. Typically, the oxidation of graphite disrupts the sp2-hybridised carbon network of the stacked graphene sheets in graphite and gives rise to defects and wider spacing between adjacent sheets as a result of oxygen functionalities on both sides of the carbon basal plane. It has been revealed that the distance between the adjacent sheets increases from 0.334 nm in graphite powder to 0.68 nm in graphite oxide powder [46]. Owing to the hygroscopic nature of the stacked-structure of GO sheets in graphite oxide, the intercalation of water molecules between the GO sheets readily occurs. The spacing between the GO sheets varies significantly, ranging from 0.6 nm to 1.2 nm depending on the relative humidity level within the stacked-sheets [47]. Thus, the increase in spacing

between the sheets weakens the interaction between the sheets, which in turn facilitates the exfoliation of the graphite oxide into individual GO sheets upon sonication [48] or mechanical stirring [49,50] in water or polar solvent. However, several studies claimed that excessive ultrasonication could result in a decrease in lateral dimension [51] and hole defects [52,53]. The individual GO sheets obtained from the exfoliation process consist mainly of singlelayer sheets or few-layer sheets that are readily dispersed in water to form a stable aqueous GO colloidal suspension. The stability of the aqueous GO colloidal suspension is believed to be attained through negative electrostatic repulsion as a result of ionisation of the carboxylic groups and phenolic hydroxyl groups that exist on the GO sheets [54]. The thickness of a single-layer GO sheet has been reported to be approximately 1–1.4 nm [55,56] as a result of the introduction of oxygen functionalities on the carbon basal plane. In other words, it is approximately 3 times thicker than that of an ideal single-layer graphene sheet. In fact, graphite oxide can also be exfoliated into individual GO sheets in some polar organic solvents, such as ethylene glycol (EG), N-methylpyrrolidone (NMP) and N,N-dimethylformamide (DMF) to yield a non-aqueous GO colloidal suspension that is comparable to aqueous GO colloidal suspensions [57]. Typically, the GO sheets are dispersible in water up to a concentration of 3 mg/ml [23,48]. Nevertheless, the aqueous GO colloidal suspension provides a more convenient setting for the implementation of the electrochemical route toward the conversion of GO into ERGO when an ionic buffer is introduced to the electrochemical system.

3. Synthesis of ERGO As a result of impairment and disruption to the sp2 carbon bonding network arising from a plenty of oxygen functionalities on its carbon basal plane, GO is electrically insulating [58]. Deoxygenation treatment of GO [29] is required to restore the p-network, which in turn recovers the electrical conductivity of the resulting graphene materials. However, the resulting graphene materials are not expected to exhibit the same electronic quality as that of pristine graphene as a result of residual oxygen functionalities and defects [52]. Nevertheless, the deoxygenation of GO can be accomplished via electrochemical reduction techniques. Typically, the electrochemical reduction of GO can be carried out via two different routes as illustrated in Fig. 2. Both electrochemical routes,

Table 1 Summary of the Hummers and their modified version of methods used to synthesise graphite oxide. No

a b c d e f

Synthesis method

Dimension of graphite

Oxidants

Weight ratio of KMnO4 to graphite

Reaction time (oxidation)

C/O ratio

References

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