The reduction of graphene oxide
Introduction
A report in 2004 by Geim and Novoselov et al. of a method to prepare individual graphene sheets has initiated enormous scientific activity [1], [2], [3]. Graphene is a two dimensional (2D) crystal that is stable under ambient conditions; it has a special electronic structure, which gives it unusual electronic properties such as the anomalous quantum Hall effect [4] and astonishing high carrier mobility at relatively high charge carrier concentrations and at room temperature [1], [5]. As a new material, the uses of graphene are very attractive since many interesting properties, mechanical [6], thermal [7] and electrical [8] have been reported to confirm the superiority of graphene to traditional materials [9]. Following this trend, graphite oxide, first reported over 150 years ago [10], has re-emerged as an intense research interest due to its role as a precursor for the cost-effective and mass production of graphene-based materials.
Graphite oxide has a similar layered structure to graphite, but the plane of carbon atoms in graphite oxide is heavily decorated by oxygen-containing groups, which not only expand the interlayer distance but also make the atomic-thick layers hydrophilic. As a result, these oxidized layers can be exfoliated in water under moderate ultrasonication. If the exfoliated sheets contain only one or few layers of carbon atoms like graphene, these sheets are named graphene oxide (GO).1 The most attractive property of GO is that it can be (partly) reduced to graphene-like sheets by removing the oxygen-containing groups with the recovery of a conjugated structure. The reduced GO (rGO) sheets are usually considered as one kind of chemically derived graphene. Some other names have also been given to rGO, such as functionalized graphene, chemically modified graphene, chemically converted graphene, or reduced graphene [11]. The most straightforward goal of any reduction protocol is to produce graphene-like materials similar to the pristine graphene obtained from direct mechanical exfoliation (i.e. the “Scotch tape method”) of individual layers of graphite both in structure and properties. Though numerous efforts have been made, the final target is still a dream. Residual functional groups and defects dramatically alter the structure of the carbon plane, therefore, it is not appropriate to refer to rGO, even today, simply as graphene since the properties are substantially different.
Nowadays, in addition to reduction from GO, graphene can be produced by micro-mechanical exfoliation of highly ordered pyrolytic graphite [1], epitaxial growth [12], [13], [14], and chemical vapor deposition (CVD) [13], [15], [16]. These three methods can produce graphene with a relatively perfect structure and excellent properties. While in comparison, GO has two important characteristics: (1) it can be produced using inexpensive graphite as raw material by cost-effective chemical methods with a high yield, and (2) it is highly hydrophilic and can form stable aqueous colloids to facilitate the assembly of macroscopic structures by simple and cheap solution processes, both of which are important to the large-scale uses of graphene. As a result, GO and rGO are still hot topics in the research and development of graphene, especially in regard to mass applications.
Therefore, the reduction of GO is definitely a key topic, and different reduction processes result in different properties that in turn affect the final performance of materials or devices composed of rGO. Though the final target to achieve perfect graphene is hard to reach, research efforts have continuously made it closer. Here we review work on the reduction of GO, and because there are many review papers on synthesis methods [13], [17], [18], [19], [20], [21], [22], [23], and the physical [2], [3], [24], [25], [26] and chemical [9], [27], [28], [29], [30], [31] characteristics of graphene, details on them will not be repeated.
Section snippets
Preparation and characteristics of GO
GO was firstly reported in 1840 by Schafhaeutl [10] and 1859 by Brodie [32]. The history of the evolution of synthesis methods and chemical structure of GO has been extensively reviewed by Dreyer et al. [9] and Compton and Nguyen [19]. Currently, GO is prepared mostly based on the method proposed by Hummers and Offeman [33] in 1958, where the oxidation of graphite to graphite oxide is accomplished by treating graphite with a water-free mixture of concentrated sulfuric acid, sodium nitrate and
Criteria used in determining the effect of reduction
Since reduction can make a great change in the microstructure and properties of GO, some obvious changes can be directly observed or measured to judge the reducing effect of different reduction processes.
Thermal annealing
GO can be reduced solely by heat treatment and the process is named thermal annealing reduction. In the initial stages of graphene research, rapid heating (>2000 °C/min) was usually used to exfoliate graphite oxide to achieve graphene [35], [45], [74], [75]. The mechanism of exfoliation is mainly the sudden expansion of CO or CO2 gases evolved into the spaces between graphene sheets during rapid heating of the graphite oxide. The rapid temperature increase makes the oxygen-containing functional
Reduction mechanism
Though numerous strategies have been proposed to reduce GO, there are still many questions without clear answers. For example, can the functional groups of a GO sheet be fully eliminated? Can the lattice defects formed during oxidation be restored during reduction? Does a reduction process decrease or increase the defect density in a graphene sheet? The answers and further improvements in GO reduction will rely on an improved understanding of reduction mechanisms. But only limited work has
Summary and prospects
We have reviewed the reduction of GO to prepare graphene-like rGO. This is an attractive route for the mass-scale production and applications of graphene. Though the full reduction of GO to graphene is still hard to achieve, partial reduction of GO is rather easy and tens of reduction methods have been proposed. The accumulation of experimental phenomena and theoretical simulation results has provided clearer views of the structure and chemistry of graphene, GO and rGO, and this may be helpful
Acknowledgements
This work was supported by the Key Research Program of Ministry of Science and Technology, China (No. 2011CB932604), the National Natural Science Foundation of China (Nos. 51102243 and 50921004), and by the Chinese Academy of Sciences (KGCX2-YW-231).
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