The reduction of graphene oxide

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The reduction of graphene oxide Songfeng Pei, Hui-Ming Cheng

*

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China

A R T I C L E I N F O

A B S T R A C T

Article history:

Graphene has attracted great interest for its excellent mechanical, electrical, thermal and

Received 28 September 2011

optical properties. It can be produced by micro-mechanical exfoliation of highly ordered

Accepted 8 November 2011

pyrolytic graphite, epitaxial growth, chemical vapor deposition, and the reduction of graph-

Available online xxxx

ene oxide (GO). The first 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. A key topic in the research and applications of GO is the reduction, which partly restores the structure and properties of graphene. Different reduction processes result in different properties of reduced GO (rGO), which in turn affect the final performance of materials or devices composed of rGO. In this contribution, we review the state-of-art status of the reduction of GO on both techniques and mechanisms. The development in this field will speed the applications of graphene. 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

A report in 2004 by Geim and Novoselov et al. of a method to prepare individual graphene sheets has initiated enormous scientific activity [1–3]. 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

* Corresponding author: Fax: +86 24 2390 3126. E-mail address: [email protected] (H.-M. Cheng). 1 ‘GO’ in this paper refers only to graphene oxide, while graphite oxide is not abbreviated in this paper. 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.11.010

Please cite this article in press as: Pei S, Cheng H.-M. The reduction of graphene oxide. Carbon (2011), doi:10.1016/j.carbon.2011.11.010

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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 [12–14], 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 [13,17–23], and the physical [2,3,24– 26] and chemical [9,27–31] characteristics of graphene, details on them will not be repeated.

2.

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 potassium permanganate. Though some modification has been proposed [34–37], the main strategy is unchanged. As a result, these methods are usually named modified Hummers methods. Though it has been developed for over a century, the precise chemical structure of GO is still not quite clear, which contributes to the complexity of GO due to its partial amorphous character. Several early investigations have proposed structural models of GO with a regular lattice composed of discrete repeat units [38], and the widely accepted GO model proposed by Lerf and Klinowski [39,40] is a nonstoichiometric

model (shown in Fig. 1), wherein the carbon plane is decorated with hydroxyl and epoxy (1,2-ether) functional groups. Carbonyl groups are also present, most likely as carboxylic acids along the sheet edge but also as organic carbonyl defects within the sheet. Recent nuclear magnetic resonance (NMR) spectroscopy studies [41,42] of GO have made slight modifications to the proposed structure including the presence of 5and 6-membered lactols on the periphery of graphitic platelets as well as the presence of esters and tertiary alcohols on the surface, though epoxy and alcohol groups on the plane are still dominant. More detailed information on this evolution can be found in the review by Dreyer et al. [9]. An ideal sheet of graphene consists of only trigonally bonded sp2 carbon atoms and is perfectly flat [43] apart from microscopic ripples [44]. The heavily decorated GO sheets consist partly of tetrahedrally bonded sp3 carbon atoms, which are displaced slightly above or below the graphene plane [45]. Due to the structure deformation and the presence of covalentlybonded functional groups, GO sheets are atomically rough [46–48]. Mkhoyan et al. [47] examined the oxygen distribution on a GO monolayer using high-resolution annular dark field (ADF) imaging in a scanning transmission electron microscope (STEM), as shown in Fig. 2. The results indicate that the degree of oxidation fluctuates at the nanometer-scale, suggesting the presence of sp2 and sp3 carbon clusters of a few nanometers. Several groups [46,49–51] have studied the surface of GO with scanning tunneling microscopy (STM) and observed highly defective regions, probably due to the presence of oxygen and other areas are nearly intact. Surprisingly, a report shows that the graphene-like honeycomb lattice in GO is preserved, albeit with disorder, that is, the carbon atoms attached to functional groups are slightly displaced but the overall size of the unit cell in GO remains similar to that of graphene [52]. As a result, GO can be described as a random distribution of oxidized areas with oxygen-containing functional groups, combined with non-oxidized regions where most of the carbon atoms preserve sp2 hybridization. The conductivity of graphene mainly relies on the longrange conjugated network of the graphitic lattice [53,54]. Functionalization breaks the conjugated structure and localizes p-electrons, which results in a decrease of both carrier mobility and carrier concentration. Though there are conjugated areas in GO, long-range (>lm) conductivity is Copyright 1998 Elsevier. 1998

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Fig. 1 – Lerf–Klinowski model of GO with the omission of minor groups (carboxyl, carbonyl, ester, etc.) on the periphery of the carbon plane of the graphitic platelets of GO [39,40].


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Fig. 2 – (a) AFM image of GO sheets. (b) STEM-ADF image of a GO film where mono-, bi- and tri-layers are labeled as a, b, and c. The round opening in the middle is a hole through the single film. (c) High-magnification ADF image of a monolayer GO film. (d) Simple drawing of monolayer and possible packing of bi- and tri-layers [47].

blocked by the absence of percolating pathways between sp2 carbon clusters to allow classical carrier transport to occur. As a result, as-synthesized GO sheets or films are typically insulating, exhibiting a sheet resistance of about 1012 X/sq or higher [34,55]. The attached groups and lattice defects modify the electronic structure of graphene and serve as strong scattering centers that affect the electrical transport. Therefore, the reduction of GO is not only concerned with removing the oxygen-containing groups bonded to the graphene and removing other atomic-scale lattice defects, but is also aimed at recovering the conjugated network of the graphitic lattice. These structure changes result in the recovery of electrical conductivity and other properties of graphene.

with a brown color and semi-transparency, as shown in Fig. 3a. The reduction in a colloid state by chemical reduction, e.g. hydrazine reduction, usually results in a black precipitation from the original yellow–brown suspension, which is probably a result of an increase in the hydrophobicity of the


3. 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.

3.1.

Visual characteristics

Optical observation is a direct way to see the changes in GO before and after reduction. Since a reduction process can dramatically improve the electrical conductivity of GO, the increased charge carrier concentration and mobility will improve the reflection to incident light, which makes a rGO film have a metallic luster compared to its GO film precursor

Fig. 3 – Typical optical images of (a) a GO film and rGO film [58], Copyright 2011 Elsevier. (b) GO solution and rGO solution [59], Copyright 2009 ACS. (c, d) GO and rGO sheets on a 300 nm SiO2/Si substrate [34].


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material caused by a decrease in polar functionality on the surface of the sheets [56]. To improve the processibility of rGO, some strategies have been proposed to keep the colloid state by adding surfactants or adjusting solvent properties, while the change in color to black can be an obvious visible characteristic of the effect of reduction, as shown in Fig. 3b. The related change can be also observed on the microscale by optical microscopy of GO/rGO sheets lying on a properlyselected substrate like a SiO2/Si wafer. As shown in Fig. 3c and d, the as-prepared GO sheets are almost transparent with a very subtle optical contrast with the substrate, which confirms the insulating nature of the GO sheets. The small blue regions near the edges, corresponding to a larger thickness, are attributed to the commonly-observed edge folding [57], while after reduction, the rGO sheets show much improved contrast with the substrate, which is the same as that of pristine graphene sheets lying on the same substrate.

3.2.

Electrical conductivity

Graphene is reported to have a high electrical conductivity. A few-layer graphene sheet (thickness < 3 nm) has a sheet resistance (Rs) of around 400 X/sq at room temperature [1]. Recently, Bae et al. [60] have reported the production of graphene films by CVD. After transferring them to transparent substrate, a graphene-based transparent conductive film (TCF) composed of 4 layers has a sheet resistance of around 30 X/sq with transparency around 90% [60]. Assuming the film thickness was 2 nm, the calculated bulk conductivity of this film is 1.6 · 105 S/cm (107 S/m), which is much higher than for indium tin oxide (ITO) or metal films with the same thickness [61]. Since the purpose of reduction is mainly to restore the high conductivity of graphene, the electrical conductivity of rGO can be a direct criterion to judge the effect of different reduction methods. The electrical conductivity of rGO can be described in several ways: Rs of an individual rGO sheet (Rs-is), Rs of a thin film assembly of rGO sheets (Rs-f), powder conductivity (rp) and bulk conductivity (r) of rGO. Sheet resistance (Rs; X/sq) is a measure of the electrical resistance of a sheet, independent of its thickness. It is related to bulk conductivity by Eq. (1), where r is bulk conductivity (unit: S/cm) and t is sample thickness (unit: cm): Rs ¼

1 rt

reduction of hydroiodic acid (HI) [34] with a transparency of 78% at 550 nm wavelength, the calculated bulk conductivity of the film is about 1190 S/cm. Stankovich et al. [56] has used powder conductivity to describe the conductivity of rGO. In their measurement, rGO powders are compressed to pellets with different apparent densities and then measured by a two-probe method [56].

3.3.

Carbon to oxygen atomic ratio (C/O ratio)

Depending on the preparation method, GO with chemical compositions ranging from C8O2H3 to C8O4H5, corresponding to a C/O ratio of 4:1–2:1, is typically produced [38,64,65]. After reduction, the C/O ratio can be improved to approximately 12:1 in most cases [45,66], but values as large as 246:1 have been recently reported [42]. The C/O ratio is usually obtained through elemental analysis measurements by combustion, and also by X-ray photoelectron spectrometry (XPS) analysis. It has been proved that the data obtained by elemental analysis are reasonably consistent with the data by XPS, considering the fact that elemental analysis gives the bulk composition while XPS is a surface analysis technique [45]. Furthermore, XPS spectra can give more information on the chemical structures of GO and rGO. Since it is p-electrons from the sp2 carbon that largely determine the optical and electrical properties of carbonbased materials [67], the fraction of sp2 bonding can provide insight into structure–property relationships. Briefly, as shown in Fig. 4, the C1s XPS spectrum of GO clearly indicates a considerable degree of oxidation with four components that correspond to carbon atoms in different functional groups: the non-oxygenated ring C (284.6 eV), the C in C–O bonds (286.0 eV), the carbonyl C (287.8 eV), and the carboxylate carbon (O–C = O, 289.0 eV) [68]. Although the C1s XPS spectrum of rGO also exhibits these oxygen functional groups, their peak intensities are much weaker than those in GO. Table 1 summarizes the electrical conductivity and C/O ratio of typical reports on the reduction of GO. The details on each reduction method will be discussed in Section 4.

ð1Þ

Rs-is can be measured by a two-probe method or four-probe method using an in situ fabricated microelectrode pair on an individual rGO sheet with the assistance of delicate photo- or electro-lithography. The lowest Rs-is was reported to be about 14 kX/sq (350 S/cm) by Lopez et al. [62], about two order higher than that of pristine graphene [1]. The highest bulk conductivity of a rGO sheet was reported to be 1314 S/cm by Su et al. [63]. Both values are obtained from rGO by thermal annealing at high temperature, and the details will be discussed in Section 5. Because graphene is usually used in the form of thin films, like TCF, Rs-f by a four-probe method on the surface of a macroscopic film is often used to describe its electrical conductivity when prepared using different ways. The lowest Rs-f (0.84 kX/sq) of rGO-based TCF (10 nm in thickness) was achieved by Zhao et al. by chemical

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Fig. 4 – The C1s XPS spectra of (a) GO and (b) rGO [56].


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Table 1 – Comparison of the reducing effect of GO by different methods. Ref. no. [56] [69] [70] [71] [55] [72] [42]

[73]

[58] a b

Reduction method

Form

C/O ratio

r (S/cm)

Hydrazine hydrate Hydrazine reduction in colloid state 150 mM NaBH4 solution, 2 h Hydrazine vapor Thermal annealing at 900 C, UHVa Thermal annealing at 1100 C, UHV Thermal annealing at 1100 C in Ar/H2 Multi-step treatment: (I) NaBH4 solution (II) Concentrated H2SO4 180 C, 12 h (III) Thermal annealing at 1100 C in Ar/H2 Vitamin C Hydrazine monohydrate Pyrogallol KOH 55% HI reduction

Powder Film TCF Film

10.3 NAb 8.6 8.8 14.1 NA NA (I) 4.78 (II) 8.57 (III) >246

2 72 0.045 NG NG 103 727 (I) 0.823 (II) 16.6 (III) 202

12.5 12.5 NA NA >14.9

77 99.6 4.8 1.9103 298

TCF TCF Powder

Film

Film

UHV: ultra high vacuum. NA: not available.

In addition to the three parameters presented above, some other analysis techniques, such as Raman spectroscopy, solid-state FT-NMR spectroscopy, transmission electron microscopy (TEM), and atomic force microscopy (AFM), are also used to show the structure and property changes of GO after reduction. These analyses can give more detailed information on the structure of GO and rGO, and be helpful to understand the mechanisms of reduction processes, but in most cases, these results are not as clear in showing the reducing effect as are the three parameters mentioned earlier.

4.

Reduction strategies

4.1.

Thermal reduction

4.1.1.

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 oxygencontaining functional groups attached on carbon plane decompose into gases that create huge pressure between the stacked layers. Based on state equation, a pressure of 40 MPa is generated at 300 C, while 130 MPa is generated at 1000 C [74]. Evaluation of the Hamaker constant predicts that a pressure of only 2.5 MPa is enough to separate two stacked GO platelets [74]. The exfoliated sheets can be directly named graphene (or chemically derived graphene) rather than GO, which means that the rapid heating process not only exfoliates graphite oxide but also reduces the functionalized graphene sheets by decomposing oxygen-containing groups at elevated temperature. This dual-effect makes thermal expansion of graphite oxide a good strategy to produce bulk quantity graphene.

However, this procedure is found only to produce small size and wrinkled graphene sheets [45]. This is mainly because the decomposition of oxygen-containing groups also removes carbon atoms from the carbon plane, which splits the graphene sheets into small pieces and results in the distortion of the carbon plane, as shown in Fig. 5. A notable effect of thermal exfoliation is the structural damage to graphene sheets caused by the release of carbon dioxide [49]. Approximately 30% of the mass of the graphite oxide is lost during the exfoliation process, leaving behind lattice defects throughout the sheet [45]. Defects inevitably affect the electronic properties of the product by decreasing the ballistic transport path length and introducing scattering centers. As a result, the electrical conductivity of the graphene sheets has a typical mean value of 10–23 S/cm that is much lower than that of perfect graphene, indicating a weak effect on reduction and restoration of the electronic structure of carbon plane. An alternative way is to exfoliate graphite oxide in the liquid phase, which enables the exfoliation of graphene sheets with large lateral sizes [34]. The reduction is carried out after the formation of macroscopic materials, e.g. films or powders, by annealing in inert or reducing atmospheres. In this strategy, the heating temperature significantly affects the effect of reduction on GO [45,55,66,71,72,76]. Schniepp et al. [45] found that if the temperature was less than 500 C, the C/O ratio was no more than 7, while if the temperature reached 750 C, the C/O ratio could be higher than 13. Li et al. have monitored the chemical structure variation with annealing temperature, and the XPS spectrum evolution shown in Fig. 6 reveals that high temperature is needed to achieve the good reduction of GO. Wang et al. [72] annealed GO thin films at different temperatures, and showed that the volume electrical conductivity of the reduced GO film obtained at 500 C was only 50 S/cm, while for those at 700 C and 1100 C it could be 100 S/cm and 550 S/cm (Fig. 7), respectively. Wu et al. [76] used arc-discharge treatment to exfoliate graphite oxide to prepare graphene. Since the arc-discharge could provide temperatures above 2000 C in a short time,


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Fig. 5 – Pseudo-3D representation of a 600 nm · 600 nm AFM scan of an individual graphene sheet showing the wrinkled and rough structure of the surface, and an atomistic model of the graphite oxide to graphene transition [45].


Fig. 6 – XPS spectra of GO sheets annealed in 2 Torr of (a) NH3/Ar (10% NH3) and (b) H2 at various temperatures [77].

Fig. 7 – Increase of the average conductivity of graphene films from 49, 93, 383 to 550 S/cm, along with the temperature increasing from 550 C, 700 C, 900 C to 1100 C, respectively [72].

the typical sheet electrical conductivity of graphene sheets was about 2000 S/cm, and elemental analysis revealed that the exfoliated graphene sheets had a C/O ratio of 15–18.

In addition to annealing temperature, annealing atmosphere is important for the thermal annealing reduction of GO. Since the etching of oxygen will be dramatically increased at high temperatures, oxygen gas should be excluded during annealing. As a result, annealing reduction is usually carried out in vacuum [55], or an inert [72] or reducing atmosphere [35,72,75,77]. Becerril et al. [55] have reduced GO films by thermal annealing at 1000 C, and found that a quality vacuum (