Fast and cost-effective room temperature synthesis of high quality graphene oxide with excellent structural intactness
Introduction
Since the discovery in 2004, graphene, a monolayer of carbon atoms arranged in hexagonal rings, has been sitting in the spotlight owing to its fascinating properties [1,2]. As the most important graphene derivatives, graphene oxide (GO) and its reduced form, reduced graphene (rGO), have been the subject of intense exploration and achieved great success in a diverse range of applications, ranging from energy-storage and -conversion devices [3,4] to ionic and molecular sieving membranes [5,6] to biomedical applications [[7], [8], [9], [10]]. The ultimate goal of developing these applications is to promote the broad commercialization of graphene-based products with high performance and cost-effectiveness [11]. In order to realize this goal, the scalable and cost-effective fabrication of GO materials is of significant importance [12]. Currently, the exfoliation of chemically oxidized graphite into single layers using the KMnO4-based Hummers method [13], namely the graphite oxide route, is the most extensively used method to produce GO in bulk [12]. Typically, it involves the edge-to-core diffusion of oxidizing agents in the narrow graphite interlayer galleries, which determines the reaction rate of forming GO [14]. To facilitate the conversion of graphite into GO, one common strategy is to pre-intercalate graphite with foreign species which enables to expand the interlayer spacings [[15], [16], [17], [18]]. The spacing expansion is more dramatic when gasifying the thermo-decomposable intercalants, namely forming the expanded graphite [19,20]. In addition, applying external heating (≥ 35 °C) can thermodynamically favor the diffusion process [21,22]. Owing to the inexpensive sources, good dispersibility, rich oxygen functional groups and high product yield, graphite oxide route has been successfully scaled up from lab-scale researches to industrial mass production [12,23].
Despite its preliminary commercialization, the graphite oxide route with harsh oxidation conditions, such as the use of strong acids, oxidants and heating, usually introduces uncontrollable functionalities and enormous permanent defects into final products. This will result in poor electronic properties of rGO, including electrical conductivity and charge-carrier mobility, which are undesirable for electroconductive applications [24]. To preserve the sp2 honeycomb structure in graphene basal plane, using low reaction temperature (<0 °C) in the graphite oxide route can effectively minimize the loss of carbon atoms in the GO. The resulting highly intact rGO exhibited excellent electronic properties that are comparable to the pristine graphene [[25], [26], [27]]. However, the low temperature modified methods are of higher cost due to the necessity for low temperature control, poor GO yield (<50%) and significantly more time-consuming process (>1 day per batch).
To obtain high quality GO without compromising cost-effectiveness, our group recently reported a simple but effective method to synthesize GO with rich thermally labile oxygen functional groups from pre-intercalated graphite at room temperature (25 °C) [28]. In comparison to the conventional graphite oxide routes (reaction temperature ≥ 35 °C), the advantages of our approach are that the resultant GO is not only less defective but also restores more electrical conductivity after simple thermal annealing at only 150 °C in ambient air. This makes the room temperature GO (RT-GO) more conducive and cost-effective to electroconductive applications relative to the traditional GO. For example, RT-GO can be blended well into composite slurry and converted to conductive binders in lithium ion batteries after mild annealing at 150 °C [28]. The benefits of the facile deoxygenation and high structural integrity of RT-GO have also been demonstrated by Li and co-workers [29]. Furthermore, it is worth noting that the RT-GO prepared by Li’s group, which was oxidized for 3 h, exhibited comparable defect density and electrical conductivity to the GO oxidized at 5 °C for 12 h (low T-GO) after chemical reduction.
The diffusion of oxidants in natural graphite or pre-intercalated graphite proceeds in a noticeably slower manner at room temperature and below, thus requiring much longer reaction time [14,28,30]. To address this issue, the use of expanded graphite with larger inter-galleries spacing as the starting material is likely to facilitate the propagation of oxidation under mild conditions [19,20,[31], [32], [33], [34]]. Herein, we demonstrated that the conversion of expanded graphite into monolayered GO can be accomplished within only 5 h at both 5 °C (low temperature) and 25 °C (room temperature). Further, this enabled the direct comparisons between RT-GO and low T-GO under the same reaction conditions except for temperature. We further found that in spite of its higher oxidation degree and lower electrical conductivity, RT-GO could regain considerably more electrical conductivity as compared to the low T-GO after a simple thermal treatment at 150 °C or mild chemical reduction by vitamin C (VC). In addition, VC was found to be more efficient for the restoration of conductivity than the mild thermal annealing process. Extensive characterizations revealed that more conjugated sp2 carbon networks and less defective structure recovered from VC reduction led to the greater increase in the conductivity of the RT-GO with respect to that of the low-T GO. On account of the mild but efficient oxidation process coupled with eco-friendly reduction methods, it is expected that the RT-GO derived from expanded graphite will offer significant advantages towards the bulk-scale, sustainable production of graphene materials at low cost for electroconductive applications.
Section snippets
Materials
Expanded graphite powder (≥ 99% carbon, 6 μm) was purchased from Qingdao FRT Graphite Co. Ltd. KMnO4 (99.0%), concentrated H2SO4 (98%), aqueous H2O2 solution (30% w/w), N,N-dimethylformamide (DMF), ethanol (undenatured, 100%) and hydrochloric acid (32 wt%) were all purchased from Chem-Supply. Vitamin C (l-ascorbic acid, C6H8O6) was purchased from Ajax Chemicals. Deionized (DI) water was used in all experimental procedures, including the synthesis reaction and purification.
Synthesis of RT-GO and low-T GO
GO samples were
Chemical properties of as-prepared RT-GO and low T-GO
In this work, we prepared GO samples using two different oxidation temperatures, namely 5 °C and 25 °C, from expanded graphite (see experimental detail). XRD was used to investigate the conversion of expanded graphite to graphite oxide, as shown in Fig. 1a. As compared to the original expanded graphite, all GO samples lost the strong graphitic peak at 26.4°, which can be associated with the interlayer spacing between unoxidized graphene sheets (3.46 Å). Instead, the main characteristic peak at
Conclusions
In this paper, we have further studied the chemical method to produce monolayered GO with excellent solution processability from expanded graphite at room temperature. The use of expanded graphite significantly facilitated the conversion to GO within only 5 h at both 5 °C and 25 °C. This allows a more rational comparison between low T-GO (at 5 °C) and RT-GO (at 25 °C) using the same synthesis conditions. We found that GO produced at 25 °C was able to recover considerably more electrical
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors acknowledge the support from the Australian Research Council (DP190100120, LP160101521 and FT160100207).
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