High-quality few layer graphene produced by electrochemical intercalation and microwave-assisted expansion of graphite

Abstract

Few-layer graphene is synthesized from electrochemically-produced graphite intercalation compounds in aqueous perchloric acid. Although anodic intercalation is more efficient in terms of time, cathodic pre-treatment is preferred to avoid the formation of graphite oxide. The materials are characterized by high resolution transmission electron microscopy and scanning electron microscopy, UV–visible, infrared and Raman spectroscopy. We demonstrate that the method, under the experimental conditions used in this work, does not produce damage to the sp² carbon lattice. The synthetic approach using electrochemical-potential control is very promising to obtain, in a controllable manner, graphene with different degrees of oxidation.

Introduction

Graphene, a one-atom-thick planar sheet of sp²-bonded carbon atoms, is a quasi-2-dimensional (2D) material. The fascinating properties of single-layer graphene (SLG) and few-layer graphene (FLG) have made it one of the most promising materials of the first decade of the 21st century [1]. It has been observed that graphene presents an ambipolar field effect [2], a quantum Hall effect at room temperature [3], [4], [5], very high charge carrier mobility that is temperature-independent and translates into ballistic transport [6], [7], and very high surface area [8]. Graphene and its derivatives are promising candidates as components for applications in the field of energy-storage and energy conversion materials, thermally and electrically conductive reinforced nanocomposites, nanoelectronics and sensors, among many others.

However, graphene suffers from a problem which is common to many novel nanomaterials; the lack of effective methods for large-scale production. Consequently, full exploitation of the properties of graphene still requires the development of methods to produce large quantities of FLG with low density of defects in the crystal lattice [9]. The exfoliation of expandable graphite (EG) via graphite intercalation compounds (GIC) may be one of the most attractive methods proposed because this process, which has been extensively studied in the past, is cheap and scalable [10], [11]. Nevertheless, the possibility of its use as a route to obtain FLG without damage to the sp² structure – denominated as high quality graphene, HQG – is still not clear. Stankovich et al. [12] reported numerous failed attempts to produce HQG from GIC, including the synthesis by exfoliation of potassium-intercalated graphite [13].

Recently, electrochemical techniques have been employed in the production of graphene. Guo et al. reported the electrochemical reduction of exfoliated graphite oxide (GO) in PBS [14] and sodium sulfate [15] solutions. However, this method presents the same disadvantages as all synthetic approaches where GO is used as starting material; the sp³ defects cannot be efficiently transformed back to sp² as was evident from Raman spectroscopy [14]. Liu et al. synthesized ionic-liquid-functionalized graphite sheets by applying 15 V between two graphite rods immersed in a water/ionic–liquid mixture [16], but the nature of the method produced graphene with high defect densities. In addition, Lu et al. reported the synthesis of fluorescent carbon nanostructures – including graphene – by using ionic liquid-assisted electrochemical exfoliation of graphite electrodes, also under oxidative conditions [17].

The exfoliation of graphite associated to the charging/discharging process is a well known phenomenon in the field of lithium-ion batteries using carbonaceous material. In fact, it is an undesired phenomena occurring during lithium intercalation/de-intercalation, leading to severe battery failure. In this context, much effort has been invested in engineering methods to avoid graphite exfoliation [18], [19], [20]. Under appropriate experimental conditions, graphite can be electrochemically oxidized and reduced to give ${\text{C}}_n^{+}$ and ${\text{C}}_n^{-}$ lattices which can hold anions and cations respectively [21], [22]. In addition, when a sufficiently negative (positive) potential is applied to a graphite-working electrode in aqueous solution, molecular hydrogen (O₂, CO₂ and GO) can be produced simultaneously to the intercalation process. To the best of our knowledge, the electrochemical production and subsequent intercalation of hydrogen on a graphite electrode coupled with the expansion and exfoliation assisted by hydrogen gas evolution has not been studied to date. Moreover, the electrochemical intercalation of anions – in this work, perchlorate – and posterior exfoliation of graphite in aqueous acid media has been investigated in more detail [23], [24], [25], [26], [27], [28]. For example: (a) it has been shown that, the perchlorate ion is one of the best intercalating species at low acid concentrations [23] and (b) the anion intercalation can subsequently damage the sp² lattice due to side reactions such as GO and carbon dioxide (CO₂) formation [24], [29]. However, by employing concentrated perchloric acid and with a careful selection of the intercalation potential, the contribution of those reactions, and consequently the creation of defects in the carbon network, can be minimized [26], [28].

Here we report a simple method to produce FLG that combines the anodic and cathodic electrochemical intercalation of graphite in aqueous perchloric acid and posterior expansion by microwave radiation. This method has several advantages: First, strong oxidizing conditions are avoided, thus irreversible sp³ defects caused under oxidation are not generated. Second, complex and expensive organic compounds are not necessary. Third, contaminants in the final product – hydrogen, protons, perchloric acid, perchlorate and water – can be eliminated by simple water-washing and further evaporation. Fourth, if some degree of functionalization – oxidation – is required, this can be tuned by choosing the appropriate potential and time applied during the electrochemical treatment.