Facile synthesis of graphene by ultrasonic-assisted electrochemical exfoliation of graphite

Abstract

Graphene, a 2-dimensional form of carbon, attracted significant attention in a wide range of applications such as energy storage, power generation, chemical sensors, composite materials owing to its unmatched physical and chemical properties. In this study, graphene powder was synthesized by ultrasonic-assisted electrochemical exfoliation of the graphite electrode from acidic bath. An external ultrasonic bath (ultrasonic frequency of 40 kHz and ultrasonic power of 180 W) was employed to provide ultrasonic assistance during the electrochemical exfoliation process. The synthesized graphene powder was characterized with FTIR spectroscopy, Raman spectroscopy, XRD, and SEM techniques to study the chemical, microstructural and morphological properties. FTIR spectrum exhibited the C–O and O–H functional groups and the C=C stretching of the hexagonal ring of graphene. Raman spectrum showed two sharp peaks for I_D and I_G bands at ∼1350 cm⁻¹ and ∼1580 cm⁻¹, respectively. The XRD results revealed the polycrystalline nature of graphene powder. The SEM results showed various sizes and shapes of graphene powder. Our proposed method shows huge potentials for facile synthesis of graphene powder on a large scale.

Introduction

Graphene has been widely employed in several fields such as solar cell, energy storage, catalysis, energy storage, power generation, water desalination, bio or gas sensors and advanced composite materials [1], [2], [3], [4], [5], [6], [7], [8], [9] due to its extraordinary properties like excellent flexibility, biocompatibility, good chemical stability, high electrical and thermal conductivity, high strength and large specific surface area [10], [11], [12], [13], [14], [15], [16], [17]. Graphene can be synthesized in various forms, such as reduced graphene oxide, graphene oxide, graphene powder, large-area graphene, etc. These various forms of graphene have different properties, which enables a large number of applications. Graphene was successfully obtained from graphite via mechanical exfoliation with the help of scotch tape in 2004 [18]. This method has been reported to be the production of single-layer graphene with high electronic and structural quality. However, the main drawbacks of this method are high time consumption and low production yields, which makes it unfavorable [19]. To overcome above-mentioned drawbacks, several researchers are taking efforts to develop a facile, economical and scalable method for the synthesis of graphene.

In the last 15 years, various methods have been developed for the synthesis graphene, such as mechanical exfoliation [20], electrochemical exfoliation [21], chemical vapor deposition [15], epitaxial growth on SiC [22], liquid-phase exfoliation through sonication [23] and chemical or thermal reduction of graphite oxide [24], [25]. Specifically, the electrochemical exfoliation method is widely used to prepare graphene due to its several advantages over other methods such as simple in operation, economical, operates at ambient temperature, single-step process and non-toxic, etc. In the electrochemical exfoliation method, the quality and yield of graphene depend upon the type of electrolyte used for exfoliation of graphene from the graphite electrode. Several electrolytes have been used in electrochemical exfoliation method such as inorganic solution, surfactant, polar solvent, high molecular polymer and ionic liquid [26]. However, inorganic solution (especially H₂SO₄) has been most preferred because it provides a higher yield of graphene. An electrical current is provided during the electrochemical exfoliation process to drive reduction or oxidation, exfoliation and intercalation of graphite rod to prepare graphene powder. Chen et al. [27] have reported the synthesis of colloidal graphene by electrochemical exfoliation of graphite anode using (NH₄)₂SO₄ electrolyte. The synthesized colloidal graphene can be used in several applications such as graphene ink in printed electronics, supercapacitors, solar cells, sensors and lithium-ion batteries. Ching-Yuan Su et al. [28] synthesized thin graphite sheets by electrochemical exfoliation of the highly oriented pyrolytic graphite anode electrode. The electrolyte was prepared using H₂SO₄ and de-ionized (DI) water. A (-10 V to + 10 V) D.C. power supply was supplied during the electrochemical exfoliation process. The obtained graphene sheets were filtered, washed with DI water and then dried. Finally, dried graphene sheets were dispersed in dimethylformamide (DMF) solution. The prepared graphene sheets exhibited up to 30 μm lateral size. Tripathi et al. [19] used the electrochemical exfoliation technique for the synthesis of graphene using the alkaline electrolyte. The electrolyte was prepared using KOH and DI water. They have studied the effect of pH variation (11, 12 and 13) on the quality of graphene. The electrochemical exfoliation of graphite was carried out in two steps. In the first step, +3 V power supply was provided for 300 sec and in the second step (-10 V to + 10 V) were applied on the anode at an interval of 5 sec. After the completion of the process, thick graphite pieces were found at the bottom of the flask and nearly transparent few layers of graphene was found floats on the electrolyte surface. Obtained results revealed that prepared graphene consists of 1–4 layers with a higher lateral extent and minimum disorder. Rao et al. [29] have reported preparation of few layer graphene by electrochemical exfoliation method using a electrolyte of sodium hydroxide/hydrogen peroxide/water (NaOH/H₂O₂/H₂O). Pure graphite electrode was used as anode (source of graphene) and platinum sheet was used as cathode. The obtained results confirmed that the H₂O₂ plays vital role in the efficient electrochemical exfoliation of graphite. Parvez et al. [30] have employed electrochemical exfoliation technique for the fabrication of graphene. The electrolyte was prepared using H₂SO₄ and DI water. A positive 10 V supply was provided in between the graphite electrode (anode) and platinum wire (cathode). The produced graphene sheets exhibited ∼10 µm in lateral size. However, synthesis of graphene powder by ultrasonic assisted electrochemical exfoliation of graphite electrode from acidic bath (H₂SO₄ and H₂O) is not studied.

In this paper, we present the synthesis of graphene powder by ultrasonic-assisted electrochemical exfoliation using an acidic bath. Pure graphite rod and platinum coated titanium electrode are used as anode and cathode, respectively and vertically dipped into the electrolyte (H₂SO₄ and H₂O). The external ultrasonic force will result in the separation of graphene layers from graphite electrodes and a decrease in the agglomeration of graphene in the electrolyte. The morphological, microstructural and chemical properties of graphene powder were systematically studied and presented in detail. We have found that few layer graphene was produced successfully due to the generation of O₂ and SO₂ within the graphite sheets.