The role of microwave absorption on formation of graphene from graphite oxide

Abstract

By means of manipulating the oxygen content in graphite oxides (GO) and/or graphene-based materials, we demonstrate that the microwave absorption capacity of carbon materials is highly dependent on their chemical composition and structure. The increase of oxygen in GO remarkably decreases its microwave absorption capacity due to the size decrease of the π–π conjugated structure in these materials, and vice versa. It was revealed that graphene is an excellent microwave absorbent while GO with poor microwave absorption capacity, the unoxidized graphitic region “impurities” in GO act as the microwave absorbents to initiate the microwave-induced deoxygenation. The addition of a small amount graphene to GO leads to avalanche-like deoxygenation reaction of GO under microwave irradiation (MWI) and graphene formation, which was used for electrode materials in supercapacitors. The interaction between microwaves and graphene or graphene-based materials may be used for the fabrication of a variety of graphene-based nanocomposites with exceptional properties and a wealth of practical applications.

Introduction

Graphene has attracted tremendous attention for its remarkable electronic and thermal conductivity, large specific surface area, high mobility of charge carriers, excellent chemical stability and mechanical strength due to its unique structure made of sp² carbon atoms tightly packed into a honeycomb lattice [1]. For the sake of practical application, it is critical to find versatile methods that can produce graphene abundantly and efficiently at low cost. Until now, several strategies, such as micromechanical cleavage [2], epitaxial growth [3], [4], chemical vapor deposition [5], and exfoliation of graphite oxide (GO) [6], [7], [8], [9], have been pursued to synthesize graphene sheets. Among these methods, exfoliation of GO is the most promising method for low-cost and scalable production, and has been researched intensively [10], [11]. Thermal expansion of GO represents one of the most attractive approaches because of its straight forwardness, high efficiency, and high degree of exfoliation [12]. However, this process is energy-consuming and must be carried out at high temperature.

Microwaves, an alternative energy input source, have been widely used because of their internal and volumetric heating of materials, in the field of organic synthesis [13], environmental remediation [14], preparation of catalysts [15] and activated carbon [16]. A variety of nanostructures with different compositions have also been fabricated via the energy-efficient microwave irradiation (MWI) approach [17]. In this process, microwave energy is transformed into heat by using a microwave absorbent. For this very reason, it is necessary to select a proper microwave absorbent to efficiently convert microwave energy into heat. MWI has demonstrated the ability to successfully produce graphene from GO, where the main attention has been focused on the production efficiency [18], [19], [20], [21]. However, the interaction of microwaves with GO and graphene, which is very important for the further development of this method, is not fully understood.

In the present work, we investigate the response of oxidized graphite with varying degrees of oxidation under MWI. Graphene is demonstrated to behave as an excellent microwave absorbent. By dispersing tiny amounts of graphene into a GO matrix, a local heating stimulated by graphene under MWI creates an avalanche-like deoxygenating reaction of GO, thus giving rise to graphene, which shows a high specific surface area and good electrochemical performance.