Chemical functionalization of graphene and its applications

Abstract

Functionalization and dispersion of graphene sheets are of crucial importance for their end applications. Chemical functionalization of graphene enables this material to be processed by solvent-assisted techniques, such as layer-by-layer assembly, spin-coating, and filtration. It also prevents the agglomeration of single layer graphene during reduction and maintains the inherent properties of graphene. Therefore, a detailed review on the advances of chemical functionalization of graphene is presented. Synthesis and characterization of graphene have also been reviewed in the current article. The functionalization of graphene can be performed by covalent and noncovalent modification techniques. In both cases, surface modification of graphene oxide followed by reduction has been carried out to obtain functionalized graphene. It has been found that both the covalent and noncovalent modification techniques are very effective in the preparation of processable graphene. However, the electrical conductivity of the functionalized graphene has been observed to decrease significantly compared to pure graphene. Moreover, the surface area of the functionalized graphene prepared by covalent and non-covalent techniques decreases significantly due to the destructive chemical oxidation of flake graphite followed by sonication, functionalization and chemical reduction. In order to overcome these problems, several studies have been reported on the preparation of functionalized graphene directly from graphite (one-step process). In all these cases, surface modification of graphene can prevent agglomeration and facilitates the formation of stable dispersions. Surface modified graphene can be used for the fabrication of polymer nanocomposites, super-capacitor devices, drug delivery system, solar cells, memory devices, transistor device, biosensor, etc.

Introduction

Graphene is a one-atom-thick planar sheet of sp²-bonded carbon atoms densely packed in a honeycomb crystal lattice. It is the mother element of some carbon allotropes, including graphite, carbon nanotubes, and fullerenes (Fig. 1) [1], [2]. Naturally occurring graphite has been known as a mineral for nearly 500 years. A large number of experiments focus on the insertion of additional chemical species between the basal planes of graphite [3], [4], [5]. Graphene, the building block of graphite, was theoretically established in 1940 [6]. Boehm and co-workers separated thin lamellae of carbon by heating and chemical reduction of graphite oxide in 1962 [7]. However, until 2004, single-layer graphene was believed to be thermodynamically unstable under ambient conditions [8], [9]. Geim and co-workers at Manchester University successfully identified single layers of graphene in a simple tabletop experiment in 2004 [10]. This revolutionary discovery has added a new dimension of research in the fields of physics, chemistry, biotechnology, and materials science. The “thinnest” known material graphene exhibits excellent electrical conductivity, mechanical flexibility, optical transparency, thermal conductivity, and low coefficient of thermal expansion (CTE) behavior [10], [11], [12], [13], [14], [15]. The unique features of graphene have attracted tremendous interest both in academics and industry. Recently, graphene has been used as alternative carbon-based nanofiller in the preparation of polymer nanocomposites [16], [17], [18], [19], [20], [21], [22], [23]. Graphene-based polymer nanocomposites exhibit improved mechanical, thermal, and electrical properties [16], [17], [18], [19], [20], [21], [22], [23]. Graphene is expected to play an important role in the fabrication of nano-electronic and bio-electronic devices in near future [2]. Graphene is also capable of replacing metal conductors in electronic and electrical devices due to its excellent electrical conductivity and mechanical flexibility [24], [25], [26]. Ongoing research shows that graphene can replace brittle and chemically unstable indium tin oxide in flexible displays and touch screens. Therefore, graphene as an electronic circuit material is considered to be potentially superior to other carbon-based nanofillers [27], [28], [29]. It is well established that the superior properties of graphene are associated with its single-layer. However, the fabrication of single-layer graphene is difficult at ambient temperature. Graphene sheets with a high specific surface area tend to form irreversible agglomerates or even restack to form graphite through π–π stacking and van der Waals interactions if the sheets are not well separated from each other [30], [31]. Aggregation can be reduced by the attachment of other small molecules or polymers to the graphene sheets. The presence of hydrophilic or hydrophobic groups prevents aggregation of graphene sheets by strong polar-polar interactions or by their bulky size [31], [32]. The attachment of functional groups to graphene also aids in dispersion in a hydrophilic or hydrophobic media, as well as in the organic polymer. The use of pristine graphene in nanoelectronic devices (especially for transistor purposes) is also not suitable for practical applications. Graphene based field effect transistors cannot be turned off effectively due to the absence of a band gap in pristine graphene [33]. Therefore, the problems associated with the band gap of pristine graphene should be solved before using the material as a transistor [34]. The controlled functionalization of graphene with electron-donating or electron-withdrawing groups (n-doping or p-doping) can be achieved very easily [34]. The molecular level doping of graphene via charge transfer between electron donor and electron acceptor molecules gives rise to significant changes in the electronic structure of graphene, as evidenced by changes in the Raman and photoelectron spectra [34], [35], [36]. Therefore, an efficient approach to the production of surface-functionalized graphene sheets in large quantities has been a major focus of many researchers, with the goal of exploiting the most frequently proposed applications of graphene in the areas of polymer nanocomposites, super-capacitor devices, drug delivery systems, solar cells, memory devices, transistor devices, biosensors etc.

A general review on the synthesis, characterization, properties, and applications of graphene is given by Singh et al. [1]. It describes a detailed overview of the synthesis of graphene by mechanical exfoliation of graphite, chemical vapor deposition (CVD), and chemical approaches. The application of functionalized graphene in various areas has also been suggested. In the present review, we present an updated overview that focuses on the procedures of surface modification, characterization, properties, and applications of functionalized graphene. Depending on the nature of chemical reactions occurring between the graphene oxide (GO) and organic modifier, a classification of the chemical functionalization of graphene has been presented in the present review. This article also provides a short review of graphene synthesis and different characterization techniques used to identify single-layer graphene. Extensive research has been carried out on the surface modification of graphene. However, limitations with respect to dispersibility, electrical conductivity, and the complications in the experimental procedures have been encountered. The present article discusses different surface treatment processes, such as covalent and non-covalent functionalization techniques, stabilization in ionic medium, and in situ surface treatments starting from graphite. This article also highlights present and future trends for application of surface-modified graphene.