The chemical and structural analysis of graphene oxide with different degrees of oxidation

Abstract

Graphene oxide (GO) with various degrees of oxidation was synthesized using a modified Hummers method. The formation of different types of oxygen containing functional groups in GO, and their influences on its structure were analyzed using X-ray diffraction (XRD), Fourier transform infra-red spectra, X-ray photoelectron spectra (XPS), zeta potential analysis and Raman spectroscopy. XRD studies showed a disruption of the graphitic AB stacking order during the increase in oxidation levels. XPS analysis revealed the formation of hydroxyl and carboxyl groups at lower oxidation levels and epoxide groups at higher oxidation levels. The influence of the oxidation degree on the properties of GO was evaluated by zeta potential analysis, which showed a linear increase in the zeta potential with increasing oxidation levels. Raman spectroscopy analysis revealed that increasing oxidation levels results in a transition from a crystalline to an amorphous structure. The electrochemical properties of GO is highly influenced by the variation in degree of oxidation. Our results suggest that the properties of GO can be tuned by varying the oxidation degree, which may pave the way to new developments in the GO-based applications.

Introduction

Graphene oxide (GO) is an atomic sheet of graphite decorated by several oxygenated functional groups on its basal planes and at its edges, resulting in a hybrid structure comprising a mixture of sp² and sp³ hybridized carbon atoms [1]. GO can be synthesized by the oxidation of graphite into graphite oxide followed by the exfoliation of this graphitic oxide into GO [2]. It is already well known that chemically both graphite oxide and GO have similar or identical structures. Both possess stacked structures with chemical functionality on their basal planes and at their edges [3]. The only difference between them is the number of stacked layers, GO possess a monolayer or just a few stacked layers, while graphite oxide contains a greater number of stacked layers [3], [4]. The formation of oxygenated functional groups in graphite oxide makes them easier to exfoliate into monolayers of GO by simple stirring or mild sonication [5]. Even though, this material has been well known since 1859 when it was first synthesized by Brodie [6], the material only becomes of widespread interest after the discovery of graphene and is due to the fact it acts as a major precursor for the synthesis of graphene sheets by suitable reduction techniques either chemically or thermally [7].

Apart from the synthesis of graphene, GO has several standalone applications in various fields such as optoelectronics, supercapacitors, memory devices, composite materials, photocatalysis and as a drug delivery agent [8], [9], [10], [11]. This has drawn the attention of researchers to explore the intriguing properties of GO nanosheets. Most of the outstanding properties of GO arise from its hybrid electronic structure as it contains both the conducting π states from the sp² carbon domains and also the σ states from the sp³ carbon domains [12]. Theoretical studies have revealed that the properties of GO can be altered by tuning the sp²/sp³ ratios of the carbon atoms [13]. Previous experimental reports showed the observation of quantum confinement phenomena in GO due to the formation of unoxidized sp² islands between the oxidized sp³ regions [14]. The formation of sp³ domains in GO is due to the oxidation reaction which results in the decoration of different types of functional groups such as hydroxyl, epoxyl, carbonyl and carboxyl groups. The presence of these oxygenated functional groups in GO makes it hydrophilic in nature and also enables them to functionalize other materials with suitable chemistry [15].

The sp²/sp³ ratios in GO can be tuned by varying the oxidation degree using suitable chemical reactions. GO with various ratio of sp²/sp³ domains may provide novel properties that can be useful for making several improvements in the development of graphene based research applications such as biosensors, supercapacitors, and optoelectronic devices etc., With the motivation of altering the properties of GO by varying the oxidation levels, we used a modified Hummers method employing various quantities of oxidizing agent. To date, three major methods have been used to synthesize GO viz. (i) the Brodie method [6], (ii) the Staudenmaier method [16], and (iii) the Hummers method [17]. Of these methods, the Hummers method is generally considered to be the best and most researchers follow it since it has the advantage of non-toxicity compared to the former two which involve highly toxic reactions due to the liberation of toxic gases and highly reactive species [3]. In this paper, we report on the synthesis of GO with different oxidation levels and investigated their chemical and structural analysis using X-ray diffraction (XRD), Fourier transform infra red spectra (FTIR), X-ray photoelectron (XPS) analysis, zeta potential, transmission electron microscopy (TEM) and Raman spectroscopy. The degree of oxidation is also studied by analyzing their electrochemical properties.