Facile electrochemical approach for the production of graphite oxide with tunable chemistry
Graphical abstract
Introduction
Chemically-derived graphite oxide (CGO) prepared via the oxidative chemical method (e.g. the Hummers method) is a popular precursor for production of graphene, because of its scalability, high yield and dispersibility in various solvents [1], [2], [3]. However, CGO not only suffers from contamination by metal ions (e.g. K+ and Mn2+) [4], [5], but also the presence of inherent hole defects on the carbon basal plane [6], caused by strong oxidizing agents such as KMnO4, and KClO3 [7], [8], [9], which ultimately lead to a drastic drop in CGO's conductivity. Even after reduction by chemical or thermal methods [10], [11], [12], [13], the electrical conductivity of the reduced CGO would be ultimately limited by those structural defects [14], [15].
Alternatively, graphite oxide could be prepared via electrochemical methods [16], [17], [18]. Fundamentally, the intercalation of the graphite through the electrochemical oxidation process is similar to the chemical oxidation in which the graphite was first activated via the extraction of its electrons [19]. Although the electrochemical methods are considered to be greener compared to the chemical methods [20], [21], a limitation of the paradigm to date has been the inability to tune the oxidation level of electrochemically-derived graphite oxide (EGrO) over a wide range. For instance, Bartosz et al. carried out electrochemical oxidation of graphite flakes held in a platinum mesh via linear sweep voltammetry; however, their EGrO displayed a relatively small degree of oxidation (C:O ratio of 9.8:1) [18]. The structure and chemical tunability of EGrO are very critical for its eventual applications, as different oxidation degrees or oxygen functional groups will lead to considerable variation in its properties, including dispersibility, physical morphology, conductivity, toxicity and heavy ion adsorption behaviour [22], [23], [24], [25].
A further limitation of previously reported electrochemical methods is the challenge to produce EGrO with high reproducibility. The difficulty lies in the ineffective distribution of electrical current to all graphite flakes for complete electrochemical intercalation and oxidation, especially after the initial graphite expansion caused by the intercalation and oxidation processes. Very frequently, the accompanied water/solvent electrolysis process exacerbates the detachment of multi-layered graphene flakes before the desired electrochemical reaction (e.g. oxidation) process can be achieved [21], [26]. To address these limitations, we describe a procedure for the controllable and reproducible synthesis of EGrO using a Tee-cell setup. In this study, the effects of charging time and electrolyte (perchloric acid) concentration were investigated, and the EGrO products were characterized in detail using X-ray diffraction (XRD), nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Raman spectroscopy.
Section snippets
Materials
The graphite flakes used in the experiments were purchased from Sigma-Aldrich (Product Number: 332461). The lateral size of the flakes was determined by scanning electron microscope (SEM) to be approximately 500 μm (Fig. S1). Perchloric acid (11.6 M, 70 wt %) and sulfuric acid (18.0 M, 98 wt %) were obtained from Sigma-Aldrich and used as received. Other concentrations of acid were prepared by dilution with ultrapure Millipore water.
Sample preparation
EGrO was prepared in a two-electrode Tee-cell purchased from
Anodic oxidation of graphite
The galvanostatic charging curves for various time periods are shown in Fig. 1b. They exhibit a sequence of characteristic slopes and plateaus which can be divided into three charging segments. In general, the good overlap among the charging curves across the five different experiments suggests that the technique has good batch-to-batch reproducibility. This good reproducibility originated from the spatial confinement of graphite in the Tee-cell, thereby maintaining the electrical connectivity
Conclusion
In summary, EGrO can be readily produced in a reproducible and controlled manner in the Tee-cell setup for the detailed study of the electrochemical graphite intercalation and oxidation processes. Notably, for the most part, the electrochemical oxidation process did not proceed to a higher degree of graphite oxidation (to produce carboxyl groups) even after extended period of reaction (in 11.6 M perchloric acid). Furthermore, we found that the degree of oxidation can be further limited by
Acknowledgements
The authors acknowledge funding support from Baosteel-Australia Joint Research and Development Centre (BA11006) and the Australian Research Council (DE 140101662). This work made use of the facilities at the Monash Centre for Electron Microscopy and Monash X-ray platform.
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