Graphene Oxide Chemistry Management via the Use of KMnO4/K2Cr2O7 Oxidizing Agents

1. Introduction

In recent years, graphene has attracted much attention from both experimental and theoretical research groups due to its unique structural, mechanical, heat-conducting, and electricity-conducting properties [1,2,3,4,5]. This makes graphene attractive for application in various fields, such as microelectronics [6,7,8,9,10], sensors [11,12,13,14,15], biomedicine [16], and the accumulation of electrical energy [17,18], etc.

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It should be noted that the ideal graphene structure is not required in all potential applications. An increasing number of studies have been devoted to the synthesis and application of graphenes modified with organic groups and having a defect structure, which—in many publications—are also called ‘functionalized graphenes’ or ‘chemically modified graphenes’ (CMGs) [19,20,21,22]. One of the most important CMGs is graphene oxide (GO), a nonstoichiometric derivative of graphene, the edges and surfaces of which are covered with various oxygen-containing functional groups. The interest in GO is primarily due to the fact that it forms stable dispersions in water and a number of other polar solvents, and can also be reduced by thermal or chemical action to a graphene-like material (reduced graphene oxide, rGO) [23,24,25,26,27] with a given composition of organic groups. By controlling the chemical composition, one can adjust the electronic structure of materials, their sorption capacity, ability to electrical and thermal conductivity, affinity for composite components, and ability to participate in ion exchange reactions and bind to various biological objects (proteins, antibodies, aptamers, etc.). This opens up new opportunities to optimize the physical properties of graphene for practical applications, and to form new graphene-based smart materials.

Brodie, for the first time, synthesized GO by the treatment of graphite with a mixture of HNO3 and KClO3, with the subsequent product isolation and re-treatment with a reaction mixture several times. The thus-produced GO had low oxidation degree, and was described by the formula C11H4O5 [28]. An application of the Staudenmaier method, based on the use of concentrated H2SO4 as a reaction medium and an increased amount of KClO3 [29,30], made it possible to achieve a high content of hydroxyl groups (C-OH) with a small amount of the edge-located carbonyls (C=O) [31]. According to the X-ray photoelectron spectroscopy data, the C/O ratio of the resulting product was 2.47. However, the method turned out to be laborious and dangerous: despite only one stage being involved, the addition of potassium chlorate lasted more than 1 week (due to the explosiveness of the reaction mixture), and the released chlorine dioxide had to be removed with an inert gas [29].

The classical method for the synthesis of GO is considered to be the method proposed in 1958 by Hummers and Offeman [32]. The method is based on the application of a concentrated H2SO4, NaNO3, and KMnO4 mixture at temperatures below 45 °C. The entire oxidation process is completed within 2 h, and provides a higher oxidation degree (C/O ratio 2.05) than the Staudenmaier method, along with a higher content of C=O groups in the resulting GO [31]. In order to reduce the amount of the unoxidized fraction after graphite oxidation via the Hummers method, Kovtyukhova et al. proposed, in 1999, to pretreat graphite with a mixture of H2SO4, K2S2O8, and P2O5 at 80 °C for several hours [33]. Modified Hummers methods are currently the most common way to synthesize GO. The use of one of the most common modifications, the Marcano method [34], leads to a product with an even higher oxidation state (C/O ratio 1.95) compared to other methods. Furthermore, it contains an increased content of C-OH and C=O functional groups, with the appearance of a noticeable amount of carboxyls (COOH) [31]. Other works on various modifications of the Hummers method also suggest an increase in the amount of potassium permanganate, and changes in temperature conditions and processing time, etc. [25,26,27].

In 2010, Chandra et al. [35] proposed an alternative approach for the synthesis of GO by the replacement of KMnO4 with K2Cr2O7. Graphite was mixed with NaNO3 and concentrated H2SO4 in an ice bath. Then, K2Cr2O7 was slowly added and kept under stirring for 5 days at room temperature. In 2018, Martin Rosillo-Lopez and Christoph G. Salzmann [36] presented an optimized technique for oxidizing graphite with K2Cr2O7. The authors reduced the reaction time from 5 days to 20 h, and also showed that sodium nitrate does not affect the reaction. A key feature of the GO obtained by the aforementioned method is the many times higher content of the edge oxygen-containing groups, namely carboxyls and ketones/aldehydes, with the simultaneous drastic decrease in the content of the basal plane hydroxyl and epoxy (C-O-C) groups.

Furthermore, in order to obtain GO with various oxidation degrees, electrochemical methods of oxidation of thermally-expanded graphite have been used [37,38]. Such methods are environmentally-friendly and fairly simple to implement; however, the functional composition of the oxidation products obtained in the above articles has not been studied.

Despite these results, the question of a simple, scalable synthesis method of GO with a given oxidation state (C/O ratio), a relative concentration of given oxygen-containing groups, and the absence of contaminants remains open. Furthermore, for the relationship between the parameters of synthesis and the chemistry of graphene oxide, in particular, the ratio of oxygen-containing groups on the surface and edges of the graphene layer remains unclear.

In this article, we propose a novel approach for the synthesis of GO, which allows the management of GO chemistry via the use of a combination of KMnO4 and K2Cr2O7 oxidizing agents. The novelty of the presented work is in a simple, previously-undescribed approach to the production of GO with a controlled, predetermined functional composition through the selection of the ratio of oxidants in the mixture directly in the process of GO synthesis. It should be noted that the results obtained indicate the possibility of the fine quantitative tuning of the content of basal-plane and edge-located functional groups of GO. This is an excellent opportunity for the development of the field of functionalized graphene compounds, as most of them are obtained from GO, and the possibilities of its functionalization directly depend on its initial functional composition. Using a set of spectroscopic methods, we have studied in detail the interplay between the KMnO4/K2Cr2O7 ratio in the oxidizing mixture and the composition of basal-plane and edge-located oxygen-containing groups. The structural, morphological, and optical features of the synthesized GO samples due to the changes in the chemistry of the material have also been studied via a set of microscopic methods, the laser diffraction method, and UV-Vis spectroscopy. Given all of the results, the facile method for the one-step synthesis of CMGs with the desired composition of oxygen-containing groups and optical properties is presented, opening up new possibilities for advances in the optoelectronic (e.g., organic solar cells based on rGO with covalent-bonded perovskite quantum dots) and electrochemical (batteries, li-ion accumulators, hybrid supercapacitors) applications of graphene-related materials.

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