20 July, 2020 14:02

Mysterious mechanism of graphene oxide formation explained

Project lead Ayrat Dimiev has been working on this topic since 2012, when he was a part of Professor James Tour's group at Rice University. First results saw light in 2014. That paper, which has amassed 490 citations at this moment, dealt with the mechanism of turning graphite into graphene oxide (GO). Dr. Dimiev later transferred to the private sector and resumed his inquiries in 2017, after returning to Kazan Federal University and opening the Advanced Carbon Nanomaterials Lab. The experimental part of this new publication was conducted by Dr. Ksenia Shukhina and Dr. Artur Khannanov.

Natural graphite, used as the precursor for graphene oxide production, is a highly ordered crystalline inorganic material, which is believed to be formed by decay of organic matter. It is extremely thermodynamically stable and resistant to be converted to the organic-like metastable graphite oxide. On this route, it goes through several transformations, resulting in respective intermediate products. The first intermediate product is graphite intercalation compound (GIC). GICs have been intensively studied in the second half of the 20th century. In recent years they gained renewed interest due to the discovery of graphene and related materials. The second step of the complex reaction, i.e. the conversion of GIC to pristine graphite oxide, remained mysterious. The most interesting question was about the nature of species attacking carbon atoms to form covalent C-O bonds. For many years, it was conventionally assumed that the attacking species are the manganese derivatives like Mn2O7 or MnO3+. In this study, the authors unambiguously demonstrated that the manganese derivatives do not even penetrate graphite galleries; they only withdraw electron density from graphene, but the actual species attacking carbon atoms are water molecules. Thus, reaction cannot proceed in fully anhydrous conditions, and speeds up in presence of small quantities of water.

Another new finding, registered by Ksenia Shukhina for the first time, was the imaginary reversibility of the C-O bond formation, as long as the graphite sample remains intercalated with sulfuric acid. The as-formed C-O bonds can be easily cleaved by the laser irradiation, converting GO back to stage-1 GIC in the irradiated areas of the graphite flake.  After careful analysis, this “reversibility” was interpreted by the authors as the mobility of the C-O bonds, i.e. the bonds do not cleave, but freely migrate along the graphene plane for micron-scale distances. The discovered phenomena and proposed reaction mechanism provide rationale for a range of the well-known but yet poorly understood experimental observations in the graphene chemistry. Among them is the existence of the oxidized and graphenic domains in the GO structure.

The results of this fundamental study give a comprehensive view on the driving forces of the complex processes occurring during the transformation of graphite into graphene oxide. This is the first time such a multifaceted des­cription of a dynamic system has been made, and this is the result not only of newly obtained experimental data, but also of many years of reflection on the issue by the project lead. Understanding these processes will finally let one to control this reaction and get products with desired properties. This applies not only to the final product of graphene oxide, but also to the entire family of materials obtained by exposing graphite to acidic oxidizing mixtures: expanded graphite, graphene nano-platelets containing from 3 to 50 graphene sheets, graphite intercalates, and doped graphene. As for graphene oxide itself, its successful use has already been repeatedly demonstrated in such areas as composite materials, selective membranes, catalysis, lithium-ion batteries, etc. However, the use of graphene oxide is hampered by the high cost of its production and the lack of control over the properties of the synthesized product. The published research addresses both of these problems.

Currently, work is ongoing to study the interaction of graphene oxide with metals. The researchers are firmly convinced that this process is based not just on electrostatic attraction, or on non-specific adsorption, as it is commonly believed, but on a chemical interaction with bond formation through the coordination mechanism. The objective now is to describe the complex reaction mechanism of the rearrangements, leading to the metal bonding in the dynamic structure of graphene oxide.

The research is supported by Russian Science Foundation, grant 16-13-10291.

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