Novosibirsk State Technical University (NETI) has modified one of the methods for the synthesis of graphite oxide (GO) and studied the effect of hydrogen peroxide on the properties of GO. The resulting material was heated, and due to its high porosity, it became possible to use it as a sorbent.
Oxidized carbon materials such as graphite oxide and its related materials are of particular interest to scientists due to their unique combinations of physical and chemical properties. Graphite oxide contains a large number of oxygen-containing functional groups, which positively affect its ability to disperse in water and allow for various modifications of the surface of the material. There are several basic methods for obtaining graphite oxide, one of them is the Hummers method, which is most widely used due to its relatively fast synthesis process (about 2 hours) and the ability to control surface chemistry by modifying the synthesis technique. Precisely because of the latter, there is a huge variety of modified Hammers methods for producing graphite oxide.
Scientists at NSTU-NETI synthesized graphite oxide using a modified Hammers method. Fine graphite powder of high quality was placed in a flask and five reagents were added: sodium nitrate (NaNO3), sulfuric acid (H2SO4), potassium permanganate (KMnO4), water and hydrogen peroxide (H2O2). The quality and properties of GO formed during synthesis are influenced by many factors, including the reagents used, the reaction temperature, the pretreatment of the starting material, and the holding time of the reaction mixture. In this work, the role of controlling the properties of the material was assigned to hydrogen peroxide as a reagent, which is added last to the system.
"Hydrogen peroxide was added to the reaction mixture in various volumes to study its role in the synthesis and formation of surface functional groups. The volume of H2O2 was varied in order to obtain an expanded set of experimental data. We also studied the role of the long—term effect of hydrogen peroxide on the transformations of functional groups in graphite oxide. For this purpose, material samples were obtained that were kept in the reaction mixture for 24 hours and seven days after the addition of H2O2," said Alexander Bannov, Professor of the Department of Chemistry and Chemical Technologies at NSTU-NETI, Doctor of Chemical Sciences.
According to him, the new synthesis control method is the ability to control functional groups and their composition, that is, the surface chemistry of a material, and thereby give it various properties. In particular, the scientists were faced with the task of maximizing the porosity of graphite oxide.
"Graphite oxide itself is not porous, but it contains a lot of oxygen-containing functional groups, intercalated water, and in order for it to become porous, it must be heated to 350°C. However, it is important to observe a certain condition: The heating rate should be moderate so that the gas phase exits gradually, loosening the material, giving it porosity. The specific surface area of the material can be increased by 30-40 times," notes Alexander Bannov.
As a result of the heating, reduced graphite oxide (rGO) was formed. The degree of expansion was assessed by changes in bulk density, and the texture properties of porous samples were measured. To show the effect of the oxidation state of GO on the porosity of reduced graphite oxide (rGO), additional thermal reduction experiments were conducted.
The resulting highly porous graphite material is suitable as a sorbent for liquid absorption and water purification. The property of reduced graphite oxide to swell when heated, forming a protective layer of foam with low thermal conductivity, can be used to create non-toxic flame-retardant materials and functional materials based on it.