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Mysterious mechanism of graphene oxide formation explained

Posted By Graphene Council, Monday, July 20, 2020
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.

Tags:  Advanced Carbon Nanomaterials Lab  Artur Khannanov  Ayrat Dimiev  Graphene  graphene oxide  graphite  Kazan Federal University  Ksenia Shukhina  Rice University 

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Study puts spin into quantum technologies

Posted By Graphene Council, Thursday, February 27, 2020

A team of international scientists investigating how to control the spin of atom-like impurities in 2D materials have observed the dependence of the atom's energy on an external magnetic field for the first time.

The results of the study, published in Nature Materials, will be of interest to both academic and industry research groups working on the development of future quantum applications, the researchers say.


Researchers led by Prof Vladimir Dyakonov at the University of Würzburg in collaboration with scientists from the University of Technology Sydney (UTS), the Kazan Federal University and the Universidade Federal de Minas Gerais, demonstrated the ability to control the spin of atom-like impurities in 2D material hexagonal boron-nitride. By combining laser and microwave excitation the researchers were able to change the spin states, for example "up" to "down", of atom-like impurities hosted in the material and show the dependence of their energy on an external magnetic field.

This is the first time that the phenomenon has been observed in a material that is made of a single sheet of atoms like graphene. The researchers say that this newly demonstrated quantum spin-optical properties, combined with the ease of integrating with other 2D materials and devices, establishes hexagonal boron-nitride as an intriguing candidate for advanced quantum technology hardware.

"2D atomic crystals are currently some of the most studied materials in condensed matter physics and materials science," says UTS physicist Dr Mehran Kianinia, a co-author of the study.

"Their physics is intriguing from a fundamental point of view, but beyond that, we can think of stacking different 2D crystals to create completely new materials, heterostructures and devices with specific designer properties," he says.

UTS researcher, Dr Carlo Bradac, a senior co-author of the study says that in addition to adding another unique property, to an already impressive range of properties for a 2D material, the discovery has enormous potential for the field of quantum sensing.

"What really excites me is the potential [in the context of quantum sensing]. These spins are sensitive to their immediate surroundings. Unlike 3D solids, where the atom-like system can be as far as a few nanometres from the object to sense, here the controllable spin is right at the surface. Our hope is to use these individual spins as tiny sensors and map, with unprecedented spatial resolution, variations in temperature, as well as magnetic and electric fields onto variations in spin" Dr Bradac says.

"Imagine, for instance, being able to measure minuscule magnetic fields with sensors as small as single atoms. The possibilities are far reaching and range from nuclear magnetic resonance spectroscopy for nanoscale medical diagnostic and material chemistry to GPS-free navigation using the Earth's magnetic field," he says.

However quantum-based nanoscale magnetometry is "just one area where controlling single spins in solids is useful" says senior author of the study UTS Professor Igor Aharonovich.

"Beyond quantum sensing, many quantum computing and quantum communication applications rely on our ability to control the spin-state--zero, one and anything in between--of single atom-like systems in solid host materials. This allows us to encode, store and transfer information in the form of quantum bits or qubits," he says.

Amongst many others, this research highlights how scientists are quickly becoming masters in the craft of manipulating objects in the quantum regime. In fact, achievements like Lockheed Martin's Black Ice project and Google's quantum supremacy are proof that we are striding away from mere proof-of-concept experiments towards real world, quantum-enabled solutions to practical problems.

Tags:  2D materials  Graphene  Hexagonal boron nitride  Kazan Federal University  Nature Materials  Universidade Federal de Minas Gerais  University of Technology Sydney  University of Wurzburg  Vladimir Dyakonov 

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