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Soil check: How much water does your soil contain?

Posted By Graphene Council, 22 hours ago
Researchers use ultra-small graphene particles to develop a new soil moisture sensor. Anyone who has tried their hand at growing plants, be it an amateur gardener or a seasoned farmer, would be familiar with the perils of under- or over-watering a sapling. Plants require the right amount of water for their healthy growth, and to figure out when and how much to water one has to know the existing moisture levels in the soil. When it comes to keeping track of the watering schedule for a large number of plants, such as for a field of crops, there is a need for an affordable, easy-to-use soil moisture sensor that can accurately measure the water content in the soil. 

A recent study, published in the journal Carbon, demonstrates the workings of a soil moisture sensor made from graphene quantum dots, which are nanometer-sized fragments of graphene. The study was conducted by a team of researchers from the Indian Institute of Technology, Bombay (IIT Bombay), Gauhati University, and Dhirubhai Ambani Institute of Information and Communication Technology, Gandhinagar. It was funded by the Department of Science and Technology, the University Grants Commission and the Assam Science Technology and Environmental Council.

Graphene is made up of a sheet of carbon atoms arranged in a honeycomb-like pattern. Over the years, studies have explored the use of graphene quantum dots — disc-shaped materials made of a few layers of graphene, measuring mere nanometers — for a variety of sensing applications. While extensive research is being carried out on the synthesis of graphene quantum dots, the challenge remains in designing a method that results in a good yield of uniformly-sized particles. Additionally, the process must be scalable and easily adaptable for its  commercialisation.

“Our motivations behind this study was to devise a simple, inexpensive and scalable approach for synthesising graphene quantum dots, and to develop an affordable soil moisture sensor that is suitable for large scale use,” says Prof Hemen Kalita, who is the lead author of this study. He is an Assistant Professor at the Gauhati University and previously was a doctoral student with Prof M Aslam at IIT Bombay. 

The researchers have proposed a method to produce graphene quantum dots as small as 3–5 nanometre from easily available and low-cost graphene oxide. They coated a thin film of graphene oxide onto a carbon electrode and placed it inside an electrolyte solution. When an electric current is applied to the setup, the carbon bonds in the graphene oxide get cleaved, and molecules of the electrolyte occupy those gaps in the graphene oxide layer. Eventually, they form quantum dots of graphene having oxygen-containing chemical groups. 

“At a laboratory scale, we were successful in synthesising graphene quantum dots through our novel approach, and we have filed a patent for the synthesis method,” says Prof Kalita. 

Using the graphene quantum dots, the researchers fabricated a soil moisture sensor which is smaller in size than a lentil seed. The moisture content value displayed by the sensor depends on the resistance measured across it, and with an increasing percentage of water content, there is a fall in resistance. When the sensor is inserted into moist soil, the oxygen atoms present in the graphene quantum dots interact with the hydrogen atoms of the water and form a layer of water molecules on the surface of the sensor. When an external voltage is applied to the sensor via a source meter, the loosely held water molecules in the upper layers get ionised and conduct electrical charge. This leads to a decrease in resistance of the sensor.  

The researchers tested the soil moisture sensors on samples of black and red soil. They found that the moisture content measured by the sensor closely matched the known water content of the soil samples. The sensor gives the final reading within 3 minutes and can be used again after 20 seconds.  

Further, the researchers tested the stability of the sensor by continuously using it over five months to measure the water content in soil samples. They found that the sensor gives a consistent reading throughout this time and works well for a range of soil water levels.  

“With extensive field testing and improved packaging, our sensors will be suitable for commercialisation. A few companies have approached us and initiated discussions with our team to take this project to the industry front,” says Prof Kalita. “We are aiming to develop stable and affordable sensors for the middle-class farmer community,” he signs off. 

Tags:  Gauhati University  Graphene  graphene oxide  Hemen Kalita  Sensors 

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Energy harvesting goes organic, gets more flexible

Posted By Graphene Council, Thursday, September 17, 2020
Nanogenerators capable of converting mechanical energy into electricity are typically made from metal oxides and lead-based perovskites. But these inorganic materials aren't biocompatible, so the race is on to create natural biocompatible piezoelectric materials for energy harvesting, electronic sensing, and stimulating nerves and muscles.

University College Dublin and University of Texas at Dallas researchers decided to explore peptide-based nanotubes, because they would be an appealing option for use within electronic devices and for energy harvesting applications.

In the Journal of Applied Physics, from AIP Publishing, the group reports using a combination of ultraviolet and ozone exposure to generate a wettability difference and an applied field to create horizontally aligned polarization of nanotubes on flexible substrates with interlocking electrodes.

"The piezoelectric properties of peptide-based materials make them particularly attractive for energy harvesting, because pressing or bending them generates an electric charge," said Sawsan Almohammed, lead author and a postdoctoral researcher at University College Dublin.

There's also an increased demand for organic materials to replace inorganic materials, which tend to be toxic and difficult to make.

"Peptide-based materials are organic, easy to make, and have strong chemical and physical stability," she said.

In the group's approach, the physical alignment of nanotubes is achieved by patterning a wettability difference onto the surface of a flexible substrate. This creates a chemical force that pushes the peptide nanotube solution from the hydrophobic region, which repels water, with a high contact angle to the hydrophilic region, which attracts water, with a low contact angle.

Not only did the researchers improve the alignment of the tubes, which is essential for energy harvesting applications, but they also improved the conductivity of the tubes by making composite structures with graphene oxide.

"It's well known that when two materials with different work functions come into contact with each other, an electric charge flows from low to high work function," Almohammed said. "The main novelty of our work is that controlling the horizontal alignment of the nanotubes by electrical field and wettability-assisted self-assembly improved both the current and voltage output, and further enhancement was achieved by incorporating graphene oxide."

The group's work will enable the use of organic materials, especially peptide-based ones, more widely within electronic devices, sensors, and energy harvesting applications, because two key limitations of peptide nanotubes -- alignment and conductivity -- have been improved.

"We're also exploring how charge transfer processes from bending and electric field applications can enhance Raman spectroscopy-based detection of molecules," Almohammed said. "We hope these two efforts can be combined to create a self-energized biosensor with a wide range of applications, including biological and environmental monitoring, high-contrast imaging, and high-efficiency light-emitting diodes."

Tags:  Energy  Graphene  graphene oxide  LED  Sawsan Almohammed  University College Dublin  University of Texas 

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E-Beam atomic-scale 3D 'sculpting' could enable new quantum nanodevices

Posted By Graphene Council, Thursday, September 17, 2020

By varying the energy and dose of tightly-focused electron beams, researchers have demonstrated the ability to both etch away and deposit high-resolution nanoscale patterns on two-dimensional layers of graphene oxide. The 3D additive/subtractive “sculpting” can be done without changing the chemistry of the electron beam deposition chamber, providing the foundation for building a new generation of nanoscale structures.

Based on focused electron beam-induced processing (FEBID) techniques, the work could allow production of 2D/3D complex nanostructures and functional nanodevices useful in quantum communications, sensing, and other applications. For oxygen-containing materials such as graphene oxide, etching can be done without introducing outside materials, using oxygen from the substrate.

“By timing and tuning the energy of the electron beam, we can activate interaction of the beam with oxygen in the graphene oxide to do etching, or interaction with hydrocarbons on the surface to create carbon deposition,” said Andrei Fedorov, professor and Rae S. and Frank H. Neely Chair in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “With atomic-scale control, we can produce complicated patterns using direct write-remove processes. Quantum systems require precise control on an atomic scale, and this could enable a host of potential applications.”

The technique was described in the journal ACS Applied Materials & Interfaces ("High-Resolution Three-Dimensional Sculpting of Two-Dimensional Graphene Oxide by E-Beam Direct Write"). The work was supported by the U.S. Department of Energy Office of Science, Basic Energy Sciences. Co-authors included researchers from Pusan National University in South Korea.

Creation of nanoscale structures is traditionally done using a multistep process of photoresist coating and patterning by photo- or electron beam lithography, followed by bulk dry/wet etching or deposition. Use of this process limits the range of functionalities and structural topologies that can be achieved, increases the complexity and cost, and risks contamination from the multiple chemical steps, creating barriers to fabrication of new types of devices from sensitive 2D materials.

FEBIP enables a material chemistry/site-specific, high-resolution multimode atomic scale processing and provides unprecedented opportunities for “direct-write,” single-step surface patterning of 2D nanomaterials with an in-situ imaging capability. It allows for realizing a rapid multiscale/multimode “top-down and bottom-up” approach, ranging from an atomic scale manipulation to a large-area surface modification on nano- and microscales.

“By tuning the time and the energy of the electrons, you can either remove material or add material,” Fedorov said. “We did not expect that upon electron exposure of graphene oxide that we would start etching patterns.”

With graphene oxide, the electron beam introduces atomic scale perturbations into the 2D-arranged carbon atoms and uses embedded oxygen as an etchant to remove carbon atoms in precise patterns without introduction of a material into the reaction chamber. Fedorov said any oxygen-containing material might produce the same effect. “It’s like the graphene oxide carries its own etchant,” he said. “All we need to activate it is to ‘seed’ the reaction with electrons of appropriate energy.”

For adding carbon, keeping the electron beam focused on the same spot for a longer time generates an excess of lower-energy electrons by interactions of the beam with the substrate to decompose the hydrocarbon molecules onto the surface of the graphene oxide. In that case, the electrons interact with the hydrocarbons rather than the graphene and oxygen atoms, leaving behind liberated carbon atoms as a 3D deposit.

“Depending on how many electrons you bring to it, you can grow structures of different heights away from the etched grooves or from the two-dimensional plane,” he said. “You can think of it almost like holographic writing with excited electrons, substrate and adsorbed molecules combined at the right time and the right place.”

The process should be suitable for depositing materials such as metals and semiconductors, though precursors would need to be added to the chamber for their creation. The 3D structures, just nanometers high, could serve as spacers between layers of graphene or as active sensing elements or other devices on the layers.

“If you want to use graphene or graphene oxide for quantum mechanical devices, you should be able to position layers of material with a separation on the scale of individual carbon atoms,” Fedorov said. “The process could also be used with other materials.”

Using the technique, high-energy electron beams can produce feature sizes just a few nanometers wide. Trenches etched in surfaces could be filled with metals by introducing metal atoms contained in precursors.

Beyond simple patterns, the process could also be used to grow complex structures. “In principle, you could grow a structure like a nanoscale Eiffel Tower with all the intricate details,” Fedorov said. “It would take a long time, but this is the level of control that is possible with electron beam writing.”

Though systems have been built to use multiple electron beams in parallel, Fedorov doesn’t see them being used in high-volume applications. More likely, he said, is laboratory use to fabricate unique structures useful for research purposes.

“We are demonstrating structures that would otherwise be impossible to produce,” he said. “We want to enable the exploitation of new capabilities in areas such as quantum devices. This technique could be an imagination enabler for interesting new physics coming our way with graphene and other interesting materials.”

Tags:  2D materials  Andrei Fedorov  Graphene  graphene oxide  nanomaterials  Pusan National University 

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Researchers Synthesize Ag2S/Reduced Graphene Oxide Composite Catalysts

Posted By Graphene Council, Tuesday, September 8, 2020
The development of efficient electrocatalyst to produce molecular hydrogen from water is receiving considerable attention, in an effort to decrease our reliance on fossil fuels. Silver sulfide (Ag2S) nanocrystals have attracted enormous interests due to its excellent properties.

At present, the performance of hydrogen generation reaction (HER) catalysts based on Ag2S nanocrystals are still at a certain distance from expectations. One of the main reasons is that the impeded charge transfer and decreased active sites caused by aggregation of Ag2S nanocrystals.

To solve this problem, a research team led by Prof. YU Weili from the Changchun Institute of Optics, Fine Mechanics, and Physics (CIOMP) of the Chinese Academy of Sciences and Prof. Hicham Idriss of King Abdullah University of Science and Technology (KAUST) synthesized Ag2S/rGO composite catalysts with smaller crystal size and better charge transfer properties by the solution fabrication strategy.

The study was published in Catalysts on August 19.

The researchers combined the high quality Ag2S nanocrystals with reduced graphene oxide with high carrier mobility, and prepared efficient HER catalysts. 

Compared to catalyst based on pure Ag2S nanocrystals, the Ag2S/rGO composites catalyst showed a significant decrease of overpotential, Tafel slope and electrochemical resistance. Transmission electron microscope (TEM) images showed that the induced rGO provided abundant nucleation sites, thus preventing the aggregation of Ag2S nanocrystals.

The average size of Ag2S nanocrystals grown on rGO was calculated to be about 7 nm.

Time-resolved photoluminescence (TRPL) studies showed that the improvement of the catalystic performance was mainly attributed to the efficient charge transfer of the Ag2S/rGO composites. 

In this study, Ag2S nanocrystals and two-dimensional material rGO are effectively combined to improve the catalytic performance through the synergistic advantages of the two materials in carrier transfer and aggregation inhibition. 

The study provides a new way for the preparation of high performance composite HER catalysts.

Tags:  and Physics  Changchun Institute of Optics  Fine Mechanics  Graphene  Graphene Oxide  Hicham Idriss  King Abdullah University of Science and Technology  YU Weili 

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Associate professor receives grant from National Science Foundation

Posted By Graphene Council, Thursday, September 3, 2020
The National Science Foundation has awarded more than $300,000 for water treatment research to Dr. Lucy Mar Camacho, a Texas A&M University-Kingsville associate professor of Environmental Engineering.

The grant will fund the research project “Collaborative Research: Dry-Wet Phase Inversion Pathway of Graphene Oxide (GO)-Based Mixed-Matrix Membranes for Mineral Ions Separation by Membrane Distillation.”

Membrane distillation is an energy-efficient alternative to multi-stage flash and multi-effect distillation processes and can be configured to concentrate brines, according to the project description. Graphene oxide is a versatile anti-fouling nanomaterial that will be used in the synthesis of mixed-matrix membranes with properties specific to the application in membrane distillation.

Camacho said membrane technology for water desalination and treatment of produced water has the potential to fundamentally alter the way society views water reuse.

“Augmenting water treatment capacity will allow rural, arid, and isolated regions with limited access to water, to have potable and reliable membrane systems for treating water,” the project description states.

The goal of the project is to establish and understand the dry-wet phase inversion membrane development approach to overcome limitations in utility for produced water purification.

“I am glad to have the National Science Foundation recognizing my effort and funding my research ideas, which also means recognizing Texas A&M-Kingsville,” Camacho said. “My research on nanomembrane technology for desalination of impaired waters will allow me to contribute to solve one of the most challenging paradigms of the 21st century, which is the lack of potable water for a growing population. As a researcher, I am very glad to been able to help solve these issues.”

Tags:  Graphene  Graphene Oxide  Lucy Mar Camacho  National Science Foundation  Texas A&M University-Kingsville 

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An improved wearable, stretchable gas sensor using nanocomposites

Posted By Graphene Council, Friday, August 28, 2020
A stretchable, wearable gas sensor for environmental sensing has been developed and tested by researchers at Penn State, Northeastern University and five universities in China.

The sensor combines a newly developed laser-induced graphene foam material with a unique form of molybdenum disulfide and reduced-graphene oxide nanocomposites. The researchers were interested in seeing how different morphologies, or shapes, of the gas-sensitive nanocomposites affect the sensitivity of the material to detecting nitrogen dioxide molecules at very low concentration. To change the morphology, they packed a container with very finely ground salt crystals.

Nitrogen dioxide is a noxious gas emitted by vehicles that can irritate the lungs at low concentrations and lead to disease and death at high concentrations.

When the researchers added molybdenum disulfide and reduced graphene oxide precursors to the canister, the nanocomposites formed structures in the small spaces between the salt crystals. They tried this with a variety of different salt sizes and tested the sensitivity on conventional interdigitated electrodes, as well as the newly developed laser-induced graphene platform. When the salt was removed by dissolving in water, the researchers determined that the smallest salt crystals enabled the most sensitive sensor.

“We have done the testing to 1 part per million and lower concentrations, which could be 10 times better than conventional design,” says Huanyu Larry Cheng, assistant professor of engineering science and mechanics and materials science and engineering. “This is a rather modest complexity compared to the best conventional technology which requires high-resolution lithography in a cleanroom.”

Ning Yi and Han Li, doctoral students at Penn State and co-authors on the paper in Materials Today Physics, added, “The paper investigated the sensing performance of the reduced graphene oxide/moly disulfide composite. More importantly, we find a way to enhance the sensitivity and signal-to-noise ratio of the gas sensor by controlling the morphology of the composite material and the configuration of the sensor-testing platform. We think the stretchable nitrogen dioxide gas sensor may find applications in real-time environmental monitoring or the healthcare industry.”

Other Penn State authors on the paper, titled “Stretchable, Ultrasensitive, and Low-Temperature NO2 Sensors Based on MoS2@rGO Nanocomposites,” are Li Yang, Jia Zhu, Xiaoqi Zheng and Zhendong Liu.

Tags:  composite  Graphene  graphene oxide  Healthcare  Huanyu Larry Cheng  nanocomposites  Northeastern University  Penn State  Sensors 

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New Device Can Measure Toxic Lead Within Minutes

Posted By Graphene Council, Wednesday, August 26, 2020
Rutgers researchers have created a miniature device for measuring trace levels of toxic lead in sediments at the bottom of harbors, rivers and other waterways within minutes – far faster than currently available laboratory-based tests, which take days.

The affordable lab-on-a-chip device could also allow municipalities, water companies, universities, K-12 schools, daycares and homeowners to easily and swiftly test their water supplies. The research is published in the IEEE Sensors Journal.

“In addition to detecting lead contamination in environmental samples or water in pipes in homes or elementary schools, with a tool like this, someday you could go to a sushi bar and check whether the fish you ordered has lead or mercury in it,” said senior author Mehdi Javanmard, an associate professor in the Department of Electrical and Computer Engineering in the School of Engineering at Rutgers University–New Brunswick.

“Detecting toxic metals like lead, mercury and copper normally requires collecting samples and sending them to a lab for costly analysis, with results returned in days,” Javanmard said. “Our goal was to bypass this process and build a sensitive, inexpensive device that can easily be carried around and analyze samples on-site within minutes to rapidly identify hot spots of contamination.”

The research focused on analyzing lead in sediment samples.  Many river sediments in New Jersey and nationwide are contaminated by industrial and other waste dumped decades ago. Proper management of contaminated dredged materials from navigational channels is important to limit potential impacts on wildlife, agriculture, plants and food supplies. Quick identification of contaminated areas could enable timely and cost-effective programs to manage dredged materials.

The new device extracts lead from a sediment sample and purifies it, with a thin film of graphene oxide as a lead detector. Graphene is an atom thick layer of graphite, the writing material in pencils.

More research is needed to further validate the device’s performance and increase its durability so it can become a viable commercial product, possibly in two to four years.

This project was done in collaboration with the Department of Electrical and Computer Engineering and Rutgers’ Center for Advanced Infrastructure and Transportation (CAIT). It was funded by CAIT, the USDOT-University Transportation Research Center–Region II.

Tags:  Graphene  graphene oxide  Mehdi Javanmard  Rutgers University  Sensors  water purification 

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Chrysalis-shaped graphene oxide cathodes make magnesium batteries cleaner and greener

Posted By Graphene Council, Friday, August 14, 2020
Scientists at Graphene Flagship partners the University of Padova, the University of Trieste and CNR-IMM, Italy, in collaboration with researchers from other European institutions, have developed a new strategy to boost the performance of magnesium-based rechargeable batteries. Combining vanadium and graphene oxide, they obtained a high-power cathode that shows excellent promise for sustainable energy storage.

Rechargeable batteries are widespread in modern electronics, as they can repeatedly accumulate, store and discharge energy through a reversible electrochemical reaction. This makes them vital for the lasting function of mobile phones, laptops and electric vehicles, all of which endure hundreds of charge cycles over their lifetime. Typical rechargeable batteries are made using lithium anodes, but magnesium anodes have a number of properties that make them promising alternatives.

"Several factors make magnesium-based rechargeable batteries attractive," begins first author Vito Di Noto, from Graphene Flagship partner the University of Padova, Italy. "They have a higher volumetric capacity than those made with lithium, and they can be safely handled in air." Moreover, magnesium is a cheaper and more abundant raw material. "In fact, it is one of the most abundant elements in the Earth's crust," explains Di Noto. Magnesium anodes also represent a safer alternative: they are less prone to dendrite formation, a phenomenon that can lead to short circuits and, in rare circumstances, battery explosion.

However, the development of magnesium batteries has been hindered by their poorly performing cathodes, which often result in significantly worse-performing devices than their lithium-based counterparts.

To tackle this challenge, the researchers developed an all-new cathode material for magnesium batteries based on graphene and vanadium oxides. The material exhibits a peculiar chrysalis-like microstructure that enhances the performance of the battery. Graphene oxide flakes encircle a nanoparticle core based on vanadium oxide: "the structures are fixed together thanks to a layer of ammonium ions," explains Di Noto. The chrysalis-like material combines vanadium's high redox activity and graphene oxide's electrical properties. "This yields a cathode with very strong chemical and electrochemical stability," he continues.

The new graphene-enhanced cathode has allowed researchers to operate a coin cell at very high current rates and power, with a promisingly high specific capacity. "The synergistic effects provided by graphene oxide, vanadium and the chrysalis morphology enable the coin cell to operate with 500% more sustained current than state-of-the-art magnesium batteries, at a 40% higher working potential." These properties could be exploited to make batteries for mobile devices that last longer between charges or deliver more power.

Furthermore, magnesium's high natural abundance means that magnesium-based rechargeable batteries could be an environmentally friendly solution. This work brings graphene batteries one step closer to the market. "Magnesium is one of the most sustainable metals in the world, and can be easily recycled – up to 100%," Di Noto continues. "We hope that our work will contribute to the turning point towards the establishment of a greener and more sustainable energy economy."

Daniel Carriazo, Graphene Flagship Work Package Deputy for Energy Storage, comments: "As the production of lithium-ion batteries increases exponentially to fulfil the demand of new applications, it is necessary to develop alternative energy storage technologies made out of accessible and environmentally friendly materials." Carriazo says that this work shows very promising results when a vanadium-based graphene composite is used as the positive electrode in a potassium-ion battery. "The incorporation of graphene enables fast charging, overcoming one of the limitations associated with this technology," he continues.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "Graphene and layered materials have recognised potential in energy storage, and graphene is already present in commercial devices. This approach tackles the need to produce more environmentally sustainable batteries, and thanks to the introduction of graphene oxide into the cathode, shows how magnesium could be used, which is easier to recycle. Sustainable development always guides the technology and innovation roadmap of the Graphene Flagship, and this research is yet another promising example." 

Tags:  Andrea C. Ferrari  Daniel Carriazo  Graphene  Graphene Flagship  graphene oxide  University of Padova  Vito Di Noto 

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Researchers Propose Novel Method for Heavy Metal Sensing with Graphene Oxide

Posted By Graphene Council, Wednesday, July 22, 2020
With the development of technology and the progress of urbanization, heavy metal pollution has become a huge problem facing society today. At the same time, heavy metal detection has become one of the most concerning issues of the public.  

How to solve this problem? A research team led by Prof. Dr. KONG Depeng from the Xi'an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences (CAS) has given us a reliable answer. 

They proposed a Ni2+ heavy metal sensor based on graphene oxide (GO) functionalized micro-tapered long-period fiber grating (MTLPG) where light-matter interaction is enhanced. The results were published in Applied Physics Express.  

How did they do it? 

In fact, the long-period fiber gratings (LPFGs) and two-dimensional (2D) materials have shown extensive applications in optical sensing, optical communications, light processing, chemical, and biochemical sensors.  

Besides, GO as a very important derivate of graphene, it has similar properties to graphene. But the significant difference between GO and graphene is that hydrophilic oxygen functional groups such as epoxy, hydroxyl, and carboxyl are distributed on the basal planes and edges of GO. These oxygen-containing groups enable GO to be water-soluble and hydrophilic. 

In this work, they deposited a GO supernate on the cylindrical surface of the MTLPG, which fabricated by a CO2 laser heating source, by a chemical bonding method associated with physical adsorption and assisted by the optical tweezer effect.  

In terms of results, GO-fiber optic architecture features high sensitivity, real-time monitoring, stability, and reusability, and it provides a remarkable analytical platform for chemical and biochemical applications. 

Tags:  2D materials  Graphene  graphene oxide  KONG Depeng  Sensors  Xi'an Institute of Optics and Precision Mechanics 

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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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