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​Graphene: how it became the new Teflon of last decade

Graphene is one of the most used materials in today’s world and with all the exceptions that it is being used, it is being proven as one of the best materials for almost all industries. Ever since its usage began, it has evolved in various ways all of which are remarkable and an excellent working choice. The characteristics and properties of graphene are highly unique which enables it to perform the given tasks so perfectly.

However, graphene utilized as a Teflon has also proved to be nothing less than a perfect material as it is benefiting the industry so much that its production has massively increased.

Introduction

In 2004, graphene, a new material, was isolated for the first time. Graphite’s single layer makes up graphene, and it is the same graphite that is utilized in the pencil lead.Geim and Novoselov repeatedly used sticky tape to separate graphite fragments until one-atom-thick flakes were created by them and that’s how they isolated the graphene for the first time.

Remarkable structure

The discovery of graphene may look simple but graphene’s structure is excellent. A two-dimensional crystalline structure is possessed by graphene; the flat layer of atoms consists of hexagonal rings of carbon, giving a honeycomb structure. 0.33 nanometer is the approximate thickness of the layer itself. There was an old belief before graphene that due to thermal instability, the existence of 2-dimensional molecules is not possible.

Unique properties

Graphene has remarkable characteristics due to its structure. According to experiments, graphene is the most robust material ever known. Because of the lack of defects in graphene and Graphene’s strong electrostatic forces, graphene is at least 200 times stronger as compared to steel. Due to graphene’s flat and hexagonal structure, graphene is a remarkable conductor of electricity and heat as there is very little resistance to the movement of electrons.

Despite being 200 times stronger than steel, the weight of graphene is only 0.77 milligrams per square meter, making it a lightweight material. According to researches, graphene is extremely flexible, as it can stretch to 25 percent of its original length without any breakage. All of these characteristics of graphene are excellent, even individually but all of them combined in one material, that’s what made graphene a remarkable material with potential applications in possibly all of the various kinds of industries.

Characteristics of graphene

Electronic properties

Graphene has an extremely high electrical conductivity and its most useful characteristic is that it is a zero-overlap semimetal (with both holes and electrons as charge carriers).

A total of 6 electrons are possessed by the carbon atoms; the outer shell has 4 electrons whereas the inner shell has 2. Every individual carbon atom has 4 outer shell electrons that are available for chemical bonding. Each atom in graphene is linked to 3 of the other carbon atoms on a 2-dimensional plane, resulting in 1 electron being free for electronic conduction in the 3rd dimension.

Pi (π) electrons are these highly mobile electrons and they are placed below and above the graphene sheet. There is an overlapping of these pi orbitals as they help in improving the carbon to carbon bonds in graphene. The anti-bonding (conduction and valance bands) and bonding of these pi orbitals fundamentally dictate graphene’s electrical characteristics.

Mechanical properties

Graphene has many remarkable characteristics but its inherent strength is one of its best. Graphene’s carbon bonds strength makes graphene the strongest material that has ever been discovered. 130,000,000,000 Pascals (or 130 gigapascals) is graphene’s ultimate tensile strength whereas the tensile strength is 375,700,000 for Aramid (Kevlar), or 400,000,000 for A36 structural steel.

Graphene is not only remarkably strong but is also very light as it weighs 0.77 mg/m². If you take 1 m² of paper it will be roughly 1000 times heavier than graphene. There are sayings that one single sheet of graphene (thickness of only 1 atom) can be enough to cover a whole football field but it will still weigh less than a gram.

Optical properties

Another interesting and remarkable characteristic of graphene is its capability of absorbing a large 2.3% white light, specifically keeping in mind that its thickness is only 1 atom. This is probably possible because ofits aforementioned electronic characteristics; the electrons functioning like massless charge carriers having extremely high mobility.

It has been proven in the past that the amount of absorbed white light is based on the Fine Structure Constant. If we add another graphene layer to it, the absorption of white light will be increased by almost 2.3%. The opacity of graphene is πα ≈ 2.3% and over the visible frequency range, it equates to a universal dynamic conductivity value of G=e2/4ℏ (±2-3%).

 

What is graphene currently being utilized for?

The characteristics of graphene are the reason for graphene’s huge amount of usages in numerous applications. It has been more than 15 years since its isolation for the first time, the market has been filled with many graphene products and graphene’s expansion isn’t stopping and going into new sectors.

Equipment

Graphene first appeared in applications with low entry barriers like sports equipment. For instance, in Korea, standard graphene displayed an extremelylightweight bicycle frame infused with graphene. Graphene-enhanced bicycle tires have been launched by Vittoria and Goodyear. Grays incorporated their hockey sticks with graphene. The flexibility and strength of graphene were used bythe multimillion-dollar company Head in the same industry as they incorporated graphene into the frame of a new tennis racket line.

Thermal regulation

Beyond sports equipment, graphene was also adopted early in sports clothing because of graphene’s thermal regulation and durability in textiles. A sportswear brand, known as Inov-8, worked with the National Graphene Institute that was based in Manchester, United Kingdom, for releasing the first-ever graphene-enhanced running shoe of the company in 2018, which eventually expanded into the full range.

According to Inov-8, in comparison with regular rubber, graphene-enhanced rubber is 50% more elastic and 50% more strength. Other than the sports industry, companies like Graphene-X have been taking to Kickstarter for launching the daily usage of graphene-enhanced jackets and pants with great success.

Coatings

Graphene has been trialed in one area and that is coatings, but there has been the development of both barrier coatings and electronically conductive coatings, a huge amount of which have been seen to be utilized to protect the hulls of the ships.

From Graphene to Fluorographene

Two complementary approaches have been employed by us to obtain FG. One of them is GrF’s mechanical cleavage. Bulk GrF is obtained by using harsh fluorination conditions, leading to many defects in the structure, which is the reason for its monolayers being prone to rupture and very fragile. Moreover, extraction of 1 μm GrF monolayers was successful and they were then utilized in Raman studies.

Configurations of graphone, fluorographene and fluorographone: (a) 4×4... |  Download Scientific Diagram

Preparation

There was a need for an alternative approach for preparing large FG samples that most of our experiments are suited for, that’s why the need was both a necessity and a convenience. In that alternative approach, graphene was exposed to atomic F. Xenon difluoride’s decomposition forms atomic F. Graphene is stable in molecular F2 at room temperature. A clear advantage is possessed by this approach as using XeF2 ignores the potential damage that the ion bombardment can cause. In addition, another simple low-hazard procedure is the implementation of fluorination by XeF2 that can be carried out in any laboratory.

Standard cleavage technique

In short, the standard cleavage technique was used to make large graphene crystals of more than 100 μm in size. It seemed impossible to use Si wafers in the fluorination procedures due to the ability of XeF2 to rapidly etch to Si and easily diffuse through even amorphous SiO2 thick layer. Also, chemically inert support is necessarily needed and complete fluorination needs the graphene from both sides to be exposed, so cleaved crystals were transferred by us on the Au grids that are utilized for TEM (transmission electron microscopy).

Use of Au grids

Quantifoil is a lithographically patterned polymer film and they cover the Au grids. Au grids covered with Quantifoil were used for providing sufficient support to graphene. Then the samples were heated to 70 degrees Celsius after being placed with XeF2 in a Teflon container (reaction was speeded up by the elevated T whereas Au grids were destroyed by the usage of even higher T). The products were later utilized for Optical, TEM, and Raman studies and probed by AFM (atomic force microscopy).

FG was transferred back from TEM grids on an oxidized Si wafer for electrical characterization. Either the Capillary transfer method was used or the grids were pressed against the wafer to do the latter.

Raman Spectroscopy of Fluorinated Graphene

Due to the consecutive exposures to atomic F, the Raman spectra of graphene goes through evolution and it can be seen, along with an emergence of a prominent D peak, indicating the presence of the atomic-scale defects. The double-resonance band (G’ or 2D peak) disappears however the G and D peak intensities stay almost the same as the time of fluorination increases. Gradually, all the Raman features disappear on an increase in the time of fluorination (some days).

Difference of behavior

There is a radical difference between the behavior of this and the behavior of hydrogenated graphene. In hydrogenated graphene, the strength of the 2-dimensional band stays the same. A Raman spectrum is displayed by the partially fluorinated graphene (10-20 hours) and it is similar to the Raman Spectra of GO that has a comparatively small 2D band and relatable intensities of the D and G peaks. The disappearance of all of the characteristic peaks ensures more dramatic changes caused by fluorination like those reported for GO and hydrogenated graphene.

FF graphene’s optical transparency to our green laser light was completely explained. Theories claim that GrF should possess Eg ≈3.5 eV. There are no previous measurements of this gap mainly because the material normally comes in opaque white powder shape.

Comparison between the observations

Comparing FG’s observed spectrum with Raman spectra of bulk GrF and a monolayer that’s extracted is instructive. The former two are identical and are corresponding to the partially fluorinate graphene’s spectrum (near a state that’s obtained after 20 to 30 hours) within the noise level, which is surprising as normally, GrF displays fluorine-to-carbon ratios that are more than unity (the ratio is ≈1.1 here) and are fully fluorinated according to assumptions.

Non-stoichiometric F/C ratios are because of the presence of various structural defects, allowing more carbon bonds to be terminated with fluorine (CF3 and CF2 bonding). According to Raman data, GrF planes stays not completely fluorinated despite F/C >1, and GrF spectra should not be used as it is a reference for obtaining a FF state.

Stability and Structure

TEM was used to obtain structural information regarding FG. There is an electron diffraction micrograph for a FF membrane and it displayed a remarkable hexagonal symmetry and has the same quality which is possessed by pristine graphene. FG’s unit cell is a little expanded as compared to the cell of graphene, in comparison with the case of the hydrogenated graphene as a compressed lattice was displayed by it. The lattice of the FG is similar for all the studied FF membranes and its expansion was isotropic.

TEM’s limited accuracy

Expansion of the recorded values is because of the TEM’s limited accuracy in d’s precision measurements. FG has a unit cell almost 1 percent more than graphene. There is supposed to be an increase in d, as fluorination results in sp3-type bonding, corresponding to a larger interatomic distance as compared to sp2. Although, the increase is less than the increase in GrF, where, as compared to graphite, d was 2.8-4.5% larger.

In FG, the smaller d is because of the 2D sheet’s possibility to undergo strong interatomic corrugations if the surrounded 3D matrix doesn’t restrict out-of-plane displacements of carbon atoms, same as graphene’s case which predicts the value of d to be close to the one that was observed for FG.

The Raman signatures

FG stability was studied at increased T by the Raman signatures for partial and complete fluorination. The process was largely reversible for graphene that has been fluorinated for only some hours. After fluorination for more than 20 hours, even at 450 degrees Celsius, the annealing was not able to restore the 2D peak but the G and D peaks significantly grew and became similar in intensity.

For FF graphene

Raman spectra of FF graphene didn’t change and losses of F turned discernable only for extensive annealing at more than 400 degrees Celsius. FG was also stable in liquids like propanol, acetone, water, etc. under ambient conditions. Despite the tests not being exhaustive, the chemical stability of FG is the same as the chemical stability of Teflon and graphite fluoride.

There were also investigations on the digraphite fluoride (C2F), which was a stage II intercalation graphite compound, which enabled comparatively easy exfoliation but it was not stable in the liquids that are mentioned above, for instance, propanol, acetone, and water. Even under ambient conditions, its few and single-layer crystals were not stable, lessening fastly to the state the same as lessened GO or strongly damaged graphene.

Fluorination

Micromechanical cleavage was used to prepare large graphene crystals on top of an oxidized silicon wafer. Graphene had to be transferred onto nickel and gold grids that could sustain the procedures of fluorination due to Si’s high reactivity with the atomic fluorine, also allowing the exposure of graphene from both of the sides to F. Firstly, a thin polymer layer was deposited with graphene crystals on top of the wafer.

During further processing, mechanical support was provided by the PMMA film for the graphene. After that, the layer of SiO2 was etched away in a solution of 3% potassium hydroxide, lifting off the PMMA film together with graphene crystals. The film floating in the water was picked up on a TEM grid after a complete cleaning in the deionized water.

Dissolving the acetone

At last, a critical point dryer was used to dry the samples and PMMA was dissolved in acetone. There is an optical micrograph that shows oneof our Quantifoil-Au grids. 7 μm is the size of the Quantifoil mesh, and the complete Au cell is covered by graphene. At 70 C, graphene membranes on Quantifoil were exposed to XeF2. This procedure took place in a glove box for leaving no chance of moisture as that would lead to the production of HF.

One of the fully covered Quantifoil cells with FG can be seen in a TEM micrograph, and it is as small dust particles within the aperture. Graphene crystals that were cleaved on the top of quartz wafers were also fluorinated, and these samples were utilized for optical spectroscopy measurements.

Increasing fluorination’s speed

According to findings, a major increase in speed is possible by utilizing higher T. Also, a PTFE-lined stainless steel container was utilized. Graphene needs to be placed on the Ni grids as the requirements of the high-T procedure. At 200 C, unlike gold, Ni grids can sustain XeF2. If this approach is used, then we can reach the FF state in a few hours instead of weeks.

According to the above report, showing changes gradually from graphene to fluorographene was more instructive, and following them utilizing the low T fluorination was easier. Using higher T has significance when it comes to applications, and it also confirms that the FF state that was told above was final. At 200 C, prolonged fluorination resulted in the same transport, optical, and Raman characteristics as those achieved at 70 C for extremely long exposures.

Fluorographene paper 2D Teflon

Graphene and graphene laminates were fluorinated on SiC for demonstrating that scaling up FG’s production for applications is possible. Filter deposition was used to attain laminates from a graphene suspension that was made by graphite’s sonication. At 200 C, laminate was exposed to XeF2 for speeding up the process of fluorination that involves F’s diffusion between crystallites.

There was no change in the saturated state even with more fluorination. 10 hours were enough for reaching a saturated state. Graphite can’t be fluorinated under conditions like these. GrF can be produced by using higher T, implying that the presence of the multilayer graphene in laminates possibly stays not FF.

PDF> Fluorographene: a two-dimensional counterpart of Teflon. | Semantic  Scholar

Results

This resulting product is different in visuals than the original material as the original one is black and has a metallic shine whereasFG paper is yellowish, and is also transparent. FG is a broad gap material according to this direct visual proof. An onset was displayed by the light transmission at ≈3.1 eV.

Ranges of gaps

The color of GO and FG is somewhat alike. GO paper turns nontransparent. Under ambient conditions, FG paper is stable like Gr, and it is extremely hydrophobic.

Molecular electrostatic potential (MESP)

The MESP of GO/PANi/Teflon structures, and GO, PANi/Teflon structures can be used to describe the stability of the surface of GO/PANi/Teflon, and GO, PANi/Teflon structures. Their molecular reactivity is described by the MESP.

Color mapping

Differently charged regions are presented in different colors. The color of the regions will be red if they are extremely reached with electrons, for instance, highly electro-negative regions. However, the neutral regions are yellow whereas the blue color is for the high positivity regions. Due to GO’s interaction with PANi/Teflon’s supposed structure, the red color’s intensity increased.

Enhancement in the activity

This interaction caused a significant enhancement in the reactivity and an increase in the electronegativity of GO and the blended PANi/Teflon. Such changes have ensured that these GO/PANi/Teflon models are suitable for many applications like drug delivery systems, sensors, and energy storage materials.

Conclusion

Conversion of graphene to Teflon has not been an easy road to follow but the results that came out have been exceptional. A lot of industries are now using this material and it is because of the properties that it possesses which play a major role in making its characteristics stronger.

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