Production Of High-quality Graphene Research Articles

Study of Anodic Electrochemical Exfoliation of Graphite Under Acidic Electrolyte for Scalable Production of Graphene

Graphene has attracted many attentions due to its exceptional mechanical, thermal, electrical, and chemical properties. There has been active research for scalable production of high-quality graphene from graphite. Anodic electrochemical exfoliation of graphite is one of the promising methods for fast and simple production of graphene at low cost with moderate quality. Sulfate containing electrolytes are known for highest exfoliation efficiency among anions, however mechanistic details of exfoliation process remain poorly understood. Here, the role of sulfate anion during the exfoliation process is studied by comparing with other anions under acid conditions. We reveal that water molecules are the main source of gaseous species that exfoliates graphite via mass spectroscopy. The formation and binding energy of anions between graphene layers are also calculated by DFT computations. The fundamental understanding of the exfoliation mechanism of graphite can be applicable to the electrochemical production of other 2D materials.

Ultrafast microwave reduction process for high-quality graphene foam with outstanding electromagnetic interference shielding and good adsorption capacity

Mass production of high-quality graphene and its assemblies is a key step for practical application of graphene in many fields. Herein, a facile and ultrafast microwave reduction process with the aid of catalytic pre-reduction at room temperature is reported towards the preparation of high-quality graphene foam. The catalytic pre-reduction aided microwave method enables an efficient reduction process within a few seconds, whilst the original architecture of three-dimensional graphene foam remains, along with a superhigh C/O ratio (31.3), low lattice defects, and significant electrical conductivity (50,000 S/m) as well as excellent dispersibility and processability. Due to the intrinsic structure advantages of MW-H-GF, when applied to electromagnetic shielding, it could deliver a shielding efficiency of around 50 dB in the low frequency region while 70 dB in the high frequency region, indicating excellent electromagnetic shielding performance. Furthermore, MW-H-GF also demonstrated remarkable oil adsorption capacity which can reach an adsorption capacity of 480 times chloroform of the self-weight.

Direct Liquid Phase Exfoliation of Graphite to Produce Few-Layer Graphene by Microfluidization.

Graphene has attracted a great number of attentions due to the excellent physical and chemical properties. For the convenience of investigations and applications, it is crucial to produce the grapheme with high-quality and high-yield by an easy-obtained method. In this research, a promising method is demonstrated to produce a high-concentration few-layer graphene (FLG) dispersion by direct microfluidization in water/surfactant systems. The effects of surfactant selection, chamber pressure and microfluidization cycles on the graphitic material exfoliation efficiency are systematically studied. The FLG concentration and the quality of the as-prepared FLG were determined by a series of characterizations. The graphene dispersions, with an average lateral size of 0.6 μm and a few-layer structure, were stabilized by surfactants at a high concentration of up to 1.7 mg/mL and exhibited a relatively high quality (ID/IG = 0.07-0.56, C/O ~ 19.36) within a processing time of a few hours. This method should facilitate the mass production of high-quality graphene by liquid-phase exfoliation and promote the industrial application of graphene.

Tunable solution-processable anodic exfoliated graphene

Electrochemical exfoliation of graphite is an emerging approach to provide large scale, low cost, efficient and reliable production of high quality, low defect and solution-processable graphene. The use of a sulphuric acid electrolyte allows rapid and efficient anodic graphite exfoliation. However, the highly oxidative process in this electrolyte promotes hydroxyl radicals. These degrade the carbon lattice structurally, hence compromising the quality of graphene produced. Here, we report a simple and effective way to overcome this challenge; the use of an additive to scavenge hydroxyl radicals. The addition of a small volume percentage of ethylene glycol acts to scavenge hydroxyl radicals. Furthermore, it is readily adsorbed onto the intercalated graphite surface, and therefore also serves as an oxidative prevention layer. As a result, the damage on the graphitic structure is greatly minimized, resulting in a low defect and high quality few layers graphene, with high yield (>80%) and a low degree of oxidation with a C/O ratio of 16.9. The graphene solution-processability is readily tuned and improved with stable dispersions up to 5mg/mL in dimethylformamide. This new strategy of a simple and low cost dual purpose additive in scavenging hydroxyl radicals and providing an oxidative prevention layer to efficiently tune the graphene quality and solution-processability; holds promise in industrial scale mass production of high quality graphene.

Mass production of low-boiling point solvent- and water-soluble graphene by simple salt-assisted ball milling.

Developing a mass production method for graphene is essential for practical usage of this remarkable material. Direct exfoliation of graphite in a liquid is a promising approach for production of high quality graphene. However, this technique has three huge obstacles to be solved; limitation of solvent, low yield and low quality (i.e., multilayer graphene with a small size). Here, we found that soluble graphite produced by mechanochemical reaction with salts overcomes the above three drawbacks. Soluble graphite was exfoliated into monolayer graphene with more than 10% yield in five minutes of sonication. The modified graphite was easily exfoliated in a low-boiling point solvent such as acetone, alcohol and water without the aid of a surfactant. Molecular simulation revealed that the salt is adsorbed to the active carbon at the graphite edge. In the case of weak acid salts, the original bonding nature between the alkali ion and the base molecule is retained after the reaction. Thus, alkali metals are easily dissociated in a polar solvent, leading to negative charge of graphene, enabling the exfoliation of graphite in low boiling point solvents. The approach proposed here opens up a new door to practical usage of the attractive 2D material.

Green and facile production of high-quality graphene from graphite by the combination of hydroxyl radicals and electrical exfoliation in different electrolyte systems.

A novel, simple and efficient method by the combination of hydroxyl radicals and electrical exfoliation of graphite for the green production of high-quality graphene from graphite was first developed in our self-manufactured exfoliation apparatus. In this work, we focused on the investigation of the roles of various electrolyte systems for the exfoliation of graphite. Sodium chloride, sodium hydroxide, poly vinyl pyrrolidone (PVP), dodecyl trimethyl ammonium bromide (DTAB) and sodium dodecyl benzene sulfonate (SDBS) were tested as the electrolyte. The yields of the graphene product in sodium hydroxide, PVP, DTAB, sodium chloride and SDBS electrolyte system were 32.9%, 34.0%, 45.2%, 77.5% and 83.5%, respectively. The experimental result demonstrated that graphite can be successfully exfoliated to graphene in these electrolytes, with SDBS showing the best exfoliation effect. We further investigated the effects of process parameters on the graphite exfoliation in the SDBS system by single factor experiments. The obtained optimal process parameters were as follows: graphite dosage, 5.0 g; SDBS solution concentration, 10.0% (w/v); applied current strength, 10 mA; air flow, 1.0 L h−1; and exfoliation time, 3 h. At these conditions, the yield of the graphene product was 89.7%. TEM results revealed that the graphene product possessed the characteristic features of a thin-layer graphene sheet. XRD results showed that the graphene products still maintained the structures of carbon atoms or molecules. FT-IR and Raman results indicated that the products exhibited the characteristic peaks and the absorption peaks of graphene. AFM test results revealed that the layer number of graphene product obtained was about 2, while the layer numbers of the graphene products obtained from sodium hydroxide, PVP, DTAB and sodium chloride systems were 30, 20, 4 and 3, respectively, at the same experimental conditions. The observed exfoliation effect in the SDBS system was due to its good electrical conductivity, which was favorable for the formation of hydroxyl radicals in exfoliation. Furthermore, SDBS has good hydrophilic properties and can enable even dispersion of graphite in the system. These two effects facilitated the exfoliation of graphite to form good-quality graphene. SDBS as the electrolyte did not corrode the electrode, and it could be recycled; also, it does not pollute the environment and reduces the production cost, which is favorable for mass production.

Efficient room-temperature production of high-quality graphene by introducing removable oxygen functional groups to the precursor.

The excellent solution-processability of graphene oxide (GO) has provided a collection of strategies for the construction of functional graphene assemblies. To improve the performance of graphene-based materials, structurally intact GO should be prepared as a precursor for high-quality graphene; however, solution chemical methods have been constantly challenged by a structural integrity versus fabrication yield trade-off. Here, we report a wet chemical method for the high-efficiency production of a high-quality graphene oxide precursor, with all steps conducted at room-temperature. The functionalization of graphite was performed under temperature and water content control in a concentrated sulfuric acid-potassium permanganate system, and the resulting GO showed a monolayer yield of over 120%. We show that the increased production yield comes from the high functionalization degree and, more interestingly, the functional groups on GO were proven to be removable upon reduction with hydroiodic acid, which produced high-quality graphene-based materials.

Biosynthesis and characterization of graphene by using non-toxic reducing agent from Allium Cepa extract: Anti-bacterial properties

Graphene based materials have attracted huge interest in recent years due to their outstanding properties and applications in various fields including bioengineering, electronics, nanotechnology, composite materials and many more. Despite numerous reports on synthesis of graphene, the mass production of high quality graphene in an inexpensive and eco-friendly method has remained as a challenge. In this work, we present a simple and green method for biosynthesis of graphene by using nontoxic reducing agent from Allium Cepa (onion) extracts. Modified Hummers' method was used to synthesis the Graphene oxide (G0) and extracts from Allium Cepa was used as reducing agent. The prepared graphene was analyzed by Raman spectroscopy, XRD, FTIR, SEM, TEM and XPS. The experimental results showed that GO was successfully reduced to graphene using onion extract. The Raman spectroscopy results, XPS results and XRD results confirmed the reduction of GO to graphene. The SEM and TEM results also reconfirmed the reduction of GO into graphene, where GO exhibited different morphologies, i.e. hexagonal larger sheets than graphene. The antibacterial properties of the graphene were studied against two gram-negative and gram-positive bacteria. Graphene inhibited cell growth, which proves that our prepared graphene can be useful as an antimicrobial agent against different microorganisms. This work thus reports the design of a novel, facile synthetic route for a new production method of graphene.

Optical constants of restored and etched reduced graphene oxide: a spectroscopic ellipsometry study

Structural and defective information in graphene are usually detected by Raman spectroscopy and electrical techniques; however, less attention has been paid to a spectroscopic ellipsometry investigation on graphene. Here, optical responses of graphene grown by chemical vapor deposition (G), reduced graphene oxide (RGO), restored RGO, and etched RGO are characterized by spectroscopic ellipsometry. The Lorentz oscillator model is used to fit the ellipsometric parameters. The resulting optical constants, refractive index and extinction coefficient are higher in less defective graphene, i.e. G and restored RGO. The results can be correlated with the Raman spectroscopic observation – fewer defects leading to higher density and more optical absorption. Our investigation opens a new angle of view to probe the structural information in the defective graphene and thus favors the production of high-quality graphene.

High-yield production of high-quality graphene by novel electrochemical exfoliation at air-electrolyte interface

An end-face electrochemical exfoliation (EFEE) strategy for scalable graphene preparation is established, which has the confined area and thus guarantees uniform current density at the air-electrolyte interface, different from the traditional electrode set-up. The production of graphene is presented that has excellent yield (∼100%) and outstanding conductivity (3.8 × 104 S m−1 with a graphene loading of 0.7 mg cm−2). The exfoliation mechanism is also discussed.

Salt and water co-assisted exfoliation of graphite in organic solvent for efficient and large scale production of high-quality graphene

Graphene has attracted enormous attention due to its unique physical properties and attractive applications in many fields. However, it is an ongoing challenge to develop a facile and low-cost method for the large scale preparation of high-quality graphene (HQGr). In this work, we have developed an improved liquid-phase exfoliation method to mass produce HQGr. This method is quite simple but efficient by exfoliation of graphite in organic solvent with the co-assistance of sodium citrate and water. Remarkably, the concentration of as-exfoliated HQGr was as high as 0.71 mg/mL under optimal conditions, while the oxygen content in HQGr was only 2.39%. After annealing at 500 °C for 2 h in argon atmosphere, the mean conductivity of annealed HQGr was as high as 1.4 × 104 S m−1. Therefore, this facile method for liquid-phase exfoliation of graphite has excellent potential in the industrial-scale production of HQGr for numerous applications in energy storage, optical and electronic fields.

Thickness-controlled direct growth of nanographene and nanographite film on non-catalytic substrates

Metal-catalyzed chemical vapor deposition (CVD) has been broadly employed for large-scale production of high-quality graphene. However, a following transfer process to targeted substrates is needed, which is incompatible with current silicon technology. We here report a new CVD approach to form nanographene and nanographite films with accurate thickness control directly on non-catalytic substrates such as silicon dioxide and quartz at 800 °C. The growth time is as short as a few seconds. The approach includes using 9-bis(diethylamino)silylanthracene as the carbon source and an atomic layer deposition (ALD) controlling system. The structure of the formed nanographene and nanographite films were characterized using atomic force microscopy, high resolution transmission electron microscopy, Raman scattering, and x-ray photoemission spectroscopy. The nanographite film exhibits a transmittance higher than 80% at 550 nm and a sheet electrical resistance of 2000 ohms per square at room temperature. A negative temperature-dependence of the resistance of the nanographite film is also observed. Moreover, the thickness of the films can be precisely controlled via the deposition cycles using an ALD system, which promotes great application potential for optoelectronic and thermoelectronic-devices.

Tension-controlled single-crystallization of copper foils for roll-to-roll synthesis of high-quality graphene films

It has been known that the crystalline orientation of Cu substrates plays a crucial role in chemical vapor deposition (CVD) synthesis of high-quality graphene. In particular, Cu (1 1 1) surface showing the minimum lattice mismatch with graphene is expected to provide an ideal catalytic reactivity that can minimize the formation of defects, which also induces larger single-crystalline domain sizes of graphene. Usually, the Cu (1 1 1) substrates can be epitaxially grown on single-crystalline inorganic substrates or can be recrystallized by annealing for more than 12 h, which limits the cost and time-effective synthesis of graphene. Here, we demonstrate a new method to optimize the crystalline orientations of vertically suspended Cu foils by tension control during graphene growth, resulting in large-area recrystallization into Cu (1 1 1) surface as the applied tension activates the grain boundary energy of Cu and promotes its abnormal grain growth to single crystals. In addition, we found a clue that the formation of graphene cooperatively assists the recrystallization into Cu (1 1 1) by minimizing the surface energy of Cu. The domain sizes and charge carrier mobility of graphene grown on the single-crystalline Cu (1 1 1) are 5 times and ~50% increased, respectively, in comparison with those of graphene from Cu (1 0 0), indicating that the less lattice mismatch and the lower interaction energy between Cu (1 1 1) and graphene allows the growth of larger single-crystalline graphene with higher charge carrier mobility. Thus, we believe that our finding provides a crucial idea to design a roll-to-roll (R2R) graphene synthesis system where the tension control is inevitably involved, which would be of great importance for the continuous production of high-quality graphene in the future.

Green and facile production of high-quality graphene from graphite by the combination of hydroxyl radical and electrical exfoliation.

A novel, simple and efficient method, involving the combination of hydroxyl radicals and electrical exfoliation of graphite for the green production of high-quality graphene from graphite, was developed for the first time. The process parameters were optimized by single factor experiments; the optimal conditions were 4.0 g graphite dosage, sodium chloride solution with concentration of 5.0% (w/v), applied current strength of 10 mA, and air flow rate of 1.0 L h−1 for 3 h. Under the optimized conditions, graphite was successfully exfoliated to graphene. SEM and TEM results revealed that the graphene product has the characteristic features of a thin-layer graphene sheet. XRD results showed that the graphene products still maintained the structure of the carbon atoms or molecules. FT-IR and Raman results indicated that the products have the characteristic peaks and absorption peaks of graphene. AFM imaging results reveal that the layer number of the graphene product obtained by this method is about 3, while the graphene products from the individual hydroxyl radical oxidation and electrical exfoliation processes were 50 and 133 layers, respectively, under the same experimental conditions. The good quality of the graphene product can be attributed to the synergistic effect between the strong oxidation of the hydroxyl radicals and electrical exfoliation. The proposed method has the advantages of simple operation, mild preparation conditions, non-utilization of aggressive reagents, recycling of the reaction medium, etc. Thus, this method could serve as a green and efficient alternative for the production of graphene and its derivatives in industry.

Preparation of high-quality graphene using triggered microwave reduction under an air atmosphere

High-quality intrinsic graphene can be prepared by a simple triggered microwave reduction method under air conditions, which provides a simple and low-cost route for large-scale production of high-quality graphene.

Effect of intermittent oxygen exposure on chemical vapor deposition of graphene

Abstract

Direct exfoliation of graphite into graphene in aqueous solution using a novel surfactant obtained from used engine oil

Large-scale production of high-quality graphene is very critical for practical applications of graphene materials and devices. Exfoliation of graphite in an aqueous solution of surfactants is one of the most promising approaches to produce graphene. In this study, a novel anionic surfactant [sulfonated used engine oil (SUEO)], which was prepared from used engine oil, was employed to exfoliate the graphite nanoplatelets into graphene sheets in an aqueous solution under sonication to form a stable dispersion. The efficiency of SUEO for exfoliating and dispersing graphene was investigated and compared with that of traditional surfactants, such as sodium dodecyl sulfate, sodium dodecyl benzene sulfate, cetyl trimethyl ammonium bromide, and polyvinylpyrrolidone. Result showed that the graphene dispersion with excellent stability had a higher concentration (0.477 mg/mL) than others at 0.5 g/L optimal SUEO dosage in 4 h sonication time. The superior performance of SUEO can be attributed to its special molecular structures, whose hydrophobic moieties contain cycloalkanes/aromatics with different molecular weights and/or side chain –R with different lengths. Structural diversities are very helpful to the “jigsaw-puzzle” process on the graphene surface, where the total interfacial energy of the mixture system was minimized. Microscopic (SEM, TEM, and AFM) and spectroscopic (XRD, XPS, and Raman) measurements revealed that the dispersion consisted of few-layer graphene sheets with lower levels of defects or oxidation. This study presents a new class of dispersing agents for graphene that assists in the exfoliation process in water with high concentration and the stabilization of the graphene sheets against reaggregation.

Effect of Residual Gas Composition on Epitaxial Growth of Graphene on SiC

In recent years, graphene growth optimization has been one of the key routes towards large-scale, high-quality graphene production. We have measured in-situ residual gas content during epitaxial graphene growth on silicon carbide (SiC) to find detrimental factors of epitaxial graphene growth. The growth conditions in high vacuum and purified argon are compared. The grown epitaxial graphene is studied by Raman scattering mapping and mechanical strain, charge density, number of graphene layers and graphene grain size are evaluated. Charge density and carrier mobility has been studied by Hall effect measurements in van der Pauw configuration. We have identified a major role of chemical reaction of carbon and residual water. The rate of the reaction is lowered when purified argon is used. We also show, that according to time varying gas content, it is preferable to grow graphene at higher temperatures and shorter times. Other sources of growth environment contamination are also discussed. The reaction of water and carbon is discussed to be one of the factors increasing number of defects in graphene. The importance of purified argon and its sufficient flow rate is concluded to be important for high-quality graphene growth as it reduces the rate of undesired chemical reactions and provides more stable and defined growth ambient.

Large-scale production of high-quality graphene sheets by a non-electrified electrochemical exfoliation method

Graphene materials have recently experienced rapid developments, but large-scale production of high-quality graphene is still desired. Electrochemical exfoliation method is considered as a promising approach to meet this requirement. Here we demonstrate a non-electrified electrochemical exfoliation method to produce high-quality graphene (NEEG) sheets. By direct electrochemical reaction between graphite powders and metallic Li in 1 M LiPF6/propylene carbonate electrolyte, continuous graphite exfoliation is achieved with a high yield of excess 80% without any consumption of electric energy. The as-prepared NEEG has high quality with few defects (ID/IG = 0.45), a high C/O ratio of 27.74 and excellent electronic conductivity of 102.5 S cm−1. As conductive additives in Li4Ti5O12 electrode for lithium ion batteries, NEEG contributes to higher capacity and initial coulombic efficiency than common thermally reduced graphene oxide. Given the advantages of NEEG, the non-electrified electrochemical exfoliation method is promising for the large-scale preparation of high-quality graphene sheets.

Scalable graphene production from ethanol decomposition by microwave argon plasma torch

A fast, efficient and simple method is presented for the production of high quality graphene on a large scale by using an atmospheric pressure plasma-based technique. This technique allows to obtain high quality graphene in powder in just one step, without the use of neither metal catalysts and nor specific substrate during the process. Moreover, the cost for graphene production is significantly reduced since the ethanol used as carbon source can be obtained from the fermentation of agricultural industries. The process provides an additional benefit contributing to the revalorization of waste in the production of a high-value added product like graphene. Thus, this work demonstrates the features of plasma technology as a low cost, efficient, clean and environmentally friendly route for production of high-quality graphene.