A review on mechanical exfoliation for the scalable production of graphene

The preparation of graphene with high quality serves as a prerequisite for its usage. Traditional methods of graphene production, represented by liquid-phase exfoliation and chemical vapor depositi...

Graphene, a newly discovered and extensively investigated material, has many unique and extraordinary properties which promise major technological advances in fields ranging from electronics to mechanical engineering and food production. Unfortunately, complex techniques and high production costs hinder commonplace applications. Scaling of existing graphene production techniques to the industrial level without compromising its properties is a current challenge. This article focuses on the perspectives and challenges of scalability, equipment, and technological perspectives of the plasma-based techniques which offer many unique possibilities for the synthesis of graphene and graphene-containing products. The plasma-based processes are amenable for scaling and could also be useful to enhance the controllability of the conventional chemical vapour deposition method and some other techniques, and to ensure a good quality of the produced graphene. We examine the unique features of the plasma-enhanced graphene production approaches, including the techniques based on inductively-coupled and arc discharges, in the context of their potential scaling to mass production following the generic scaling approaches applicable to the existing processes and systems. This work analyses a large amount of the recent literature on graphene production by various techniques and summarizes the results in a tabular form to provide a simple and convenient comparison of several available techniques. Our analysis reveals a significant potential of scalability for plasma-based technologies, based on the scaling-related process characteristics. Among other processes, a greater yield of 1 g × h(-1) m(-2) was reached for the arc discharge technology, whereas the other plasma-based techniques show process yields comparable to the neutral-gas based methods. Selected plasma-based techniques show lower energy consumption than in thermal CVD processes, and the ability to produce graphene flakes of various sizes reaching hundreds of square millimetres, and the thickness varying from a monolayer to 10-20 layers. Additional factors such as electrical voltage and current, not available in thermal CVD processes could potentially lead to better scalability, flexibility and control of the plasma-based processes. Advantages and disadvantages of various systems are also considered.

Large scale production of graphene has recently received a great deal of attention because of its interesting properties and wide range of potential applications. Conventional chemical exfoliation approaches, such as Hummer’s method,provides cost-effective pathway for large scale production, however, the method suffers from several key challenges such as the usage of corrosive and toxic solution, high processing temperature, long duration, and the poor quality of graphene produced which severely affects its potential applications. Recently, electrochemical exfoliation of graphene has been proposed for a high production yield with better quality. The electrochemical method involves two steps: 1) intercalation of ionic species between the graphene layers, followed by 2) oxidation of these ionic species under an anodic voltage into gas molecule enabling exfoliation of graphene flakes from the graphite electrode.1 In this study we examine a new approach to the intercalation, using an intercalating ion that has received very little attention and has not been evaluated for electrochemical exfoliation. The intercalation was followed by exfoliation in (NH4)2SO4 solution leading to the production of graphene flakes. The resultant graphene was found to have less disrupted sp2 lattice structure, with a high yield of about 95 % of graphene, along with high electrical conductivity and a low oxidation level (with a C:O ratio of about 15, determined by XPS). In addition, the presence of nitrogen and sulfur moieties in the resulting graphene structure can act as an electrocatalyst2 for applications such as the oxygen reduction reaction. Thus, the as-prepared graphene could be used for a range of electrochemical applications.

Large scale production of graphene has recently received a great deal of attention because of its interesting properties and wide range of potential applications. Conventional chemical exfoliation approaches, such as Hummer’s method,provides cost-effective pathway for large scale production, however, the method suffers from several key challenges such as the usage of corrosive and toxic solution, high processing temperature, long duration, and the poor quality of graphene produced which severely affects its potential applications. Recently, electrochemical exfoliation of graphene has been proposed for a high production yield with better quality. The electrochemical method involves two steps: 1) intercalation of ionic species between the graphene layers, followed by 2) oxidation of these ionic species under an anodic voltage into gas molecule enabling exfoliation of graphene flakes from the graphite electrode.1 In this study we examine a new approach to the intercalation, using an intercalating ion that has received very little attention and has not been evaluated for electrochemical exfoliation. The intercalation was followed by exfoliation in (NH4)2SO4 solution leading to the production of graphene flakes. The resultant graphene was found to have less disrupted sp2 lattice structure, with a high yield of about 95 % of graphene, along with high electrical conductivity and a low oxidation level (with a C:O ratio of about 15, determined by XPS). In addition, the presence of nitrogen and sulfur moieties in the resulting graphene structure can act as an electrocatalyst2 for applications such as the oxygen reduction reaction. Thus, the as-prepared graphene could be used for a range of electrochemical applications.

A review on liquid-phase exfoliation for scalable production of pure graphene, wrinkled, crumpled and functionalized graphene and challenges

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.

Regulating cations and solvents of the electrolyte for ultra-efficient electrochemical production of high-quality graphene

Top-down synthesis of graphene: A comprehensive review

Graphene, the two‐dimensional form of carbon, has received a great deal of attention across academia and industry due to its extraordinary electrical, mechanical, thermal, chemical, and optical properties. In view of the potential impact of graphene on numerous and diverse applications in electronics, novel materials, energy, transport, and healthcare, large‐scale graphene production is a challenge that must be addressed. In the past decade, top–down production has demonstrated high potential for scale‐up. This review features the recent progress made in top–down production methods that have been proposed for the manufacturing of graphene‐based products. Fabrication methods such as liquid‐phase mechanical, chemical and electrochemical exfoliation of graphite are outlined, with a particular focus on nonoxidizing routes for graphene production. Analysis of exfoliation mechanisms, solvent considerations, key advantages and issues, and important production characteristics including production rate and yield, where applicable, are outlined. Future challenges and opportunities in graphene production are also highlighted. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3246–3276, 2018

The production of unfunctionalized and nonoxidized graphene by exfoliation of graphite ina volatile solvent, 1-propanol, is reported. A stable homogeneous dispersion of graphenewas obtained by mild sonication of graphite powder and subsequent centrifugation. Thepresence of a graphene monolayer was observed by atomic force microscopy andtransmission electron microscopy. The solvent, 1-propanol, from the depositeddispersion was simply and quickly removed by air drying at room temperature,without the help of high temperature annealing or vacuum drying, which shortensproduction time and does not leave any residue of the solvent in the graphene sheets.

Graphite oxide has attracted much interest as a possible route for preparation of natural graphite in the large-scale production and manipulation of graphene as a material with extraordinary electronic properties. Graphite oxide was prepared by modified Hummers method from purified natural graphite sample from West Kalimantan. We demonstrated that natural graphite is well-purified by acid leaching method. The purified graphite was proceed for intercalating process by modifying Hummers method. The modification is on the reaction time and temperature of the intercalation process. The materials used in the intercalating process are H2SO4 and KMNO4. The purified natural graphite is analyzed by carbon content based on Loss on Ignition test. The thermo gravimetricanalysis and the Fouriertransform infrared spectroscopy are performed to investigate the oxidation results of the obtained GO which is indicated by the existence of functional groups. In addition, the X-ray diffraction and energy dispersive X-ray spectroscopy are also applied to characterize respectively for the crystal structure and elemental analysis. The results confirmed that natural graphite samples with 68% carbon content was purified into 97.68 % carbon content. While the intercalation process formed a formation of functional groups in the obtained GO. The results show that the temperature and reaction times have improved the efficiency of the oxidation process. It is concluded that these method could be considered as an important route for large-scale production of graphene.

Electrochemical exfoliation has emerged as a green, effective and scalable route for mass production of graphene. Cathodic exfoliation of graphite, offers a direct production of high quality and low defect graphene. However, the protocols for cathodic exfoliation reported to date, use mostly non-aqueous electrolytes which require extended period of intercalation, and post-treatment to further exfoliate the intercalated graphene layers. In contrast, the employment of environment friendly aqueous-based electrolytes, coupling with a shorter exfoliation period would be attractive features of any new protocol. Here, we demonstrate efficient cathodic electrochemical exfoliation of graphite to graphene nanoplatelets in aqueous electrolytes using common and inexpensive alkali-metal salts such as KCl. The key driving force to exfoliate graphite successfully in aqueous electrolyte is applying a sufficiently high voltage, and a high salt concentration which facilitate cation intercalation, and promotes hydrogen evolution to exfoliate the graphene. The cathodic exfoliated graphene nanoplatelets using KCl aqueous electrolyte exhibits a low defect density (ID/IG of 0.06, a C/O ratio of 57.8), high graphite exfoliation yields (> 80%) in short times (< 10 min for a graphite foil of 1 cm × 1 cm × 0.0254 cm, 30.0 mg). The highly conductive structure consists mainly of 10–13 layers graphene sheets that serve as an excellent support material for electrocatalytic reactions. This environment-benign aqueous-based cathodic electrochemical exfoliation of graphite opens a new opportunity in large-scale and low-cost production of high-quality graphene nanoplatelets.

<p indent="0mm">Graphene is a planar sheet composed of densely arranged sp²-bonded carbon atoms in a honeycomb lattice structure. It exhibits remarkable properties, including a high specific surface area <sc>(~2650 m²/g),</sc> excellent mechanical properties, and superior thermal and electrical conductivity. These characteristics make it an ideal candidate for applications in diverse fields such as electromagnetic interference shielding, sensors, advanced electronic devices, energy storage, water purification, gas separation, and biomedical applications. It is no exaggeration to say that the successful preparation of high-yield and high-quality graphene in future developments and utilizations is directly related to the extent of its in-depth expansion and application in various related fields. Microwave-assisted preparation of graphene has emerged as a burgeoning research direction in the field of graphene preparation due to its environmental friendliness, high efficiency, and convenient characteristics, and notable progress has been achieved in this area. Specifically, various methods such as microwave-assisted liquid-phase exfoliation, microwave-assisted reduction of graphene oxide, and microwave-assisted chemical vapor deposition can be employed to prepare graphene with diverse properties. These methods are capable of meeting the requirements of different application scenarios, thereby providing robust support for technological innovation and development in related fields. This review aims to offer a comprehensive view of the research advancements in microwave-assisted graphene preparation. First, we introduce the principle of microwave heating and briefly describe the characteristics and advantages of microwave-assisted graphene preparation. Subsequently, we summarize the current research progress in various microwave-assisted graphene preparation methods, mainly including two types: top-down and bottom-up. The principles, process parameters, advantages and disadvantages of each method are elucidated, as well as their applications in related fields. In the top-down approach, we elaborate on the utilization of microwaves in liquid-phase exfoliation and graphene oxide reduction methods. A notable feature of these methods is the significant shortening of the reaction time. Moreover, the merits of microwaves in solid-state reaction systems are emphasized during the microwave-assisted reduction of graphene oxide. This is because microwave heating depends on the wave penetration and the absorption properties of materials. In contrast, the presence of liquid-phase solvents may result in the dissipation of microwave energy. Consequently, the reduced graphene oxide (rGO) prepared by this approach exhibits qualities such as high quality, high purity, low oxygen content, and a limited number of layers. This effectively preserves the inherent characteristics of graphene and shows promise for the continuous large-scale production of high-quality rGO. In the bottom-up approach, the focus is mainly on presenting the microwave plasma vapor deposition technique for graphene preparation. Subsequently, the production of graphene is expounded upon from the aspects of both graphene thin films and powders, and the influence of certain pertinent parameters, such as gas source type, proportion, flow rate, reaction temperature, microwave power, etc on the quality of graphene is elaborated in detail. Additionally, a brief introduction is provided on the method of preparing graphene by microwave pyrolysis conversion of other carbon-containing waste materials, such as biomass, waste plastics, and waste batteries. We also review the related application fields of graphene prepared by microwave methods. Finally, the challenges and future development trends of microwave-assisted preparation of graphene are discussed, aiming to provide a reference for the large-scale production and further application research of graphene.

Graphene is critical for applications in electronics, optical devices, thermal management, energy, and biosystems, while at the same time cost-effective and large-scale production of graphene is a challenge. In this regard, vapor phase graphene synthesis is a bottom-up approach, which could be compatible with device industry fabrication methods. Here, we review the state-of-the-art techniques developed for the scalable production of graphene in bottom-up approaches. These mainly include the epitaxial growth and chemical vapor deposition methods. Product quality, structure, and yields for different graphene growth techniques are discussed and specific examples are described. The article also emphasizes promising methods for scalable graphene production but still needing a deeper research understanding.

This paper reports a pH-mediated hydrothermal reduction method that combines with moulding methods, allowing controllable fabrication of high density (up to 1.6 g cm???3) isotropic graphene macrostructures with various shapes. This ???graphene??? (CG) has a compressive strength of 361 MPa (6 times higher than conventional graphite products) and electrical conductivity of 7.6 S cm???1. The processing steps are scalable.