Advances in two-dimensional heterostructures by mono-element intercalation underneath epitaxial graphene

Two-dimensional (2D) materials have displayed many remarkable physical properties, including 2D superconductivity, magnetism, and layer-dependent bandgaps. However, it is difficult for a single 2D material to meet complex practical requirements. Heterostructures obtained by vertically stacking different kinds of 2D materials have extensively attracted researchers’ attention because of their rich electronic features. With heterostructures, the constraints of lattice matching can be overcome. Meanwhile, high application potential has been explored for electronic and optoelectronic devices, including tunneling transistors, flexible electronics, and photodetectors. Specifically, graphene-based van der Waals heterostructures (vdWHs) by intercalation are emerging to realize various functional heterostructures-based electronic devices. Intercalating atoms under epitaxial graphene can efficiently decouple graphene from the substrate, and is expected to realize rich novel electronic properties in graphene. In this study, we systematically review the progress of the mono-element intercalation in graphene-based vdWHs, including the intercalation mechanism, intercalation-modified electronic properties, and the practical applications of 2D intercalated heterostructures. This work would inspire edge-cutting ideas in the scientific frontiers of 2D materials.     

Sonochemical preparation of functionalized graphenes

A convenient sonochemical method is described for the preparation of polystyrene functionalized graphenes starting from graphite flakes and a reactive monomer, styrene. Ultrasonic irradiation of graphite in styrene results in the mechanochemical exfoliation of graphite flakes to single-layer and few-layer graphene sheets combined with functionalization of the graphene with polystyrene chains. The polystyrene chains are formed from sonochemically initiated radical polymerization of styrene and can make up to ~18 wt % of the functionalized graphene, as determined by thermal gravimetric analysis. This one-step protocol can be generally applied to the functionalization of graphenes with other vinyl monomers for graphene-based composite materials.     

Graphene-based materials for the electrochemical determination of hazardous ions

The use of graphene in the field of electrochemical sensors is increasing due to two main properties that make graphene and derivatives appealing for this purpose: their conductivity and high surface area. In addition, graphene materials can be easily functionalized with nanoparticles (Au, Pt, etc.) or organic molecules (DNA, polymers, etc.) producing synergies that allow higher sensitivity, lower limit of detection as well as increased selectivity. The present review focuses on the most important works published related to graphene-based electrochemical sensors for the determination of hazardous ions (such as As(III), Cd²⁺, Pb²⁺, Hg²⁺, Cr(VI), Cu²⁺, Ag⁺, etc.). The review presents examples of the use of graphene-based electrodes for this purpose as well as important parameters of the sensors such as: limit of detection, linear range, sensitivity, main interferences, stability, and reproducibility. The application of these graphene-based electrodes in real samples (water or food matrices) is indicated, as well. There is room for improvement of these type of sensors and more effort should be devoted to the use of doped graphene (doped for instance with N, B, S, Se, etc.) since electrochemically active sites originated by doping facilitate charge transfer, adsorption and activation of analytes, and fixation of functional moieties/molecules. This will allow the sensitivity and the selectivity of the electrodes to be increased when combined with other materials (nanoparticles/organic molecules).     

Graphene/inorganic nanocomposites: Evolving photocatalysts for solar energy conversion for environmental remediation

Current energy crisis and environmental issues, including depletion of fossil fuels, rapid industrialization, and undesired CO₂ emission resulting in global warming has created havoc for the global population and significantly affected the quality of life. In this scenario the environmental problems in the forefront of research priorities. Development of renewable energy resources particularly the efficient conversion of solar light to sustainable energy is crucial in addressing environmental problems. In this regard, the synthesis of semiconductors-based photocatalysts has emerged as an effective tool for different photocatalytic applications and environmental remediation. Among different photocatalyst options available, graphene and graphene derivatives such as, graphene oxide (GO), highly reduced graphene oxide (HRG), and doped graphene (N, S, P, B-HRG) have become rising stars on the horizon of semiconductors-based photocatalytic applications. Graphene is a single layer of graphite consisting of a unique planar structure, high conductivity, greater electron mobility, and significantly very high specific surface area. Besides, the recent advancements in synthetic approaches have led to the cost-effective production of graphene-based materials on a large-scale. Therefore, graphene-based materials have gained considerable recognition for the production of semiconducting photocatalysts involving other semiconducting materials. The graphene-based semiconductors photocatalysts surpasses electron-holes pairs recombination rate and lowers the energy band gap by tailoring the valence band (VB) and conduction band (CB) leading to the enhanced photocatalytic performance of hybrid photocatalysts. Herein, we have summarized the latest developments in designing and fabrication of graphene-based semiconducting photocatalysts using a variety of commonly applied methods such as, post-deposition methods, in-situ binding methods, hydrothermal and/or solvothermal approaches. In addition, we will discuss the photocatalytic properties of the resulting graphene-based hybrid materials for various environmental remediation processes such as; (i) clean H₂ fuel production, photocatalytic (ii) pollutants degradation, (iii) photo-redox organic transformation and (iv) photo-induced CO₂ reduction. On the whole, by the inclusion of more than 300 references, this review possibly covered in detail the aspects of graphene-based semiconductor photocatalysts for environmental remediation processes. Finally, the review will conclude a short summary and discussion about future perspectives, challenges and new directions in these emerging areas of research.     

An in situ oxidation route to fabricate graphene nanoplate-metal oxide composites

We report our studies on an improved soft chemical route to directly fabricate graphene nanoplate-metal oxide (Ag₂O, Co₃O₄, Cu₂O and ZnO) composites from the in situ oxidation of graphene nanoplates. By virtue of H⁺ from hydrolysis of the metal nitrate aqueous solution and NO₃⁻, only a small amount of functional groups were introduced, acting as anchor sites and consequently forming the graphene nanoplate-metal oxide composites. The main advantages of this approach are that it does not require cumbersome oxidation of graphite in advance and no need to reduce the composites due to the lower oxidation degree. The microstructures of as-obtained metal oxides on graphene nanoplates can be dramatically controlled by changing the reaction parameters, opening up the possibility for processing the optical and electrochemical properties of the graphene-based nanocomposites.     

Self-assembly of graphene oxide nanosheets in t-butanol/water medium under gamma-ray radiation

Graphene oxide (GO) nanosheets dispersed in strong acidic t-butanol/water medium can be reduced and self-assembled into a self-standing graphene hydrogel under γ-ray radiation, providing a facile and economical preparation method for hydroxylalkylated graphene-based aerogel.     

Graphene oxide dispersions in organic solvents

The dispersion behavior of graphene oxide in different organic solvents has been investigated. As-prepared graphite oxide could be dispersed in N, N-dimethylformamide, N-methyl-2-pyrrolidone, tetrahydrofuran, and ethylene glycol. In all of these solvents, full exfoliation of the graphite oxide material into individual, single-layer graphene oxide sheets was achieved by sonication. The graphene oxide dispersions exhibited long-term stability and were made of sheets between a few hundred nanometers and a few micrometers large, similar to the case of graphene oxide dispersions in water. These results should facilitate the manipulation and processing of graphene-based materials for different applications.     

Preparation and Properties of Elastomer Composites Containing “Graphene”-Based Fillers: A Review

Elastomers are materials showing exceptional elasticity and are used for numerous applications. However, their low stiffness as well as their insulating behavior can be limiting so the incorporation of graphene-based materials can help and improve drastically their properties. With high Young's modulus, high electrical and thermal conductivities, graphene and graphene-like fillers seem ideal fillers to effectively tune elastomers properties. With low graphene-like loadings, most elasticity properties of elastomers could be preserved while increasing or adding new properties to the composites to enable new applications. Herein, we focus on the effects of “graphene” incorporation into elastomers and we will highlight the key parameters to effectively monitor the changes.     

Electrochemically Exfoliated Graphene and Graphene Oxide for Energy Storage and Electrochemistry Applications

Top‐down methods are of key importance for large‐scale graphene and graphene oxide preparation. Electrochemical exfoliation of graphite has lately gained much interest because of the simplicity of execution, the short process time, and the good quality of graphene that can be obtained. Here, we test three different electrolytes, that is, H₂SO₄, Na₂SO₄, and LiClO₄, with a common exfoliation procedure to evaluate the difference in structural and chemical properties that result for the graphene. The properties are analyzed by means of scanning transmission electron microscopy (STEM), Raman spectroscopy, and X‐ray photoelectron spectroscopy. We then tested the graphene materials for electrochemical applications, measuring the heterogeneous electron transfer (HET) rates with a Fe(CN)₆^3?/4? redox probe, and their capacitive behavior in alkaline solutions. We correlate the electrochemical features with the presence of structural defects and oxygen functionalities on the graphene materials. In particular, the use of LiClO₄ during the electrochemical exfoliation of graphite allowed the formation of highly oxidized graphene with a C/O ratio close to 4.0 and represents a possible avenue for the mass production of graphene oxide as valid alternative to the current laborious and dangerous chemical procedures, which also have limited scalability.     

A Study of Graphene‐Based Copper Catalysts: Copper(I) Nanoplatelets for Batch and Continuous‐Flow Applications

The use of graphene derivatives as supports improves the properties of heterogeneous catalysts, with graphene oxide (GO) being the most frequently employed. To explore greener possibilities as well as to get some insights into the role of the different graphenic supports (GO, rGO, carbon black, and graphite nanoplatelets), we prepared, under the same standard conditions, a variety of heterogeneous Cu catalysts and systematically evaluated their composition and catalytic activity in azide–alkyne cycloadditions as a model reaction. The use of sustainable graphite nanoplatelets (GNPs) afforded a stable Cu^I catalyst with good recyclability properties, which are compatible with flow conditions, and able to catalyze other reactions such as the regio‐ and stereoselective sulfonylation of alkynes (addition reaction) and the Meerwein arylation (single electron transfer process).     

Facile synthesis of wide-bandgap fluorinated graphene semiconductors

The bandgap opening of graphene is extremely important for the expansion of the applications of graphene-based materials into optoelectronics and photonics. Current methods to open the bandgap of graphene have intrinsic drawbacks including small bandgap openings, the use hazardous/harsh chemical oxidations, and the requirement of expensive chemical-vapor deposition technologies. Herein, an eco-friendly, highly effective, low-cost, and highly scalable synthetic approach is reported for synthesizing wide-bandgap fluorinated graphene (F-graphene or or fluorographene) semiconductors under ambient conditions. In this synthesis, ionic liquids are used as the only chemical to exfoliate commercially available fluorinated graphite into single and few-layer F-graphene. Experimental and theoretical results show that the bandgap of F-graphene is largely dependent on the F coverage and configuration, and thereby can be tuned over a very wide range.     

石墨烯基杂化材料在微生物燃料电池电极中的应用

微生物燃料电池（MFC）是利用生物催化剂将污水有机物中的化学能直接转化为电能的技术,因其功率密度和能量转化效率低,电极制作成本高,限制了其大规模实际应用。因此如何提高电极的催化性能并降低电极制作成本成为MFC的研究重点方向。由于石墨烯基杂化材料具有良好的导电性和催化特性,因此石墨烯基杂化材料成为在MFC电极应用中的热点之一。本文综述了近年来MFC石墨烯基杂化电极材料的最新研究进展,重点讨论了改性石墨烯电极、金属及非金属/石墨烯杂化电极、金属氧化物/石墨烯杂化电极、聚合物/石墨烯杂化电极和石墨烯凝胶电极的设计思路和制备方法及其催化性能,着重分析了石墨烯基阳极和阴极杂化材料对MFC产电性能的影响。最后对石墨烯基杂化材料在MFC应用中存在的问题及研究前景进行了总结和展望。     

Inherent Electrochemistry and Activation of Chemically Modified Graphenes for Electrochemical Applications

Graphene research is currently at the frontier of electrochemistry. Many different graphene‐based materials are employed by electrochemists as electrodes in sensing and in energy‐storage devices. Because the methods for their preparation are inherently different, graphene materials are expected to exhibit different electrochemical behaviors depending on the functionalities and density of defects present. Electrochemical treatment of these “chemically modified graphenes” (CMGs) represents an easy approach to alter surface functionalities and consequently tune the electrochemical performance. Herein, we report a preliminary electrochemical characterization of four common chemically modified graphenes, namely: graphene oxide, graphite oxide, chemically reduced graphene oxide, and thermally reduced graphene oxide. These CMGs were compared with graphite as a reference material. Cyclic voltammetry was used to ascertain the chemical functionalities present and to understand the potential ranges in which the materials were electroactive. Electrochemical treatment with either an oxidative or a reductive fixed potential were then carried out to activate these chemically modified graphenes. The effects of such electrochemical treatments on their electrocatalytic properties were then investigated by cyclic voltammetry in the presence of well‐known redox probes, such as [Fe(CN)₆]^4?/3?, Fe^3+/2+, [Ru(NH₃)₆]^2+/3+, and ascorbic acid. Thermally reduced graphene oxide exhibited the best electrochemical behavior amongst all of the CMGs, with the fastest rate of heterogeneous electron transfer (HET) and the lowest overpotentials. These findings will have far‐reaching consequences for the evaluation of different CMGs as electrode materials in electrochemical devices.     

石墨烯的功能化及其在储能材料领域中的应用

石墨烯是由sp~2杂化的碳原子紧密堆积成的单原子层二维碳材料,由于其优异的物理和化学性质被视为最有前景的新型材料之一。但由于石墨烯片层之间在范德华力的作用下易发生不可逆团聚,丧失其单层二维纳米片的结构特性,以及石墨烯表面呈现惰性状态,致使其与其他介质的相互作用较弱,难以均匀分散在极性或非极性的溶剂中,因而石墨烯的应用受到限制。对石墨烯进行功能化可以调控其分子结构、电子能级和化学性质,不仅可以有效抑制石墨烯的团聚而且能够改善其在溶剂中的分散性和稳定性,从而实现石墨烯基材料的多元化应用。本文综述了近年来共价键和非共价键功能化石墨烯以及其复合材料在储能领域的研究进展,并对功能化石墨烯的发展前景进行了展望。     

Molecularly engineered graphene surfaces for sensing applications: A review

Graphene is scientifically and commercially important because of its unique molecular structure which is monoatomic in thickness, rigorously two-dimensional and highly conjugated. Consequently, graphene exhibits exceptional electrical, optical, thermal and mechanical properties. Herein, we critically discuss the surface modification of graphene, the specific advantages that graphene-based materials can provide over other materials in sensor research and their related chemical and electrochemical properties. Furthermore, we describe the latest developments in the use of these materials for sensing technology, including chemical sensors and biosensors and their applications in security, environmental safety and diseases detection and diagnosis.     

Understanding Self-assembly,Colloidal Behavior and Rheological Properties of Graphene Derivatives for High-performance Supercapacitor Fabrication

Graphene derivatives, such as graphene oxide(GO) and reduced graphene oxide(RGO), have been widely used as promising twodimensional nanoscale building blocks due to their fascinating properties, cost-effective production, and good processability. Understanding the intrinsic self-assembling, colloidal, and rheological features of graphene derivatives is of critical importance to establish the formation-structureproperty relationship of graphene-based materials. This article reviews recent progresses in our studies of these interesting properties of graphene derivatives for developing high performance supercapacitors. The content is organized to include characteristics of the dispersions of graphene derivatives, self-assembly of nanosheets from liquid medium, colloidal behavior, rheological properties of the dispersions, processing methods based on the properties, and performance of the fabricated supercapacitors. GO and RGO nanosheets are proved to form different types of assembled structures with unique morphologies, such as ultrathin layer-by-layer films, porous aggregates, and nanoscrolls. The unique rheological properties of GO dispersions and hydrogels, feasible for both the traditional wet-processing and newly-developed technology like three-dimensional printing, are highlighted for their potential in structural manipulation and scalable fabrication of graphene-based devices. The research devoted to up-grading the performance of supercapacitors is presented in some details, which could be applicable for fabricating other graphene-based energy storage devices. Some challenges and perspectives in our point of view are given in the last part of this feature article.     

锂硫电池用石墨烯基材料的研究进展

锂硫电池因其理论能量密度高、资源丰富和环境友好等优势,被认为是最有发展前景的下一代电化学储能系统之一。然而,硫的绝缘性、充放电中间产物多硫化物的溶解和扩散、硫的体积膨胀以及锂负极安全性等问题,严重制约着锂硫电池的商业应用。石墨烯因其具有高导电、高柔性等诸多优异特性而被广泛研究,将其用于锂硫电池的正极载体、隔膜涂层和集流体中,以期实现高比能、高稳定性的锂硫电池。本文综述了石墨烯基材料,包括石墨烯、功能化石墨烯、掺杂石墨烯和石墨烯复合物,在锂硫电池中应用的研究进展,并展望了锂硫电池用石墨烯基材料的未来发展方向。     

Graphene-based electrochemical energy conversion and storage: fuel cells, supercapacitors and lithium ion batteries

Graphene has attracted extensive research interest due to its strictly 2-dimensional (2D) structure, which results in its unique electronic, thermal, mechanical, and chemical properties and potential technical applications. These remarkable characteristics of graphene, along with the inherent benefits of a carbon material, make it a promising candidate for application in electrochemical energy devices. This article reviews the methods of graphene preparation, introduces the unique electrochemical behavior of graphene, and summarizes the recent research and development on graphene-based fuel cells, supercapacitors and lithium ion batteries. In addition, promising areas are identified for the future development of graphene-based materials in electrochemical energy conversion and storage systems.     

功能化石墨烯材料：定义、分类及制备策略

自2004年被成功制备后,石墨烯因其独特迷人的性质在近十几年来备受关注,同时也引发了二维纳米材料的研究热潮。单原子层厚度的二维结构赋予石墨烯非同寻常的光学、电子学、磁学及力学等性质,使得石墨烯在生物学、医学、化学、物理学和环境科学等多个领域展现出极大的应用潜力。制得注意的是,石墨烯在应用时通常需要进行功能化,调节其组成、大小、形状和结构等,以便于加工处理或满足不同的应用需求。石墨烯功能化方法多样,功能化产物也是种类繁多。然而,到目前为止,石墨烯功能化产物并没有系统全面的分类和精确的定义。因此,本文在系统总结现有石墨烯功能化研究的基础上,给出了石墨烯功能化产物的系统分类、各类的精确定义和相应的制备策略,并通过典型示例进行了详细地阐述。石墨烯功能化的产物统称为“功能化石墨烯材料”,分为两类：“功能化石墨烯”和“功能化石墨烯复合材料”。功能化石墨烯材料的制备可由“自上而下”和“自下而上”两种策略实现。制备策略的选择取决于应用需求。系统分类、精确命名和制备策略的归纳必将有助于功能化石墨烯材料的进一步发展。