Graphene chemistry

Tomás Torres

Tomás Torres is the Director of the Institute for Advanced Research in Chemical Sciences (IAdChem) and Full Professor of Organic Chemistry at the Autónoma University of Madrid (UAM) and Associated Senior Scientist at the IMDEA Nanoscience in Madrid. In addition to various aspects of synthetic and supramolecular chemistry, his current research interests include the preparation and study of optical properties of organic functional materials. His group, that presently consists of twenty five researchers, is currently exploring several areas of basic research and applications of phthalocyanines, porphyrins and carbon nanostructures (fullerenes, carbon nanotubes, graphene), including organic and hybrid solar cells, with a focus on nanotechnology. He has published more than 500 articles, reviews and patents, and his H-index is 75.

Geim and Novoselov were jointly awarded the 2010 Nobel Prize in Physics “for groundbreaking experiments regarding the two-dimensional material graphene”. Currently, graphene is one of the most representative low-dimensional materials and shows particular electronic, optical, mechanical and chemical properties, among others, which have found application in several scientific and technological areas. The graphene family has gained the attention of an increasing number of researchers working in different areas, from composites to molecular electronics. Thus, many promising applications based on graphene are being developed currently. Several themed issues and monographs focusing on this research space have been published. This themed issue of Chemical Society Reviews incorporates both critical and tutorial reviews and aims at providing an overview of recent developments in the expanding field of graphene chemistry. Thus, the issue covers a panorama of the latest progress related to different aspects of graphene species, ranging from theory, chemical functionalization, electron transfer chemistry and catalysis, to graphene composite materials, “graphene-on-surface” structures, 2D heterostructures, graphene derivatives and biomedical applications. Moreover, it has a differential character of major importance regarding previous collections: each review is written by authors from two or more research groups, who have been able to share and to contrast their particular experiences in a joint paper.

The tutorial review by Roldan, Guinea and colleagues (DOI: 10.1039/C7CS00210F) presents an overview of the basic theoretical aspects of two-dimensional (2D) crystals. The authors revise not only the essential aspects of graphene but also the new families of semiconducting 2D materials, like transition metal dichalcogenides or black phosphorus. Additionally, some of the exciting new possibilities offered by 2D crystals are discussed, such as manipulation and control of quantum degrees of freedom (spin and pseudospin), confinement of excitons, or unconventional superconducting phases.

The joint work of five research groups, namely, the teams of Guldi, Hirsch, Martín, D'Souza and Torres (DOI: 10.1039/C7CS00229G) is dedicated to graphene-based materials (GBMs). In this review, the authors offer a general overview of the most relevant synthetic approaches for the covalent and non-covalent functionalization and characterization of GBMs. Moreover, they describe some representative examples of the incorporation to GBMs of electroactive units such as porphyrins, phthalocyanines, or ferrocene, among others, affording donor–acceptor (D–A) hybrids. For these latter systems, the photophysical characterization of their ground- and excited-state features has also been included. Finally, for some of the presented architectures, their application in solar energy conversion schemes and energy production has also been discussed.

Wang, Strano and colleagues (DOI: 10.1039/C7CS00181A) highlight the progress made in electron transfer chemistry of graphene. The participation of graphene in this kind of reactions, where an electron is transferred between graphene and other species, encompasses many important processes that have shown versatility and potential for being used in several applications. In this article, the authors review recent developments in this area, ranging from covalent functionalization of graphene, for modifying its properties and incorporating different functional groups, to electrochemical reactions. They also review the selective etching of graphene to form edges and nanopores, which have unique chemical and physical properties, for example for new membrane and filtration applications.

Graphenes and related materials have also attracted growing interest as metal-free catalysts. Antonietti, García and colleagues (DOI: 10.1039/C7CS00156H) report on this connection. They list the active sites that have been proposed to be responsible for the catalytic activity observed for such systems. It is shown that diverse defects and chemical functionalities on the graphene layers can catalyze reactions, including oxygenated functional groups, carbon vacancies and holes, edge effects, and the presence of dopant elements. Besides discrete active sites, the catalytic activity arising from the collective properties of graphenes as materials, by adsorbing substrates and reagents and activating them by charge transfer, is also reviewed.

The joint contribution of the groups of Kong, Xu, Ruoff and Zhu (DOI: 10.1039/C7CS00256D) gives an overview of the physics and chemistry of the interfaces in graphene involved systems. Graphene films on different surfaces are expected to exhibit significant variations in their properties, which further lead to changes in its morphology, electronic structure, surface chemistry/physics, and surface/interface states. A rational design of the interfacial interactions between graphene and its support layers is of great significance. Therefore, a thorough understanding of surface/interface properties of graphene is fundamental. In this review, the authors present the major “graphene-on-surface” structures and examine the roles of their properties and related phenomena in governing the overall performances for specific applications, including coating, anti-friction and superlubricity, and surface catalysis.

The review by Bisett, Ago and colleagues (DOI: 10.1039/C7CS00160F) deals with the synthesis, structure and applications of graphene-based 2D heterostructures. The field of 2D heterostructures is gaining increased interest in recent years. Such systems not only overcome some limitations of graphene-based materials, but also allow us to realize novel properties. In this review, the authors discuss the evolution and current state of the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, it also covers heterostructures realized with lower dimensionality materials, along with the intercalation in few-layer graphene as a special case of heterostructure. Additionally, graphene heterostructures produced by liquid phase exfoliation techniques and their applications for energy storage have been also reviewed.

The review of Pumera and Sofer (DOI: 10.1039/C7CS00215G) deals with graphene derivatives, like hydrogenated graphene (graphane), fluorographene, hydroxygraphene (graphol), and graphene acid among others. Thus, stoichiometric derivatives of graphene, having well-defined chemical structure and well-defined chemical bonds, are of great interest to the 2D materials research. This review article describes various synthetic methods and discusses the chemistry and properties of these graphene derivatives, showing that there is an immense potential of utilization for required applications.

The second joint tutorial review from the groups of Vázquez, Bianco and Prato (DOI: 10.1039/C7CS00363C) focuses on the area of biomedical applications of graphene. Graphene is especially involved in drug delivery, biosensing and tissue engineering, with strong contributions to the whole nanomedicine area. Pending problems to solve, on the way to the manufacturing of biomedical devices, include for example the lack of standardization in the production of the graphene family members. In this review, the authors critically describe the latest developments of the graphene family materials in the biomedical field. Thus, they analyze graphene-based devices starting from graphene synthetic strategies, functionalization and processability protocols up to the final in vitro and in vivo applications. The authors also address the toxicological impact and the limitations in translating graphene materials into advanced clinical tools. This review concludes with a summary and some invigorating perspectives on the challenges, new trends and guidelines for future developments.

In summary, I am pleased to introduce an outstanding collection of articles that aim to furnish an accurate vision of the state-of-the-art of graphene chemistry. In my opinion, there is no doubt that graphene is in a pole position for innovative future developments at the forefront of the chemical sciences.

Last but not least, I would like to thank all the authors of this themed issue for their precious contributions and the great effort that they have invested. I hope that the reviews will be enjoyed by many readers. Finally, I would like to express my appreciation to the staff of Chemical Society Reviews for helping to carry out this project.

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