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Introduction

The large family of nanostructured carbons

In this article, I reflect on 20 years of my research in the field of carbon nanomaterials and provide my vision of this exciting and quickly growing field. Graphene is the latest example and is now the most widely researched carbon structure, but other carbons deserve attention as well. I first describe carbon nanomaterials in general, to present an overview of the world of carbon nanostructures and to highlight their most promising applications. Then, I use the example of supercapacitor electrodes to show how to select different materials, design the required structures from carbon, and match them to specific electrolytes to achieve target properties. Structural and energy-related applications of carbon nanotubes and graphene are described in more detail in the sidebars. At the end, I briefly introduce a new family of carbon-based materials, namely, two-dimensional (2D) transition-metal carbides (MXenes), which have a planar 2D structure similar to that of graphene, MoS2, or clay and show great potential for energy storage and a variety of other applications.

About three years ago, I wrote a commentary titled “Not Just Graphene.”1 This title was chosen because we scientists behave like children when we have new toys. Whenever there is a new material, we get excited, and we start playing with it. First were fullerenes2 and carbon nanotubes;3 today, everyone works on graphene.4 However, what is exciting about the world of carbon is that it is so rich; there are so many different materials available (Figure 1). Whereas graphene is currently “hot,” this article discusses a great variety of carbons. The special electronic structure of the carbon atom allows it to adopt sp, sp2, and sp3 hybridizations, resulting in the ability to form a diversity of nanostructures.5 Carbon is unique in this sense. We could build a world made entirely of carbon-based structures if we knew how to design with carbon, and a few examples are shown here. But it is exactly this variety, this richness of the carbon nanoworld, that also makes it very difficult for engineers to select the right material for a specific application.

Yury Gogotsi, Department of Materials Science and Engineering and A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, PA, USA; [email protected] DOI: 10.1557/mrs.2015.272

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MRS BULLETIN • VOLUME 40 • DECEMBER 2015 • www.mrs.org/bulletin

© 2015 Materials Research Society

NOT JUST GRAPHENE: THE WONDERFUL WORLD OF CARBON AND RELATED NANOMATERIALSS

built. By introducing pentagonal defects, one can produce fullerenes (closed carbon shells), or a graphene sheet can be rolled into a seamless cylinder to make a carbon nanotube. Yet, within every family, there is again an enormous richness of structures. A large variety of fullerene shells can be formed starting from C20 and going to a much larger number of carbon atoms in each shell, with C60 being the most stable.9 Figure 2 shows various carbon nanotube forms. There are single-wall tubes (Figure 2