20 Years of Carbon Nanotubes

Carbon nanotubes (CNTs) have been known since 1976,1 and single-walled carbon nanotubes since 1993.2 However, the observation of "helical microtubules of graphitic carbon" by electron microscopy, reported by Sumio Iijima from the Japanese electronic company NEC in a 1991 publication in Nature,3 signaled the birth of a new research area and inspired a flood of scientific activities worldwide, crossing several disciplines. Soon after the report in Nature, CNTs were found to have excellent mechanical, physical, and chemical properties, and in fact were presumed to combine all the best properties.4 A wide range of methods has been developed to produce CNTs on a large scale, modify them, and integrate them into devices or use them in materials science, catalysis, and energy research. Billions of dollars, funded by governments, organizations, and foundations, have been invested in these research activities. Many applications of CNTs have been demonstrated on the basis of these efforts and innovations. About ten years later, at the beginning of this millennium, mankind faced (and is facing) a new, crucial problem: the volume of new fossil energy reserves discovered is lower than the volume being produced; an indication that these energy sources will be depleted. A side-effect of the increasing energy consumption is the increasing emission of greenhouse gases that cause climate change, which has recently become noticeable. The awareness of the finite resources of fossil energy and of climate change resulting from fossil fuel consumption have sensitized society and, hence, the scientific community.5 Chemistry and materials are two key areas of science that are closely related to the production, transformation, transfer, storage, and application of energy.5, 6 In this context we should ask how we can make use of CNTs, the shooting star in nanoscience and nanotechnology over the last twenty years, for sustainable chemistry or, in a broader meaning, for the sustainable development of our society? As a material with excellent properties CNTs have been explored as catalyst supports and even as catalysts to increase the selectivity and conversion rate of many current industrial reactions, in order to save resources, decrease the energy consumption of a process, and decrease carbon dioxide emissions. The advantage of using CNTs instead of activated carbon or other supports lies in the high dispersion of catalysts on CNTs and their solid anchoring, hindering the sintering of metal particles at high temperatures. The use of CNTs as support in photocatalysis is based on their good conductivity, allowing to effectively separate electrons and holes. A relatively new use for CNTs in catalysis is the possibility to replace transition metal (oxides) in homogenous and heterogeneous reactions and to replace noble metals, mostly as electrocatalysts for oxygen reduction reaction. This development is of high interest for fuel cell applications. Another important application area of CNTs aimed at sustainable development is their use in energy storage and conversion devices, for example in organic light-emitting diodes, lithium ions batteries, supercapacitors, and fuel cells. For energy storage, the use of CNTs as electrode material seems less promising than their use as additives for electrodes. This has been realized in many commercial devices to increase the rate performance of the batteries. When talking about the applications of CNTs for progressing to a sustainable society, we should not forget their use in composite materials, for example, as additive in rubber or in polymers to enhance or improve their mechanical properties. Their use as biocompatible materials and in sensors deserves mention, also. However, their large-scale synthesis and mass production are prerequisites for all of these applications. The production must be environmentally friendly, controllable in quality, low-cost, and resource-saving, that is, the production of CNTs itself must be a sustainable process. This has stimulated many efforts to understand the fundamental aspects of the growth mechanism of CNTs,7 optimize the process, and scale-up the reactors. The most significant progress achieved in CNT technology might be their production on an industrial scale: 70 kg h−1 and a yield 179 g per gram of catalyst used.8 Even so, the large-scale production of single-walled CNTs and multiwalled CNTs with homogeneous diameters and a single, uniform structure remains challenging. Another prerequisite for the broad application of CNTs is knowledge about their health and safety aspects. With mass-scale production and many applications, persons handling CNTs in the production process and consumers are more and more exposed to the material. Concern about potential adverse health effects have increased. Many biomedical applications also intend to make use of CNTs, and therefore their biocompatibility and -distribution must be assessed carefully. Recent studies on the cytotoxicity and hazardous aspects of CNTs still cannot provide a consensus on this material when exposed to humans.9 This remains an important issue for the safe application of CNTs. We are now in 2011, and twenty years have passed since Iijima's 1991 paper. The major events and milestones of CNTs with respect to synthesis and applications in materials science and chemistry are summarized in Figure 1. CNTs have become mature, and this is an excellent time to think about where we are and in which direction we will go. This special issue contains Reviews, a Concept, Communications, and Full Papers on all of the aspects mentioned above, especially those using CNTs for sustainable chemistry and renewable energy applications, and their synthesis and production. The guest editor holds the opinion that CNTs are playing, and will be playing, a much more important role in sustainable chemistry than they are thought to play in electronics or, for example, space applications. However, CNTs alone are not the solution of our future energy or chemical industries. They are a component, a building block in a key future technology; for its realization a concerted cooperation of chemists, materials scientists, and engineers is needed. The next twenty years will decide the fate of CNTs, the former star in nanomaterials and nanoscience, in a time when "graphenes" seem to complement or challenge their role. Science remains exciting. Selected major events and milestones during twenty years of CNTs. The illustration shows that the major advances in the first ten years were made in synthesis, while in the second ten-year period advances were made in their application. Due to the limited space, only one or two papers were selected per year, and only one author for each paper, and only the journal is mentioned, while hundreds of papers are published each year. The illustration starts in 1991, but CNTs were already reported in 1976. Dang Sheng Su completed his Ph.D. at the Technical University of Vienna (Austria) in 1991, and then moved to the Fritz Haber Institute (FHI) of the Max Planck Society in Berlin (Germany) as a post-doctoral fellow in the Department of Electron Microscopy. After a short stay at the Hahn-Meitner Institut GmbH and the Humboldt Universität zu Berlin (Germany), he joined the FHI in 1999, where he works on nanomaterials in heterogeneous catalysis and energy storage. He has newly been appointed as Professor at the Institute of Metal Research and head of the Catalysis and Energy Materials Division of the National Laboratory of Materials Science, Chinese Academy of Sciences, Shenyang (PR China).