Theme issue: carbon nanostructures

Overview

Since the discovery of fullerenes in 1985 by Sir Harold W. Kroto, Robert F. Curl and the late Richard E. Smalley—the three scientists awarded with the Nobel Prize in Chemistry in 1996—these new carbon allotropes have received a lot of attention from the scientific community and still exhibit an unabated interest. In addition to the more explored fullerenes, the most recent multi-wall and single-wall carbon nanotubes (CNTs)—firstly reported by Iijima in 1991—as well as other carbon nanostructures such as nanohorns, graphenes and other less common carbon nano-objects (nano-onions, nanobuds, peapods, nanocups, nanotori) have received the attention of scientists all over the world. In fact, a look at SciFinder reveals that around five thousand scientific papers with references to fullerenes and around three times more to CNTs have appeared in the literature during the last five years. These numbers are significantly higher when considering patents and those papers containing the concept of fullerene or CNT. This huge amount of work stems from the expectations that these carbon nanostructures have for potential applications in different fields ranging from materials science and molecular electronics to biological and biomedical applications.

In this theme issue dedicated to “Carbon Nanostructures” we have collected nineteen papers from groups active in the research of fullerenes and related carbon structures, and try to show the state of the art in this field.

Fullerenes

Because of the abundance and availability of fullerenes, and in particular [60]fullerene, this icosahedral molecule has been among the most studied chemical compounds during the last two decades. Despite the common idea that the reactivity and properties of this fascinating molecule are already known, still a variety of important reactions from the arsenal of organic chemistry are being tried with fullerenes. New properties are expected from the reaction of fullerenes with functional addends exhibiting interesting properties. Thus, the linkage of C₆₀ to electron donor units has resulted in a topic where many groups have been intensively working during the last decade. Several articles are devoted to the study of new C₆₀-based donor–acceptor dyads in this issue. Thus, Fukuzumi et al. in a Feature Article report photofunctional nanomaterials composed of multiporphyrins and carbon-based π-electron acceptors, namely fullerenes and CNTs and a new type of nanocarbon, which are utilized to construct efficient light energy conversion systems such as photovoltaic devices. The recent advances in self-assembled supramolecular architectures based on tetrapyrrole macrocycles as donors and C₆₀ as well as CNTs as electron acceptors for electron and energy transfer applications is highlighted by Chitta et al. in another Feature Article. Quintiliani et al. report on a tripodal architecture involving three phthalocyanine arms and a fullerene cage as a different scenario to study the observed electron transfer processes.

Campidelli et al. describe how fullerene–peralkylated ferrocene dyads show, in addition to the photoinduced electron transfer process, interesting liquid-crystalline behaviour displaying an enantiotropic smectic A phase. In a second paper dedicated to liquid crystals involving fullerenes, Maringa et al. describe the preparation of liquid-crystalline methanofullerodendrimers through a Bingel reaction with mesomorphic malonate derivatives. Based on a convergent and modular synthesis, all the prepared malonates gave rise to hexagonal columnar phases, while the fullerodendrimers presented a rectangular columnar phase for the second generation hemidendrimer.

As stated above, the chemistry of fullerenes is full of excitement and Delavaux et al. present new fullerene rich nanostructures by using organotin chemistry, in one of those rare examples combining fullerenes with a stannoxane central cage.

Carbon nanotubes

Among the many aspects to consider in the investigation of carbon nanotubes, their formation remains still an open question closely related to other important questions such as purity, composition and electronic properties. As a result, there is general interest in this topic which is also discussed in this theme issue by Wen et al. in a theoretical study directed at unravelling the formation processes of catalyst nanoparticles and SWNTs in a premixed flame doped with pentacarbonyl iron. Interestingly, the carbon, hydrogen and oxygen atoms formed at the surface of the catalyst affect the growth of SWNTs. This article is nicely complemented by the article of Vander Sande's group in which the formation process of catalyst nanoparticles and SWNTs in a premixed CH₄–O₂–Ar flame. Other aspects such as flame temperature seem also to have a strong influence on the inception and growth of the carbon nanotube.

Functionalization of carbon nanotubes is another topic which is currently under investigation provided that it represents a way to solubilise them in common organic solvents. Gómez-Escalonilla et al. report on the sidewall covalent functionalization of SWNTs by using a Heck cross-coupling reaction of pre-functionalized p-iodophenyl-SWNTs. The presence of anthracene units results in photoinduced electron transfer processes.

Wunderlich et al. have also carried out an investigation of the covalent sidewall functionalization of SWNTs by reductive alkylation (Billups reaction). Interestingly, SWNTs with smaller diameter were found to be considerably more reactive than tubes with larger diameters.

Carbon nanotubes have also been connected to electron donor units, thus showing the electron acceptor character of these carbon allotropes. On this topic, Ehli et al. report the supramolecular linkage of a well-known electron donor unit, tetrathiafulvalene (TTF), to different types of carbon nanotubes through the pyrene unit. Photoinduced charge injection into the conduction band of CNT probes afforded stable radical ion pairs for multiwall CNTs and shorter lifetimes for SWNTs. These data are of interest for the design of functional materials for photovoltaic applications.

Finally, efficient photosensitized energy transfer and near-IR fluorescence from porphyrin/SWNT complexes is described by Casey et al.

Other carbon nanostructures

A general overview of the different forms shown by elemental carbon at the nanometer scale is reported by Delgado et al. in which, in addition to fullerenes, including endohedral fullerenes, and carbon nanotubes, other carbon nanoforms are presented for a wider readership. In particular, this Feature Article gives an idea of the different fascinating forms in which carbon can be found, namely graphenes, nanohorns, nano-onions, nanobuds, nanotori, nanocups, and peapods.

Fig. 1 Computer generated image of a SWCNT filled with [60]fullerene units.

Graphenes are specifically considered by Zhi and Müllen in a Feature Article which is mainly focussed on the bottom-up approach for preparing nanographenes as well as other unconventional carbon materials, highlighting the importance of the thermolysis procedures for preparing such materials.

Subrahmanyam et al. complement the above Feature Article with a paper devoted to the preparation of graphene by different routes, namely pyrolysis of camphor under reducing conditions, exfoliation of graphitic oxide, conversion of nanodiamonds and arc evaporation of SiC. Furthermore, water soluble graphene has been produced by extensive acid treatment of exfoliation of graphitic oxide or treatment with polyethylene glycol.

Diamond nanoparticles are another form of carbon materials with singular properties and applications. The progress made in this field has been reviewed by Krueger in a Feature Article which reveals the usefulness of these materials in biomedical or electronic applications.

Sano et al. report on the synthesis of carbon nanohorns by a gas-injected arc-in-water method with varied gas flow rate. The characteristics for use as catalyst supports for polymer electrolyte fuel cell electrodes is related to the arc current in their production method.

Fig. 2 A typical TEM image of single-wall carbon nanohorns synthesized by the GI-AIW method with N₂ injection.

The electronic and magnetic properties of individual carbon clusters, clusters inside fullerenes and graphitic nanoribbons have been studied by López-Urías et al. using many-body schemes in which the correlated π-electrons are treated by means of the single band Hubbard model.

Cattaruzza et al. discuss new hybrid materials for molecular-based devices, which have been prepared by covalent immobilization, involving Si–C bonds, of fullerene derivatives on flat silicon surfaces by following several synthetic protocols.

This theme issue does not intend to give an exhaustive picture of this very active and fast-moving field. Rather, a few representative examples try to show the wide variety of forms and the exciting materials aspects that have been evolving in the recent past. The Guest Editors want to express their gratitude to all the contributors for their efforts to make this an outstanding issue. Finally, we want to express our gratitude to the staff of Journal of Materials Chemistry, as well as to The Royal Society of Chemistry, for their support and help in achieving this goal.

Plate1 Dirk M. Guldi

Plate2 Nazario Martín

Plate3 Maurizio Prato

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