Nano “onions

Ding, L. et al. (2005) Molecular characterization of the cytotoxic mechanism of multiwall carbon nanotubes and nano-onions on human skin fibroblast. Nano Letters, 5 (12), 2448-64. 

The discovery of fullerenes in 1985 led to the era of nanomaterials.1 The three-dimensional geometry of these molecules as well as their unique properties distinguishes them from conventional molecules encountered in organic chemistry. Due to recent discoveries in this field, the horizons of this area have broadened to encompass various new molecules such as endohedral fullerenes, nanotubes, carbon nanohorns, and carbon nano-onions. This chapter discusses the electrochemical behavior of some of these carbon nanoparticles with special emphasis on endohedral fullerenes. Since a large number of fullerene derivatives have been prepared and their various electrochemical studies in different solvents and electrolytes have been reported, the electrochemistry of these derivatives is beyond the scope of this text.2 3 Among the other carbon nanoparticles, the electrochemistry of derivatives of carbon nanotubes has been reported. These studies have been highlighted in the final part of the chapter. 

A novel template effect of SiNWs tvas discovered accidentally trying to disperse SiNWs in common solvents such as CHCI3, CH2CI2 and CH3I. A 15 min bath sonication resulted in a colloidal solution, the products of which were characterized by HRTEM, EELS and Raman. The analysis revealed that under sonication the SiNWs acted as templates on which carbon nanotubes and carbon nano-onions formed (figure 10.32). Moreover, in addition to these known carbon structures. 

Fig. 10.33. High magnifications of (a) carbon nanotube and (b) carbon nano-onion grown on the SiNW template...

Joly-Pottuz L, Vacher B, Ohmae N, et al. Anti-wear and friction reducing mechanism of carbon nano-onions as lubricant additives. Tribol Lett 2008 30 69-80. 

Figure 2.15 High-resolution TEM micrograph of carbon nano-onions. (Reprinted from Ref. [74] with permission from Elsevier.)...

Roy, D., Chhowalla, M., Wang, H., et al. (2003). Characterisation of carbon nano-onions using Raman spectroscopy. Chem. Phys. Lett., 373, 52-6. 

It has been menhoned before (Section 4.2.1) that apart from the spherical carbon onions, there are also markedly faceted structures. Some of these exhibit a large central cavity (Figure 4.7a). They may be generated, for instance, by heating spherical nano-onions, or by direct methods like arc discharge or others. Structures of this kind are furthermore observed as faceted shells of metal nanoparticles that fill the void within the carbon structure. 

The fact of the nano-onions formally being multiwalled nanotubes with a tubular length of zero is of particular relevance here. The growth of carbon species takes place inside a plasma zone. This has to be designed in a way to prevent (or at least disfavor) linear growth of occasionally emerging nanotubes and thus to promote the formation of spherical structures instead. One possible method consists in raising the pressure inside the reaction chamber to ensure a quick dissipation of heat. 

The carbon onions can be isolated from the metal substrate by thermal treatment The silver or copper evaporate in vacuo at temperatures above 850 °C to release the onions. EspedaUy when using a thin film of silver on a silica support, the nano-onions can easily be collected from the silica gel left after heating as there are virtually no interactions between them (Figure 4.12). If, on the other hand, sihcon or steel is employed as a support for the silver layer, a strong binding of... 

Furthermore succeeds, with the support of hydrogen, the reductive transformation of supercritical carbon dioxide into nano-onions (CO2 -r2H2 C-F2H20). A platinum catalyst [Pt(T] -C,S-Ci2H8)(PEt3)2] is employed here which presumably releases [Pt(PEt3)2] as reactive species. The resultant carbon onions exhibit a partially spiral structure, or they form aggregates enclosed in a common shell. 

Carbon soot, as might be seen in Figure 1.10, already possesses a structure very much alike that of nano-onions. Only the roof tile arrangement of the graphene platelets in soot differs from the concentric pattern of intercalated fullerenes in the onions. Hence, it is self-suggesting to try preparing carbon onions from diverse soot materials. 

The resulting carbon onions normally consist of five to ten shells and agglomerate to be grape-like structures. The individual nano-onions are spherical in parts only, while others are oval in shape (Figure 4.20). In the center of the particles, there is often a shell corresponding in diameter to a C )-molecule. At very high conversion temperatures, however (>1900 °C in general), the carbon onions... 

Apart from the methods described so far, there are several other procedures mainly or partly leading to the formation of nano-onions or onion-like structures. Some of these experiments will be presented below. 

It is clearly to be seen from electron micrographs that the conversion of carbon spiroids into nano-onions really proceeds from the core to the periphery. They show entirely spiral objects at first. These are transformed into completely concentric onions via the hybrid form of an onion core with a spiral shell (Figure 4.30). It is presumably even just a part of the spiroid structures actually present in the sample that are detected in these examinations as their projections appear onion-like at unfavorable orientation. 

A signal of delocalized it-electrons, on the other hand, is not observed in spherical nano-onions. This means that the dimensions of conjugated spMomains are rather limited and that most it-elecfrons are locahzed instead. An additional broad signal arises, however, for the better graphitized, polyhedral nanoparticles obtained... 

Owing to their curved and defective structure, carbon onions are quite easily converted into other forms of carbon. The transformation of spherical particles into faceted nanoparticles by heating to at least 1900 °C has already been described in Section 4.3.5.3 on the thermal produchon of nano-onions from diamond particles. 

Still, when using porous carbons, the desorption of newly generated styrene from the catalyst surface was found to be hindered, and so the conversion was Hmited. The lack of any porosity in carbon onions should clearly be beneficial here, and indeed the conversion and the yields of styrene could be increased when using nano-onions or onion-Uke materiaL In samples of catalyst that had already been used, the onion stracture was found to be partly destroyed. Actually, the catalyst reaches full activity only in this state, which also accounts for the induction period observed. On the onions surface, presumably carbonyl groups and quinoid structures constituting the real active sites are formed. 

Little is known so far about the chemical properties, yet first results suggest a reactivity similar to that of multiwalled carbon nanotubes. Furthermore, a transformation of nano-onions into other forms of carbon can be achieved by heating (equihbration as faceted nanoparticles) or electron bombardment. In large carbon onions, a formation of small diamond clusters due to internal self-compression has been observed. These grow up to be nanoscale diamond particles under complete consumption of the onion structure. 

The latest development in the subfield of surface-modified semiconductor nanocrystals is the synthesis of three-layered colloidal particles [55-58]. The novel structures consist of a size-quantized semiconductor particle acting as the core spherically covered by several monolayers of another semiconductor material, which by themselves are surrounded by several monolayers of, again, the core material acting as the outermost shell. These particles are called quantum dot quantum wells (QDQWs) or, metaphorically, nano-onions. The more scientific naming is motivated by the analogy to real quantum wells, which are semiconductor structures with alternating layers of two semiconductor materials exhibiting quantum confinement in one dimension in at least one of the materials. 

Recently, a functionalization of carbon nano-onions (CNO, multilayer fullerenes) was carried out by [2+1] cycloaddition of nitrenes. The grafted products were prepared by the grafting-from method combining in situ ring-opening polymerization (ROP) and ATRP [127]. 

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