Carbon onions characterization

Today nanotechnology includes the synthesis, characterization, and application of a variety of nanostructured materials. Different carbon nanostrucmres exist simultaneously at the nanoscale, including carbon nanotubes, carbon onions, nanodiamond (ND), and diamondoids, all showing unique and novel properties [1]. 

The Raman characterization of different carbon nanomaterials in inert (Ar) atmosphere reveals a strong influence of the laser power (energy density) on the Raman spectra [59]. In general, an increase in the laser power leads to a decrease in the Raman frequency. For example, when using the most common excitation source in Raman spectroscopy - the 514-nm line of an Ar-ion laser - the G Band in the Raman spectrum of carbon onions (Fig. 12.22a) shifts from 1,594 cm (0.1 mW) down to 1,565 cm (0.7 mW). The downshift is related to an increase in the sample temperature and has been measured for other carbon materials including graphite and CNTs. 

Multiwalled nanotubes suggest themselves as a comparison structure for onions the same as the fullerenes do. like with the MWNT, the poor solubUity of carbon onions poses problems in chemical conversions. Further compUcations arise from the inhomogeneities regarding diameter and number of shells as instead of defined products, there will always be a mixture that is much harder to characterize. 

Core-shell carbons can also be produced by floating catalysts. For example, a picric-acid-detonation-induced pyrolysis of FcH or Fe(CO)5 gave carbon-encapsulated Fe NPs (5-20 nm) [29]. The reaction is characterized by a self-heating and extremely fast process. Tubular structures are formed at a high C/Fe ratio in this reaction. The pyrolysis of metallocenes such as FcH, cobaltocene, or nickelocene is known to yield CNTs and metal-fiUed onion-like structures [21]. The wall thickness (diameter) is controlled by the FcH content. Carbons onions have been synthesized in a CVD reactor at a temperature of 900 °C using Fe3(CO)i2 as the catalyst under an Ar/02 atmosphere [29]. 

In contrast to the spherical carbon onions observed in the first experiments by Ugarte, OLC particles were subsequently produced with polyhedral facets, more closely matching the polyhedral structures predicted from the consideration of nested fullerene structures described above. These polyhedral onion-like particles were synthesized by vacuum heat treatment of carbon sooF and diamond nanoparticles." Figure 10.5 presents HRTEM images of the polyhedral OLC particles produced in the experiments of Kuznetsov et al. The range of synthesis methods available has led to the production of different types of OLC. In addition to their shape, such carbon onions can be characterized by other parameters, such as the number of concentric shells, the spacing between adjacent shells, the size of the innermost shell, and the presence of different types of defects. 

Synthesis of carbon onions through heat treatment of soot by de Heer and Ugarte represented the first approach in producing carbon onions in macroscopic quantities. This is an important requirement for many further characterization steps and for any potential applications of this material. These carbon onions consist of hollow carbon onions, with between 2 and 8 graphitic shells, and their production in macroscopic quantities by this experimental approach enabled their examination by ultraviolet-visible (UV-Vis) and Raman spectroscopy. It was proposed that carbon onions could be a possible carrier of the 2175 A interstellar absorption bump. ° Raman spectra of this material revealed pronounced differences to other graphitic materials. ... 

Cabioc h et al. developed a method based on carbon ion implantation into a metal matrix (Ag, Cu), resulting in onions with typical diameters in the 3-15 nm range. Snfficient quantities could be produced for investigation of their optical, electronic, and tribological properties. Fourier transform infrared (FTIR) spectroscopy measurements on these carbon onions demonstrated that the most stable state for the onions consists of concentric spheres of fnllerenes. The electronic properties of the onions were characterized by spatially resolved electron energy loss spectroscopy (EELS) in transmission, and reflection mode. ... 

Spectra obtained at 244 nm are presented in Figure 3.15. This comparison clearly shows the presence of nanodiamond only in the carbon onions containing a diamond core. These results thus confirm observations made by TEM and EELS at the nano scale. It also proves the usefulness of UV Raman spectroscopy to characterize carbon compounds at the macroscopic scale. 

A characterization of wear particles collected at the end of the friction test with a carbon onion containing a diamond core confirms that wear occnrs during friction. EELS chemical mapping reveals that wear particles are composed of carbon and iron (Figure 3.23). 

PREPARATION AND CHARACTERIZATION OF FULLERENES, CARBON NANOTUBES, AND CARBON ONIONS... 

We hope that macroscopic samples of quasi-spherical onion-like particles will soon become available, and then we will be able to characterize these systems in detail. Probably a new generation of carbon materi-aks can be generated by the three-dimensional packing of quasi-spherical multi-shell fullerenes. 

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. 

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. 

The sp -hybridized constituents of the nanodiamond sample (mostly the outer layers of the particles and, in parts, the material in the interstices of the agglomerates under examination) are characterized by two bands that have already been mentioned in the carbon nanotubes and onions the G- and the D-band. The exact position of the G-band (observed at about 1580 cm" ) depends on both the excitation wavelength (dispersion) and the particle size. With decreasing wavelength of... 

Fig. 2. Traces along the proton dimension of 2D-WISE experiments performed on 10% hydrated (left) and 35% hydrated (right) onion cell wall material. The corresponding carbon resonances are given. Proton spectral widths of 150 and 70 kHz were used for the 10 and 35% hydrated samples respectively. Reprinted from Carbohydr. Res., Vol. 322(1-2), S. Hediger, L. Emsley and M. Ficher, Solid-state NMR characterization of hydration effects on polymer mobility in onion cell wall material , pp. 102-112, Copyright 1999, with permission from Elsevier Science.

Results TEM characterization indicated a uniform distribuhon of metal parhcles on Vulcan XC 72 (Fig. 2.24). The carbon-supported homogeneously alloyed Cu/Pt nanoparticles (Fig. 2.24(a)) exhibit a mean particle size of 3.6nm (occasionally agglomerated) the onion-type Pt Cu type catalyst (Fig. 2.24(b)), in contrast, shows a broad particle size distribution (up to 12 nm). 

Figure 12.7 Structure of carbon fiber, (a) A schematic iiiustralion of tree trunk or onion skin structure (ieft) and radial structure (right), (b) A typicai opticai micrograph of carbon fiber cross sections under polarized light in crossed nicols condition showing maltose cross patterns. Polarizer and analyzers are parallel to picture edges. Source Reprinted from Nyo H, Heckler AJ, Hoemschemeyer DL, Characterizing the structures of PAN based carbon fibers, 24 Nat Symposium, San Francisco, 179, 51-60, May 8-10.

Figure 3.10 TEM characterization of the as-prepared carbon Fe-LiF composites. Top ieft BF-TEM overview image. Top right iron-rich nanoparticie inside a tubuiar graphitic structure. Bottom iron-rich particies confined by onion-iike graphite. Based on the observed iattice fringes, the two particies indicated were identified to be a-Fe [001] and FesC [001].

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