Faceted carbon nanoparticles

Figure 4.24 Mechanism of the formation of carbon onions from faceted nanoparticles.

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. 

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. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

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. 

Due to this largely graphitic character, the faceted carbon nanoparticles are more stable than their spherical analogs. In the latter, the graphitic interaction between layers is much less pronounced due to the multitude of defects and the random arrangement of shells. In the parallel domains of the nanoparticles, on the other... 

Figure 4.7 (a) HRTEM image of a faceted carbon nanoparticle ( Royal Soc. 1996) ... 

For carbon-based electrocatalyst supports, there is no question that advanced supports have been made that enhance the dispersion of Pt nanoparticles. Much of the time, this advanced dispersion has led to higher EGA compared to Pt/Vulcan and Pt/TKK however, this has not always translated into an increase in the Pt mass activity. Observations on Pt mass activity appear to be a balance between EGA and particle faceting, which plays a significant role in the ORR [166], as the particle size is altered at the same loading as Pt/G. In cases where the Pt mass activity was increased, the specific activity (when reported) always decreased, calling into... 

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. 

The arrows in the figure indicate the hydrogen desorphon peaks for the different facets. The shape-selected nanoparticles show very pronoimced peaks compared to the carbon-supported... 

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