Polyhedral particles

Electron diffraction studies [3] have revealed that hexagons within the sheets are helically wrapped along the axis of the nanotubes. The interlayer spacing between sheets is 0.34 nm which is slightly larger than that of graphite (0.3354 nm). It was dso reported [2] that the helicity aspect may vary from one nanotube to another. Ijima et al. [2] also reported that in addition to nanotubes, polyhedral particles consisting of concentric carbon sheets were also observed. 

The thermal treatment of fullerene black generates nanometric polyhedral particles] 13]. This experiment... 

Fig. 3. Schematic illustration of the growth process of a graphitic particle (a)-(d) polyhedral particle formed on the electric arc (d)-(h) transformation of a polyhedral particle into a quasi-spherical onion-like particle under the effect of high-energy electron irradiation in (f) the particle collapses and eliminates the inner empty space[25j. In both schemes, the formation of graphite layers begins at the surface and progresses towards the center.

Figure 20. A TEM snapshot of large polyhedral particles prepared by the inverse micelle system being broken by the ligand addition. The ligand featured here is decanethiol. (Reprinted with permission from Ref. [32], 2005, American Chemical Society.)...

The authors (iSl) and Kmetko (ISS) have confirmed definitely, from electron micrographs, that graphitization of carbon blacks of low surface area produces polyhedral particles. 

Usually it is difficult to separate the effect of ciystallite size on carbon reactivity from the effects of crystallite orientation and impurity content. However, Armington (62) attempted to do so by reacting a series of graphi-tized carbon blacks with oxygen and carbon dioxide, as discus.sed earlier in this article. Assuming that upon graphitization all the carbon blacks are converted to polyhedral particles with the surface composed almost completely of basal plane structure, it is possible to eliminate crystallite orientation as a variable. Spectroscopically, the total impurity content of all the graphitized carbon blacks is quite low and to a first approximation, the analyses of the individual constituents are similar. 

The dependence of the amount of H2SO4 that has reacted with 10 g of 3BS or 4BS paste as a function of soaking time is shown in Fig. 3.10. 4BS pastes react with H2SO4 much more slowly than do 3BS pastes. The smaller surface area and the greater thickness of the crystals in 4BS pastes cause the rate of the reaction of sulfation to slow down within the first 30 min of soaking [11-16]. The structure of the lead sulfate layer that covers the 4BS crystals has been examined [11-14]. First, polyhedral particles with an average size of up to 2 pm are formed on the surface of... 

SEM studies on the cathode deposit reveal multiple features. Curved and compact graphite layers are deposited on the outer shell. Inner core deposits contain bundle-like structures, which contain randomly distributed nanotubes and polyhedral particles (Fig. 9). It is possible that, as a result of carbon vapour... 

In the presence of air, the onset of nanotube weight loss occurs at ca. 700°C (Fig. 11). The mass then decreases rapidly and, at ca. 860°C, the nanotubes are completely oxidised to CO and CO2. By comparison, Cgo is less resistant to oxidation. Figure 11 compares the weight loss vs temperature for inner core deposits (containing nanotubes and polyhedral particles) and Cgo- Thus nanotubes constitute one of the forms of carbon most resistant towards oxidation. 

Fortunately, non-destructive methods for separating carbon nanotubes from polyhedral particles have been reported [87,88]. These involve the use of well-... 

Fig. lOa-c. TEM images a bulk material containing nanotubes and polyhedral particles, 1 2 ratio b oxidised sample (10.5 % of the original mass remaining), exhibiting a decrease in the number of polyhedral particles c oxidised sample (1% of the mass remaining), showing essentially pure nanotubes... 

Another recent report describes the large scale synthesis of ahgned carbon nanotubes, of uniform length and diameter, by passage of acetylene over iron nanoparticles embedded in mesoporous silica [107]. The latter two methods, based on the pyrolysis of organic precursors over templated/catalysts supports, are by far superior by comparison with plasma arcs, since other graphitic structures such as polyhedral particles, encapsulated particles and amorphous carbon are notably absent (Fig. 16). 

Silica is reported to be an essential component even in a virus. Faust and Adams (30) isolated a crystalline virus consisting of polyhedral particles from lepidopterous larvae (Bombyx mori, etc) and found that it contained silicon,- corresponding to 0.2-0.6% SiO as an integral part of the protein matm. 

Terrones, H., Terrones, M., 1997, The transformation of polyhedral particles into graphitic onions, J. Phys. Chem. Solids 58, 1789-1796. 

All the polyhedral particles show more than one resonance in contrast to sphere, where a single peak can be found. 

Cohen-Stuart, M.A. (2008) Synthesis of polyhedral particles by dispersion polymerization in supercritical carbon dioxide. Colloid and Polymer Science, 286,1435 536. 

Figure 3. (A) a-A Oa polyhedra particles formed at 36 ipm. (B) a-A Oa polyhedral particles formed at 36 rpm and 4x10 seeds/ml. (Reproduced with permission from ref. 23. Copyright 1997 The American Ceramic Society.)...

The importance of the particle shape is related to the development of facets with differing photocatalytic efficiencies [78-80]. Consequently, polyhedral particles have been reported to exhibit, in general, a higher efficiency than spherical ones (Fig. 10.11) [81], In addition to the effects of facets per se, the shape can also influence the density of surface structural defects [82], These defects have been shown to modify the efficiency by both attracting or repelling one of the charge carriers and changing the adsorption properties. 

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