Fullerene-diamond transformation

Fullerene-Diamond Transformation. The rapid compression of Cqo powder, to more than 150 atm in less than a second, caused a collapse of the fullerenes and the formation of a shining and transparent material which was identified as a polycrystalline diamond in an amorphous carbon matrix.O Thus the fullerenes are the first known phase of carbon that transforms into diamond at room temperature. Graphite also transforms into diamond but only at high temperatures and pressures (see Ch. 12, Sec. 3.0). 

Carbon is unique among chemical elements since it exists in different forms and microtextures transforming it into a very attractive material that is widely used in a broad range of electrochemical applications. Carbon exists in various allotropic forms due to its valency, with the most well-known being carbon black, diamond, fullerenes, graphene and carbon nanotubes. This review is divided into four sections. In the first two sections the structure, electronic and electrochemical properties of carbon are presented along with their applications. The last two sections deal with the use of carbon in polymer electrolyte fuel cells (PEFCs) as catalyst support and oxygen reduction reaction (ORR) electrocatalyst. 

Fullerenes are the third natural form of carbon. These have been found to exist in interstellar dust and in geological formations on Earth, but only in 1985 did Smalley, Kroto and co-workers discovered this class of carbon solids and their unusual properties [447, 448]. It has been shown that Ceo, the most common fullerene, could be transformed under high pressure into the other forms of carbon, diamond, and graphite [449] or, at moderately high pressures and temperatures, into new various metastable forms [450 53]. Ceo crystals, fullerites, have/cc structure with weak van der Waals interactions. This structure is stable at ambient temperature up to 20 GPa and at ambient pressure up to 1800 K [454, 455]. 

For synthetic chemists, who are interested in the transformation of known and the creation of new matter, elemental carbon as starting material once played a minor role. This situahon changed dramatically when the family of carbon allotropes consisting of the classical forms graphite and diamond became enriched by the fullerenes. In contrast to graphite and diamond, with extended solid state stmctures, fullerenes are spherical molecules and are soluble in various organic solvents, an important requirement for chemical manipulations. 

Carbon is likely to congeal to high-molecular-weight polymers as H2 distills off. In extraterrestrial environments, we expect lower hydrocarbons eventually to transform into pure carbon, either diamond (in which all the carbons are singly bonded to other carbons), fullerenes and graphite (in which each interaction between a pair of carbons is the approximate equivalent of 1.5 bonds), or carbon bonded to other elements that cannot be converted to a volatile form. 

Diamond, graphite, and the fullerenes differ in their physical and chemical properties because of differences in the arrangement and bonding of the carbon atoms. Diamond is the densest (3.51 vs 2.22 and 1.72 g cm-3 for graphite and Cw, respectively), but graphite is more stable than diamond, by 2.9 kJ mol-1 at 300 K and 1 atm pressure it is considerably more stable than the fullerenes (see later). From the densities it follows that to transform graphite into diamond, pressure must be applied, and from the thermodynamic properties of the two allotropes it can be estimated that they would be in equilibrium at 300 K under a pressure of —15,000 atm. Of course, equilibrium is attained extremely slowly at this temperature, and this property allows the diamond structure to persist under ordinary conditions. 

Considerable attention has been paid to possible mechanisms of formation since a firm understanding of this aspect could lead to the development of more effective synthetic routes to the individual fullerenes. It is also known that, when thin films of Cgo and C70 are laser-vaporized into a rapid stream of an inert gas, individual molecules of Ceo or C70 can themselves coalesce to form stable larger fullerenes such as Cno or C140, and higher multiples. Even more dramatically, when a sample of C o is subjected to a pressure of 20 GPa (i.e. 200 kbar), it apparently immediately transforms into polycrystalline diamond. 

Fullerene crystals can be produced at high yield. By counter diffusion from fullerene solution to pure isopropyl alcohol solvent, fullerene single crystal fibers with needle shape were formed. Needle diameters were found to be 2-100 pm and their lengths were 0.15 = 5 mm. Buckyball-based sintered carbon materials can be transformed into polycrystalline diamonds at less severe conditions using powder metallurgy methods. 

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