2.1 Carbon nanomaterials  (Page 18/9)

Introduction

Although nanomaterials had been known for many years prior to the report of C ₆₀ the field of nanoscale science was undoubtedly founded upon this seminal discovery. Part of the reason for this explosion in nanochemistry is that while carbon materials range from well-defined nano sized molecules (i.e., C ₆₀ ) to tubes with lengths of hundreds of microns, they do not exhibit the instabilities of other nanomaterials as a result of the very high activation barriers to their structural rearrangement. As a consequence they are highly stable even in their unfunctionalized forms. Despite this range of carbon nanomaterials possible they exhibit common reaction chemistry: that of organic chemistry.

The previously unknown allotrope of carbon: C ₆₀ , was discovered in 1985, and in 1996, Curl, Kroto, and Smalley were awarded the Nobel Prize in Chemistry for the discovery. The other allotropes of carbon are graphite (sp ² ) and diamond (sp ³ ). C ₆₀ , commonly known as the “buckyball” or “Buckminsterfullerene”, has a spherical shape comprising of highly pyramidalized sp ² carbon atoms. The C ₆₀ variant is often compared to the typical soccer football, hence buckyball. However, confusingly, this term is commonly used for higher derivatives. Fullerenes are similar in sheet structure to graphite but they contain pentagonal (or sometimes heptagonal) rings that prevent the sheet from being planar. The unusual structure of C ₆₀ led to the introduction of a new class of molecules known as fullerenes, which now constitute the third allotrope of carbon. Fullerenes are commonly defined as “any of a class of closed hollow aromatic carbon compounds that are made up of twelve pentagonal and differing numbers of hexagonal faces.”

The number of carbon atoms in a fullerene range from C ₆₀ to C ₇₀ , C ₇₆ , and higher. Higher order fullerenes include carbon nanotubes that can be described as fullerenes that have been stretched along a rotational axis to form a tube. As a consequence of differences in the chemistry of fullerenes such as C ₆₀ and C ₇₀ as compared to nanotubes, these will be dealt with separately herein. In addition there have also been reports of nanohorns and nanofibers, however, these may be considered as variations on the general theme. It should be noted that fullerenes and nanotubes have been shown to be in flames produced by hydrocarbon combustion. Unfortunately, these naturally occurring varieties can be highly irregular in size and quality, as well as being formed in mixtures, making them unsuitable for both research and industrial applications.

Fullerenes

Carbon-60 (C ₆₀ ) is probably the most studied individual type of nanomaterial. The spherical shape of C ₆₀ is constructed from twelve pentagons and twenty hexagons and resembles a soccer ball ( [link] a). The next stable higher fullerene is C ₇₀ ( [link] b) that is shaped like a rugby or American football. The progression of higher fullerenes continues in the sequence C ₇₄ , C ₇₆ , C ₇₈ , etc. The structural relationship between each involves the addition of six membered rings. Mathematically (and chemically) two principles define the existence of a stable fullerene, i.e., Euler’s theorem and isolated pentagon rule (IPR). Euler’s theorem states that for the closure of each spherical network, n (n ≥ 2) hexagons and 12 pentagons are required while the IPR says no two pentagons may be connected directly with each other as destabilization is caused by two adjacent pentagons.