The third allotrope of carbon fullerene was discovered in 1985 by Harold W. Kioto (University of Sussex, Brighton, England), Robert F. Curl, and Richard E. Smalley (Rice University, Houston, Texas, the United States).168 In fullerenes each carbon atom is bonded to three other carbon atoms with sp2 hybridization and is composed of 20 hexagons and 12 pentagons as shown in Figure 1.7.
FIGURE 1.7 Schematic representation of the (a) C60 fullerene structure also called buckminsterfullereue and (b) Geodesmic dome built by American architect Richard Buckminster Fuller.
The similarity in the icosahedral symmetry and closed cage structure of fullerene to that of geodesic dome build by American architect Richard Buckminster Fuller, fullerene is called buckminsterfullerene . It consists of 60 carbon atoms arranged in the shape of a soccer ball, hence it is also known as buckyball. The discovery of fullerene received appreciation with Nobel Prize in Chemistry in 1996.169170
SYNTHESIS OF FULLERENE
The invention of fullerenes was first reported in 1985 when Smalley and Kioto noticed a carbon cluster containing 60 atoms generating from their laser vaporization apparatus.171 Since then the research on this new class of allotrope flourished rapidly. The most common synthesis methods of fullerene involve laser vaporization, electric arc heating, laser irradiation of polycyclic hydrocarbons (PAHs), and resistive arc heating. Fullerenes have gained incredible importance in nanoscience due to their outstanding properties and valuable applications. They have been produced from graphite, burning of benzene, combustion of hydrocarbon, and pyrolysis of naphthalene. The high cost and harmful precursors in synthesis is a major concern limiting its application in various fields. Therefore, it is essential to search better alternatives for the production of fullerene using safer and greener sources.
There are only very few reports on the synthesis of fullerene from natural resources. According to the literature review', coal, coke, and camphor wrere the main natural precursors used for its synthesis.172-177 Also, there are reports on natural occurrence of fullerene in ancient carbons and suggested to be present in celestial objects such as meteorites. The accidental discovery of natural fullerenes was made by geochemists at Arizona State University at Tempe during the study of a coal-like mineral taken from rock sediments apparently formed during the Precambrian era, more than 600 million years ago.
Coal fines from tailings in coal preparation plants which are regarded as an ongoing pollution hazard were chosen as the precursor for fiiller- enes. Use of low ash coal as carbon source for fullerene synthesis not only reduces environmental pollution but also transforms into a valuable form with immense potential of applications. The main synthesis method used for the preparation of fiillerene from coal w'as electric arc heating. In electric arc heating, an electric arc was generated between two rods in an inert atmosphere which produces a fluffy soot. The rod should be prepared and it can be changed according to the precursor which is to be used for the synthesis.178
Fullerene was also synthesized from camphor (an extract of a tree usually found in Asia) by the vacuum evaporation of toluene extract of camphor soot. In that method, they replaced the tedious chromatographic separation method with hot filament CVD technique.
PROPERTIES OF FULLERENE
The exceptional chemical properties of fullerene are attributed to its electronic structure.179 Each carbon atom in fullerene is connected to three other carbon atoms resulting in two single bonds and one double bond. Actually there are two types of bonds in C60 fullerene namely, a bond shared between a hexagon and pentagon which acts like single bond and that is shared between two hexagons that acts as a double bond. If the fullerene structures have less number of hexagons in it, then it exhibits more sp3 characteristics such as high reactivity and higher strain. The conjugated system in fullerene is different from that of classical aromatic compounds since there are no replaceable hydrogen atoms that could facilitate substitution reactions. The main two chemical reactions that occur in fullerenes include addition reactions that results in exohedral adduct and redox reactions leading to the formation of salts.180181 Fullerenes behave as electron deficient alkenes irrespective of their extreme conjugation. This is evident from their reactions with nucleophiles, homolytic reagents, and free radicals to produce stable adducts.182
Fullerenes also have outstanding mechanical properties, high pressure resistance, and capacity to return to original shape even after subjecting to more than 3000 atm. The high bulk modulus indicates that they are harder than steel and diamond.183 Fullerenes are allotrope of carbon having huge applications especially in medical, engineering, and pharmaceutical fields. But the high cost of synthesis, use of expensive precursors, and catalysts are hindering the practicability of fullerenes. In this perspective, there is high need for the development of inexpensive, renewable, and environment friendly precursors for the synthesis of fullerenes. Unluckily, the green steps taken toward this material are still in its infancy and need to be modified for a better future. The main synthesis methods used for synthesis of fullerene using natural precursors namely coal, coke, and camphor, their advantages, and properties are discussed.
Carbon, one among the nature abundant element can exist in more than one crystalline form called allotropes. Among the different existing carbon nanomaterials (CNs), NDs is considered as the novel member to nanocarbon family. The term “NDs” include materials consisting of nano-sized tetrahedral networks.184 Diamond nanoparticles usually called as NDs are diamonds with a size below 1 pm.185 The ND structure is a network comprising of nano-sized tetrahedral frameworks of sp3 hybridized carbon atoms with small amounts of sp2 carbons at their surface boundaries.186 Indifferent to other nanocarbon particles, NDs exhibit unique features like inertness, hardness, conductivity, presence of я-electron network, and easiness in huge surface functionalization that allow for tunability of surface.186 A wide variety of diamond-based materials at the nanoscale ranging from single diamond clusters to bulk nanocrystalline films have been produced recently.187 188 Based on molecular arrangements, NDs are classified into nanocrystalline diamond (NCD) particles of size less than 100 mil, ultrananocrystalline diamond (UNCD) particles of size less than 10 inn, and diamondoids having particles in the 1-2 mn range. Diamond-like carbon (DLC) films, is a veiy closely related teim is defined by IUPAC186 as, “are hard, amorphous films with a significant fraction of sp3-hybndized carbon atoms and which can contain a significant amount of hydrogen.”
Nanometer sized diamond are of extra-terrestrial origin, are found in meteorites, protoplanetary nebulae and interstellar dusts, residues of detonation, and in diamond films.189 It is known that primitive chon- dritic meteorites contain approximately 1500 ppm of nanometer-sized diamonds, containing isotopically anomalous noble gases like nitrogen and hydrogen.188 Meteoritic NDs shows potency to provide insights into early solar system formation conditions.188 ND synthesis was first discovered in 1963 by three scientists K.V. Volkov, V.V. Danilenko, and V.I. Elin in All-Union Research Institute of Technical Physics. This synthesis of NDs was discovered accidentally while studying diamond synthesis by shock compression of nondiamond carbon modifications in blast chambers.190
STRUCTURE AND PROPERTIES OF NANODIAMOND
The structure of a ND crystallite consists of a diamond core of sp3 hybrid carbon atoms that are protected by an amorphous surface layer of carbon atoms in the sp: hybridization state (Fig. 1.8). A hybrid layer which exists between the core and the amorphous layer comprises of carbon atoms in both sp3 and sp: hybridization states.191 The sp3 organized carbon elements in the core structure of NDs is responsible for its toughness, hardness, chemical stability, high surface areas, biocompatibility, and resistance to harsh conditions.192193
FIGURE 1.8 (a) Model structure of carbon nanodiamond, (b) closer view of nanodiamond surface covered with surface functional groups and sp:hybridized carbon, (c) spJ hybridized carbon diamond core.
NDs show fascinating optical properties due to the significant band gap (5.49 eV) and high transparency in UY and IR regions.192 193 The diamond core is responsible for high refractive index of 2.4, which provides high light scattering and hence make them advantageous as nontoxic UV protective nanoadditives in sunscreens and polymer coatings.194 Optically active imperfection sites present in nandiamond crystal is responsible for many fluorescent applications.195 196 Stable imperfection sites of nitrogen vacancy can be created in NDs through high energy ion and electron irradiation. In addition to optically active fluorescent defect centers, doping ND core with boron and tritium helps to produce diamonds with applications in the fields of electrical conductivity196 and bioimaging,197 respectively.
The significant advantage of NDs on comparison with other nanocarbon materials is the existence of unpaired electrons in the surface layer which allows tunable surface functionalizations, without compromising the diverse properties of the diamond core.19SND surface functionalization is of eminent relevance regarding the improvement of particles dispersion, changing their wettability and adhesion, enhancement in catalytic properties. The 1,3-propanediamine functionalized ND shows high solubility in acetone, CH2C1„NMP, DMF, DMAc, and DMSO. The interaction of ND with sodium bis (2-ethylhexyl) sulfosuccinate resulted in good aqueous dispersions.197
SYNTHETIC METHODS FOR NANODIAMOND
Several methods like detonation technique, hydrothermal synthesis, CVD, microwave plasma chemical vapor deposition (MWPCVD), laser bombarding, ion bombarding, autoclave using supercritical fluids, ion irradiation of graphite, chlorination of carbides, ultrasound cavitation, electron irradiation of pure carbon sheets, and EC methods are available for the synthesis of NDs.199-201 High temperature and pressure conditions are a major requirement for the afore mentioned methods.