Chemical precursors, carbon atom generation

While evaporative methods of generating C atoms utilize bulk carbon and a great deal of thermal or photochemical energy, another approach is to use the exother-micity of the decomposition of a suitable precursor to produce C atoms. These methods yield what are called chemically generated carbon atoms and will be outlined below. 

Sol-gel processing almost exclusively involves metal alkoxides as precursor chemicals. An alkoxide is an alkane with an oxygen interposed between at least one of the carbon atoms and the metal—for example, (OC2H5)4Si, known as tetraethoxysilane (TEOS, a.k.a. tetratethyl orthosilicate). Other common alkoxide precursors are listed in Table 7.15. TEOS is used to produce the sol-gel-derived silica that generated the X-ray diffraction pattern in Figure 1.50. Let us use silica as an example to study the steps in the sol-gel process. 

These complexes are of great interest as model compounds for understanding the functions and relationships of several biological macromolecules, as well as for their chemical reactivity. In the latter case the insertion of small molecules between the metal ion and the carbon atom may result in activation of the inserted molecule or may generate new monomeric or polymeric materials. In addition, metal-carbon o-bonded porphyrins can act as precursors in the synthesis of metal-metal bonded derivatives. 

The synthesis, chemical properties, and electrochemistry of metallo-porphyrins with metal-metal and metal-carbon bonds have recently been described in two extensive reviews Porphyrins are known to coordinate with most metallic and pseudo-metallic elements and, in theory, synthesis of numerous organometallic porphyrins is possible. However, to date, the synthesis of metal-carbon a-bonded complexes has been limited to metallo-porphyrins with the following central metals Fe, Ru, Co, Rh, Ir, Ti, Al, Ga, In, Tl, Si, Ge, Sn and Zn. These organometallic complexes are of importance as model compounds for understanding the functions and relationships of several biologically important macromolecules, as well as for their chemical reactivity. The insertion of small molecules between the metal ion and the carbon atom of a metalloporphyrin may result in activation of the inserted molecule or may generate new monomeric or polymeric materials. In addition, metal-carbon o-bonded porphyrins can act as precursors in the synthesis of metal-metal bonded derivatives. 

A more extensively investigated precursor to chemically generated C atoms is diazotetrazole (6), which is easily prepared from readily available 5-aminotetrazole (7). " In this method, 7 is converted into the corresponding diazonium chloride 8, which is coated on the walls of a flask and pyrolysed in the presence of a gaseous substrate (Eq. 6). This technique has the drawback that 8 is extremely explosive and only small quantities can be prepared at a time. " However, the synthesis of 7 with a labeled carbon is quite simple, allowing convenient evaluation of the fate of the reacting carbon. " ... 

The thermal decomposition of organic compounds can also be employed to generate small carbon clusters or atoms. The borderline with chemical vapor deposition (CVD) as presented in the next section is not really fix. In both cases, the method is based on the thermal decomposition of organic precursors. Processes both with and without catalyst have been reported. Contrary to the chemical vapor deposition, however, the catalyst (if applied) is not coated onto a substrate, but the substance or a precursor is added directly to the starting material ( floating catalyst ). The resulting mixture is then introduced into the reactor either in solid or in liquid state by a gas stream. From this point of view the HiPCo-process could also be considered a pyrolytic preparation of SWNT, but due to its importance it is usually regarded as autonomous method. 

The first MWNTs have been obtained as early as 1976 by iron-catalyzed pyrolysis of benzene. Apart from that, there is a number of methods to produce MWNT, which all of them differ in the way of generating small carbon clusters or atoms from the respective starting materials. They include arc discharge, laser ablation, chemical vapor deposition with and without plasma enhancement or the catalytic decomposition of various precursor compounds. It turned out that MWNTs from low-temperature syntheses bear more defects and, as a whole, are less ordered than those generated at high temperatures. However, these drawbacks can still be compensated by subsequent recuperation of defective samples at elevated temperatures. 

Like in the preparation of single-walled carbon nanotubes, the chemical vapor deposition of MWNT consists in the generation of small carbon clusters or atoms from precursor compounds. The products precipitate in the shape of different carbon materials with the reaction conditions determining the specific stracture... 

The generation of such intermediate products does not take place in the case of MBE. Here the sources are either purely elemental or at least do not contain hydro-organic compounds. This means that reactive CHj or H radicals cannot be produced during or before MBE growth. The formation of such chemical byproducts in the form of carbon hydrides or hydrogen occurs during the pyrolysis of aU the precursors mentioned in Table 13.2 and likewise influence the surface formation of aU III-V and II-VI compounds. This influence on the atomic structure of the growing surface can take place in two ways ... 

---