Pyrolysis in Mesophase

In contrast to the common gas-phase and solid-phase pyrolytic syntheses for CNTs, fullerene-like carbon nano-materials can also be prepared with a newly developed pyrolytic method in which well-defined polycyclic aromatic hydrocarbons (PAHs) are used under mild reaction conditions [74]. This approach, to some extent, is similar to the classical liquid-phase carbonization of organic compounds (aromatics, petroleum, and coal-tar pitches) [75, 76]. Very recently, this method has been demonstrated for synthesis of various nano- and microcarbons using the compound hexa-alkyl-substituted hexa-peri-hexabenzocoronene (HBC), whose molecular structure is detailed in Figure 6, as a

Figure 6. Molecular structures of substituted HBC (R = CH2-CH2-C10H21) and unsubstituted HBC (R = H) used as precursor compounds for pyrolysis in the mesophase.

starting precursor. All pyrolysis experiments were carried out in evacuated and sealed quartz ampules by electric heating (rate 2 °C/min). The starting compound HBC has a large two-dimensional disk-like graphite subunit and exceptional mesomorphic properties. The flexible alkyl chains (—R) around the graphite disks lower the melting points and induce the formation of stable discotic mesophases in a columnar arrangement of the disk-like molecules resulting from a stacking of the w-layers. During the thermolysis at moderate temperatures (e.g., 400 °C, 72 h), even larger structures can be further built up, preserving the liquid crystalline order. In this step, it is believed that the prolonged heat treatment will provide highly oriented structural units, as a consequence of their mobility in the melt. The synthesis also includes thermal oligomerization of the HBC subunits following the cleavage of alkyl side chains with the preservation of the initial mesomorphic order. After these pretreat-ments, graphitization of the extended PAH structures can occur at a temperature (e.g., 800 °C or 650 °C, 24 h) much lower than the normally used range of 2000-3000 °C [75], which leads to generation of different carbon micro- and nanoparticles in the absence of metal-based catalysts. Different substrates (e.g., quartz and mica) for carbon deposition have been investigated. Many of the carbon objects, such as nanocolumns, nanodonuts, nanosticks, zigzag, and root-like micro-objects, synthesized with this method are new nano-carbon morphologies observed for the first time with this novel method.

One important note in these syntheses that must be mentioned is that the liquid crystalline properties of the precursor organics are required. Without the alkyl side chains (i.e., they are replaced by H, i.e., R = H) in the HBC molecules, an ordered mesophase cannot be formed, which yields black carbon products (disordered and spongy structures) that are completely different from those obtained with the alkyl-substituted HBC after the same pyrolysis treatments. Therefore, a proper control of alkyl chain cleavage and a directional cross-coupling of radical intermediates in an ordered mesophase are key prerequisites for the formation of large mesoscopic structures in the above carbon material preparations [74].

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