Nanocarbon synthesis

One synthesis approach that does not rely on CNT formation from the gas phase is molten salt synthesis. The reactor consists of a vertically oriented quartz tube that contains two graphite electrodes (i.e. anode is also the crucible) and is filled with ionic salts (e.g. LiCl or LiBr). An external furnace keeps the temperature at around 600 °C, which leads to the melting of the salt. Upon applying an electric field the ions penetrate and exfoliate the graphite cathode, producing graphene-type sheets that wrap up into CNTs on the cathode surface. Subsequently, the reactor is allowed to cool down, washed with water, and nanocarbon materials are extracted with toluene [83]. This process typically yields 20-30 % MWCNTs of low purity. 

Fig. 5.1 Schematic representation of templated synthesis of nanocarbon hybrids with (a) nanocarbon as template and (b) nanocarbon as hybridizing component.

In general, the various synthesis strategies for nanocarbon hybrids can be categorized as ex situ and in situ techniques [3]. The ex situ ( building block ) approach involves the separate synthesis of the two components prior to their hybridization. One can rely on a plethora of scientific work to ensure good control of the component s dimensions (i.e. size, number of layers), morphology (i.e. spherical nanoparticles, nanowires) and functionalization. The components are then hybridized through covalent, noncovalent or electrostatic interactions. In contrast, the in situ approach is a one-step process that involves the synthesis of one of the components in the pres-... 

In situ growth via covalent binding of a hybridizing component to a nanocarbon can be achieved in the case of polymers, dendrons and various other macromolecules which are synthesized in a stepwise manner. The in situ synthesis of such macromolecules potentially increases binding site density while steric effects of the nanocarbon can lead to increased variation in average polymer chain length (polydispersity) [101 103]. 

The highlighted examples of gas phase hybridization clearly show that they are well suited for the deposition of metals and metal oxides. They are most practical when the nanocarbons are either surface bound or grown from a surface, such as CVD grown graphene and CNT forests. Improved control of layer thickness can be obtained, in comparison to wet chemical approaches, but the synthesis strategies generally require sophisticated equipment not often present in a laboratory. 

The greatest advantage of in situ methods over ex situ processes is the benefit of using the nanocarbon as a substrate, template and heat sink for stabilizing metastable phases and small particle sizes and creating hybrids with unusual morphologies [232]. This enables the synthesis of new hybrid materials that may offer new properties and unknown potential for future research and application. 

This chapter demonstrates the huge variety of synthesis techniques available for the preparation of nanocarbon hybrids, which can be categorized into ex situ and in situ approaches. 

As subsequent chapters will document, the type, structure and quality of the nanocarbon have a considerable impact on the final performance of the nanocarbon hybrid. Currently, most publications on the synthesis of nanocarbon hybrids focus on GO, which is both easy to prepare and simple to hybridize. However, the mechanical and electrical properties of GO (and also RGO) are often inferior to their pristine counterparts and in fact closer to those of activated carbon. Hence, we recommend always synthesizing and comparing various types of nanocarbons with different features and functionalizations. 

Finally, the reproducibility of the nanocarbons and their hybrids is of paramount importance when implementing them into commercial devices. This will require the definition of key characteristics and the development of standard synthesis methodology that will also enable better comparison of results between research groups. 

Fig. 8.9 Different methods for spinning CNT fibers and scanning electron micrographs of representative samples, (a) Wet spinning of nanocarbons dispersed in liquid, (b) drawing from a forest of aligned CNTs and (c) direct spinning from the gas phase during CNT synthesis by CVD. Images from references [53,59, 60, 61,62], With kind permission from AMS (2000, 2013), Elsevier (2007, 2011), Wiley (2010).

This approach, however, requires the absence of ill-defined carbon deposits originating from defect-induced soot formation on the surface of nanocarbons during their synthesis. Pyrolytic structures often counteract the control over activity and selectivity in catalytic applications of well-defined nanocarbons by offering an abundance of highly reactive sites, however, in maximum structural diversity. Although some nanocarbons are equipped with a superior oxidation stability over disordered carbons [25], such amorphous structures can further induce the combustion of the well-ordered sp2 domains by creating local hotspots. Thermal or mild oxidative treatment,... 

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