Metal-free Functionalized Carbons in Catalysis: Synthesis, Characterization and Applications
CHAPTER 2: Non-covalent Methodologies for the Preparation of Metal-free Nanocarbons for Catalysis
Published:24 May 2018
Anastasios Stergiou, Nikolaos Karousis, Dimitrios Tasis, 2018. "Non-covalent Methodologies for the Preparation of Metal-free Nanocarbons for Catalysis", Metal-free Functionalized Carbons in Catalysis: Synthesis, Characterization and Applications, Alberto Villa, Nikolaos Dimitratos
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Carbon nanotubes (CNTs), graphenes, carbon dots, nanodiamonds and fullerene buckyballs are dimensionally confined sp2/sp3 hybridized carbon allotropes which constitute the nanocarbon family. The exotic and often unprecedented intrinsic properties of nanocarbons (structural, conductive, electrocatalytic) have garnered tremendous interest for both fundamental and applied research. These properties are further coupled with a high surface area, high optical transmittance, curvature-dependent chemical reactivity and thermal stability. All properties are dependent upon a variety of factors such as C–C bond hybridization, nanocarbon size, dimensionality, layer number, density/type of defect sites and synthesis strategy. For example, multiwalled carbon nanotubes (MWCNTs) produced by arc-discharge have a more graphitic structure than the corresponding material prepared by a chemical vapor deposition process. Concerning the effect of defect sites, graphene oxide (GO) (a single layer of highly oxidized graphene) possesses a high population of sp3-hybridized carbon atoms and a corrugated morphology. The presence of such defect sites significantly increases electrical resistance, making the nanomaterial an actual insulator. This behavior is in sharp contrast with the conductive properties of pristine graphene, in which the basal plane consists solely of sp2-hybridized carbon atoms, thus permitting the electron cloud to be delocalized onto the graphitic lattice. Depending on the targeted application, it is desirable that some intrinsic properties of the nanocarbon are kept as in the parent form, whereas another family of properties should be altered in a beneficial optimization. This is a key strategy to enhance the performance and the multifunctional character of nanocarbons. For example, in energy conversion/storage applications, the carbon nanostructures should possess high electrical conductivity combined with optimized electrocatalytic properties for enhanced charge transfer interaction with their chemical environment. The performance, as well as the number of potential applications, of nanocarbons can be further increased by combining them with an additional component, thus forming a composite material. Many nanocarbon hybrid materials display enhanced and even new properties in relation to their individual components; these are commonly attributed to synergistic effects between the two components. Fabrication of nanocarbon hybrid structures can be achieved by surface modification in two general ways: (i) covalent attachment of functionalities through oxidation/addition reactions onto a graphitic network or (ii) physical adsorption of substances onto a nanostructured carbon surface through hydrophobic interactions. The latter approach of non-covalent modification mostly relies on stacking interactions between the adsorbates and carbon surfaces. Common chemical structures that exhibit stacking interactions with nanocarbon surfaces include pyrenes, macrocyclic complexes (for example, porphyrins), surfactants, polymers and metal-based nanoparticles among others. This chapter is dedicated to the synthesis of metal-free carbon-based hybrid nanostructures, obtained by non-covalent functionalization schemes. We will first present an overview of the various synthesis strategies of nanocarbon hybrids for each class of carbon allotropes. This includes physical adsorption of substances in a liquid environment through hydrophobic interactions, in situ processes, utilization of sacrificial templates, heteroatom doping and others. The hybrid nanomaterials, which will be discussed in this chapter, are utilized as multifunctional components in (electro)catalytic processes. Such nanocarbon hybrids show promise in a range of key applications, with the most promising ones highlighted in this chapter, including energy conversion, (photo)catalysis, energy storage, and sensing. For each application, we will discuss current challenges in the respective field and their potential solution by using nanocarbon hybrids and present some of the most intriguing examples.