Graphene planar structure

Graphene is also used as catalyst support in PEFCs as it offers high conductivity, facile electron transfer and large surface area [151,152]. The planar structure of graphene allows its edge and basal planes to interact with the nanoparticles of the electrocatalyst [100],... 

Graphene is by no means the only possible two-dimensional arrangement of carbon. Recently a variety of planar structures containing carbon—carbon triple bonds, graphynes, have been proposed and considerable effort has been devoted to preparing them. Examples of these structures are in Figure 8.23. 

Itami and coworkers established that pyrene can be selectively coupled with an arylboronic acid in an intermolecular fashion in the presence of a Pd-catalyst and stoichiometric amounts of chloranil as an oxidant [73]. With suitable coupling partners small graphene-type structures are accessible, which can be planarized using cyclodehydrogenation conditions (Scheme 28). 

Thus, we tried to synthesize a rGO-based catalyst by preserving the characteristic planar structure of the graphene sheets, to maintain the same electric properties of the starting support, and by incorporating surface active ensembles between the sheets, to favor the catalytic activity, as in the ideal scheme of Fig. 5a. The RDE measured ORR activity of the Fe-Nx/rGO catalyst was not as good as expected with an half-wave potential of 0.58 V versus RHE, its performance was 60 mV lower compared to the Fe-Nx/MPC catalyst previously developed (Fig. 6, Table 2). Interestingly, the Nx/iGO, produced without iron, showed a very limited activity toward the ORR, 0.27 V versus RHE, much lower compared to the Fe-Nx/rGO catalyst, sign that probably also N/C ensembles show activity toward ORR. 

Graphene The structure of graphene consists of two-dimensional (2D) layers of carbon atoms ordered into a honeycomb lattice as shown in Fig. la. This planar monolayer of carbon atoms with carbon-carbon bond length of 0.142 nm is one of the allotropes (carbon nanotube, fullerene, diamond) of elemental carbon [13]. The free electrons in graphene behave like massless relativistic particles, which... 

Polyacetylene can be considered as a linear, one-dimensional, conductor. To achieve longer range electron mobility, it is desirable to construct a two-dimensional conductor in which the 7t electron cloud extends in a plane.To visualise this, consider a number of polyacetylene chains laid alongside each other, and then replace the carbon to hydrogen bonds on each chain by interchain carbon to carbon bonds. Such sheets are made up of only carbon atom and are relatively inert to oxidation. If the sheet is present as a simple planar structure then it is called graphene, which is an exfoliated form of graphite (Figure 14.3). 

In recent years, as a two-dimensional material (2D), graphene has garnered increasing attention on the horizon of materials science [126]. They possess remarkable and unique properties, such as unique planar structure, remarkable mechanical properties, extremely high specific surface area, exceptional optical properties, and fascinating electrraiic transfer at room temperature [127]. These outstanding... 

Dai, ]., Zhao, Y., Wu, X., Zeng, X.C., and Yang, J. (2013) Organometallic hexahapto-functionalized graphene band gap engineering with minute distortion to the planar structure. /. Phys. Chem. C, 117, 22156-22161. 

The basic building block of carbon is a planar sheet of carbon atoms arranged in a honeycomb structure (called graphene or basal plane). These carbon sheets are stacked in an ordered or disordered manner to form crystallites. Each crystallite has two different edge sites (Fig. 2) the armchair and zig-zag sites. In graphite and other ordered carbons, these edge sites are actually the crystallite planes, while in disordered soft and hard carbons these sites, as a result of turbostratic disorder, may not... 

Noncovalent functionalization of graphene is important, as it does not affect the electronic structure and planarity of this 2D material. Stable aqueous dispersions of polymer-coated graphitic nanoplatelets can be prepared through an exfoliation and... 

Figure 9.5(d) gives an impression about the topo-chemical nature of the hydrogen atom s attack on carbon. Even these highly reactive species attack carbon not in an isotropic form but react from the edges and thus decorate, after some extent of conversion, the planar shape of the BSU as stacks of graphene layers with uneven but identical outer shapes. The rounded protrusions into the edge structure arise from defect clusters that would manifest themselves in a perpendicular view as etch pits . 

One way to define the structure of single-walled CNT is to think of each CNT as a result of rolling a graphene sheet specifying the direction of rolling and the circumference cross-section (Figure la). The unit vectors of the planar... 

Fullcrenc black is of a similar constitution and secondary structure as normal carbon black, but with the important difference that its BSUs are composed of non-planar graphene layers. Additionally, the stacks arc not straight but curved, giving rise to a more disordered and less dense carbon material. The chemical reactivity is higher as the stability of nonplanar graphene layers is reduced. In addition, the irregular shape... 

Figure 9. Organization of carbon sheets in nanocrystalhne carbons. Case (A) represent bent sheets which are stacked concentrically and randomly. In the high-resolution TEM of fullerene black filamentous structures from carbon macrocycles which did not react to fullerene molecules can also be seen (weak contrasts). Case (B) shows the arrangement of stacks of planar graphene units. Only few of these stacks are oriented with the o-axis parallel to the electron beam and can be imaged as illustrated in the sketch, the majority are randomly orientated and give hence an amorphous contrast. Case (C) is the same as (B) with the addition of covalent bonding interactions between the stacks. These bonds are invisible by electron microscopy but influence the graph-iti/ability of the carbon.

The source of all carbon relevant to the present context is the feedstock of hydrocarbon molecules (aliphatic, aromatic, with and without heteroatoms). Figure 10 summarizes the possibilities for their conversion into black carbon. The chemical route comprises polymerization into aromatic hydrocarbons with final thermal dehydrogenation. This process often includes a liquid crystalline phase immediately before final solidification. In this phase large aromatic molecules can sclf-organizc into parallel stacks and form well-ordered precursors for graphitic structures with large planar graphene layers. This phase is referred to... 

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