7 Computational and Experimental Analysis of Carbon Functional Nanomaterials
275
as O, N, B, S, P, and their corresponding functional groups [59]. Catalytic active
sites on graphene carbon nanomaterials may be generated by tuning electronic
structures. Several approaches are reported. First of all, introducing intrinsic defects
and the edge topological structures [60] may facilitate the electron transfer which
is beneficial to the conductivity and catalytic activity of carbon nanomaterials.
Therefore, to enhance the catalytic activities is probable by optimizing the topology
through the charge density analysis on these defective carbon materials by DFT
methods. Doping atoms in graphene-based nanomaterials is an alternative approach,
based on which carbon atoms are substituted by heteroatoms, such as N, P, and B,
or bonded with heteroatoms (O, S, Se, Cl, Br, and I) on carbon surfaces or edges
[61]. Such an atomic replacement results in distinct difference of electronegativity,
thus leading to localization of, the charge and spin densities and enhancement of
the catalytic activities. The active sites are located at the carbon atoms nearby the
dopant or at edges, or dopant atom itself. The DFT calculations verified that doping
heteroatom (N, O, B, S, Cl, P, Cl, Br, and I) in graphene remarkably changed
the charge density and spin density distributions on the doped carbon materials
and consequently enhanced their catalytic and electrocatalytic properties [62, 63].
Carboxyl (-COOH), carbonyl (-C=O), and hydroxyl (–C-OH) containing oxygen
functional group exhibited the synergistic effect of edge defects on graphene to
enhance catalytic activities [64]. The final approach is to physically adsorb organic
molecules on graphene or hybrid structure of N-doped graphene [62]. Defects on
carbon materials can induce the electron transfer between the organic molecule
and the graphene or between the graphitic carbon nitride (g-C 3 N 4 ) and the doped
graphene [60]. However, the DFT calculation shows that an electron transfer occurs
from graphene sheets to the adsorbed tetracyanoethylene (TCNE) molecules. In
addition, the carbon atoms with higher charge density can generate the catalytic
active sites due to the electron transfer from the graphene to TCNE molecule [64].
The oxygen functionalities for GO can be introduced via chemical oxidation.
These oxygen functionalities with acidic and oxidative nature render GO to function
as a solid acid or green oxidant. Besides, the edges or defect sites in GO are found to
carry carboxylic acids, quinones, and aromatic CH, as well as the spin electrons. The
resulting quinone and diol redox sites play the role as active sites in carbon materials
for oxygen-activation reactions. In addition, the amphiphilic character of GO should
be advantageous for acting as a phase-transfer catalyst in oil-water biphasic systems
[65]. The potential different active sites of graphene scaffold are displayed in Fig.
7.3 [66].
In order to alter the identities of the N moieties, the content of graphitic N
species, pyridinic N centers, and pyrrolic N moieties could be tuned through
annealing of GO with various N-rich precursors such as ammonia, polyaniline, or
polypyrrole, thereby making this process more suitable for practical applications,
and the schematic diagram is shown in Fig. 7.4 [67].
275
as O, N, B, S, P, and their corresponding functional groups [59]. Catalytic active
sites on graphene carbon nanomaterials may be generated by tuning electronic
structures. Several approaches are reported. First of all, introducing intrinsic defects
and the edge topological structures [60] may facilitate the electron transfer which
is beneficial to the conductivity and catalytic activity of carbon nanomaterials.
Therefore, to enhance the catalytic activities is probable by optimizing the topology
through the charge density analysis on these defective carbon materials by DFT
methods. Doping atoms in graphene-based nanomaterials is an alternative approach,
based on which carbon atoms are substituted by heteroatoms, such as N, P, and B,
or bonded with heteroatoms (O, S, Se, Cl, Br, and I) on carbon surfaces or edges
[61]. Such an atomic replacement results in distinct difference of electronegativity,
thus leading to localization of, the charge and spin densities and enhancement of
the catalytic activities. The active sites are located at the carbon atoms nearby the
dopant or at edges, or dopant atom itself. The DFT calculations verified that doping
heteroatom (N, O, B, S, Cl, P, Cl, Br, and I) in graphene remarkably changed
the charge density and spin density distributions on the doped carbon materials
and consequently enhanced their catalytic and electrocatalytic properties [62, 63].
Carboxyl (-COOH), carbonyl (-C=O), and hydroxyl (–C-OH) containing oxygen
functional group exhibited the synergistic effect of edge defects on graphene to
enhance catalytic activities [64]. The final approach is to physically adsorb organic
molecules on graphene or hybrid structure of N-doped graphene [62]. Defects on
carbon materials can induce the electron transfer between the organic molecule
and the graphene or between the graphitic carbon nitride (g-C 3 N 4 ) and the doped
graphene [60]. However, the DFT calculation shows that an electron transfer occurs
from graphene sheets to the adsorbed tetracyanoethylene (TCNE) molecules. In
addition, the carbon atoms with higher charge density can generate the catalytic
active sites due to the electron transfer from the graphene to TCNE molecule [64].
The oxygen functionalities for GO can be introduced via chemical oxidation.
These oxygen functionalities with acidic and oxidative nature render GO to function
as a solid acid or green oxidant. Besides, the edges or defect sites in GO are found to
carry carboxylic acids, quinones, and aromatic CH, as well as the spin electrons. The
resulting quinone and diol redox sites play the role as active sites in carbon materials
for oxygen-activation reactions. In addition, the amphiphilic character of GO should
be advantageous for acting as a phase-transfer catalyst in oil-water biphasic systems
[65]. The potential different active sites of graphene scaffold are displayed in Fig.
7.3 [66].
In order to alter the identities of the N moieties, the content of graphitic N
species, pyridinic N centers, and pyrrolic N moieties could be tuned through
annealing of GO with various N-rich precursors such as ammonia, polyaniline, or
polypyrrole, thereby making this process more suitable for practical applications,
and the schematic diagram is shown in Fig. 7.4 [67].