4.4 MXenes
71
small grain size and high porosity. This unique structure can reduce the aggregation
of Mo 2 C nanocrystals, increase the active site, and enhance the interaction between
Mo 2 C nanocrystals and C-N frameworks, making it exhibit superior hydrogen evolution reaction (HER) activity in both acidic and alkaline media. With regard to
large-scale production, the synthetic strategy is facile and inexpensive and can also
be extended to fabricate various 2D hierarchical nanomaterials for electrocatalytic
applications.
Kumar et al. (2018) reported the preparation of 2D nanosheets heterojunction
composed of N-doped ZnO nanosheets (anchored) on graphitic carbon nitride (gC 3 N 4 ) nanosheets for enhanced photocatalytic hydrogen evolution. The procedure
is schematically illustrated in Fig. 4.4f. The N-ZnO nanosheets are loaded over the
g-C3N4 nanosheets and they are anchored perfectly over its surface to form 2D-2D
heterojunction with intimate face-to-face contact. It is worth mentioning here that
such type of 2D-2D heterojunctions can effectively improve the specific surface area
and provide more active sites to adsorb reactant species which promote the separation of photogenerated charge carriers significantly to improve the photocatalytic
activity (Fig. 4.4g). The optimal heterojunction photocatalyst with 30 wt% of g-C 3 N 4
nanosheets (NZCN30) show hydrogen evolution rate of 18,836 μmol h
−1 g cat
−1 in
presence of Na 2 S and Na 2 SO 3 as sacrificial agents under simulated solar light irradiation (Fig. 4.4h). Supported well by photoluminescence and photoelectrochemical
investigations, the enhanced photocatalytic performance of NZCN30 heterojunction
reveals the minimum recombination rate and high photoinduced current density,
respectively. Moreover, by high face-to-face contact surface area for separation of
photogenerated charge carriers in space which facilitates their transfer for H 2 generation, the existence of 2D-2D interfacial contact plays a major role in enhanced H 2
evolution. This work expands the space for the progress of 2D-2D heterojunctions
for diverse applications.
4.5 Polymer
2D polymer has properties well fitted for energy storage applications on account of
its combination of porosities and layered structure, providing ion diffusion routes
through the 2D planes and 1D channels. In addition, if the polymer has an aromatic
conjugated scaffold, it mediates charge transfer. The key challenge of 2D polymer
synthesis is how to confine chain evolution in two dimensions.
Liu et al. (2017b) reported a C–C coupling based 2D conjugated polymer via
endogenous polymerization of phenazine-based monomer carried out under catalystand solvent-free condition (Liu et al. 2017a) They extend the endogenous polymerization strategy to different phenazine-based monomers 2-TBTBP and 3-TBQP that
prepack into single crystals with different topologies. These are converted by endogenous polymerization into π-conjugated, nanoporous aromatic polymers (CAPs). The
products of polymerizing 2-TBTBP and 3-TBQP are denoted as CAP-1 and CAP2, respectively. Monomers 3-TBQP are densely packed into a quasi-2-D structure,
71
small grain size and high porosity. This unique structure can reduce the aggregation
of Mo 2 C nanocrystals, increase the active site, and enhance the interaction between
Mo 2 C nanocrystals and C-N frameworks, making it exhibit superior hydrogen evolution reaction (HER) activity in both acidic and alkaline media. With regard to
large-scale production, the synthetic strategy is facile and inexpensive and can also
be extended to fabricate various 2D hierarchical nanomaterials for electrocatalytic
applications.
Kumar et al. (2018) reported the preparation of 2D nanosheets heterojunction
composed of N-doped ZnO nanosheets (anchored) on graphitic carbon nitride (gC 3 N 4 ) nanosheets for enhanced photocatalytic hydrogen evolution. The procedure
is schematically illustrated in Fig. 4.4f. The N-ZnO nanosheets are loaded over the
g-C3N4 nanosheets and they are anchored perfectly over its surface to form 2D-2D
heterojunction with intimate face-to-face contact. It is worth mentioning here that
such type of 2D-2D heterojunctions can effectively improve the specific surface area
and provide more active sites to adsorb reactant species which promote the separation of photogenerated charge carriers significantly to improve the photocatalytic
activity (Fig. 4.4g). The optimal heterojunction photocatalyst with 30 wt% of g-C 3 N 4
nanosheets (NZCN30) show hydrogen evolution rate of 18,836 μmol h
−1 g cat
−1 in
presence of Na 2 S and Na 2 SO 3 as sacrificial agents under simulated solar light irradiation (Fig. 4.4h). Supported well by photoluminescence and photoelectrochemical
investigations, the enhanced photocatalytic performance of NZCN30 heterojunction
reveals the minimum recombination rate and high photoinduced current density,
respectively. Moreover, by high face-to-face contact surface area for separation of
photogenerated charge carriers in space which facilitates their transfer for H 2 generation, the existence of 2D-2D interfacial contact plays a major role in enhanced H 2
evolution. This work expands the space for the progress of 2D-2D heterojunctions
for diverse applications.
4.5 Polymer
2D polymer has properties well fitted for energy storage applications on account of
its combination of porosities and layered structure, providing ion diffusion routes
through the 2D planes and 1D channels. In addition, if the polymer has an aromatic
conjugated scaffold, it mediates charge transfer. The key challenge of 2D polymer
synthesis is how to confine chain evolution in two dimensions.
Liu et al. (2017b) reported a C–C coupling based 2D conjugated polymer via
endogenous polymerization of phenazine-based monomer carried out under catalystand solvent-free condition (Liu et al. 2017a) They extend the endogenous polymerization strategy to different phenazine-based monomers 2-TBTBP and 3-TBQP that
prepack into single crystals with different topologies. These are converted by endogenous polymerization into π-conjugated, nanoporous aromatic polymers (CAPs). The
products of polymerizing 2-TBTBP and 3-TBQP are denoted as CAP-1 and CAP2, respectively. Monomers 3-TBQP are densely packed into a quasi-2-D structure,