Page 121 - Multifunctional Photocatalytic Materials for Energy

Carbon nitride photocatalysts 109

their photocatalytic performances in water splitting. In addition to the precursor, the
annealing temperature, rate, duration, and atmosphere are also important parameters
that will significantly determine the photocatalytic activities of carbon nitride.
As previously mentioned, a series of N-rich precursors were used for the synthe-
sis of carbon nitride, and the polymerization parameters were thoroughly optimized.
However, because the lower hydrogen production rate does not meet practical require-
ments, more suitable precursors and optimum synthesis conditions are still in high
demand.

6.2.2 Copolymerization

As a modification strategy at the molecular level, copolymerization is a fascinating
approach to engineering the CB and VB and enhancing either the redox potentials or
visible light absorption of the polymeric semiconductors. Because of the presence of
functional groups, such as amino and cyano groups, in the termination of precursor
molecules, reactions with other organic monomers bearing amino, carboxyl, anhy-
dride, and so forth are possible. Also, the introduction of a monomer into a carbon
nitride framework can enhance the charge separation ability to some extent. Thus
the exclusive modification of a polymeric semiconductor can effectively extend the
delocalization of the π electrons of graphitic carbon nitride and alter the intrinsic
physiochemical properties [29]. Based on this concept, Zhang and coworkers [30]
successfully incorporated aromatic groups into carbon nitride polymers by synthesiz-
ing organic molecules containing amino and/or cyano functionalities. Further analyt-
ical results verified that both the optical and the electrical properties were enhanced
simultaneously. A remarkable red shift of light absorption to 700 nm and a fast charge
immigration were observed from UV-vis and photocurrent spectra, respectively. When
the modified g-C 3 N 4 was applied in the hydrogen evolution reaction, a reinforced
−1
hydrogen evolution (TOF of 52 h per added Pt atom) and improved stability were
obtained. Fig. 6.3A shows the results of a similar work reported by Zou et al. [31],
in which a novel carbon nitride network was fabricated through a facile bottom-up
strategy of grafting an electron-deficient pyromellitic dianhydride monomer into the
tri-s-triazine unit of carbon nitride. The extended delocalization of π electrons led to
a H 2 production rate three times greater than that of pristine g-C 3 N 4 . Also, utilization
of the supramolecular for preparation of carbon nitride opens a new pathway to the
modification of carbon nitride. To briefly summarize, assembling the precursors of
carbon nitride into supramolecules by hydrogen bonding makes the molecules align
in a designated direction, thus boosting the electron flow in a specific orientation. As
illustrated in Fig. 6.3B, Shalom et al. [32] utilized cyanuric acid, melamine, and barbi-
turic acid in water to establish a new supramolecular complex. An efficient hydrogen-
generated photocatalyst was obtained through calcination of this complex, with a
−1
much higher turnover frequency of almost 6 h per added Pt atom. This fascinating
result was attributed to better light harvesting, higher charge separation efficiency, and
more active sites.
In general, as a unique modification method of polymeric semiconductor, copo-
lymerization with a proper organic monomer can efficiently strengthen the water