Page 132 - Multifunctional Photocatalytic Materials for Energy

118 Multifunctional Photocatalytic Materials for Energy

multi-walled carbon nanotubes and carbon nitride was obtained by a facial thermal
process (Fig. 6.5C). Further investigation indicated that the presence of the carbon
nanotube effectively facilitated the charge migration. Therefore a 3.7-fold hydrogen
evolution rate was observed when the optimized composite photocatalyst was entered
into water splitting, compared with that of pristine carbon nitride. More strikingly,
a photocatalyst with a high photocatalytic performance and stability was exploited
by Liu and coworkers [78]. With the advantages of 0D carbon dots—for example,
excellent visible light absorption ability, high electron transportation capability, and
especially the unique H 2 O 2 decomposition—the 0D carbon was successfully immobi-
lized on the surface of carbon nitride. The presence of carbon dots could significantly
improves the hydrogen evolution rate, even in the absence of a non-metal co-catalyst.
The quantum efficiencies can reach as high as 16% at a wavelength of 420 ± 20 nm.
And the low-cost photocatalyst exhibited an excellent stability with no obvious de-
cline after 50 cycles of 24 h each. The synthesis of this metal-free composite can open
up a venue for designing and exploiting novel, environmentally benign, and low-cost
photocatalysts.
Isotype heterostructure, also named homojunction, is another new type of carbon
hybrid designed for facilitating charge migration using the same substance with differ-
ent crystal phases. For the first time, Zhang et al. [79] employed a band alignment ap-
proach to combine two different types of carbon nitride into a CNS/CN homojunction.
Similarly to heterojunction, these carbon nitrides derived from two different precur-
sors possessed slightly different inherent band structures that could promptly accel-
erate the electrons’ dissociation and prohibit electron-hole recombination. Therefore,
with this advantage, the novel homojunction showed improved H 2 -generated activity
−1
with almost 50 μmol h .
Thus it is clear that both the heterojunctions and carbon hybridization can sig-
nificantly improve hydrogen production, an improvement that is attributed to the
enhanced electron mobility in the intimate interface. Although a great variety of
heterostructures have been fabricated in recent years, there is still room for im-
provement by using some budding composite (such as MOF-based heterojunction)
for energy.

6.2.7 Dye-sensitized carbon nitride
Sensitization of a semiconductor with dyes is a classical modification approach that
has been widely used in solar cells, photocatalysis, and so forth [80]. Sensitization
systems can be constructed by adsorption or immobilization with metals, which can
greatly extend the visible light response region and improve the charge separation.
The pioneering study on dye sensitization of carbon nitride was done by Takanabe and
coworkers [71]. They used magnesium phthalocyanine as the sensitizer to load on the
host of Pt/carbon nitride. The introduction of dye remarkably extended the light ab-
sorption to a near-IR region at about 820 nm. With the application of this sensitization
system, a quantum efficiency of approximately 5.6% at 420 nm was observed in water
splitting. Moreover, a mechanism was proposed in which the dye was excited by elec-
trons injected into the CB of carbon nitride for hydrogen reduction. The excited dye