Page 131 - Multifunctional Photocatalytic Materials for Energy

Carbon nitride photocatalysts 117

(A) (B)
H 2
NHE (eV) –2 2 H +
H + Pt e -
–2.0 LUMO Vis –1 e -
- - - -
–1.30 eV e e e e H 2
–1.0 - - - -
–0.52 eV Vis e e e e 0
0.0 Energy vs. NHE
1.0 1.40 eV h h h h H 2 O 1 + h +
+ + + +
+ + + +
1.88 eV HOMO h h h h h
2.0 2
Ni(OH) 2
O 2 CdS g-C 3 N 4
g-C 3 N 4 CdS 3
(D) MgPc
(C) C 3 N 4
–2 H + Pt e – e – LUMO hv
Potential vs. NHE /V +1 0 H 2 + CB h + 1.8 eV TEOA
–1
> 600 nm
H /H 2
HOMO

+2 O 2 /H 2 O
VB
Fig. 6.5 (A) Transferring path of photogenerated electron and hole in the intimate interface
of binary CdS/carbon nitride heterojunction [14]; (B) Mechanism illustration of hydrogen
evolution in ternary Ni(OH) 2 /CdS/carbon nitride composite [69]; (C) TEM image of multi-
walled carbon nanotubes/carbon nitride hybrid [70]; and (D) Mechanism illustration of
hydrogen generation with MgPc-sensitized carbon nitride system [71].

apparent quantum efficiency reached as high as ~16.7% at a wavelength of 450 nm.
Moreover, the presence of Ni(OH) 2 exhibited an activity in H 2 evolution that was
seven times higher than that of the noble co-catalysts.

6.2.6 Carbonaceous/carbon nitride hybrids

Carbon hybrids are special heterojunctions without any metals and are well matched
for the requirement of sustainable development [75]. In this configuration, carbon
serves as an electron transportation medium because of its excellent conductivity
[76]. Based on this, Li et al. [77] successfully synthesized a variety of heteroatom
(O, S, B, N)-doped graphenes coupled with graphitic carbon nitride. The enhanced
H 2 -generated rate of the metal-free heterojunctions revealed the positive influence of
the presence of graphene. Moreover, N-graphene/carbon nitride was found to possess
the best photocatalytic behavior among all the prepared samples in the report. Also,
a possible reason proposed was that the introduction of the N element altered the
electronic structure of graphene. In addition, a 1D nanotube was used for the hybrid-
ization with carbon nitride. Ge et al. [70] reported that a novel composite made of