Page 127 - Multifunctional Photocatalytic Materials for Energy

114 Multifunctional Photocatalytic Materials for Energy

More interestingly, because of the surface plasmon resonance effect, noble metals al-
ways show an excellent capability to capture photo-excited electrons and a strong vis-
ible light absorption ability when they are deposited on the surface of carbon nitride.
Thus deposition of noble metal(s) onto the catalyst surface has been very popular in
the field of catalysis despite the high cost. The modification of carbon nitride using
noble metals can be done by either thermal reduction or photodeposition. Employing
0
the former way, Ge et al. [53] obtained Ag -doped carbon nitride. With the synergic ef-
fect between silver and carbon nitride, the H 2 evolution rate surpassed that of pristine
carbon nitride by more than 11.7-fold.
In addition to metal doping [54], nonmetal [55] doping [56] is also an effective
approach [57] in the modification of semiconductors and can significantly enhance
the intrinsic electronic property and engineer the band structure of carbon nitride
photocatalysts. Phosphorus has been used widely to enhance the conductivity of car-
bon nitride in recent years. For example, Ran and coworkers [58] successfully pre-
pared P-doped carbon nitride by heating a mixture of 2-aminoethylphosphonic acid
and melamine. The extended light absorption spectrum caused by the empty midgap
states from P was confirmed by further theoretical calculations and experimental
studies. Moreover, the reinforced photocatalytic activity endowed the modified pho-
−1 −1
tocatalyst with a robust H 2 generation rate (1596 μmol h g ). Also, Li et al. [59]
chose H 2 O 2 as the O source to modify carbon nitride. Using a hydrothermal route,
O was successfully introduced into the matrix to partially substitute N atoms in
g-C 3 N 4 . Then the O-doped carbon nitride was applied in the hydrogen evolution
reaction, showing 2.5 times higher efficiency than that of unmodified carbon ni-
tride. The enhanced photocatalysis was mainly attributed to the larger surface area,
extended visible light absorption, and increased electron mobility. In another study,
Liu et al. [60] obtained sulfur-doped carbon nitride by treating dicyandiamide-
derived carbon nitride in a H 2 S atmosphere. More noticeably, the introduction of the
S element not only upshifted the VB but also significantly widened the CB minimum
because of the reduced particle size after doping. Therefore an 8.0 times higher H 2
generation rate at λ > 420 nm than that of bulk carbon nitride was achieved because
of the strengthened photoreduction ability.
Table 6.2 provides a summary of different elements doped with carbon nitride
along with their relevant doping method and the corresponding bandgap energies and
their photocatalytic performances in hydrogen evolution. It was found that doping
with either metal or non-metal elements can engineer the intrinsic optical properties
of pristine carbon nitride, enhancing its visible light absorption. Meanwhile, the intro-
duction of metal and some non-metal elements can improve the conductivity of carbon
nitride, accelerating the separation of electron-hole pairs. Therefore the synergistic
effect effectively boosts the photocatalytic ability of g-C 3 N 4 in H 2 evolution.
It has been found that substitution of either C or N atoms [61] or imbedding [62] the
surface [63] of carbon nitride [64] with metal [65] or non-metal element [66] can sig-
nificantly tune the band structure of pristine carbon nitride and boost the H 2 -generated
rate. Whereas, dual doping for carbon nitride has been rarely reported, it can be a sally
port in the following works.