Page 126 - Multifunctional Photocatalytic Materials for Energy

Carbon nitride photocatalysts 113

of bulk carbon nitride. The same strategy was also employed in the fabrication of
nanohelical carbon nitride. In this work [49], chiral mesoporous silica was chosen as
a sacrificial template. The helical carbon nitride was successfully prepared through
a facial nanocasting method. The excellent light-harvesting capability and charge
carrier separation rate of helical carbon nitride led to a higher hydrogen generation
−1
rate (74 μmol h ).
In addition to the 1D, 2D, and porous structure, establishment of a 0D nanostructure
is also a promising pathway in semiconductor development. To prevent the distortion of
hollow carbon nitride during polycondensation of a N-rich precursor, Sun and coworkers
[42] employed a novel template of silica nanoparticles to confine the polymerization
process. By controlling the space of the template, a series of hollow carbon nitride pho-
tocatalysts with different thicknesses were achieved (Fig. 6.4C–E). In the presence of a
co-catalyst, the hydrogen production capability of hollow spheres was remarkably en-
hanced, with an overall AQY reaching approximately 7.5%. In addition to nanospheres,
quantum dots, which are less than 10 nm in size, endow fascinating optical properties
because of the strong quantum confinement effect [50]. For example, Wang and cowork-
ers [43] adopted a thermal-chemical etching process to break a carbon nitride layer down
to nanoribbons and quantum dots successively (Fig. 6.4F). Interestingly, the obtained
quantum dots showed strong up-conversion behavior that could convert NIR light to
visible light. As a result, carbon nitride quantum dots were applied in hydrogen gener-
ation together with pristine carbon nitride, and the rate of H 2 production was enhanced
roughly 2.87 times than occurred without carbon nitride quantum dots.
As mentioned, nanostructured carbon nitride in a porous structure, 2D, 1D,
and 0D morphologies are fabricated and utilized for hydrogen evolution reaction.
Nevertheless, the reported morphologies of carbon nitride are less explored than
those of other inorganic semiconductors because of deformation of the polycon-
densation process. Therefore further endeavors should be made to overcome this
obstacle.

6.2.4 Doped carbon nitride

Doping plays predominant roles in engineering the band gap, enhancing the intrinsic
optical, and improving electrochemical properties by incorporating heteroatoms into
the framework of carbon nitride. In the area of carbon nitride modifications, a myriad
of studies on metal doping (Fe and Ag, etc.) and non-metal doping (B, O, F, and S,
etc.) have been reported.
Yue et al. [51] used ZnCl 2 as the Zn source to synthesize the Zn-doped carbon ni-
tride. The X-ray photoelectron spectroscopy (XPS) and diffusion reflectance spectra
confirmed the successful introduction of Zn metal into g-C 3 N 4 and the extended ab-
sorption threshold by the red shift after doping, respectively. Moreover, the enhanced
visible light absorption resulted in a higher photocatalytic capability in water splitting,
with a maximum quantum yield of 3.2% at 420 nm. In another work, Gao and cowork-
ers [52] successfully incorporated Fe ion into the matrix of carbon nitride without
destroying the host. Further measurements proved that ion-doping favored the elec-
tron mobility and resulted in an efficient H 2 generation (quantum efficiency of 0.8%).