Page 130 - Multifunctional Photocatalytic Materials for Energy

116 Multifunctional Photocatalytic Materials for Energy

6.2.5 Carbon nitride-based heterojunctions

Hybridization of carbon nitride with appropriate candidates is also considered as an
efficient technique for accelerating the separation of the photogenerated electron and
hole pairs. Inspired by this fact, numerous carbon nitride-based heterojunctions were
fabricated and applied in hydrogen evolution reactions. TiO 2 , a typical inorganic semi-
conductor, is widely used as a hybridization candidate with carbon nitride because
of its non-response ability to visible light region [67]. For example, Wang et al. [68]
synthesized TiO 2 /carbon nitride heterojunctions simply by annealing the mixture of
melamine with the precursor of TiO 2 . UV-vis results revealed that the presence of
carbon nitride can make up the non-response ability of TiO 2 , and the PL spectra indi-
cated that the presence of TiO 2 can significantly solve the low electron transportation
rate of carbon nitride. Therefore the synergistic effect provided by the semiconductor
heterojunctions resulted in an enhancement of hydrogen evolution rate 5 times that of
pristine g-C 3 N 4 . In addition, CdS [7] is regarded as another promising photocatalyst
because of its high efficiency in solar storage and conversion. However, the stability is
2−
generally unsatisfactory because the S tends to be self-oxidized by the light- excited
holes. To overcome this obstacle, a novel core-shell photocatalyst of hybridized car-
bon nitride with CdS was reported by Liu and coworkers [14] via a chemisorption
method. As illustrated in Fig. 6.5A, the CB and VB of CdS are suitable for that of car-
bon nitride; thus the photogenerated electron in carbon nitride can transfer through the
intimate interface to the surface of CdS. Meanwhile, the corresponding photo-excited
holes move to carbon nitride, with remarkably improved charge carrier mobility. Thus
the self-oxidation process in CdS can be effectively prohibited. As a result, a H 2 pro-
duction rate 2.5 and 2.2 times higher than pure CdS and graphitic carbon nitride,
respectively, can be observed in the hydrogen evolution reaction. An excellent repro-
ducibility was also obtained in the core@shell structure. In addition to metal oxide
and sulfide, a metal organic framework (MOF) [72] is attracting increasing attention
to construct heterostructures together with carbon nitride. For instance, Wang and co-
workers [73] reported a novel heterostructure made by coupling carbon nitride with
Zr-containing MOF UiO-66 octahedrons. The as-obtained visible light photocatalyst
was first used for water splitting, showing a H 2 generation rate more than 17 times
higher than that of carbon nitride alone. The significant enhancement was mostly at-
tributed to the boosted electron-hole separation rate within the intimate contact. He
and coworkers [74] reported a facial annealing approach to fabricate a novel hetero-
junction that combined graphitized polyacrylonitrile nanosheets with layered carbon
nitride. The results indicated that the introduction of a polymer can effectively accel-
erate the charge migration; therefore a H 2 production rate 3.8 times that of pristine
carbon nitride was observed. Apart from the previously mentioned binary composite,
studies on ternary heterojunctions have expanded as well in recent years. For instance,
a noble metal-free ternary composite was reported by Yan et al. [69] They first con-
structed a CdS/carbon nitride core/shell structure, loaded Ni(OH) 2 on the surface of
the core/shell hybrid via a hydrothermal method, and used the as-fabricated composite
in the hydrogen evolution reaction. Fig. 6.5B clearly shows that recombination of the
photogenerated electron and hole pairs can be effectively reduced. As a result, the