Page 100 - Multifunctional Photocatalytic Materials for Energy

Graphene photocatalysts 89

Graphene derivatives serve as excellent electron acceptors and transport channels
to suppress the recombination of photoinduced electrons and holes, thus enhancing
the efficiency of photocatalytic H 2 production. Photocatalytic H 2 production over
graphene-supported ZnS nanoparticles under visible light illumination (λ > 420 nm)
was studied [92]. The composite photocatalyst with an optimum 0.1 wt.% of GO
−1 −1
achieved a H 2 -production rate of 7.42 μmol h g , which was eight times higher than
that of bare ZnS. The high photocatalytic H 2 production activity was attributed to
the photosensitization of graphene. In this case, the electrons photogenerated from
graphene could be transferred to the CB of ZnS to participate in the photocatalytic
process under visible light illumination. In another work [108], rGO was coupled with
Zn x Cd 1−x S photocatalysts for photocatalytic H 2 production under simulated solar irra-
diation using Na 2 S and Na 2 SO 3 as sacrificial agents. The photocatalytic H 2 -production
rate of the optimized rGO-Zn 0.8 Cd 0.2 S photocatalyst (0.25 wt.% rGO content) was
−1 −1
1824 μmol h g with an apparent QE of 23.0% at 420 nm. The performance was
even better than that of optimized Pt-Zn 0.8 Cd 0.2 S under the same reaction conditions.
It was observed that the introduction of rGO could effectively promote the transfer and
separation of charge carriers and increase the surface-active sites for water reduction,
thus leading to the enhanced performance. This work also indicated the promising
potential of graphene to replace noble metals as a co-catalyst in specific photocatalytic
systems for H 2 production. The presence of a small amount of rGO (1.0 wt.%) could
lead to a significant increase of specific surface area in rGO-ZnIn 2 S 4 nanocomposites
2 −1
(e.g., 150 and 99.8 m g for 1.0 wt.% rGO-ZnIn 2 S 4 and bare ZnIn 2 S 4 , respectively)
−1
[93]. This rGO-ZnIn 2 S 4 nanocomposite showed a H 2 evolution rate of 40.9 μmol h ,
−1
whereas the rate of bare ZnIn 2 S 4 was only 9.5 μmol h under visible-light illumina-
tion. The strong interaction between ZnIn 2 S 4 nanosheets and rGO in the nanocompos-
ites facilitated the electron transfer from ZnIn 2 S 4 to rGO, with the latter serving as a
good electron acceptor and mediator, as well as the co-catalyst for H 2 evolution.
The H 2 production efficiency of those graphene-based binary photocatalysts can
be improved by introducing an additional component to form graphene-based ter-
nary composite photocatalysts, as reported for NiS/Zn 0.5 Cd 0.5 S/rGO ternary compos-
ites, where the three components were well-connected with each other [109]. Such a
connection enables rGO to be an effective electron acceptor and transporter to cap-
ture photoinduced electrons from the CB of Zn 0.5 Cd 0.5 S and simultaneously offer
reduction- active centers for H 2 evolution. At the optimal amount of 0.25 wt.% rGO
and 3 mol % NiS, the ternary composite photocatalyst exhibited a H 2 -production rate
−1
of 376 μmol h with a high apparent QE of 31% at 420 nm.
An effective strategy to obtain an intimate and large contact interface is to con-
struct 2D-2D layered junctions to provide abundant surface-active sites and achieve
efficient interfacial charge transfer [110]. A graphene/g-C 3 N 4 composite was applied
in photocatalytic H 2 production under visible light illumination [95]. The successful
formation of 2D–2D layered junctions between g-C 3 N 4 and graphene led to a very
efficient interfacial charge separation, which enabled spatial accumulation of pho-
toinduced electrons and holes on the sides of graphene and g-C 3 N 4 , respectively. The
−1 −1
highest photocatalytic H 2 -production rate (451 μmol h g ) was achieved with the
composite containing 1.0 wt.% graphene.