Page 99 - Multifunctional Photocatalytic Materials for Energy

88 Multifunctional Photocatalytic Materials for Energy

graphene was developed by alginate modified with various amounts of H 2 PO 4 at a
neutral pH, followed by pyrolysis at a high temperature (900°C) under an inert atmo-
sphere [98].

5.4 Energy applications

Artificial photosynthesis is regarded as a potential long-term solution to mitigate the
world's energy problems because it may produce solar-based fuels via photocatalytic
water splitting and/or CO 2 photoreduction, among other methods. The incorporation
of graphene-based materials with semiconductor photocatalysts can lead to different
promoting effects through one or more of the following effects: (i) supports material
for enhanced structure stability; (ii) increases adsorption and active sites toward the
reagents; (iii) uses electron acceptor and transport channel to suppress the recombi-
nation of photo-excited electron-hole pairs; (iv) involves a co-catalyst; (v) involves
photosensitization; and (vi) has photocatalyst and band gap narrowing effect [16,107].
In the following section, the main applications of graphene-based semiconductor pho-
tocatalysts in the two referred processes are briefly reviewed.

5.4.1 Photocatalytic hydrogen generation
During the past decade, graphene has shown great ability to enhance the photocata-
lytic H 2 production performance of semiconductor photocatalysts [38].
GO/TiO 2 photocatalysts were studied for water splitting under UV/vis irradiation
[49]. XRD results showed that the average crystal size of TiO 2 (anatase) was ∼11 nm
for all samples with various GO contents. The GO/TiO 2 composite with a 5 wt.% of
−1
GO exhibited a H 2 evolution rate of 8.6 μmol h , 1.9 times higher than that obtained
−1
for the TiO 2 benchmark, P25 (4.5 μmol h ). The larger surface area as well as the
excellent electronic conductivity of graphene that suppressed the recombination of
photoinduced electrons and holes were the main factors enhancing the photocatalytic
activity of the GO/TiO 2 composite.
It is known that graphene as a H 2 -evolution co-catalyst can greatly boost the photo-
catalytic activity of metal sulfides. CdS/GO photocatalysts with a uniform distribution
of CdS clusters led to a more efficient transfer of photoinduced electrons from CdS
−1
to GO [89]. A high H 2 -production rate of 1.12 mmol h was obtained at an optimal
GO content of 1.0 wt.% (about 4.87 times higher than that of bare CdS), presenting an
apparent quantum efficiency (QE) of 22.5% at λ = 420 nm. Graphene-supported CdS
nanoparticles for photocatalytic H 2 production were also prepared by a hydrothermal
−1
method [88]. In this case, the H 2 -production rate was 70 μmol h for the graphene/
CdS composite at an optimal mass ratio of 0.01/1 under Xe lamp irradiation (200 W,
−1
λ ≥ 420 nm), while bare CdS only showed a rate of 14.5 μmol h . Significant band-gap
narrowing was observed due to the strong interactions between CdS and graphene.
Combined with the advantage of more efficient charge separation, the graphene-
modified CdS photocatalyst exhibited much better photocatalytic H 2 -production per-
formance than bare CdS.