Page 101 - Multifunctional Photocatalytic Materials for Energy

90 Multifunctional Photocatalytic Materials for Energy

The synergetic effect of MoS 2 and graphene as co-catalysts for enhanced photo-
catalytic H 2 production activity of TiO 2 nanoparticles was also evaluated [94]. It was
found that a layered MoS 2 /graphene (MG) hybrid served as a highly active co-catalyst
for photocatalytic H 2 production under Xe arc lamp irradiation, using TiO 2 as the pho-
tocatalyst and ethanol as the sacrificial agent (Fig. 5.6A). The graphene/MoS 2 /TiO 2
composite (TiO 2 /MG) photocatalyst with an optimal amount of 5.0 wt.% graphene and
−1
0.5 wt.% MoS 2 exhibited a high H 2 -production rate of 165.3 μmol h with an apparent
QE of 9.7% at 365 nm (Fig. 5.6B). This is logical because graphene possesses a high
work function (∼−0.08 eV versus NHE) to capture photoinduced electrons from the
CB of TiO 2 [38]. The photogenerated electrons in the CB of TiO 2 can be transferred
+
to MoS 2 nanosheets through the graphene sheets and then react with the adsorbed H
ions at the edges of the MoS 2 to generate H 2 . In fact, nanoscale MoS 2 is highly active
for H 2 evolution as a result of the quantum-confinement effect. This indicates that,
because of a notable synergetic effect between MoS 2 nanosheets and graphene, the
composite co-catalyst has several advantages, including suppression of charge recom-
bination, improvement of interfacial charge transfer, and an increase in the number of
active adsorption sites, as well photocatalytic reaction centers.
Doped graphene materials have been used to improve the photocatalytic activity
of graphene-based semiconductor composites in the photocatalytic generation of H 2
because of a high intimate interfacial contact between doped graphene and semicon-
ductor nanoparticles [111–114]. The fabrication of TiO 2 nanoparticles-functionalized
N-doped graphene composites for photocatalytic H 2 generation for water (Fig. 5.7)
has been studied [113]. N-doped rGO showed higher electrical conductivity than
rGO because of its efficient structural restoration and smaller populations of defects
in the graphitic structure. Moreover, the N atoms in N-doped graphene played im-
portant roles as nucleation and anchor sites for TiO 2 nanoparticles, resulting in their
uniform distribution on the graphene sheet. The photocatalytic activities for H 2

Fig. 5.6 (A) Photocatalytic H 2 evolution of TiO 2 /MG composites with different MoS 2 and
rGO contents in the MG hybrid as co-catalyst under UV irradiation; (B) schematic illustration
of the charge transfer in TiO 2 /MG composites.
Reproduced with permission from Q. Xiang, J. Yu, M. Jaroniec, Synergetic effect of MoS2
and graphene as cocatalysts for enhanced photocatalytic H 2 production activity of TiO 2
nanoparticles, J. Am. Chem. Soc. 134 (15) (2012) 6575–6578. Copyright 2012, American
Chemical Society.