be achieved by UV reduction [145], sonochemical deposition [146], or KBH 4
reduction [147]. The SPR effect of Ag, enhanced solar light absorption and promoted charge separation of Ag/TiO 2–x are responsible for the high visible-lightdriven photocatalytic degradation of organic pollutants [147, 148].
In order to reduce the high cost of noble metals, researchers are seeking new
approaches for the replacement with non-noble metals. Among all the non-noble
metal elements, Cu is regarded as a competitive candidate to substitute noble metals
and works as cocatalyst with TiO 2 photocatalysts, because of its low cost and
considerable catalytic performance. In general, nanosized Cu nanoparticles are not
stable, and they are easy to be oxidized to Cu 2 O or CuO while being exposed in air.
Zhao et al. synthesized TiO 2–x @Cu particles, in order to prevent the oxidation of
metallic Cu in the photocatalytic reaction process [143]. However, it is also reported
that the Cu
+ species are more active than Cu or Cu
2+ , and the high photocatalytic
performance of the Cu-grafted TiO 2 catalysts is related to the presence of stabilized
Cu
+ species [149]. In addition, the generation of oxygen vacancies in the preparation
process could promote the stabilization of Cu 2 O species on the surface of
TiO 2 [149].
Liu et al. ascribed the high photocatalytic CO 2 reduction performance of Cu/TiO 2
catalysts to the synergistic effect of Ti
3+ species and Cu
+
/Cu
0 couples, which were
formed during the thermal treatment in H 2 atmosphere [150]. The formation of Ti
3+
species contributes to the enhanced adsorption of CO 2 and improved separation of
electrons and holes, while the Cu
+ generation also promotes the electron transfer
during the photocatalytic reaction process. Besides, the Cu
+ species were also
reported to be the main active phase for CO oxidation, where the Cu
+ species
resulted from the reduction of Cu
2+ by Ti
3+ species on TiO 2–x [94].
4.5.3 TiO 2–x Composited with Carbon
Graphene is a kind of single-layered carbon material [151] and basic building block
for creating other graphitic materials: wrapped into 0D fullerenes, rolled into 1D
carbon nanotubes, and stacked into 3D graphite [152]. Recently, it has attracted
plenty of interest for compositing with other semiconductors because of its superior
conductivity property and unique structure.
Xing et al. synthesized TiO 2–x /graphene composites via a simple hydrothermal
process using NaBH 4 as the reducing agent and also boron dopant source [99]
(Fig. 4.14a). Electron transfer was achieved from TiO 2–x nanorods to graphene
through Ti–O–C bonds. As the result, electrons were collected on the graphene,
while holes were lying on the surface of TiO 2–x , promoting the efficient separation of
photogenerated electrons and holes. Qiu et al. employed vacuum activation process
to achieve self-doping of TiO 2 , reduction of graphene, and the compositing of TiO 2–
x on the surface of graphene simultaneously [90]. It was demonstrated that the selfdoping of Ti
3+ and the bonding between TiO 2–x and graphene were responsible for
92
4 Preparation of Reduced TiO 2–x for Photocatalysis
reduction [147]. The SPR effect of Ag, enhanced solar light absorption and promoted charge separation of Ag/TiO 2–x are responsible for the high visible-lightdriven photocatalytic degradation of organic pollutants [147, 148].
In order to reduce the high cost of noble metals, researchers are seeking new
approaches for the replacement with non-noble metals. Among all the non-noble
metal elements, Cu is regarded as a competitive candidate to substitute noble metals
and works as cocatalyst with TiO 2 photocatalysts, because of its low cost and
considerable catalytic performance. In general, nanosized Cu nanoparticles are not
stable, and they are easy to be oxidized to Cu 2 O or CuO while being exposed in air.
Zhao et al. synthesized TiO 2–x @Cu particles, in order to prevent the oxidation of
metallic Cu in the photocatalytic reaction process [143]. However, it is also reported
that the Cu
+ species are more active than Cu or Cu
2+ , and the high photocatalytic
performance of the Cu-grafted TiO 2 catalysts is related to the presence of stabilized
Cu
+ species [149]. In addition, the generation of oxygen vacancies in the preparation
process could promote the stabilization of Cu 2 O species on the surface of
TiO 2 [149].
Liu et al. ascribed the high photocatalytic CO 2 reduction performance of Cu/TiO 2
catalysts to the synergistic effect of Ti
3+ species and Cu
+
/Cu
0 couples, which were
formed during the thermal treatment in H 2 atmosphere [150]. The formation of Ti
3+
species contributes to the enhanced adsorption of CO 2 and improved separation of
electrons and holes, while the Cu
+ generation also promotes the electron transfer
during the photocatalytic reaction process. Besides, the Cu
+ species were also
reported to be the main active phase for CO oxidation, where the Cu
+ species
resulted from the reduction of Cu
2+ by Ti
3+ species on TiO 2–x [94].
4.5.3 TiO 2–x Composited with Carbon
Graphene is a kind of single-layered carbon material [151] and basic building block
for creating other graphitic materials: wrapped into 0D fullerenes, rolled into 1D
carbon nanotubes, and stacked into 3D graphite [152]. Recently, it has attracted
plenty of interest for compositing with other semiconductors because of its superior
conductivity property and unique structure.
Xing et al. synthesized TiO 2–x /graphene composites via a simple hydrothermal
process using NaBH 4 as the reducing agent and also boron dopant source [99]
(Fig. 4.14a). Electron transfer was achieved from TiO 2–x nanorods to graphene
through Ti–O–C bonds. As the result, electrons were collected on the graphene,
while holes were lying on the surface of TiO 2–x , promoting the efficient separation of
photogenerated electrons and holes. Qiu et al. employed vacuum activation process
to achieve self-doping of TiO 2 , reduction of graphene, and the compositing of TiO 2–
x on the surface of graphene simultaneously [90]. It was demonstrated that the selfdoping of Ti
3+ and the bonding between TiO 2–x and graphene were responsible for
92
4 Preparation of Reduced TiO 2–x for Photocatalysis