TiO 2 /CdS, TiO 2 /CdTe, and TiO 2 /CdSTe heterostructured core/shell nanowire
have also been successfully synthesized by physical vapor deposition of CdS,
CdTe, and the alloyed CdSTe layer onto the hydrothermally pre-grown TiO 2
nanowires (Ai et al. 2015). Compared to CdS- or CdTe-coated TiO 2 NWAs electrode, CdS and CdTe on TiO 2 induce a higher photocurrent density. Moreover, the
increase of the photocurrents indicates that the good photoresponse properties of the
electrodes are mainly due to a wider light absorption range of the material.
Ai and coworkers reported that TiO 2 /CdS x Se 1-x (0 < x < 1) core/shell NWAs
photoelectrode significantly enhances the PEC hydrogen generation performance,
especially in the case of TiO 2 /CdS 0.52 Se 0.48 composite electrode, due to both an
enhanced light absorption and an efficient charge separation. The doping causes an
increase in the photocurrent in the PEC cell, which is beneficial for the final
hydrogen generation (Zakrzewska et al. 2015). On the other hand, the
photoreactivity of TiO 2 codoped was investigated by other authors with different
results (Liu and Li 2011; Li et al. 2011). Therefore, the incorporation of metal
leads to a decrease in the band-gap energy and increase the light absorption from
the UV region to the visible spectral region, leading to enhanced visible light
photoreactivity.
Incorporating WO 3 into TiO 2 not only efficiently inhibits the recombination
between the photogenerated (e
À /h
+ ) but also reduces the band-gap energy of TiO 2
(Kobayashi and Katsunori 2007; Yang et al. 2010). WO 3 -doped TiO 2 has been used
as an efficient photoanode in PEC cells to yield charge carriers with a longer lifetime
and enhanced H 2 evolution (Lai and Sreekantan 2013). Hierarchical photoanodes
WO 3 –TiO 2 nanotube (WTNs) composites have been fabricated by an in situ anodization of titanium in a single-step methodology (Momeni et al. 2015). A method to
prepare WTNs composites with a tube diameter of 80–110 nm and a wall thickness
of 20–40 nm has also been developed (see Fig. 7.15).
The effect of WO 3 doping becomes clearer from the transient photoresponse
behavior (Fig. 7.16a). The potentiostatic plot of TiO 2 modified with different
amounts of WO 3 is subjected to on/off cycles of illumination at 0.2 V. All the
samples demonstrated stable and instantaneous changes in photocurrent upon
illumination on/off cycles. On the other hand, the PEC H 2 evolution using
WTNs-2 decreased drastically from 2.14 mLÁcm
À2 compared to WTNs-1
(1.20 mLÁcm
À2 ) and WTNs-3 (1.72 mLÁcm
À2 ). These findings agree with the
photocurrent density results (Fig. 7.16b).
Recently, the coupling of TiO 2 with carbon materials such as activated carbon,
(Hu et al. 2014) C 60 fullerene, (Grandcolas et al. 2014) carbon nanotubes, (Ong et al.
2013) graphene, (Zhang et al. 2010b; Tan et al. 2013; Xiang et al. 2011) etc. has
attracted much attention for the synthesis of efficient photocatalysts. Therefore, all
these studies show that the presence of carbon introduces several important effects
which improve the photocatalytic efficiency of the hybrid materials. These effects
may be due to the stabilization of the TiO 2 in the carbon support improving the
charge transport to reduce recombination reactions at the TiO 2 /carbon interfaces
(Yu et al. 2014). The enhancement in photocatalytic activity of carbon materials is
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
241
have also been successfully synthesized by physical vapor deposition of CdS,
CdTe, and the alloyed CdSTe layer onto the hydrothermally pre-grown TiO 2
nanowires (Ai et al. 2015). Compared to CdS- or CdTe-coated TiO 2 NWAs electrode, CdS and CdTe on TiO 2 induce a higher photocurrent density. Moreover, the
increase of the photocurrents indicates that the good photoresponse properties of the
electrodes are mainly due to a wider light absorption range of the material.
Ai and coworkers reported that TiO 2 /CdS x Se 1-x (0 < x < 1) core/shell NWAs
photoelectrode significantly enhances the PEC hydrogen generation performance,
especially in the case of TiO 2 /CdS 0.52 Se 0.48 composite electrode, due to both an
enhanced light absorption and an efficient charge separation. The doping causes an
increase in the photocurrent in the PEC cell, which is beneficial for the final
hydrogen generation (Zakrzewska et al. 2015). On the other hand, the
photoreactivity of TiO 2 codoped was investigated by other authors with different
results (Liu and Li 2011; Li et al. 2011). Therefore, the incorporation of metal
leads to a decrease in the band-gap energy and increase the light absorption from
the UV region to the visible spectral region, leading to enhanced visible light
photoreactivity.
Incorporating WO 3 into TiO 2 not only efficiently inhibits the recombination
between the photogenerated (e
À /h
+ ) but also reduces the band-gap energy of TiO 2
(Kobayashi and Katsunori 2007; Yang et al. 2010). WO 3 -doped TiO 2 has been used
as an efficient photoanode in PEC cells to yield charge carriers with a longer lifetime
and enhanced H 2 evolution (Lai and Sreekantan 2013). Hierarchical photoanodes
WO 3 –TiO 2 nanotube (WTNs) composites have been fabricated by an in situ anodization of titanium in a single-step methodology (Momeni et al. 2015). A method to
prepare WTNs composites with a tube diameter of 80–110 nm and a wall thickness
of 20–40 nm has also been developed (see Fig. 7.15).
The effect of WO 3 doping becomes clearer from the transient photoresponse
behavior (Fig. 7.16a). The potentiostatic plot of TiO 2 modified with different
amounts of WO 3 is subjected to on/off cycles of illumination at 0.2 V. All the
samples demonstrated stable and instantaneous changes in photocurrent upon
illumination on/off cycles. On the other hand, the PEC H 2 evolution using
WTNs-2 decreased drastically from 2.14 mLÁcm
À2 compared to WTNs-1
(1.20 mLÁcm
À2 ) and WTNs-3 (1.72 mLÁcm
À2 ). These findings agree with the
photocurrent density results (Fig. 7.16b).
Recently, the coupling of TiO 2 with carbon materials such as activated carbon,
(Hu et al. 2014) C 60 fullerene, (Grandcolas et al. 2014) carbon nanotubes, (Ong et al.
2013) graphene, (Zhang et al. 2010b; Tan et al. 2013; Xiang et al. 2011) etc. has
attracted much attention for the synthesis of efficient photocatalysts. Therefore, all
these studies show that the presence of carbon introduces several important effects
which improve the photocatalytic efficiency of the hybrid materials. These effects
may be due to the stabilization of the TiO 2 in the carbon support improving the
charge transport to reduce recombination reactions at the TiO 2 /carbon interfaces
(Yu et al. 2014). The enhancement in photocatalytic activity of carbon materials is
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
241