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and/or a tailor adsorption band in the visible and near-infrared region. Reduced TiO 2
(Ti 3+ doped TiO 2 ) has been demonstrated to exhibit visible light photoreactivity due to
the existence of a vacancy band of electronic states just below the CB. Recently, Chen et
al. reported that the disorder-engineered nanophase TiO 2 exhibited substantial solar-
driven photocatalytic activities, in which a lower-energy mid-gap state was derived from
hybridization of the O 2p orbital with the Ti 3d orbital as the result of disorders stabilized
by hydrogen. 21 Indeed, the formation of these point defects also accompanies the metal/
nonmetal doping process, which needs further investigation.
11.2.2.4 Continuously Tuning Band Structure through Solid Solutions
Compared with elemental doping, forming solid solutions between wide- and narrow-
band-gap semiconductors is promising for precisely controlling the electronic structures
by varying the ratio of the compositions to achieve both effective visible light absorption
and adequate redox potential. An important requirement for solid solutions is that the
compositions should have similar crystal phase structure and lattice parameters. Three
typical strategies have been explored to manipulate the band structure of the solid solu-
tion systems: continuously tuning the VB, continuously modulating the CB, and simulta-
neously adjusting both VB and CB.
A well-known example of VB manipulation is the (ZnO) x (GaN) 1− x system (Figure 11.8a).
Both GaN and ZnO have wurtzite structures with a band gap over 3 eV, while the solid solu-
tion (Ga 1− x Zn x )(N 1− x O x ) exhibits visible light photoreactivity. The narrowed band gap can be
explained by the p-d repulsion between Zn3 d and N2 p electrons, which shifts the VB upward
without affecting CB composed of Ga4 s 4 p hybridized orbital. The quantum eficiency for
water splitting at 420-450 nm reached 5% with Rh 2− x Cr x O 3 as a cocatalyst. 22 ZnGeN 2 -ZnO, a n
analog solid system, also behaves as a visible light photocatalyst for water splitting. Another
example of continuous modulation of the VB is the NaNbO 3 -AgNbO 3 solid solution. An
enhanced visible light photoreactivity was achieved by varying the ratio of Ag/O. 23
Continuously modulating the CB using solid solutions is also extensively explored for
speciic photocatalytic reactions. In the AgGa 1− x In x S 2 solid solution, the participation of
In5 s 5 p orbitals leads the CB upward and then allows for tuning the band gaps continu-
ously. AgGa 0.9 In 0.1 S 2 exhibits the optimal photoreactivity for H 2 evolution. 25 Recently, a
(a)
(b)
AgAlO 2
AgAl 1- x Ga x O 2
AgGaO 2
Ga4 s ,4 p
Ga4 s ,4 p
Ga4 s ,4 p
CB
Ag 5 s 5 p
+ Al 3 s 3 p
Ag 5 s 5 p
+ Al 3 s 3 p
+ GA 4 s 4 p
2.8 eV
2.6 eV
3.4 eV
Ag 5 s 5 p
+ GA 4 s 4 p
N2 p
+
Zn3 d , O2 p
N2 p
+
Zn3 d , O2 p
2.83 eV
VB
2.19 eV
N2 p
GaN
(Ga 1- x Zn x )(N 1- x O x )
with x = 0.05
(Ga 1- x Zn x )(N 1- x O x )
with x = 0.22
Ag 4 d + O2 p
Ag 4 d + O2 p
Ag 4 d + O2 p
FIGURE 11.8
(a) Schematic band structures of GaN and (Ga 1− x Zn x )(N 1− x O x ) with x = 0.05-0.22. (b) Schematic electronic
structures of AgAlO 2 , AgGaO 2 , and AgAl 1− x Ga x O 2 solid solutions. (Adapted with permission from Maeda, K.,
Teramura, K., Takata, T., Hara, M., Saito, N., Toda, K., Inoue, Y., Kobayashi, H., Domen, K., J. Phys. Chem . B, 109,
20504 and Ouyang, S. X., Ye, J. H., J. Am. Chem. Soc ., 133, 7757, respectively. Copyright 2005 and 2011, American
Chemical Society.)
 
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