Environmental Engineering Reference
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by the electrons in the conduction band to form H
2
gas, and oxidized by holes
in the valence band to generate O
2
gas [5]. To serve as good photocatalysts,
semiconductor materials should satisfy several key requirements [7]. First,
the band edge positions should straddle the electrolysis potentials of H
+
/H
2
(0 V vs. NHE) and O
2
/H
2
O (1.23 V vs. NHE). Therefore, the theoretical
minimum energy required for water splitting is 1.23 eV. However, taking
account of the presence of overpotential for water reduction and oxidation
on semiconductor surfaces, the actual energy for water splitting should be at
least 1.8 eV [5]. Second, the bandgap of photocatalysts should be small
enough to absorb a significant portion of solar light. Third, the semiconduc-
tors must be chemically stable during water oxidation and reduction in
aqueous solution [7].
A number of strategies have been developed to improve the photocatalytic
water-splitting performance of semiconductors. One strategy is bandgap
engineering to improve photon absorption efficiency. The overall solar-to-
hydrogen efficiency is closely related to the light absorption efficiency. For
example, nitrogen doping has been demonstrated to increase visible light
photoactivity of TiO
2
in water splitting by introducing nitrogen as impurities
to narrow the bandgap [8]. A second strategy is improvement of charge sepa-
ration and suppression of the electron-hole recombination loss. For instance,
considerable efforts have been placed to develop semiconductor nanostruc-
tures that have a large surface area and short carrier diffusion length, which
are beneficial for charge separation and suppression of electron-hole recom-
bination [7]. A third strategy is the construction of surface reaction sites for
water oxidation or reduction. For example, Pt-modified semiconductors have
been used to reduce the overpotential of water reduction and to improve the
efficiency of hydrogen generation [9].
A number of semiconductors have been studied for solar hydrogen genera-
tion. These semiconductors can be classified into metal oxides such as TiO
2
[10, 11], ZnO [12, 13], and SrTiO
3
[14], metal oxynitrides such as TaON [15,
16], metal nitrides/phosphide such as Ta
3
N
5
and InP
3
[17, 18], metal chalco-
genides [9, 12], and silicon [19, 20]. The conduction bands of all these
semiconductors are more negative than water reduction potential (0 vs.
NHE), which ensure spontaneous water reduction when the semiconductors
are illuminated with light energy larger than their bandgap energies. In this
chapter, we will discuss the recent progress in developing different kinds of
semiconductors for photocatalytic hydrogen generation.
3.2.2 Metal Oxides
Metal oxides have been extensively studied as photocatalysts for solar
hydrogen generation, since the first demonstration of photocatalytic study
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