Environmental Engineering Reference
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FIGURE 3.1 (a) Photocatalytic hydrogen generation rate collected for ZnO nanorod arrays (NRA) film,
hydrogen-treated ZnO (H:ZnO) NRA film, and H:ZnO nanorod (NR) powder in a solution containing
0.1 M Na 2 SO 3 and 0.1 M Na 2 S under white light irradiation. (b) Cycling performance of H : ZnO NRA
films. Source : Reproduced with permission from Lu et al. [24]. (See color insert.)
performance of ZnO for water splitting was attributed to the increased elec-
trical conductivity as a result of introduction of oxygen vacancies, which is
a shallow donor for ZnO.
Despite the various approaches that have been demonstrated to be effec-
tive in improving the optical and electronic properties of metal oxides such
as ZnO and TiO 2 , the large bandgap of these metal oxides is still one of the
major factors limiting their performance. Therefore, it is highly desirable to
develop small bandgap semiconductors for photocatalytic hydrogen
3.2.3 Metal Oxynitrides/Metal Nitrides/Metal Phosphides
Metal nitrides or metal oxynitrides such as Ta 3 N 5 and TaON have attracted
increasing attention due to the relatively smaller bandgap energies compared
with their oxide counterparts [15, 26]. It is known that the valence band of
metal oxides is dominated by O2p orbital, and the conduction band edges
consist predominantly of the empty orbitals of metal ions. The low lying O
2p orbital imposes an intrinsic limitation on the electronic band structure for
simultaneously achieving favorable bandgap energy and band edge positions.
In comparison with metal oxides, metal nitrides and metal oxynitrides have
more negative valence band edge due to the hybridization of N 2p with O
2p orbitals [27]. For example, Ta 2 O 5 has a bandgap of 4.2 eV, while Ta 3 N 5
and TaON have narrower bandgap energies of 2.1 and 2.5 eV, respectively
[28]. TaON and Ta 3 N 5 can be prepared by nitridation of Ta 2 O 5 at elevated
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