303
Water Dissociation Technologies for Hydrogen
agent. The physical doping of transition metal ions into TiO 2 by the advanced
ion-implantation technique also allowed modified TiO 2 to work under visible light
radiation. The ion implantation technique is, however, very expensive for commercial use. The visible light response can also be obtained by doping of anions such as
N, S, or C [62–64] as substitutes for oxygen in the TiO 2 lattice. When TiO 2 is fused
with metal oxides such as SrO, BaO, and Ln 2 O 3 , metal titanates and intermediate
band gaps are obtained [44]. Materials such as SrTiO 3 , La 2 Ti 2 O 7 , and Sm 2 Ti 2 O 7 have
shown some promising results. Promising results have also been shown by using
Sm 2 Ti 2 S 2 O 5 , where sulfur anion is substituted for oxygen [44]. Under visible light
radiation, the last material works as a stable photocatalyst for the reduction of H + to
H 2 or the oxidation of H 2 O to O 2 in the presence of sacrificial electron donor Na 2 S–
Na 2 SO 3 or methanol or acceptor Ag + [44]. A new class of titanium semiconductors, titanium disilicide (TiSi 2 ) that absorbs a wide range of solar light, has recently
been proposed as a prototype photocatalyst for the water dissociation reaction. More
description of this catalyst is given in an excellent review by Navarro et al. [44].
11.3.1.2 tantalates and niobates
Layered and tunneling structures of oxides are considered as promising materials
for water dissociation reaction. Tantalates and niobates oxides with corner-sharing
octahedral MO 6 (M = Ta or Nb) have been examined as photocatalysts for water
dissociation [44]. Kato and Kudo [42] observed that MTaO 3 (M  =  Li, Na, K) are
effective photocatalysts for water dissociation under UV light. The oxides crystallize in pervoskite structure type. Lin et al. [43] showed that NaTaO 3 produced by
sol–gel method gave higher activity for water dissociation than the same material
prepared by the high-temperature solid-state synthesis. The most active photocatalysts were those that achieve higher nitrogen substitution, maintaining the original
layered structure of Sr 2 Nb 2 O 7 . More detailed discussion of these types of catalysts is
given by Navarro et al. [44].
11.3.1.3 transition-metal Oxides, nitrides, and Oxynitrides
Certain vanadium and tungsten compounds were found to be active in water dissociation reaction. BiVO 4 with scheelite structure and Ag 3 VO 4 with pervoskite
structure showed photocatalytic activity in visible light for oxygen evolution from
an aqueous silver nitrate solution [61,65]. The WO 3 system also oxidizes water
at moderately high rates in the presence of Ag + and Fe 3
+ ions [44]. Under visible light, Pt–WO 3 alone with NaIO 3 produces oxygen at high rate but produced
no hydrogen [44]. Some other catalysts in this category are also examined by
Navarro et al. [44].
Navarro et al. [44] also reported that nitrides and oxynitrides of transition metal
cations with d10 electronic configurations (Ga 3
+ and Ga 4
+ ) constitute a class of photocatalysts suitable for water dissociation in visible light without sacrificial reagents.
Among various cocatalysts examined, the largest improvement in activity was
obtained when (Ga 1−x Zn x )(N 1−x O x ) was loaded with a mixed oxide of Rh and Cr [44].
This semiconductor evolves hydrogen and oxygen steadily and stoichiometrically
under visible light from pure water in the absence of sacrificial agent. The solid solution between ZnO and ZnGeN 2 (Zn 1+x Ge) − (N 2 O x ) has also been found to be active