The effects of metal-ion doping or replacement on the photocatalytic performance for water splitting of d10 and d0 metal
oxides and d10 metal nitride were studied. The photocatalysts examined were (1) α-Ga2-2xIn2xO3 and ZnGa2-2xIn2xO4 in
which In3+ was added to Ga2O3 and ZnGa2O4, respectively, (2) YxIn2-xO3 being a solid solution of In2O3 and Y2O3, (3)
metal ion doped CeO2, and (4) metal ion doped GaN. The photocatalytic activity of 1 wt % RuO2-loaded α-Ga2-2xIn2xO3
increased sharply with increasing x, reached a maximum at around x=0.02, and considerably decreased with further
increase in x. The DFT calculation showed that the band structures of α-Ga2-2xIn2xO3 had the contribution of In 4d
orbital to the valence band and of In5s orbital to the conduction band. Similar effects were observed for ZnGa2-2xIn2xO4.
RuO2-dispersed YxIn2-xO3 had a capability of producing H2 and O2 in the range x=1.0-1.5 in which the highest activity
was obtained at x=1.3. The structures of both InO6 and YO6 octahedra were deformed in the solid solution,, and the
hybridization of In5s5p and Y4d orbitals in the conduction band was enhanced. Undoped CeO2 was photocatalytically
inactive, but metal ion-doped CeO2 showed a considerable photocatalytic activity. The activation occurred in the case
that metal ions doped had larger ion sizes than that of Ce4+. The small amount doping of divalent metal ions (Zn2+ and
Mg2+) converted photocatalytically inactive GaN to an efficient photocatalyst. The doping was shown to produce p-type
GaN which had the large concentration and high mobility of holes. The roles of metal ion doping and replacement in
the photocatalytic properties are discussed.