Effects of doping on photocatalytic activity for water splitting of metal oxides and nitride

Abstract

The effects of metal-ion doping or replacement on the photocatalytic performance for water splitting of d 10 and d 0 metal oxides and d 10 metal nitride were studied. The photocatalysts examined were (1) α-Ga 2-2x In 2x O 3 and ZnGa 2-2x In 2x O 4 in which In 3+ was added to Ga 2 O 3 and ZnGa 2 O 4 , respectively, (2) Y x In 2-x O 3 being a solid solution of In 2 O 3 and Y 2 O 3 , (3) metal ion doped CeO 2 , and (4) metal ion doped GaN. The photocatalytic activity of 1 wt % RuO 2 -loaded α-Ga 2-2x In 2x O 3 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-Ga 2-2x In 2x O 3 had the contribution of In 4d orbital to the valence band and of In5s orbital to the conduction band. Similar effects were observed for ZnGa 2-2x In 2x O 4 . RuO 2 -dispersed Y x In 2-x O 3 had a capability of producing H 2 and O 2 in the range x=1.0-1.5 in which the highest activity was obtained at x=1.3. The structures of both InO 6 and YO 6 octahedra were deformed in the solid solution,, and the hybridization of In5s5p and Y4d orbitals in the conduction band was enhanced. Undoped CeO 2 was photocatalytically inactive, but metal ion-doped CeO 2 showed a considerable photocatalytic activity. The activation occurred in the case that metal ions doped had larger ion sizes than that of Ce 4+ . The small amount doping of divalent metal ions (Zn 2+ and Mg 2+ ) 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.