Photocatalytic water splitting by RuO2-loaded metal oxides and nitrides with d(0)- and d(10)-related electronic configurations

This article reviews photocatalysis for water splitting by various kinds of metal oxides and nitrides such as ferroelectric metal oxides, different kinds of titanates with d(0)(Ti4+) electronic configuration as a core metal ion, and various typical metal oxides with d(10)(In3+, Ga3+, Ge4+, Sn4+, Sb5+) configuration, and d(10) (Ga3+) metal nitride, together with d(10)s(2) (Pb2+) and d(0)f(0) (Ce4+) metal oxides. Ferroelectric metal oxides with single domain structure showed anomalous photovoltaic effects that controlled the behavior of photoexcited electrons. Various metal oxides involving ionic alkaline metal/alkaline earth metal ions showed good correlation between photocatalytic activity and the distortion of octahedral XO6/tetrahedral XO4 (X core metal ion). When the metal oxides involved covalent metal ions such as Zn2+ and Pb2+, the activity was invoked even in distortion-free structures: this is due to strong electronic effects that affect the band structure. The activity of d(10) metal oxides and nitrides is associated with conduction bands of hybridized sp orbitals with large dispersion that are able to generate photoexcited electrons with large mobility. A feature of the photocatalytically active metal oxides and nitrides discovered so far, which is their closed shell electronic structures is discussed.