Photophysical and Photocatalytic Properties of Three Isostructural Oxide Semiconductors In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 with Different 3d Transition Metals

Three oxide semiconductor photocatalysts, namely, In 6 NiTi 6 O 22, In 3 CrTi 2 O 10, and In 12 NiCr 2 Ti 10 O 42, have been investigated systematically to clarify the effects of transition metal cations (Ni and/or Cr) with partially filled 3d orbitals on the photophysical and photocatalytic properties. It was found that the three compounds all crystallized in a monoclinic crystal system with the same space group P 2 1 / a irrespective of the minor constituent cations Ni and/or Cr. However, their band-gap energies differed greatly depending on the minor cations: ∼2.48 eV for In 6 NiTi 6 O 22, ∼2.0 eV for In 3 CrTi 2 O 10, and ∼2.14 eV for In 12 NiCr 2 Ti 10 O 42 . In terms of photocatalytic H 2 evolution, In 12 NiCr 2 Ti 10 O 42, with both Ni and Cr as the minor cations, showed a much higher activity (∼8.2 μM/h) than either In 6 NiTi 6 O 22 (∼0.3 μM/h) or In 3 CrTi 2 O 10 (∼0.2 μM/h), with Ni or Cr, respectively, as the sole minor cation. In accordance with the photophysical and photocatalytic properties, we suggest that discontinuous interbands are formed by the split Ni 3d orbital in In 6 NiTi 6 O 22 or Cr 3d orbital in In 3 CrTi 2 O 10 individually, whereas continuous conduction and valence bands are formed in In 12 NiCr 2 Ti 10 O 42 by the hybridization of split Ni 3d and Cr 3d orbitals with Ti 3d/In 5sp and O 2p orbitals, respectively. The improved photocatalytic activity of In 12 NiCr 2 Ti 10 O 42 can be attributed to the enhanced mobility of photoexcited charge carriers.