Substitutional mechanism of Ni into the wide-band-gap semiconductor InTaO ₄ and its implications for water splitting activity in the wolframite structure type

The mechanism of Ni substitution into the oxide semiconductor InTaO ₄ has been studied through a combination of structural and spectroscopic techniques, providing insights into its previously reported photoactivity. Magnetic susceptibility and X-ray absorption near-edge spectroscopy (XANES) measurements demonstrate that nickel is divalent within the host lattice. The combined refinement of synchrotron X-ray and neutron powder diffraction data indicates that the product of Ni doping has the stoichiometry of (In _1-xNi _2x/3Ta _x/3)TaO ₄ with a solubility limit of x ≈ 0.18, corresponding to 12% Ni on the In site. Single-phase samples were only obtained at synthesis temperatures of 1150 °C or higher due to the sluggish reaction mechanism that is hypothesized to result from small free energy differences between (In _1-xNi _2x/3Ta _x/3)TaO ₄ compounds with different x values. Undoped InTaO ₄ is shown to have an indirect band gap of 3.96 eV, with direct optical transitions becoming allowed at photon energies in excess of 5.1 eV. Very small band-gap reductions (less than 0.2 eV) result from Ni doping, and the origin of the yellow color of (In _1-xNi _2x/3Ta _x/3)TaO ₄ compounds instead results from a weak ³A _2g → ³T _1g internal d → d transition not associated with the conduction or valence band that is common to oxide compounds with Ni ²⁺ in an octahedral environment.