title: Substitutional mechanism of Ni into the wide-band-gap semiconductor InTaO4 and its implications for water splitting activity in the wolframite structure type.
abstract: 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-x)Ni(2x/3)Ta(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-x)Ni(2x/3)Ta(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-x)Ni(2x/3)Ta(x/3))TaO(4) compounds instead results from a weak (3)A(2g) → (3)T(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.
authors.affiliation: Department of Chemistry, State University of New York-Stony Brook, Stony Brook, New York 11794-3400, USA.
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Malingowski
authors.forename: Peter W
authors.initials: PW
authors.lastname: Stephens
authors.forename: Ashfia
authors.initials: A
authors.lastname: Huq
authors.forename: Qingzhen
authors.initials: Q
authors.lastname: Huang
authors.forename: Syed
authors.initials: S
authors.lastname: Khalid
authors.forename: Peter G
authors.initials: PG
authors.lastname: Khalifah