Single- and Double-Site Substitutions in Mixed-Metal Oxides: Adjusting the Band Edges Toward the Water Redox Couples

New mixed-metal oxide solid solutions, i.e., the single-metal substituted Na₂Ta_4-yNb_yO₁₁ (0 ≤ y ≤ 4) and the double-metal substituted Na_2-2xSn_xTa_4-yNb_yO₁₁ (0 ≤ y ≤ 4; 0 ≤ x ≤ 0.35), were investigated and used to probe the impact of composition on their crystalline structures, optical band gaps, band energies, and photocatalytic properties. The Na₂Ta₄O₁₁ (y = 1) phase was prepared by flux-mediated synthesis, while the members of the Na₂Ta_4-yNb_yO₁₁ solid solution (1 ≤ y ≤ 4) were prepared by traditional high-temperature reactions. The Sn(II)-containing Na_2-2xSn_xTa_4-yNb_yO₁₁ (0 ≤ y ≤ 4) solid solutions were prepared by flux-mediated ion-exchange reactions of the Na₂Ta_4-yNb_yO₁₁ solid solutions within a SnCl₂ flux. The crystalline structures of both solid solutions are based on the parent Na₂B₄O₁₁ (B = Nb, Ta) phases and consist of layers of edge-shared BO₇ pentagonal bipyramids that alternate with layers of isolated BO₆ octahedra surrounded by Na(I) cations. Rietveld refinements of the Na₂Ta_4-yNb_yO₁₁ solid solution showed that Nb(V) cations were disordered equally over both the BO₇ and BO₆ atomic sites, with a symmetry-lowering distortion from R3c to C2/c occurring at ∼67-75% Nb (y = ∼2.7-3.0). A red-shift in the optical band gaps from ∼4.3 to ∼3.6 eV is observed owing to a new conduction band edge that arises from the introduction of the lower-energy Nb 4d-orbitals. Reactions of these phases within a SnCl₂ flux yielded the new Na_2-2xSn_xTa_4-yNb_yO₁₁ solid solution with Sn-content varying from ∼11% to ∼21%. However, significant red-shifting of the band gap is found with increasing Nb-content, down to ∼2.3 eV for Na_1.4Sn_0.3Nb₄O₁₁, because of the higher energy valence band edge upon incorporation of Sn(II) into the structure. Aqueous suspensions of the particles irradiated at ultraviolet-visible energies yielded the highest photocatalytic hydrogen production rates for Na_1.3Sn_0.35Ta_1.2Nb_2.8O₁₁ (∼124 μmol H₂·g^-1·h^-1) and Na_1.4Sn_0.3Ta₃NbO₁₁ (∼105 μmol H₂·g^-1·h^-1), i.e., for the compositions with the highest Sn(II)-content. Further, polycrystalline films show n-type anodic photocurrents under ultraviolet-visible light irradiation. These results show that the valence and conduction band energies can be raised and lowered, respectively, using single-metal and double-metal substituted solid solutions. Thus, a novel approach is revealed for achieving smaller visible-light bandgap sizes and a closer bracketing of the water redox couples in order to drive total water splitting reactions that are critical for efficient solar energy conversion.