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 Na2Ta4-yNbyO11 (0 <= y <= 4) and the double-metal substituted Na2-2xSnxTa4-yNbyO11 (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 Na2Ta4-yNbyO11, solid solution (1 <= y <= 4) were prepared by traditional high-temperature reactions. The Sn(II)-containing Na2-2xSnxTa4-yNbyO11 (0 <= y <= 4) solid solutions were prepared by flux-mediated ion-exchange reactions of the Na2Ta4-yNbyO11 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 Na2Ta4-yNbyO11 solid solution showed that Nb (V) cations were disordered equally over both the BO7 and. BO6 atomic sites, with a symmetry-lowering distortion from R (3) over barc to C2/c occurring at similar to 67-75% Nb (y = similar to 2.7-3.0). A red-shift in the optical band gaps from similar to 4.3 to similar 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 Na2-2xSnxTa4-yNbyO11 solid solution with Sn-content varying from similar to 11% to similar to 21%. However, significant red-shifting of the band gap is found with increasing Nb-content, down to similar to 2.3 eV for Na1.4Sn0.3Nb4O11, because of the higher energy valence band edge upon incorporation of Sn(Il) into. the structure. Aqueous suspensions of the particles irradiated at ultraviolet-visible energies yielded the highest photocatalytic hydrogen production rates for Na1.3Sn0.35Ta1.2Nb2.8O11 (similar to 424 mu mol H-2.g(-1).h(-1)) and Na1.4Sn0.3Ta3NbO11 ), (similar to 105 mu mol H-2.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 conchiction 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.