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

Abstract

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 R3̅c 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 Na2–2xSnxTa4–yNbyO11 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 Na1.4Sn0.3Nb4O11, 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 Na1.3Sn0.35Ta1.2Nb2.8O11 (∼124 μmol H2·g–1·h–1) and Na1.4Sn0.3Ta3NbO11 (∼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.