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

• Published in: Journal of physical chemistry. C 2016-09, Vol.120 (34), p.19175-19188
• Main Authors: Boltersdorf, Jonathan, Zoellner, Brandon, Fancher, Chris M, Jones, Jacob L, Maggard, Paul A
• Format: Article
• Language: English
• Subjects: electrical conductivity; energy; heat transfer; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; solutions; thermodynamic modeling
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b05758

New mixed-metal oxide solid solutions, i.e., the single-metal substituted Na2Ta4–y Nb y O11 (0 ≤ y ≤ 4) and the double-metal substituted Na2–2x Sn x Ta4–y Nb y O11 (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–y Nb y O11 solid solution (1 ≤ y ≤ 4) were prepared by traditional high-temperature reactions. The Sn­(II)-containing Na2–2x Sn x Ta4–y Nb y O11 (0 ≤ y ≤ 4) solid solutions were prepared by flux-mediated ion-exchange reactions of the Na2Ta4–y Nb y O11 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–y Nb y O11 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 Na2–2x Sn x Ta4–y Nb y O11 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 i