Title: 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 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 R$$\overline{3c}$$ 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. Furthermore, 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.

Authors:

Boltersdorf, Jonathan ^[1]; Zoellner, Brandon ^[1]; Fancher, Chris M. ^[1]; Jones, Jacob L. ^[1]; Maggard, Paul A. ^[1]

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+ Show Author Affiliations

1. North Carolina State Univ., Raleigh, NC (United States)

Publication Date:

Mon Aug 01 00:00:00 EDT 2016

Research Org.:

Argonne National Laboratory (ANL), Argonne, IL (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)

OSTI Identifier:

1322361

Grant/Contract Number:  

AC02-06CH11357

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Physical Chemistry. C

Additional Journal Information:

Journal Volume: 120; Journal Issue: 34; Journal ID: ISSN 1932-7447

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

ENGLISH

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; solutions; thermodynamic modeling; heat transfer; energy; electrical conductivity