We report on the synthesis and characterization of a novel manganese vanadate, Mn1.5(H2O)(NH4)V4O12, with rare in situ disorder of Mn(H2O)22+/2NH4+. We show that vacancies created by ammonium ions and coordinating water molecules within the manganese vanadate crystal structure yield high-charge capacity, favorable rate capability, and long cycle life in Li-ion half-cells.

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.

Molten-salt reactions can be used to prepare single-crystal metal-oxide particles with morphologies and sizes that can be varied from the nanoscale to the microscale, subsequently enabling a growing number of novel investigations into their photocatalytic activities. Crystal growth using flux-mediated methods facilitates finer synthetic manipulation over particle characteristics. The synthetic flexibility that flux synthesis affords for the growth of metal-oxides has led to the stabilization of phases with limited stability, the discovery of new compositions, and access to alternate crystal morphologies and sizes that exhibit significant changes in photocatalytic activities at their surfaces, such as for the reduction of water to hydrogen in aqueous solutions. This approach has significantly impacted the current understanding of the optical and photocatalytic properties of metal-oxides, such as the dependence of band gap energies on the structure and chemical composition (i.e., obtained from flux-mediated ion-exchange reactions). Thus, flux preparations of metal-oxide photocatalysts assist in the growth and optimization of their particles in order to understand and tune the photocatalytic reaction rates at their surfaces.

Layered Dion–Jacobson phases RbLaNb2O7 and RbA2Nb3O10 (A = Ca, Sr) and the Ruddlesden–Popper phase Rb2La2Ti3O10 were prepared by solid-state methods at a reaction time of 50 h and a temperature of 1100 °C. The products were silver-exchanged within a AgNO3 flux at a reaction time of 24 h and a temperature of 250 °C. Substitution of silver cations into the interlayer spacing of the layered structures is found to decrease the optical bandgap sizes on average by ∼0.5 to ∼1.0 eV. The products were found by scanning electron microscopy (SEM) to exhibit irregularly shaped platelet morphologies with an average size of ∼1–5 μm across their lateral dimensions and stepped edges ranging from ∼20 to ∼300 nm in height. Significant increases in photocatalytic hydrogen production rates for all silver-exchanged products were observed. The silver-exchanged RbA2Nb3O10 layered structures exhibited the highest photocatalytic hydrogen formation rates under ultraviolet and visible irradiation (∼13,616 μmol H2·g–1·h–1). These rates were 10 times higher than prior to silver exchange (∼1,418 μmol H2·g–1·h–1). However, photocatalytic activity under only visible light irradiation is not observed. It is also found that the silver cations located at the surfaces are reduced to Ag(s) after prolonged UV and visible light exposure in solution, which functions to increase their activity under UV irradiation. Electronic-structure calculations based on density functional theory show that the highest-energy valence band states are composed of Ag 4d-orbital and O 2p-orbital contributions within the interlayer spacing of the structure. The lowest-energy conduction band states arise from the Nb/Ti d-orbital and O 2p-orbital contributions that are confined to the two-dimensional niobate/titanate sheets within the structures and along which the excited-electrons can preferentially migrate.Keywords: band engineering; flux synthesis; layered-niobate; photocatalysis; solar energy

The Ag(I) and Bi(III) tantalates Ag2Ta4O11, BiTa7O19, and Bi7Ta3O18 were prepared by solid-state methods at 1000 °C for 24–48 h. The Pb(II)-containing tantalate PbTa2O6 was prepared at 1100 °C for 24 h, whereas Pb3Ta4O13 and PbTa4O11 were synthesized from a reaction of A2Ta4O11 (A = Na, Ag) precursors with a PbCl2 flux (at 1:1, 5:1, and 10:1 molar ratios) at 700 °C from 24 to 96 h. The PbTa2O6, Pb3Ta4O13, and Bi7Ta3O18 structures consist of TaO6 layers and TaO6 chains/rings with Pb(II) ions located within the cavities. The structures of Ag2Ta4O11, PbTa4O11, and BiTa7O19 consist of layers of TaO7 pentagonal bipyramids that alternate with Ag(I), Pb(II), and Bi(III) cations, respectively. UV–vis diffuse reflectance data were used to measure bandgap sizes for Ag2Ta4O11 (∼3.9 eV), PbTa4O11 (∼3.8–3.95 eV), Pb3Ta4O13 (∼3.0 eV), PbTa2O6 (∼3.6 eV), BiTa7O19 (∼3.6 eV), and Bi7Ta3O18 (∼2.75 eV). A decrease in the band gap was observed with an increase in the Pb(II) or Bi(III) content. Photocatalytic activities of the platinized samples in aqueous solutions under ultraviolet irradiation were found to range from ∼7 to ∼194 μmol H2·g–1·h–1 in aqueous methanol and from ∼42 to ∼213 μmol O2·g–1·h–1 in aqueous silver nitrate. Electronic-structure calculations based on density functional theory show the highest-energy valence band states consist of the respective Ag 4d orbital/Pb 6s orbital/Bi 6s orbital and O 2p orbital contributions, and the lowest-energy conduction band states arise from the Ta 5d orbital contributions. The latter are delocalized over the TaO7 pentagonal bipyramid layers within the A2Ta4O11 (A = Na, Ag), PbTa4O11, and BiTa7O19 structures. Nearly all of the tantalates exhibit significant water oxidation photocatalytic activity. However, higher activity for water reduction was found for tantalates consisting of TaO7 pentagonal bipyramid layers that can serve as charge-migration pathways.Keywords: band engineering; layered tantalate; lead exchange; photocatalysis; solar energy

We report on the synthesis and characterization of a novel manganese vanadate, Mn1.5(H2O)(NH4)V4O12, with rare in situ disorder of Mn(H2O)22+/2NH4+. We show that vacancies created by ammonium ions and coordinating water molecules within the manganese vanadate crystal structure yield high-charge capacity, favorable rate capability, and long cycle life in Li-ion half-cells.