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 metaloxides has led to the stabilization of phases 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 ionexchange reactions). Thus, flux preparations of metal-oxide photocatalysts assist in the growth 2 and optimization of their particles in order to understand and tune the photocatalytic reaction rates at their surfaces.

A nanoparticle synthetic strategy for the preparation of a new metastable Cu(I)-niobate is described, and that involves multipored Li₃NbO₄ nanoparticles as a precursor. A hydrothermal reaction of HNbO₃ and LiOH·H₂O in PEG200 and water at ∼180 °C yields ∼15-40 nm Li₃NbO₄ particles. These particles are subsequently used in a solvothermal copper(I)-exchange reaction with excess CuCl at 150 °C. Heating these products within the used CuCl flux (mp = 430 °C) to 450 °C for 30 min yields ∼4-12 nm Cu₂Nb₈O₂₁ crystalline nanoparticles, and for a heating time of 24 h yields μm-sized, rod-shaped crystals. The new structure was characterized by single-crystal X-ray diffraction to have a condensed network consisting of NbO₇ polyhedra and chains of elongated CuO₄ tetrahedra. The compound thermally decomposes starting at ∼250 °C and higher temperatures, depending on the particle sizes, owing to the loss of the weakly coordinated Cu(I) cations from the structure and a concurrent disproportionation reaction at its surfaces. Thus, conventional solid-state reactions involving higher temperatures and bulk reagents have proven unsatisfactory for its synthesis. The measured bandgap size is ∼1.43-1.65 eV (indirect) and shows a dependence on the particle sizes. Electronic structure calculations based on density functional theory show that the bandgap transition results from the excitation of electrons at the band edges between filled Cu(I) 3d¹⁰-orbitals and empty Nb(V) 4d⁰-orbitals, respectively. The p-type nature of the Cu₂Nb₈O₂₁ particles was confirmed in photoelectrochemical measurements on polycrystalline films that show a strong photocathodic current under visible-light irradiation in aqueous solutions. These results demonstrate the general utility of reactive nanoscale precursors in the synthetic discovery of new Cu(I)-based semiconducting oxides and which also show promise for use in solar energy conversion applications.

The p-type semiconductor CuNb 3 O 8 has been synthesized by solid-state and flux reactions and investigated for the effects of copper extrusion from its structure at 250−750 °C in air. High purity CuNb 3 O 8 could be prepared by solid-state reactions at 750 °C at reaction times of 15 min and 48 h, and within a CuCl flux (10:1 molar ratio) at 750 °C at reaction times of 15 min and 12 h. The CuNb 3 O 8 phase grows rapidly into well-faceted micrometersized crystals under these conditions, even with the use of Cu 2 O and Nb 2 O 5 nanoparticle reactants. Heating CuNb 3 O 8 in air to 450 °C for 3 h yields Cu-deficient Cu 0.79(2) Nb 3 O 8 that was characterized by powder X-ray Rietveld refinements (Sp. Grp. P2 1 /a, Z = 4, a = 15.322(2) Å, b = 5.0476(6) Å, c = 7.4930(6) Å, β = 107.07(1) o , and V = 554.0(1) Å 3 ). The parent structure of CuNb 3 O 8 is maintained with ∼21% copper vacancies but with notably shorter Cu−O distances (by 0.16−0.27 Å) within the Cu−O−Nb1 zigzag chains down its b-axis.Copper is extruded at high temperatures in air and is oxidized to form ∼100−200 nm CuO islands on the surfaces of Cu 1−x Nb 3 O 8 , as characterized by electron microscopy and X-ray photoelectron spectroscopy (XPS) techniques. XPS measurements show only the Cu(II) oxidation state at the surfaces after heating in air at 450 and 550 °C. Magnetic susceptibility of the bulk powders after heating to 350 and 450 °C is consistent with the percentage of Cu(II) in the compound. Electronic structure calculations find that an increase in Cu vacancies from 0 to 25% shifts the Fermi level to lower energies, resulting in the partial oxidation of Cu(I) to Cu(II). However, higher amounts of Cu vacancies lead to a significant increase in the energy of the O 2p contributions, and which cross the Fermi level and become partially oxidized at the top of the valence band. These oxygen contributions occur over the bridging Cu−O−Nb neighbors when the Cu site is vacant. After heating to 550 °C, XPS data show the formation of a new higher energy O 1s peak that corresponds to the formation of "O − " species at this higher concentration of Cu vacancies. Light-driven bandgap transitions between the valence and conduction band edges are predicted to occur between regions of the structure having Cu vacancies to regions of the structure without Cu vacancies, respectively. This perturbation of the electronic structure of Cu-deficient Cu 1−x Nb 3 O 8 could serve to drive a more effective separation of excited electron/hole pairs. Thus, these findings help shed new light on p-type Cu(I)-niobate photoelectrode films, i.e., CuNb 3 O 8 and CuNbO 3 , that exhibit significant increases in their cathodic photocurrents after being heated to increasing temperatures in air.

The Cu(I)-tantalate, Cu 2 Ta 4 O 11 , has been synthesized by flux methods in high purity and characterized by singlecrystal X-ray diffraction (space group R3̅ c (167), a = 6.219(2) Å, c = 37.107(1) Å). The compound is a new n = 1 member of the Cu(I)-tantalate Cu x Ta 3n+1 O 8n+3 series of structures and can be prepared in a molten CuCl flux within a relatively low temperature range of ∼625−700 °C, in comparison to the synthesis of Cu 5 Ta 11 O 30 (n = 1.5) and Cu 3 Ta 7 O 19 (n = 2) at ∼800 to 1000 °C. The structure consists of layers of TaO 7 pentagonal bipyramids that alternate with layers of isolated TaO 6 octahedra and linearly coordinated Cu(I) cations. An increasing Cu-site vacancy across this series from Cu 3 Ta 7 O 19 (100%), to Cu 5 Ta 11 O 30 (83.3%), to Cu 2 Ta 4 O 11 (66.7%) leads to an increasing fraction of O atoms that are not locally charge balanced by the Ta(V)/Cu(I) cations and thus yields decreased stability of Cu 2 Ta 4 O 11 . Thermal analysis shows that Cu 2 Ta 4 O 11 decomposes in air or under flowing nitrogen at temperatures above ∼550 °C (in the absence of the CuCl flux) into a mixture of known tantalates and Cu(II)tantalate phases. The compound exhibits a bandgap size of ∼2.55 eV (indirect), with higher-energy direct transitions starting at ∼2.73 eV. Electronic structure calculations confirm the indirect nature of the lowest-energy bandgap transition, which arises from valence and conduction band states that are primarily composed of Cu 3d 10 and Ta 5d 0 orbital contributions, respectively.

A new low-temperature polymorph of the copper(I)-tantalate, α-Cu 2 Ta 4 O 11 , has been synthesized in a molten CuCl-flux reaction at 665 o C for 1 h and characterized by powder X-ray diffraction Rietveld refinements (space group (#9), a = 10.734(1) Å, b = 6.2506(3) Å, c = 12.887(1) Å, β = 106.070(4) o). Th e α-Cu 2 Ta 4 O 11 phase is a lower-symmetry monoclinic polymorph of the rhombohedral Cu 2 Ta 4 O 11 structure (i.e., β-Cu 2 Ta 4 O 11 space group 3 � (#167), a = 6.2190(2) Å, c = 37.107(1) Å), and related crystallographically by a hex = a mono /√3, b hex = b mono , and c hex = 3c mono sinβ mono. Its structure is similar to the rhombohedral β-Cu 2 Ta 4 O 11 and is composed of single layers of highly-distorted and edge-shared TaO 7 and TaO 6 polyhedra alternating with layers of nearly linearly-coordinated Cu(I) cations and isolated TaO 6 octahedra. Temperature dependent powder X-ray diffraction data show the α-Cu 2 Ta 4 O 11 phase is relatively stable under vacuum at 223 K and 298 K, but reversibly transforms to β-Cu 2 Ta 4 O 11 by at least 523 K and higher temperatures. The symmetry-lowering distortions from β-Cu 2 Ta 4 O 11 to α-Cu 2 Ta 4 O 11 arise from the out-of-center displacements of the Ta 5d 0 cations in the TaO 7 pentagonal bipyramids. The UV-Vis diffuse reflectance spectrum of the monoclinic α-Cu 2 Ta 4 O 11 shows an indirect bandgap transition of ~2.6 eV, with the higher-energy direct