@article{king_sullivan_watkins-curry_chan_maggard_2016, title={Flux-mediated syntheses, structural characterization and low-temperature polymorphism of the p-type semiconductor Cu2Ta4O11}, volume={236}, ISSN={["1095-726X"]}, DOI={10.1016/j.jssc.2015.08.041}, abstractNote={A new low-temperature polymorph of the copper(I)-tantalate, α-Cu2Ta4O11, has been synthesized in a molten CuCl-flux reaction at 665 °C for 1 h and characterized by powder X-ray diffraction Rietveld refinements (space group Cc (#9), a=10.734(1) Å, b=6.2506(3) Å, c=12.887(1) Å, β=106.070(4)°). The α-Cu2Ta4O11 phase is a lower-symmetry monoclinic polymorph of the rhombohedral Cu2Ta4O11 structure (i.e., β-Cu2Ta4O11 space group R3̅c (#167), a=6.2190(2) Å, c=37.107(1) Å), and related crystallographically by ahex=amono/√3, bhex=bmono, and chex=3cmonosinβmono. Its structure is similar to the rhombohedral β-Cu2Ta4O11 and is composed of single layers of highly-distorted and edge-shared TaO7 and TaO6 polyhedra alternating with layers of nearly linearly-coordinated Cu(I) cations and isolated TaO6 octahedra. Temperature dependent powder X-ray diffraction data show the α-Cu2Ta4O11 phase is relatively stable under vacuum at 223 K and 298 K, but reversibly transforms to β-Cu2Ta4O11 by at least 523 K and higher temperatures. The symmetry-lowering distortions from β-Cu2Ta4O11 to α-Cu2Ta4O11 arise from the out-of-center displacements of the Ta 5d0 cations in the TaO7 pentagonal bipyramids. The UV–vis diffuse reflectance spectrum of the monoclinic α-Cu2Ta4O11 shows an indirect bandgap transition of ∼2.6 eV, with the higher-energy direct transitions starting at ∼2.7 eV. Photoelectrochemical measurements on polycrystalline films of α-Cu2Ta4O11 show strong cathodic photocurrents of ∼1.5 mA/cm2 under AM 1.5 G solar irradiation.}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={King, Nacole and Sullivan, Ian and Watkins-Curry, Pilanda and Chan, Julia Y. and Maggard, Paul A.}, year={2016}, month={Apr}, pages={10–18} }
 @article{boltersdorf_king_maggard_2015, title={Flux-mediated crystal growth of metal oxides: synthetic tunability of particle morphologies, sizes, and surface features for photocatalysis research}, volume={17}, ISSN={["1466-8033"]}, DOI={10.1039/c4ce01587h}, abstractNote={Flux crystal growth of mixed-metal oxide photocatalysts with (A) rod- and (B) platelet-shaped morphologies grown under varied flux conditions.}, number={11}, journal={CRYSTENGCOMM}, author={Boltersdorf, Jonathan and King, Nacole and Maggard, Paul A.}, year={2015}, pages={2225–2241} }
 @article{king_sommer_watkins-curry_chan_maggard_2015, title={Synthesis, Structure, and Thermal Instability of the Cu2Ta4O11 Phase}, volume={15}, ISSN={["1528-7505"]}, DOI={10.1021/cg500987c}, abstractNote={The Cu(I)-tantalate, Cu2Ta4O11, has been synthesized by flux methods in high purity and characterized by single-crystal X-ray diffraction (space group R3̅c (167), a = 6.219(2) Å, c = 37.107(1) Å). The compound is a new n = 1 member of the Cu(I)-tantalate CuxTa3n+1O8n+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 Cu5Ta11O30 (n = 1.5) and Cu3Ta7O19 (n = 2) at ∼800 to 1000 °C. The structure consists of layers of TaO7 pentagonal bipyramids that alternate with layers of isolated TaO6 octahedra and linearly coordinated Cu(I) cations. An increasing Cu-site vacancy across this series from Cu3Ta7O19 (100%), to Cu5Ta11O30 (83.3%), to Cu2Ta4O11 (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 Cu2Ta4O11. Thermal analysis shows that Cu2Ta4O11 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 3d10 and Ta 5d0 orbital contributions, respectively.}, number={2}, journal={CRYSTAL GROWTH & DESIGN}, publisher={American Chemical Society (ACS)}, author={King, Nacole and Sommer, Roger D. and Watkins-Curry, Pilanda and Chan, Julia Y. and Maggard, Paul A.}, year={2015}, month={Feb}, pages={552–558} }
 @article{king_sahoo_fuoco_stuart_dougherty_liu_maggard_2014, title={Copper Deficiency in the p-Type Semiconductor Cu1–xNb3O8}, volume={26}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/CM404147J}, DOI={10.1021/cm404147j}, abstractNote={The p-type semiconductor CuNb3O8 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 CuNb3O8 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 CuNb3O8 phase grows rapidly into well-faceted micrometer-sized crystals under these conditions, even with the use of Cu2O and Nb2O5 nanoparticle reactants. Heating CuNb3O8 in air to 450 °C for 3 h yields Cu-deficient Cu0.79(2)Nb3O8 that was characterized by powder X-ray Rietveld refinements (Sp. Grp. P21/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) Å3). The parent structure of CuNb3O8 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 Cu1–xNb3O8, 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 Cu1–xNb3O8 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., CuNb3O8 and CuNbO3, that exhibit significant increases in their cathodic photocurrents after being heated to increasing temperatures in air.}, number={6}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={King, Nacole and Sahoo, Prangya Parimita and Fuoco, Lindsay and Stuart, Sean and Dougherty, Daniel and Liu, Yi and Maggard, Paul A.}, year={2014}, month={Mar}, pages={2095–2104} }
 @article{choi_king_maggard_2013, title={Metastable Cu(I)-Niobate Semiconductor with a Low-Temperature, Nanoparticle-Mediated Synthesis}, volume={7}, ISSN={["1936-086X"]}, DOI={10.1021/nn305707f}, abstractNote={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.}, number={2}, journal={ACS NANO}, author={Choi, Jonglak and King, Nacole and Maggard, Paul A.}, year={2013}, month={Feb}, pages={1699–1708} }