@article{sohag_o'donnell_fuoco_maggard_2021, title={A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode}, volume={26}, ISSN={["1420-3049"]}, url={https://www.mdpi.com/1420-3049/26/22/6830}, DOI={10.3390/molecules26226830}, abstractNote={A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3-x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm-2 up to >0.5 mA cm-2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm-2) at an applied bias of -0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents.}, number={22}, journal={MOLECULES}, author={Sohag, Zahirul and O'Donnell, Shaun and Fuoco, Lindsay and Maggard, Paul A.}, year={2021}, month={Nov} }
 @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{sullivan_sahoo_fuoco_hewitt_stuart_dougherty_maggard_2014, title={Cu-Deficiency in the p-Type Semiconductor Cu5–xTa11O30: Impact on Its Crystalline Structure, Surfaces, and Photoelectrochemical Properties}, volume={26}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/CM502891T}, DOI={10.1021/cm502891t}, abstractNote={The p-type semiconductor Cu5Ta11O30 has been investigated for the effect of Cu extrusion on its crystalline structure, surface chemistry, and photoelectrochemical properties. The Cu5Ta11O30 phase was prepared in high purity using a CuCl-mediated flux synthesis route, followed by heating the products in air from 250 to 750 °C in order to investigate the effects of its reported film preparation conditions as a p-type photoelectrode. At 650 °C and higher temperatures, Cu5Ta11O30 is found to decompose into CuTa2O6 and Ta2O5. At lower temperatures of 250 to 550 °C, nanosized CuIIO surface islands and a Cu-deficient Cu5–xTa11O30 crystalline structure (i.e., x ∼ 1.8(1) after 450 °C for 3 h in air) is found by electron microscopy and Rietveld structural refinement results, respectively. Its crystalline structure exhibits a decrease in the unit cell volume with increasing reaction temperature and time, owing to the increasing removal of Cu(I) ions from its structure. The parent structure of Cu5Ta11O30 is conserved up to ∼50% Cu vacancies but with one notably shorter Cu–O distance (by ∼0.26 Å) and concomitant changes in the Ta–O distances within the pentagonal bipyramidal TaO7 layers (by ∼0.29 Å to ∼0.36 Å). The extrusion and oxidation of Cu(I) to Cu(II) cations at its surfaces is found by X-ray photoelectron spectroscopy, while magnetic susceptibility data are consistent with the oxidation of Cu(I) within its structure, as given by CuI(5–2x)CuIIxTa11O30. Polycrystalline films of Cu5–xTa11O30 were prepared under similar conditions by sintering, followed by heating in air at temperatures of 350 °C, 450 °C, and 550 °C, each for 15, 30, and 60 min. An increasing amount of copper deficiency in the Cu5–xTa11O30 structure and CuIIO surface islands are found to result in significant increases in its p-type visible-light photocurrent at up to −2.5 mA/cm2 (radiant power density of ∼500 mW/cm2). Similarly high p-type photocurrents are also observed for Cu5Ta11O30 films with an increasing amount of CuO nanoparticles deposited onto their surfaces, showing that the enhancement primarily arises from the presence of the CuO nanoparticles which provide a favorable band-energy offset to drive electron–hole separation at the surfaces. By contrast, negligible photocurrents are observed for Cu-deficient Cu5–xTa11O30 without the CuO nanoparticles. Electronic structure calculations show that an increase in Cu vacancies shifts the Fermi level to lower energies, resulting in the depopulation of primarily Cu 3d10-orbitals as well as O 2p orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and CuIIO surface islands on the p-type photoelectrode films for solar energy conversion systems.}, number={23}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Sullivan, Ian and Sahoo, Prangya P. and Fuoco, Lindsay and Hewitt, Andrew S. and Stuart, Sean and Dougherty, Daniel and Maggard, Paul A.}, year={2014}, month={Nov}, pages={6711–6721} }
 @article{mclamb_sahoo_fuoco_maggard_2013, title={Flux Growth of Single-Crystal Na2Ta4O11 Particles and their Photocatalytic Hydrogen Production}, volume={13}, ISSN={["1528-7483"]}, DOI={10.1021/cg301859d}, abstractNote={Single-crystal particles of the layered natrotantite, i.e., Na2Ta4O11, were prepared within a K2SO4/Na2SO4 flux for flux-to-reactant molar ratios from 12:1 to 1:1 at a reaction temperature of 1000 °C for 2 h. Depending on the conditions, the flux reactions yielded crystals of Na2Ta4O11 that ranged in size from ∼100 nm to ∼1000 nm. The highest and lowest flux amounts yielded more isolated single crystals with sharper facets and surfaces, whereas intermediate flux amounts yielded more aggregates of particles with smooth and rounded surface features. All products were characterized by UV–vis diffuse reflectance techniques and were found to exhibit an indirect bandgap size of ∼4.1–4.3 eV and a larger direct bandgap transition of ∼4.5 eV. When the crystals are suspended in aqueous solutions and irradiated by ultraviolet light, they exhibit stable photocatalytic rates for hydrogen production of ∼13.4 μmol of H2·g–1·h–1 to ∼34.1 μmol of H2·g–1·h–1. The higher photocatalytic rates are found for the single crystals with the highly faceted and nanoterraced surfaces. Electronic structure calculations based on density functional theory confirm the lowest-energy bandgap transition is indirect and between the Γ and M k-points in the valence and conduction band states, respectively. The bandgap excitation is found to result in delocalization of the excited electrons over a layer of condensed TaO7 pentagonal bipyramids, which is a relatively unexplored structural feature for photocatalytic metal oxides.}, number={6}, journal={CRYSTAL GROWTH & DESIGN}, author={McLamb, Nathan and Sahoo, Prangya Parimita and Fuoco, Lindsay and Maggard, Paul A.}, year={2013}, month={Jun}, pages={2322–2326} }
 @article{arney_fuoco_boltersdorf_maggard_2013, title={Flux Synthesis of Na2Ca2Nb4O13: The Influence of Particle Shapes, Surface Features, and Surface Areas on Photocatalytic Hydrogen Production}, volume={96}, ISSN={["0002-7820"]}, DOI={10.1111/jace.12122}, abstractNote={The layered perovskite (n = 4) Ruddlesden-Popper phase Na2Ca2Nb4O13 was prepared within molten NaCl and Na2SO4 fluxes, yielding either rod-shaped or platelet-shaped particles, respectively. The flux-to-reactant molar ratios of 5:1 or 20:1 were found to significantly influence particle sizes and surface areas, while still maintaining the overall particle shapes. Measured surface areas of flux-prepared Na2Ca2Nb4O13 particles ranged from ∼0.36 to 4.6 m2/g, with the highest surface areas obtained using a 5:1 (NaCl-to-Na2Ca2Nb4O13) molar ratio. All samples exhibited a bandgap size of ∼3.3 eV, as determined by UV–Vis diffuse reflectance measurements. Photocatalytic rates for hydrogen production under ultraviolet light for platinized Na2Ca2Nb4O13 particles in an aqueous methanol solution ranged from ∼230 to 1355 μmol H2 g−1 h−1 when using the photochemical deposition (PCD) method of platinization, and ∼113–1099 μmol H2 g−1 h−1 when using the incipient wetness impregnation (IWI) method of platinization. The higher photocatalytic rates were obtained for the rod-shaped particles with the highest surface areas, with an apparent quantum yield (AQY) measured at ∼6.5% at 350 nm. For the platelet-shaped particles, the higher photocatalytic rates were observed for the sample with the lowest surface area but the largest concentration of stepped edges and grooves observed at the particle surfaces. The latter origin of the photocatalytic activity is confirmed by the significant enhancement of the photocatalytic rates by the PCD method that allows for the preferential deposition of the surface Pt cocatalyst islands at the stepped edges and grooves, while the photocatalytic enhancement is much smaller when using the more general IWI platinization method.}, number={4}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, author={Arney, David and Fuoco, Lindsay and Boltersdorf, Jonathan and Maggard, Paul A.}, year={2013}, month={Apr}, pages={1158–1162} }
 @article{fuoco_joshi_maggard_2012, title={Preparation and Photoelectrochemical Properties of p-type Cu5Ta11O30 and Cu3Ta7O19 Semiconducting Polycrystalline Films}, volume={116}, ISSN={["1932-7447"]}, DOI={10.1021/jp300267v}, abstractNote={New p-type polycrystalline films of semiconducting Cu5Ta11O30 and Cu3Ta7O19 were prepared on fluorine-doped tin oxide (FTO) glass starting from their CuCl-flux synthesis as highly faceted micrometer-sized particles. The particles were annealed on FTO at 400–500 °C, followed by a mild oxidation in air at between 250 and 550 °C. In an aqueous 0.5 M Na2SO4 electrolyte solution (pH = 6.3), the films exhibit strong cathodic photocurrents under irradiation by visible and/or ultraviolet light, which increased with higher annealing and oxidation temperatures owing to increased p-type carrier concentration and better electrical contact between particles. Thermogravimetric analyses show that the oxidation treatments result in an oxygen uptake at concentrations of ∼3 × 1020 cm–3 at 250 °C, to ∼4 × 1021 cm–3 at 550 °C, with the higher temperatures leading to the decomposition of the film. The Cu5Ta11O30 and Cu3Ta7O19 bulk powders exhibit band-gap sizes of ∼2.59 and ∼2.47 eV, respectively, and show an onset of their cathodic photocurrents at wavelengths of ∼500–550 nm. Mott–Schottky measurements of their flat-band potentials have been used to determine the valence band positions at approximately +1.06 and +1.19 V versus RHE (pH = 6.3), and thus conduction band positions of about −1.53 and −1.28 V for Cu5Ta11O30 and Cu3Ta7O19, respectively. The band positions are thus suitably located for the photon-driven reduction and oxidation of water. The highest observed incident photon-to-current efficiencies (IPCE %) for hydrogen production were ∼5% at 350 nm and ∼1–2% at 500–600 nm. Electronic structure calculations based on density functional theory methods show that the conduction band states are delocalized within layers of TaO7 pentagonal bipyramids, whereas the valence band states originate within layers of linearly coordinated Cu(I) cations. The lowest-energy band-gap transitions involve a metal-to-metal charge transfer between Cu(I) and Ta(V) cations in these two types of layers. Compared to other Cu(I) oxides, these structures possess sufficiently disperse bands for high carrier mobility within these layers, and thus the strong cathodic photocurrents of the films.}, number={19}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Fuoco, Lindsay and Joshi, Upendra A. and Maggard, Paul A.}, year={2012}, month={May}, pages={10490–10497} }
 @article{fuoco_rodriguez_peppel_maggard_2011, title={Molten-Salt-Mediated Syntheses of Sr2FeReO6, Ba2FeReO6, and Sr2CrReO6: Particle Sizes, B/B ' Site Disorder, and Magnetic Properties}, volume={23}, ISSN={["1520-5002"]}, DOI={10.1021/cm202545z}, abstractNote={The half-metallic double-perovskites Sr2FeReO6, Ba2FeReO6, and Sr2CrReO6 were synthesized in high purity and homogeneity using a NaCl/KCl molten flux at 750–800 °C in as little as 3–6 h. The particle sizes could be varied from ∼50 nm to >1 μm depending on the specific flux conditions and the double-perovskite composition. Powder X-ray diffraction refinements were used to characterize the extent of B/B′ site disorder (i.e., Fe/Re or Cr/Re sites), and that ranged from ∼83–85% for Sr2FeReO6, ∼90–97% for Ba2FeReO6, and ∼84–90% for Sr2CrReO6. The magnetic properties were measured as a function of magnetic field strength and analyzed with respect to the extent of B/B′ site ordering and resultant particle sizes that ranged from ∼50–500 nm for Sr2FeReO6, ∼100–800 nm for Sr2CrReO6, and ∼250 nm to 2 μm for Ba2FeReO6. Further, large temperature-dependent magnetoresistivites were observed for the flux-prepared Sr2FeReO6, Ba2FeReO6, and Sr2CrReO6, which are attributed to high concentrations of grain boundaries present.}, number={24}, journal={CHEMISTRY OF MATERIALS}, author={Fuoco, Lindsay and Rodriguez, Dianny and Peppel, Tim and Maggard, Paul A.}, year={2011}, month={Dec}, pages={5409–5414} }