Single-crystal particles of the layered natrotantite, i.e., Na 2 Ta 4 O 11 , were prepared within a K 2 SO 4 /Na 2 SO 4 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 Na 2 Ta 4 O 11 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.. 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 TaO 7 pentagonal bipyramids, which is a relatively unexplored structural feature for photocatalytic metal oxides.
The p-type semiconductor Cu 5 Ta 11 O 30 has been investigated for the effect of Cu extrusion on its crystalline structure, surface chemistry, and photoelectrochemical properties. The Cu 5 Ta 11 O 30 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, Cu 5 Ta 11 O 30 is found to decompose into CuTa 2 O 6 and Ta 2 O 5 . At lower temperatures of 250 to 550 °C, nanosized Cu II O surface islands and a Cu-deficient Cu 5−x Ta 11 O 30 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 Cu 5 Ta 11 O 30 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 TaO 7 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 Cu I(5−2x) Cu II x Ta 11 O 30 . Polycrystalline films of Cu 5−x Ta 11 O 30 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 Cu 5−x Ta 11 O 30 structure and Cu II O surface islands are found to result in significant increases in its p-type visiblelight photocurrent at up to −2.5 mA/cm 2 (radiant power density of ∼500 mW/cm 2 ). Similarly high p-type photocurrents are also observed for Cu 5 Ta 11 O 30 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 Cu 5−x Ta 11 O 30 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 3d 10 -orbitals as well as O 2p orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and Cu II O surface islands on the p-type photoelectrode films for solar energy conversion systems.
Formation of surface nanoparticles on p-type Cu3VO4 (shown) and their critical role in enhancing its photocurrents for solar energy conversion.
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.
Key indicatorsSingle-crystal X-ray study T = 290 K Mean (C-C) = 0.005 Å R factor = 0.062 wR factor = 0.143 Data-to-parameter ratio = 12.2 For details of how these key indicators were automatically derived from the article, see
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