A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode

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 −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

Preparation and Photoelectrochemical Properties of p-type Cu₅Ta₁₁O₃₀ and Cu₃Ta₇O₁₉ Semiconducting Polycrystalline Films

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

Flux Growth of Single-Crystal Na₂Ta₄O₁₁ Particles and their Photocatalytic Hydrogen Production

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 <i>k</i>-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

Specific Chemistry of the Anions: [TaOF₅]^2–, [TaF₆]⁻, and [TaF₇]^2–

The controlled crystallization of specific tantalum oxide-fluoride and tantalum fluoride anions ([TaOF₅]^2–, [TaF₆]⁻, and [TaF₇]^2–) is demonstrated using organic reagents with varied corresponding p<i>K</i>_a values in the presence of aqueous hydrofluoric acid. The identity of tantalum oxide-fluoride or fluoride anions of [TaOF₅]^2–, [TaF₆]⁻, and [TaF₇]^2– are shown to crystallize successively from solution to solid state by increasing the corresponding p<i>K</i>_a of organic reagents, which lead to the subsequent increase of fluoride concentration in the hydrofluoric acid solution. With the use of this methodology, three new hybrid crystal structures were targeted: [H₂(2,2′-bpy)]­TaOF₅ (2,2′-bpy = 2,2′-bipyridyl) <b>1</b>, [Hdpa]­TaF₆ (dpa = 2,2′-dipyridylamine) <b>2</b>, and [H₂En]­TaF₇ (En = ethylenediamine) <b>3</b>, respectively. The applicability and comparison of this methodology for tantalum and previously prepared niobium compounds show that it can be broadly used to design new materials with specific functionalities for other transition metal oxide-fluorides

Copper Deficiency in the p‑Type Semiconductor Cu_1–<i>x</i>Nb₃O₈

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 Cu_0.79(2)Nb₃O₈ that was characterized by powder X-ray Rietveld refinements (Sp. Grp. <i>P</i>2₁/<i>a</i>, <i>Z</i> = 4, <i>a</i> = 15.322(2) Å, <i>b</i> = 5.0476(6) Å, <i>c</i> = 7.4930(6) Å, β = 107.07(1)^o, and <i>V</i> = 554.0(1) ų). 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 <i>b</i>-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–<i>x</i>Nb₃O₈, 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–<i>x</i>Nb₃O₈ 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

Cu-Deficiency in the <i>p</i>‑Type Semiconductor Cu_5–<i>x</i>Ta₁₁O₃₀: Impact on Its Crystalline Structure, Surfaces, and Photoelectrochemical Properties

The <i>p</i>-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 <i>p</i>-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 Cu^IIO surface islands and a Cu-deficient Cu_5–<i>x</i>Ta₁₁O₃₀ crystalline structure (i.e., <i>x</i> ∼ 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 Cu^I_{(5–2<i>x</i>)}Cu^II_<i>x</i>Ta₁₁O₃₀. Polycrystalline films of Cu_5–<i>x</i>Ta₁₁O₃₀ 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–<i>x</i>Ta₁₁O₃₀ structure and Cu^IIO surface islands are found to result in significant increases in its <i>p</i>-type visible-light photocurrent at up to −2.5 mA/cm² (radiant power density of ∼500 mW/cm²). Similarly high <i>p</i>-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 Cu_5–<i>x</i>Ta₁₁O₃₀ 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 3<i>d</i>¹⁰-orbitals as well as O 2<i>p</i> orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and Cu^IIO surface islands on the <i>p</i>-type photoelectrode films for solar energy conversion systems

Reversible Magnesium Intercalation into a Layered Oxyfluoride Cathode

Reversible Magnesium Intercalation into a Layered Oxyfluoride Cathod