Ta Cations Research Articles

Advanced electrocatalytic performance of the configuration entropy cobalt-free Bi0.5Sr0.5FeO3–δ cathode catalysts for solid oxide fuel cells

The medium-entropy perovskite oxide Bi0.5Sr0.5Fe0.85Nb0.05Ta0.05Sb0.05O3–δ (BSFNTS) is evaluated as a potential cathode catalyst for solid oxide fuel cells (SOFCs). The crystal structure, electrocatalytic activity, oxygen reduction kinetics, and CO2 tolerance are systematically investigated. At 700 °C, the BSFNTS cathode exhibits excellent electrochemical performance with a polarization resistance as low as 0.095 Ω cm2. The maximal power density of the fuel cell with the BSFNTS cathode is 900 mW cm−2. Furthermore, the rate control step for the oxygen reduction reaction (ORR) of the electrode is primarily identified as the adsorbed and diffusion process of the molecule oxygen. The BSFNTS electrode presents excellent CO2 tolerance and durability in a CO2-containing atmosphere, which is related to the high acidity of Bi, Nb, Ta, and Sb cations and the larger average bonding energy of BSFNTS. The preliminary results indicate that BSFNTS medium-entropy oxide is an attractive cathode electrocatalyst for SOFCs.

Effect of Covalence and Degree of Cation Order on the Luminous Efficacy of Mn4+ Luminescence in the Double Perovskites, Ba2BTaO6 (B = Y, Lu, Sc).

The spectroscopic properties of the Mn4+ ion are investigated in the series of isostructural double perovskite compounds, Ba2BTaO6 (B = Y, Lu, Sc). A comparison of these properties highlights the influence of covalent bonding within the perovskite framework and the degree of order between the B3+-Ta cations on the energy and intensity of the Mn4+2E → 4A2 emission transition (R-line). These two parameters of the emission spectrum are of importance for practical application since they determine the phosphor luminous efficacy. The influence of covalent bonding within the corner shared BO6/2 and TaO6/2 perovskite framework on the energy of the R-line energy is investigated. From the spectroscopic data, we have derived information on the influence of the degree of order between the B3+ and Ta5+ cations on the intensity of the R-line. The lowest energy and the highest intensity of the R-line are found in the double perovskite, Ba2ScTaO6. The purpose of this work is to propose for first time an explanation of these effects in the considered double perovskites. The obtained results are useful guidelines for practical improvement and tuning of key parameters of phosphors to the desired values.

A theoretical–experimental study of KSr2Nb(3-x)TaxO10 (x = 0,1,2) systems as photocatalysts

Understanding the influence of structural changes arising from electronic characteristics is a significant challenge in novel material development. The Nb(V) and Ta(V) cations are pertinent due to their similar ionic radii across various coordinations. Layered perovskites (KSr2Nb(3-x)TaxO10, where x = 0,1, and 2) were synthesized using the solid-state method. These compounds were characterized employing experimental techniques and subjected to photocatalytic testing in a model reaction. Density Functional Theory simulations were conducted, confirming experimental data accuracy and prediction reliability. Evaluation of intra and inter-octahedral distortion and charge carrier effective masses were correlated with experimental outcomes. Remarkably, substituting Nb with Ta increased photocatalytic activity, suggesting a promising avenue for tailoring material properties.

High‐entropy yttrium pyrochlore ceramics with glass‐like thermal conductivity for thermal barrier coating application

AbstractA2B2O7‐type oxides with low thermal conductivities are potential candidates for next‐generation thermal barrier coatings. The formation of high‐entropy ceramics is considered as a newly effective way to further lower their thermal conductivities. High‐entropy Y2(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)2O7 (5HEO) and Y2(Ti0.25Zr 0.25Hf0.25Ta0.25)2O7 (4HEO) ceramics were prepared by in situ solid reaction sintering, considering the important roles of B‐site cations on thermal conductivities of the A2B2O7‐type oxides. Reaction process, phase structures, microstructures, and thermal conductivities of the as‐sintered ceramics were investigated. Lattice distortion effects on their thermal conductivities were also discussed by using the proposed criterion based on the supercell volume difference of the individual compounds. Near fully‐dense 5HEO and 4HEO ceramics were obtained after being sintered at 1600°C. The former one had a dual‐phase structure containing high‐entropy Y2(Ti0.227Zr0.227Hf0.227Nb0.136Ta0.182)2O7.318 pyrochlore oxide (5HEO‐P) and Y(Nb, Ta)O4 solid solution, while the latter one was a single‐phase pyrochlore oxide (4HEO‐P) with homogeneous element distribution. The formed 5HEO‐P oxide has larger lattice distortion than 4HEO‐P oxide due to the larger total amounts of Nb and Ta cations at B sites in the 5HEO‐P oxide. It results in lower thermal conductivity of 5HEO ceramics (keeping at 1.8 W·m–1·K–1) than those of 4HEO ceramics (ranging from 1.8 to 2.5 W·m–1·K–1) at temperatures from 25°C to 1400°C. Their glass‐like thermal conductivities were determined by the selection of B site cations and high‐entropy effects. These results provide some useful information for the material design of novel thermal barrier coating materials.

Quasi‐1D Antiferroelectricity in Centrosymmetric CsTaS3 Crystal

Abstract1D ferroelectric (FE) materials are expected to be promising low‐dimensional materials for nanometer devices, but are rarely reported. The previous experiment discovered that quasi‐1D CsTaS3 crystalizes in centrosymmetric space group, but in which the Ta cations have a strange shift along the c axis from the center of TaS6 octahedron. Here, the crystal structure, electronic structure, ferroelectric, and piezoelectric properties of CsTaS3 by using first‐principles calculations are studied. It is found that the empty d orbitals of Ta cations induce the distortions of TaS6 octahedra, which bring the polarity along a given [TaS3]− chain. The adjacent chains are antiferroelectric (AFE) coupling making CsTaS3 being an AFE material. However, the FE phase of CsTaS3 is supposed to be accessible since its total energy is only 0.3 meV per formula higher than that of the AFE phase. The polarization of FE CsTaS3 is estimated to be 16.8 μC cm−2, which is comparable to those of well‐known FE perovskites. The work predicts a quasi‐1D AFE material, which would be helpful for subsequent experimental work.

Large electrocaloric response via percolation of polar nanoregions

A first-principle-based effective Hamiltonian model is developed to investigate electrocaloric effects (ECE) of ferroelectric $\mathrm{Pb}({\mathrm{Sc}}_{0.5}{\mathrm{Ta}}_{0.5})\mathrm{O}3$ (PST) possessing different degrees of chemical ordering between Sc and Ta cations. It is found that PST exhibits large electrocaloric effects when the electric field drives a paraelectric-to-ferroelectric phase transition isothermally above the Curie temperature. More precisely, for any specific temperature above the Curie temperature, the electrocaloric coefficient exhibits its maximum at a critical electric field that is precisely the threshold of percolation for which the polar nanoregions begin to propagate inside the whole sample, with dipoles being parallel to the field's direction. This percolation-induced maximal ECE occurs for all the possible degrees of chemical ordering, therefore making it a general and novel mechanism, based on which a strategy is further proposed to improve the ECE.

Investigation on Optimal Ta/Cr Ratio of a Single Crystal Ni-Base Superalloy in View of the Isothermal Oxidation Behavior

The relative content of strengthening element tantalum (Ta) and oxidation-resistant element chromium (Cr) is an essential value for superalloys to obtain an excellent combination of oxidation resistance and mechanical properties. In the present paper, the isothermal oxidation behavior of several single crystal Ni-base superalloys with different Ta/Cr (wt. %, similarly hereinafter) ratios at 1000 °C in static air has been systematically investigated to explore the optimal Ta/Cr for excellent oxidation resistance. A detailed microstructure study using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and an electro-probe microanalyzer (EPMA) was performed to reveal the oxidation products and mechanisms. For all alloys, a three-layer structured scale consisting of an outer (Cr, Al, Ti, Ni, Ta)-O layer, an inner Al2O3 layer and an inner nitride layer was formed. As Ta/Cr increased, the amounts of Ta-containing products, cracks, holes and inner nitride increased. Meanwhile, the completeness of the Al2O3 layer got worse. It was shown that if Ta/Cr ≤ 0.5, Ta increased the growth rate of Cr2O3 via the doping effect induced by Ta cations. If Ta/Cr > 0.5, Ta reduced the completeness of Cr2O3 through competitive growth of Ta2O5 and Cr2O3. A good oxidation performance can be expected with the value Ta/Cr ≤ 0.5.

Analysis of two-level systems and mechanical loss in amorphous ZrO2-doped Ta2O5 by non-cage-breaking and cage-breaking transitions.

The energy landscape of ZrO2-doped amorphous Ta2O5 is explored in this work. With models corresponding to experimental concentrations of 50% Zr and 50% Ta cations, we search for, gather, and analyze two-level systems (TLSs) from molecular dynamic simulations. The mechanical loss function is calculated for each TLS individually. The results show that TLS with low asymmetry and large elastic coupling constants contribute the most to mechanical loss. We identify these as "bad actors." The higher barriers relate to the mechanical loss at higher temperatures. The concept of the oxygen cage that describes the local structural environment surrounding a metal ion is introduced. The existence of a drastic change in local environment, or a cage-breaking process, enables us to understand the double peaks present in the asymmetry distribution and provides a pictorial interpretation to distinguish two types of TLS. Quantitatively, a cage-breaking event is related to at least one large distance change in an atom-atom pair, and non-cage-breaking transitions have only small rearrangements. The majority of TLSs are cage-breaking transitions, but non-cage-breaking TLS transitions show higher average mechanical loss in ZrO2-doped Ta2O5. By decomposing the contributions to mechanical loss, we find that the low temperature loss peak near 40 K mainly comes from non-cage-breaking TLS transitions and the second loss peak near 120 K originates from cage-breaking TLS transitions. This finding is important for understanding the interplay between the atomic structure of TLS and mechanical loss.

2DEG and 2DHG in NaTaO3 polar thin films: thickness and strain dependency

Two-dimensional (2D) carrier gases in perovskite surfaces and interfaces have been intensely studied since their properties are attractive to many functional devices and applications. Here, we demonstrate through ab initio DFT calculations that surface 2D carries gases can be found in NaTaO3 ultrathin films. Furthermore, we show the thickness dependence of such phenomenon and how it can be tuned when biaxial in-plane strain is applied. Tensile does not alter the valence and conduction character of the films but promotes 2D electron and hole gases in the (TaO2)+ and (NaO)− surfaces, respectively. Because of the competition between surface and strain effects to deal with the cleavage-induced polarity, biaxial compression is able to generate 2D hole gases in the (TaO2)+ surface instead. Such carrier-type and layer switching are explained through changes in the electrostatic potential balancing along the [001] direction and (Na,Ta) cations displacements. The presented results concern not only nanoelectronics but also catalytic applications where modulating bandgap and valence/conduction states is desired.

The role of doping and microstructure on hydrogen solubility in monoclinic ZrO2: Experimental validations of simulated defect chemistry

The effect of chemical doping and ceramic microstructure on H solubility was measured systematically for the first time in monoclinic ZrO2. Excitingly, the influence of chemical doping by Fe, Cr and Ta cations on bulk H solubility was in qualitative agreement with defect chemistry simulations based on density functional theory and statistical thermodynamics. This is the first experimental validation of the recently-developed modeling framework, which is capable of high-throughput defect chemistry computations of light elements in metal oxides. H solubility was quantified using temperature-programmed desorption and was measured for various microstructures and under varied oxygen partial pressure, enabling us to identify the microstructural origin (i.e. bulk, surfaces and grain boundaries) of sorbed H defects and infer their defect type. Doping the oxide resulted in a significant increase in the H-uptake time, indicating that solute cations slowed H defect diffusion. Ta doping offered the lowest bulk H solubility at all measurement temperatures (410 °C–60 °C), though Fe doping also lowers bulk solubility at lower temperatures and yielded the most tightly bound H defects of all oxides. This could be significant in applications where one wishes to minimize H permeation via limiting diffusion kinetics. Based on this work we emphasize the role of both doping and microstructure on the overall H uptake and highlight the importance of elucidating the relative nuances of each oxide system; we thus reiterate the value of a high-throughput computational analysis.

Valence state of B and Ta cations in the AB1/2Ta1/2O3 ceramics (A = Ca, Sr, Ba, Pb; B = Fe, Sc) from X-ray photoelectron and Mössbauer spectroscopy data

Valence states of iron and tantalum ions in eight ceramic compounds with а perovskite structure AxByTazO3+σ (A = Pb, Ba, Sr, Ca; B = Fe, Sc) were studied, including the compounds AxFeyTazO3+σ (A = Ba, Sr, Ca) annealed in a reducing atmosphere of CO. In a number of samples, the presence of Fe2+ ions was detected along with Fe3+. Relative fractions of the Fe2+ and Fe3+ ions were obtained by a full-profile fitting of the Fe2p X-ray photoelectron spectra (XPS) by superposition of experimental spectra of Fe2+ and Fe3+ ions, and by the Mössbauer spectroscopy method. The presence of Ta4+ ions along with Ta5+ ions was detected. The samples subjected to high-temperature annealing in the atmosphere of CO contain only Ta4+ ions. A modification of a standard methodology of the XPS determination of elemental compositions is proposed for the cases when the O1s-spectra have high-energy “tails”; it is based on ensuring the electrical neutrality of the samples. Changes in the elemental and ionic composition of the samples reflect the effect of the annealing of samples in a reducing atmosphere of CO.

Direct Visualization of Independent Ta Centers Supported on Two-Dimensional TiO2 Nanosheets.

Highly dispersed, supported oxides are ubiquitous solid catalysts but can be challenging to characterize with atomic precision. Here, it is shown that crystalline anatase TiO2 nanosheets (∼5 nm thick) are ideal supports for imaging highly dispersed active sites. Ta cations were deposited by several routes, and high-resolution high angle annular dark-field scanning transmission electron microscopy was used to determine the location of Ta with respect to the TiO2 lattice and quantify Ta-Ta distances. In the best case, it is shown that >80% of Ta atoms are isolated from one another, whereas other techniques are blind to this critical catalytic property or give only qualitative estimates. TiO2 nanosheets may prove to be a useful platform for other types of catalysis studies.

Sodium Tantalate Photocatalysts Doped with Metal Cations: Why Are They Active for Water Splitting?

Sodium tantalate (NaTaO3) is the first semiconductor photocatalyst that produces hydrogen–oxygen mixed bubbles in the overall water splitting reaction, when appropriately doped with metal cations. A series of studies are reviewed herein to answer the question of why doping with metal cations raised the quantum efficiency of the reaction. Infrared absorption of band‐gap‐excited photocatalysts demonstrated that cation doping reduced the electron–hole recombination rate and the steady‐state population of charge carriers accordingly increased. In‐depth studies are focused on Sr cations incorporated through solid‐state, flux, and hydrothermal reactions. The recombination rate was reduced when Ta cations were exchanged with Sr cations. Sodium cations were simultaneously exchanged to balance the cationic and anionic charges with no need for creating oxygen anion vacancies. NaTaO3Sr(Sr1/3Ta2/3)O3 solid solution was formed as a result of the simultaneous doping. In addition to doping at the appropriate sites, the intraparticle distribution of Sr cations played an essential role to reduce the recombination rate. Strontium cations segregated to produce graded composition from the Sr‐rich surface to the Sr‐poor core. The bottom of the conduction band was raised at the Sr‐rich surface and the excited electrons were driven to the Sr‐poor core, leaving holes at the surface. However, the graded composition had a dual purpose; the excited electron population increased and its fractional contribution to the surface reaction decreased.

Influence of B site-cations on phase transition, magnetic switching and band-gap modulation in Pb(B′0.5B″0.5)O3–Pb(Zr0.53Ti0.47)O3 ceramics

The xPb(Fe0.5Nb0.5)O3–(1-x)Pb(Zr0.53Ti0.47)O3 and yPb(Fe0.5Ta0.5)O3–(1-y)Pb(Zr0.53Ti0.47)O3 multiferroic ceramics synthesized by a solid state reaction method have been investigated by means of structural characterizations, optical and magnetic measurements. X-ray diffraction (XRD) and Raman scattering analyses show that all the samples have the perovskite structure, but exhibit different phase transition behaviors resulting from the influence of B-cations. The internal lattice strain of the ceramics is estimated from XRD data by Williamson-Hall analysis method. The optical measurement indicates that the modification of optical properties can be achieved by manipulating the composition accurately in these perovskite oxides. Pb(Fe0.5Nb0.5)O3 or Pb(Fe0.5Ta0.5)O3 doping reduces the band-gap of Pb(Zr0.53Ti0.47)O3 from 3.32eV to 2.51eV. Furthermore, the different magnetic behaviors of the two compounds are observed, which mainly stem from the effects of nonmagnetic Nb and Ta cations on the interaction strength among the magnetic Fe ions. These results are helpful for a deeper understanding of structural, optical and magnetic properties and show the potential role, such materials can play, in multiferroic applications and solar energy devices.

Synergistic Resistive Switching Mechanism of Oxygen Vacancies and Metal Interstitials in Ta2O5

Ta2O5 is extensively studied as a data-storage material for resistance random access memory (RRAM). The resistive switching (RS) in Ta2O5-based RRAM is generally believed to be due to the diffusion of oxygen vacancy (Vo) inside the oxide, while the role of metal interstitials is paid less attention. Here, on the basis of first-principles calculations, we show that the role of interstitial Ta (Tai) is competitive under the oxygen-poor condition and should also contribute to RS in Ta2O5. This is obvious by our calculated comparable energy barriers for the diffusion of Vo and Tai, which are 3.5 and 3.7 eV, respectively. Furthermore, the presence of electric field in working devices will enhance the migration of Tai due to its higher charge states compared to Vo. Meanwhile, Tai will introduce more defect states closer to the conduction bands and, thus, is more effective on tuning the electronic structure of Ta2O5. The present work unravels the contribution of Ta cations in RS of tantalum oxide-based RRAM, pre...

Structure and thermodynamic stability of UTa3O10, a U(v)-bearing compound.

Heating a mixture of uranyl(vi) nitrate and tantalum(v) oxide in the molar ratio of 2 : 3 to 1400 °C resulted in the formation of a new compound, UTa3O10. The honey colored to yellow brown crystals of UTa3O10 crystallize in an orthorhombic structure with the space group Fddd (no. 70), lattice parameters a = 7.3947(1), b = 12.7599(2), c = 15.8156(2) Å, and Z = 8. Vertex sharing [TaO6]7- octahedra of two crystallographically distinct Ta cations form a three dimensional tantalate framework. Within this framework, six membered rings of [TaO6]7- octahedra are formed within the (001) plane. The center of these rings is occupied by the uranyl cations [UO2]+, with an oxidation state of +5 for uranium. The pentavalence of U and Ta was confirmed by X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy. The enthalpy of formation of UTa3O10 from Ta2O5, β-U3O7, and U3O8 has been determined to be 13.1 ± 18.1 kJ mol-1 using high temperature oxide melt solution calorimetry with sodium molybdate as the solvent at 700 °C. The close to zero enthalpy of formation of UTa3O10 can be explained by closely balanced structural stabilizing and destabilizing factors, which may also apply to other UM3O10 compounds.

Nonstoichiometric control of tunnel-filling order, thermal expansion, and dielectric relaxation in tetragonal tungsten Bronzes Ba0.5-xTaO3-x.

Ordering of interpolated Ba(2+) chains and alternate Ta-O rows (TaO)(3+) in the pentagonal tunnels of tetragonal tungsten bronzes (TTB) is controlled by the nonstoichiometry in the highly nonstoichiometric Ba0.5-xTaO3-x system. In Ba0.22TaO2.72, the filling of Ba(2+) and (TaO)(3+) groups is partially ordered along the ab-plane of the simple TTB structure, resulting in a √2-type TTB superstructure (Pbmm), while in Ba0.175TaO2.675, the pentagonal tunnel filling is completely ordered along the b-axis of the simple TTB structure, leading to a triple TTB superstructure (P21212). Both superstructures show completely empty square tunnels favoring Ba(2+) conduction and feature unusual accommodation of Ta(5+) cations in the small triangular tunnels. In contrast with stoichiometric Ba6GaTa9O30, which shows linear thermal expansion of the cell parameters and monotonic decrease of permittivity with temperature within 100-800 K, these TTB superstructures and slightly nonstoichiometric simple TTB Ba0.4TaO2.9 display abnormally broad and frequency-dependent extrinsic dielectric relaxations in 10(3)-10(5) Hz above room temperature, a linear deviation of the c-axis thermal expansion around 600 K, and high dielectric permittivity ∼60-95 at 1 MHz at room temperature.

High symmetry Nbn and Tan (n = 12, 15, and 17) clusters: High magnetic moments and the finding of superatoms with doping

Using ab initio calculations, we study Nbn and Tan clusters with n=12, 15, and 17 and find superatoms made of transition metals. Nb12 and Ta12 have an empty cage icosahedral structure. Neutral Ta12 has icosahedral symmetry and 2μB magnetic moments while the cage for Nb12 is slightly distorted and has no magnetic moment. These clusters behave like a divalent superatom. Accordingly an oxygen atom interacts exohedrally on a 3-fold site of the icosahedral cage like an MgO molecule and leaves the cage intact while Fe, Ru, and Os atoms can be endohedrally doped in Ta12 to produce electronically closed shell clusters with large highest occupied-lowest unoccupied molecular orbital (HOMO–LUMO) gap. Also isoelectronic Co@X12 and Rh@X12 (X=Nb and Ta) cations have a large HOMO–LUMO gap of up to about 1eV. In these cases the magnetic moment is quenched. On the other hand doping with Ni leaves the magnetic behavior of Ta12 the same. Further anions of Nb15 and Ta15 have a closed electronic shell structure and a singlet ground state with a large HOMO–LUMO gap. We show that isoelectronic Nb14Mo, Nb14W, Ta14Mo, Ta14W, Nb14Mn+, Nb14Re+, Ta14Mn+, and Ta14Re+ all have a large HOMO–LUMO gap of up to about 1.0eV and are magic clusters. Most interestingly Ta17 has a tetrahedrally symmetric Z16 Frank–Kasper polyhedral structure with a large magnetic moment of 5μB. On the other hand the magnetic moment on Nb17 cluster is 1μB. The magnetic moment on Ta17 is surprisingly large and it is counter intuitive as normally the magnetic moments decrease as one goes down in a column in the periodic table but some tantalum clusters behave differently. Doping of the n=17 clusters with Zr at the center leaves Zr@Ta16 magnetic with 2μB and 6μB magnetic moments nearly degenerate while Zr@Nb16 has zero magnetic moment.

CO oxidation on Ta-Modified SnO2 solid solution catalysts

Co-precipitation method was adopted to prepare Sn–Ta mixed oxide catalysts with different Sn/Ta molar ratios and used for CO oxidation. The catalysts were investigated by N2-Brunauer–Emmett–Teller (N2-BET), X-ray diffraction patterns (XRD), H2-temperature programmed reduction (H2-TPR), Thermal Gravity Analysis – Differential Scanning Calorimetry (TGA–DSC) techniques. It is revealed that a small amount of Ta cations can be doped into SnO2 lattice to form solid solution by co-precipitation method, which resulted in samples having higher surface areas, improved thermal stability and more deficient oxygen species on the surface of SnO2. As a result, those Sn rich Sn–Ta solid solution catalysts with an Sn/Ta molar ratio higher than 4/2 showed significantly enhanced activity as well as good resistance to water deactivation. It is noted here that if tantala disperses onto SnO2 surface instead of doping into its lattice, it will then have negative effect on its activity.

NaCu(Ta1−yNby)4O11 solid solution: A tunable band gap spanning the visible-light wavelengths

The new solid-solution NaCu(Ta1−yNby)4O11 (0≤y≤0.7) was synthesized by solid-state methods in the form of bulk powders that ranged from light-yellow to brown colored and were characterized by powder X-ray diffraction techniques (Space group R-3c (#167); Z=6; a=6.214(1)–6.218(1)Å and c=36.86(1)–36.94(1)Å). Full-profile Rietveld refinements confirmed a site-differentiated ordering of the Cu(I) and Na cations, i.e., occupying the 12c (linear environment) and 18d (seven-coordinate environment) crystallographic sites respectively. Conversely, a statistical mixture of Ta(V) and Nb(V) cations occurred over the 6b (octahedral environment) or the 18e (pentagonal-bipyramidal environment) crystallographic sites, without any preferential segregation. The UV-Vis diffuse reflectance spectra showed a significant red-shift of the optical bandgap size (indirect) from ∼2.70eV to ∼1.80eV across the solid solution with increasing Nb(V) content. Electronic-structure calculations using the tight-binding linear-muffin-tin-orbital approach showed that the reduction in bandgap size arises from the introduction of the lower-energy Nb 4d0 orbitals into the conduction band and consequently a lower energy of the conduction band edge. The lowest-energy bandgap transitions were found to be derived from electronic transitions between the filled Cu(I) and the empty Nb(V) d-orbitals, with a small amount of mixing with the O 2p orbitals. The resulting conduction and valence band energies are found to approximately bracket the redox potentials for water reduction and oxidation, and meeting the thermodynamic requirements for photocatalytic water-splitting reactions.