Ta 5d Orbitals Research Articles

Visible-light harvesting SrTiO3 solid solutions for photocatalytic hydrogen evolution from water.

The fabrication of solid solutions represents a compelling approach to modulating the physicochemical properties of materials. In this study, we achieved the successful synthesis of solid solutions comprising SrTiO3 and SrTaO2N (denoted as (SrTiO3)1-x-(SrTaO2N)x, 0 ≤ x ≤ 1) using the magnesium powder-assisted nitridation method. The absorption edge of (SrTiO3)1-x-(SrTaO2N)x is tunable from 500 to 600 nm. The conduction band minimum (CBM) of (SrTiO3)1-x-(SrTaO2N)x comprises the Ti 3d orbitals and the Ta 5d orbitals, while the valence band maximum (VBM) consists of the O 2p and N 2p orbitals. The microstructure of the (SrTiO3)1-x‑(SrTaO2N)x consists of small nanoparticles, exhibiting a larger specific surface area than the parent compounds of SrTiO3 and SrTaO2N. In the photocatalytic hydrogen evolution reaction (HER) with sacrificial reagents, the activity of solid solutions is notably superior to that of nitrogen-doped SrTiO3 and SrTaO2N. This superiority is mainly attributed to its broad light absorption range and high charge separation efficiency, which indicates its potential as a promising photocatalytic material. Moreover, the magnesium powder-assisted nitridation method exhibits obvious advantages for the synthesis of oxynitrides and bears instructional significance for the synthesis of other nitrogen-containing compounds and even sulfur-containing compounds.

Bandgap engineering and antiferroelectric stability of tantalum doped silver niobate ceramics from first-principles

Extensive research has been conducted on silver niobite (AgNbO3)-based antiferroelectric ceramics for their promising applications in energy storage applications, with various compositional modifications explored to improve their energy storage capabilities. In this theoretical study, we have systematically investigated the electronic, structural, and chemical bonding properties of AgNb1-xTaxO3 (x = 0.00, 0.125, 0.25, 0.375, 0.50, abbreviated as ANT100x) solid solutions based on first-principles calculation. Our results reveal that the bandgap increases from 1.82 eV to 1.89 eV, due to the higher energy level of Ta 5d orbitals compared to Nb 4d orbitals. The enlarged bandgap, accompanied with oxygen vacancy formation energy (ΔEf,vac), contributes to the enhancement of Eb. The Ta substitution of Nb site suppresses the cation displacement, oxygen octahedral distortion, and bond length and angles, indicating an improved stability of antiferroelectric phase. In addition, the electron localization function (ELF) and Bader charge values show weakened covalent bonding of Ta−O bonds compared to Nb−O bonds. These theoretical findings have the potential to aid in the advancement and creation of novel energy storage applications using lead-free AFE perovskites, as well as facilitate the manipulation of their breakdown electric field through bandgap engineering.

First-principles investigation on structural and photoelectric properties of (Nb, Ta) codoped anatase TiO2

The structural and photoelectric properties of (Nb, Ta) codoped anatase TiO2 were studied by first-principles calculations based on density functional theory and the Hubbard U correction. The results demonstrated that hybridized states composed of Ta 5d and Nb 4d orbitals at the bottom of the conduction band enabled the Fermi level to enter the conduction band and facilitated (Nb, Ta) codoped anatase TiO2 to exhibit n-type metallic characteristics, and the conductivity of Ti0·875Nb0·0625Ta0·0625O2(I) is approximately 3.2 times that of Ti0·9375Nb0·0625O2. Noted that both conductivity and mobility decreased with the increase of the distance between impurity Nb and Ta atoms and the Nb doping concentration. Moreover, the calculated results of optical properties presented the shifts of the absorption edges towards the ultraviolet region, and (Nb, Ta) codoped anatase TiO2 possessed the high transmittance thanks to its low absorption and reflection of visible light.

Overall Water Splitting Under Visible Light Irradiation Over Ir and A‐Codoped KTaO3 (A=Ca, Sr, Ba, La) Photocatalysts

Metal oxides are an attractive material group as photocatalysts, while the number of visible‐light‐responsive metal oxide photocatalysts especially for overall water splitting is quite limited. Herein, overall water splitting under visible light irradiation over Ir and A‐codoped KTaO3 (A = Ca, Sr, Ba, La) photocatalysts is demonstrated. The Ir and A‐codoped KTaO3 works by the electron transition from impurity levels formed by Ir3+ to the conduction band formed by Ta5d orbitals. The codoping of Ca, Sr, Ba, and La contributes to the formation of fine photocatalyst particles and stabilization of doped Ir as trivalent. An Rh‐core/Cr2O3‐shell cocatalyst loaded by photodeposition is effective for overall water splitting over Ir and La‐codoped KTaO3 compared to a RhCrO3 cocatalyst loaded by impregnation and other metal‐core/Cr2O3‐shell cocatalysts. The Rh/Cr2O3‐loaded KTaO3 codoped with Ir and La is also active for solar water splitting.

A member of p-type TCO family: Sn2TaxNb2-xO7 with a tunable band gap and controllable hole mobility

Via density functional theory, we considered a new member of p-type transparent conducting oxides (TCOs) family, Sn2TaxNb2-xO7 (x = 0, 0.5, 1, 1.5, 2), to modify the carrier mobility, band gaps, optical properties by Ta5+ substitution at Nb5+ sites. The calculated effective masses indicate that the carrier mobility is expected to be controlled by varying the ratio of Ta and Nb elements in Sn2TaxNb2-xO7 compounds. Nb 4d orbital considerably exists in the valence band maximum (VBM) of Sn2TaxNb2-xO7 (x = 0.5, 1, 1.5), which impedes the overlap between Sn 5s and O 2p orbitals. It offers reliable evidence that non (n-1) d10ns2 cations in the VBM would be detrimental to the hole transport in TCOs. The introduction of Ta 5d orbitals promotes the conduction band edge toward a higher level, thus broadening the band gaps of Sn2TaxNb2-xO7. A structural distortion has been observed when Ta5+ ions replaced Nb5+ sites since the two cations have different bindings with the surrounding oxygen atoms.

Atom-Scale Dispersed Palladium in a conductivePd0.1TaS2 Lattice with a Unique Electronic Structurefor Efficient Hydrogen Evolution

Noble metal catalysts have extraordinary catalytic activity but suffer from high cost; therefore single atom catalysis attracts tremendous attention. Here, we propose atom-scale dispersed palladium (Pd) in a layered TaS2 lattice to form new compounds of PdxTaS2, whose crystal structure was resolved by powder X-ray diffraction. The electrical conductivity of metallic Pd0.1TaS2 (1.36 * 104 S m-1) is twice as large as that of TaS2 (6.18 * 103 S m-1). This compound applied as a new electrocatalyst for the hydrogen evolution reaction (HER) exhibits excellent catalytic performance with an onset overpotential of 77 mV, superior to those of other catalysts such as TaS2 (295 mV) and Pd-loaded TaS2 (114 mV). Density functional theory calculations reveal that the interaction of Pd-4d and Ta-5d orbitals results in the upshift of the Fermi level (TaS2 versus Pd0.1TaS2) to decrease the barrier height for the HER. Furthermore, the hybridization of Pd-4d and S-3p orbitals builds up pathways along the c axis to improve the conductivity and promote HER performance. It is noted that the amount of Pd used in Pd0.1TaS2 is only 4 wt%, compared with 20 wt% in commercial Pd/C catalysts, which is beneficial for its practical application.

Dopant-related electron trap states in Lu2O3:Ta

Electronic structure of Ta-doped cubic (bixbyite-type) lutetium oxide was analyzed using augmented plane wave with local orbitals density functional theory (DFT) calculations with meta-generalized gradient approximation (meta-GGA, mGGA) Räsänen, Pittalis and Proetto functional. Two doping sites, three oxidation states of the dopant and optional interstitial oxygen (Oi) were considered. The calculations indicate that introduction of the dopant results in energy levels below conduction band, with strong contribution of Ta 5d orbitals. The position of the levels depends strongly on the oxidation state of the dopant. Depending on the composition, two distinct electron traps with depths of 1.5–1.2 eV and 0.9–0.2 eV were revealed. Introduction of Oi at proximity of the Ta dopant results in partial overlap of the Ta-related defect states with conduction band, which makes the traps very shallow, deteriorating the electron trapping properties. In some cases, shallow hole traps can also be attributed to the Oi. The calculated traps structure is in good accord with the experimental data.

First-principles study of elastic, electronic, thermodynamic, and thermoelectric transport properties of TaCoSn

Abstract In this paper, we have performed a comprehensive set of first-principles calculations to study elastic, electronic, thermodynamic and thermoelectric properties of TaCoSn using density functional theory (DFT). Half-heusler, TaCoSn has been found to be elastically and thermodynamically stable, ductile and hard material. The Debye temperature of TaCoSn has been found to be 375.39 K. The calculated energy bands indicate that TaCoSn is an indirect band gap semiconductor and the value of gap is 1.107 eV using PBE functional and it is 1.153 eV by TB-mBJ potentials. Such small increase of band gap by TB-mBJ potential has no significant effect on the transport properties of TaCoSn. In TaCoSn, no significant spin-orbit interaction is found. The density of states at the Fermi energy is dominated by Ta-5d and Co-3d orbitals due to strong hybridization between them. We also calculate the relaxation time and lattice thermal conductivity. The lattice thermal conductivity of TaCoSn (4.95 W/mK at 300 K) is relatively small than that of other half-heusler compounds. The maximum Seebeck coefficient at 500 K is 249.41 μV/K. The obtained power factor (S2σ/τ) at 600 K is ∼12.5 × 1011 W/msK2. The calculated maximum figure of merit (ZT) is 0.73 at 600 K indicates that TaCoSn is a promising material for thermoelectric device applications.

Interfacial magnetic-phase transition mediated large perpendicular magnetic anisotropy in FeRh/MgO by a heavy transition-metal capping

Stacking a magnetic memory junction in spintronic devices necessarily involves making contacts with a transitional-metal capping electrode. Herein, by means of first-principles calculations, we reveal the importance of heavy transition-metal capping on magnetic-phase transition from antiferromagnetic (AFM) to ferromagnetic (FM) order and the large perpendicular magnetic anisotropy (PMA) found in Ta-capped FeRh films on MgO substrate. While magnetization of FeRh films reorients from in-plane to PMA when in contact with MgO, the presence of Ta capping further enhances the magnitude of the PMA energy by at least five times. This large PMA is associated with the AFM-FM transition at the interface, which in turn modifies the out-of-plane Fe 3d orbital states through the hybridization with the strong spin-orbit coupled Ta 5d orbitals. Furthermore, the magnetic-phase transition at the interface is the result of the mutual mechanisms of the capping-induced volume/tetragonal expansion in the interfacial FeRh layers and the competition between the direct and indirect exchange interactions. These findings suggest that Ta/FeRh/MgO multilayers may represent highly favourable memory materials with net interfacial ferromagnetism and large PMA in antiferromagnet spintronics.

Creating Two-Dimensional Electron Gas in Polar/Polar Perovskite Oxide Heterostructures: First-Principles Characterization of LaAlO3/A(+)B(5+)O3.

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) at the polar/polar (LaO)(+)/(BO2)(+) interface in the LaAlO3/A(+)B(5+)O3 (A = Na and K, B = Nb and Ta) heterostructures (HS). Unlike the prototype polar/nonpolar LaAlO3/SrTiO3 HS system where there exists a least film thickness of four LaAlO3 unit cells to have an insulator-to-metal transition, we found that the polar/polar LaAlO3/A(+)B(5+)O3 HS systems are intrinsically conducting at their interfaces without an insulator-to-metal transition. The interfacial charge carrier densities of these polar/polar HS systems are on the order of 10(14) cm(-2), much larger than that of the LaAlO3/SrTiO3 system. This is mainly attributed to two donor layers, i.e., (LaO)(+) and (BO2)(+) (B = Nb and Ta), in the polar/polar LaAlO3/A(+)B(5+)O3 systems, while only one (LaO)(+) donor layer in the polar/nonpolar LaAlO3/SrTiO3 system. In addition, it is expected that, due to less localized Nb 4d and Ta 5d orbitals with respect to Ti 3d orbitals, these LaAlO3/A(+)B(5+)O3 HS systems can exhibit potentially higher electron mobility because of their smaller electron effective mass than that in the LaAlO3/SrTiO3 system. Our results demonstrate that the electronic reconstruction at the polar/polar interface could be an alternative way to produce superior 2DEG in the perovskite-oxide-based HS systems.

Nb and Ta layer doping effects on the interfacial energetics and electronic properties of LaAlO3/SrTiO3 heterostructure: first-principles analysis.

The two-dimensional electron gas (2DEG) formed at the n-type (LaO)(+1)/(TiO2)(0) interface in the polar/nonpolar LaAlO3/SrTiO3 (LAO/STO) heterostructure (HS) has emerged as a prominent research area because of its great potential for nanoelectronic applications. Due to its practical implementation in devices, desired physical properties such as high charge carrier density and mobility are vital. In this respect, 4d and 5d transition metal doping near the interfacial region is expected to tailor electronic properties of the LAO/STO HS system effectively. Herein, we studied Nb and Ta-doping effects on the energetics, electronic structure, interfacial charge carrier density, magnetic moment, and the charge confinements of the 2DEG at the n-type (LaO)(+1)/(TiO2)(0) interface of LAO/STO HS using first-principles density functional theory calculations. We found that the substitutional doping of Nb(Ta) at Ti [Nb(Ta)@Ti] and Al [Nb(Ta)@Al] sites is energetically more favorable than that at La [Nb(Ta)@La] and Sr [Nb(Ta)@Sr] sites, and under appropriate thermodynamic conditions, the changes in the interfacial energy of HS systems upon Nb(Ta)@Ti and Nb(Ta)@Al doping are negative, implying that the formation of these structures is energetically favored. Our calculations also showed that Nb(Ta)@Ti and Nb(Ta)@Al doping significantly improve the interfacial charge carrier density with respect to that of the undoped system, which is because the Nb(Ta) dopant introduces excess free electrons into the system, and these free electrons reside mainly on the Nb(Ta) ions and interfacial Ti ions. Hence, along with the Ti 3d orbitals, the Nb 4d and Ta 5d orbitals also contribute to the interfacial metallic states; accordingly, the magnetic moments on the interfacial Ti ions increase significantly. As expected, the Nb@Al and Ta@Al doped LAO/STO HS systems show higher interfacial charge carrier density than the undoped and other doped systems. In contrast, Nb@Ti and Ta@Ti doped systems may show higher charge carrier mobility because of the lower electron effective mass.

Electronic structures of anatase (TiO2)1-x(TaON)x solid solutions: a first-principles study.

Sensitizing wide band gap photo-functional materials under visible-light irradiation is an important task for efficient solar energy conversion. Although nitrogen doping into anatase TiO2 has been extensively studied for this purpose, it is hard to increase the nitrogen content in anatase TiO2 because of the aliovalent nitrogen substituted for oxygen, leading to the formation of secondary phases or defects that hamper the migration of photoexcited charge carriers. In this paper, electronic structures of (TiO2)1-x(TaON)x (0 ≤ x ≤ 1) solid solutions, in which the stoichiometry is satisfied with the co-substitution of Ti for Ta along with O for N, are investigated within the anatase crystal structure using first-principles calculations. Our computational results show that the solid solutions have substantially narrower band gaps than TiO2, without introducing any localized energy states in the forbidden gap. In addition, in comparison with the pristine TiO2, the solid solution has a direct band gap when the content of TaON exceeds 0.25, which is advantageous to light absorption. The valence band maximum (VBM) of the solid solutions, which is mainly composed of N 2p states hybridized with O 2p, Ti 3d or Ta 5d orbitals, is higher in energy than that of pristine anatase TiO2 consisting of non-bonding O 2p states. On the other hand, incorporating TaON into TiO2 causes the formation of d-d bonding states through π interactions and substantially lowers the conduction band minimum (CBM) because of the shortened distance between some metal atoms. As a result, the anatase (TiO2)1-x(TaON)x is expected to become a promising visible-light absorber. In addition, some atomic configurations are found to possess exceptionally narrow band gaps.

Improved visible-light photocatalytic activity of NaTaO3 with perovskite-like structure via sulfur anion doping

A novel sulfur anion-doped NaTaO3 photocatalyst with nanocubic morphology, at an average size of 200–500nm or so, was synthesized by a simple hydrothermal process. The as-prepared samples were characterized by various techniques, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–vis diffuse reflectance spectra (UV–vis DRS) and photocatalytic degradation evaluation. The results indicate that the as-prepared NaTaO3 is assigned to monoclinic lattice, which is much closer to cubic phase than conventional orthorhombic perovskite structure. And doping of S anions would cause, to some extent at least, expanded unit cell and distorted lattice structure from monoclinic into cubic phase, whose TaOTa band angle is much closer to 180°, resulting in the higher mobility of photo induced charge carriers. Narrowed band gap and splitting slopes are observed in S-doped NaTaO3 by UV–vis DRS, confirming the visible-light adsorption capability and the upward shifted valence band edge, which come from the overlapping of S 3p orbitals and Ta 5d orbitals. In virtue of the crystalline and electronic structures, the resultant S-doped NaTaO3 samples, as compared with pure NaTaO3, exhibit remarkably improved visible-light photocatalytic activity and similar UV-light photocatalytic activity, during the photocatalytic degradation of methyl orange. In addition, S-doped NaTaO3 also displays reliable recycling photocatalytic performance and satisfying solar catalytic activity for the organic pollutant. On the basic of above experimental phenomena, a possible mechanism for sulfur anion doping in NaTaO3 perovskite-like structure is also presented.

Major Difference in Visible-Light Photocatalytic Features between Perfect and Self-Defective Ta3N5 Materials: A Screened Coulomb Hybrid DFT Investigation

Relevant properties to visible-light overall water splitting reactions of perfect and self-defective bulk Ta3N5 semiconductor photocatalysts are investigated using accurate first-principles quantum calculations on the basis of density functional theory (DFT, including the perturbation theory DFPT) within the screened coulomb hybrid (HSE06) exchange-correlation formalism. Among the various explored self-defective structures, a strong stabilization is obtained for the configuration displaying a direct interaction between the created N- and Ta-vacancies. In the lowest-energy structure, each of the three created Ta-vacancies and the five created N-vacancies is found to be in aggregated disposition, leading to the formation of cages into the lattice. Although the calculated structural, electronic, and optical properties of the two materials are found to be very similar and in good agreement with available experimental works, their photocatalytic features for visible-light overall water splitting reactions show completely different behaviors. On the basis of calculated band edge positions relative to water redox potentials, the perfect Ta3N5 (calculated band gap of 2.2 eV) is predicted by HSE06 to be a good candidate only for H+ reduction while the self-defective Ta3N5 (calculated band gap of 2.0 eV) reveals suitable band positions for both water oxidation and H+ reduction similar to the experimental data reported on Ta3N5 powders. Its ability to reduce H+ is predicted to be lower than the perfect one. However, the strongly localized electronic characters of the valence band (VB) and conduction band (CB) edge states of the self-defective material only on the N 2p and Ta 5d orbitals surrounding the aggregated N- and Ta-vacancies are expected to strongly limit the probability of photogenerated carrier mobility through its crystal structure.

Intrinsic defects and electronic conductivity of TaON: First-principles insights

Using hybrid functional calculations we show that TaON has different defect properties from the binary tantalum oxide and nitride: (i) instead of O or N vacancies or Ta interstitials, the ON antisite is the dominant defect, which determines its intrinsic n-type conductivity and the p-type doping difficulty; (ii) the ON antisite has a shallower donor level than O or N vacancies, with a delocalized distribution composed mainly of the Ta 5d orbitals, which gives rise to better electronic conductivity in the oxynitride than in the oxide and nitride. The phase stability analysis reveals that further oxidation of TaON is possible when the oxygen chemical potential is high (O rich); thus, a low oxygen partial pressure is required to synthesize stoichiometric TaON samples.

Photophysical and Photocatalytic Properties of Zn3M2O8 (M = Nb, Ta)

The Zn3M2O8 (M = Nb, Ta) compounds with layered crystal structures were prepared by a conventional solid‐state reaction method, and their electronic‐band structures, optical properties, and photocatalytic activities were investigated. The prepared compounds were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis diffuse reflectance, and Mott–Schottky measurement. The UV–Vis diffuse reflectance spectra revealed Zn3Nb2O8 and Zn3Ta2O8 exhibited band gaps of 4.1 and 4.5 eV, respectively. From the electronic‐band‐structure calculations using DFT method, it was found that the valence band in both compounds was constructed by the hybridization of Zn 3d and O 2p orbitals, whereas the conduction band was consisted of Nb 4d (Zn3Nb2O8) and Ta 5d (Zn3Ta2O8) orbitals. The photocatalytic activity of both compounds was examined by the measurement of the H2 production rate in an aqueous methanol solution under UV light irradiation. The Zn3Ta2O8 with a larger band gap exhibited higher activity than Zn3Nb2O8, which was attributed to its higher reduction potential resulting from the difference of its electronic‐band configuration.

Morphology-controlled synthesis and efficient photocatalytic performances of a new promising photocatalyst Sr0.25H1.5Ta2O6·H2O

A new photocatalyst Sr0.25H1.5Ta2O6·H2O (HST) with high surface area was successfully prepared by a facile and mild hydrothermal reaction using Ta2O5·nH2O as a precursor. TEM images revealed that their different morphologies, from nanoplate to nanopolyhedron, were formed under different pH values of the reactive solutions. The formation mechanism of HST was also studied and proposed. Growth of HST nanocrystallites followed a reaction-crystallization model. By analyzing the results of XRD, DRS, XPS, electrochemistry and theoretical calculation, the crystal and electronic structural characterizations of HST was established. Compared with isostructural Sr0.4H1.2Nb2O6·H2O (HSN), the Ta 5d orbitals were the main contribution to the bottom of the conduction band of HST, inducing the energy level more negative while the top of valence band, which was dominated by O 2p states, remained almost unchanged. Due to the more suitable electronic band structure and various morphologies, the photocatalyst showed superior photocatalytic activities for water splitting to generate H2 and for degrading benzene as compared with HSN. The rate of H2 evolution and the conversion ratio of benzene were 81 times and 4 times higher than that of TiO2 (Degussa P25), respectively. The proposed mechanisms for photocatalytic reactions based on the experimental results were discussed.

Site-Differentiated Solid Solution in (Na1−xCux)2Ta4O11 and Its Electronic Structure and Optical Properties

The (Na(1-x)Cu(x))(2)Ta(4)O(11) (0 ≤ x ≤ 0.78) solid-solution was synthesized within evacuated fused-silica vessels and characterized by powder X-ray diffraction techniques (space group: R3c (#167), Z = 6, a = 6.2061(2)-6.2131(2) Å, c = 36.712(1)-36.861(1) Å, for x = 0.37, 0.57, and 0.78). The structure consists of single layers of TaO(7) pentagonal bipyramids as well as layers of isolated TaO(6) octahedra surrounded by Na(+) and Cu(+) cations. Full-profile Rietveld refinements revealed a site-differentiated substitution of Na(+) cations located in the 12c (Wyckoff) crystallographic site for Cu(+) cations in the 18d crystallographic site. This site differentiation is driven by the linear coordination geometry afforded at the Cu(+) site compared to the distorted seven-coordinate geometry of the Na(+) site. Compositions more Cu-rich than x ~ 0.78, that is, closer to "Cu(2)Ta(4)O(11)", could not be synthesized owing to the destabilizing Na(+)/Cu(+) vacancies that increase with x up to the highest attainable value of ~26%. The UV-vis diffuse reflectance spectra show a significant red-shift of the bandgap size from ~4.0 eV to ~2.65 eV with increasing Cu(+) content across the series. Electronic structure calculations using the TB-LMTO-ASA approach show that the reduction in bandgap size arises from the introduction of Cu 3d(10) orbitals and the formation of a new higher-energy valence band. A direct bandgap transition emerges at k = Γ that is derived from the filled Cu 3d(10) and the empty Ta 5d(0) orbitals, including a small amount of mixing with the O 2p orbitals. The resulting conduction and valence band energies are determined to favorably bracket the redox potentials for water reduction and oxidation, meeting the thermodynamic requirement for photocatalytic water-splitting reactions.

Visible-Light Photocatalytic Degradation of BiTaO4 Photocatalyst and Mechanism of Photocorrosion Suppression

BiTaO4 photocatalyst for methylene blue (MB) degradation, which worked under visible-light irradiation, was systematically investigated for the first time. BiTaO4 possess similar degradation ability of TiO2−XNX under visible-light irradiation (λ > 420 nm). Density functional calculations revealed that Bi 6s orbitals contributed to the formation of valence band, resulting in narrowing the band gap. Moreover, comparing with BiNbO4, we found BiTaO4 showed photocorrosion suppression performance during the photocatalytic process. The differences in the photocorrosion suppression between BiTaO4 and BiNbO4 could be attributed to the conduction curvature and bandwidth, leading to differences in the mobility of photogenerated electrons formed in the conduction bands, which mainly consisted of Ta 5d and Nb 4d orbitals, respectively.

Crystal Structure and Electron Density of Tantalum Oxynitride, a Visible Light Responsive Photocatalyst

Tantalum oxynitride (TaON) is investigated by neutron and synchrotron diffraction analysis to clarify the structural and electronic features responsible for the visible-light activity of this material. In combination with density functional theory (DFT) calculations, the present measurements confirm the TaON photocatalyst to be monoclinic with space group P21/c (a = 4.94941(4) Å, b = 5.01662(4) Å, c = 5.16430(2) Å, α = 90°, β = 99.6107(4)°, γ = 90°, V = 126.426(2) Å3 at 298 K). Complete occupational ordering of the anions occurs in alternate layers normal to the [100] direction. Oxygen and nitrogen atoms in TaON are coordinated with three and four tantalum atoms, respectively. Bond valence sums for the oxygen and nitrogen atomic sites are calculated to be 2.1 and 3.2, respectively. Electron density analyses based on synchrotron data in combination with DFT calculations reveal covalent bonds between Ta and O atoms and between Ta and N atoms. The hybridization of anion 2p and Ta 5d orbitals attributed to the covalent bonds between Ta and O or N atoms results in increased dispersion of the valence band, raising the top of the valence band and leading to a visible-light response. The minimum electron density at the Ta−O bond decreases with increasing bond length, whereas no such trend was observed for the Ta−N bond.