Nb 4d Research Articles

Visible-light-driven photocatalysts: (La/Bi + N)-codoped NaNbO3 by first principles

To improve the photocatalytic activity of NaNbO3 for water splitting, the bandgap and the band edges of NaNbO3 should be tailored to match the visible part of the solar spectrum and hydrogen and oxygen redox potentials. By analyzing the band structures of La/Bi-doped and (La/Bi + N)-codoped NaNbO3, we found that the pseudointermediate band (PIB) was formed in the bandgap in all the doped systems because of the orbital splitting of the Nb 4d induced by the dramatically enlarged O-Nb-O angles. The PIB could make the wide bandgap semiconductors absorb visible-light photons as long as it was degenerate or partially degenerate. Considering that the appropriate band edges and absorption properties, we believe that (La/Bi + N)-codoped NaNbO3 materials are promising photocatalysts for hydrogen production through water splitting under visible-light irradiation without other modifications.

Fat body catalase activity as a biochemical index for the recognition of thermotolerant breeds of mulberry silkworm, Bombyx mori L.

The fifth instar larvae of the silkworm Bombyx mori L. were exposed to selected high temperatures (35 and 40 °C) in order to understand the changes in the level of catalase activity in the three tissues of fat body, midgut and haemolymph of the five selected bivoltine breeds and their 9 quantitative traits, namely larval weight, cocoon weight, shell weight, shell ratio, filament length, filament weight, denier, renditta and effective rearing rate (ERR), also the correlation between them under high temperature conditions were examined. Catalase activity resulting in fat body revealed a positive correlation between the control (28±1 °C) and 40±1 °C. The CSR 2 breed showed the most level of thermotolerance and catalase activity, compared with the CSR 4, JROP, NB 4D 2 and KA breeds. It was found that the level of catalase activity in fat body may be a reliable biochemical index to recognize thermotolerant breeds in order to develop resistant hybrids for tropical areas.

Electronic Structure of Sr2AlNbO6: Dielectric and X-ray Photoemission Spectroscopy Study

The dielectric property and the electronic structure of double perovskite oxide strontium aluminium niobate, Sr2AlNbO6 (SAN), synthesised by a solid state reaction technique are investigated. The Rietveld refinement of the X-ray diffraction data of the sample shows the cubic phase with the Fm3m structure (lattice parameter, a = 7.7789 Å). The compound shows significant frequency dispersion in its dielectric properties. The Cole–Cole model is used to study the dielectric relaxation of SAN. The scaling behavior of loss tangent suggests that the relaxation describes the same mechanism at various temperatures. The electronic structure of SAN is studied by the first principles full potential linearized augmented plane wave method. Its valence band consists mainly of the O 2p states hybridised with the Nb 4d states. The calculated density of states is used to compare the valence band X-ray photoemission spectra of the sample.

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.

Band structure and photocatalytic activities for H 2 production of ABi 2Nb 2O 9 (A = Ca, Sr, Ba)

A new series of layered perovskite photocatalysts, ABi 2Nb 2O 9 (A = Ca, Sr, Ba), were synthesized by the conventional solid-state reaction method and characterized by X-ray diffraction (XRD) and UV-visible spectrometer. The results showed that the structure of ABi 2Nb 2O 9 (A = Ca, Sr) is orthorhombic, while that of BaBi 2Nb 2O 9 is tetragonal. The band gaps of CaBi 2Nb 2O 9, SrBi 2Nb 2O 9, and BaBi 2Nb 2O 9 were estimated to be 3.46, 3.43, and 3.30 eV, respectively. It was found from the electronic band structure study, using the density functional theory (DFT) with planewave basis, that the conduction bands of these photocatalysts mainly consist of Nb 4d + Bi 6p + O 2p orbitals and the valence bands are composed of hybridization with O 2p + Nb 4d + Bi 6s orbitals. The photocatalytic activities for water splitting were investigated under UV-light irradiation and indicated that these photocatalysts showed photocatalytic activity for H 2 and O 2 evolution from aqueous solutions containing sacrificial reagents (methanol and Ag +).

New Series of Solid-Solution Semiconductors (AgNbO3)1−x(SrTiO3)x with Modulated Band Structure and Enhanced Visible-Light Photocatalytic Activity

A novel series of solid-solution semiconductors (AgNbO3)1−x(SrTiO3)x (0 ≤ x ≤ 1) have been developed as highly visible-light-active photocatalysts for efficient O2 evolution and decomposition of organic pollutants. Rietveld refinement reveals that the perovskite-type solid solutions (AgNbO3)1−x(SrTiO3)x are crystallized in an orthorhombic system (0 ≤ x < 0.9) or a cubic system (0.9 ≤ x ≤ 1). In the mixed valent perovskites (AgNbO3)1−x(SrTiO3)x, the hybridization behaviors between the Ag 4d and O 2p orbitals and between the Nb 4d and Ti 3d orbitals play a crucial role in tuning the energy band structure (band gap, band edge, and bandwidth, etc.) and, thus, in tailoring the photophysical and photocatalytic properties. As a result of competition between the absorption ability to visible-light and the reductive/oxidative abilities, the highest visible-light activities for both O2 evolution and decomposition of gaseous 2-propanol (IPA) are realized over (AgNbO3)0.75(SrTiO3)0.25. In addition, very fine Ag parti...

NMR and Seebeck coefficient studies of the electronic band structure in Nb1−xB2

With the aim of providing experimental information for the Nb deficiency enhanced superconducting temperature (TC) in the Nb1−xB2 samples, we carried out a study on Nb1−xB2 (x=0, 0.13, 0.20, and 0.26) by means of nuclear magnetic resonance (NMR) and Seebeck coefficient (S) measurements. From the N93b NMR spin-lattice relaxation rates, we can deduce the Nb 4d partial Fermi level density of states (DOS) Nd(EF) for each individual composition. The result indicates that Nb0.74B2 and Nb0.80B2 possess large Nd(EF) while the lowest one appears in Nb0.87B2. The Seebeck coefficient also shows smaller absolute values in Nb0.74B2 and Nb0.80B2, associated with higher Fermi level DOS in both compounds. Interestingly, the temperature variation in S exhibits a broad minimum in Nb0.87B2, which is attributed to the presence of a pseudogap near the Fermi level. These observations were found to be in good agreement with the prediction from band structure calculations based on the appearance of the Nb vacancies in Nb1−xB2. In addition, the present study clearly reveals that the observed Tc enhancement by Nb deficiency has no direct relevance to their electronic Fermi level DOS.

Electronic structure and photocatalytic water splitting of lanthanum-doped Bi 2AlNbO 7

Bi 2− x La x AlNbO 7 (0 ≤ x ≤ 0.5) photocatalysts were synthesized by the solid-state reaction method and characterized by powder X-ray diffraction (XRD), infrared (IR) spectra and ultraviolet–visible (UV–vis) spectrophotometer. The band gaps of the photocatalysts were estimated from absorption edge of diffuse reflectance spectra, which were increased by the doping of lanthanum. It was found from the electronic band structure study that orbitals of La 5d, Bi 6p and Nb 4d formed a conduction band at a more positive level than Bi 6p and Nb 4d orbitals, which results in increasing the band gap. Photocatalytic activity for water splitting of Bi 1.8La 0.2AlNbO 7 was about 2 times higher than that of nondoped Bi 2AlNbO 7. The increased photocatalytic activity of La-doped Bi 2AlNbO 7 was discussed in relation to the band structure and the strong absorption of OH groups at the surface of the catalyst.

Crystallographic phase stabilities and electronic structures in AgNbO3 by first-principles calculation

Silver niobate (AgNbO3) assumes several phases that crystallise in the perovskite structure similar to sodium niobate (NaNbO3). In order to investigate the phase stability of AgNbO3, the formation enthalpies of both physically the realised (real) cubic , tetragonal (P4/mbm) and orthorhombic (Cmcm, Pbcm) phases as well as the virtual phases, the orthorhombic (Pc21 b) and rhombohedral (R3cR) phases, which are observed in NaNbO3, have been obtained using a plane-wave pseudopotential method. Although the orthorhombic (Pbcm) phase has the lowest formation enthalpy among the real phases, the virtual rhombohedral (R3cR) phase has the lowest symmetry as well as the lowest formation enthalpy among all phases. We thus speculate that the rhombohedral (R3cR) phase may be stable at extremely low temperatures provided that it is accessible kinetically. In addition, the electronic structures of the various phases of AgNbO3 are calculated within the generalised gradient approximation with the Perdew–Burke–Ernzerhof correction (GGA-PBE). The valence band top was found to consist of predominately localised Ag 4d and O 2p orbitals constituting the O 2p–Nb 4d bonding orbital, while the bottom of the conduction band was found to mainly consist of the anti-bonding orbital of Nb 4d–O 2p under the Ag 5s orbital.

Electron low energy-loss functions of Pb(Mg1/3Nb2/3)O3: Theory and experiment

Local low energy-loss functions (ELFs) of chemical ordered nanodomains in relaxor Pb(Mg1/3Nb2/3)O3 (PMN) were obtained by using Kramers–Krönig analysis of experimental electron low energy-loss spectra. Electronic structures and dielectric functions were calculated via ab initio calculation using the full-potential linearized augmented plane-wave method on a 30 atom PMN supercell with B-site cation ordering along the [111] direction. The theoretical ELFs are fundamentally in agreement with the experimental ELFs. Furthermore, we systematically elucidate all the peaks in the low ELF based on the ab initio calculated densities of states. It is found that the Nb 4d–O 2p hybridization is enhanced through ferroelectric transition, which makes the O 2p collective excitation energy upper shift in ELF, while Mg, also as the B-site ion, presents no covalent bonding effect with O.

Superconducting niobium nitride films deposited by unbalanced magnetron sputtering

Niobium nitride (NbN) thin films were deposited under different configurations of the magnetic field using a magnetron sputtering system. The magnetic field configuration varied from balanced to unbalanced leading to different growth conditions and film properties. The aim of the paper was to identify correlations between deposition conditions, film properties and the electrical properties, specially the superconductive critical temperature ( T C). The results suggested that there is a critical deposition condition, having an optimum ion–atom arrival ratio that promotes a well ordered and textured nanocrystalline structure (cubic phase) with the minimum residual stress and only under this condition a high critical temperature (16K) was obtained. Lower T C values around 12K were obtained for the NbN samples having a lower degree of structural perfection and texture, and a larger fraction of intergranular voids. On the other hand, analysis of valence-band spectra showed that the contribution of the Nb 4d states remained essentially constant while the higher T C was correlated to a higher contribution of the N 2p states.

Effects of Ta5+ Substitution on the Structure and Photocatalytic Behavior of the Ca2Nb2O7 Photocatalyst

Nanosized Ca2Nb2-xTaxO7 (0 ≤ x ≤ 2) photocatalysts were synthesized at 160 °C by hydrothermal methods. The absorption onset of Ca2Nb2-xTaxO7 (0 ≤ x ≤ 2) photocatalysts shifted to shorter wavelengths with an increase of tantalum molar ratio. On the basis of theoretical calculations, the conduction band level of Ca2Nb2-xTaxO7 became higher, and the curvature of the conduction band became larger with an increase of tantalum content due to the different electronegativities of Ta 5d and Nb 4d. The photocatalytic activity of Ca2Nb2O7 slightly decreased with a small molar ratio of tantalum substitution (x ≤ 0.8), while the activity can be effectively improved with a large molar ratio of tantalum substitution (x > 0.8). The increase of activity as x > 0.8 can be attributed to more effective photoelectron transfer in the conduction band with larger curvature, and the slight decrease of activity as x ≤ 0.8 is attributed to the decreased amount of absorbed photons and lattice distortion, which can act as recombination centers.

The electronic structure and the photoluminescence property of the photocatalyst BaZn1/3Nb2/3O3

The photocatalyst BaZn1/3Nb2/3O3 with ABO3 perovskite structure has been synthesized by using a solid-state reaction process. It was characterized by x-ray diffraction and photoluminescence spectroscopy. The luminescence band centers around 285 nm and shows a large Stokes shift compared with the excitation spectrum, indicating a strong electron–phonon interaction in the photocatalyst BaZn1/3Nb2/3O3. The electronic structure of BaZn1/3Nb2/3O3 was calculated by using the pseudopotential method of the density function theory. It shows that the conduction band should be mainly composed of the Nb 4d states, and the valence band should be mainly composed of the O 2p state. The densities of the O 2p states and the Zn 4s states at the bottom of the conduction band are very low. The Zn 4s states show an expanded structure, which was proposed to be helpful for the migration of the photoexcited carriers, thus favoring the photocatalytic activity of BaZn1/3Nb2/3O3.

Investigation of hole states near the Fermi level in Nb 1−xMg xB 2 by electron energy-loss spectroscopy and first-principles calculations

The fine structures of the electron energy-loss spectra (EELS) for the B-K edge have been examined in NbB 2 and superconducting Nb 0.75Mg 0.25B 2. The experimental results are analyzed based on the calculations of density functional theory (DFT) using the Wien2k code. The results of the EELS spectra and the angular decomposition of the density of states (DOS) reveal that both the B p z and B p x +p y states in NbB 2 have large weights at the Fermi energy due to intersheet covalent bonding with notable hybridization between the Nb 4d and B 2p states. This kind of hybridization also results in different core-hole behaviors for the B-K edge in two orthogonal crystallographic orientations. The best fit between experimental and theoretical data is achieved with consideration of the core-hole effect of the B 1 s states, in particular for the q⊥ c spectra. Analysis of the electronic structure of the Nb 1− x Mg x B 2 superconductors suggests that confinement of the intersheet covalent bonding is likely to be favorable for the improvement of superconductivity in this kind of materials.

K 4Ce 2M 10O 30 (M = Ta, Nb) as visible light-driven photocatalysts for hydrogen evolution from water decomposition

A series of single-phase metal oxide photocatalysts K 4Ce 2M 10O 30 (M = Ta, Nb), capable of evolving H 2 and O 2 from aqueous solutions containing a sacrificial electron donor (Na 2SO 3) and acceptor (AgNO 3), respectively, under visible light irradiation ( λ > 420 nm) without any co-catalyst were presented. The activities were greatly enhanced by the incorporation of Pt, RuO 2 and NiO (NiO x ) as co-catalysts on the prepared oxides. The photocatalysts have an appropriate band gap energy ca. 1.8–2.3 eV (corresponding to absorption edge of 540–690 nm) and excellent chemical potential level for utilization of solar energy, representing candidates of photocatalysts for hydrogen evolution from water decomposition. Density function theory (DFT) calculation indicated that while their valence bands are composed of hybridization with O 2p + Ta 5d (or Nb 4d) and occupied Ce 4f orbitals, the conduction bands of these photocatalysts K 4Ce 2M 10O 30 (M = Ta, Nb) are mainly attributable to the Ta 5d (or Nb 4d) orbitals. Although the unoccupied Ce 4f orbitals have overlap in the bottom of conduction band, they are less effective in transferring electrons and photocatalytic activities for their high localized nature. The contribution of these orbitals to the energy bands affects the electronic structure of the both photocatalysts and gives rise to their differences in light absorption and photocatalytic activities.

Preparation, Properties, and Reactivities of Unprecedented Oxo-Sulfido Nb(IV) Aqua Ions and Crystal Structure of (Me2NH2)6[Nb5(μ3-S)2(μ3-O)2(μ2-O)2(NCS)14]·3.5H2O

By treatment of Zn-reduced ethanolic solutions of NbCl5 with HCl in the presence of sulfide followed by cation-exchange chromatography, two oxo-sulfido niobium aqua ions, the red [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ and the yellow-brown [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+, were isolated. Both readily form their respective thiocyanate complexes, for which the structure for the former has been previously reported. Brown crystals of (Me2NH2)6[Nb5S2O4(NCS)14].3.5H2O (1) were isolated in the case of the latter, and the structure was determined by X-ray crystallography (space group: a = 15.4018(5) A, b = 21.1932(8) A, c = 22.0487(8) A, alpha=gamma = 90 degrees , beta = 103.4590(10) degrees , and R(1) = 0.0659). An unprecedented pentanuclear Nb5S2O48+ core is revealed in which short Nb-Nb distances (2.7995(8)-2.9111(8) A) are consistent with metal-metal bonding. A stopped-flow kinetic study of the 1:1 equilibration of NCS- with [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ has been carried out. Equilibration rate constants are independent of [H(+)] in the range investigated (0.5-2.0 M) and at 25 degrees C; kf= 9.5 M(-1) s(-1), kaq = 2.6 x 10(-2) s(-1), and K = 365 M1). Conditions with first NCS- and then [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ in excess revealed a statistical factor of 4, suggesting the presence of four kinetically equivalent Nb atoms. Attempts to study the 1:1 substitution of NCS- with [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+ showed signs of saturation kinetics. Quantum chemical calculations using the density functional theory (DFT) approach were performed on both the Nb4O5S4+ and Nb5O4S28+ naked clusters. The highest occupied and lowest unoccupied molecular orbitals have dominant Nb(4d) character. The HOMO for Nb4O5S4+ is a nondegenerate fully filled MO, whereas for Nb5O4S28+, it is a nondegenerate partially filled MO with one unpaired electron. EPR spectroscopy on [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+ shows that the molecule has total anisotropy (C2v), with all three tensors, gx= 2.399, gy= 1.975, and gz= 1.531, resolved. No hyperfine interaction expected from the nuclear moment of I = 9/2 for 93Nb was observed.

X-ray photoelectron spectroscopy studies of non-stoichiometric superconductingNbB2+x

Polycrystalline samples of NbB2+x with nominal composition (B/Nb) = 2.0, 2.1, 2.2, 2.3, 2.4 and 2.5 were studied by x-ray photoelectron spectroscopy (XPS).The spectra revealed Nb and B oxides on the surface of the samples, mainlyB2O3 and Nb2O5. After Ar ion etching the intensity of Nb and B oxides decreased. The Nb3d5/2 and B 1s core levels associated with the chemical states (B/Nb) were identifiedand they do not change with etching time. The binding energies of the Nb3d5/2 and B 1s core levels increase as boron content increases, suggesting a positive chemical shiftin the core levels. On the other hand, analysis of valence band spectra showed that thecontribution of the Nb 4d states slightly decreased while the contribution of the B2pπ states increased as the boron content increased. As a consequence, the electronic andsuperconducting properties were substantially modified, in good agreement withband-structure calculations.

Electronic, energy, and thermal properties of the Möbius strip and related ring nanostructures of NbS3

The electronic structure and energy characteristics of the Mobius strip and related ring nanostructures of niobium trisulfide NbS3 are investigated using the density functional tight-binding (DFTB) method. The results obtained are analyzed and compared with those obtained for the crystal and planar ribbon structures of NbS3. The formation of NbS3 ring structures leads to a considerable increase in the density of Nb 4d states at the Fermi level. The simulation of the behavior of the ring structures in the temperature range T = 0–700 K demonstrates that the Mobius strip has a lower thermal stability as compared to the NbS3 ring.

Additional transition induced by magnetoelectric interaction in ferroelectromagnet Pb(Fe 1/2Nb 1/2)O 3

Abstract Besides the two phase transitions known in ferroelectromagnets, an additional transition was observed experimentally in Pb(Fe 1/2Nb 1/2)O 3 and the possible mechanism was explored. The magnetic moment shows that anomalous increases when the temperature goes down away from the Néel temperature of 20–30 K. The existence of the hysteresis verifies the weak ferromagnetic property. This additional phase transition is considered to originate from the magnetoelectric coupling between the ferroelectric and antiferromagnetic orders in this compound. From the Mössbauer parameter analysis, it is confirmed that the hybridization of the O 2p, Fe 3d and Nb 4d electron states makes the realization of magnetoelectric coupling possible.

Electronic structure of AgNbO 3 and NaNbO 3 studied by X-ray photoelectron spectroscopy

The XPS examinations of the AgNbO 3 and NaNbO 3 single crystals and ceramics allowed estimate their average composition as Ag 1.1Nb 0.9O 3 and Na 1.2Nb 0.9O 2.9. The valence bands of the AgNbO 3 compound, formed mainly of the Nb 4d, Ag 4d and O 2p states, show an energy gap about 3 eV while for the NaNbO 3 compound consist of the O 2p states hybridized with the Nb 3d states and show an energy gap about 4 eV. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. The broadening of the core level lines of AgNbO 3 suggests a stronger structural disorder in comparison with NaNbO 3 compound.