Nb Vacancies Research Articles

Vacancies engineering in ultrathin porous g-C3N4 tubes for enhanced photocatalytic PMS activation for imidacloprid degradation

Precise morphology and vacancy engineering is a promising strategy to enhance the degradation performance of refractory contaminants in photocatalysis coupling peroxymonosulfate activation (PC-PMS) systems. In this context, we developed ultrathin porous g-C3N4 tubes with three-coordinated nitrogen (Nb) vacancies between the heptazine units through hydrogen intercalated exfoliation and selective nitrogen removal at high temperatures. The ultrathin porous tubular architecture provides an ultrahigh-specific surface area with abundant exposed active sites, ensures effective mass transfer, and shortens the diffusion distance of photogenerated carriers. The Nb vacancies act as unsaturated sites, inhibiting the photogenerated carrier’s recombination and facilitating the adsorption/activation of O2 and its conversion to ∙O2– via photogenerated electrons. Then, the PMS is subsequently oxidated by photogenerated holes selectively producing 1O2. Together, these continuously released free radicals unlock the complete degradation of imidacloprid within 20 min in the PC-PMS system, with a degradation rate about 350 % higher than conventional g-C3N4 tubes and 10-fold that of pristine g-C3N4. This study provides a novel insight into the efficient utilization of photogenerated electron-hole pairs conversion to ∙O2– and 1O2 for the removal of pesticide contaminants through the rational design of photocatalysts in the PC-PMS system.

Effect of doping and defects on the electronic properties of MoS2/WSe2 bilayer heterostructure: a first-principles study.

This work studies the effect of Nb, Mo, Re dopant, and Se vacancy in WSe2 on the electronic and optical properties of the MoS2/WSe2 bilayer heterostructure based on first-principles calculations. Our research shows that the MoS2/WSe2 bilayer heterostructure exhibits a type-II band alignment with a valence band offset (VBO) of 1.07 eV and a conduction band offset (CBO) of 1.00 eV. It also shows that different dopants or defects can considerably modulate the energy band alignment and interlayer charge transfer of the heterostructure. Owing to the orbital hybridization of the dopant atoms with other atoms and the consequent enhancement of the coupling between the two structural layers, a transition of the band alignment from type-II to type-I is realized with the Re dopant. The effect of doping and defects on the electronic properties of heterojunctions contributes to applications in high-performance optoelectronic devices.

Improved energy storage performance of NaNbO3‐based antiferroelectrics by tuning polarizability and defect engineering

AbstractNaNbO3‐based antiferroelectric ceramics are promising candidates for high‐performance energy storage capacitors due to their environmental friendliness and low cost despite their current energy storage properties being inferior to those of their lead‐based and AgNbO3‐based counterparts. Typically, the antiferroelectric phase in NaNbO3 ceramics is not stable and the characteristic double hysteresis loops are seldom observed. In this work, the antiferroelectricity was improved by reducing the polarizability, resulting in well‐developed double hysteresis loops in 0.94NaNbO3–0.06(K0.5Bi0.5)SnO3 ceramics. To further improve the energy storage properties, defect engineering was performed for 0.94NaNbO3–0.06(K0.5Bi0.5)SnO3 by substituting Na+ with La3+ and introducing Nb vacancies. As a result, the stability of the field‐induced ferroelectric phase was reduced, and the energy storage properties were enhanced. The strategy of combining polarizability design and defect engineering can be a promising approach for optimizing NaNbO3‐based antiferroelectric ceramics.

Cation-deficient T-Nb2O5/graphene Hybrids synthesized via chemical oxidative etching of MXene for advanced lithium-ion capacitors

Orthorhombic niobium pentoxide (T-Nb2O5) is widely acknowledged as a fast pseudocapacitive material. Nevertheless, its application is hindered by the narrow voltage window (1–3 V vs. Li/Li+) that arises from irreversible phase transformation and sluggish kinetics during deep lithiation. Herein, we demonstrate a unique method for introducing Nb vacancies in T-Nb2O5 nanoparticles via amine-assisted oxidative etching of Nb2CTx MXene, providing extra storage sites and improving structural flexibility by introducing cationic defects. Subsequently reduced graphene oxide (rGO) is employed as substrate to disperse T-Nb2O5 nanoparticles and construct T-Nb2O5/rGO nanohybrids. Multiple characterizations and computational simulations demonstrate that the resulting T-Nb2O5/rGO hybrid anode exhibits rapid and stable multi-electron transfer lithium storage. Owing to the enrichment of Nb vacancies and nanoparticle morphology, even when voltage window of 0.01–3 V (vs. Li/Li+) is extended, T-Nb2O5 exhibits a pseudocapacitive mechanism and integrity of partial crystal structure; effectively tackling the structural collapse and sluggish kinetics of T-Nb2O5. Consequently, the T-Nb2O5/rGO anode shows a superior rate capacity (148 mAh/g at 10 A/g) and cycling stability (3000 cycles at 5 A/g). Remarkably, the assembled lithium-ion capacitors achieve a high energy density of 123.7 Wh/kg, a power density of 22.5 kW/kg, and a capacity retention of 83.6% after 20,000 cycles.

First-principles calculations for the effect of energetic point defect formation on electronic properties of the Weyl MX family (M = Nb, Ta; X = P, As)

• The unfolding technique was used for the first time to highlight the impact of point defects on the electronic band structure. • The transition metal Nb and Ta vacancies are energetically nearer to stability in the cases of NbAs and TaAs. • The pnictide P vacancy is energetically favorable among four possible point defects in both NbP and TaP. • For TaAs, the Ta vacancy concentration is significantly higher than all other types of defect concentrations. • All four of these compounds are thermodynamically, dynamically, and mechanically stable. By the first-principles of the density functional theory (DFT) approach, the structural parameters, elastic, vibrational, electronic properties, and enthalpies of formation of four stoichiometric Weyl semimetal compounds were evaluated. Besides, we calculated point defect formation energies and electronic band structures of defect-containing supercells by using an unfolding technique to highlight the effects of point defects (vacancies and anti-sites) on Weyl points in the electronic structure of these compounds. The transition metal Nb and Ta vacancies are energetically nearer to stability in the cases of NbAs and TaAs, but the pnictide P vacancy is energetically favorable among four possible point defects in both NbP and TaP. The band structure results, obtained from the unfold method, agree with defect formation energies at the ground state. Moreover, a true and accurate description of defects is based on the well-ordered compounds (NbAs, TaAs, NbP, and TaP). A relationship between temperature (K based on defect formation energies) and defect concentrations for these Weyl compounds was derived. In particular, for TaAs, the Ta vacancy concentration is significantly higher than all other types of defect concentrations.

Improved Thermoelectric Performance of NbCoSb with Intrinsic Nb Vacancies and Ni-Doping-Induced Band Degeneracy

Lately, the nominal 19-electron half-Heusler compound NbCoSb, historically viewed as a metal, has attracted reacquaintance and widespread attention because of its unexpected high thermoelectric (TE) performance. Here, the electronic structures of NbxCo1–yNiySb (x = 0.8, 1; y = 0, 0.1) have been systematically investigated by using the first-principles method and semiclassical Boltzmann transport theory. We demonstrate that Ni doping at Co sites in NbCoSb with 20% intrinsic Nb vacancies (Nb0.8CoSb) leads to a large band degeneracy, and the conduction band edge are mainly provided by the d-orbitals of the Ni, Co, and Nb atoms. The relative localization of the d orbitals not only remarkably increased the density-of-states effective mass near the Fermi energy but also retained a high mobility, which resulted in an optimal electrical conductivity without significant reduction of the Seebeck coefficient (∼−411.08 μV K –1 at 800 K) and thereby a large power factor. Meanwhile, Ni doping still preserved a low lattice thermal conductivity. As a result, a peak zT of 1.18 was achieved at 1100 K in the compound Nb0.8Co0.9Ni0.1Sb with a carrier concentration of 4.35 × 1020 cm–3. The present work identifies that Ni doping is an effective method to improve the TE properties of nominally 19-electron NbCoSb compounds with Nb vacancies and demonstrates the great potential for searching of nonstoichiometric 19-electron half-Heusler compounds with vacancies.

The impact of the simultaneous presence of Li and Nb(Ta) vacancies in the defect structure of pure LiBO3 (B=Nb, Ta) on the curie temperature

The lithium niobate (LiNbO3, "LN") and lithium tantalate (LiTaO3, "LT") crystals are piezoelectric materials that exhibit intrinsic or extrinsic defect structures, which immediately impact their optical and electrical properties. In this study, we examine the influence of point defects on the Curie temperature (Tc) for different samples of pure LN and LT, using three vacancy models: the tantalum (niobium) vacancy model, the lithium vacancy model and the mixed lithium and tantalum (niobium) vacancy model. Based on the Safaryan approach, it was found that the calculated values for the tantalum (niobium) vacancy model did not agree with the experimental data. However, the lithium vacancy model and the mixed vacancy model showed good agreement between experimental and theoretical results. In conclusion, the proposed mixed vacancy model provides the best description of the defect structure in LN and LT materials, and the Tc, is influenced by the structural defects present in the LN and LT compounds.

Molecular dynamics simulation of seeded crystal growth in glass

The mechanism of non-congruent growth of a crystal from glass has been sought using molecular dynamics simulations. Specifically, as a model of this process, the growth of a lithium niobate (LiNbO 3 ) crystal seed sandwiched between two lithium niobosilicate (LNS) glass slabs has been simulated as a function of time and temperature. The growth of pre-existing crystal is strongly affected by the orientation of crystal seed, temperature, and the SiO 2 concentration in the surrounding LNS glass matrix. The orientation of LiNbO 3 seed surface that has inherently larger interplanar distance results in a relatively slower crystal growth. The addition of SiO 2 to LNS system significantly decreases the crystal growth, which primarily occurs in the region devoid of Si. The suppressive effect of SiO 2 on growth rate can be traced to the existence of defect complex comprising of Si substituted at the Nb site and a nearby Nb vacancy. • Crystal growth is observed in interfacial structures containing lithium niobosilicate glasses and lithium niobate crystals. • Crystal orientation with large interplanar space results in high crystal growth rate. • High SiO 2 content suppresses the formation of ordered structures. • [Si Nb -V Nb ] defect complex exists in LiNbO 3 crystal at low SiO 2 concentration.

Giant permittivity in Nb-doped SrTiO3 single crystal: Compositional gradient and local structure

Nb-doped SrTiO3 single crystal exhibited a giant permittivity (>6.5 × 105) with an acceptably low dielectric loss (<10−1) in a wide temperature range from −120 to 200 °C, making this material a good candidate for energy storage devices and modern microelectronics components. The mechanisms responsible for the giant permittivity of Nb-doped SrTiO3 single crystals were studied by means of microstructure characterizations, dielectric measurements, and density-functional theory calculations. A chemical compositional gradient extending from the surfaces was found, forming the internal barrier layer capacitance (IBLC) effect. Polar nanoregions (PNRs) were observed because of local fluctuations in distributions of Nb and oxygen vacancies. While both compositional gradients and local chemistry fluctuations increased polarization of the Nb-doped SrTiO3 single crystals, the local fluctuations dominated enhanced polarizability. This work suggests that optimizing local structures and chemistries in dielectrics is an effective way to tailor the desired dielectric performance.

Study of Niobium Mononitride Thin Films Grown Using High Power Impulse Magnetron Sputtering

Herein, the effect of microstructure on the electronic, and superconducting properties of niobium mononitride (NbN) thin films grown using a high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (dcMS) is studied. X‐ray reflectivity, cross‐sectional scanning electron microscopy and atomic force microscopy measurements suggest that the film grown with dcMS has a non‐uniform distribution of islands with loosely packed columns while the HiPIMS grown film has a smoother surface and a denser microstructure. Although the X‐ray diffraction measurements show a single‐phase rock‐salt‐type crystal structure in both cases, the local and electronic structure analyzed using N K‐edge X‐ray absorption near edge structure measurements reveals the evidence of a large amount of Nb vacancies in dcMS‐NbN while HiPIMS‐NbN film is closer to stoichiometry. The ordered structure of HiPIMS‐NbN sample results in a relatively higher superconducting transition temperature of 15.2 K and lower normal state resistivity of 90 μΩ cm with a moderate critical field of 18 T and smaller coherence length of 4.2 nm. These results suggest HiPIMS can be utilized to grow high‐quality superconducting thin films of few nanometers required in modern technological devices such as single‐photon detectors, superconducting quantum interference devices.

Intrinsic Complex Vacancy-Induced d0 Magnetism in Ca2Nb2O7 PLD Film

Introducing magnetism into the ferroelectric Ca2Nb2O7 with high Curie temperature can make it a potential multiferroic material at room temperature. Stoichiometric Ca2Nb2O7, nonstoichiometric Ca1.9Nb2O7-δ and Ca2Nb1.9O7-δ single phase films were deposited on STO (110) substrate by pulsed laser deposition under appropriate conditions. The films were characterized by XRD, FE-SEM, Element mapping and XPS. Both stoichiometric Ca2Nb2O7 and Ca1.9Nb2O7-δ films were diamagnetic in the magnetic measurement and ab initio calculations, while the Ca2Nb1.9O7-δ film with the complex vacancy of VNb+O exhibited ferromagnetic behavior at room temperature, with the saturated magnetization of 3.6 emu/cm3. Calculations on the Ca2Nb2O7 (010) surface indicate that the VNb+O can induce spin polarization on the residual O atoms around the Nb vacancies, and the system was most stable when the Nb and O vacancies were the 4th nearest-neighbored, with FM coupling energetically more stable than the AFM coupling. Our work verified experimentally and theoretically the feasibility of introducing ferromagnetism into Ca2Nb2O7 film by the intrinsic complex vacancy of VNb+O.

Enhanced thermoelectric performance of nominal 19-electron half-Heusler compound NbCoSb with intrinsic Nb and Sb vacancies

Since 2015, when we first reported that the half-Heusler alloy NbCoSb with a nominal 19-valence-electrons composition exhibits the moderate thermoelectric properties of a heavily doped n-type semiconductor, we have been studying the preparation of pure-phase materials. Here we report, for the first time, the effects of Nb and Sb deficiencies on the phase composition and thermoelectric properties of NbCoSb. The experimental results show that phase-pure material can be achieved if the composition is slightly Nb and Sb deficient and that intrinsic vacancy defects have a remarkable effect on the thermoelectric performance. The ZT value is doubled and a maximum ZT of 0.8 is achieved in Nb0·97CoSb0.99 at 973 K, benefiting from the elimination of impurity phases and the optimization of carrier concentration. This work proves that introducing vacancy defects is an effective strategy to improve the thermoelectric properties of some half-Heusler alloys, especially of nominal 19-electron half-Heusler compounds.

Influences of Vacancy Concentration and Al Substitution on Structural, Electronic, and Elastic Properties of Nb5Si3 from First‐Principles Calculations

The effects of vacancy concentration and Al substitution on the structural, electronic, and elastic properties of Nb5Si3 are studied using first‐principles calculations. The formation energy, elastic modulus, and electronic properties of three different Nb vacancies in Nb5Si3 are discussed in detail. With the increase in vacancy concentration, the obtained shear modulus, Young's modulus, and hardness of Nb vacancy decrease, which are lower than those of pristine Nb5Si3. The bulk modulus/shear modulus (B/G) ratio increases with the increase in Nb vacancy concentration. However, these vacancies result in the transition from brittleness to ductility, and Nb5Si3 with 5% vacancy concentration exhibits ductile behavior. The calculation of electronic structure shows that these Nb vacancies change the local hybridization between Nb and Nb atoms. As the concentration increases, the position of the peak moves toward lower energy. It is predicted that vacancies can improve the ductility behavior of Nb5Si3.

Stoichiometry-related defect structure in lithium niobate and lithium tantalate

Congruently grown LiNbO3 (LiTaO3) is known to be highly defective due to its significant Li2O deficiency. We present in this work a comparative study between normal LiNbO3 (LiTaO3) and ilmenite structural LiNbO3 (LiTaO3). Namely, the normal cation stacking sequence is replaced by ilmenite ordering ‘…Nb (Ta) Li vacancy Li Nb (Ta) vacancy Nb (Ta) Li vacancy Li Nb (Ta) vacancy…’. From Safaryan’s approach which combines a ferroelectric phase transition theory and vacancy models, we calculated the Curie temperature in ilmenite LiNbO3 (LiTaO3). We have shown that ilmenite structural LiNbO3 (LiTaO3) is in excellent agreement with the result of the experiment compared to normal LiNbO3 (LiTaO3).

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

Effects of Native Vacancies on Nb-Doped MgH2 Using Density Functional Theory Calculations

Fil: Gaztanaga, Francisco. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca. Instituto de Fisica del Sur. Universidad Nacional del Sur. Departamento de Fisica. Instituto de Fisica del Sur; Argentina

Characterization of Nb Interface Segregation During Welding Thermal Cycle in Microalloyed Steel by Atom Probe Tomography

Coarse-grained, welding heat-affected zone microstructure was simulated in a Nb-bearing microalloyed steel. The granular bainite with a great number of martensite-austenite (M-A) constituents was the predominant phase. Using atom probe tomography (APT), the distributions of niobium at prior austenite grain boundary (PAGB), ferrite/martensite-austenite (M-A) constituent interface (FMAI), and ferrite/ferrite interface (FFI) were investigated. The binding energy of Nb atom and vacancy was predicted to be 0.45 eV, indicating that Nb segregation by welding thermal cycle is probably a result of the nonequilibrium mechanism. The maximum enrichment of Nb was found at FMAI with enrichment factor of 3.50. Intermediate enrichment of Nb was at PAGB with enrichment factor of 3.12. The interfacial excess of Nb solute element ГNb at PAGB determined by APT was 0.27 × 1019 atoms/m2. The segregation energy was calculated to be 22.91 kJ/mol. The minimum enrichment of Nb was at FFI with an enrichment factor of 1.80.

Enhanced Thermoelectric Performance in 18‐Electron Nb0.8CoSb Half‐Heusler Compound with Intrinsic Nb Vacancies

AbstractTypical 18‐electron half‐Heusler compounds, ZrNiSn and NbFeSb, are identified as promising high‐temperature thermoelectric materials. NbCoSb with nominal 19 valence electrons, which is supposed to be metallic, is recently reported to also exhibit thermoelectric properties of a heavily doped n‐type semiconductor. Here for the first time, it is experimentally demonstrated that the nominal 19‐electron NbCoSb is actually the composite of 18‐electron Nb0.8+δCoSb (0 ≤ δ &lt; 0.05) and impurity phases. Single‐phase Nb0.8+δCoSb with intrinsic Nb vacancies, following the 18‐electron rule, possesses improved thermoelectric performance, and the slight change in the content of Nb vacancies has a profound effect on the thermoelectric properties. The carrier concentration can be controlled by varying the Nb deficiency, and the optimization of the thermoelectric properties can be realized within the narrow pure phase region. Benefiting from the elimination of impurity phases and the optimization of carrier concentration, thermoelectric performance is remarkably enhanced by ≈100% and a maximum zT of 0.9 is achieved in Nb0.83CoSb at 1123 K. This work expands the family of half‐Heusler thermoelectric materials and opens a new avenue for searching for nominal 19‐electron half‐Heusler compounds with intrinsic vacancies as promising thermoelectric materials.

Impact of Nb vacancies and p-type doping of the NbCoSn-NbCoSb half-Heusler thermoelectrics.

The half-Heuslers NbCoSn and NbCoSb have promising thermoelectric properties. Here, an investigation of the NbCo1+ySn1-zSbz (y = 0, 0.05; 0 ≤ z ≤ 1) solid-solution is presented. In addition, the p-type doping of NbCoSn using Ti and Zr substitution is investigated. Rietveld analysis reveals the gradual creation of Nb vacancies to compensate for the n-type doping caused by the substitution of Sb in NbCoSn. This leads to a similar valence electron count (∼18.25) for the NbCo1+ySn1-zSbz samples (z > 0). Mass fluctuation disorder due to the Nb vacancies strongly decreases the lattice thermal conductivity from 10 W m-1 K-1 (z = 0) to 4.5 W m-1 K-1 (z = 0.5, 1). This is accompanied by a transition to degenerate semiconducting behaviour leading to large power factors, S2/ρ = 2.5-3 mW m-1 K-2 and figures of merit, ZT = 0.25-0.33 at 773 K. Ti and Zr can be used to achieve positive Seebeck values, e.g. S = +150 μV K-1 for 20% Zr at 773 K. However, the electrical resistivity, ρ323K = 27-35 mΩ cm, remains too large for these materials to be considered useful p-type materials.

Research of physicochemical properties and structure of strongly doped LiNbO3:ZnO ([ZnO] ~ 4.02–8.91 mol %) crystals

In LiNbO3:ZnО crystals grown from melts containing ~4.0–9.0 mol % ZnO, the evolution of structure was studied, the lattice periods were detected, and the models of atomic structure were analyzed. Raman spectroscopy and full-profile analysis of XRD patterns were used to obtain the results. The rise in ZnO concentration in the melt from 4.0 to ~6.99 mol % leads to the decrease in the unit cell volume of the crystals. The further rise in ZnO concentration in the melt to ~7.8 mol % results in the increase in the unit cell volume of the crystals. Refinement of the profile characteristics of XRD patterns and structural characteristics of the researched crystal samples was performed by the Rietveld method using the PDWin software package. All samples were considered single-phase, taking into consideration the absence of intense bands coinciding with the lithium niobate bands. Zn2+ cations were detected to occupy Li sites (which are vacant in the congruent LiNbO3 crystal). In this case, Nb vacancies are absent. Zinc displaces all excess Nb atoms in Li sites. In the sample grown from melt with ~6.12 mol % ZnO. In this case, Li vacancies and electroneutrality stay the same in the sample. Thus, NbLi antistructure defects are absent in the LiNbO3:ZnО crystal (~6.12 mol % ZnО in the melt). In LiNbO3:ZnО crystals with higher ZnO concentration, NbLi antistructure defects appear again. In this case, the concentration of Zn in Li sites almost coincides with the one in the LiNbO3:ZnО crystal. Unit cell parameters are equal in crystals grown from melts with ~7.8 and ~6.76 mol %. The biggest changes in the Raman spectra of these crystals are observed in the regions of vibrations of cations (200–300 cm–1) located in oxygen octahedra ВО6 (В: Nb5+, Li+, Zn2+) and in the region of vibrations of oxygen atoms in oxygen octahedra (500–900 cm–1). This indicates a change in order of alteration of intrinsic, doping cations and vacancies along the polar axis and deformation of oxygen octahedra during doping. This corresponds to anisotropic expansion of oxygen octahedra along the polar axis in LiNbO3:ZnО crystals.