Vanadium Vacancies Research Articles

In Situ Photoelectrochemical‐Induced Surface Reconstruction of BiVO4 Photoanodes for Solar Fuel Production

BiVO4 has been widely concerned due to its great potential in photoelectrochemical (PEC) water splitting. However, low carrier mobilities and high recombination efficiency of photogenerated carriers impede its photocatalytic performance. Herein, an in situ PEC cyclic‐voltammetry‐induced surface reconstruction of BiVO4 photoanodes (BVO pristine) is developed with significantly enhanced efficiency for solar water splitting. A series of in situ characterizations (including in situ X‐ray diffraction, in situ Raman), together with electrochemical tests and density‐functional theory calculations, reveal that during the photoelectrical activation process, the BVO pristine surfaces undergo a crystal plane reconstruction with greatly increased {040} crystal face to promote the separation of photogenerated carriers. In addition, abundant vanadium vacancies and oxygen vacancies are also introduced into the BiVO4 surface during the crystal face reconstruction process with more favorable surface water adsorption and increased injection efficiency of photogenerated carriers. Therefore, the charge‐transfer resistance (Rct) between BVO pristine and electrolyte under AM 1.5G illumination substantially reduced from the original 15 200 to 2820 Ω after the activation. Moreover, the photocurrent density of activated BVO pristines increases more than 12 times, relative to the original BiVO4. In this work, a new horizon for in situ photoelectric activation of semiconductor photoelectrodes with significantly enhanced PEC water splitting is provided.

Pre-removing partial ammonium ion induces vanadium vacancy assist NH4V4O10 as a high-performance aqueous zinc ion battery cathode

Ammonium vanadate (NH4V4O10) is regarded as a promising cathode material for aqueous zinc-ion batteries, given its considerable theoretical capacity and tunable interlayer spacing. However, due to its inherent low electrical conductivity and the excessive presence of ammonium ions between layers, NH4V4O10 exhibits unsatisfactory electrochemical performance. In this study, it is proposed that the electrochemical performance of NH4V4O10 can be significantly enhanced by removing part of the ammonium cation and increasing the vanadium vacancy. The decrease of ammonium further increases the layer spacing, reduces the irreversible deamination and accelerates the diffusion of Zn2+. Density functional theory (DFT) calculations reveal that increasing vanadium vacancies significantly diminishes the strong electrostatic interactions between the V-O layers and Zn2+, enhances the material’s electrical conductivity, and stabilizes the crystal structure during zinc ion (de)intercalation, thus obtaining excellent electrochemical properties. As a result, the Vd-NVO cathode demonstrates a high reversible capacity of 371 mAh g−1, excellent rate performance, and remarkable cyclic stability, maintaining a reversible capacity of 131 mAh g−1 even after 1000 cycles at 5 A g−1.

Influence of vacancies on the stability and mechanical properties of V2AlC: Guidelines for etching MAX phases via ab initio calculations

This study focuses on the ideal etching of defective MAX phases in a mechanical exfoliation process to produce vanadium-based MXenes. Two-dimensional MXenes exhibit high structural stability and excellent electrical conductivity, and can be applied as electrode materials. Mechanical exfoliation can be facilitated by modifying the vacancy in the V2AlC MAX phase, which makes etching challenging. We report the effects of M (vanadium), A (aluminum), and X (carbon) vacancies on the mechanical behavior of V2AlC via ab initio calculations. To this end, the anisotropic mechanical properties and strain–stress curves were calculated for MAX phases with different vacancies (x = 0.08, which corresponds to 8.33 at.%). The presence of vanadium vacancies significantly deteriorated the structural and mechanical properties compared to those of aluminum and carbon vacancies. Thus, it was confirmed that vanadium vacancies had a significant effect on the mechanical exfoliation of the MAX phases. These findings provide further insights into the design of MXenes.

Atomically Dispersed Cerium Sites Immobilized on Vanadium Vacancies of Monolayer Nickel‐Vanadium Layered Double Hydroxide: Accelerating Water Splitting Kinetics

AbstractRational design of efficient single‐atom catalysts is a potential avenue to mitigate the sluggish oxygen evolution reaction (OER) kinetics. Adopting appropriate matrixes to stabilize the single‐atom active centers with the optimized geometric and electronic structure plays an essential role in enhancing catalytic activities. Herein, massive isolated Ce atoms are successfully anchored on monolayer nickel‐vanadium layered double hydroxide support (Ce SAs/m‐NiV LDH) via the vanadium defects trapping strategy, resulting in stabilized Ce single‐atom with the maximum loading of 8.07 wt.%. Benefitting from the strong synergetic electronic interaction between Ce single atoms and monolayer NiV LDH matrix, thus‐prepared catalyst possesses favorable OER (209 mV @ 10 mA cm−2) and water electrolysis performance (1.47 V @ 10 mA cm−2), surpassing other catalysts and even the commercial RuO2 catalyst. Density functional theory (DFT) calculations in combination with in situ electrochemical impedance spectroscopy analysis reveal that the immobilization of monatomic Ce can effectively narrow the band gap and strengthen the density states near the Fermi level as well as more easily adsorb the surficial OH–, leading to a lower charge transfer barrier and faster water splitting kinetics.

Annealing Enhanced Phase Transition in VO2 Thin Films Deposited on Glass Substrates via Chemical Vapor Deposition

In this work, VO2 is deposited on borosilicate glass substrates via a single step Chemical Vapor Deposition (CVD). Though the structural characterization revealed the presence of phase pure polycrystalline VO2 in simple monoclinic crystal structure (P21/c), electrical measurements did not show a sharp transition. To understand the ambiguity, surface characterization was performed with X-ray Photoelectron Spectroscopy (XPS) on the as-deposited VO2 thin films. Vanadium was present in V3+, V4+ and V5+ states. Following these measurements, annealing was performed in ambience at 200 °C for 10 minutes. Subsequent electrical measurements showed an improved phase transition, enhanced by an order of magnitude. We attribute the enhancement in transition to annealing of oxygen vacancies present in VO2 thin films because of the deposition conditions. Decrease in oxygen vacancies was correlated with a decrease in the fraction of V3+ from XPS measurements. The sample annealed at 200°C for 10 minutes exhibited a higher resistance ratio (ΔA = 305) with an optimum hysteresis (4°C) and transition width(12°C), whereas the as-deposited sample exhibited a resistance ratio of ΔA = 32. The thermal treatment affected the characteristics of the transition. A narrow hysteresis of 4°C and a sharp transition width with a reasonable resistance ratio were obtained for the sample annealed at 200°C for 10 minutes. From this study, we understand that our synthesis of VO2 thin films via CVD is prone to be off-stoichiometric and therefore, contains defects such as oxygen vacancies. We could mitigate the effect of these vacancies and engineer the stoichiometry by performing a simple annealing which in turn improved the strength of the phase transition. Drawbacks such as a poor-quality transition, resulting from synthesis parameters can be mitigated if we understand the effect of vanadium and oxygen vacancies.

Controlling effect of TiO2 carrier on formed vanadium species for effective catalytic oxidization of chlorobenzene

Catalytic oxidization of the dioxins is critical to the atmosphere since medical-waste incineration is globally increased due to COVID-19. Vanadium supported TiO2 is the most-widely investigated catalyst. However, previous reports usually investigated V-Ti relationships after vanadate was loaded on TiO2 and an activation was completed. In comparison, this work emphasizes on the influence of TiO2 surface chemistries on properties of formed vanadium species during a thermal activation. The vanadate loaded TiO2 are detailedly analyzed by both experimental characterizations and theoretical calculations. As a result, TiO2 with more Ti4+ is inclined to be reductive, and promotes the formation of more low-valence vanadium and surface vacancies. On the contrary, TiO2 with more Ti3+ is inclined to be oxidative, and promotes the formation of more lattice oxygens and high-valence vanadium. When they were compared in catalytic oxidization of chlorobenzene, the Ti4+-induced catalyst attains a conversion of 98.0 % at 250 °C, far bigger than that (11.3 %) of the Ti3+-induced catalyst. The main result of this work helps readers to understand the process of the catalyst preparation, which is in favor of producing a more effective catalyst by regulating the catalyst carrier.

Cationic vanadium vacancy-enriched V2−xO5 on V2C MXene as superior bifunctional electrocatalysts for Li-O2 batteries

The development of cationic vacancies has been extensively examined as an effective strategy to improve the activity of electrocatalysts. However, it is a challenge to effectively introduce cationic vacancies on the material surface. Their specific effects on the electrochemical performance of lithium-oxygen (Li-O2) batteries are rarely reported. In this work, vanadium pentoxide with abundant vanadium vacancies (V2−xO5) is in situ prepared on the V2C MXene (V2−xO5@V2C MXene) surface, and their bifunctional catalytic activity toward the oxygen electrode reaction in Li-O2 batteries is systematically examined. The results show that the V2−xO5@V2C MXene-based Li-O2 battery exhibits excellent performance. It delivers a high energy efficiency of 83.4% at 100 mA g−1 and excellent cycling performance of more than 500 cycles. Furthermore, density functional theory calculations confirm that the presence of cationic vanadium vacancies can provide abundant active sites to reduce the reaction barrier and optimize the adsorption of reactants, increasing the oxygen electrode reactions in the Li-O2 battery. This work provides a meaningful view that modulating the electronic structure by creating cationic metal vacancies can improve the electrocatalytic activity of transition metal oxides.

The behaviors of He atoms and vacancies at TiO/V interface: A first-principles study

We study the behaviors of He atoms and vanadium vacancies at TiO/V interface by first-principles calculation to interpret the effects of TiO-precipitate on the He bubble nucleation. At TiO/V interface, He atom prefers to dissolve in the interstitial sites between TiO and adjacent V layer, and monovacancy tends to form in the adjacent V layer of TiO-precipitate. The TiO-precipitate promotes the formation of monovacancy and enhances the attractive interaction between He atom and monovacancy. Whether in monovacancy or multi-vacancy cluster, He atoms can be captured persistently in thermodynamics. In conclusion, TiO/V interface is a potential nucleation site in vanadium alloys for He bubbles. In addition, we also study the behaviors of He atom and monovacancy in the vanadium crystal as a contrast. Our simulations provide a theoretical insight for further studying the formation of the He bubbles in vanadium alloys.

Revealing the Origin of Highly Efficient Polysulfide Anchoring and Transformation on Anion‐Substituted Vanadium Nitride Host

AbstractMetal nitrides and quasi‐metallic compounds have been extensively employed as sulfur hosts for confining polysulfide shuttling and improving the electronic conductivity. Their electronic structures and surface chemical bonds significantly determine the adsorption and catalytic abilities for polysulfide. However, the surface compositions of the reported metal nitrides and their sulfur anchoring mechanisms are still controversial. Herein, the authors demonstrate the anion‐substituted mechanism from vanadium oxide, oxynitride to nitride during ammonia‐annealing process and systematically unravel the long‐range disorder rock‐salt structure of vanadium oxynitride with abundant vanadium (V) and nitrogen (N) vacancies by synchrotron X‐ray absorption spectra, atomic pair distribution function, and density functional theory calculation. The defect‐rich vanadium oxynitride that is previously considered as vanadium nitride possesses the enhanced electron delocalization of V, N, and oxygen (O) atoms. It strengthens the polar LiN/O and VS bonds, especially near V vacancy, resulting in a strong polar adsorption for polysulfide. Meanwhile, the vanadium oxynitride effectively catalyzes the breaking and conversion of polysulfide, improving the reduction kinetics during discharge process. The bifunctional effects render the excellent cycling and rate performances. This work deeply understands the sulfur redox mechanisms on vanadium oxynitride and nitride and promotes the developments of the quasi‐metallic compounds/sulfur cathodes in Lithium‐sulfur battery.

Phase equilibria in the Fe‐V‐O system near “FeO”‐V2O3 isopleth

AbstractPhase equilibria in the “FeO”‐V2O3 system from 1273 to 1808 K and in the range of oxygen partial pressure from 10−15 to 10−4 atm are investigated. High‐temperature quenching, XRD, SEM‐EDS, and DSC are used to determine the phase relations. Stable regions of (FeO)s.s., (V2O3)s.s., and spinel phases are considerably effected by the oxygen partial pressure, and structural models are proposed as (Fe2+, Fe3+, V2+)1‐xO, (V2+, V3+, V4+, Fe3+)2O3+x, and (Fe2+, Fe3+, V3+)(Fe2+, Fe3+, V3+, Va)2O4. Continuous solid solution FeV2O4‐Fe3O4 is formed. The nonstoichiometry of FeV2O4 is attributed to the appearance of vanadium vacancies for electroneutrality due to the oxidation of Fe2+. The standard Gibbs energy of formation for FeV2O4 and component activities in FeV2O4‐Fe3O4 solid solution at 1623 and 1773 K are derived based on the equilibrium oxygen partial pressure. The cation distribution in FeV2O4 at different temperatures is obtained according to site preference energy.

Solution plasma boosts facet-dependent photoactivity of decahedral BiVO4

Decahedral BiVO4 has attracted increasing attention recently due to its fascinating manipulation of charge separation across exposed {0 1 0} and {1 1 0} facets. However, the charge separation driven solely by facet-junction is limited by the relatively small energy difference of the two exposed facets. Further promotion of charge separation remains a challenge through simple post-processed surface activation. Here, solution plasma is utilized to activate the as-synthesized decahedral BiVO4 leading to a 1.5-fold higher oxygen evolution rate from water oxidation under visible light. The system uses pulsed high-voltage discharge in BiVO4 suspension to generate a plasma in solution, which is particularly efficient for introducing vanadium vacancies. This leads to electron-trapping by vanadium vacancies, and the charge separation of BiVO4 is substantially promoted with spatial separation of active sites on {1 1 0} and {0 1 0} facets as manifested by the selective deposition of Pt and MnOx on {0 1 0} and {1 1 0} exposed facets, respectively. The as-adopted strategy indicates that solution plasma works well in surface activation and is promising for boosting facet-dependent photoactivity of polyhedron photocatalysts.

Energetics and kinetics of metal impurities in the low-temperature ordered phase of V2C from first-principles calculations

The site preference of Fe, Ni, Ce, Nd and U impurities and their migration behaviors in the low-temperature ordered orthorhombic phase of V2C are calculated using density functional theory. It is found that all impurities prefer the substitutional vanadium site over the octahedral interstitial site. The energy required to incorporate an impurity into the lattice increases with increasing impurity size. Binding between an impurity at the substitutional vanadium site and a neighboring vanadium vacancy is shown to be much stronger for large impurities, U, Ce and Nd, than for small impurities, Fe and Ni. The strong binding can be attributed to the openness of the V2C structure, which allows large impurity atoms to relax by shifting towards neighboring empty octahedral sites. The proximity of each impurity-vacancy pair to nearby carbon interstitial atoms leads to a high degree of anisotropy in binding strength with binding energies differing by > 2 eV for the same impurity type. Small impurities, Fe and Ni, have negative impurity volumes at the substitutional vanadium site, so binding with neighboring vanadium vacancies is much weaker. Migration barriers for direct exchange of an impurity with a vanadium vacancy are calculated for Fe and Ni. On average migration barriers for Ni are lower than those for Fe.

Highly Efficient and Exceptionally Durable CO2 Photoreduction to Methanol over Freestanding Defective Single-Unit-Cell Bismuth Vanadate Layers.

Unearthing an ideal model for disclosing the role of defect sites in solar CO2 reduction remains a great challenge. Here, freestanding gram-scale single-unit-cell o-BiVO4 layers are successfully synthesized for the first time. Positron annihilation spectrometry and X-ray fluorescence unveil their distinct vanadium vacancy concentrations. Density functional calculations reveal that the introduction of vanadium vacancies brings a new defect level and higher hole concentration near Fermi level, resulting in increased photoabsorption and superior electronic conductivity. The higher surface photovoltage intensity of single-unit-cell o-BiVO4 layers with rich vanadium vacancies ensures their higher carriers separation efficiency, further confirmed by the increased carriers lifetime from 74.5 to 143.6 ns revealed by time-resolved fluorescence emission decay spectra. As a result, single-unit-cell o-BiVO4 layers with rich vanadium vacancies exhibit a high methanol formation rate up to 398.3 μmol g-1 h-1 and an apparent quantum efficiency of 5.96% at 350 nm, much larger than that of single-unit-cell o-BiVO4 layers with poor vanadium vacancies, and also the former's catalytic activity proceeds without deactivation even after 96 h. This highly efficient and spectrally stable CO2 photoconversion performances hold great promise for practical implementation of solar fuel production.

Electronic structure of the Heusler compound Fe2VAl and its point defects by ab initio calculations

Density functional‐based first principles calculations are performed to study the electronic structure of Fe2VAl and the formation energy of the intrinsic point defects such as vacancies, antisites, and interstitials. The ab initio calculations performed using an usual exchange‐correlation functional show that pure Fe2VAl is a non‐magnetic semimetal. Defects always change the density of states (DOS) near the Fermi level and alter the electronic properties of the system which could explain the anomalous nature of Fe2VAl which is found semiconducting in experiments and when using a hybrid functional . The relative stability of the different defects has been calculated and antisites appear to be the most favorable defects to form. Spin‐polarized electronic structure calculations reveal that defects like Vanadium vacancies and FeV and VFe antisites induce a magnetic character to the system in agreement with experimental results.

Ab initio study of shallow acceptors in bixbyite V2O3

We present the results of our study on p-type dopability of bixbyite V2O3 using the Heyd, Scuseria, and Ernzerhof hybrid functional (HSE06) within the density functional theory (DFT) formalism. We study vanadium and oxygen vacancies as intrinsic defects and substitutional Mg, Sc, and Y as extrinsic defects. We find that Mg substituting V acts as a shallow acceptor, and that oxygen vacancies are electrically neutral. Hence, we predict Mg-doped V2O3 to be a p-type conductor. Our results also show that vanadium vacancies are relatively shallow, with a binding energy of 0.14 eV, so that they might also lead to p-type conductivity.

The identification of defect structures for oxygen pressure dependent VO2 crystal films

The defect structures of vanadium dioxide (VO2) films prepared at different oxygen pressures have been investigated using positron annihilation spectroscopy for the first time. It is found that the concentration of vanadium vacancies is not dependent on oxygen pressure for the range studied, implying that at high oxygen pressure, the point defects which have an effect on transition properties are O-interstitials. The variations of oxygen pressures are more probable to cause changes of the type or concentration of oxygen related defects (such as O-interstitials or O-vacancies) and further influence the transition characters. No matter what kind of the defects they are, the localized point defects have a great impact on the metal-insulator phase transition (MIT) process and the corresponding influence mechanisms are discussed. In addition, the stability of the VO2 films is also studied. The present results are valuable for the achievement of VO2-based devices.

First-principles study of phase stability of stoichiometric vanadium nitrides

First-principles pseudopotential total-energy and phonon-spectrum calculations of various phases of stoichiometric vanadium nitrides with a small number of vanadium and nitrogen vacancies, as well as with small carbon and oxygen admixtures, were carried out. It was found that the stoichiometric hexagonal and tetragonal vanadium nitrides (VN) were dynamically stable. The NaCl-type VN exhibited lattice instability. The total energy for the computed ground-state phases of VN increased in the sequence hexagonal (WC type, $P\overline{6}m2$)--hexagonal (AsNi type, $P{6}_{3}/mmc$)--tetragonal ($P{4}_{2}/mcm$)--cubic (NaCl type, $Fm\overline{3}m$)--cubic (ZnS type, $F\overline{4}3m$)--cubic (CsCl type, $Pm\overline{3}m$). At low temperatures, the vacancies in both sublattices stabilize the triclinic phases of V${}_{x}$N${}_{x}$ for $x<0.94$, which is consistent with tetragonal symmetry. At high temperatures, the stability of the NaCl- and WC-type VN structures was estimated using an approach based on band-energy smearing. The results obtained indicate that the cubic structure of VN with the fully occupied sublattices will be stable only at high temperatures. The small carbon and oxygen admixtures do not stabilize the cubic phase compared to the hexagonal one. The phase stability of the cubic and hexagonal structures of V${}_{x}$N${}_{x}$ was explained on the basis of the peculiarities of their electronic densities of states near the Fermi level. The predicted structural transformations in stoichiometric vanadium nitrides are compatible with those observed experimentally.

Microtwinning in highly nonstoichiometric VO x thin films

Both pulsed-DC biased and commercial ion-beam sputtered VO x thin films maintain a face-centered-cubic nanocrystalline phase, even for stoichiometries of x > 1.5, which is well outside the bulk equilibrium solubility range for cubic VO x . Many of these highly nonstoichiometric films exhibit a high density of microtwins, which give rise to unusual fine structure in the selected-area electron diffraction patterns, namely: an additional defect ring; a significant broadening of the {2 0 0} ring; pairs of parallel rod features which are tangent to the additional defect ring; and additional fine-structure features between the {2 0 0} and {2 2 0} rings. The formation of the microtwins is correlated with the coalescence of vanadium vacancies along the {1 1 1} twin planes in the crystalline lattice.

Changes in VO2 band structure induced by charge localization and surface segregation

Vanadium vacancies introduce acceptor doping with hole localization, while oxygen vacancies cause electron localization and donor doping. As deposition temperature increases, donor concentration stays constant, whereas acceptor concentration significantly increases, leading to enhanced (011) lattice-plane compression and surface segregation. Localized charges result in shifts of O 1s and V4+ 2p core levels toward higher binding energies, and O 2p and V4+ 3d valence bands toward the Fermi level, but egπ bands lifting and a1g bands splitting energies are both insensitive to charge localization. Particularly, band-gap energy decreases with increasing V–V pair distance, and is significantly reduced by band tailing.

Theoretical study of the influence of cation vacancies on the catalytic properties of vanadium antimonate

We have theoretically studied the influence of antimony and vanadium cation vacancies in the electronic structure and reactivity of vanadium antimonate, using molecular orbital methods. From the analysis of the electronic properties of the VSbO 4 crystal structure, we can infer that both antimony and vanadium vacancies increase the oxidation state of closer V cations. This would indicate that, in the rutile-type VSbO 4 phase the Sb and V cations defects stabilize the V in a higher oxidation state (V 4+). Calculations of the adsorption energy for different toluene adsorption geometries on the VSbO 4(1 1 0) surface have also been performed. The oxidation state of Sb, V and O atoms and the overlap population of metal–oxygen bonds have been evaluated. Our results indicate that the cation defects influence in the toluene adsorption reactions is slight. We have computed different alternatives for the reoxidation of the VSbO 4(1 1 0) surface active sites which were reduced during the oxygenated products formation. These calculations indicate that the V cations in higher oxidation state (V 4+) are the species, which preferentially incorporate lattice oxygen to the reduced Sb cations. Thus, the cation defects would stabilize the V 4+ species in the VSbO 4 structure, determining its ability to provide lattice oxygen as a reactant.