Shape-Controlled First-Row Transition Metal Vanadates for Electrochemical and Photoelectrochemical Water Splitting.

Transition metal vanadates (MVs) possess abundant electroactive sites, short ion diffusion pathways, and optical properties that make them suitable for various electrochemical (EC) and photoelectrochemical (PEC) applications. While these materials are commonly used in energy storage devices like batteries and capacitors, their shape-controlled 1D and 2D morphologies have gained equal popularity in water splitting (WS) technology in recent times. This review focuses on recent progress made on various first-row (3d, 4 s) transition metal vanadates (t-MVs) having controlled one-dimensional (fiber, wire, or rod) and two-dimensional (layered or sheet) morphologies with a specific emphasis on copper vanadates (CuV), cobalt vanadates (CoV), iron vanadates (FeV), and nickel vanadates (NiV). The review covers different aspects of shape-controlled 1D and 2D t-MVs including optoelectrical properties, wet chemistry synthesis, and electrochemical (EC-WS) and photoelectrochemical water splitting (PEC-WS) performance in terms of onset potential, overpotential, and long-term stability or high cyclic performance. The review concludes by providing some possible thoughts on how to promote the water-splitting attributes of shape-controlled t-MVs more effectively.

Electrochemical and Structural Characterization of Soft Landed Tungsten-Substituted Lindqvist Polyoxovanadate-Alkoxides.

Lindqvist polyoxovanadate-alkoxide (POV-alkoxide) clusters are excellent candidates for applications in energy storage and conversion due to their rich electrochemical profiles. One approach to tune the redox properties of these cluster complexes is through substitutional cationic doping within the hexavanadate core. Here, we report the synthesis of a series of tungsten-substituted POV-alkoxide clusters with one and two tungsten atoms. Soft landing of mass-selected ions was used to purify heterometal POV-alkoxides that cannot be readily separated using conventional approaches. The soft landed POV-alkoxides are characterized using infrared reflection-absorption spectroscopy and electrospray ionization mass spectrometry. The redox properties of the isolated ions are examined using an in situ electrochemical cell which enables traditional in vacuo electrochemical measurements inside of an ion soft landing instrument. Although the overall cluster core retains redox activity after tungsten doping, vanadium-based redox couples (VV /VIV ) are shifted substantially, indicating a pronounced effect of a heteroatom on the electronic structure of the core.

The Influence of Defects on the Luminescence of Trivalent Terbium in Nanocrystalline Yttrium Orthovanadate.

Terbium-doped YVO4 has been considered a nonluminescent solid since the first classic studies on rare-earth-doped phosphors in the 1960s. However, we demonstrate that defect engineering of YVO4:Tb3+ nanoparticles overcomes the metal-metal charge transfer (MMCT) process which is responsible for the quenching of the Tb3+ luminescence. Tetragonal (Y1-xTbx)VO4 nanoparticles obtained by colloidal precipitation showed expanded unit cells, high defect densities, and intimately mixed carbonates and hydroxides, which contribute to a shift of the MMCT states to higher energies. Consequently, we demonstrate unambiguously for the first time that Tb3+ luminescence can be excited by VO43- → Tb3+ energy transfer and by direct population of the 5D4 state in YVO4. We also discuss how thermal treatment removes these effects and shifts the quenching MMCT state to lower energies, thus highlighting the major consequences of defect density and microstructure in nanosized phosphors. Therefore, our findings ultimately show nanostructured YVO4:Tb3+ can be reclassified as a UV-excitable luminescent material.

Assembling and Regulating of Transition Metal-Based Heterophase Vanadates as Efficient Oxygen Evolution Catalysts.

Developing efficient, durable, and low-cost earth-abundant elements-based oxygen evolution reaction (OER) catalysts by rapid and scalable strategies is of great importance for future sustainable electrochemical hydrogen production. The earth-abundant high-valency metals, especially vanadium, can modulate the electronic structure of 3d metal oxides and oxyhydroxides and offer the active sites near-optimal adsorption energies for OER intermediates. Here, the authors propose a facile assembling and regulating strategy to controllably synthesize a serial of transition metal (CoFe, NiFe, and NiCo)-based vanadates for efficient OER catalysis. By tuning the reaction concentrations, NiFe-based vanadates with different crystallinities can be facilely regulated, where the catalyst with moderate heterophase (mixed crystalline and amorphous structures) shows the best OER catalytic activity in terms of low overpotential (267 mV at the current density of 10 mA cm-2 ), low Tafel slope (38 mV per decade), and excellent long-term durability in alkaline electrolyte, exceeding its noble metal-based counterparts (RuO2 ) and most current existing OER catalysts. This work not only reports a facile and controllable method to synthesize a series of vanadates-based catalysts with heterophase nanostructures for high-performance OER catalysis, but also may expand the scope of designing cost-effective transition metal-based electrocatalysts for water splitting.

Synthesis and Characterization of Magnesium Vanadates as Potential Magnesium-Ion Cathode Materials through an Ab†Initio Guided Carbothermal Reduction Approach.

Many technologically relevant materials for advanced energy storage and catalysis feature reduced transition-metal (TM) oxides that are often nontrivial to prepare because of the need to control the reducing nature of the atmosphere in which they are synthesized. Herein, we show that an ab initio predictive synthesis strategy can be used to produce multi-gram-scale products of various MgVx Oy -type phases (δ-MgV2 O5 , spinel MgV2 O4 , and MgVO3 ) containing V3+ or V4+ relevant for Mg-ion battery cathodes. Characterization of these phases using 25 Mg solid-state NMR spectroscopy illustrates the potential of 25 Mg NMR for studying reversible magnesiation and local charge distributions. Rotor-assisted population transfer (RAPT) is used as a much-needed signal-to-noise enhancement technique. The ab initio guided synthesis method is seen as a step forward towards a predictive synthesis strategy for targeting specific complex TM oxides with variable oxidation states of technological importance.

Host-Guest Complexation Between Cyclodextrins and Hybrid Hexavanadates: What are the Driving Forces?

Host-guest complexes between native cyclodextrins (α-, ß- and γ-CD) and hybrid Lindqvist-type polyoxovanadates (POVs) [V6 O13 ((OCH2 )3 C-R)2 ]2- with R = CH2 CH3 , NO2 , CH2 OH and NH(BOC) (BOC = N-tert-butoxycarbonyl) were studied in aqueous solution. Six crystal structures determined by single-crystal X-ray diffraction analysis revealed the nature of the functional R group strongly influences the host-guest conformation and also the crystal packing. In all systems isolated in the solid-state, the organic groups R are embedded within the cyclodextrin cavities, involving only a few weak supramolecular contacts. The interaction between hybrid POVs and the macrocyclic organic hosts have been deeply studied in solution using ITC, cyclic voltammetry and NMR methods (1D 1 H NMR, and 2D DOSY, and ROESY). This set of complementary techniques provides clear insights about the strength of interactions and the binding host-guest modes occurring in aqueous solution, highlighting a dramatic influence of the functional group R on the supramolecular properties of the hexavanadate polyoxoanions (association constant K1:1 vary from 0 to 2 000 M-1 ) while isolated functional organic groups exhibit only very weak intrinsic affinity with CDs. Electrochemical and calorimetric investigations suggest that the driving force of the host-guest association involving larger CDs (ß- and γ-CD) is mainly related to the chaotropic effect. In contrast, the hydrophobic effect supported by weak attractive forces appears as the main contributor for the formation of α-CD-containing host-guest complexes. In any cases, the origin of driving forces is clearly related to the ability of the macrocyclic host to desolvate the exposed moieties of the hybrid POVs.

A Congruent-Melting Mid-Infrared Nonlinear Optical Vanadate Exhibiting Strong Second-Harmonic Generation.

Study of mid-infrared (mid-IR) nonlinear optical (NLO) materials is hindered by the competing requirements of optimized second-harmonic generation (SHG) coefficient dij and laser-induced damage threshold (LIDT) as well as the harsh synthetic conditions. Herein, we report facile hydrothermal synthesis of a polar NLO vanadate Cs4 V8 O22 (CVO) featuring a quasi-rigid honeycomb-layered structure with [VO4 ] and [VO5 ] polyhedra aligned parallel. CVO possesses a wide IR-transparent window, high LIDT, and congruent-melting behavior. It has very strong phase-matchable SHG intensities in metal vanadate family (12.0 × KDP @ 1064 nm and 2.2 × AGS @ 2100 nm). First-principles calculations suggest that the exceptional SHG responses of CVO largely originate from virtual electronic transitions within [V4 O11 ]∞ layer; the excellent optical transmittance of CVO arises from the special characteristics of vibrational phonons resulting from the layered structure.

Metal-Organic Framework-Derived Co3 V2 O8 @CuV2 O6 Hybrid Architecture as a Multifunctional Binder-Free Electrode for Li-Ion Batteries and Hybrid Supercapacitors.

Metal-organic frameworks (MOFs) are promising materials in diverse fields because of their constructive traits of varied structural topologies, high porosity, and high surface area. MOFs are also an ideal precursor/template to derive porous and functional morphologies. Herein, Co3 V2 O8 nanohexagonal prisms are grafted on CuV2 O6 nanorod arrays (CuV-CoV)-grown copper foam (CF) using solution-processing methods, followed by thermal treatment. Direct preparation of active material on CF can potentially eliminate electrochemically inactive and non-conductive binders, leading to improved charge-transfer rate. Furthermore, solution-processing methods are simple and cost-effective. Owing to versatile valence states and good redox activity, the vanadium-incorporated mixed metal oxides (CuV-CoV) exhibited superior electrochemical performance in lithium (Li)-ion battery and supercapacitor (SC) studies. Furthermore, hollow carbon particles (HCPs) derived from MOF particles (MOF-HCPs) are used as the anode material in SCs. A hybrid SC (HSC) fabricated with CuV-CoV and MOF-HCP materials exhibited noteworthy electrochemical properties. Moreover, a solid-state HSC (SSHSC) is constructed and its real-time feasibility is investigated by harvesting the dynamic energy of a bicycle with the help of a direct current generator. The charged SSHSCs potentially powered various electronic components.

High Areal Capacity Li-Ion Storage of Binder-Free Metal Vanadate/Carbon Hybrid Anode by Ion-Exchange Reaction.

Storing more energy in a limited device area is very challenging but crucial for the applications of flexible and wearable electronics. Metal vanadates have been regarded as a fascinating group of materials in many areas, especially in lithium-ion storage. However, there has not been a versatile strategy to synthesize flexible metal vanadate hybrid nanostructures as binder-free anodes for Li-ion batteries so far. A convenient and versatile synthesis of Mx Vy Ox+2.5y @carbon cloth (M = Mn, Co, Ni, Cu) composites is proposed here based on a two-step hydrothermal route. As-synthesized products demonstrate hierarchical proliferous structure, ranging from nanoparticles (0D), and nanobelts (1D) to a 3D interconnected network. The metal vanadate/carbon hybrid nanostructures exhibit excellent lithium storage capability, with a high areal specific capacity up to 5.9 mAh cm-2 (which equals to 1676.8 mAh g-1 ) at a current density of 200 mA g-1 . Moreover, the nature of good flexibility, mixed valence states, and ultrahigh mass loading density (over 3.5 mg cm-2 ) all guarantee their great potential in compact energy storage for future wearable devices and other related applications.

Eu3+ -Activated Gd8 V2 O17 : Energy Transfer, Luminescence, and Temperature-Dependence Characteristics.

A new phosphor of the type Gd8(1-x) Eu8 x V2 O17 (x=0-1.0) was synthesized through the solid-state reaction ceramics method. A pure phase formation was verified by using X-ray powder diffraction measurements. The luminescence of Gd8 V2 O17 :Eu3+ was investigated through optical and laser excitation spectroscopy. The luminescence curves were investigated in the temperature region 10 to 300 K. Gd8 V2 O17 shows a self-activated luminescence under excitation with UV- and near-UV light. The spectra, the decay lifetimes, and the thermal stability of Gd8(1-x) Eu8 x V2 O17 (x=0.005-1.0) strongly depend on both the Eu3+ concentration and the temperature. The tunable luminescence is realized by controlling the Eu3+ -doping level to adjust the host energy-transfer efficiency from the VO43- groups to the Eu3+ activators. At low Eu3+ concentrations (<30 mol %), the intensity and lifetime show an unusual change with an increase of the temperature from 10-300 K, that is, the luminescence experiences a straightforward enhancement. The energy transfer from the VO43- group to the Eu3+ ions could be accelerated with an increase of the temperature resulting in an unusual enhancement of the Eu3+ luminescence and lifetime. However, the emission of the Eu3+ ions decreased for highly Eu-doped samples (>30 mol %) with an increase of the temperature. The luminescence mechanism was discussed on the basis of the charge-transfer band of the Eu3+ ions, the doping concentration, and the proposed microstructures in the lattices.

Colossal Negative Thermal Expansion in Electron-Doped PbVO3 Perovskites.

Colossal negative thermal expansion (NTE) with a volume contraction of about 8 %, the largest value reported so far for NTE materials, was observed in an electron-doped giant tetragonal perovskite compound Pb1-x Bix VO3 (x=0.2 and 0.3). A polar tetragonal (P4mm) to non-polar cubic structural transition took place upon heating. The coefficient of thermal expansion (CTE) and the working temperature could be tuned by changing the Bi content, and La substitution decreased the transition temperature to room temperature. Pb0.76 La0.04 Bi0.20 VO3 exhibited a unit cell volume contraction of 6.7 % from 200 K to 420 K. Interestingly, further gigantic NTE of about 8.5 % was observed in a dilametric measurement of a Pb0.76 La0.04 Bi0.20 VO3 polycrystalline sample. The pronounced NTE in the sintered body should be attributed to an anisotropic lattice parameter change.

Formation of NiCo2 V2 O8 Yolk-Double Shell Spheres with Enhanced Lithium Storage Properties.

Complex nanostructures with multi-components and intricate architectures hold great potential in developing high-performance electrode materials for lithium-ion batteries (LIBs). Herein, we demonstrate a facile self-templating strategy for the synthesis of metal vanadate nanomaterials with complex chemical composition of NiCo2 V2 O8 and a unique yolk-double shell structure. Starting with the Ni-Co glycerate spheres, NiCo2 V2 O8 yolk-double shell spheres are synthesized through an anion-exchange reaction of Ni-Co glycerate templates with VO3- ions, followed by an annealing treatment. By virtue of compositional and structural advantages, these NiCo2 V2 O8 yolk-double shell spheres manifest outstanding lithium storage properties when evaluated as anodes for LIBs. Impressively, an extra-high reversible capacity of 1228 mAh g-1 can be retained after 500 cycles at a high current density of 1.0 Ag-1 .

Induced Rate Performance Enhancement in Off-Stoichiometric Na3+3x V2-x (PO4 )3 with Potential Applicability as the Cathode for Sodium-Ion Batteries.

Off-stoichiometric Na3+3x V2-x (PO4 )3 samples have been prepared by a sol-gel route. X-ray diffraction and XPS revealed the flexibility of the NASICON framework to accommodate these deviations of the stoichiometry; at least for low x values. X-ray photoelectron spectra evidenced the presence of Na4 P2 O7 impurities. The synergic combination of the structural deviations and the presence of Na4 P2 O7 impurities induce a significant improvement of the electrochemical performance and cycling stability at high rates, as compared to the stoichiometric Na3 V2 (PO4 )3 sample. The fast kinetic response provided by the induced off-stoichiometry involves a decrease of the cell resistance.

Photoluminescence, cathodoluminescence and thermal stability of Sm3+ -activated Sr3 La(VO4 )3 red-emitting phosphors.

A series of Sm3+ -activated Sr3 La(VO4 )3 phosphors were synthesized by a facile sol-gel method. X-ray diffraction patterns and photoluminescence (PL)/cathodoluminescence (CL) spectra as well as PL decay curves were employed to characterize the obtained samples. Upon 402 nm light excitation, the characteristic emissions of Sm3+ ions corresponding to 4 G5/2 →6 HJ transitions were observed in all the as-prepared products. The PL emission intensity was increased with increase in Sm3+ ion concentration, while concentration quenching occurred when the doping concentration was over 4 mol%. The non-radiative energy transfer mechanism for concentration quenching of Sm3+ ions was dominated by dipole-dipole interaction and the critical distance was around 21.59 Å. Furthermore, temperature-dependent PL emission spectra revealed that the obtained phosphors possessed good thermal stability with an activation energy of 0.19 eV. In addition, the CL spectra of the samples were almost the same as the PL spectra, and the CL emission intensity showed a tendency to increase with increase in accelerating voltage and filament current. These results suggest that the Sm3+ -activated Sr3 La(VO4 )3 phosphors with good color coordinates, high color purity and superior thermal stability may be a potential candidate for applications in white light-emitting diodes and field-emission displays as red-emitting phosphors.

0D/2D Heterojunctions of Vanadate Quantum Dots/Graphitic Carbon Nitride Nanosheets for Enhanced Visible-Light-Driven Photocatalysis.

0D/2D heterojunctions, especially quantum dots (QDs)/nanosheets (NSs) have attracted significant attention for use of photoexcited electrons/holes due to their high charge mobility. Herein, unprecedent heterojunctions of vanadate (AgVO3 , BiVO4 , InVO4 and CuV2 O6 ) QDs/graphitic carbon nitride (g-C3 N4 ) NSs exhibiting multiple unique advances beyond traditional 0D/2D composites have been developed. The photoactive contribution, up-conversion absorption, and nitrogen coordinating sites of g-C3 N4 NSs, highly dispersed vanadate nanocrystals, as well as the strong coupling and band alignment between them lead to superior visible-light-driven photoelectrochemical (PEC) and photocatalytic performance, competing with the best reported photocatalysts. This work is expected to provide a new concept to construct multifunctional 0D/2D nanocomposites for a large variety of opto-electronic applications, not limited in photocatalysis.

Cu3 V2 O8 Nanoparticles as Intercalation-Type Anode Material for Lithium-Ion Batteries.

Cu3 V2 O8 nanoparticles with particle sizes of 40-50 nm have been prepared by the co-precipitation method. The Cu3 V2 O8 electrode delivers a discharge capacity of 462 mA h g(-1) for the first 10 cycles and then the specific capacity, surprisingly, increases to 773 mA h g(-1) after 50 cycles, possibly as a result of extra lithium interfacial storage through the reversible formation/decomposition of a solid electrolyte interface (SEI) film. In addition, the electrode shows good rate capability with discharge capacities of 218 mA h g(-1) under current densities of 1000 mA g(-1) . Moreover, the lithium storage mechanism for Cu3 V2 O8 nanoparticles is explained on the basis of ex situ X-ray diffraction data and high-resolution transmission electron microscopy analyses at different charge/discharge depths. It was evidenced that Cu3 V2 O8 decomposes into copper metal and Li3 VO4 on being initially discharged to 0.01 V, and the Li3 VO4 is then likely to act as the host for lithium ions in subsequent cycles by means of the intercalation mechanism. Such an "in situ" compositing phenomenon during the electrochemical processes is novel and provides a very useful insight into the design of new anode materials for application in lithium-ion batteries.

A promising RVO4:Eu(3+), Li(+)@SiO2 (R = Gd, Y and Gd/Y) red-emitting phosphor with improved luminescence (cd/m(2)) and colour purity for optical display applications.

Red emission intensity was optimized in three stages, by investigating the effects of: (i) host composition (Gd, Y and Gd/Y), (ii) codoping Li(+) as a sensitizer and, finally, (iii) with a SiO2 shell coating as a protecting layer. Lanthanide vanadate powder phosphors were synthesized using a modified colloidal precipitation technique. The effects of SiO2 coating on phosphor particles were characterized using scanning electron microscopy (SEM)-EDAX, transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and photoluminescence (PL) measurements. An improvement in the PL intensity on Li codoping was due to improved crystallinity, which led to higher oscillating strengths for the optical transitions, and also a lowering of the inversion symmetry of Eu(3+) ions. Red emission intensity due to (5)D0 → (5)D2 transition of the phosphor Y0.94VO4:Eu(3+)0.05, Li(+)0.01 was enhanced by 22.28% compared with Y0.95VO4:Eu(3+)0.05, and was further improved by 58.73% with SiO2 coating. The luminescence intensity (I) and colour coordinates (x, y) of the optimized phosphor Y0.94VO4:Eu(3+)0.05, Li(+)0.01@SiO2, where I = 13.07 cd/m(2) and (x = 0.6721, y = 0.3240), were compared with values for a commercial red phosphor (Y2O2S:Eu(3+)), where I = 27 cd/m(2) and (x = 0.6522, y = 0.3437). The measured colour coordinates are superior to those of the commercial red phosphor, and moreover, match well with standard NTSC values (x = 0.67, y = 0.33).

Fast and Stable Zinc Anode-Based Electrochromic Displays Enabled by Bimetallically Doped Vanadate and Aqueous Zn2+/Na+ Hybrid Electrolytes.

Vanadates are a class of the most promising electrochromic materials for displays as their multicolor characteristics. However, the slow switching times and vanadate dissolution issues of recently reported vanadates significantly hinder their diverse practical applications. Herein, novel strategies are developed to design electrochemically stable vanadates having rapid switching times. We show that the interlayer spacing is greatly broadened by introducing sodium and lanthanum ions into V3O8 interlayers, which facilitates the transportation of cations and enhances the electrochemical kinetics. In addition, a hybrid Zn2+/Na+ electrolyte is designed to inhibit vanadate dissolution while significantly accelerating electrochemical kinetics. As a result, our electrochromic displays yield the most rapid switching times in comparison with any reported Zn-vanadate electrochromic displays. It is envisioned that stable vanadate-based electrochromic displays having video speed switching are appearing on the near horizon.

Correlation-Driven Topological Transition in Janus Two-Dimensional Vanadates.

The appearance of intrinsic ferromagnetism in 2D materials opens the possibility of investigating the interplay between magnetism and topology. The magnetic anisotropy energy (MAE) describing the easy axis for magnetization in a particular direction is an important yardstick for nanoscale applications. Here, the first-principles approach is used to investigate the electronic band structures, the strain dependence of MAE in pristine VSi2Z4 (Z = P, As) and its Janus phase VSiGeP2As2 and the evolution of the topology as a function of the Coulomb interaction. In the Janus phase the compound presents a breaking of the mirror symmetry, which is equivalent to having an electric field, and the system can be piezoelectric. It is revealed that all three monolayers exhibit ferromagnetic ground state ordering, which is robust even under biaxial strains. A large value of coupling J is obtained, and this, together with the magnetocrystalline anisotropy, will produce a large critical temperature. We found an out-of-plane (in-plane) magnetization for VSi2P4 (VSi2As4), and an in-plane magnetization for VSiGeP2As2. Furthermore, we observed a correlation-driven topological transition in the Janus VSiGeP2As2. Our analysis of these emerging pristine and Janus-phased magnetic semiconductors opens prospects for studying the interplay between magnetism and topology in two-dimensional materials.

Mg2+ Ion Pre-Insertion Boosting Reaction Kinetics and Structural Stability of Ammonium Vanadates for High-Performance Aqueous Zinc-Ion Batteries.

Aqueous zinc-ion batteries (AZIBs) attract much attention owing to their high safety, environmentally friendliness and low cost. However, the unsatisfactory performance of cathode materials is one of the unsolved important factors for their widespread application. Herein, we report NH4 V4 O10 nanorods with Mg2+ ion preinsertion (Mg-NHVO) as a high-performance cathode material for AZIBs. The preinserted Mg2+ ions effectively improve the reaction kinetics and structural stability of NH4 V4 O10 (NHVO), which are confirmed by electrochemical analysis and density functional theory calculations. Compared with pristine NHVO, the intrinsic conductivity of Mg-NHVO is improved by 5 times based on the test results of a single nanorod device. Besides, Mg-NHVO could maintain a high specific capacity of 152.3 mAh g-1 after 6000 cycles at the current density of 5 A g-1 , which is larger than that of NHVO (only exhibits a low specific capacity of 30.5 mAh g-1 at the same condition). Moreover, the two-phase crystal structure evolution process of Mg-NHVO in AZIBs is revealed. This work provides a simple and efficient method to improve the electrochemical performance of ammonium vanadates and enhances the understanding about the reaction mechanism of layered vanadium-based materials in AZIBs.