Metal-to-Metal Charge Transfer in AWO4 (A = Mg, Mn, Co, Ni, Cu, or Zn) Compounds with the Wolframite Structure

0D/1D CuWO4/Mn0.3Cd0.7S S-scheme heterojunctions for full-spectrum bifunctional photocatalytic degradation and hydrogen production

Integrating photocatalytic oxidation for pollutant removal with hydrogen production via photocatalysis presents a promising approach for sustainable water purification and renewable energy generation, circumventing the sluggish multi-electron transfer inherent in photocatalytic water oxidation. This study introduces novel zero-/one-dimensional (0D/1D) CuWO4/Mn0.3Cd0.7S step-scheme (S-scheme) heterojunctions that exhibit exceptional bifunctional capabilities in photocatalytic degradation and hydrogen production under full-spectrum illumination. The degradation efficiency for tetracycline (TC) using 5 %-CuWO4/Mn0.3Cd0.7S reaches 94.3 % and 94.5 % within 60 min and 6 h, respectively, under ultraviolet-visible (UV-Vis) and near-infrared (NIR) light. Notably, these 0D/1D CuWO4/Mn0.3Cd0.7S S-scheme heterojunctions demonstrate superior hydrogen production, achieving rates of 12442.03 μL g-1h-1 and 2418.54 μL g-1h-1 under UV-Vis light and NIR light irradiation, respectively-these rates are 2.3 times and 55.2 times higher than that of Mn0.3Cd0.7S alone. This performance enhancement is attributed to the intrinsic dimensional effects, transitions of transition metal d-d orbitals, and S-scheme hole/electron (h+/e-) separation characteristics. Additionally, experimental results and density functional theory (DFT) calculations have clarified the modulation of electronic configurations, band alignment, and interfacial interactions via 0D/1D S-scheme heterojunction engineering. This study sheds light on the electron transfer mechanism within S-scheme heterojunction and enhances the effectiveness, economy, and sustainability of recalcitrant pollutant removal and hydrogen production.

Efficient wide-spectrum one-dimensional MWO4 (M = Mn, Co, and Cd) photocatalysts: Synthesis, characterization and density functional theory study

Broadening the absorption region to near-infrared (NIR) light is critical for the photocatalysis due to the larger proportion and stronger penetration of NIR light in solar energy. In the present paper, one-dimensional (1D) MWO4 (M = Mn, Co, and Cd) materials synthesized by electrospinning technique, were studied by combining the density functional theory (DFT) with experiment results, which possessed the enhanced light absorption capability within the range of 200-2000 nm. It was proved that in the ultraviolet-visible (UV-Vis) region, the absorption bands of CoWO4 and MnWO4 samples were attributed to the metal-to-metal charge transfer mechanism, while the absorption of CdWO4 sample may be referable to the ligand-to-metal charge transfer mechanism. In the near-infrared (NIR) region, the absorption of CoWO4 and MnWO4 primarily originated from the d-d orbital transitions of Mn2+ and Co2+. The photocatalytic experimental results showed that the degradation rates for bisphenol A (BPA) over CoWO4, MnWO4, and CdWO4 photocatalysts under UV-Vis/NIR light irradiation for 140 min/12 h were 78.8 %/75.9 %, 23.8 %/21.3 %, 12.8 %/8.7 %, respectively. This research offers the novel insights into the precise construction of tungstate catalytic systems and contributes to the advancement of UV-Vis-NIR full spectrum photocatalytic technology, and lays a foundation for a cleaner and more environmental-friendly future.

Constructing core-shell structured Co3O4-MnWO4 composite photoelectrode with superior PEC water purification performance

Semiconductor photoelectrocatalytic (PEC) technology is one of the most effective methods for removing organic pollutants from wastewater in advanced oxidation processes(AOPs). The selection of suitable semiconductor materials as photoanodes is a crucial factor for achieving superior PEC performance. Here, a core-shell structured Co3O4-MnWO4 architecture is created by enveloping MnWO4 nanoparticles onto the surface of Co3O4 nanowires through a two-step hydrothermal process. The optimized Co3O4-MnWO4-5 photoelectrode showed superior PEC degradation efficiency for KN-R (∼91.2% in 2 h) and durable stability (the accelerated lifetime reached ∼9100 s at a current density of 50 mA cm-2). Three actual wastewaters were also collected to verify the practical applicability of the photoelectrode.The energy consumption was measured at 4.48 kWhm-3, with a COD removal efficiency of 83% and a decolorization rate of 98%. These results demonstrate the excellent performance and promising application of the photoelectrode. The enhancement of PEC performance for the core-shell structured Co3O4-MnWO4 architecture can be attributed to the suitable energy band structure of the Co3O4-MnWO4 composite, higher OEP, larger electrochemical active surface area, accelerated transport of interface carriers, and lower charge transfer resistance. The energy band structure of the Co3O4-MnWO4 composite showed a strong redox ability to induce electrons/holes (e-/h+), which enhances the generation of intermediate active species (hydroxyl radical ·OH and superoxide radicals ·O2-). Therefore, the rationally designed core-shell structured Co3O4-MnWO4 architecture exhibited excellent practical applicability in the degradation of organic pollutants.

Synthesis, Optical, Dielectric, SHG, Magnetic and Visible Light Driven Catalytic Studies on Compounds Belonging to the Swedenborgite Structure

A new compound, InBaZn3 GaO7 , with swedenborgite structure along with transition metal (TM) substituted variants have also been prepared. The structure contains layers of tetrahedral ions (Zn2+ /Ga3+ ) connected by octahedrally coordinated In3+ ion forming the three-dimensional structure with voids where the Ba2+ ions occupy. The TM substituted compounds form with new colors. The origin of the color was understood based on the ligand-field transitions. The near IR reflectivity studies indicate that the Ni - substituted compounds exhibit good near - IR reflectivity behavior, making them possible candidates for 'cool pigments'. The temperature dependent dielectric studies indicate that the InBaZn3 GaO7 compound undergoes a phase transition at ~360 °C. The compounds are active towards second harmonic generation (SHG). Magnetic studies show the compounds, InBaZn2 CoFeO7 and InBaZn2 CuFeO7 to be anti-ferromagnetic in nature. The copper containing compounds were found to be good catalysts, under visible light, for the oxidation of aromatic alkenes. The many properties observed in the swedenborgite structure-based compounds suggests that the mineral structure offers a fertile ground to investigate newer compounds and properties.

Ni(II)-doped CuWO4 photoanodes with enhanced photoelectrocatalytic activity

CuWO4 has emerged in the last years as a ternary metal oxide material for photoanodes application in photoelectrochemical cells, thanks to its relatively narrow band gap, high stability and selectivity toward the oxygen evolution reaction, though largely limited by its poor charge separation efficiency. Aiming at overcoming this limitation, we investigate here the effects that Cu(II) ion substitution has on the photoelectrocatalytic (PEC) performance of copper tungstate. Optically transparent CuWO4 thin-film photoanodes, prepared via spin coating and containing different amounts of Ni(II) ions, were fully characterized via UV-Vis spectroscopy, XRD and SEM analyses, and their PEC performance was tested via linear sweep voltammetry, incident photon to current efficiency and internal quantum efficiency analyses. From tests performed in the presence of a hole scavenger-containing electrolyte, the charge injection and separation efficiencies of the electrodes were also calculated. Pure-phase crystalline and/or heterojunction materials were obtained with higher PEC performance compared to pure CuWO4, mainly due to a significantly enhanced charge separation efficiency in the bulk of the material.

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

CuWO4 is a ternary semiconductor oxide with excellent visible light harvesting properties up to 550 nm and stability at high pH values, which make it a suitable material to build photoanodes for solar light conversion to hydrogen via water splitting. In this work, we studied the photoelectrochemical (PEC) performance of transparent CuWO4 electrodes with tunable light absorption and thickness, aiming at identifying the intrinsic bottlenecks of photogenerated charge carriers in this semiconductor. We found that electrodes with optimal CuWO4 thickness exhibit visible light activity due to the absorption of long-wavelength photons and a balanced electron and hole extraction from the oxide. The PEC performance of CuWO4 is light-intensity-dependent, with charge recombination increasing with light intensity and most photogenerated charge carriers recombining in bulk sites, as demonstrated by PEC tests performed in the presence of sacrificial agents or cocatalysts. The best-performing 580 nm thick CuWO4 electrode delivers a photocurrent of 0.37 mA cm-2 at 1.23 VSHE, with a 7% absorbed photon to current efficiency over the CuWO4 absorption spectrum.

Band-Gap Engineering of Layered Perovskites by Cu Spacer Insertion as Photocatalysts for Depollution Reaction

A multi-step ion-exchange methodology was developed for the fabrication of Cu(LaTa2O7)2 lamellar architectures capable of wastewater depollution. The (001) diffraction line of RbLaTa2O7 depended on the guest species hosted by the starting material. SEM and TEM images confirmed the well-preserved lamellar structure for all intercalated layered perovskites. The UV–Vis, XPS, and photocurrent spectroscopies proved that Cu intercalation induces a red-shift band gap compared to the perovskite host. Moreover, the UV–Vis spectroscopy elucidated the copper ions environment in the Cu-modified layered perovskites. H2-TPR results confirmed that Cu species located on the surface are reduced at a lower temperature while those from the interlayer occur at higher temperature ranges. The photocatalytic degradation of phenol under simulated solar irradiation was used as a model reaction to assess the performances of the studied catalysts. Increased photocatalytic activity was observed for Cu-modified layered perovskites compared to RbLaTa2O7 pristine. This behavior resulted from the efficient separation of photogenerated charge carriers and light absorption induced by copper spacer insertion.

Green Fabrication of Heterostructured CoTiO3/TiO2 Nanocatalysts for Efficient Photocatalytic Degradation of Cinnamic Acid

In this work, CoTiO₃/TiO₂ (CTO/Ti) heterostructures were prepared by a hydrothermal procedure in a neutral medium using perovskite CoTiO₃ and tetraisopropyl titanate. Characteristics of the synthesized catalysts were analyzed by various techniques including X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, UV-vis diffuse reflectance spectroscopy, Brunauer-Emmett-Teller adsorption-desorption, energy-dispersive X-ray spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and point of zero charges. The activity in the photodegradation of cinnamic acid (CA) under UV-A irradiation of the CTO/Ti heterostructure was investigated and compared with individual materials TiO₂ (Ti-w) and CoTiO₃ (CTO). The investigation showed that the heterostructured CoTiO₃/TiO₂ catalyst with optimal composition (5% CTO) exhibited much higher photocatalytic activity for degradation of cinnamic acid than individual CoTiO₃ and TiO₂. Under the optimal conditions (C _cat = 0.75 g/L, Q _air = 0.3 L/min, and pH = 3.8) the 90 min conversion of cinnamic acid reached 80.9% on 5CTO/Ti, much higher than those of CTO (4.6%) and Ti-w (75.2%). It was found that the enhancement in activity for the CA removal of the CTO/Ti heterostructure was due to the construction of a heterojunction structure between TiO₂(Ti-w) and CoTiO₃ that resulted in an increase in the specific surface area and porosity, reduction of the band gap energy, and higher efficient separation of charge carriers on the surface to prevent recombination. Alternatively, a comparison of the recyclability of 5CTO/Ti and Ti-w was made for CA degradation. The results showed a decrease in the CA conversion by 38% on 5CTO/Ti and 48% on Ti-w after six reaction cycles.

Pyrazinamide photodegradation on NiWO4-palygorskite nanocomposites under polychromatic irradiation

In this work, antibiotic pyrazinamide (PZA) photodegradation on palygorskite (Pal), NiWO₄ crystals, and NiWO₄-Pal (2, 6, and 10%) nanocomposites was evaluated under polychromatic irradiation. In the characterization of the samples, XRD patterns displayed good crystallinity for NiWO₄ crystals and nanocomposites. In addition, the diffractograms were used in the Rietveld refinement for phase indexing, revealing a wolframite-type monoclinic structure with the space group P2/c. The active vibrational modes related to the characteristic groups of the samples were identified using Raman and FTIR spectroscopy. Photoluminescence (PL) spectra revealed that NiWO₄ and NiWO₄-Pal (2%) nanocomposite have the highest electron-hole pair recombination rate, and the contribution of the green component in the NiWO₄-Pal (2%) nanocomposite indicates a greater contribution of deep energy levels to the PL profile. DRS in the UV-visible region indicated that NiWO₄ crystals have indirect band-gap energy (E_gap) 2.64 eV; NiWO₄-Pal (2, 6, and 10%) nanocomposites have 2.62, 2.58, and 2.59 eV, respectively; and Pal has 2.83 eV. The catalytic tests showed that the NiWO₄-Pal (2%) nanocomposite samples, under polychromatic radiation, exhibit greater efficiency in photodegradation at 110 min, with yield of 98.5%. The ROS tests indicated that the studied reactive species play a similar role in PZA photodegradation.

Ternary Oxide CuWO4/BiVO4/FeCoOx Films for Photoelectrochemical Water Oxidation: Insights into the Electronic Structure and Interfacial Band Alignment

Photoelectrochemical (PEC) water oxidation using ternary oxide systems has been considered a promising approach for investigating the effective utilization of sunlight and the production of green fuel. Herein, we report a ternary-oxide-based CuWO4/BiVO4/FeCoOx film deposited entirely by RF-magnetron sputtering using homemade ceramic targets. Our CuWO4/BiVO4 photoanode exhibits a significant photocurrent density of 0.82 mA cm-2 at 1.23 V vs RHE under AM 1.5G illumination, which is a record 382% increase compared to that of the bare CuWO4 film. To further boost the PEC performance, we deposited an ultrathin layer of amorphous FeCoOx cocatalyst, resulting in a triple CuWO4/BiVO4/FeCoOx heterojunction with a significant reduction in onset potential and a 500% increase in the photocurrent density of bare CuWO4. Experimental and theoretical approaches were used to provide insights into the interfacial band alignment and photoinduced charge carrier pathway across heterojunctions. Our results reveal noticeable interface potential barriers for charge carriers at the CuWO4/BiVO4 heterojunction, potentially limiting its application in tandem systems. Conversely, the deposition of the FeCoOx ultrathin layer over the CuWO4/BiVO4 heterojunction induces a p-n junction on the BiVO4/FeCoOx interface, which, when combined with the abundant FeCoOx oxygen vacancies, results in improved charge separation and transport as well as enhanced photoelectrochemical stability. Our study provides a feasible strategy for producing photocatalytic heterojunction systems and introduces simple tools for investigating interface effects on photoinduced charge carrier pathways for PEC water splitting.

High Near-Infrared Reflective Zn1-xAxWO4 Pigments with Various Hues Facilely Fabricated by Tuning Doped Transition Metal Ions (A = Co, Mn, and Fe)

A series of novel transition metal ion-substituted Zn_1-xA_xWO₄ (A = Co, Mn, and Fe, 0 < x ≤ 0.1) inorganic pigments with blue, yellow, brown, and pale green colors have been prepared by a solution combustion method and exhibit extremely high near-infrared reflectance (R > 85%). X-ray energy-dispersive spectroscopy analysis makes it clear that transition metal ions have already been incorporated into the host ZnWO₄ lattice and do not change the lattice's initial wolframite structure. The optical absorption spectrum in the UV region of the ZnWO₄ pigment calcined at 800 °C for 3 h is a ligand-to-metal charge transfer from O 2p nonbonding orbits to antibonding W 5d orbits. On account of the doping Co²⁺ (3d⁷), Mn²⁺ (3d⁵), and Fe³⁺ (3d⁵) transition metal ions, these chromophore ions have occupied the distorted octahedral site of Zn²⁺, leading to d-d transition and metal-to-metal charge transfer from the occupied 3d orbits of A²⁺ to unoccupied W 5d orbits in UV and visible ranges and generating some bright colors. Significantly, these inorganic pigments are also endowed with excellent thermal and chemical stability and are conducive to harsh working conditions. All of the analysis results have offered some design strategies for various colorful inorganic pigments with high near-infrared reflectance.

Ultrafast Charge Carrier Dynamics in CuWO4 Photoanodes

CuWO₄ is a ternary metal oxide semiconductor with promising properties for photoelectrochemical (PEC) water splitting and solar light conversion, due to its quite low band gap (2.3 eV) and high stability in an alkaline environment. Aiming at understanding the origin of the relatively low PEC efficiency attained with CuWO₄ photoanodes, we here investigate transparent CuWO₄ electrodes prepared by a simple solution-based method through the combination of femtosecond transient absorption spectroscopy with electrochemical, PEC, and photochromic characterizations. The very fast recombination dynamics of the charge carriers photogenerated in CuWO₄, which is the reason for its low efficiency, is discussed in relation with its PEC performance and with the recently calculated band structure of this material, also in comparison with the behavior of other semiconductor oxides employed in PEC applications, in particular Fe₂O₃.

CdS decorated MnWO4 nanorod nanoheterostructures: a new 0D-1D hybrid system for enhanced photocatalytic hydrogen production under natural sunlight

Constructing a heterostructure is an effective strategy to reduce the electron-hole recombination rate, which enhances photocatalytic activity. Here, we report a facile hydrothermal method to grow CdS nanoparticles on MnWO₄ nanorods and their photocatalytic hydrogen generation under solar light. A structural study shows the decoration of hexagonal CdS nanoparticles on monoclinic MnWO₄. Morphological studies based on FE-TEM analysis confirm the sensitization of CdS nanoparticles (10 nm) on MnWO₄ nanorods of diameter-35 nm with mean length ∼100 nm. The lower PL intensity of MnWO₄ was observed with an increasing amount of CdS nanoparticles, which shows inhibition of the charge carrier recombination rate. A CdS@MnWO₄ narrow band gap semiconductor was employed for photocatalytic hydrogen generation from water under solar light and the highest amount of hydrogen, i.e. 3218 μmol h^-1 g^-1, is obtained which is 21 times higher than that with pristine MnWO₄. The enhanced photocatalytic activity is ascribed to the formation of a CdS@MnWO₄ nanoheterostructure resulting in efficient spatial separation of photogenerated electron-hole pairs due to vacancy defects. More significantly, direct Z-scheme electron transfer from MnWO₄ to CdS is responsible for the enhanced hydrogen evolution. This work signifies that a CdS decorated MnWO₄ nanoheterostructure has the potential to improve the solar to direct fuel conversion efficiency.

Targeted synthesis of a CrIII–O–VV core oxo-bridged complex: spectroscopic, magnetic and electrical properties

The results of vibrational, electronic, structural, thermal, magnetic and impedance spectroscopy studies are presented in the first reported compound with a Cr^III–O–V^V bridge.

Visible-Light-Activated C-C Bond Cleavage and Aerobic Oxidation of Benzyl Alcohols Employing BiMXO5 (M=Mg, Cd, Ni, Co, Pb, Ca and X=V, P)

The synthesis, structure, optical and photocatalytic studies of a family of compounds with the general formula, BiMXO₅ ; M=Mg, Cd, Ni, Co, Pb, Ca and X=V, P is presented. The compounds were prepared by regular solid-state reaction of constituents in the temperature range of 720-810 °C for 24 h. The compounds were characterized by powder X-ray diffraction (PXRD) methods. The Rietveld refinement of the PXRD patterns have been carried out to establish the structure. The optical absorption spectra along with the colors in daylight have been explained employing the allowed d-d transition. In addition, the observed colors of some of the V⁵⁺ containing compounds were explained using metal-to-metal charge transfer (MMCT) from the partially filled transition-metal 3d orbitals to the empty 3d orbitals of V⁵⁺ ions. The near IR (NIR) reflectivity studies indicate that many compounds exhibit good NIR reflectivity, suggesting that these compounds can be employed as 'cool pigments'. The experimentally determined band gaps of the prepared compounds were found to be suitable to exploit them for visible light activated photocatalysis. Photocatalytic C-C bond cleavage of alkenes and aerobic oxidation of alcohols were investigated employing visible light, which gave good yields and selectivity. The present study clearly demonstrated the versatility of the Paganoite family of compounds (BiMXO₅ ) towards new colored inorganic materials, visible-light photocatalysts and 'cool pigments'.

Structural Complexity and Magnetic Orderings in a Large Family of 3d-4f Heterobimetallic Sulfates

The synthesis of a large family of heterobimetallic lanthanide copper sulfates was realized via stoichiometric hydrothermal reactions among Ln₂O₃, CuO, and H₂SO₄, giving rise to four distinct phases, namely Ln₂Cu(SO₄)₂(OH)₄ (Ln = Sm-Ho) (LnCuSO₄-1), Ln₄Cu(SO₄)₂(OH)₁₀ (Ln = Tm-Lu) (LnCuSO₄-2), LnCu(SO₄)(OH)₃ (Ln = Nd-Gd, except Pm) (LnCuSO₄-3), and LnCu(SO₄)(OH)₃ (Ln = Dy-Lu) (LnCuSO₄-4), with completely different topologies. The passage from LnCuSO₄-1 and LnCuSO₄-3 to LnCuSO₄-2 and LnCuSO₄-4 across the 4f series, respectively, can be ascribed to the effect of lanthanide contraction, which progressively induces shrinking of the Ln-O distance, reduction in the Ln coordination number, and eventually structural transitions. The incorporation of identical 3d-4f metal ions into different spin-lattices, in conjunction with substitution of diverse Ln³⁺ cations within the same spin-lattice, gives rise to tunable magnetic properties varying from ferromagnetic ordering in GdCuSO₄-3 and HoCuSO₄-4 to antiferromagnetic ordering in YbCuSO₄-4, and to paramagnetic correlations found in GdCuSO₄-1 and YbCuSO₄-2.

Antiferromagnetism of Double Molybdate LiFe(MoO4)2

The magnetic properties of the spin-5/2 double molybdate LiFe(MoO₄)₂ have been characterized by heat capacity, magnetic susceptibility, and neutron powder diffraction techniques. Unlike the multiferroic system LiFe(WO₄)₂ which exhibits two successive magnetic transitions, LiFe(MoO₄)₂ undergoes only one antiferromagnetic transition at T_N ∼ 23.8 K. Its antiferromagnetic magnetic structure with the commensurate propagation vector k = (0, 0.5, 0) has been determined. Density functional theory calculations confirm the antiferromagnetic ground state and provide a numerical estimate of the relevant exchange coupling constants.

Structural and Optical Properties of Ca0.9Cu0.01WO4 Solid Solution Synthesized by Sonochemistry Method at Room Temperature

In this work, we report the room-temperature synthesis of pure calcium tungstate (CaWO₄) and copper-doped calcium tungstate solid solution (Ca_0.99Cu_0.01WO₄) by using a sonochemistry method. These materials were structurally characterized by X-ray diffraction (XRD) and Raman spectroscopy. The obtained XRD patterns were submitted to a Rietveld refinement showing, in both materials, a tetragonal phase with space group and point group of I4₁/a and C_4h⁶, respectively. Microscopy images of both materials, obtained by field emission scanning electron microscopy, showed spherical agglomerated structures composed by spherical nanoparticles, while calcium and tungstate elements were identified by energy-dispersive X-ray spectroscopy for pure calcium tungstate and copper, calcium, and tungstate for Ca_0.99Cu_0.01WO₄ solid solution. The decrease of optical band gap (E_gap) from 4.0 eV (CaWO₄) to 3.45 eV (Ca_0.99Cu_0.01WO₄) confirmed the substitution of calcium atoms for copper atoms in the clusters [CaO₈]. Maximum photoluminescence (PL) emission was shifted from 522 nm in the pure CaWO₄ to 475 nm in the Ca_0.99Cu_0.01WO₄ solid solution. Consequently, there was an increase of PL emissions intensity in the blue and green regions of the visible spectrum, due to electronic transitions between the orbitals O 2p, Cu 3d, and W 5d.

Mn4+ activated Al2O3 red-emitting ceramic phosphor with excellent thermal conductivity

An Al₂O₃:Mn⁴⁺, Mg²⁺ red emitting ceramic phosphor, which can be effectively excited by ultraviolet and blue light, was successfully synthesized via solid-state reaction in an oxygen and air atmosphere. The ceramic sintered in oxygen atmosphere has higher optical transmittance and stronger luminescence intensity than the ceramic sintered in the air, which is more suitable for LED application. Since the structure of α-Al₂O₃ is very simple, it is convenient to study the factors affecting the Mn⁴⁺ luminescence. The crystal-strength parameter Dq, Racah parameters B and C, and the nephelauxetic ratio β₁ were calculated to investigate the influence of crystal field strength and nephelauxetic effect on the emission of Mn⁴⁺ in the Al₂O₃ host. The ratio of Dq to B was 1.74, which was lower than 2.2. This indicated that the Mn⁴⁺ ions in the α-Al₂O₃ host were in a weak crystal field environment. Under the 395 nm and 460 nm excitations, quantum yields (QY) of the sample were measured to be 46% and 28.7%, respectively. The density measured by the Archimedes method was 3.61 g/cm³. The ceramic also showed an excellent thermal conductivity value, which was as high as 26.27 W·m^-1·K^-1@30 °C.

Non-Noble Cobalt Tungstate Catalyst for Effective Electrocatalytic Oxidation of Borohydride

Morphologically tuned cobalt tungstate (CoWO₄), a new entrant toward borohydride oxidation reaction (BOR), was explored as it exhibited negligible H₂ evolution while enabling rapid BOR. A simple synthetic strategy was employed and fine-tuned to obtain different morphologies of CoWO₄ whose urchin-shaped variant gave exciting activity toward BOR. An early and quite negative onset potential of -1.14 V was observed giving a maximum obtainable specific current density of 105.3 mA mg^-1. The synthesized variants were investigated in depth by various electrochemical measurements and assessed in light of previous reports toward BOR activity. Hydrodynamic studies were also performed to ascertain the nature of these static electrochemical measurements. Quantitative assessment of the evolved H₂, a prominent competitive reaction to BOR, was performed suggesting minimal interference. The probable origin of such morphology-dependent activity was subsequently studied in detail by high-resolution transmission electron microscopy (HR-TEM) analysis, revealing nanometric structures in the urchin-like variant, which enhance the obtainable BOR activity.

Evidence of Wolframite-Type Structure in Ultrasmall Nanocrystals with a Targeted Composition MnWO4

Here, we report a study of white-ochre powders with targeted composition MnWO₄ prepared via a coprecipitation method. Through X-ray total scattering combined with pair distribution function analysis and Rietveld refinement of X-ray diffraction data, we find that their crystal structure is similar to that of bulk-MnWO₄, despite a mean crystallite size of 1.0-1.6 nm and a significant deviation of the average chemical composition from MnWO₄. The chemical formula derived from elemental and thermogravimetric analyses is Mn_0.8WO_3.6(OH)_0.4·3H₂O. X-ray absorption and magnetic susceptibility measurements show that Mn and W have the same oxidation states as in MnWO₄. No magnetic ordering or spin glass or superparamagnetic behavior is observed above 2 K, unlike in the case of MnWO₄ nanocrystals having a mean size higher than 10 nm.

Origins of the odd optical observables in plutonium and americium tungstates

A series of f-block tungstates show atypical coloration for both the Ce(iii) and Pu(iii) compounds; whereas the other lanthanide and Am(iii) compounds possess normal absorption features. The different optical properties are actually derived from the tungstate component rather than from 5f electrons/orbitals.

A series of trivalent f-block tungstates, MW₂O₇(OH)(H₂O) (M = La, Ce, Pr, Nd, and Pu) and AmWO₄(OH), have been prepared in crystalline form using hydrothermal methods. Both structure types take the form of 3D networks where MW₂O₇(OH)(H₂O) is assembled from infinite chains of distorted tungstate octahedra linked by isolated MO₈ bicapped trigonal prisms; whereas AmWO₄(OH) is constructed from edge-sharing AmO₈ square antiprisms connected by distorted tungstate trigonal bipyramids. PuW₂O₇(OH)(H₂O) crystallizes as red plates; an atypical color for a Pu(iii) compound. Optical absorption spectra acquired from single crystals show strong, broadband absorption in the visible region. A similar feature is observed for CeW₂O₇(OH)(H₂O), but not for AmWO₄(OH). Here we demonstrate that these significantly different optical properties do not stem directly from the 5f electrons, as in both systems the valence band has mostly O-2p character and the conduction band has mostly W-5d character. Furthermore, the quasi-particle gap is essentially unaffected by the 5f degrees of freedom. Despite this, our analysis demonstrates that the f-electron covalency effects are quite important and substantially different energetically in PuW₂O₇(OH)(H₂O) and AmWO₄(OH), indicating that the optical gap alone cannot be used to infer conclusions concerning the f electron contribution to the chemical bond in these systems.

Electronic structure of CuWO4: dielectric-dependent, self-consistent hybrid functional study of a Mott-Hubbard type insulator

CuWO₄ is a semiconducting oxide with interesting applications in photocatalysis. In this paper we present an accurate study of the electronic properties of stoichiometric and oxygen deficient CuWO₄ based on a dielectric dependent hybrid density functional. In CuWO₄ the Cu ions (Cu²⁺) are in a 3d⁹ configuration, so that the material must be classified as a magnetic insulator. Various magnetic configurations of CuWO₄ have been considered, the most stable configuration being anti-ferromagnetic. The band structure, described in terms of density of states (DOS), exhibit the presence of a wide band dominated by W 5d states, separated by about 5 eV from the top of the valence band (VB), consisting of O 2p states partly mixed with Cu 3d states. The empty component of the Cu 3d orbitals forms a narrow band 3.6 eV above the VB maximum. The electronic structure emerging from the DOS curves and the Kohn-Sham energies is hard to reconcile with an experimental band gap of 2.1-2.3 eV. This gap can be rationalized within the Mott-Hubbard model of magnetic insulators, and has been computed from the total energies of the system with one electron removed from the O 2p band and one electron added to the Cu 3d states. Computing the charge transition levels for CuWO₄, we come to a theoretical band gap of 2.1 eV, in excellent agreement with the experimental observations. We also studied the nature of the oxygen vacancy in CuWO₄ with particular attention to the electron redistribution following the oxygen removal. The excess electrons, in fact, can occupy the localized 3d states of Cu or the localized 5d states of W. The resulting solution depends on various factors, including the concentration of oxygen vacancies.

Ag-Functionalized CuWO4/WO3 nanocomposites for solar water splitting

A new plasmonic Ag-functionalized CuWO₄/WO₃ hetero-structured photoanode was successfully prepared via a PVP-assisted sol–gel (PSG) route and electrophoretic deposition which reveals 4 times enhanced photocurrent density compared with pristine WO₃.

Selective Crystallization of Four Tungstates (La2W3O12, La2W2O9, La14W8O45, and La6W2O15) via Hydrothermal Reaction and Comparative Study of Eu3+ Luminescence

Hydrothermal reaction at 200 °C was systematically undertaken in wide ranges of solution pH (4-13) and W/La molar ratio ( R = 0.5-2), without using any organic additive, to investigate the effect of hydrothermal parameter on product property and the underlying mechanism. Combined analysis by X-ray diffraction (XRD), inductively coupled plasma (ICP) spectroscopy, elemental mapping, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that either a decreasing pH or increasing R value yielded a product richer in W and, conversely, richer in La. The results were interpreted from the solution chemistry of La³⁺ and tungstate ions. As an outcome of our 40 well-designed experiments, four La tungstates-La₂W₃O₁₂, La₂W₂O₉, La₁₄W₈O₄₅, and La₆W₂O₁₅-were successfully obtained in a phase-pure form by calcining their hydrothermal precursors. Phase and morphology evolution, structure features, and properties of Eu³⁺ emission were, for the first time, comparatively investigated for the four compounds. Spectral analysis found that the 5 at. % Eu³⁺-doped La₂W₃O₁₂ phosphor exhibits the highest quantum efficiency (∼47%), more red component, and the shortest fluorescence lifetime of luminescence (∼0.72 ms).

Quantitative Attachment of Bimetal Combinations of Transition-Metal Ions to the Surface of TiO2 Nanorods

We report the sequential, quantitative loading of transition-metal ions (Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺) onto the surface of rod-shaped anatase TiO₂ nanocrystals in bimetallic combinations (₆ C₂ = 15) to form M,M'-TiO₂ nanocrystals. The materials were characterized with transmission electron microscopy (TEM), powder X-ray diffraction (XRD), elemental analysis, X-ray photoelectron spectroscopy (XPS), and UV-visible spectroscopy. TEM and XRD data indicate that the sequential adsorption of metal ions occurs with the retention of the phase and morphology of the nanocrystal. Atomistic models of the M,M'-TiO₂ nanocrystals were optimized with density functional theory calculations. Calculated UV-visible absorption spectra and partial charge density maps of the donor and acceptor states for the electronic transitions indicate the importance of metal-to-metal charge transfer (MMCT) processes.

Electronic structure, polaron formation, and functional properties in transition-metal tungstates

Transition-metal tungstates MWO₄ (M = Co, Ni, Cu, Zn) have applications in many areas, including supercapacitors. A good understanding of the electronic structure is essential to understanding their functional properties. Here, we report a first-principles study of the materials using hybrid density-functional calculations. The electronic structure is analyzed with a focus on the nature of the electronic states near the band edges. We find that hole polarons can form at the Co lattice site in CoWO₄ and the O site in NiWO₄, CuWO₄, and ZnWO₄, resulting in the formation of Co³⁺ in the former and O^- in the latter. The electrochemical activity observed in certain tungstate compounds, but not in others, appears to be related to the ability to form hole polarons on the transition-metal ions. The formation energy and migration barrier of the hole polaron in CoWO₄ are also calculated and the results are employed to understand the reported p-type conductivity.

Nanostructured Ternary Metal Tungstate-Based Photocatalysts for Environmental Purification and Solar Water Splitting: A Review

Visible-light-responsive ternary metal tungstate (MWO₄) photocatalysts are being increasingly investigated for energy conversion and environmental purification applications owing to their striking features, including low cost, eco-friendliness, and high stability under acidic and oxidative conditions. However, rapid recombination of photoinduced electron-hole pairs and a narrow light response range to the solar spectrum lead to low photocatalytic activity of MWO₄-based materials, thus significantly hampering their wide usage in practice. To enable their widespread practical usage, significant efforts have been devoted, by developing new concepts and innovative strategies. In this review, we aim to provide an integrated overview of the fundamentals and recent progress of MWO₄-based photocatalysts. Furthermore, different strategies, including morphological control, surface modification, heteroatom doping, and heterojunction fabrication, which are employed to promote the photocatalytic activities of MWO₄-based materials, are systematically summarized and discussed. Finally, existing challenges and a future perspective are also provided to shed light on the development of highly efficient MWO₄-based photocatalysts.

Ultrafast NH3 Sensing Properties of WO3@CoWO4 Heterojunction Nanofibres at Room Temperature

Highly selective detection, quick response times (<5 s), and superior response (|Rn – Ra|/Ra = 1.17) to NH3 gas, particularly at room temperature (RT), are still enormous challenges in gas sensor applications. In this paper, a rational design and facile synthesis for a NH3 sensor have been proposed. Massage ball-like WO3@CoWO4 (Co-W) nanofibres (NFs) were prepared by a facile one-step synthesis utilising an electrospinning approach, followed by appropriate calcination. A Co-W NF sensor with a Co-to-W atomic ratio of 3 : 10 (Co-W-3), which consisted of nano-sized WO3 protrusions (10–15 nm) on submicrometre-sized single crystal CoWO4 particles (100–150 nm) exhibited excellent gas-sensing properties at RT due to the single crystal CoWO4–CoWO4 homojunction structure and distinct massage ball-like WO3–CoWO4 heterojunction. The approach developed in this work will be important for the low-cost and large-scale production of a Co-W-3 ultrafast sensing material with highly promising applications in gas sensors.

Charge transport, interfacial interactions and synergistic mechanisms in BiNbO4/MWO4 (M = Zn and Cd) heterostructures for hydrogen production: insights from a DFT+U study

In the 21st century, the growing demand of global energy is one of the key challenges. The photocatalytic generation of hydrogen has attracted extensive attention to discuss the increasing global demand for sustainable and clean energy. However, hydrogen evolution reactions normally use the economically expensive rare noble metals and the processes remain a challenge. Herein, low-cost BiNbO₄/MWO₄(010) heterostructures are studied for the first time to check their suitability towards photocatalytic hydrogen production. A theoretical study with the aid of density functional theory (DFT) is used to investigate the synergistic effect, ionisation energy, electron affinities, charge transfer, electronic properties and the underlying mechanism for hydrogen generation of BiNbO₄/MWO₄(010) heterostructures. The experimental band gaps of bulk ZnWO₄, CdWO₄ and BiNbO₄ are well reproduced using the DFT+U method. The calculated band edge position shows a type-II staggered band alignment and the charge transfer between BiNbO₄ and MWO₄ monolayers results in a large interfacial built-in potential, which will favour the separation of charge carriers in the heterostructures. The effective mass of the photoinduced holes is higher compared to the electrons, making the heterostructures useful in hydrogen production. The relatively low ionisation energy and electron affinity for the heterostructures compared to the monolayers make them ideal for photocatalysis applications due to their small energy barrier for the injection of electrons and creation of holes. The BiNbO₄/MWO₄(010) heterostructures are more suitable for photocatalytic hydrogen production due to their strong reducing power relative to the H⁺/H₂O potential. This study sheds light on the less known BiNbO₄/ZnWO₄(010) heterostructures and the fully explored electronic and optical properties will pave way for future photocatalytic water splitting applications.

Structural evolution, growth mechanism and photoluminescence properties of CuWO4 nanocrystals

Copper tungstate (CuWO₄) crystals were synthesized by the sonochemistry (SC) method, and then, heat treated in a conventional furnace at different temperatures for 1h. The structural evolution, growth mechanism and photoluminescence (PL) properties of these crystals were thoroughly investigated. X-ray diffraction patterns, micro-Raman spectra and Fourier transformed infrared spectra indicated that crystals heat treated and 100°C and 200°C have water molecules in their lattice (copper tungstate dihydrate (CuWO₄·2H₂O) with monoclinic structure), when the crystals are calcinated at 300°C have the presence of two phase (CuWO₄·2H₂O and CuWO₄), while the others heat treated at 400°C and 500°C have a single CuWO₄ triclinic structure. Field emission scanning electron microscopy revealed a change in the morphological features of these crystals with the increase of the heat treatment temperature. Transmission electron microscopy (TEM), high resolution-TEM images and selected area electron diffraction were employed to examine the shape, size and structure of these crystals. Ultraviolet-Visible spectra evidenced a decrease of band gap values with the increase of the temperature, which were correlated with the reduction of intermediary energy levels within the band gap. The intense photoluminescence (PL) emission was detected for the sample heat treat at 300°C for 1h, which have a mixture of CuWO₄·2H₂O and CuWO₄ phases. Therefore, there is a synergic effect between the intermediary energy levels arising from these two phases during the electronic transitions responsible for PL emissions.

Unexpected formation of scheelite-structured Ca1-xCdxWO4 (0 ≤ x ≤ 1) continuous solid solutions with tunable photoluminescent and electronic properties

Design of a solid solution with tunable functionality is an attractive strategy toward realizing novel devices with multi-functionalities. In this work, a series of Ca_1-xCd_xWO₄ solid solutions in the entire range 0 ≤ x ≤ 1 with tetragonal scheelite structure have been successfully prepared for the first time. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Fourier transform Raman (FT-Raman) spectroscopies indicated that all the nanocrystals have a tetragonal scheelite structure without wolframite phase. Structural refinement data revealed that the lattice volume decreased with the replacement of Ca²⁺ by Cd²⁺ ions. UV-Vis diffuse reflectance spectra indicated that optical band gap reduced with the replacement of Ca²⁺ by Cd²⁺ ions. Scanning electron microscopic (SEM) images showed that morphologies of the nanocrystals changed with the chemical compositions. The structure evolution of the solid solutions was further investigated by high-resolution transmission electron microscopy (HRTEM). Moreover, the influence of chemical compositions on the photoluminescent and electric performance has been performed and discussed. The reported synthetic approach and findings reported here are important to understand the structure and structure-property relation of scheelite-structured tungstate and molybdate compounds, which has potential applications in the design of other kinds of novel functional materials.