Structural characterization,energy transfer,and Judd–Ofelt analysis of pure and the Eu3+-doped Ca2LiMg2V3O12 phosphors

A novel vanadate host Ca₂LiMg₂V₃O₁₂ (CLMV) and the Eu³⁺-doped samples were synthesized via a solid-state reaction method. The phase formation and the morphological analysis were studied in detail. The Rietveld refinement result shows that the host belongs to cubic space group Ia-3d (230) with lattice parameter, a = 12.3948 Å, V = 1904.23 Å³, and Z = 8. The diffuse reflectance spectroscopy measurement estimated the bandgap of the host and the CLMV:0.05Eu³⁺ phosphors. The host exhibits a broad absorption band (peak at 345 nm) ranging from 240 to 380 nm, which is attributed to the charge transfer in the O²⁻–V⁵⁺ complex. Under near UV excitation (λ_exc = 345 nm), the host gives a broad emission band covering the visible region from 400 to 730 nm and the emission is in the bluish–green region of the CIE diagram. When the host is doped with the Eu³⁺ ions and excited at 345 nm, the emission spectrum depicts the superimposition of the characteristic emission bands (red emission) of the Eu³⁺ ions corresponding to the f–f transitions over the broad emission band of the host. The calculated color coordinates (9600 to 2280 K) demonstrated the color tuning ability of the phosphor as the dopant concentration is increased in the host. This is because the VO₄³⁻ group plays the sensitiser role and partially transfers energy with the Eu³⁺ ions. When the same set of phosphors were excited at the dominant characteristic excitation band (λ_exc = 394 nm) of the Eu³⁺, the characteristic emission bands of the Eu³⁺ in the orange–red region were observed. As the electric dipole transition of the Eu³⁺ was found to be dominant, the prepared phosphors possessed high color purity (CP). The energy transfer mechanism and the lifetime values were also presented. The temperature-dependent PL studies showed good thermal stability of the optimum sample. Various radiative transition properties were analyzed by the Judd–Ofelt theory. The photometric results reveal the color tuning ability and CP of the CLMV:xEu³⁺ phosphors.     

Active BiVO4 Swimmers Propelled by Depletion Gradients Caused by Photodeposition

Artificial active matter often self-propels by creating gradients of one or more species or quantities. For chemical swimmers, most frequently either O₂ or H⁺ that are created in certain catalytic reactions are causing the interfacial flows which drive the self-propulsion. While the palette of reactions is extending constantly, especially toward more bio-compatible fuels, the depletion of species is often overlooked. Here, the photodeposition of metal species on BiVO₄ micro swimmers is considered. During the photodeposition reaction, metal ions are removed from the solution creating a depleted region around the particle. The ability of this depletion to drive active motion of artificial micro swimmers, as well as the influences of different metal ions and counter ions on the motion are investigated and cross compared.     

Effect of chlorine-repulsive molecular fragments on photocatalytic seawater splitting performance of BiVO4 for oxygen evolution

Photocatalysis to produce clean energy by splitting seawater have great practical importance for dealing with the energy crisis. However, seawater contains many Cl^? ions whose oxidation competes with the oxygen evolution reaction. In this work, we present a photocatalyst modified with S-containing molecular fragments on the surface to improve its efficiency in the oxygen evolution reaction in seawater splitting. We found that the oxygen evolution performance of BiVO₄ modified with S-containing molecular fragments was 1.7 times higher than the unmodified material. Based on this finding, the modification didn't affect the light absorption and charge separation efficiency of BiVO₄, but lead to a marked (83.6%) decrease of the effective chlorine concentration in the reactor after photocatalytic reaction. The results indicate that the surface modification with S-containing molecular fragments is an effective method to repel chlorine. This work provides a useful reference to improve the efficiency of photocatalysts in overall seawater splitting.     

Thin Zinc Oxide Layer Passivating Bismuth Vanadate for Selective Photoelectrochemical Water Oxidation to Hydrogen Peroxide (Small 33/2023)

Selective photoelectrochemical (PEC) water oxidation to hydrogen peroxide is an underexplored option as opposed to the mainstream oxygen reduction reaction. Albeit interesting, selective H₂O₂ production via oxidative pathway is plagued by the noncontrollable two-electron transfer reaction and the overoxidation of the thus-formed H₂O₂ to O₂. Here, ZnO passivator-coated BiVO₄ photoanode is reported for selective PEC H₂O₂ production. Both the H₂O₂ selectivity and production rate increase in the range of 1.0–2.0 V versus RHE under simulated sunlight irradiation. The photoelectrochemical impedance spectra and open-circuit potentials suggest a flattened band bending and positively shifted quasi-Fermi level of BiVO₄ upon ZnO coating, facilitating H₂O₂ generation and suppressing the competitive reaction of O₂ evolution. The ZnO overlayer also inhibits H₂O₂ decomposition, accelerates charge extraction from BiVO₄, and serves as a hole reservoir under photoexcitation. This work offers insights into surface states and the role of the coating layer in manipulating two/four-electron transfer for selective H₂O₂ synthesis from PEC water oxidation.     

Structural Engineering of BiVO4/CoFe MOF Heterostructures Boosting Charge Transfer for Efficient Photoelectrochemical Water Splitting

Boosting charge separation and transfer of photoanodes is crucial for providing high viability of photoelectrochemical hydrogen (H₂) generation. Here, a structural engineering strategy is designed and synthesized for uniformly coating an ultrathin CoFe bimetal-organic framework (CoFe MOF) layer over a BiVO₄ photoanode for boosted charge separation and transfer. The photocurrent density of the optimized BiVO₄/CoFe MOF(NA) photoanode reaches a value of 3.92 mA cm⁻² at 1.23 V versus reversible hydrogen electrode (RHE), up to 6.03 times that of pristine BiVO₄, due to the greatly increased efficiency of charge transfer and separation. In addition, this photoanode records one onset potential that is considerably shifted negatively when compared to BiVO₄. Transient absorption spectroscopy reveals that the CoFe MOF(NA) prolongs charge recombination lifetime by blocking the hole-transfer pathway from the BiVO₄ to its surface trap states. This work sheds light on boosting charge separation and transfer through structural engineering to enhance the photocurrent of photoanodes for solar H₂ production.     

Synthesis,crystal structure,and characterization of Na2SrV4O12: A low-firing dielectric vanadate

In this work, cyclotetravanadate Na₂SrV₄O₁₂ was synthesized at a relatively low sintering temperature of ∼500°C using a solid-state reaction method. X-ray diffraction and a transmission electron microscope characterization featured a tetragonal structure that was built by a 3D frame of isolated tetracyclic (V₄O₁₂)⁴⁻. Dielectric measurements demonstrated strong dependence on frequency and temperature. A low relative permittivity of ε_r ∼ 8 ± 0.2 and a dielectric (loss tanδ) ∼ 0.4 ± 0.01 was achieved at a frequency of 10 kHz and room temperature. ac impedance and conductivity analysis revealed a thermally activated migration behavior of charge carriers with a short-range hopping feature. XPS analysis validated the existence of oxygen vacancy and reduction in vanadium (from V⁵⁺ to V⁴⁺), which gave rise to charged lattice defects. The migration or hopping of such charged defects was responsible for the observed electrical behaviors. Owing to the simple composition, inexpensive raw materials and low density (2.99 g/cm³) make Na₂SrV₄O₁₂ ceramic a potential candidate for lightweight devices and in photocatalytic degradation and all-solid-state ion batteries.