Effect of the electrode material on the oxygen reduction at the nonstoichiometric lanthanum fluoride/electrode interface

The voltammetry method with a linear potential scan is used for investigating the effect the electrode material (Ni, Co, electrodes on the basis of cobalt oxides modified with carbon) exerts on the reduction of gaseous oxygen at interfaces solid fluoride-conducting electrode LaF₃:Eu²⁺/electrode, O₂, and conjugated processes. Properties of the modified electrodes are characterized by the impedance spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy methods. The oxygen reaction is irreversible at the LaF₃:Eu²⁺|Ni (or Co) interfaces. At the interface of LaF₃:Eu²⁺ with modified electrodes Co (C n at %), where n = 5 and 9, mobile forms of oxygen are reversible and the reduction of gaseous and chemisorbed oxygen is controlled by diffusion with different effective kinetic parameters.     

Electrochemical Behavior of the LaF3:Eu2+/Me Interface (Me = Sm,Ce, Gd,V, Ag)

The electrochemical behavior of the LaF₃ : Eu²⁺| interface (Me = Sm, Ce, Gd, V, Ag) is studied using stripping voltammetry. Current–voltage curves for cells Ag, AgCl|KCl and KF|LaF₃ : Eu²⁺|Sm (or Ce) may be viewed as background curves for analyzing solid-phase reactions with other metallic electrodes. The substrate material is found to affect properties of interfaces LaF₃ : Eu²⁺/Gd (or V), which are characterized either by solid-phase reactions involving the fluoride ion of a solid electrolyte or by reactions involving oxygen. Regularities, obtained for silver in contact with LaF₃ : Eu²⁺, link electrochemical signals of the reaction of the silver fluoride formation with the contact area.     

Galvanostatic investigation of electrochemical processes at the LaF<Subscript>3</Subscript>/M (M: Ag,Bi) interface

The electrochemical processes at the interface between solid fluorine-conducting electrolyte LaF₃(Eu²⁺ 0.8 mol %) and silver or bismuth electrodes in the two-electrode cell with nonpolarizable reference electrode are studied using the galvanostatic method. The anodic galvanostatic transients of LaF₃: Eu²⁺/Ag and LaF₃: Eu²⁺/Bi interfaces are linearized on the log(η ? η_max), vs. t coordinates, i.e. the rate of LaF₃|MF _n |M electrode formation is limited by slow surface diffusion of metal adions. The initial portions of cathodic galvanostatic transients in the range of solid-electrolyte lanthanum reduction are approximated by the linear dependence of η on log(1 ? √t/τ). The plots of logI vs. 1/η are linear both for the lanthanum reduction and for silver and bismuth oxidation involving mobile fluoride ion of solid electrolyte, which is typical for two-dimensional growth of new phase.     

Effect of the Electrode Material on the Reduction of La<Superscript>3+</Superscript> and Ce<Superscript>3+</Superscript> in Fluoride-Conducting Solid Electrolytes

The reduction of immobile cations La³⁺ and Ce³⁺ in fluoride-conducting solid electrolytes (FSE) LaF₃ (Eu²⁺ 0.8 mol %), LaF₃ (Sr²⁺ 5 mol %), and CeF₃ (Sr²⁺ 5 mol %) in contact with Ag, Bi, Si, La, Ce, and Sm working electrodes is studied by chronoamperometry and voltammetry with linear potential scan. Discovered is linear dependence of initial segments of potentiostatic transients of cathodic current on t ^1/2 at FSE interfaces with Ag, Bi, La, Ce, and Sm. The dependence is due to diffusion-controlled instantaneous nucleation of Ln and Ce. The La³⁺ and Ce³⁺ reduction at the FSE/Ag interface is reversible in a narrow region. The reduction and oxidation of La³⁺ and Ce³⁺ (cations of the FSE rigid lattice) at the FSE/Me (Me = La, Ce and Sm, Bi, Si) interface is irreversible and involves a chemical reaction.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 662–672.Original Russian Text Copyright © 2005 by Turaeva, Kot, Urchukova, Murin.     

Electrochemical formation of a low-dimensional phase on the boundary between the LaF<Subscript>3</Subscript>:Eu<Superscript>2+</Superscript> single crystal and the metal

The kinetics of the formation of a new phase at the interface between the LaF₃:Eu²⁺ single crystal and the (Sn, Bi, or Sb) metallic electrodes was studied using potentiostatic transient measurements and voltammetry with a linear variation of voltage. A comparison of the theoretical and experimental reduced transients that define two-dimensional instantaneous nucleation on a plot of I/I _m vs. t/t _m and three-dimensional growth of the instantaneous and progressive types of nucleation on a plot of I ²/I _m² vs. t/t _m showed that the model was not fully consistent with the experiment. The dependence of the stationary current logI _A(max) of the potentiostatic transients on 1/η during the formation of the intermediate phase on the boundaries of LaF₃:Eu²⁺|Sn and LaF₃:Eu²⁺|Bi was found to be linear, which corresponds to two-dimensional nucleation and growth of the new phase.     

Voltammetry with a Fluoride-Selective Electrode with Solid-Phase Reference System

Experimental results obtained in a study of the voltammetric response of an all-solid fluoride-selective electrode based on LaF₃ (Eu²⁺ 0.8 mol %), LaF₃ (Sr²⁺ 5 mol %) and CeF₃ (Sr²⁺ 5 mol %) single crystals brought in contact with Ag, Bi, and Sn metal samples are presented. The method of cathodic inversion voltammetry was applied to study the reduction of La³⁺ and Ce³⁺-cations from the rigid sublattice of solid electrolytes, which determines the threshold of the electrochemical stability of a membrane. Anodic inversion voltammetry was used to investigate the characteristics of solid-phase generation of metal fluorides at the interface between the fluoride-selective electrode and metals.     

Ein Beitrag zur Strukturchemie der Selten-Erd-Trifluoride

Some Aspects of the Structural Chemistry of the Rare Earth Trifluorides The preparation and characterization of the rare earth trifluorides crystallizing in the hexagonal LaF₃-type and the orthorhombic YF₃-type is described. The lattice parameters were derived from GUINIER powder data of high precision. These lattice parameters served to calculate the effective ionic radii of the tripositive rare earth ions for coordination numbers 8 and 9. It is shown that both structure types can be stabiblized by the formation of nonstoichiometric compounds of the general formula (Me, RE)F_x (Me ? Eu²⁺, Sr²⁺ and RE ? Eu³⁺, Gd³⁺) with x varying approximately from 2,77 to 2,94 for the hexagonal and from 2,94 to 3,00 for the orthorhombic nonstoichiometric phases. For the pure trifluorides and the nonstoichiometric phases a critical radius ratio of 0.830±0.008 was found for the changeover from the LaF₃- to the YF₃-structure.     

Cyclic Voltammetry of a Fluoride-Selective Electrode in Contact with Metals

Electrochemical behavior of the interface between a LaF₃ : Eu²⁺ single-crystal fluoride-selective membrane and Bi, Sn, Pb, Sb, and Cd metallic microelectrodes is considered with the aim to apply a fluoride-selective electrode in solid-state voltammetry. The ionization-discharge processes on the metals in air, involving mobile fluoride ions from the LaF₃ membrane, were studied by cyclic voltammetry.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 2, 2005, pp. 265–272.Original Russian Text Copyright © 2005 by Turaeva, Pegova, Vasilevskii.     

Kinetics of intermediate phase growth at the interface of fluoride-conducting solid electrolytes with metal electrodes

The effect of the reference electrode (Ag, AgCl∣KF + KCl or Sn, SnF₂) on the kinetics of new phase growth at interfaces formed by LaF₃:Eu²⁺ or CeF₃:Sr²⁺ single crystal solid electrolytes with metal electrodes of Ag and Bi is studied by chronoamperometry and linear scan voltammetry. It is shown that when a solid-phase reference electrode is used, the current transients reflect hindrances associated with instantaneous two- or three-dimensional nucleation as well as limitations for fluoride ion diffusion in the solid electrolyte.     

Thermal annealing of LaF3:Eu3+ nanocrystals synthesized by a solvothermal method and their luminescence properties

In this work, we used a low temperature solvothermal method to synthesize Eu³⁺-doped LaF₃ (LaF₃:Eu³⁺) nanocrystals. The effect of thermal annealing on their phase structures and luminescence properties was studied. Transformation from LaF₃ to LaOF was observed after the annealing, and the initial transformation process was studied using a rapid thermal annealing technique. It was found that a sufficiently high annealing temperature is required for the transformation of LaF₃ to LaOF. LaOF phase started to be formed after annealing at 500 °C for as short as 5 min, and higher annealing temperatures and longer annealing time led to a larger amount of LaOF formed. With the increase of the formation of LaOF, the luminescence was greatly enhanced. Strong O^2? → Eu³⁺ charge transfer band was present in these samples annealed at 500 °C and higher temperatures, and greatly enhanced ⁷F₀ → ⁵D₂ transition of Eu³⁺ was also observed.     

Synthesis of highly fluorescent LaF3:Ln/LaF3 core/shell nanocrystals by a surfactant-free aqueous solution route

A surfactant-free aqueous solution route has been established for the synthesis of LaF₃:Ln³⁺/LaF₃ core/shell nanocrystals (Ln=Ce, Tb, Nd) heated at 75 °C at ambient pressure. All the as-prepared nanocrystals with spherical shape have an average size around 20 nm, and consist of well crystallized hexagonal phases. The X-ray photoelectron spectra was used to confirm that the LaF₃ shells have coated the LaF₃:Ce³⁺, Tb³⁺ cores. Compared with that of the original cores under the same conditions, the emission intensity of the LaF₃:Ce³⁺, Tb³⁺/LaF₃ and LaF₃:Nd³⁺/LaF₃ core/shell nanocrystals increased significantly of 120% and 60%, respectively. The quantum yield of the LaF₃:Ce³⁺, Tb³⁺/LaF₃ core/shell nanocrystals reached about 27% in aqueous solution. These results indicate that a significant reduction of the quenching from the surface of the core nanocrystals can be obtained by the synthesis of the core/shell structures, and this method can provide more desirable lanthanide-doped nanocrystals for potential biological applications.     

Crystal structure of polymer hexaaqua-hexakis(2-thiobarbiturato)dieuropium(III)

Complex [Eu₂(HTBA)₆(H₂O)₆] _n (I), where H₂TBA is 2-thiobarbituric acid C₄H₄N₂O₂S, is synthesized. Its structure is determined by X-ray diffraction analysis (CIF file CCDC 987519). The crystals of complex I are monoclinic: a = 14.1033(4) Å, b = 10.0988(4) Å, c = 15.4061(5) Å, β = 110.003(1)°, V = 2061.9(1) ų, space group P2/n, Z = 2. All three independent ligands HTBA^? are coordinated to Eu³⁺ through oxygen atoms. Six HTBA^? ions (two terminal and four bridging) and two water molecules are coordinated to one of the independent Eu³⁺ ions. The second Eu³⁺ ion is bound to four bridging HTBA^? ions and four water molecules. The coordination polyhedra are square antiprisms. The bridging HTBA^? ions join the antiprisms into layers. The structure is stabilized by numerous hydrogen bonds and the π-π interaction between HTBA^?.     

Cyclic voltammetric investigation of Eu<Superscript>3+</Superscript> on a MWCNTs/SDS-modified glassy carbon (GC) electrode

Electrochemical reduction behavior of Eu³⁺ on a multi-walled carbon nanotubes (MWCNTs)/sodium lauryl sulfate (SDS) (MWCNTs/SDS)-modified glassy carbon (GC) electrode was investigated by cyclic voltammetry (CV). Results indicated that the electrochemical reduction process of Eu³⁺ at the MWCNTs/SDS-modified GC electrode is a quasi-reversible and diffusion-controlled process. The value of standard rate constant (k _s) at the MWCNTs/SDS-modified GC electrode was estimated to 1.96 × 10⁻² cm s⁻¹. CV studies showed that the electrochemical response of Eu³⁺ was directly related to the ratio of MWCNTs to SDS, and the tube diameter of MWCNTs had a slight influence on the electrochemical behavior of Eu³⁺, whereas the tube length. of MWCNTs had a strong influence. CVs results also proved that s-MWCNTs (with shorter tube length)-modified GC electrode showed better response to the electrochemical reaction of Eu³⁺.     

Eu(fod)(3) binding on the (1)H-NMR spectra of some benzo[3n]crown-n and dibenzo[3n+2]crown-n

The complexing of Eu(fod)₃ with macrocyclic ethers, benzo[15]crown-5, benzo[12]crown-4, dibenzo[20]crown-6, dibenzo[23]crown-7 and dibenzo[26]crown-8 was observed on their ¹H-NMR spectra and the selective binding constants at 400 MHz and 305 K in CDCl₃ were reported. The Eu(fod)₃ interaction displayed the selective binding role of oxygen on macrocyclic, H₂COCH₂, backbones with o- or m-dioxyphenyl groups referring the ¹H chemical shifts. The estimated equilibrium constants, K_a of 1:1 ratio of interactions were in accordance with the Eu³⁺ ionic radii to bind the oxygen sites depending on the macrocyclic size and conformation of the ethers. The minimum lanthanide-macrocyclic ether distance displayed the maximum stability so that benzo[3n]crown-n (n=4, 5) group was found to bind the Eu(fod)₃ moderately whilst dibenzo[3n+2]crown-n (n=6-8) oligomer chemical shifts were induced largely since the such Eu³⁺ complexes are more stable with larger ethyleneoxy groups.     

Crystal structure and luminescence property of novel three-dimensional network of {Eu2(p-BDC)3(Phen)2(H2O)2}n

The single crystal of a supramolecule, {Eu₂(p-BDC)₃(Phen)₂(H₂O)₂}_n (p-BDC=1,4-benzenedicarboxylate), with characteristic luminescence of Eu³⁺ was obtained by means of soft chemistry. The crystal structure determination reveals that each Eu³⁺ ion is coordinated by five oxygen atoms of p-BDC anions, one oxygen atom from water molecule, and two nitrogen atoms of Phen, respectively, resulting in an eight-coordinated Eu³⁺ center and a distorted square antiprism coordination polyhedron. Four bridges, two carboxylates of μ₄-p-BDC and two of μ₃-p-BDC, connect two Eu atoms into a binuclear unit. Moreover, the μ₃-p-BDC integrates the binuclear building blocks at the direction of b axis and the μ₄-p-BDC polymerizes the structure roughly along the direction of the sum vector of axis b and c, respectively, forming two-dimensional layers. Hydrogen bonds between layers make the structure a three-dimensional network. The luminescence spectra measured under 77 K demonstrate the antenna effect of Phen and the ⁵D₁→⁵D₀ energy transfer path within Eu³⁺ ion. Both luminescence spectra and crystal structure lead to the conclusion that the local symmetry around the Eu³⁺ ion is C₁ and that more than one Eu³⁺ ion sites having slight environmental difference are present.     

Significant enhancement of luminescence intensity of CaTiO3:Eu3+ red phosphor prepared by sol–gel method and co-doped with Bi3+ and Mg2+

In this study, red phosphors Ca_1?n Mg _n TiO₃:Eu³⁺,Bi³⁺ were prepared by the sol?Cgel method and the impact of single dopant, co-dopants and solid solutions on the photoluminescence of the samples has been also investigated. Our results show that the crystal structure of the host does not have distinct changes when doped with Eu³⁺, Bi³⁺ and/or Mg²⁺. The emission intensity at 615?nm of Eu³⁺ increased at the presence of Bi³⁺ ions owing to the energy transfer from Bi³⁺ ion to Eu³⁺ ion. Moreover, with the addition of Mg²⁺, the red emission of the phosphor was further enhanced due to the stronger absorption at 399 and 467?nm, which match well with the emission of near-UV (395?C400?nm) and blue-LED (450?C470?nm), respectively. Under the near-UV (399?nm) or blue light (467?nm) excitation, the fluorescence quantum yield of the optimal composition Ca_0.9Mg_0.1TiO₃:0.18Eu³⁺,0.018Bi³⁺ is 0.36 and 0.41, respectively, which possesses the higher photoluminescence intensity than CaMoO₄:0.2Bi³⁺,0.05Eu³⁺ and the commercially available Y₂O₂S:Eu³⁺ phosphors under near-UV excitation. Based on these results, we are currently considering the potential application of Ca_0.9Mg_0.1TiO₃:Eu³⁺,Bi³⁺ as a near-UV or blue-chip convertible red-emitting phosphor.     

Spectroelectrochemistry of EuCl3 in Four Molten Salt Eutectics; 3 LiCl−NaCl, 3 LiCl−2 KCl,LiCl−RbCl,and 3 LiCl−2 CsCl; at 873 K

Key electrochemical properties affecting pyroprocessing of nuclear fuel were examined in four eutectic melts using Eu^3+/2+ as a representative probe. We report the electrochemical and spectroelectrochemical behavior of EuCl₃ in four molten salt eutectics (3 LiCl?NaCl, 3 LiCl?2 KCl, LiCl?RbCl and 3 LiCl?2 CsCl) at 873 K. Cyclic voltammetry was used to determine the reduction potential for Eu^3+/2+ and the applied potentials for spectroelectrochemistry. Single step chronoabsorptometry and thin‐layer spectroelectrochemistry were used to obtain the number of electrons transferred, reduction potentials and diffusion coefficients for Eu³⁺ in each eutectic melt. The reduction potentials determined by thin‐layer spectroelectrochemistry were essentially the same as those obtained using cyclic voltammetry. The diffusion coefficient for Eu³⁺ was the largest in the 3 LiCl?NaCl melt, showed a negative shift in the 3 LiCl?2 KCl melt, and was the smallest in the LiCl?RbCl and 3 LiCl?2 CsCl eutectic melts. The basic one‐electron reversible electron transfer for Eu^3+/2+ was not affected by melt composition.     

(Ca,Me)La4Si3O13:Eu3+(Me=Sr,Ba)荧光粉的结构和发光特性

采用高温固相法制备了(Ca,Me)La4Si3O13∶Eu3+(Me=Sr,Ba)系列红色荧光体,考察了Eu3+掺杂浓度和Sr2+,Ba2+置换对荧光体结构和发光特性的影响。Eu3+最佳掺杂浓度为nEu3+∶nLa3+=1∶7,5D0-7F2与5D0-7F1跃迁发射强度比为2.55。Eu3+掺杂使晶胞参数a和c呈线性变小,对c的影响大于a,使a/c比增大。Sr2+和Ba2+分别置换基质中的Ca2+可以形成完全固溶体,晶胞参数随Sr2+或Ba2+的置换量增加呈线性增大,使a/c比减小。各发射峰强度在Sr2+置换量为0.4 mol时出现极大值,但随Ba2+置换量的增加而不断增强,全置换后荧光强度最大。荧光体的色坐标为(0.638 5,0.353 0)。     

Reduction of Eu to Eu in MAl2Si2O8 (M=Ca, Sr, Ba) in air condition

In general, the reduction of Eu³⁺ to Eu²⁺ in solids needs an annealing process in a reducing atmosphere. In this paper, it is of great interest and importance to find that the reduction of Eu³⁺ to Eu²⁺ can be realized in a series of alkaline-earth metal aluminum silicates MAl₂Si₂O₈ (M=Ca, Sr, Ba) just in air condition. The Eu²⁺-doped MAl₂Si₂O₈ (M=Ca, Sr, Ba) powder samples were prepared in air atmosphere by Pechini-type sol-gel process. It was found that the strong band emissions of 4f⁶5d¹-4f⁷ from Eu²⁺ were observed at 417, 404 and 373 nm in air-annealed CaAl₂Si₂O₈, SrAl₂Si₂O₈ and BaAl₂Si₂O₈, respectively, under ultraviolet excitation although the Eu³⁺ precursors were employed. In addition, under low-voltage electron beam excitation, Eu²⁺-doped MAl₂Si₂O₈ also shows strong blue or ultraviolet emission corresponding to 4f⁶5d¹-4f⁷ transition. The reduction mechanism from Eu³⁺ to Eu²⁺ in these compounds has been discussed in detail.     

The preparation,characterization, photoelectrochemical and photocatalytic properties of lanthanide metal-ion-doped TiO2 nanoparticles

Lanthanide metal-ion-doped TiO₂ nanoparticles were prepared with hydrothermal method and characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma (ICP) and fluorescence spectrum. The results showed that a small part of metal ions entered into the lattice of TiO₂ and others adsorbed on the surface of TiO₂. The photoelectrochemical and photocatalytic properties of these lanthanide metal-ion-doped TiO₂ nanoparticles were investigated and the results showed that the photoresponse of Eu³⁺-, La³⁺-, Nd³⁺- and Pr³⁺-doped TiO₂ electrodes were much larger and that of Sm³⁺-doped TiO₂ electrode was a little larger than that of undoped TiO₂ electrode, indicating that the photogenerated carriers were separated more efficiently in Eu³⁺-, La³⁺-, Nd³⁺- and Pr³⁺-doped TiO₂ nanoparticles than in undoped TiO₂ nanoparticles. The photocatalytic degradation of rhodamine B (RB) was conducted in the suspension of lanthanide metal-ion-doped TiO₂ nanoparticles, and its first-order reaction rate constant (k) and average initial rate (r_ini) were significantly higher in the presence of Eu³⁺-, La³⁺-, Nd³⁺- and Pr³⁺-doped TiO₂ nanoparticles than those in the presence of undoped TiO₂ nanoparticles. The enhanced photocatalytic degradation rate of RB in the presence of Eu³⁺-, La³⁺-, Nd³⁺- and Pr³⁺-doped TiO₂ nanoparticles is attributed to increased charge separation in these systems. The effect of the content of La³⁺ on the reaction parameters (k and r_ini) was also investigated and the result showed that there was an optimal value (ca. 0.5%) of the content of La³⁺ to make the rate constant (k) and average initial rate (r_ini) reach the maxima.