Alain Wattiaux

Sonochemical Approach to the Synthesis of Fe3O4@SiO2 Core−Shell Nanoparticles with Tunable Properties

In this study, we report a rapid sonochemical synthesis of monodisperse nonaggregated Fe(3)O(4)@SiO(2) magnetic nanoparticles (NPs). We found that coprecipitation of Fe(II) and Fe(III) in aqueous solutions under the effect of power ultrasound yields smaller Fe(3)O(4) NPs with a narrow size distribution (4-8 nm) compared to the silent reaction. Moreover, the coating of Fe(3)O(4) NPs with silica using an alkaline hydrolysis of tetraethyl orthosilicate in ethanol-water mixture is accelerated many-fold in the presence of a 20 kHz ultrasonic field. The thickness of the silica shell can be easily controlled in the range of several nanometers during sonication. Mossbauer spectra revealed that nonsuperparamagnetic behavior of obtained core-shell NPs is mostly related to the dipole-dipole interactions of magnetic cores and not to the particle size effect. Core-shell Fe(3)O(4)@SiO(2) NPs prepared with sonochemistry exhibit a higher magnetization value than that for NPs obtained under silent conditions owing to better control of the deposited silica quantities as well as to the high speed of sonochemical coating, which prevents the magnetite from oxidizing.

A new superconductor obtained by electrochemical oxidation of La2CuO4

Abstract The electrochemical oxidation of La 2 CuO 4 in alkaline solution (1N KOH) has been used for preparing the superconducting phase La 2 CuO 4+δ (δ⋍0.07). Crystallographic data show an enhanced orthorhombic distortion and an increase of the unit cell volume. The transport properties reveal a sharp transition from a metallic behaviour to a superconducting state below 44 K which is corroborated by the noteworthy magnetic properties.

Transport and magnetic properties of the superconducting La2CuO4+δ phases (0 < δ < 0.09) prepared by electrochemical oxidation

Abstract A preliminary phase diagram is given for the system La 2 CuO 4+δ , o ≤ δ ≤ 0.09, prepared electrochemically at room temperature. Room-temperature X-ray data distinguish two orthorhombic phase fields, O I for δ M for δ > 0.055. The orthorhombicity of O I decreases with increasing δ, that of O M increases with δ. A spinodal phase segregation into an oxygen-rich superconductive phase and an oxygen-poor antiferromagnetic phase occurs in the temperature interval 220K T ≤ T sp ; below 220 K, the mobility of the interstitial oxygen becomes too low for a further static phase segregation. The phase segregation causes an increase in resistivity and a decrease in the Seebeck coefficient with decreasing temperature in the interval 220 T T sp . A similar transport behavior occurs in the range T c T T ϱ ≤ 120 K in samples 0.02 ≤ δ ≤ 0.04; from comparisons with the La 2− x Sr x CuO 4 system, we suggest that at lower temperatures this behavior is the signature for a further dynamic hole segregation, induced by atomic displacements, into domains more rich in mobile holes within the superconductive phase. The diamagnetic AC susceptibility associated with this superconductivity is independent of the magnetic field below 0.01 mT. In the compositional range 0.06 ≤ δ ≤ 0.09, the Seebeck data indicate a nearly constant concentration of itinerant holes in the superconductive CuO 2 planes. We conclude that in the O M phase oxygen-oxygen interactions within the superconductive layers trap out at oxygen-atom clusters the excess mobile holes Δ P = 2 δ −0.12 from the CuO 2 planes. An H c1 = 64 mT was obtained for δ = 0.09. A step in the resistivity drop at T c signals a temperature interval T c T T on within which the superconductive pairs are confined to small domains. Whether these domains represent a second superconductive phase correlated to additional oxygen ordering that fails to develop with increasing δ or should be considered a pair-fluctuation phenomenon is not resolved.

Electrochemical Oxygen Intercalation into Oxide Networks

The electrochemical oxidation of Sr 2 Fe 2 O 5 brownmillerite-type ferrite and La 2 M O 4 ( M = Ni, Cu) oxides is reviewed. The oxidation process appears as a very powerful method for preparing high oxidation states of transition-metal oxides. The characterization of the reaction products shows that they are well described as intercalation compounds of oxygenated anions. The fact that the intercalated species are the same as the host anions distinguishes these materials from most other intercalated compounds, in which guest species usually are chemically different from the host ions. The question of the nature and site of the intercalated species as well as that of their diffusion into the sample bulk is discussed on the basis of various characterizations and physical measurements as well as within the scope of recent considerations of the electronic distribution in oxides of transition elements in a high oxidation state.

A novel preparation method of the SrFeO3 cubic perovskite by electrochemical means

Abstract The SrFeO 3 cubic perovskite has until now been prepared under high pressure of oxygen (340 atm) at 550°C. A novel preparation method of this phase has been investigated using electrochemical oxidation of the SrFeO 2.50 brownmillerite. The starting material SrFeO 2.53 ± 0.02 has been characterized by powder X-ray diffraction, chemical analyses and 57 Fe Mossbauer spectroscopy. The electrochemical oxidation was carried out at a potential of 400 mV for 60 hours, at room temperature using a three electrode system in air with a solution of 1N, KOH as the electrolyte. The phase obtained after this electrochemical processing has been characterized. Powder X-ray diffraction shows a cubic unit cell (a c =3.845 A ) . The chemical analysis as well as Mossbauer spectroscopy data reveal the presence of only Fe(IV), leading to the formula SrFeO 3.00 ± 0.02 . Moreover the value of the quadrupole splitting ( Δ =0) accounts for a symmetrical octahedral site for Fe(IV) and obviously corroborates the cubic structure of this phase.

Microdomain texture and oxygen excess in the calcium-lanthanum ferrite: Ca2LaFe3O8

Abstract The analysis by TEM and electron diffraction of the anion-deficient perovskite Ca2LaFe3O8 confirms the model previously proposed by J. C. Grenier et al. (Mater. Res. Bull. 11, 1219 (1976) ) with a structure intermediate between perovskite and brownmillerite. The unit cell parameters are ∼√2ac, 3ac, √2ac (where ac is the cubic perovskite unit cell parameter). However, the unit cell is sometimes doubled along the b axis. When the sample is treated in air at temperatures around 1400°C, an oxidation process is observed and the unit cell becomes cubic (ac = 3.848(3) A). Nevertheless, electron diffraction investigations suggest the existence of a much more complex situation in which three-dimensional microdomains intergrow within one crystal. Each of these microdomains appears to have a structure clearly related to the low-temperature sample, but the superstructure is randomly found along each of the three cubic subcell directions (i.e., the unit cell √2ac, √2ac, 3ac alternates randomly with 3ac, √2ac, √2a, and with √2ac, 3ac, √2ac). High-resolution electron microscopy allows one to ascertain this microdomain texture of the real crystal.

Electrolytic Oxygen Evolution in Alkaline Medium on La1 − x Sr x FeO3 − y Perovskite‐Related Ferrites I . Electrochemical Study

Oxygen evolution by electrolysis of aqueous solutions has been studied on nonstoichiometric perovskite-related ferrites of composition La/sub 1-x/Sr/sub x/Fe/sub 1-tau//sup 3+/Fe/sub tau//sup 4+/O/sub 3-y/(O .. M(O/sup -//sub ads/ + H/sub 2/O is rate determining whereas at high potential it is the first electron transfer M(OH/sup -//sub ads/ ..-->.. M(OH)/sub ads/ + e/sup -/. This change with potential implied that the transport properties of the materials play a decisive role in the reaction mechanism.

O3–NaxMn1/3Fe2/3O2 as a positive electrode material for Na-ion batteries: structural evolutions and redox mechanisms upon Na+ (de)intercalation

The electrochemical properties of the O3-type NaxMn1/3Fe2/3O2 (x = 0.77) phase used as positive electrode material in Na batteries were investigated in the 1.5–3.8 V, 1.5–4.0 V and 1.5–4.3 V ranges. We show that cycling the Na cells in a wider voltage range do not induce a significant gain on long term cycling as the discharge capacities reached for the three experiments are identical after the 14th cycle. The structural changes the material undergoes from 1.5 V (fully intercalated state) to 4.3 V were investigated by operando in situ X-ray powder diffraction (XRPD) and were further characterized by ex situ synchrotron XRPD. We show that the low amount of Mn3+ ions (≈33% of total Mn+ ions) is enough to induce a cooperative Jahn–Teller effect for all MO6 octahedra in the fully intercalated state. Upon deintercalation the material exhibits several structural transitions: O′3 → O3 → P3. Furthermore, several residual phases are observed during the experiment. In particular, a small part of the O3 type is not transformed to P3 but is always involved in the electrochemical process. To explain this behaviour the hypothesis of an inhomogeneity, which is not detected by XRD, is suggested. All phases converge into a poorly crystallized phase for x ≈ 0.15. The short interslab distance of the resulting phase strongly suggests an octahedral environment for the Na+ ions. X-ray absorption spectroscopy and 57Fe Mossbauer spectroscopy were used to confirm the activity of the Mn4+/Mn3+ and Fe4+/Fe3+ redox couples in the low and high voltage regions, respectively. 57Fe Mossbauer spectroscopy also showed an increase of the disorder into the material upon deintercalation.

Electrolytic Oxygen Evolution in Alkaline Medium on La1 − x Sr x FeO3 − y Perovskite‐Related Ferrites II . Influence of Bulk Properties

The influence of the bulk properties upon the oxygen reaction in alkaline medium has been considered for La/sub 1-x/Sr/sub x/Fe/sub 1-tau//sup 3+/Fe/sub tau//sup 4+/O/sub 3-y/ phases (0.10 less than or equal to x less than or equal to 0.95). The evolution of electrocatalytic properties is discussed as a function of composition ratio (x) of Fe/sup 4+/ content (tau), and of vacancy concentration (y). The electrocatalytic activity goes through a maximum for x approx. = 0.90 and increases continuously with tau. The active sites of the reaction mechanism are Fe/sup 4+/ cations in high spin configuration located in surface fivefold coordinated sites of C/sub 4v/ symmetry; a seven-step mechanism is proposed involving the variation of both coordination and oxidation state of iron. It has been shown that the activity increases with the covalency of the Fe - O bonds in the bulk. From transport properties, it follows that n-type materials are the most efficient, and that the activity increases when the concentration c/sub e/ of delocalized electrons in the sigma* band decreases. This enhancement can also be associated with an increase in electron mobility.

Spin crossover or intra-molecular electron transfer in a cyanido-bridged Fe/Co dinuclear dumbbell: a matter of state†

The design of molecule-based systems displaying tuneable optical and/or magnetic properties under external stimuli has received a great deal of attention in the past few years. This interest is driven by the potential applications in high-performance molecule-based electronics in the areas of recording media, switches, sensors, and displays. As an example, three-dimensional Fe/Co Prussian blue compounds exhibit a concomitant change in magnetic and optical properties due to a temperature- or light-induced metal-to-metal electron transfer. The foregoing remarkable properties in Prussian blues prompted us to design soluble molecular fragments of these coordination networks through a rational building-block approach in order to mimic their properties on a single molecule. With a judicious choice of the ligands for the iron and cobalt molecular precursors, we prepared a dinuclear cyanido-bridged Fe/Co complex that exhibits an unexpected temperature-dependent spin crossover in the solid state while an intramolecular electron transfer triggered by protonation is observed in solution.