Search

International audience; A new luminescent composite film resulting from the dispersion of luminescent organic dyes in a single-layered hydroxide (SLH)-type inorganic matrix has been developed. Two fluorescent organic dyes emitting visible light upon blue LED excitation were investigated in this study: dicyanomethylene (DCM) and pyranine (HPTS). These dyes exhibit broad emission bands that cover a large part of the visible spectrum. The concept developed in our work consisted in keeping SLH in its wet form to ensure a good dispersion of the fluorescent dyes prior to immobilizing the hybrid materials in a silicone polymer to achieve luminescent composite films. We demonstrate that these coatings stacked upon each other and placed above a blue LED lead to white-light emission with suitable photometric parameters for applications in lighting or display devices: colour temperature of 5409 K and colour rendering index (CRI) of 81.

International audience; A series of hybrid metal (Zn, Cu, and Co) layered simple hydroxides (LSH) functionalized by surfactants, namely, dodecylsulfonate (DS0) or dodecylsulfate (DS–) and by tetrasulfonato metal (Cu, Co)-phthalocyanines, was synthesized by anionic exchange reactions and characterized by ancillary techniques. The electrochemical properties of these compounds were investigated by cyclic voltammetry in phosphate buffer solutions over a wide potential range including that suitable for oxygen reduction. The electroactivity of intralayer metal cations (Cu and Co) was evidenced as a function of the phosphate buffer pH. The redox properties of Co- and Cu-PcTS0 grafted in LSH (Zn, Co, and Cu) were also investigated in neutral pH solution, under argon, air, or O2 atmospheres. Interestingly, these hybrid compounds display an electrocatalytic behavior toward the reduction of oxygen. In particular with CoPcTS0-LSH, a sharp electrocatalytic reduction wave is observed at −0.40 V/Ag-AgCl with an intensity that depends on the oxygen concentration. The formation of a CoPc-O2 grafted species in the presence of oxygen was confirmed by electron paramagnetic resonance spectroscopy.

Through a topochemical oxidative reaction (TOR) under air, a β-Co(OH)2 brucite type structure is converted into a monometallic CoIICoIII–CO3 layered double hydroxide (LDH). The structural and morphological characterizations are performed using powder X-ray diffraction, Fourier-transformed IR spectroscopy, and scanning and transmission electron microscopy. The local structure is scrutinized using an extended X-ray absorption fine structure, X-ray absorption near-edge structure, and pair distribution function analysis. The chemical composition of pristine material and its derivatives (electrochemically treated) are identified by thermogravimetry analysis for the bulk and X-ray photoelectron spectroscopy for the surface. The electrochemical behavior is investigated on deposited thin films in aqueous electrolyte (KOH) by cyclic voltammetry and electrochemical impedance spectroscopy, and their capacitive properties are further investigated by Galvanostatic cycling with potential limitation. The charge capacity is found to be as high as 1490 F g−1 for CoIICoIII–CO3 LDH at a current density of 0.5 A g−1. The performances of these materials are described using Ragone plots, which finally allow us to propose them as promising supercapacitor materials. A surface-to-bulk comparison using the above characterization techniques gives insight into the cyclability and reversibility limits of this material.

A series of (CoxNi1−x)2Al–NO3 (0 ≤ x ≤ 1) Layered Double Hydroxides (LDH) has been synthesized by the coprecipitation method. Their chemistry, structure and morphology were characterized using Inductively Coupled Plasma Atomic Emission and Energy-Dispersive X-ray Spectroscopies, Powder X-Ray Diffraction, Fourier Transformed Infrared spectroscopy, Raman spectroscopy and Scanning Electron Microscopy. The electrochemical behaviors of the LDH were investigated by Cyclic Voltammetry, Electrochemical Impedance Spectroscopy and Galvanostatic Charge/discharge with Potential Limitation measurements. The relationship between chemical composition of the LDH compounds (i.e. substituted samples vs physical mixtures) and their electrochemical properties is discussed. Special attention was paid to i) the role of the electrolyte cation (0.1 M COH, C+ = K+, Li+ and Na+) and to ii) the contribution of (CoxNi1−x)2Al–NO3@graphene nanocomposites (x = 0, 0.5, 1) in improving electron transfer in the materials.

International audience; This work investigated in situ changes in the structure of the layered double hydroxides (LDH) during electrochemical processes as well as followed the ion motion related to the charge balancing process. Electrogravimetric measurements, using an electrochemical quartz crystal microbalance (EQCM), allowed one to monitor simultaneously changes in current and in microbalance frequency when electroactive films of Co2Al–NO3 LDH were subjected to voltammetry potential cycling in alkaline solutions (0.1 M COH, C = Li+, Na+, and K+). EQCM results showed a strong dependence of the electrochemical process on the nature of electrolyte cations. Operando X-ray diffraction measurements, carried out during potential cycling of Co2Al–NO3 films in KOH electrolyte, showed the diffusion of OH– species gradually replacing NO3–. Total X-ray scattering experiments recorded ex situ on Co2Al–NO3 after successive oxidation and reduction revealed the presence of a high level of structural disorders even in the pristine material. A quantitative analysis of the local distribution of the cations by the analysis of the pair distribution functions highlighted the presence of different Co sites with distinct modification of Co–OH distances in the first coordination shells after oxidation/reduction.

A series of monometallic Layered Double Hydroxides (LDH) using electroactive cation, i.e. divalent or trivalent cobalt, was prepared by Topochemical Oxidation Reaction (TOR) under O-2 atmosphere at 40 degrees C from pristine beta-Co(OH)(2) platelets. The oxidation state of the ill-defined layered materials was evaluated by coupling thermal measurements and chemical titration (iodometry). Their characterization by ancillary techniques was completed by the study of their magnetic behavior. The obtained magnetic moments suggest the presence of structural local deformation around the Co-II ions, unhomogeneous charge distribution yielding to clustering effects cannot be discarded. Their pseudo-faradic properties as supercapacitor in KOH solution was thoroughly investigated by using Cyclic Voltammetry (CV), Galvanostatic Cycling with Potential Limitation (GCPL) and Electrochemical Impedance Spectroscopy (EIS) techniques. As a function of the oxygen treatment, the relative amount of Co-II/Co-III was found to range into 5.3 and 13.3, which is unusually high when compared to classical LDH charge distribution. Pseudocapacitance as high as 1540 F g(-1) was obtained underlining a high percentage of Co-II, approximate to 40%, involved in electrochemical process. This high percentage is tentatively explained by an extended outer-active electrochemical surface which demonstrates that TOR is a quick and easy process to get a high pseudocapacitive performance.

International audience; Co2Al-ABTS layered double hydroxides and associated Co2Al-ABTS@graphene composite were prepared in one pot technique by in situ coprecipitation. The as-obtained materials were then fully characterized by means of Powder X-Ray Diffraction, Fourier Transformed InfraRed and Scanning Electron Microscopy confirming the intercalation of azino-bis(3-ethylbenzothiazoline-6-sulphonate) (ABTS) between the LDH layers. Their electrochemical properties, according to Cyclic Voltammetry and Electrochemical Impedance Spectroscopy data, were improved compared to Zn2Al-ABTS reference material. Co2Al-ABTS hybrid LDH was found to combine both electronic transfers: interlayer provided by the presence of ABTS and intralayer due to the Co redox species. Moreover, an improvement of electronic transfer between the LDH particles was further achieved by addition of graphene. The resulting composite assemblies were tested for the first time as oxygen bioelectrode based on bilirubin oxidase. This original approach gives rise to enhanced electroenzymatic currents (×2.5) for oxygen reduction at 0 V and pH 7.0 as regard to that obtained for the reference laccase/LDH-ABTS based bioelectrode at pH 5.5.

International audience; The electrochemical behavior of layered double hydroxides (LDHs) containing 3d metal is presented here by analyzing the recent research progress described in the literature. Electron transfer within these inorganic lamellar materials can be promoted by redox-active cation sites (nickel, cobalt, iron, or manganese) located within the layer structure, with a potential window varying between −0.1 and 0.8 V/SCE in aqueous electrolyte, depending on the nature of metal cations and on the LDH composition. Additives, such as metal complexes, metal oxides, or nanocarbons to form hybrid or composite LDH-based materials, can further improve these redox properties. The electrochemical behavior of those LDH materials and their hybrid derivatives and composites, investigated mainly by cyclic voltammetry and electrochemical impedance spectroscopy, is described. A special attention is paid on NiAl-LDH and CoAl-LDH, as illustrative examples of LDH electrochemistry.