J.P. Besse

Delamination and restacking of layered doublehydroxides

Layered double hydroxides recovered after exfoliation have been ncharacterized by solid state chemistry techniques. The nature of the recovered nmaterials is highly dependant on the drying process; when gently dried, a nwell-ordered phase is obtained, either by freeze-drying or reconstruction, nbut the material becomes amorphous after evaporation of the solvent. Delamination nwas used to prepare interstratified LDHs, making this technique a new route nto the formation of a wide range of tunable materials.

Rehydration of Zn-Al layered double hydroxides

Rehydration is shown to be straightforward for the reconstruction of polyoxometallate-pillared layered double hydroxides. Zn-Al hydrotalcite-like minerals were prepared with Zn/Al ratios of 1 to 5 by coprecipitation at pH 7. Good crystallinity was obtained for samples with Zn/Al ratios above 2. Thermal decomposition was achieved by calcining the samples at 300 to 900 °C. The calcined samples were exposed to decarbonated water, with or without hydrothermal treatment to evaluate reconstruction of the hydrotalcite-like minerals by rehydration. Restoration of the hydrotalcite-like structure was found to be independent of the Zn/Al ratios for samples calcined between 300 and 400 °C; however, asecond phase, aluminum hydroxide or zinc oxide, was generally detected. A spinel phase, formed during the calcination of samples at temperatures above 600 °C, inhibited reconstruction of the hydrotalcite-like phase. The rehydrated hydrotalcite-like minerals had Zn/Al ratios close to 2, irrespective of the chemistry of the starting material.

Structural approach and conductivity of lamellar hydroxides Zn2Cr(OH)6X.nH2O (x = anion) by xanes, exafs and X-ray diffraction

Abstract Powder diffractograms of Zn 2 Cr(OH) 6 X.nH 2 O (X = anion) are characteristic of lamellar and ill-organized materials. A structural approach has been investigated using XANES and EXAFS analysis of the K edge spectra of Cr, Zn and Br. Results of X-ray absorption analysis indicates an octahedral coordination for Cr and Zn and a delocalisation of X anion. At 293 K we measured conductivities in the range 3.10 −3 to 2.10 −6 Ω −1 . cm −1 . The fact that these materials can be obtained as strongly oriented thick layers suggests possible applications in microionic technology, particularly in the development of hydrogen sensors.

Realization of humidity sensors based on a screen-printed anionic clay

Abstract Anionic clays, also referred as lamellar-double hydroxides (LDHs) are well known to display a high ionic conductivity for a large variety of interlamellar anionic species. In previous works, it was shown that the conductivity was mainly protonic and closely related to the hydration state of the compound. Such phases can therefore be used as humidity sensors providing that a reliable device could be formed. A Zn0.67Al0.33(OH)2Cl0.33 · nH2O compound is studied. The forming of this material by the screen printing technique is optimized in order to obtain adherent thick layers with regular levelling and a good resolution on alumina substrates. Two devices are studied, one working on A.C. with a two electrodes geometry and the other on D.C. with a four electrodes geometry. The results about sensitivity, stability and response time measurements carried out on each device are compared and discussed.

Preparation, thermal behavior and characterization of a zinc-aluminium-bromate lamellar double hydroxide

Abstract A [Znue5f8Al] Lamellar Double Hydroxide intercalated with bromate anions was studied by TGA, FTIR and PXRD. This compound displays an unusual 1H stacking sequence for this kind of anionic species. By moderate heating (140°C), an expansion of the basal spacing from 9.29A to 9.52A is observed whereas with other interlamellar anions the structure habitually contracts by partial dehydration of the interlamellar domains. It is also shown that the bromate anions are never grafted on the hydroxylated framework.

Evolution related to hygrometry of two lamellar double hydroxide pillared structures [CuCrS04] and [ZnAlS04]

Abstract In Lamellar Double Hydroxide (LDH) intercalated by tetrahedral oxo-anions, several polytypes exist. In a previous publication, we have shown the influence of a moderate thermal treatment on the nature of the obtained polytypes. In this paper, we study by PXRD the isothermal evolution of the stacking sequence and basal spacing of the hydroxylated sheets related to relative humidity of ambient atmosphere for two LDH compounds [Cuue5f8Crue5f8SO 4 ] and [Znue5f8Alue5f8S0 4 ]. Stacking sequences 3R,1 H and perhaps 2H are evidenced. Various basal spacings close to 7.2A, 8.2A, 8.8A, 11A and perhaps 17.6A are observed for the hydroxylated sheets.