Zn Al Cl Research Articles

Intercalation of Tetracyanoquinodimethane in [Zn–Al] Layered Double Hydroxide

The radical anion of tetracyanoquinodimethane has been intercalated into a layered double hydroxide by two different methods: constant pH coprecipitation and calcination–rehydration of a [Zn–Al–Cl] layered double hydroxide. The relationship between synthesis and characterization of products obtained is discussed.

Chloride-hydrogenophosphate ion exchange into the zinc–aluminium–chloride layered double hydroxide

The ion exchange of chloride by hydrogenophosphate in the [Zn–Al–Cl] layered double hydroxide was investigated by using X-ray diffraction and infrared spectroscopy. The effects of [Zn 2+/[AI 3+] ratio in [Zn–Al–Cl], anion concentration in the solution, aging time and temperature of the solution on the ion exchange were studied. The best sample in terms of crystallinity and extent of ion exchange was obtained by carrying out the exchange reaction at 25°C in a 0.01 M HPO 4 2− solution at pH 8 with at least 5 h of aging time. A sample prepared under these conditions was further characterized by X-ray microanalysis.

Anionic clay modified electrodes: electron transfer mediated by electroactive nickel, cobalt or manganese sites in layered double hydroxide films

A voltammetric wave was observed in an electrode modified with an MgMnCO3 layered double hydroxide (LDH). It was attributed to Mn(III)/Mn(IV) redox processes in the film. Iodometry showed that the LDH manganese centres, initially present in an average valence of 3.52 +, could be oxidized to a maximum of approximately 3.8 +, or reduced down to 3+. Reduction below 3 + did not occur even after several hours at negative potentials. Co(II) sites in a CoAlCl LDH could be oxidized up to a maximum of about 3.8 +. This oxidation was only reversible between 3.8+ and 3.0+. Complete reduction back to 2+ only occurred for CoAlCl films in which the average valence of the cobalt had never exceeded 3+. Much larger voltammetric waves were obtained for solutions of [Fe(CN)6]4−, [Mo(CN)8]4− or [Ru(CN)6]4− in redox active manganese, cobalt or nickel LDH films than in redox inactive ZnAlCl films. Improved charge transport in LDH films containing redox active metals allowed a much larger fraction of the absorbed anions to participate in the electrochemical reaction. When the potential of nickel, cobalt or manganese LDH-modified electrodes were scanned in mixtures of [Fe(CN)6]4− and [Ru(CN)6]4− or [Mo(CN)8]4−, electron transfer from [Fe(CN)6]4− to electrochemically oxidized [Ru(CN)6]3− or [Mo(CN)8]3−, mediated by the LDH electroactive metal centres was observed. No such transfer was found for redox inactive ZnAlCl LDH films.

Electrodes modified with synthetic anionic clays

Anionic clays Zn 2Cr(OH) 6C1·2H 2O, denoted [ZnCrCI], and Zn 3A1(OH) 8C1·2H 2O, denoted [ZnAlCl], are rapidly exchanged with electroactive organic sulfonate anion such as 2,2′-azinobis 3-ethylbenzothiazoline-6-sulfonate (ABTS). These samples are prepared by a simple exchange reaction or by the coprecipitation method. High degrees of exchange are obtained and pillar-like layered structures are formed. Clay-carbon paste modified electrodes have been produced using these materials and the precursor [ZnCrCl]. The electrochemical behaviour of the intercalated substrate has been studied using cyclic voltammetry. The electrogenerated radical of intercalated [ZnAl-ABTS] has been characterized by EPR.

Realization of a humidity sensor

A new type of humidity sensor based on the hydrotalcite-type protonic conductor Zn 2Al(OH) 6Cl. nH 2O (henceforth called [ZnAlCl]) deposited as a thick film by the screen-printing technique, is described. By using an original ink composed of a concentrated aqueous suspension of [ZnAlCl] powder, very adherent deposits with good resolution and levelling are achieved. X-ray patterns show a high orientation of the (001) reticular planes in a direction parallel to the substrate; this property must enhance conductivity values. The small dimensions of the sensor allow a 'hygrometric cell' to be realized, with which we can entirely characterize the sensor by means of both its calibration and response time measurements.