FE-DOPED SODIUM ALUMINOSILICATE THIN-FILMS - CONDUCTIVITY, MICROSTRUCTURAL ORGANIZATION AND SENSOR PROPERTIES

In order to get an interfacial layer providing reversible and fast ion ic and electronic exchange between metallic contact and Na+ ion-conduc ting sensor membrane, thin films of Fe-doped sodium aluminosilicate gl ass prepared by RF co-sputtering of the host glass and metallic iron h ave been investigated. It was found that non-reactive (Ar+) and reacti ve (Ar+/O2+) sputtered layers exhibit drastically different transport and sensor properties in accordance with Fe-57 conversion electron Mos sbauer spectroscopic study of the local environment of iron in the fil ms obtained. The main part of iron in the non-reactive sputtered mater ial forms small Fe particles or clusters of 2 to 4 nm in diameter. The se particles dispersed in the insulating glassy matrix cause an enormo us increase of the conductivity by 9 to 10 orders of magnitude with in creasing Fe content. On the other hand, room-temperature conductivity of reactive sputtered films is by a factor of 10(5) to 10(7) less than that of non-reactive sputtered samples. Both as-prepared and annealed non-reactive sputtered layers with an iron content from 3 to 12 at.% exhibit fast and reproducible redox response comparable with that of a Pt electrode. At smaller Fe concentration, redox response is hindered by low electronic conductivity. At higher iron content, oxidative and corrosion-induced phenomena affecting redox response were observed. A s-prepared films reveal no Na+ sensitivity even after conditioning in NaCl solutions for at least two weeks. Annealed non-reactive sputtered layers with 3-4 at.% Fe exhibit fast and reproducible sodium ion resp onse but only in concentrated NaCl solutions and with strongly reduced slope (20-30 mV/pNa). Small concentrations of iron do not disturb sod ium ion-exchange between solution and thin film. Na-22 tracer measurem ents of sodium uptake and loss for the obtained samples are in accorda nce with the sensor properties observed. It can be concluded that prop erly prepared and annealed films with comparable ionic and electronic conductivity, and ionic and electronic exchange current density at the interfaces are promising materials for application as an intermediate layer of an all-solid-state potentiometric sodium sensor.