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

In order to get an interfacial layer providing reversible and fast ionic and electronic exchange between metallic contact and Na+ ion-conducting sensor membrane, thin films of Fe-doped sodium aluminosilicate glass prepared by RF co-sputtering of the host glass and metallic iron have been investigated. It was found that non-reactive (Ar+) and reactive (Ar+/O2+) sputtered layers exhibit drastically different transport and sensor properties in accordance with Fe-57 conversion electron Mossbauer spectroscopic study of the local environment of iron in the films obtained. The main part of iron in the non-reactive sputtered material forms small Fe particles or clusters of 2 to 4 nm in diameter. These particles dispersed in the insulating glassy matrix cause an enormous increase of the conductivity by 9 to 10 orders of magnitude with increasing 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. As-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 response but only in concentrated NaCl solutions and with strongly reduced slope (20-30 mV/pNa). Small concentrations of iron do not disturb sodium ion-exchange between solution and thin film. Na-22 tracer measurements of sodium uptake and loss for the obtained samples are in accordance with the sensor properties observed. It can be concluded that properly 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.