Influence of surface roughness on the response time of iridium oxide pH sensors

Abstract

Iridium oxide (IrOx) formed by the carbonate melt oxidation method has been reported to exhibit excellent pH response characteristics. Although slope of the response and long-term response stability have been reported in relation to fabrication conditions and composition, the response kinetics of IrOx pH sensors have not been investigated in detail. The IrOx layers were shown to have a columnar structure with high aspect ratio, which resulted in deep recesses in the IrOx surface. In the present study, the influence of the surface roughness of IrOx on the response time of IrOx pH sensors is investigated. The 90 % response times of the sensors were found to be in the order of several hundreds of milliseconds and became longer as the IrOx layer became thicker and as the surface recesses became deeper. The results indicated that the diffusion of ions in the recessed structures strongly influenced the response time of the IrOx pH sensors. It is therefore important to lower the density of the recessed structures in the surface of the IrOx layers oxidized in lithium carbonate melts to achieve IrOx pH sensors that display rapid and reproducible responses.