Fabrication method and characterization of electrodeposited and heat-treated iridium oxide films for pH sensing

Abstract

A newly developed pH electrode that is formed on a glass substrate is demonstrated. Its fabrication involves the electrodeposition of iridium oxide (IrOx) onto a Au thin-film at a constant current of 0.01 mA, followed by sequential electrode heat treatment at 400 degrees C. The prepared electrode exhibited a near-Nernstian pH response of 59.5 (+/- 1) mV/pH, comparable to the theoretical sensitivity (59 mV/pH); this characteristic is highly desirable as it allows for the electrode predictable in measuring potentials when considering that IrOx electrodes prepared by a hydrated method are known to exhibit a super-Nernstian pH response (from 59 to 80 mV/pH). Electrodeposition of IrOx produces a porous structure on the electrode surface, while IrOx film thickness is decreased by heat treatment; these processes may also affect the oxidation state of IrOx films. Cathodic charge storage capacities (CSCC) were measured by cyclic voltammetry as 72 and 48 mC/cm(2) for the non-heat-treated and heat-treated IrOx electrodes, respectively, corresponding to a CSCC value almost equivalent to the iridium amount, which is related to IrOx film thicknesses. Open-circuit potentials were measured to evaluate performances related to pH sensitivity, response time, and reversibility in the pH range of 2.38-11.61. Furthermore, individual electrodes prepared by the same fabrication process showed consistency in terms of sensitivity, with correlation coefficients over 0.999. (C) 2014 Elsevier B.V. All rights reserved.