Effects of Thermal Oxidation Cycle Numbers and Hydration on IrOx pH Sensor

Iridium/iridium oxide pH sensors were fabricated by cyclic isothermal heating and water quenching with cycle number equals to one, two and three, respectively. Examining of Nernst response range, pH response rate, and long term stability of the electrodes, etc. suggested that the three-time-oxidized electrodes appeared to show the best integrated performance. Several approaches were adopted to explore the essence of above phenomena. Morphology and composition investigations indicate a two-layer structure of the surface oxide film, i.e., dense inner layer and porous outer layer, with oxygen concentration decreases gradually from surface to iridium substrate. Raman spectroscopy implies good crystal quality of the fabricated iridium oxide after hydration. XPS illustrates that effective compositions of the electrode surface are IrO2 and Ir(OH)(4). EIS of the IrOx electrodes were investigated in pH buffers to analyze their pH response mechanism. Combining cross section observation and EIS results, the good performance of the three-time-oxidized electrode is attributed to the thicker porous outer layer showing more effective hydration, larger active surface area, and smaller reaction resistance in pH detection. (C) 2013 The Electrochemical Society. All rights reserved.