POTENTIOMETRIC OXYGEN SENSING WITH COPPER-FILMS - RESPONSE MECHANISM AND ANALYTICAL IMPLICATIONS

The electrochemical response mechanism of a previously reported potentiometric oxygen (O-2) sensing system based on thin films of copper sputtered on single crystal Si(100) is examined. The potentiometric O-2 response of such films is shown to depend on sample stirring rate as well as the pH, ionic strength and nature of the buffer salts within the test solution. XPS studies of the copper films exposed to solution for several days confirm the presence of copper corrosion products on the surface. These findings, in conjunction with cyclic voltammetric measurements, suggest that the potentiometric O-2 response originates from slow corrosion of copper and simultaneous reduction of O-2 at the surface of the thin films. A steady-state situation exists when the rates of these two reactions are equal, resulting in a corrosion potential (also referred as rest potential E(r) for the system) that varies in a near Nernstian (1 e(-)) manner with the partial pressure of O-2 in solution. A mathematical formulation for this type of response, based on the Butler-Volmer equation, is presented. The analytical implications of these findings with respect to devising useful potentiometric O-2 sensors based on copper films are discussed.