Scanning electrochemical microscopy. 49. Gas-phase scanning electrochemical microscopy measurements with a Clark oxygen ultramicroelectrode

The use of an ultramicroelectrode tip as a basis for a Clark membrane electrode allows the sensing of oxygen concentration using the oxygen reduction signal and the application of scanning electrochemical microscopy (SECM) to pure gas-phase measurements. A 25-mum-diameter Pt wire was sealed in glass and the glass coated with silver paint near the tip to produce a Ag ring, Pt disk electrode. A drop of electrolyte provided the contact between the Pt cathode indicator electrode and the Ag anode counter/reference electrode, while the liquid phase was maintained by a high-density polyethylene membrane pulled over the tip and fastened with an O-ring. The membrane isolated the electrode both electrically and chemically from the sample environment. The electrode was tested with both solution- and gas-phase samples, and SECM approach curves were recorded. The presence of a membrane caused a deviation in electrode behavior from theory in both solution and gas phases, due in part to an increase in the electrode response time. However, the approach curves obtained could be used to determine the distance of the membrane from a surface and the thickness of the electrolyte layer within the membrane. SECM images in the gas phase, of oxygen flux through an array of 100-mum-diameter holes in a silicon wafer, were obtained by measuring the reduction current of oxygen while scanning in the x-y plane over the surface.