Reflectance Changes during Formation and Reduction of Oxide Films on Gold and Platinum: Corrections for “Double-Layer Effects”

Abstract

Hysteresis between the processes of formation and reduction of thin oxide films on Au and Pt have been investigated by ellipsometric (Δ) and relative reflectance measurements. The interpretation of results obtained by optical methods where thin films are in the course of formation towards a monolayer, or beyond, requires consideration of optical effects due to contributions from changing ion concentration and metal surface electron charge density in the double‐layer since substantial changes of optical parameters for reflectance at Au and Pt interfaces arise with changing electrode potential. Resulting changes of Δ and must therefore be corrected for continuing double‐layer effects as the ad‐layer is laid down or removed (in the reduction process). The theoretical basis for evaluation of double‐layer optical effects is reviewed but it is proposed that, at the present time, empirical correction procedures may be preferable. Three methods are examined and an optimum procedure is recommended. The procedure is applied to the evaluation of hysteresis effects in the formation and reduction of thin oxide films at Au and Pt where evaluation of the true hysteresis effects depends in an important way on a proper correction procedure for the double‐layer optical effects.