Photoelectrochemical imaging of non-planar surfaces: theinfluence of geometrical and optical factors on imageformation

Abstract

In situ scanning photoelectrochemical (PEC)microscopy is a technique for mapping the photocurrentstimulated by a focused light at an electrode-electrolyteinterface. This technique gives position-sensitive informationon the nature of passivating films (composition, type and degreeof crystallization, thickness, etc) that cover metalelectrodes. In the analysis of PEC images,further aspects related to the surface shape and/or surfaceirregularities (roughness, slope errors, surface waviness,etc) have to be taken into account. In this paper we presentthe effects of non-planar surfaces by measuring the photoresponseof passivating oxide films on cylindrical iron rods with different diameters. The variation of the angle of incidenceof the light, as the laser beam scans the surface of the rod,implies either a change in the spot size and different opticalresponse of the oxide-metal system, such as the transmittivityof the oxide, the reflectivity of the metal and the light pathinside the film. For evaluating the behaviour of thephotocurrent, we have simulated these geometrical and opticaleffects by using a simple model in geometrical optics approximation. Images have been also acquired at variousdistances from the beam focus for highlighting the degradationof the spatial resolution induced by the focusing misalignment of the curved samples.