Photoelectrochemical imaging of non-planar surfaces: the influence of geometrical and optical factors on image formation

In situ scanning photoelectrochemical (PEC) microscopy is a technique for mapping the photocurrent stimulated by a focused light at an electrode-electrolyte interface. This technique gives position-sensitive information on the nature of passivating films (composition, type and degree of crystallization, thickness, etc) that cover metal electrodes. In the analysis of PEC images, further aspects related to the surface shape and/or surface irregularities (roughness, slope errors, surface waviness, etc) have to be taken into account. In this paper we present the effects of non-planar surfaces by measuring the photoresponse of passivating oxide films on cylindrical iron rods with different diameters. The variation of the angle of incidence of the light, as the laser beam scans the surface of the rod, implies either a change in the spot size and different optical response of the oxide-metal system, such as the transmittivity of the oxide, the reflectivity of the metal and the light path inside the film. For evaluating the behaviour of the photocurrent, we have simulated these geometrical and optical effects by using a simple model in geometrical optics approximation. Images have been also acquired at various distances from the beam focus for highlighting the degradation of the spatial resolution induced by the focusing misalignment of the curved samples.