In-situ Raman spectra of solution phase electrogenerated species were recorded in a channel flow cell. A microscope objective aligned normal to the direction of flow downstream from the edge of the working electrode was used to focus the excitation laser beam near the metal-electrode|electrolyte interface and also to collect the Raman scattered light from electrogenerated species under maximum detection sensitivity. Linear correlations were found between both the gain and the loss of the integrated Raman intensity attributed to the active redox species, and the current measured at the working electrode as a function of potential.
Light-activated microelectrodes in redox electrolytes were modeled theoretically using COMSOL under strict axisymmetric geometry. Dimensioned and dimensionless steady state profiles for solid state and solution phase species were predicted by solving self-consistently the transport equations and the electrostatic potential within the semiconductor phase subject to the appropriate boundary conditions. The local flux at the interface in the direction normal to the semiconductor surface was calculated as a function of photon flux intensity and bias potential. The predicted limited currents were proportional to the photon flux intensity under high bias potential, the simulation results showed that photo-generated holes in an n-type semiconductor would escape beyond the edge of the illuminated disk, thereby increasing the effective area of the light-activated microelectrode.
The effect of the shape on properties of electrocatalytically active nanoparticles supported on inactive planar substrates was modeled theoretically. The results indicated very minor differences between the diffusion limited currents, ilim, for reaction on a hemisphere and full sphere with the same area. However, for prolate spheroids large enhancements were predicted as the aspect ratio was increased. Analyses of the predicted behavior of active microdisks dispersed on both planar and spherical inactive substrates suggested that currents very close to ilim could be achieved for coverage on the order of 1%. This effect might explain experimental observations for the onset potential for the oxygen reduction well ahead of the onset of the reduction of iron porphyrins adsorbed on carbon surfaces associated with the conversion from the inactive to the active form of the macrocycle.