Scanning photoelectrochemical microscopy (SPECM) enables the investigation of photoelectrochemical processes at a substrate by integrating an illumination system into a conventional SECM set-up. The illumination can be integrated below the photoactive substrate, irradiating the substrate from the back (back illumination) or above the substrate, irradiating the substrate from the top by using a glass fiber inserted into a glass capillary (front illumination). Generally, the photocurrent at the substrate is recorded. The illumination from the back allows the detection of e.g. oxygen, which was produced in the photoreaction at a conventional UME. In the case where the illumination is realized via a glass fiber inside a single-barrel capillary, only the photocurrent at the substrate can be detected. Inserting an Ag wire into the capillary allows additional measurements of ion conductance currents. This can be used in a SICM experiment for distance-control of the capillary. In another setup with front illumination, a double-barrel or theta capillary is used. One barrel can be modified to yield a carbon UME whereas the second barrel is used for introducing the glass fiber for illumination. Certainly, other combinations of substrate illumination and recording of photocurrents are possible.

SPECM_1
Back illumination and recording of photocurrent at the photoactive substrate and ORR current at the UME.
SPECM_2
Front illumination through a glass fiber inside a capillary and recording of the photocurrent.
SPECM_3
Front illumination through a glass fiber inside a theta capillary and recording of photocurrent at the photoactive substrate and ORR current at the UME.