The utilization of (sun-)light for the conversion of solar energy into either electrical or chemical energy is very popular. Prominent examples are photoelectrochemical water splitting and CO2 reduction or dye sensitized solar cells. Other branches of photoelectrochemistry include the investigation of semiconducting polymers or inorganic semiconductors. Scanning electrochemical microscopy (SECM) can be combined with a Synchronized Photo- Excitation System to study those materials and processes. This related technique is called scanning photoelectrochemical microscopy (SPECM).
Photoexcitation from the bottom of the sample through an inverted microscope has the advantage of using a common microelectrode from the top. Microelectrode and illuminated spot are aligned for high precision and high spatial resolution of the experiments. Thin films of photoelectrocatalysts for water splitting can be mapped simultaneously regarding photocurrent at the sample, ORR current at the microelectrode (detection of produced oxygen) and topography via shear force. This way, it is possible to gain multiple information in one scan. Areas of varying photoactivity can be correlated to topographical properties, e.g. local film thickness.
SPECM was used in a two-step experiment. In the first step a photocatalyst library was fabricated via photodeposition of FeOOH from solution to produce active water splitting catalysts. The catalyst spot size is determined by the illuminated area which was ~ 200 µm. The deposition time was varied to produce varying quantities of FeOOH. The second step is the characterization of these photocatalyst spots using a microelectrode. The most active catalyst spot could be identified by recording the ORR current at the microelectrode. The ORR current is a direct measure for oxygen production at the photoactive surface.
Quantum dots (QDs) can be used to substitute dye sensitizers in solar cells. SECM was used in a high-throughput screening of a binary QD library to identify the most active composition. Instead of a microelectrode, an optical fiber was used to illuminate the sample locally. The photocurrent is detected at the sample which is employed as working electrode.