Scanning photoelectrochemical microscopy for the investigation of local photocatalytic H2 Evolution in matrixed Langmuir films

Molecule-in-matrix systems such as Langmuir films with photosensitizer and hydrogen evolution reaction (HER) catalyst may exhibit heterogeneities in photocatalytic activity across large-area films. Here, we introduce a confined illumination approach and simultaneous localized detection of light-driven hydrogen (H2) evolution to directly correlate information on structural organization with catalytic performance at Langmuir-Blodgett and Langmuir-Schäfer films with incorporated [Mo₃S₁₃]²⁻ catalyst and Ru-based photosensitizer. Derived from coating of optical fibers used in scanning near-field optical microscopy (SNOM), the outside of the glass sheath of the microelectrode was modified with a silver coating. Coupling light into the coated glass sheath, local illumination with suppressed light loss and visible light excitation directly beneath the microsensor apex, resulting in a 3.4-fold higher photon flux density compared to the uncovered microelectrode. Using platinum-black modified microelectrodes as sensitive H2 microsensors, a bifunctional probe serving simultaneously as local light waveguide and H2 microsensor, enables quantitative detection of photogenerated H2 with high spatial resolution. This is demonstrated here for films comprising a phospholipid matrix or a π-conjugated rigid molecular scaffold co-embedding Ru-based photosensitizers and [Mo₃S₁₃]²⁻ catalysts. This enables not only the local quantification of H2, but also to determine the apparent quantum efficiencies (AQEs) of the different film architectures.