Dye-sensitized photocathodes were prepared by attaching a phosphonic acid-functionalized selenorhodamine dye (3-SeP) to delafossite CuAlO₂ thin films. The dye, 3-SeP, adsorbed with a robust stability to CuAlO₂ as both monomers and H-aggregated dyes resulting in enhanced absorbance of visible light. Transient photovoltage measurements of these photocathode systems show a long-lived positive shift of the Fermi level, measurable as a positive open-circuit voltage within 1 Hz laser illumination. This is evidence of a mechanism in which holes are transferred from photoexcited 3-SeP to CuAlO₂. In linear sweep voltammetry measurements, 3-SeP/CuAlO₂-on-FTO electrodes exhibited 7-to-8-fold greater reductive photocurrents than unfunctionalized CuAlO₂-on-FTO electrodes. This indicates excited-state hole transfer within these systems could possibly create excitonic conditions where efficient reduction of protons is possible. Through modifying conditions and electrolyte composition in chronocoulometry experiments, H₂has been successfully measured in the headspace above the closed-photoelectrochemical cell with these cathodes in a supporting electrolyte with a reduction co-catalyst, H⁺ source, and triethanolamine (as a sacrificial electron donor). These experiments show successful reduction of H⁺ to H₂ with a Faradaic efficiency of (43 ± 27)%. Our results highlight the potential of selenorhodamine-sensitized Cu(I) delafossites as dye-sensitized photocathodes for redox photocatalysis and the production of solar fuels.

Gregory Lapp

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