Light-Driven, Low-Overpotential Water Oxidation on an Alternating-Layer Architecture of Palladium-Salen/Reduced Graphene Oxide p-n Heterojunction

This work reports the rational design and in-depth characterization of a photo-active anode for the oxygen evolution reaction (OER), fabricated by the alternating electrochemical deposition of a p-type poly[Pd(Salen)] metallopolymer and n-type reduced graphene oxide (rGO). The resulting layered architecture forms a well-defined p-n heterojunction, as confirmed by Mott-Schottky analysis, which is fundamental to its high performance. Under illumination, the poly[Pd(Salen)]/rGO platform demonstrates a remarkable reduction in the OER overpotential of 241 mV, driven by the photovoltage generated at the interface. A comprehensive investigation combining electrochemical impedance spectroscopy, Tafel analysis, and direct oxygen monitoring revealed a complex, light-dependent mechanism. The reaction follows pseudo-first-order kinetics and its rate is directly proportional to the incident photon flux. Notably, the analysis suggests a light-induced shift in the rate-determining step, while Turnover Frequency (TOF) calculations show a dramatic enhancement of the intrinsic catalytic activity of each palladium site, reaching 35 s⁻¹ under 30 W illumination. Based on these results, a band-alignment diagram of the stepped-gap heterojunction is proposed to explain how efficient spatial separation of photogenerated charge carriers at the p–n interface drives the enhanced catalytic activity. This study not only presents an efficient photoanode but also provides a deep mechanistic understanding of the synergy between molecular catalysts and graphene materials, paving the way for the design of advanced materials for solar-to-fuel conversion.