Co‐Engineering Band Structure and Interfacial Electron Pathways by Etching‐Assisted Transformation of Prussian Blue Analogues into Sulfide Photocatalysts with Graphdiyne Cocatalysts for Artificial Photosynthesis of Hydrogen Peroxide

ABSTRACT Artificial photosynthesis of hydrogen peroxide in neutral water is limited by weak solar absorption and the lack of interfaces that selectively channel electrons into the two‐electron oxygen‐reduction pathway. We address these constraints by co‐engineering band structure and electron pathways through etching‐assisted transformation of Prussian blue analogues into defect‐rich metal‐sulfide photocatalysts and integration of graphdiyne carbon cocatalysts. The sulfur‐etching conversion yields defect‐rich Zn‐CdS with red‐shifted absorption and favorable band energetics, while graphdiyne provides π‐conjugated pathways and sites that stabilize *OOH, directing two‐electron reduction of O ₂ to H ₂ O ₂ . Energetic alignment from work‐function/valence‐band measurements and density‐functional theory indicates downhill electron transfer from the sulfide to graphdiyne, corroborated by photoluminescence quenching, shorter lifetimes, lower charge‐transfer resistance, and higher photocurrent. In situ diffuse‐reflectance infrared spectroscopy detects *OOH/ H ₂ O ₂ , spin‐trapping electron‐paramagnetic resonance confirms oxygenated intermediates, and rotating ring‐disk voltammetry indicates a near‐two‐electron pathway. Under UV–vis irradiation, the composite exhibits a H ₂ O ₂ formation rate of 128.7 µmol g ^{− 1} min ^{− 1} without sacrificial agents, maintains activity over cycling, and permits kinetic deconvolution of formation versus self‐decomposition. Extending the method to Ni/Mn/Co precursors reproduces the structure‐function trend. By coupling light‐harvesting control with interfacial catalytic specificity, this organic–inorganic heterojunction offers a route to efficient, sacrificial‐agent‐free photosynthesis of H ₂ O ₂ .