Interfacial Co‐Operativity Enables Ultrafast Charge Transfer Within the Co‐Fe Prussian Blue Analogue|Zno Heterostructure

Heterostructures of Cobalt‐Iron (Co‐Fe) Prussian blue analogues (PBA) and inorganic semiconductors are attractive materials for photocatalytic and photoelectrochemical water oxidation. Their efficiency is rooted in the charge transfer (CT) at the PBA|semiconductor interface. The interfacial CT, however, often suffers from sluggish kinetics, optimization of which has been elusive. In this work, we investigate PBA|ZnO heterostructures spectroscopically and show that tuning the interfacial composition of the heterostructure presents a synthetic handle to significantly improve interfacial CT. We employ ultrafast transient absorption (TA) spectroscopy to probe the CT kinetics, while interface‐sensitive vibrational spectroscopy, that is, time‐resolved and in‐situ vibrational sum‐frequency generation (VSFG), sheds light on the molecular response to the CT across the interface. These measurements reveal that cooperative intermolecular interactions at the PBA|ZnO interface are key to achieving efficient CT. Furthermore, we relate the CT observed on ps‐timescales to the functional properties of the PBA|ZnO heterostructure in terms of photocatalytic water oxidation, which increases by about 200% in absolute yield as compared to a heterostructure without interfacial co‐operativity. Thus, this work presents for the first time a molecular picture of a PBA|ZnO interface and offers a novel perspective to optimize the CT dynamics in PBA|semiconductor heterostructures by tuning the interfacial chemical structure of PBA.