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.