Probing the Interfacial Molecular Structure of a Co-Prussian Blue In Situ

Molecular-level insight into the interfacial composition of electrodes at the solid-electrolyte and the solid-electrode interface is essential to understanding the charge transfer processes, which are vital for electrochemical (EC) and photoelectrochemical (PEC) applications. However, spectroscopic access to both interfaces, particularly upon application of an external bias, remains a challenge. Here, in situ surface sensitive vibrational sum-frequency generation (VSFG) spectroscopy is used for the first time to directly access the interfacial structure of a cobalt-containing Prussian blue analog (Co-PBA) in contact with the electrolyte and TiO₂/Au surface. Structural and compositional changes of the Prussian blue layer during electrochemical oxidation are studied by monitoring the stretching vibration of the CN group. At open circuit potential, VSFG reveals a non-homogeneous distribution of oxidation states of metal sites: Fe^III–CN–Co^II and Fe^II–CN–Co^III coordination motifs are dominantly observed at the Co-PBA|TiO₂ interface, while it is only the Fe^II–CN–Co^II unit at the electrolyte interface. Upon increasing the potential applied to the electrode, the partial oxidation of Fe^II–CN–Co^II to Fe^III–CN–Co^II is observed followed by its transformation to Fe^II–CN–Co^III via charge transfer and, finally, the formation of Fe^III–CN–Co^III species at the interface with TiO₂ and the electrolyte.