Probing the interfacial molecular structure of a Co-Prussian blue in situ

Abstract

Molecular-level insight into the interfacial composition of electrodes at thesolid-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 toboth interfaces, particularly upon application of an external bias, remains achallenge. 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) incontact with the electrolyte and TiO₂/Au surface. Structural and compositional changes of the Prussian blue layer during electrochemicaloxidation are studied by monitoring the stretching vibration of the CN group. At open circuit potential, VSFG reveals a non-homogeneous distribution ofoxidation states of metal sites: Feᴵᴵᴵ–CN–Coᴵᴵ and Feᴵᴵ –CN–Co coordinationmotifs are dominantly observed at the Co-PBA|TiO₂ interface, while it is onlythe Feᴵᴵ–CN–Coᴵᴵ unit at the electrolyte interface. Upon increasing the potential applied to the electrode, the partial oxidation of Feᴵᴵ–CN–Coᴵᴵ to Feᴵᴵᴵ–CN–Coᴵᴵ is observed followed by its transformation to Feᴵᴵ–CN–Coᴵᴵᴵ via charge transfer and, finally, the formation of Feᴵᴵᴵ–CN–Coᴵᴵᴵ species at the interface with TiO2 and the electrolyte.