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 understandingthe charge transfer processes, which are vital for electrochemical (EC) andphotoelectrochemical (PEC) applications. However, spectroscopic access toboth interfaces, particularly upon application of an external bias, remains achallenge. Here, in situ surface sensitive vibrational sum-frequencygeneration (VSFG) spectroscopy is used for the first time to directly access theinterfacial structure of a cobalt-containing Prussian blue analog (Co-PBA) incontact with the electrolyte and TiO 2 /Au surface. Structural andcompositional 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: Fe III –CN–Co II and FeII –CN–Co III coordinationmotifs are dominantly observed at the Co-PBA|TiO 2 interface, while it is onlythe FeII –CN–Co II unit at the electrolyte interface. Upon increasing thepotential applied to the electrode, the partial oxidation of Fe II –CN–Co II toFeIII –CN–Co II is observed followed by its transformation to FeII –CN–Co III viacharge transfer and, finally, the formation of Fe III –CN–Co III species at theinterface with TiO2 and the electrolyte.