Correlating nanoscale structure with ion intercalation in electrodeposited nickel hexacyanoferrate thin films

The relationship between the structure and ion intercalation in electrodeposited nickel hexacyanoferrate thin films is investigated using energy-dispersive X-ray (EDS) and Raman spectroscopies combined with electrochemical control of the iron centers' oxidation states. Nickel hexacyanoferrate thin films are prepared by cathodic deposition on a platinum substrate. Potential cycling in the range -100 to +900 mV vs saturated calomel electrode is used to reversibly intercalate and deintercalate K+ from the matrix. Raman spectroscopy is used to determine the oxidation state of the iron centers, and EDS quantifies K+ intercalation. Potential step changes are made between -100 and +900 mV, with spectroscopic analysis following each step change. A number of structural analogues to "soluble" and "insoluble" Prussian Blue are tested against the experimental results. The relationship between the amount of intercalated alkali cations and the oxidation state of the iron centers suggests that this material is nickel-rich, a result supported by X-ray photoelectron spectroscopy compositional analysis. These results are more consistent with a structure analogous to "insoluble", rather than "soluble", Prussian Blue, with the approximate unit cell stoichiometry K4Ni4II[Fe-II(CN)(6)](3).