A Route to Diverse Combinatorial Libraries of Electroactive Nickel Hexacyanoferrate

Abstract

Electroactive nickel hexacyanoferrate (NiHCF) thin films are of interest for their ion sensing and ion intercalation properties; these properties depend on the material's stoichiometry and structure. A small combinatorial library of NiHCF thin films was synthesized by microdispensing 2-μL droplets of reagent with varied proportions of a divalent nickel source (nickel sulfate), a ferricyanide source (potassium ferricyanide), and water on Pt substrates. Two different mixing orders were used (Ni II /H 2 O/Fe III and Fe III /H 2 O/Ni II ) and 13 discrete compositions on the ternary reagent diagram were chosen. These 26 library members were electrochemically intercalated/deintercalated with K + ions by cyclic voltammetry. Raman spectroscopy was used to screen individual library members for electroactivity; this step eliminated 11 inactive members, most of which resulted from Ni II -rich growth conditions. Energy-dispersive X-ray spectroscopy (EDS) was used to determine the number of intercalated K + ions in the oxidized unit cell (N K,ox ) for the remaining 15 members. The quantity N K,ox is a sensitive measure of NiHCF matrix stoichiometry, with a perfectly stoichiometric matrix possessing N K,ox = 4. N K,ox for this library was found to vary from 0 to ∼3 (maximum possible variability is 0-4), with smaller N K,ox from the Ni II /H 2 O/Fe III mixing order and larger N K,ox from the Fe III /H 2 O/Ni II order. The methods used to synthesize this small, but diverse, library are easily scalable to much larger libraries.