Self-assembled films of Prussian Blue and analogues: Optical and electrochemical properties and application as ion-sieving membranes

Ultrathin films of metal hexacyanometalates were prepared upon multiple sequential adsorption of metal cations M^m+ (Fe³⁺, Fe²⁺, Co²⁺, and Ni²⁺) and hexacyanometalate anions [M(CN)₆]^n- (Fe(CN)₆^3-, Fe(CN)₆^4-, and Co(CN)₆^3-) on solid supports. The layer-by-layer deposition led to the formation of films of the metal complex salts with monolayer precision. The films were characterized using UV and IR spectroscopic methods and cyclic voltammetry. The alternating adsorption of Fe³⁺ and Fe(CN)₆^4- ions led to dense and defect-free films of Prussian Blue, which were useful as membranes for ion separation. The porous, zeolitic structure of Prussian Blue was permeable for ions with a small Stokes radius such as Cs⁺, K⁺, and Cl^-, whereas large hydrated ions such as Na⁺, Li⁺, Mg²⁺, or SO₄^2- were blocked. The effect of ion sieving increased with the thickness of the membrane. After a hundred dipping cycles, high separation factors α(CsCl/NaCl) and α(KCl/NaCl) of 6.5 and 6.2, respectively, were found. Corresponding membranes of cobalt and nickel hexacyanoferrate were also useful for ion separation, but the α-values were lower. Possible reasons for the differences in selectivity are discussed.