Two-Layer Structures of Ultra-Thin Metal Hexacyanoferrate Films: Charge Trapping Possibility of Application to Corrosion Protection

The possibility of fabrication of ultra-thin films of metal hexacyanoferrates, Prussian Blue, PB, and nickel hexacyanoferrate, NiHCNFe, by multiple sequential attachment of metal cations (self-assembled on 4-aminobenzoic acid monolayer) and hexacyanoferrate anions, has been demonstrated. These films can form a bilayer arrangement on glassy carbon when PB and NiHCNFe are deposited as inner and outer films, respectively. To avoid mixing of the NiHCNFe and PB redox sites, we introduce an polymer interlayer from poly(4-vinylpyridine) and Nafion. Since the outer NiHCNFe film is physically separated from the electrode surface, and it undergoes redox reactions at potentials characteristic of the inner PB film, an effect of the reversible charge state trapping (bistable switching) is observed. Astable bilayer structure can also be deposited on stainless steel. The whole concept may be of importance to corrosion protection since charge accumulated in the oxidized bilayer film tends to stabilize corrosion potential within passive range.