Nanoporous iron oxide membranes: Layer-by-layer deposition and electrochemical characterisation of processes within nanopores

A versatile procedure for the formation of nanoporous metal oxide membranes is reported, based on a layer-by-layer deposition procedure ('directed assembly') of metal oxide nanoparticles with appropriate 'linker' molecules; here Fe₂O₃ particles and phytic acid. Two types of nanoporous Fe₂O₃ membranes have been prepared and characterised: (A) a nanofilm deposit composed of 4-5 nm diameter Fe₂O₃ particles linked by phytic acid and (B) a nanoporous film formed after calcination of the type A deposit at 500°C in air. The nanofilm deposits are characterised by microscopy (SEM and AFM) and by electrochemical methods. Mechanically stable and homogeneous nanofilm deposits with controlled thickness (ca. 3 nm per layer deposited) were obtained. Transport of small molecules or ions through the nanoporous structure and their electrochemical conversion are shown to be fast in the presence of a sufficiently high concentration of supporting electrolyte. During the electrochemical oxidation of ferrocyanide, Fe(CN)₆^-4, the nanoporous structure of the type A deposit is shown to act as an 'active' membrane, which changes the electrode kinetics by 'double-layer superposition' effects. For the second type of nanofilm, type B, ferrocyanide is accumulated by adsorption within the porous structure.