Single sheet iron oxide based films: electrochemical properties with in situ UV-vis measurement

Abstract

Stable single sheet iron(III) oxide (SSI) colloidal suspensions can be obtained via delamination of the corresponding layered iron(III) oxides in sodium hydroxide followed by redispersion in tetrabutylammonium hydroxide. Multilayer films with alternate layers of poly(diallyldimethylammonium) (PDDA) ions and SSI ((PDDA/SSI)_n) can be fabricated via layer-by-layer (LBL) electrostatic assembly using this stable suspension. For a single PDDA/SSI film, atomic force microscopy (AFM) shows high surface coverage and an average thickness of 14 nm. Each successive PDDA/SSI film has the same iron content which is confirmed by linear enhancement of the UV-vis absorption versus the number of layers (n). Films of (PDDA/SSI)_n deposited on indium tin oxide-coated quartz electrodes are in electrical contact and the film layers are electrochemically identical to each other. The redox properties of (PDDA/SSI)_n films depend on the cathodic switching potential (E_c). E_c values less negative than −0.5 to −0.7 V (vs. Ag/AgCl) result in highly reversible scans between fully oxidized SSI and partially reduced SSI. E_c values more negative than −0.9 to −1.1 V (vs. Ag/AgCl) result in irreversible cycles and deterioration of the (PDDA/SSI)_n films. The half-wave potential of the (PDDA/SSI)_n films in the latter case is −0.12 V (vs. Ag/AgCl) at pH 7 and −0.47 V (vs. Ag/AgCl) at pH 9 similar to other iron oxide systems. UV-vis spectroelectrochemical measurements reveal that the charge transfer into the (PDDA/SSI)_n films causes a reversible in situ UV-vis spectral change when the potential is cycled between −0.7 V and +0.6 V (vs. Ag/AgCl) at pH 12. The (PDDA/SSI)_n films become bleached in the UV spectral region (200–400 nm) and colored in the visible spectral region (400–900 nm) when exposed to a cathodic potential. The high mobility of protons results in fast charge transfer during reduction and oxidation of (PDDA/SSI)_n films.