Kinetics of deposition and stability of pyrocatechol -FeIII coordinated films

Metal coordination between polyphenols and metal cations like Fe³⁺ allows to produce conformal homogeneous and robust coatings on a vast variety of materials. The deposition kinetics and the stability of the obtained films are however only poorly investigated. In the present article it is shown that rough, granular but pinhole free coatings up to 50nm in thickness can be obtained in a one pot manner using pyrocatechol (Pyr)/Fe³⁺ mixtures at different stoichiometries (with Fe³⁺/Pyr ratios equal to 0.55 or 1.10) provided the deposition time is extended up to 24h. More importantly, we show that these films are dissolved upon oxidation of Pyr in cyclic voltammetry experiments. When the films deposited during short durations are rinsed with buffer and subsequently re-exposed to Pyr containing solution, they undergo partial dissolution most probably through a ligand exchange process. Such a dissolution process does not occur anymore in the same conditions, when the deposition time is increased above 5h. All Pyr-Fe³⁺ based films can be stabilized by a post-deposition of a polyelectrolyte multilayer film based on the alternated adsorption of poly(allylamine hydrochloride) and the sodium salt of poly(styrene sulfonate). The deposition of 5 bilayers of these polyelectrolytes allows suppressing the dissolution of Pyr-Fe³⁺ based films produced for short deposition times.