Second Harmonic Generation Studies of Fe(II) Interactions with Hematite (α-Fe<sub>2</sub>O<sub>3</sub>)

Iron oxides are a ubiquitous class of compounds that are involved in many biological, geological, and technological processes, and the Fe­(III)/Fe­(II) redox couple is a fundamental transformation pathway; however, the study of iron oxide surfaces in aqueous solution by powerful spectroscopic techniques has been limited due to “strong absorber problem”. In this work, atomic layer deposition (ALD) thin films of polycrystalline α-Fe<sub>2</sub>O<sub>3</sub> were analyzed using the Eisenthal χ<sup>(3)</sup> technique, a variant of second harmonic generation that reports on interfacial potentials. By determining the surface charge densities at multiple pH values, the point of zero charge was found to be 5.5 ± 0.3. The interaction of aqueous Fe­(II) at pH 4 and in 1 mM NaCl with ALD-prepared hematite was found to be fully reversible and to lead to about 4 times more ferrous iron ions adsorbed per square centimeter than on fused-silica surfaces under the same conditions. The data are consistent with a recently proposed conceptual model for net Fe­(II) uptake or release that is underlain by a dynamic equilibrium between Fe­(II) adsorbed onto hematite, electron transfer into favorable surface sites with attendant Fe­(III) deposition, and electron conduction to favorable remote sites that release and replenish aqueous Fe­(II).