Water Oxidation on Hematite Photoelectrodes: Insight into the Nature of Surface States through In Situ Spectroelectrochemistry

Uniform planar films of hematite (α-Fe₂O₃), deposited by atomic layer deposition, were examined using in situ spectroelectrochemistry during photoinduced water oxidation. A change in the absorption spectrum of hematite electrodes during water oxidation was measured under illumination and applied potentials. The absorption was correlated to a charge measured by cyclic voltammetry and with a capacitance measured by impedance spectroscopy. Modification of the hematite surface with alumina reduced the absorption feature and the associated capacitance, suggesting that these features are associated with the surface. Comparing the spectral change of hematite to absorption features of molecular analogues allowed us to tentatively assign the absorbance and capacitive features to the oxidation of a low valent iron-aqua or iron-hydroxyl species to a high valent iron-oxo chemical species at the surface

Substrate Dependent Water Splitting with Ultrathin α‑Fe₂O₃ Electrodes

Thin films of hematite (α-Fe₂O₃) were deposited by atomic layer deposition (ALD), and the effects of metal oxide underlayers on the photocatalytic water oxidation performance were investigated. It was found that a Ga₂O₃ underlayer dramatically enhances the water oxidation performance of the thinnest hematite films. The performance enhancement is attributed to the increased crystallinity of the ultrathin films induced by the oxide underlayers. The degree of crystallinity was examined by Raman line shape analysis of the characteristic hematite phonon modes. It was found that multiple metal oxide underlayers, including Nb₂O₅, ITO, and WO₃, increase the film crystallinity compared to hematite deposited on bare FTO. The increased crystallite size was also clearly evident from the high resolution SEM images. The degree of crystallinity was found to correlate with absorbance and the photocatalytic water oxidation performance. These findings shed light on the origin of the dead layer at the interface of the FTO substrate and ultrathin hematite films and elucidate strategies at overcoming it