Sustained solar-driven water splitting to make dihydrogen must also include the formation of dioxygen. Many researchers view hematite (a-Fe2O3) as a potentially attractive photocatalytic anode for water oxidation. Hematite is inexpensive and its electronic absorption spectrum extends to ca. 600 nm, i.e. across the majority of the visible spectrum. Unfortunately, it is a weak absorber at wavelengths longer than about 450 nm � meaning that large optical pathlengths are required in order to absorb a substantial fraction of incident, visible-region photons. The distances over which photo-generated holes (i.e. potential oxidizing equivalents) can move, however, are much shorter � probably 10 nm or less. Additionally, there is a strong wavelength dependence to the internal quantum efficiency for water oxidation, even in the thin-film photo-electrode limit. This presentation will focus on how: a) atomic-layer deposition (ALD) can be used to fabricate with sub-nanometer precision thin and ultra-thin films of hematite, b) these thin films can be used as uniquely well defined model systems for investigating efficiency-relevant mechanistic questions,  c) high-area, transparent, distributed current collectors can be exploited to reconcile the competing structural requirements for photon absorption versus hole delivery, and d) well-defined, ALD-based modulation of photo-electrode chemical composition can be exploited to boost internal quantum efficiencies for water photo-oxidation by a-Fe2O3, especially at long wavelengths.

Understanding iron-oxide as a photo-catalyst for the oxidation half of the water splitting reaction

245th National Meeting (2013)

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