HETEROEPITAXIAL HEMATITE PHOTOANODES AS A MODEL SYSTEM FOR SOLAR WATER SPLITTING

Hematite is a leading candidate for use as a photoanode material in solar powered water splitting due to its stability in aqueous solution, 2.1 eV band gap, vast abundance, and low cost. Elimination of grain boundaries and reduction of defects by growth of heteroepitaxial films has potential to significantly suppress the deleterious effects of charge carrier recombination and increase water splitting efficiency. We present the growth of heteroepitaxial multilayer Pt(111)/Fe₂O₃(0001) films deposited on sapphire c-plane (0001) substrates by RF magnetron sputtering and pulsed laser deposition, respectively.

The deposited films were highly crystalline, displaying an in-plane mosaic spread of less than 1 degree and a homogenous surface morphology with roughness of ~3 A, allowing for systematic investigation of hematite properties without changes to the microstructure. Ellipsometry and UV-Vis spectroscopy measurements were shown to be in excellent agreement with modelling, demonstrating that the optics of the system including absorption in the hematite layer are well described.

For polycrystalline hematite photoanodes deposited on platinum, full characterization of the system is hampered by the inability to make measurements in alkaline electrolyte containing 0.5 M hydrogen peroxide (H2O2) due to spontaneous decomposition of H2O2 by the exposed platinum. The pin-hole free high quality of the heteroepitaxial films is demonstrated by the ability to make stable and reproducible measurements in H2O2 containing electrolyte allowing for accurate extraction of charge separation and injection efficiency. The combination of excellent crystalline quality in addition to the well characterized optics and electrochemical properties of the heteroepitaxial hematite photoanodes demonstrate that Al₂O₃(0001)/Pt(111)/Fe₂O₃(0001) is a powerful model system for systematic investigation into solar water splitting photoanodes.