Influence of annealing temperature and Sn doping on the optical properties of hematite thin films determined by spectroscopic ellipsometry

Hematite (α-Fe₂O₃) thin films were prepared by sol-gel route and investigated for application in H₂ generation by photo-assisted water splitting. The photoelectrochemical (PEC) performance was shown to increase significantly for films deposited on SnO₂:F/glass subjected to high temperature (T) annealing (>750 °C). Strong correlation was found between photogenerated current, donor concentration, and Sn concentration as determined by Mott-Schottky analysis and X-ray photoelectron spectroscopy. The effects of thermal annealing and Sn addition in the resulting microstructure and optical properties of hematite films deposited on fused silica substrates were determined by a combination of structural characterization techniques and spectroscopic ellipsometry. Thermal annealing (>600 °C) induces a higher optical absorption that is associated directly to film densification and grain growth; however, it promotes no changes in the energy positions of the main Fe₂O₃ electronic transitions. The band gap energy was found to be 2.21 eV and independent of microstructure and of Sn concentration for all studied films. On the other hand, Sn can be incorporated in the Fe₂O₃ lattice for concentration up to Sn/Fe ∼2%, leading to an increase in energy split of the main absorption peak, attributed to a distortion of the Fe₂O₃ lattice. For higher concentrations, Sn incorporation leads to a reduction in absorption, associated with higher porosity and the formation of a secondary Sn-rich phase. In summary, the variation in the optical properties induced by thermal annealing and Sn addition cannot account for the order of magnitude increase of the current density generated by photoanodes annealed at high T (>750 °C); thus, it is concluded that the major contribution for the enhanced PEC performance comes from improved electronic properties induced by the n-type doping caused by Sn diffusion from the SnO₂:F substrate.