Improvement of Hematite as Photocatalyst by Doping with Tantalum

The use of tantalum as a highly effective dopant for hematite photoelectrochemistry (PEC) has shown contradictory results in previous reports. We show here through screening of different compositions by scanning electrochemical microscopy that Ta doping significantly improves the PEC performance of dropcast films on fluorine-doped tin oxide (FTO). In studies with larger electrodes, a 2% Ta-doped hematite photoanode fabricated at 500 °C shows the highest improvement of photoactivity, which is ∼32 times higher than pure hematite even under visible light. At fabrication temperature higher than 500 °C (e.g., 600, 680 °C), the substrate FTO becomes more resistive and the dopant Ta prefers to segregate from the bulk phase (α-Fe2O3) and forms tantalum fluoride oxide (TaO2F), which may act as charge-carrier recombination centers, and the corresponding Ta-doped samples show much lower photoactivities. Ta-doped hematite samples show stronger (110) diffraction as compared with the pure α-Fe2O3. We show that the doping of Ta induced a preferential growth along the {001} basal plane, which has been reported to have good conductivity. We found the conductivity of the Ta-doped hematite was improved up to at least about one order of magnitude after the incorporation of Ta, with the improved carrier mobility decreasing recombination of the photogenerated holes and electrons.