Influence of Ti Doping Levels on the Photoelectrochemical Properties of Thin-Film Hematite (α-Fe2O3) Photoanodes

Doping with Ti enhances the electron conductivity and photoelectrochemical properties in hematite (alpha-Fe2O3) photoanodes with respect to those of undoped hematite photoanodes. However, the optimal doping level is unknown. This work examined the influence of the Ti doping level on the photoelectrochemical properties of thin-film (similar to 50-nm) hematite photoanodes. The films were deposited by pulsed laser deposition (PLD) on glass substrates coated with transparent electrodes (fluorinated tin oxide, FTO) from Ti doped Fe2O3 targets with different Ti concentrations: 0 (undoped), 0.25, 0.8, 1, and 7 cation %. The film thicknesses, morphologies, microstructures, and optical properties were nearly the same for all of the photoanodes, thereby enabling systematic comparison of the effect of the doping level without spurious side effects related to morphological variations. The photoelectrochemical performances of all of the Ti-doped photoanodes were considerably higher than that of the undoped photoanode. Among the doped photoanodes, the performance of the heavily doped (7 cation %) photoanode was found to be lower than those of the photoanodes with doping levels of <= 1 cation %. Complementary measurements with a hole scavenger (H2O2) and intensity-modulated photocurrent spectroscopy (IMPS) analysis showed that, for the doped photoanodes, both the charge-separation and charge-transfer efficiencies improved with decreasing doping levels and were considerably lower for the heavily doped photoanode than for the lightly doped photoanodes.