Effects of low temperature annealing on the photo-electrochemical performance of tin-doped hematite photo-anodes

The effects of post-deposition annealing at 400 and 500 °C on the photo-electrochemical performance of SnIV-doped α-Fe2O3 photo-anodes are reported. Samples were fabricated by spray pyrolysis on fluorine-doped tin oxide (FTO) and on titanium substrates. Photo-electrochemical, morphological and optical properties were determined to explain the shift in photocurrent densities to lower electrode potentials and the decrease of maximum photocurrent densities for alkaline water oxidation after annealing. Annealing at 400 and 500 °C in air did not affect significantly the morphology, crystallinity, optical absorption or spatial distributions of oxygen vacancy concentrations. However, XPS data showed a redistribution of SnIV near SnIV-doped α-Fe2O3 | 1 M NaOH interfaces after annealing. Thus, electron-hole recombination rates at photo-anode surfaces decreased after annealing, shifting photocurrents to lower electrode potentials. Conversely, depletion of SnIV in the α-Fe2O3 bulk could increase recombination rates therein and decrease photon absorption near 550 nm, due to an increased dopant concentration in the semiconductor depletion layer. This accounted for the decrease of maximum photocurrents when electron-hole recombination rates were suppressed using HO2− ions as a hole scavenger. The flat band potential of SnIV-doped α-Fe2O3 remained relatively constant at ca. 0.7 V vs. RHE, irrespective of annealing conditions.