Influence of annealing temperature on photoelectrochemical water splitting of α-Fe2O3 films prepared by anodic deposition

The present work explores the use and optimization of α-Fe2O3 layers prepared via anodic deposition of FeOOH on FTO. Layer thickness and annealing temperature are studied in view of morphology, structure, and solar light water splitting efficiency. Previously reported limits of such anodic structures can be overcome by optimal annealing (i.e. under conditions where the Fe2O3 layer becomes Sn-doped by diffusion from the FTO substrate). As a result the layers, when annealed at 650 °C for 1 h and 750 °C for 20 min, reach a 0.87 mA cm−2 at 1.23 V (vs. RHE) in 1 M KOH solution under simulated solar illumination AM 1.5 (100 mW cm−2) conditions – this is 2–3 times higher than previously reached with these structures. The main effect of optimized annealing (and Sn-doping) can be observed from impedance measurements in a drastically reduced charge transfer resistance. Additional modification with O2 evolution catalysts (IrO2, cobalt species) is less significant and leads mainly to a slight beneficial shift of the photocurrent onset potential.