Effect of Tin Doping on α-Fe₂O₃ Photoanodes for Water Splitting

Sputter-deposited films of α-Fe₂O₃ of thickness 600 nm were investigated as photoanodes for solar water splitting and found to have photocurrents as high as 0.8 mA/cm² at 1.23 V vs the reversible hydrogen electrode (RHE). Sputter-deposited films, relative to nanostructured samples produced by hydrothermal synthesis,, permit facile characterization of the role and placement of dopants. The Sn dopant concentration in the α-Fe₂O₃ varies as a function of distance from the fluorine-doped tin oxide (FTO) interface and was quantified using secondary ion mass spectrometry (SIMS) to give a mole fraction of cations of approximately 0.02% at the electrolyte interface. Additional techniques for determining dopant density, including energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), electrochemical impedance spectroscopy (EIS), and conductivity measurements, are compared and discussed. Based on this multifaceted data set, we conclude that not all dopants present in the α-Fe₂O₃ are active. Dopant activation, rather than just increasing surface area or dopant concentration, is critical for improving metal oxide performance in water splitting. A more complete understanding of dopant activation will lead to further improvements in the design and response of nanostructured photoanodes.