Sputtered-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). The incorporation of Sn into the α-Fe2O3 is necessary to achieve the highest photocurrents. 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. Sputter-deposited films, relative to samples produced by hydrothermal synthesis,1,2 permit facile characterization of the role and placement of dopants using techniques that cannot typically be applied to the analysis of nanostructured materials. The relative merits of different techniques for determining dopant density in nanostructured films 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 multi-faceted 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.
Citation: Journal of Physical Chemistry C
Pub Type: Journals
photoanode, hematite, tin doping, electrochemical impedance spectroscopy, dynamic secondary ion mass spectrometry