Photoelectrochemical Performance of Nanostructured Ti- and Sn-Doped α-Fe2O3 Photoanodes

Thin films of alpha-Fe2O3 doped with either Ti or Sn were prepared by coevaporating iron and titanium/tin in a reactive oxygen ambient, and their physical, chemical, and photoelectrochemical properties were studied. It was found that manipulating the deposition angle had a profound effect on the photoelectrochemical water oxidation performance of 4% Ti-doped alpha-Fe2O3 films, and a maximum in photocurrent at 1.4 V vs RHE (Reversible Hydrogen Electrode) was achieved for films grown at 75 degrees incidence. It was also found that the nanocolumnar morphology and superior porosity attained using glancing angles improved the relative conversion of visible-light (lambda > 420 nm) photons compared to dense films deposited at normal incidence. Sn-doped films were also prepared for comparison using the same deposition conditions, and although they were substantially better than undoped films, their performance was somewhat below that of Ti-doped films. The Ti-doped films deposited using optimum conditions resulted in incident photon-to-current efficiencies (IPCE) reaching 31% at 360 nm and 1.4 V vs RHE. By comparison, Sn-doped films reached only 21% under the same conditions. The increased photoconversion efficiency brought about through Ti4+ or Sn4+ incorporation appears to be due to both the improvement of electron transport within the bulk of the film and the suppression of recombination at the film-electrolyte interface due to the stronger electric field near the surface.