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

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Thin films of α-Fe 2 O 3 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 α-Fe 2 O 3 films, and a maximum in photocurrent at 1.4 V vs RHE (Reversible Hydrogen Electrode) was achieved for films grown at 75° incidence. It was also found that the nanocolumnar morphology and superior porosity attained using glancing angles improved the relative conversion of visible-light (λ > 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 Ti 4+ or Sn 4+ 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.