STUDIES ON SOME ALPHA-FE2O3 PHOTOELECTRODES

p- and n-type alpha-Fe2O3 samples were prepared and the doping limits of Mg2+, Cu2+ and Ca2+ ions ascertained from X-ray diffraction studies and electrical resistivity measurements. The doping limits are 0.2 and 0.1 at % for Mg2+ and Cu2+, respectively. alpha-Fe2O3 is not doped by Ca2+ and forms a solid-solution to give CaFe2O4. Temperature variation of resistivity and thermoelectric power between 300 and 700 K showed that d-level conduction in alpha-Fe2O3 remains unchanged by Mg2+ doping, but is changed by Cu2+ doping. Optical constants (n,k) were determined by a Kramers-Kronig analysis from the reflectance spectra recorded in the 450-800 nm wavelength region following a procedure involving a straight line approximation of the reflection attenuation. Pure alpha-Fe2O3 shows an indirect band gap of 2.135 eV and Mg2+ doped samples show an indirect band gap of approximately 2.5 eV and a direct band gap of approximately 1.6 eV. The calculated theoretical quantum efficiencies of the samples indicate Cu2+-doped alpha-Fe2O3 to be better photoanodes in a photoelectrochemical cell for water decomposition than pure alpha-Fe2O3 or Mg2+-doped alpha-Fe2O3. Experimentally, however, none of these samples show good photoresponse which is attributed to the indirect nature of the band gaps in these samples and their d-level conduction.