Studies on some α-Fe 2O 3 photoelectrodes

Abstract

p- and n-type α-Fe 2O 3 samples were prepared and the doping limits of Mg 2+, Cu 2+ and Ca 2+ ions ascertained from X-ray diffraction studies and electrical resistivity measurements. The doping limits are 0.2 and 0.1 at % for Mg 2+ and Cu 2+, respectively. α-Fe 2O 3 is not doped by Ca 2+ and forms a solid-solution to give CaFe 2O 4. Temperature variation of resistivity and thermoelectric power between 300 and 700 K showed that d-level conduction in α-Fe 2O 3 remains unchanged by Mg 2+ doping, but is changed by Cu 2+ doping. Optical constants (n, k) were determined by a Krämers-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 α-Fe 2O 3 shows an indirect band gap of 2.135 eV and Mg 2+ doped samples show an indirect band gap of 2̃.5 eV and a direct band gap of 1̃.6 eV. The calculated theoretical quantum efficiencies of the samples indicate Cu 2+-doped α-Fe 2O 3 to be better photoanodes in a photoelectrochemical cell for water decomposition than pure α-Fe 2O 3 or Mg 2+-doped α-Fe 2O 3. 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.