Photoelectrochemical Properties of Polycrystalline Mg‐Doped p‐Type Iron (III) Oxide

Abstract

Electrical Properties / Electrochemistry / Materials Properties / Photoelectrochemistry 1 Semiconductors Polycrystalline Mg-doped (0.1 -5 at.%) Fe203 photoelectrodes have been prepared by annealing at 1573 K followed by slow cooling in air to ambient temperature. These materials are essentially single-phase at Mg concentrations not exceeding 0.5 at. %. Disk-shaped (photo)-electrodes were prepared, and their electric and photoelectrochemical (PEC) properties were measured. The positive sign of the photovoltage and of the thermoelectric power 8, confirmed the materials to be p-type semiconducting. -Carrier concentrations calculated from 8, revealed each Mg2+ to introduce one electron-hole up to about 0.1 at.% Mg. The a.c. conductivities, showing conductivity activation enthalpies of about 0.5 eV in the temperature region 300-800 K, are affected by grain growth and porosity. -The currentvoltage characteristics in the dark and under illumination, the spectral response, and the flatband potential (Vh = +2.2 V vs. SCE pH = 10) have also been determined. -PEC photocurrents in the visible region correspond to very low (-0.lY0) monochromatic quantum efficiency which is due to the low electron-hole mobility. The electronic transition in the visible region is indirect, and evidence has been found for this transition to be Fe-Fe charge transfer. Surface pretreatment is found to have a major effect on the photoelectrochemical properties as deduced from current-voltage curves. The stability of the a-Fe203 electrode in alkaline solution is discussed. IntroductionThe cleavage of water into hydrogen and oxygen in a photoelectrochemical (PEC) cell using a semiconductor photoelectrode has been well established [l -41. Optimal solar energy conversion requires a chemically inert semiconducting electrode material which shows photoconductivity far into the visible, and operates with high energy-conversion efficiencies.Recently, we have studied impurity doping of the stable wide bandgap materials n-type Ti02 and SrTi03 in order to extend their photoresponse into the visible part of the solar spectrum [5, 61. It appeared that with transition metal ion dopants efficient power-conversion efficiencies could only be reached under near uv irradiation. Besides these titanates, the potential applicability of other oxides as photoelectrodes in PEC cells has been studied.The photoelectrochemical properties of iron oxide, a-Fe2O3, have attracted wide-spread attention [7 -301 due to its relatively low bandgap energy of about 2.2 eV [18,31] which is close to the ideal value for optical solar energy absorption, its stability against chemical, and photochemical corrosion [7-9, 16, 191, its vast abundance, and its low cost of preparation.Aliovalent doping is required to render a-Fe2O3 semiconducting [15 -181. However, the flat-band potential of n-type iron oxide lies far below the hydrogen reduction potential [20,27].Hence, a PEC cell with this electrode requires an external bias of about 0.6 eV. While photoassisted electrolysis of water has been dem...

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