On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures

Hematite (α-Fe ₂ O ₃ ) is regarded as one of the most promising cost-effective and stable anode materials in photoelectrochemical applications, and its performance, like other transition-metal oxides, depends strongly on its electrical and defect properties. In this work, the electrical and thermomechanical properties of undoped and Sn-doped α-Fe ₂ O ₃ nanoscale powders were characterized in situ at controlled temperatures (T = 250 to 400 °C) and atmospheres (pO ₂ = 10 ^-4 to 1 atm O ₂ ) to investigate their transport and defect properties. Frequency-dependent complex impedance spectra show that interfacial resistance between particles is negligible in comparison with particle resistance. Detailed defect models predicting the dependence of electron, hole, and iron and oxygen vacancy concentrations on temperature and oxygen partial pressures for undoped and doped α-Fe ₂ O ₃ were derived. Using these defect equilibria models, the operative defect regimes were established, and the bandgap energy of undoped α-Fe ₂ O ₃ and oxidation enthalpy of Sn-doped α-Fe ₂ O ₃ were obtained from the analysis of the temperature and pO ₂ dependence of the electrical conductivity. On the basis of these results, we are able to explain the surprisingly weak impact of donor doping on the electrical conductivity of α-Fe ₂ O ₃ . Furthermore, experimental means based on the results of this study are given for successfully tuning hematite to enhance its photocatalytic activity for the water oxidation reaction.