Physical and photoelectrochemical characterizations of hematite α-Fe2O3: Application to photocatalytic oxygen evolution

The physical properties and photoelectrochemical characterization of α-Fe2O3, synthesized by co-precipitation, have been investigated in regard to solar energy conversion. The optical gap is found to be 1.94 eV and the transition is indirectly allowed. The chemical analysis reveals an oxygen deficiency and the oxide exhibits n-type conductivity, confirmed by a negative thermopower. The plot log σ vs 1/T shows linearity in the range (400–670 K) with the donor levels at 0.14 eV below the conduction band and a break at ∼590 K, attributed to the ionization of the donors. The conduction occurs by small polaron hopping through mixed valences Fe2+/3+ with an electron mobility μ400 K of 10−3 V cm2 s−1. α-Fe2O3 exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as bias-free O2-photoanode. The flat band potential Vfb (−0.45VSCE) and the electron density ND (1.63 × 1018 cm−3) were determined, respectively, by extrapolating the linear part to C−2 = 0 and the slope of the Mott Schottky plot. At pH 6.5, the valence band (+1.35VSCE) is suitably positioned with respect to the O2/H2O level (+0.62 V) and α-Fe2O3 has been evaluated for the chemical energy storage through the photocatalytic reaction: (2SO32-+2H+→S2O32-+O2+H2O, ΔG = 213.36 kJ mol−1). The best photoactivity occurs in SO32- solution (0.025 M, pH 8) with an oxygen rate evolution of 7.8 cm3 (g catalyst)−1 h−1.