Physical and photoelectrochemical characterizations of hematite α-Fe_2O_3: Application to photocatalytic oxygen evolution

摘要

The physical properties and photoelectrochemical characterization of α-Fe_2O_3, 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 Fe~(2+/3+) with an electron mobility μ_(400 K) of 10~(-3) V cm~2 s~(-1). Α-Fe_2O_3 exhibits long term chemical stability in neutral solution and has been characterized photo-electrochemically to assess its activity as bias-free O_2-photoanode. The flat band potential V_(fb) (-0.45 V_(SCE)) and the electron density N_D (1.63 × 10~(18) 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 (+1.35 V_(SCE)) is suitably positioned with respect to the O_2/H_2O level (+0.62 V) and α-Fe_2O_3 has been evaluated for the chemical energy storage through the photocatalytic reaction: (2SO_3~(2-) + 2H~+ → S_2O_3~(2-) + O_2 + H_2O, ΔG = 213.36 kJ mol~(-1)). The best photoactivity occurs in SO_3~(2-) solution (0.025 M, pH 8) with an oxygen rate evolution of 7.8 cm~3 (g catalyst)~(-1) h~(-1).