Visible light induced hydrogen evolution on new hetero-system ZnFe2O4/SrTiO3

The physical properties and photoelectrochemical characterization of the spinel ZnFe2O4, elaborated by chemical route, have been investigated for the hydrogen production under visible light. The forbidden band is found to be 1.92 eV and the transition is indirectly allowed. The electrical conduction occurs by small polaron hopping with activation energy of 0.20 eV. p-type conductivity is evidenced from positive thermopower and cathodic photocurrent. The flat band potential (0.18 VSCE) determined from the capacitance measurements is suitably positioned with respect to H2O/H2 level (-0.85 VSCE). Hence, ZnFe2O4 is found to be an efficient photocatalyst for hydrogen generation under visible light. The photoactivity increases significantly when the spinel is combined with a wide band gap semiconductor. The best performance with a hydrogen rate evolution of 9.2 cm3 h-1 (mg catalyst)-1 occurs over the new hetero-system ZnFe2O4/SrTiO3 in Na2S2O3 (0.025 M) solution.Photoelectrochemical Spinel ZnFe2O4 Hetero-system Hydrogen

Photoelectrochemical H2-generation over Spinel FeCr2O4 in X2- solutions (X2-Â =Â S2- and )

Photocatalytic H2-production has been observed over FeCr2O4, synthesized from Cr(NO3)3 and Fe(NO3)3. The oxide crystallizes in the normal spinel structure and the transport properties exhibit p-type conductivity ascribed to metal deficiency. An optical gap of 1.33 eV is determined from the reflectance diffuse spectrum. From photoelectrochemical measurements, a flat band potential of +0.23 VSCE is found. The valence band, made up of cationic 3d orbital, is located at 5.2 eV (0.45 VSCE) below vacuum. Hence, the conduction band (-0.98 VSCE) allows a thermodynamically feasible H2 liberation under visible light. In aqueous electrolytes, the oxide is stabilized by hole consumption involving X2- species and the best photoactivity for H2-production is obtained in S2- solution with an evolution rate of 8.26 cm3 g-1  h-1. The tendency towards saturation is due to the competitive reduction of the end products namely and with water and to the yellow color of . On the other hand and for a comparative purpose, the catalytic activity of unreduced FeCr2O4 for the water-gas shift reaction (CO + H2O --> CO2 + H2) is studied in the temperature range 523-723 K. At 723 K, the rate of H2-formation and the degree of CO conversion are respectively 280 [mu]mol g-1 s-1 and 44%.Hydrogen Spinel Photoelectrochemical Water-gas shift