Synthesis and characterization of the spinel ZnFe2O4, application to the chromate reduction under visible light

The spinel ZnFe2O4 prepared from nitrate precursors is used for the reduction of chromate under visible light using TiO2 as mediator. A gap of 1.97 eV is obtained for ZnFe2O4 from the diffuse reflectance and the crystallite size (70-130 nm) is determined from TEM measurement. The current-potential characteristic indicates a good electrochemical stability. The photo electrochemical properties permit the construction of the energy diagram of the hetero-system ZnFe2O4/TiO2/HCrO4− solution. The conduction band of the spinel (−1.47 VSCE) favors the physical separation of the electron/hole pairs and the electrons injection into TiO2. The best composition ZnFe2O4 (75%)/TiO2 (25%) is used to catalyze the reaction (2 HCrO4− +  3 C2H4O4 +  1.5 O2 +  8 H+→ 2 Cr3+ +  6 CO2 +  11 H2O). 60% of Cr(VI) are reduced after 3 h of visible light irradiation and the activity obeys a first order kinetic with a half-life of 70 min. The photocurrent-photovoltage characteristic of the spinel gives an optical-to-electrical energy efficiency of 0.28% and a fill factor of 0.49.