Characterization and application of the hetero-junction ZnFe2O4/TiO2 for Cr(VI) reduction under visible light

The spinel ZnFe 2 O 4 prepared by nitrate route is used as dispersed photons collector capable to sensitize TiO 2 under visible light and to reduce Cr(VI) into trivalent state. The transport properties, optical and photo-electrochemical characterizations are correlated, to build the energetic diagram of the hetero-system ZnFe 2 O 4 /TiO 2 /CrO 4 − solution. A gap of 1.97 eV is obtained for the spinel from the diffuse reflectance. The conduction band of ZnFe 2 O 4 (−1.47 V SCE ) favors the electrons injection into TiO 2 , permitting a physical separation of the charge carriers. The oxidation of oxalic acid by photoholes prevents the corrosion of the spinel. The best configuration ZnFe 2 O 4 (75 %)/TiO 2 (25 %) is used to catalyze the downhill reaction (2HCrO 4 − + 3C 2 H 4 O 4 + 1.5O 2 + 8H + → 2Cr 3+ + 6CO 2 + 11 H 2 O, ∆ G ° = −557 kcal mol −1 ). 60 % of Cr(VI) are reduced after 3 h of visible light illumination and the photoactivity follows a first-order kinetic with a half-life of 70 min. The water reduction competes with the HCrO 4 − reduction which is the reason in the regression of the photoactivity; a hydrogen evolution rate of 24 µmol mg −1 h −1 is obtained.