Abstract
The spinel ZnFe2O4 prepared by nitrate route is used as dispersed photons collector capable to sensitize TiO2 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 ZnFe2O4/TiO2/CrO4− solution. A gap of 1.97 eV is obtained for the spinel from the diffuse reflectance. The conduction band of ZnFe2O4 (−1.47 VSCE) favors the electrons injection into TiO2, permitting a physical separation of the charge carriers. The oxidation of oxalic acid by photoholes prevents the corrosion of the spinel. The best configuration ZnFe2O4 (75 %)/TiO2 (25 %) is used to catalyze the downhill reaction (2HCrO4− + 3C2H4O4 + 1.5O2 + 8H+ → 2Cr3+ + 6CO2 + 11 H2O, ∆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 HCrO4− reduction which is the reason in the regression of the photoactivity; a hydrogen evolution rate of 24 µmol mg−1 h−1 is obtained.
Highlights
Solar energy-assisted catalysis on semiconductor materials continue to draw a great interest for the scientific community (Pekakis et al 2006; Meng et al 2007; Yaron 2010)
The spinel ZnFe2O4 prepared by nitrate route is used as dispersed photons collector capable to sensitize TiO2 under visible light and to reduce Cr(VI) into trivalent state
It is well admitted that the photocatalysis is an environmentally friendly alternative able to replace the conventional methods for the water treatment, for low pollutants concentrations (Gumy et al 2006; Lydakis-Simantiris et al 2010; Robert and Malato 2002)
Summary
Solar energy-assisted catalysis on semiconductor materials continue to draw a great interest for the scientific community (Pekakis et al 2006; Meng et al 2007; Yaron 2010). The activity is untimely dependent on the material and its preparation (Ning et al 2010; Lifeng et al 2009) and semiconductors (SCs) employed for such applications are classified into two categories: (1) those characterized by a wide band gap such as TiO2 and SrTiO3 which have the advantage of high stability toward the photocorrosion and can be polyvalent in regard to the large panel of applications (Shouai et al 2008; Jiawen et al 2011; Chen and Chen 2011) This last aspect is attributed to the electronic bands which are generally of anionic and cationic character, respectively, for the valence (VB) and conduction (CB) bands. The best performance requires a compromise between a negative flat band potential (Vfb), a small optical gap (Eg) and a chemical stability
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