Physical and photo-electrochemical properties of the spinel SrFe2O4: application to hydrogen production under visible light

SrFe 2 O 4 prepared by sol–gel method after annealing at 800 °C crystallizes in a normal spinel structure. The structural, morphological, magnetic, optical, transport, and photo-electrochemical properties were systematically investigated. The sol–gel permits to have nanocrystallites with an average size of ~ 30 nm. The UV–Visible diffuse reflectance analysis gives a direct transition of 1.80 eV, resulting from the Fe 3+ : 3d orbital splitting in octahedral site. Such degeneracy lifting into (t 2g –e g ) levels is properly matched to the sun spectrum. The capacitance–potential (C −2 − E ) characteristic of SrFe 2 O 4 plotted in basic electrolyte (KOH 0.1 M) exhibits p -type comportment with a flat band potential of ( E fb ) of 0.01 V SCE , a holes density ( N A ) of 1.4 × 10 15 cm −3 , and an extended space charge region of 0.9 µm. The electrochemical impedance spectroscopy exhibits a semicircle characteristic of the charge transfer whose diameter decreases under irradiation, thus supporting the semiconducting character of SrFe 2 O 4 . The electrons in the conduction band (− 1.70 V) have a high reducing ability and cathodically positioned with respect to the H 2 O/H 2 level, thus producing H 2 evolution under visible light illumination with a concomitant oxidation of SO 3 2− /S 2 O 6 2− . The best activity occurs at pH ~ 11 with H 2 evolution rate of 35 μmol mn −1 g −1 and a quantum yield of 1.8% under visible light (29 mW cm −2 ).