Semiconducting and electrochemical properties of the spinel FeCo2O4 synthetized by co-precipitation. Application to H2 production under visible light

Abstract

FeCo2O4 elaborated by co-precipitation was characterized photo-electrochemically for the first time in order to assess its performance for the hydrogen production. The X-ray diffraction revealed a single phase crystallizing in the spinel structure with a lattice constant of 8.1075 Å and a crystallite size of 35 nm. The UV–Visible diffuse reflectance of the black product exhibits an absorption above 650 nm and a direct optical transition at 1.53 eV was determined, assigned to the crystal field splitting of Co3+: 3d orbital hexa-coordinated. The transport properties indicated semi-conducting properties of FeCo2O4, the positive thermo-power (S300k = 666 μV K−1) demonstrated that holes are majority charge carriers. The Electrochemical Impedance Spectroscopy (EIS) realized in Na2SO4 (0.1 M) electrolyte showed two semicircles at high and intermediate frequencies, characteristic of the charge transfer and grain boundaries respectively. The first diameter (1249 Ω cm2) decreases under visible light down to 742 Ω cm2, thus supporting the semiconducting behavior and no inhibiting effect is observed due to the appearance of the photo-effect. The inverse of the square of the capacitance as a function of the potential (C−2 – E) exhibits a line, negatively slopped, characteristic of p-type behavior with a hole concentration of 21.9 × 1020 cm−3. The conduction band (−0.76 VSCE), made up of Co3+: 3d orbital is more cathodic than the H2-level leading to a spontaneous H2 evolution under visible irradiation (29 mW cm−2) with a liberation rate of 0.58 μmol H2/h−1/g. FeCo2O4 an showed excellent chemical stability after its reuse for the H2 production, as evidenced by X-ray diffraction