Enhanced photocatalytic performance of NiFe2O4 nanoparticle spinel for hydrogen production

The spinel NiFe2O4, prepared from nitrates precursors, was characterized by thermal analyses, X-Ray Diffraction, UV-Vis diffuse reflectance, Scanning electron microscopy, X-Ray Fluorescence spectrometry, X-ray photoelectron spectroscopy and photo-electrochemistry measurements. The X-ray diffrcation analysis of the powder indicates a cubic phase with a lattice constant of 8.327(8) Å and crystallite size of 19 nm. The X-Ray Fluorescence spectrometry indicates a stoichiometry, very close to NiFe2O4 catalyst calcined at 900 °C The X-ray photoelectron spectroscopy analysis confirmed the valences and crystallographic sites of the transition elements. The direct optical gap of NiFe2O4 (1.78 eV), due to the crystal field splitting of the 3d orbital in the octahedral site, is well suited for the solar spectrum and attractive for photo-electrochemical H2 production. The flat band potential (Efb = 0.47 VSCE) was obtained from the capacitance-potential (C−2 - E) characteristic in NaOH (0.1 M) electrolyte. A conduction band of −1.11 VSCE, more cathodic than the H2 level (−0.8 VSCE), enabled the use of NiFe2O4 for the water reduction into hydrogen. The H2 evolution rate of 46.5 μmol g−1 min−1 was obtained under optimal conditions (1 mg of catalyst/mL, NaOH and 50 °C) in the presence of SO32− (10−3 M) as hole scavenger under visible light flux of 23 mW cm−2. A deactivation effect of only 1% was obtained.