Visible light hydrogen production on the novel ferrite NiFe2O4

Abstract

The spinel type compounds represent a new family of photocatalysts that can be used as photoelectrodes capable to produce hydrogen under visible light. In the current study, the NiFe2O4 spinel, which is prepared by the sol–gel method, is investigated as a possible candidate, and the structure, opto-electronic, electrochemical and photoactive properties are characterized. NiFe2O4 exhibits a p-type semiconductor behaviour with a hole mobility of 1.57 × 10−10 cm2 (V s)−1. The conduction at 300 K is found to take place by electron-hopping mechanism with an activation energy ΔEσ1=0.12ev, and the lowest optical transition is observed at 1.56 eV. The valence band is of cationic character and derives from the Ni-3d orbital (5.23 eV/vacuum). On the other hand, the conduction band, ECB = −1.62 VSCE, is located above the H2O/H2 potential (−0.98 VSCE), which allows for the H2-evolution under illumination. The effects of NiFe2O4 amount, pH level and S2O32− concentration are optimized. Under these ideal conditions, the rate of H2 involvement attains 0.68 × 10−3 mL mg−1 min−1 with a quantum efficiency η(H2) = 0.53%. Electrochemical impedance spectroscopy is also used to model the photosystem and an electronic equivalent circuit is proposed.