Photoelectrochemical study of the spinel CaFe2O4 nanostructure: application to Basic Blue 41 oxidation under solar light

Ca[Fe](2)O-4 prepared by nitrate route is a narrow band gap semiconductor crystallizing in the normal spinel structure. The direct optical band gap E-g (1.93 eV), determined from the diffuse reflectance, is due to the lifting of degeneracy of Fe3+: 3d orbital, octahedrally coordinated. The semi-logarithm plot of the oxide shows a chemical stability at pH similar to 7 with an exchange current density of 2x10(-7) mA cm(-2) and a corrosion potential of -0.664 V-SCE. The photoelectrochemical properties of CaFe2O4 are investigated for the first time to assess its photocatalytic activity. A flat band potential (E-fb) of 0.33 V-SCE was obtained from the capacitance measurement in Na2SO4 (0.1 M) solution. The n-type comportment, rare for spinels, is confirmed by the chrono-amperometry and inferred to the metal insertion in the FeO6 octahedra. The electronic concentration N-D (7.1 x10(15) cm(-3)) agrees with a non-degenerate semi-conductivity and a wide space charge region (W similar to 850 nm). The electrochemical impedance spectroscopy (EIS) shows depressed semicircles with the existence of a constant phase element (CPE) and a bulk resistance of 10.89 k omega cm(2) which decreases down to 2.24 k omega cm(2) under visible irradiation. A Warburg diffusion is observed at low frequencies. The valence band (2.09 V-SCE) deriving from Fe3+: e(g) character is more anodic than the potential of H2O/(OH)-O-center dot couple. As an application, the Basic Blue 41 (BB 41), a recalcitrant dye, is successfully oxidized; an abatement of 85% is obtained within 3 h under solar light (980 W m(-2)) and the BB 41 oxidation follows a first-order kinetics with a photocatalytic half-life of 69 min.

Automated summary

Ca[Fe]（2）O-4 prepared by the nitrate route exhibits good photoelectrochemical properties and photocatalytic activity, with high oxidation efficiency towards the recalcitrant dye BB 41, making it a promising photocatalytic material.