The photocatalytic hydrogen formation and NO2− oxidation on the hetero-junction Ag/NiFe2O4 prepared by chemical route

The photoelectrochemical ability of Ag(5%)/NiFe2O4 for H2 evolution under visible light irradiation was studied. The hetero-junction prepared by sol gel method shows a much higher activity than the physical mixture (hetero-system: Ag + NiFe2O4). The electrons of NiFe2O4 activated by visible light are focalized toward Ag clusters, intimately contacted to the spinel. The bifunctional crystallite is generated by incorporation of ultrafine Ag crystallites, thereby avoiding high over-voltages. The spinel was characterized by thermal analysis, X-ray diffraction and scanning electron microscopy. The optical gap of NiFe2O4 (1.59 eV) is due to the crystal field splitting of d-d transition of Fe3+: 3d orbital in octahedral site. p-type conductivity of NiFe2O4 was demonstrated from the capacitance measurement in alkaline KOH solution (0.1 M). The conduction band (−0.87 V) is more cathodic than the H2 evolution and the hetero-junction was successfully used for the photochemical hydrogen production under visible light irradiation. The photo-activity is dependent on the nature of the hole scavenger (NO2−, Fe(CN)64− and I−). NO2− is a hazardous ion and gave the best activity, due to its optimal band bending of 0.5 V and is converted to nitrate. An evolution rate of {0.17 cm3 H2 (g catalyst)−1 mn−1} was obtained at pH ∼ 7 with an apparent quantum yield of 1.3%.