Visible-light photocatalytic activity of zinc ferrites

• In presence of zinc ferrite , the light-induced bleaching of MB is a wavelength dependent reaction with a low conversion. • No hydroxyl radicals were generated by photo-excitation of zinc ferrite in aqueous suspension. • The interfacial e − transfer from photo-excited MB to the conduction band of zinc ferrite was thermodynamically possible. • This e − transfer could lead to the observed bleaching of MB under visible light irradiation . Zinc ferrite samples were prepared at temperatures between 75 °C and 1100 °C employing published synthetic methods Phase pure zinc ferrites were, however, only obtained through high-temperature methods (more than 800 °C) as revealed by XRD and Raman analysis. Photocatalytic experiments under UV and visible light irradiation as well as a series of detailed wavelength dependent measurements applying monochromatic light sources emitting at wavelengths of 365, 455, 505, and 660 nm were performed. Visible light-induced bleaching of methylene blue in aqueous suspensions of zinc ferrites was observed. However, no photocatalytic formation of OH radicals was detected. The results of flat band potential measurements revealed the interfacial electron transfer from an excited methylene blue molecule into the conduction band of zinc ferrite to be thermodynamically possible. The bleaching of methylene blue containing suspensions under visible light irradiation is, therefore, assumed to be initiated by an interfacial electron transfer from photo-excited methylene blue molecules adsorbed on the ferrite surface into the conduction band of the semiconducting zinc ferrite.