A Type-II Heterostructure with a KBiFe2O5 Brownmillerite Core and a ZnO Nanoparticle Shell for Enhanced Optoelectronic Performance

We formed a core-shell heterostructure with a brownmillerite multiferroic KBiFe2O5 core and a wurtzite ZnO nanoparticle shell by sol-gel technique. The KBiFe2O5 behaves as a p-type semiconductor when coupled with n-type ZnO nanoparticles. The heterostructure and the individual materials were characterized by various microscopy and spectroscopy techniques. From scanning tunneling spectroscopy and correspondingly the density of states spectra, we locate the band edges to infer the nature of the individual semiconductors. The energy level diagram of the heterostructure shows the type-II band alignment at the interface, which facilitates charge separation upon illumination. In consequence, the heterostructure reveals a better degradation yield (82.5 %) of methylene blue under sunlight and photovoltaic response compared to the individual semiconductors. The ZnO nanoparticle shell plays an important role in enhancing catalytically active sites including photogenerated charge carriers to disintegrate chemical disposal from the wastewater. All oxide heterostructures thus provide a perfect combination to maximize solar light absorption, including improved charge carrier separation at the junction by appropriate core-shell interface band alignment.