Lead-Free BNT–BT0.08/CoFe2O4 Core–Shell Nanostructures with Potential Multifunctional Applications

Abstract

Herein we report on novel multiferroic core&ndash;shell nanostructures of cobalt ferrite (CoFe₂O₄)&ndash;bismuth, sodium titanate doped with barium titanate (BNT&ndash;BT_0.08), prepared by a two&ndash;step wet chemical procedure, using the sol&ndash;gel technique. The fraction of CoFe₂O₄ was varied from 1:0.5 to 1:1.5 = BNT&ndash;BT_0.08/CoFe₂O₄ (molar ratio). X&ndash;ray diffraction confirmed the presence of both the spinel CoFe₂O₄ and the perovskite Bi_0.5Na_0.5TiO₃ phases. Scanning electron microscopy analysis indicated that the diameter of the core&ndash;shell nanoparticles was between 15 and 40 nm. Transmission electron microscopy data showed two&ndash;phase composite nanostructures consisting of a BNT&ndash;BT_0.08 core surrounded by a CoFe₂O₄ shell with an average thickness of 4&ndash;7 nm. Cole-Cole plots reveal the presence of grains and grain boundary effects in the BNT&ndash;BT_0.08/CoFe₂O₄ composite. Moreover, the values of the <i>dc</i> conductivity were found to increase with the amount of CoFe₂O₄ semiconductive phase. Both X-ray photoelectron spectroscopy (XPS) and M&ouml;ssbauer measurements have shown no change in the valence of the Fe³⁺, Co²⁺, Bi³⁺ and Ti⁴⁺ cations. This study provides a detailed insight into the magnetoelectric coupling of the multiferroic BNT&ndash;BT_0.08/CoFe₂O₄ core&ndash;shell composite potentially suitable for magnetoelectric applications.