Chemical Synthesis and Structural and Magnetic Properties of Dispersible Cobalt- and Nickel-Doped ZnO Nanocrystals

Abstract

The preparation, characterization, and magnetic properties of dispersible Co- and Ni-doped ZnO nanocrystals (NC) are reported. The nanocrystals, Zn1−xTMxO where x = 0−0.07 for TM = Co and x = 0.02, 0.03 for TM = Ni have been synthesized by nonhydrolytic alcoholysis esterification elimination reaction using anhydrous metal carboxylates as precursors in the presence of oleic acid and oleyl alcohol at temperatures as low as 220 °C. The prepared nanocrystals can be easily dispersed in solvents like chloroform and toluene. Detailed X-ray diffraction (XRD) using synchrotron radiation (SR), microstructure, and Raman studies reveal that the Co-doped ZnO nanocrystals are single-phase wurtzite structure without any parasitic secondary phases and Co atoms are incorporated into the ZnO lattice and located at the substitutional sites of the Zn atoms. Magnetic studies as a function of temperature and field indicate that Co- and Ni-doped zinc oxide nanocrystals are ferromagnetic at room temperature (RT). The observed ferromagnetism in Co-doped ZnO nanocrystals is interpreted in terms of a spin-split donor impurity-band model. The detection of donor defects bound to Co sites by Raman spectroscopy and decreased magnetization in oxygen-annealed samples suggest that activation of ferromagnetism depends on defects, such as oxygen vacancies (VO) and zinc interstitials (Zni). On the other hand, the observation of secondary phases like NiO and Ni in the powder XRD patterns of Ni-doped ZnO nanocrystals suggests the extrinsic origin of ferromagnetism in this material.