Growth of Co-doped ZnO nanoparticles by porous alumina assisted sol-gel route: Structural optical and magnetic properties

We report a simple sol-gel method with spin coating to grow Co-doped ZnO nanoparticles (NPs) using porous alumina template. X-ray diffraction analysis confirms the wurtzite hexagonal structure for the Zn1-xCoxO NPs. Transmission electron microscopy (TEM) micrograph reveals clearly the formation of NPs with average diameter in the range 40-50 nm. High resolution TEM lattice images and the electron diffraction (SAED) pattern show that the NPs are single crystalline with lattice plane spacing of 2.47 angstrom for the (101) plane. UV-Vis absorption spectra show a slight blue shift in the absorption edge due to doping related modification in the band structure. PL spectra show a band edge related UV emission around 378 nm at low temperature, while defect (Zn-i, O-V) related visible emission dominates at room temperature. Room temperature ferromagnetism (RTFM) is observed in Zn1-xCoxO NPs with the moment of 450-750 memu/g. The temperature dependent magnetization (M-T) curve shows high transition temperature (T-c) as 750 K. The observed FM is explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Co ions and Zn-i, O-V related defects. This study provides an effective way to obtain HTFM with lower doping concentration as well as to have control over NP size by alumina pores in the Zn1-xCoxO system. (C) 2015 Elsevier B.V. All rights reserved.