Morphological evolution between nanorods to nanosheets and room temperature ferromagnetism of Fe-doped ZnO nanostructures

Abstract

In this work, undoped and Fe-doped single-crystalline ZnO nanostructures were successfully synthesized by a facile microwave irradiation method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that Fe-doped ZnO was comprised of a single phase nature with a hexagonal wurtzite structure up to 5% Fe doping, however, secondary phase ZnFe2O4 appeared upon further increasing the Fe dopant concentration. Field emission scanning electron microscopy (FESEM) and TEM micrographs suggested that the length and diameter of the undoped ZnO rods are about ∼2 μm and ∼200 nm, respectively. Interestingly, the morphology of ZnO changed from nanorods (1% Fe) with length ∼500 nm and diameter ∼50 nm to nanosheets (5% Fe) having thickness and lateral dimension of ∼30 nm and ∼400 nm, respectively. NEXAFS and EELS studies revealed the absence of metal clusters up to 5% and Fe is found to be in a mixed (Fe2+/Fe3+) valence state with Fe2+ as the dominant state. Optical studies depicted that the absorption peak of Fe-doped ZnO was blue-shifted as the concentration of Fe increases from 1 to 5%. However, for dopant concentration >5%, the absorption peak was found to be red-shifted with an additional absorption peak of ZnFe2O4. Also, the band gap energy decreased monotonically with the increase of Fe concentration from 1 to 5%, while increasing on further doping, band gap increased. Raman scattering spectra of Fe-doped ZnO revealed the lower frequency shift of Ehigh2 mode with doping. Magnetic studies showed that Fe doped ZnO exhibit room temperature ferromagnetism (RTFM) and the value of magnetization increased up to 5% doping and then decreased for 7 and 10% Fe-doped samples.