Co:ZnS nanostructures have been synthesized via the solvothermal method by modifying the stoichiometry of Zn and S sources in the solvothermal process. The structure and morphology analysis demonstrates that the increase of the Zn/S molar ratio leads to changes in the morphology and structure, from Co:ZnS nanorods and hexagonal nanocrystals to core–shell Co:ZnS/ZnO nanocrystals. The photoluminescence spectrum for Co:ZnS/ZnO exhibits strong visible light emissions due to the presence of a large amount of structural defects, whereas the emissions for Co:ZnS nanorods almost disappear because of the high crystallinity. All the samples display obvious room-temperature ferromagnetism. The saturated magnetic moment of the samples strongly depends on the concentration of structural defects in the nanostructures and can be effectively enhanced by the introduction of abundant defects.
We demonstrate a method to control the distribution of magnetic Co ions in Co-doped SnO2 nanorods by the surfactant. The nanorods with nanoflower morphology have been synthesized by surfactant-assisted hydrothermal method. The X-ray absorption fine structure spectra at Co K-edge reveal that Co ions substitute for Sn site in SnO2 lattice. The X-ray absorption fine structure spectra at O K-edge with its simulated spectra have been performed to investigate the local environment of Co ions. In the experimental spectra, the pre-edge peak continues to decline with the addition of the surfactant. It shows the same trend when the number of clustered Co ions in the simulated spectra is reduced, which indicates that the surfactant suppresses the formation of Co clusters. The effect of surfactant in mediating Co distribution arises from the strong interactions between the Co2+ ions and the CTA+ ions. In the Co K-edge experimental spectra, the weakened pre-edge peak reflects the electron transfer from the CTA+ ions to Co 3d band. Such electron transfer changes the charge state of Co ions and modifies the electrostatic interactions among dopant ions, leading to the uniform distribution of substitutional Co dopants. These results provide experimental guidance in the synthesis of the homogeneous-doped metal oxide semiconductors.
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