Abstract
Zinc oxide nanopowders doped with 1–15 mol % cobalt were produced by the microwave solvothermal synthesis (MSS) technique. The obtained nanoparticles were annealed at 800 °C in nitrogen (99.999%) and in synthetic air. The material nanostructure was investigated by means of the following techniques: X-ray diffraction (XRD), helium pycnometry density, specific surface area (SSA), inductively coupled plasma optical emission spectrometry (ICP-OES), extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and with magnetometry using superconducting quantum interference device (SQUID). Irrespective of the Co content, nanoparticles in their initial state present a similar morphology. They are composed of loosely agglomerated spherical particles with wurtzite-type crystal structure with crystallites of a mean size of 30 nm. Annealing to temperatures of up to 800 °C induced the growth of crystallites up to a maximum of 2 μm in diameter. For samples annealed in high purity nitrogen, the precipitation of metallic α-Co was detected for a Co content of 5 mol % or more. For samples annealed in synthetic air, no change of phase structure was detected, except for precipitation of Co3O4 for a Co content of 15 mol %. The results of the magentometry investigation indicated that all as-synthesized samples displayed paramagnetic properties with a contribution of anti-ferromagnetic coupling of Co–Co pairs. After annealing in synthetic air, the samples remained paramagnetic and samples annealed under nitrogen flow showed a magnetic response under the influences of a magnetic field, likely related to the precipitation of metallic Co in nanoparticles.
Highlights
Nanomaterials have drawn the attention of researchers from all over the world due to their new, interesting perspectives in many application areas [1,2,3]
In the present work we investigate zinc oxide (ZnO), which is an attractive material with a wide range of applications such as: transparent transistors based on semiconducting transparent oxides [4], ultraviolet (UV) light blockers [5], photocatalysts [6] or antibacterial uses [7]
scanning electron microscopy (SEM) images of undoped and 1%, 5%, 10%, and 15% Co-doped ZnO nanopowders in their as-produced state and after heat annealing in nitrogen and synthetic air are presented in Figure 1, Figure 2 and Figure 3, respectively
Summary
Nanomaterials have drawn the attention of researchers from all over the world due to their new, interesting perspectives in many application areas [1,2,3]. Using hydrothermal [32,48] and sol–gel [49,50] synthesis methods, Co-doped ZnO with both ferromagnetic and paramagnetic properties was obtained [39,41,42] These observations questioned the possibility of producing powders with controllable properties suitable for practical application as a spintronic material. We performed magnetic investigations for samples annealed under different gas environments, while keeping the same annealing temperature and process time Another question was the limit of Co content for paramagnetic behavior, and whether annealing of the synthesized powders will lead to Co clustering and the appearance of a FM response
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