Fabrication And Characterization Of Cobalt Core-Shell Nanocomposites And Hollow Compounds

In this paper, the core-shell nanostructures were prepared using simple liquid chemical methods (sol-gel method; homogeneous precipitation method; hydrothermal method). On the basis of this composite, the hollow sphere was obtained after the core template was removed by chemical etching or calcination. The shell thickness could be controlled by changing the concentration of precursor or reaction time. Based on the experiment, the formation mechanisms of core-shell structure and the mesopores were studied. It was discussed how the structure has effect on its electrochemical properties or magnetic properties.1. Fabrication and Characterization of Mesoporous Co₃O₄ Core/MesoporousSilica Shell NanocompositesThe parent Co₃O₄ particles comprised of oriented-aggregated nanocrystals was firstly prepared followed our previous report. Then, the Co₃O₄ nanoparticles were dispersed in a certain proportion of ethanol-water-ammonia-CTAB mixture. After the addition of the precursor TEOS, the silica directly deposited on the surface of Co₃O₄ nanoparticles, and the mesoporous Co₃O₄/mesoporous silica core/shell nanocomposite was formed. Based on the wide-angle X-ray diffraction results, both of the bare Co₃O₄ nanoparticles and the coated Co₃O₄ nanoparticles exhibit the same spinel structure. So it is concluded that Co₃O₄ nanoparticles has good chemical stability and thermal stability. Based on the low-angle X-ray diffraction results, it is confirmed that the Co₃O₄ core and the silica shell have mesostructures. TEM observation indicates that the presence of the silica shell effectively prevents the Co₃O₄ nanocrystals from growing. The thickness of silica shell could be controlled by changing the concentration of precursor TEOS. Based on the experiments, the formation mechanism of the nanocomposite with a mesoporous silica shell and a mesoporous Co₃O₄ core was proposed, which is based on the electrostatic attraction between negatively charged parent Co₃O₄ particles, the positively charged CTAB micelles and the negatively charged silicate species. The mesoporous structure of the Co₃O₄ core and the silica shell has been further confirmed by the N₂ adsorption-desorption isotherm. This work might provide a novel route to core/shell nanostructures and the mesoporous materials from the aggregating of the nanocrystals. Electrochemical analysis reveals that the composite shows a relatively higher discharge capacity comparing to its corresponding bare Co₃O₄ nanoparticles.2. Synthesis and Characterization of Mesoporous cobalt hydroxide nano-hollow sphereFirstly, the silica nanoparticles were synthesized according to previous report. The average size of silica particles is about 75 nm. Then, a certain amount of silica nanoparticles were dispersed in deionized water. After the addition of the urea and cobalt nitrate, the solution was heated to 80℃, and refluxed at that temperature for 2 hours. As a result, the cobalt (II) hydroxide was deposited on the surface of silica, and the silica core/mesoporous cobalt (II) hydroxide shell nanocomposite was formed. On the basis of the nanocomposite, the mesoporous cobalt (II) hydroxide hollow sphere was obtained after the core template was eroded by sodium hydroxide. The wall thickness of the mesoporous cobalt hydroxide hollow sphere could be controlled by changing the reaction time. XRD analysis demonstrated that cobalt (II) hydroxide existed as amorphous. Based on the low-angle X-ray diffraction results, it was concluded that the cobalt hydroxide hollow sphere had mesoscopic structure. XPS spectra shows the characteristic Co 2p peaks at 798.37 and 782.47eV with the separation of 15.9eV between Co 2p_1/2 and Co 2p_3/2, which confirmed that Co mainly existed as Co(OH)₂. The formation of the mesoporous cobalt hydroxide shell has been further confirmed by the nitrogen adsorption-desorption experiment. The BET surface area of the hollow sphere (wall thickness: 10nm) is 418m²/g, and BJH analysis shows that the most probable pore sizes is ca.3.9 nm with narrow size distribution. The thicker the wall thickness is, the smaller the BET surface area of the hollow sphere is. According to the experimental results, the formation mechanism of the nanocomposites with a silica core and a cobalt hydroxide shell was proposed, which is based on the electrostatic attraction between negatively charged silica particles, the positively charged cobalt ions, and the formation of cobalt hydroxide nanotubes was proposed. Firstly, the cobalt hydroxide nanoparticles were formed by hydrolysis of cobalt ions, then, the cobalt hydroxide nanosheets were formed by aggregation of nanoparticles, finally, the cobalt hydroxide nanosheets curled to form nanotubes. Electrochemical test results showed that the hollow sphere of cobalt hydroxide electrode materials demonstrated a higher capacitance and cycle stability than the corresponding bulk cobalt hydroxide. This is due to the larger specific surface area and shorter ion transmission distance of the mesoporous cobalt hydroxide hollow sphere.3. Synthesis and Characterization of CoFe₂O₄ hollow spheresFirstly, the micron carbon sphere was formed through the decomposition of glucose during the hydrothermal reaction in the glucose-cobalt sulphate-ammonium ferrous sulfate system. Due to many hydroxyl groups on the surface of carbon sphere, Fe²⁺ , Co²⁺ were directly adsorbed on the carbon sphere surface, as a result, carbon/metal salt core-shell composite was formed. After calcination, the cobalt ferrite hollow sphere could be obtained. EDS and XRD characterization demonstrated that the product was spinel cobalt ferrite. Based on the results of TEM and HRTEM observation, the Size of cobalt ferrite hollow spheres was between 600nm and 900nm, which was formed by the nanocrystalline aggregates with the size of ca.28 nm. Magnetic Properties showed that cobalt ferrite hollow spheres were ferromagnetic. The saturation magnetization of CoFe₂O₄ hollow sphere was 54 emu/g, which was less than that of the bulk cobalt ferrite (72 emu/g). This is due to the nanocrystalline's surface and size effects. The coercivity of CoFe₂O₄ hollow spheres was 860 Oe, and remanence magnetization was 18 emu/g. This material has high saturation magnetization and coercivity. Such material with hydrophilic surface is biocompatible and has potential applications in drug delivery.