Controlled Synthesis And Properties Of Cobalt Oxides Nanomaterials

Nanostructured cobalt oxides are one of the most important materials in nanotechnology because of their potential applications. Although catalysis, supercapacitors, and magnetic properties have been extensively studied in the recent years, the growth mechanism of various cobalt oxides nanostructures, the dependence of properties on structures, and the properties mechanism are still not clear. In the thesis, the growth mechanism, supercapacitance and the magnetic properties are studied. Moreover, the properties mechanism is investigated by first-principles. The main contents are summarized as follow.(1) We investigate the growth of cobalt sodium tartrate nanowires (CSTNWs) as a precursor. Diameter-adjustable CSTNWs are synthesized by a facile and environmentally friendly hydrothermal route using ethanol-water mixtures as solvents. Scanning and transmission electron microscopy as well as various spectroscopic techniques show that the CSTNWs are30micrometers long and their diameters have a narrow size distribution with smooth surfaces. The diameters of the CSTNWs are found to decrease with the reaction time, and are controllable in a range of80-250nm by the reaction time. In the synthesis of the homogeneous CSTNWs, the optimal reaction parameters include an ethanol/water ratio of1:2combined with a sodium tartrate concentration of0.225M and a pH value above13.5. A possible growth mechanism of CSTNWs is suggested. Tartrate anions act as the sustain ligands with all six oxygen atoms participating in the coordination, while the Co2+and Na+ions chelate with the O atoms from the carboxylate and hydroxyl groups in tartrate ligands. Ethanol participates in the formation of pi bonds to construct the one-dimensional networks. The amorphous nature of the CSTNWs should be attributed to the bond length differences between the Co-O and Na-0bonds and the asymmetry of tartrate molecules. The method to synthesis CSTNWs can be valuable for growth of shaped and sized metal-organic nanowires.(2) The growth mechanism and the supercapacitors properties of Co3O4nanowires are investigated. The porous Co3O4nanowires are synthesized by calcining the precursor of CSTNWs in the air. The size and shape of Co3O4nanowires are dependent on the calcining time and temperature. The optimal reaction parameters include a calcining temperature of500℃and a calcining time of1.5h, well-distributed mesopores with uniform diameters on the nanowires are obtained, as well as abundant nanopores. The sample synthesized under these parameters will have the best capacitance properties. The capacitance is923.24F/g via cyclic voltametry test with the scan rate is50mV/s. It implies that the Co3O4have good fast charge-discharge properties and high power density, can be used as high-rate electric devices.(3) The structures and electronic, magnetic properties of single crystal Co3O4nanowire are investigated by first-principles. The surfaces structures of the Co3O4nanowires are found to be{111} surfaces with the terminated Co3+. Surface states in the nanowire is found, which is come from the Co3+. This surface states cross the Fermi level, which usually appear in the semiconductors nano structures while not in metal surfaces, consistent very well with the experiments results. Furthermore, a core-shell model of magnetic exchange interaction is proposed for the Co3O4nanowires based first-principles. This model includes (ⅰ) ferromagnetic exchange interaction between Co3+in the surface plane (shell),(ⅱ) antiferromagnetic interaction in core Co2+,(ⅲ) antiferromagnetic core-shell interactions,(ⅳ) the surface plane-plane interaction dependent on the nanowire diameters. Plane-plane interaction competes with core-shell interactions as the diameter larger than15.4A. This competition leads to the complex phenomena, such as spin-glass states and magnetic-fingerprint. The magnetic interaction model also give an explanation of two order temperature and exchange bias found in nanowire(4) The magnetic properties of CoO nanoparticles are investigated by experimental measure and first-principles calculations. The different shaped particles, octahedra, spheres, and half-octahedra-spheres, are synthesized. Interestingly, ferromagnetic properties are found in octahedra and half-octahedra-spheres. There Curie temperatures are both8K. Moreover, the magnetic susceptibility of octahedra is almost two times of half-octahedra-spheres, consistent with the fact that the ordered surface plane area of octahedra is two times of that for half-octahedra-spheres. No ferromagnetism is found in spheres. The shapes driven a phase transition from ferromagnetism to antiferromagnetism exists when octahedra shaped particles changes to spheres shaped particles. First-principle calculations confirm that the octahedra surface planes is{111} surfaces terminated with Co atoms and the{111} Cr planes are the source of the ferromagnetism. The phenomena of Magnetic susceptibility of octahedra is almost two times of half-octahedra-spheres is come from that the{111} plane area of octahedra is two times of half-octahedra-spheres.