Synthesis And Properties Of Transition-Metal Oxides Nanostructures

Over the past decade, nanostrcutured transition-metal oxides have received enormous research attention, due to their great potential in a myriad of applications, such as catalysis, photocatalysis, sensing, optoelectric devices, energy storage, and so on. Since the shape of the nanostrcuture plays a pivotal role in determing the physical and chemical properties, it is significant to develope the shape-controlled synthesis for tailoring and maximizing the properties.In the present dissertation, we prepared low-dimensional Mn3O4 and CuO nanomaterials by using facile synthetic methods. By varying the experimental parameters, we are trying to reveal the mechanisms of morphology-controlled synthesis. Furthermore, We studied their physical and chemical properties and attemped to establish the relaionship between the properties and morphologies. More specifically:1. The single-unit-cell thick Mn3O4 nanosheets were synthesized in an aqueous solution at room temperature. Due to the highly shape anisotropy, these single-unit-cell thick Mn3O4 nanosheets demonstrated lower TC (39.5 K), lower TB (38 K), lower Ms (20 emu/g) and much greater HC (5800 Oe) than those of bulk Mn3O4, respectively.2. The ultrathin CuO nanowires with an average diameter of 3 nm were prepared through the topotactic transformation from Cu(OH)2 to CuO under an appropriate temperature. Because of the high specific surface area and short Li+ diffusion length, the ultrathin CuO nanowires anodes were demonstrated a superior electrochemical performance. The discharge specific capacity is 500 mAh/g after 20 cycles under a current density of 0.1 C.3. Controllable synthesis of various morphologies of CuO nanostructures tuning by hetero-metal cations is developed in aqueous solution at room temperature. The mechanism of hetero-metal cations was studied in detail, and we found:(1) The reaction kinectics is the deterministic factor to tune the morphologies of CuO nanostructures. The faster reaction rate, the elliptic CuO nanosheets with larger length ratio of long axis to short axis were produced. The slower reaction rate, the rounded CuO nanoparticles were synthesized; (2) Hetero-metal cations play a crucial role in storing OH- and controlling the releasing rate of OH- by forming hetro-metal complexes, and thus tuning the reaction kinetics; (3) The stability of the hetero-metal complexes determins the capability of the hetro-metal cations to slow the reaction kinetics; (4) The phase of final products is determined by the solubility of hetero-metal hydroxide after introducing the hetero-metal cation into the reaction solution. If the solubility of hetero-metal hydroxide is greater than that of Cu(OH)2, the final product would be CuO. However, if the solubility of hetero-metal hydroxide is smaller than that of Cu(OH)2, the final product would be hetero-metal related compound.4. The mesoporous CuO nanosheets with large specific surface area of 80.34 m2/g were synthesized in an aqueous soultion at room temperature. The mesoporous CuO nanosheets demonstrated very high catalytic activity toward CO oxidation reaction. The results showed that the CO conversion efficiency of mesoporous CuO nanosheets is 47.77 mmolcog-1 cath-1 at 200 ℃, which is 35 times higher than that of commercial CuO powder.5. To enhance the electrochemical performance of the CuO nano structure in lithium ion batteries, we proposed a facile, flexible, and large-scale technique to prepare CuO nanostructures/CNT composites with the aid of electrostatic interaction in aqueous solution.