Novel Transition Metal Oxide Nanostructures: Construction And Their Related Properties

Transition metal oxide micro/nano-materials have broad application prospects in supercapacitors, electrochemical sensors and photocatalysts, owing to their various structures and unique physical and chemical properties. Thus exploiting new synthetic method to obtain specific architecture, dimensions, size of micro/nano- materials and exploring the growth process have important practical significance on deeper realization of the relationship between structure and properties, and the broadening the application of synthesis of transition metal oxide. Typical transition metal oxides, such as Co₃O₄, NiO, NiCo₂O₄ and CuO were widely studied and applied in the field of optical, electrical and magnetic. In this paper, the controllable preparation of transition metal oxides can be reached by various strategies including hydrothermal treatment, in-situ growth and anodic oxidation, the reaction mechanisms and properties in supercapacitors, non-enzymatic glucose sensors and photocatalysts are investigated. The main results are as follows:（1） Porous hybrid nanonet/nanoflake NiCo₂O₄ with uniform morphology and high surface area have been obtained by annealing precursor synthesized by a facile solvothermal method, in which ammonium nitrate has been utilized not only to produce ammonium ions, but also to promote a similar nucleation and growth rate of Co（OH）₂ and Ni（OH）₂. The component and final morphology of NixCo_3-xO₄ can be appropriately tuned by selecting experiment parameters, such as the molar ratio of raw materials. The electrochemical performances of the as-prepared NixCo_3-xO₄ products are evaluated. It is demonstrated that tuning of the morphological structure and the pore size distribution of the products is very important in supercapacitors. Porous self-assembled nanonet/nanoflake NiCo₂O₄ electrodes exhibit excellent cyclic performance with a 94.8 % capacity retention at current density of 10 A g^-1 in a testing range of 10000 cycles.（2） By in situ growth method, the copper foil were selected as substrates for the controllable synthesis of CuO nanotube arrays with ultra-high surface area, which were transformed from corresponding Cu（OH）₂ precursor by annealing process. The morphological evolution process of the as-obtained product has been investigated by observing the intermediates during the reaction. The as-prepared CuO nanotube arrays are applied as the electrode material for supercapacitors, and the sample exhibits excellent performance with a high specific capacitance of 442 F g^-1 and a capacitance loss of only 4.6 % at current density of 1 A g^-1 after 5000 cycles. Furthermore, the electrochemical property of CuO nanotubes for glucose electrooxidation was entirely investigated by cyclic voltammetry. In the amperometric detection of glucose, the CuO nanotubes/Cu foil electrode exhibits an extraordinary limit of detection of 1.07 μM（S/N=3） and a wide dynamic range with excellent sensitive of 2231 μA mM-1 cm-2（R=0.999） at the working potential of +0.55 V. The interference from hematic acid, uric acid, urea, lactose and sodium chloride at the level of their physiological concentration were negligible, displaying the superior selectivity. These results indicate that CuO nanotubes/Cu foil composite is a promising material in various applications.（3） We have prepared three CuO nanostructures with different morphology, using a facile alkaline solution oxidation method, Cu foil as copper source by adjusting the kind of surfactant. Supercapacitor electrode based on the CuO nanostructures showed a good pseudocapacitive property. Both cyclic voltammetry and galvanostatic charge/discharge tests concluded that the flower-like electrode had a largest capacitance of 615 F g^-1 at a scan rate of 5 mV s^-1 and 520 F g^-1 at a current density of 1 A g^-1. The CuO nanoflowers have so good electrochemical performance as electrode materials for supercapacitor, which is owed to its three dimensional self-assemble structures. The novel structures within CuO nanospheres not only accommodate the volume change of charge/discharge process to maintain the structure integrity of electrodes, but also shorten OH- diffusion pathways.（4） A facile anodic oxidation method is designed for large-scale preparation of graphene-like CuO nanofilms in situ grown on conductive Cu foam, which were transformed from corresponding Cu（OH）₂ precursor by annealing process. The nanofilms had an interconnected network structure, which consisting of ultrathin CuO nanosheets perpendicularly standing on the substrate. Electrochemical results demonstrated that the nanosheet-based CuO nanofilm electrode exhibited outstanding capacitance performance with high rate property and good cycling stability. In addition, the as-prepared nanofilm electrode also showed good photocatalytic activities. Compared with commercial CuO powder, the CuO nanofilms showed an enhanced catalytic activity for the degradation of methylene blue. After 60 min, the degradation achieved by CuO nanofilms was 99 %, which was higher than that of the commercial CuO powder（54 %）.