Preparation Of Ordered Mesoporous Films By Liquid Crystal Template And The Properties Research

Ordered nanostructure materials are an important research content in nanotechnology realm. They have been extensively studied for the application in batteries, electrochemical capacitors, ultra-high-density magnetic storages, sensors and bioengineering, because of the combination of high surface area and periodic nanostructure. Template method is an efficient means for the fabrication of the ordered nanostructure materials. Among various templates, the lyotropic liquid crystalline phases from self-assembly surfactants have received much attention in recent years. Using the lyotropic liquid crystal templates, the ordered nanostructured materials have been obtained, such as nanowire arrays or mesoporous nanofilms of metals, alloys, semiconductors, metal oxides and conducting polymers. The lyotropic liquid crystal templates are various in the rich polymorphism, and their nanostructures are long-range periodical with good continuity and symmetry. The surfactants used to form the lyotropic liquid crystalline phases are low-cost. The process of preparation and the involved technologies can be artificially adjusted and controlled. During the material synthesis, the lyotropic liquid crystal template has good stability. Because of the high viscidity of the lyotropic liquid crystal, the product can be distributed evenly and seldom conglomerates. The adhering template in product can be removed efficiently only by simple means of the anneal or solution. The ordered nanostructured materials can grow in large scale, which has the significant meaning to the industrialized progress of the novel functional nanomaterials with special periodic structures.Electrochemical capacitor is a new type of energy storage equipment, which combines good power characteristic of physical capacitors with high energy density of conventional batteries. The electrochemical capacitor has many practical applications; therefore, it has been one of the hotspots in new type of chemical energy sources studies. For a supercapacitor with high performance, the former studies show that the excellent electrode material plays the most important role. It has to possess not only redox characteristic but also the matching nanoporous structure and the high special surface area to produce capacitance efficiently. In this dissertation, the research evolution and development trend of the ordered nanostructured materials and electrochemical capacitors are reviewed, with the emphasis on the latest development and the application future of the liquid crystalline templated functional mesoporous nanomaterials. Standing on the design angle of the nanostructure, the ordered nanoporous nickel hydroxides and oxides film electrodes have been successfully prepared via the constant potential electrodeposition route based on liquid crystalline template. And the research work on their application in electrochemical capacitors have been deeply performed. The main studies are as follows:1. The Brij 56/high purity water system and the Brij 56/Ni(NO₃)₂ aqueous solution system are prepared respectively. Their lyotropic liquid crystalline phase behaviors are investigated thoroughly and the obtained phase diagrams are compared carefully. Novel H_I-e Ni(OH)₂ film material has been successfully electrodeposited on titanium substrate from the hexagonal phase lyotropic liquid crystal template of Brij 56/ Ni(NO₃)₂ aqueous solution system. Low-angle X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM) studies show that the mesoporous film material has a regular nanostructure consisting of a hexagonal array of cylindrical pores with a repeat center-to-center spacing of about 7 nm. The fact indicates that the film is a direct cast of the used template. The special ordered nanoporous structure creates the fast electrochemical accessibility of the electrolyte and OH^- ions not only to the surface of the electrode materials but also to the bulk of the Ni(OH)₂ phase, which is very helpful for making full use of the electroactive sites to take place the faradic reaction, providing an important morphological foundation for a high specific capacitance. Preliminary studies show that the film has many merits such as low-cost, high special capacitance and good cycle properties, suggesting its potential application in electrochemical capacitors.2. We think that improvement in the electrochemical performance of the nickel hydroxide film electrode can be pursued by: (i) using nonionic surfactant Brij 56 as the structure-directing agent, (ii) varying the deposition potentials, and (iii) varying the anneal temperatures. To confirm the consideration, the films are physically characterized by XRD, scanning electron microscopy (SEM) and TEM to determine the effects of deposition potentials and anneal temperatures on the surface morphology. And electrochemical techniques such as cyclic voltammetry (CV) and chronopotentiometry are used to systematically study the effects of deposition potentials and anneal temperatures on the capacitance of the films. The investigation show that the roughness of the H_I-e Ni(0H)2 film is found to increase greatly with a decrease in the deposition potential. The heat-treatment of the deposited films at low temperatures seemed to cause little change in the surface morphology of the H_I-e Ni(0H)2 films. Electrochemical testing indicates that the lyotropic liquid crystal template, deposition potential and anneal temperature have played important roles in improving the electrochemical capacitive performance of the H_I-e Ni(OH)₂ film. The specific capacitance as high as 578 F g^-1 is achieved for the H_I-e Ni(OH)₂ film deposited at -0.70 V vs. SCE and heat-treated at 100℃for 1.5h, which electrochemical performance is obviously superior than the "usual" film deposited from Ni(NO₃)₂ aqueous solution system without template.3. A series of ordered mesoporous NiO films are prepared by heating the newly deposited H_I-e Ni(OH)₂ films at different temperatures in air. The films are physically characterized by thermogravimetric and differential thermal analysis (TG-DTG), TEM and XRD technologies. The studies show that the NiO films formed at low temperatures still retain the regular mesoporous structure with a uniform pore diameter of about 2.3 nm. Electrochemical investigations show that the anneal temperatures significantly affect the specific capacitance of the ordered mesoporous NiO films. The maximum specific capacitance of 590 F g^-1 is observed for the NiO film annealed at 250℃for 1.5 h when the hydroxide is just converted to non-stoichiometric nickel oxide. The non-stoichiometric nature (or defective nature) of the nickel oxide is beneficial to its pseudocapacitance. However, heat-treatment at higher temperatures may result in decreased surface area and surface activity of the oxide thereby leading to a decline of the specific capacitance. The results suggesting that this mesoporous nickel oxide film is a promising inexpensive electrode material with good capacitive characteristics for electrochemical capacitors. We believe that this lyotropic liquid crystal templated electrochemical method has the versatility of fabricating a wide range of ordered mesoporous transition metal hydroxide and oxide films materials.