Block Copolymer P123 As Soft Template To Prepare Nano-and Mesoporous Materials And Their Electrochemical Properties Study

Mesoporous materials and nanomaterials are two important parts of nano-structured materials. They have attracted extensive attention of researchers due to their great potential applications. The research in mesoporous materials now is mainly focused on the syntheses of novel structures and the functionalization of mesoporous materials. In the field of nanomaterials, the morphology control and assembly of low-dimensional nanomaterials dominate the scientific research. This thesis divides into two major parts. The first part, the mesoporous materials, mesoporous carbon and bicontinuous gyroidal mesoporous MnO₂ prepared based on "hard-templating"are introduced. In the second part, "soft-templating" synthesis of tin nanorod array films, lead tubes and silver dendrites nanostructure combining electrochemical method and nano-tin dioxide are described. In each part, we also studied the electrochemical performance of mesoporous materials and nano materials for the use in lithium ion batteries and supercapacitors.In chapter 2, mesoporous silica with 2D hexagonal structures, designated as SBA-15 was successfully synthesized in acidic media by using tri-block copolymer amphiphilic triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (copolymer EO₂₀PO₇₀EO₂₀, P123 ) as a structure-directing agent. By using the above-mentioned SBA-15 as a "hard template", novel mesoporous carbon was fabricated via an impregnation process combined with carbonization treatment. The resultant carbon shows a pore diameter of 5.3 nm, a surface area of 1300 m²g^-1, and a pore volume of 1.7 cm³g^-1. The CV curves showed the mesoporous carbon has better performance than activated carbon in electrochemical double layer capacitor within the potential range of 1.5～4.3 V vs. Li/Li⁺. The capacitance maintained 100 Fg^-1 with little attenuation after cycling 1000 times at the current of 0.29 Ag^-1 were detected.In chapter 3, large-pore 3D bicontinuous gyroidal mesoporous silica, designated as KIT-6, was successfully synthesized in acidic media via an evaporation-induced self-assembly procedure by using tri-block copolymer P123 as a structure-directing agent, and butanol as the additive. The resulted silica has large pores of 8 nm. By using the above-mentioned KIT-6 as a "hard template", large-pore mesoporous MnO₂ was fabricated via an impregnation process combined with heat treatment. The resultant MnO₂ shows a pore diameter of 7.8 nm, a surface area of 118 m²g^-1, and a pore volume of 0.35 cm³g^-1 The MnO₂ exhibited ideal capacitive behavior in 1 M Na₂SO₄ and 1 M LiClO₄, and the property of the former was better than that of thelatter. The specific capacitance was 228 Fg^-1 at the scan rate of 1mVs^-1 between -0.1 and +0.55 V vs. Hg/HgO in 6 M KOH, and after 64 times scanned, the capacitance had no obvious abatement. The results showed that the mesoporous MnO₂ can be used as the materials for supercapacitors.In chapter 4, single-crystalline tin nano-rod array films have been fabricated for the first time by electrochemical deposition method when P123 was used as surfactant at the concentration much lower than that for forming liquid crystalline phase. Comparing with those prepared without P123, the presence of amphiphilic triblock copolymer P123 results in fundamental changes in the microstructures of tin. It can be proposed that the amphiphilic species P123 can easily adsorb on various crystalloid facets of metal tin, except for (200) facet. The concentration of adsorbed P123 on (200) facet is much lower than that on other surfaces. Therefore the electrodeposition speed on (200) facet of metallic tin is much higher. In another word, the tin rods grow along the direction perpendicular to the (200) facet. The polarization in different solutions showed that P123 molecules inhibited the rate of electrodeposition of tin, the evolution potential of tin changed from ca. -0.45 to -0.48 V vs. Hg/Hg₂Cl₂, the infinite current density was about 3 mAcm^-2. The results showed the adsorption of P123 molecules fundamentally slowed the rate of tin deposition. The properties of these tin nano-rod arrays films and polycrystalline tin for lithium insertion were also investigated. The nano-rod tin array films exhibited good performance as anode material for lithium ion batteries. The first discharge capacity reached 867 mAhg^-1 at the current density of 0.1 mAcm^-2, which was beyond that of polycrystalline tin electrode. Cyclic voltammetry showed clearly lithium and tin intermetallic phase transitions, good performance of reversibility and cycleability within a potential range of 0.01～2.0 V vs. Li/Li⁺. The decay of capacity at some extent was ascribed to the main peak of Li_2.3Sn.In chapter 5, nanotubes or submicrotubes of lead have been prepared for the first time using P123 as the structure-directing agent by electrodeposition. The diameters of nanotubes are 20～70 nm for the circular tube, and about 2 μm for large quadrate tubes. XRD patterns showed the product was orthorhombic phase lead oxide (JCPDS No.05-570), indicating the lead was easily oxidized exposed to the air. The highest intensity relates to (111) showed the growth of nanotubes of lead was epitaxial and the grow direction was vertical to the (111). The polarization in different solutions showed that nitric acid was favorable to and P123 molecules inhibited to thegeneration of hydrogen. The produce of lead nanotubes was the resultant of the competition of hydrogen evolution with the electrodeposition of lead.In chapter 6, the elegant, highly ordered dendritic nanostructured silvers were successfully deposited by adding P123 into the low concentration of AgNO₃ solution. The product was always dominated by micro-sized silver particles with various shapes, whether with the current density of 0.165 or 3.3 mAcm^-2 when the concentration of AgNO₃ was 0.1 M. It was found that the increasing of concentration of P123 in the system could favor the formation of other shaped silver particles. When it reached to 5 wt%, the major product was silver particles with irregular shape and size. SEM results of the dendrites obtained in typical condition, showed that the diameter of the trunk is around 50 nm and the length of the trunk and the branch can reach 40 μm and 10 μm, respectively. Moreover, the dendrites had high bilateral symmetry. The main trunk and the side branches bilateral symmetrically growed along <211> directions and the leaves growed along <11-1> directions. Each side branches symmetrically growed from the main trunk with an angle about 60°. And this implied the silver dendrites crystal growed along a preferential direction. In contrast, under the same synthetic condition but without P123 addition, silver particles were obtained without any defined shape. It was quite obvious that P123 was very crucial for the formation of silver dendrites. It may have a structure-directing effect in the electrodeposition process. The polarization in different solutions showed that the adsorption of P123 molecules cut rate the generation of silver.In chapter 7, nano-SnO₂ was synthesized using triblock copolymer P123 as surfactant. These nanoparticles were less than 20 nm in size. The electrochemical properties of the nano-SnO₂ with lithium intercalation were studied by cyclic voltammetry and galvanostatic cycling measurements in 1M LiPF₆/ (EC+DMC). The nano-SnO₂ electrodes exhibited high reversible capacities of 660 mAhg^-1 and good cycle performance (C₁₀=93%C₁) at current density of 0.3 mAcm^-2 within potential of 0.005～1.2 V. But nano-SnO₂ couldn't solve the radical problem of the volume change. And the amorphous structure after charge/discharge wasn't preferable to the lithium ion intercalation/deintercalation.