Large Aspect Ratio Ordered Porous Alumina Template Preparation Of Its Application

Anodic alumina oxide membrane (AAO) are widely used in various fields with simple process, uniform pore size, high pore density, pore not connected with each other, consistent orientation and pore diametre can be adjucted according to practical needs. In recent years, synthesis of one-dimensional nanomaterials with the AAO has become a research hotpot. As we know, the properties of the one-dimensional nanomaterials are largely determined by its size and shape limited by the AAO template used in the synthesis.Therefore, to prepare high-quality AAO template is particularly important. In this thesis, The AAO template with ultrahigh aspect ratio was fabricated by two-step anodization process under galvanostatic conditions, in which the Ni nanowires and Pt-Ni nanowires are successfully assembled with the aspect ratio of 1000. The main research works are descried as follows:1. The preparation of AAO with ultrahigh aspect ratio. A highly purity of aluminum plate was annealed to eliminate the internal stress, then ultrasonicated in acetone, ethanol and double-distilled water respectively to remove grease. Before the anodization, the aluminum plate was first electropolished for 20 min under a constant DC voltage of 7V at 13-14℃in a solution of ethanol/perchloric acid (5:1, v/v). After the electropolish, the fist anodization was carried out in a 0.3 M of oxalic acid solution at 8 mA/cm2 and 0℃for 6 h, Then, the formed anodic oxide layer was eroded in a mixture soluction of 0.4 mol/L H3PO4 +0.2 mol/L, remaining regular pitting. Next, the plate was anodized again for 18 h under the conditions identical to those for the first anodizing step and then dipped into an aqueous solution of CuCl2/HCl to remove the Al layer backside. The barrier layer at the bottom of the AAO film was etching by the 5wt% H3PO4 solution at 30℃for 3 h. By scanning electron microscopy (SEM), the microstructure of alumina membranes prepared were abserved, the AAO arranged in order with the uniform pore size of 150 nm and the template thickness exceeding 180μm, aspect ratio of more than 1000. The anodic time which affect the nanopore morphology and thickness of the template were aslo discussed, the results showed that, under galvanostatic conditions, oxidation time did not affect the pore size of the template but the thinckness of the template. So, in the same conditions, to control the oxidation time is the key for preparation of AAO with ultrahigh aspect ratio. In addition, the barrier removal conditions were aslo optimized.2. Fabrication of Ni nanowires. Ni nanowires with different aspect ratio were fabricated by DC deposition with AAO as templates. SEM and TEM analysis showed that the nanowires were orderly arranged and uniform, its pore dimameter and length were consistent with the AAO. X-ray diffraction (XRD) structure of Ni nanowires were analyzed, results showed that Ni nanowires with fcc crystal structure, along the (220) plane preferential growth. The selected area electron diffraction (SAED) and HRTEM image showed that the nanowire is poly-crystalline and composed of ultra-fine grains with an average size about 0.2 nm. Room temperature hystersis loops of the Ni nanowires were tested by the VSM. It was shown that, the Ni nanowires with aspect ratio of 200, in parallel and perpendicular to the nanowire direction, display the coercivity 0.2 Oe and 0.33 Oe and remanence ratio 3.48% and 4.77% respectively, which show a lower magnetic anisotropy. To the Ni nanowires with aspect ratio of 1000, in parallel and perpendicular to the nanowire direction, the coercivity were 200.39 Oe and 99.75 Oe and remanence ratio were 35% and 3.5%, which displays an obvious magnetic anisotropy. To a certain extent, the magnetic properities of Ni nanowires are related with its aspect ratio.3. Fabrication of Pt-Ni nanowires. We prepared the Pt-Ni nanowires as the same method of Ni nanowires mentioned earlier. The nanowires were characterized by using SEM,EDS,TEM and HRTEM. The results show that the nanowires arranged in order and uniform, and have the unique diametre about 150 nm which consistent with the pore size of AAO. The length of the nanowire was 150μm, therefore, the aspect ratio of the nanowire achieved 1000. In addition, the Pt-Ni nanowire is poly-crystalline and composed of ultra-fine grains with an average size about 0.23 nm. Electrocatalytic activity for methanol oxidation studies have shown that the Pt-Ni nanowires show much higher catalytic activity than pure Pt nanowire because of the synergistic role of Pt and Ni in methanol oxidation reaction. When the Pt/Ni molar ratio is 2:1, the catalytic activity reaches the highest.