AAO Templating Preparation Of Mesoporous Silica Fibers & Co And Ni Metallic Nanotubes Arrays

The template synthesis method has been shown to be a versatile approach forpreparing nanostructures. Using this method, different kinds of nanomaterials havebeen fabricated, such as metals, semiconductors, polymers, carbon and othermaterials. The main idea of this method is fabricating a desired material within thepores of porous membrane. Porous anodic aluminum oxide (AAO) membrane withsatisfying physical stability and chemical inertia has attracted a great deal of interestas templates in the synthesis of various nanostructures, such as nanotubes, nanowires,nanorods, nanobelts and nanodots. It has cylindrical pores of uniform diameter fromtens to hundreds of nanometers depending on the anodizing parameters and almostparallel porous structures.For many applications, mesoporous materials should be not only controlled thenanometer scale of the pore structure but also controlled the micrometer scale of themorphology. Mesoporous silica with different morphology has been synthesis, suchas films, spheres, fibers and tubules. Due to the combination of ordered mesoporesand high aspect ratio, hollow or solid fibers have attracted much interesting forpotential application in optical devices, nanofluidics, and hosts for chemical species.Mesoporous silica fibers usually employed direct synthesis approaches throughone-phase or two phase reaction. The one-phase synthesis usually starts from a dilutesolution of silica-surfactant and the fiber diameter is in a wide range, depending onthe synthesis conditions. The two-phase reactions utilize the oil-water interface underacidic condition and the fibers have a single-crystal-like mesostructure withdiameters from 1 to 40μm. But there are many factors that influence on the products,such as pH value, temperature, stir, precursor et al. So it is difficult to control themicroscopic and macroscopic properties of the fibers independently. In this work,mesoporous silica fibers have been prepared in AAO membrane. These fibers withhierarchical organization can easily be synthesized using surfactant asstructure-directing agent and AAO membranes as hard template. The microscopicand macroscopic properties of fibers can be independently controlled. The diameterand the length of the mesoporous silica fibers is depend on the pore diameter andthickness of AAO membrane, respect. Thus, mesoporous silica fibers with differentaspect ratio can be synthesized. The orientation of the nanochannels in mesoporoussilica fibers can be readily controlled by varying the aging environment. For thesample aged with the presence of water, the nanochannels in the silica fibers werefound to be circular around the fibers axes, while for the sample aged without water,the nanochannels were found to be parallel along the fibers axes.One-dimensional (1D) magnetic nanowires have become the subject ofintensive research in view of their potential applications in ultra-high-densitymagnetic storage devices and microsensors. In recent years, many arrays of magneticnanowires including Fe, Co, Ni and their alloys have been prepared byelectrodeposition in AAO. However, there is still some deficiency in thiselectrodeposition method. First, a metal film must be coated on one face of themembrane as cathode for electroplating. Second, only nanowires can be prepared bythis method, unless the surface of the pore is modified to electrodeposit metalpreferentially on the surface of the pore. Compared with electrochemical deposition,the advantage of the electroless deposition is that there is no necessary for the surfaceto be electronically conductive. In this work, an effective method for preparingordered Co and Ni nanotube arrays in AAO membrane by electroless deposition wasdescribed. Prior to the electroless deposition, the AAO membranes were modifiedwith silane containing amine group. Then the membranes were polished to exposethe unmodified surface. Otherwise, the metal will be deposited on the membranesurface, which seals the pore entrance, resulting in the metal nanotubes with limitedlength. In order to obtain metallic tubes, a catalyst must be applied to the poresurface, so the metal preferentially deposits on the pore wall. This was accomplishedby a two-step process. The modified AAO membranes were first sensitized with Sn2+sensitizer, which binds to the pore wall by complexing with amine groups on thepore walls. The amine group functional procedure on the pore wall is a key step forpreparing metallic nanotube arrays. Because the interaction between the AAOmembrane and Sn2+ is weak, so it is difficult to immobilized Sn2+ on the pore wall.However, Sn2+ is easily complexed with amine group, thus the presence of aminegroup serves as molecular anchor on the pore wall for the metal deposition. TheSn2+-sensitized membrane was then activated with Pd2+. This leads to a redoxreaction in which Sn2+ is oxidized to Sn4+ and Pd2+ is reduced to Pd. Consequently,Pd nanoparticles were formed on the pore wall as the nanocatalyst for the electrolessdeposition. At last, the membrane was immersed in the metal plating bath, which wasdeposited metal on the pore wall. By varying the electroless deposition time, innerdiameter of the tube can be controlled. Short deposition times obtain the tube, whileif the electroless deposition time is long enough, the nanotubes will eventually closeup to form solid nanowires. The outer diameter of the tubes is determined by theAAO pore diameter. The pH value adjusted to around 7~8 is important for thenanotube growth. The higher pH value with strong alkali will not only corrode theAAO membrane, but also accelerate the metal deposition rate, making it difficult tocontrol the inner diameter of the metallic nanotube. The lower pH value with strongacid will also corrode the AAO membrane. These metallic nanotube arrays with openends have potential application in preparing novel core-shell nanocable metal alloysand other interesting nanomaterials.