Fabrication, Properties, And Applications Of Anodic Alumina Nanochannels Array

Porous anodic alumina (PAA) and its allied anodic processes have been widely studied because this material system utilizes natural self-organization for the creation of periodic arrays of nanoscaled structures. Although other porous materials, such as polymer nuclear track membranes and zeolites, are also actively used for nanotemplating, PAA films, with their natural uniform parallel pores and high pore density, offer unique opportunities for the nanostructure fabrication. These nanosized pores can serve as a perfect scaffold platform and have emerged as an important template system for synthesizing nanostructural arrays of various materials. In addition, the dimension and interval of the pores are controllable by varying the anodization voltage of high-purity aluminum foil or film with a specific electrolyte, for examples, phosphoric acid, sulfuric acid, or oxalic acid. This enables the PAAMs to be used as a convenient model system to investigate the mechanism of particle transport in nanopores and materials properties under a small space situation. The present thesis focuses on the fabrication, properties, and applications of anodic alumina nanopores array.1. Porous anodic alumina with continuously manipulated pore/cell sizeWe used polyethyleneglycol (PEG) as a modulator to manipulate pore and cell sizes in the porous anodic alumina (PAA) fabrication. It is shown for the first time that continuous manipulation of the pore size of PAA can be realized. Combined with the coexistent cell-size controlling effect, the morphology and properties of this important nanoscale template and separation membrane can be precisely regulated. The pore size modulation mechanism is proposed on the basis of the morphological and electrochemical results. The presence of PEG in the electrolyte results in a more compacted structure of the barrier layer alumina (BLA), although the barrier layer thickness does not change considerably. In addition, the additive can obviously restrain the chemical dissolution of alumina and shape smaller pores. These two effects combined with the increased viscosity of the electrolyte slow down the ion transportation and diminish the anodization current. Thus, the burning-down phenomena of the aluminum substrates can be avoided at relatively high voltage anodization, and an interpore distance up to 610 nm can be achieved.2. An environment-friendly electrochemical detachment method for porous anodic aluminaThis paper describes an improved one-step voltage pulse detachment method by using perchloric acid and ethanol mixture as detaching solution for the preparation of through-hole porous anodic alumina (PAA) membranes. The detachment of PAA from aluminum substrate and the dissolution of the barrier layer can be completed simultaneously in the detachment solution by applying a pulse voltage in situ after the anodization process. The influence of voltage pulse height and nature of the detachment solution on the efficiency of detachment have been systematically investigated. The present procedure is more environmental friendly and efficient as compared to the conventional electrochemical detachment methods and is promising for the preparation of freestanding PAA films with through-hole morphology which are important for nanomaterials synthesis and nanoscale separation.3. Simultaneous fabrication of open-ended porous membrane and microtube array in one-step anodization of aluminumA single-step, high-voltage anodization process was performed to form self-segregating two-layer porous anodic alumina films in high concentration phosphoric acid solutions in the presence of polyethylene glycol. The open-ended top layer can be directly utilized as a bio-chemical separation membrane or as a template for nanostructural construction. The porous underlying layer has another distinct morphology, which forms an integrated alumina microtubule array on aluminum substrate. The factors influencing self-segregation process were systemically studied. The formation mechanism of these distinct morphologies was proposed to be a result of accelerated dissolution of the barrier alumina induced by the locally produced heat due to the high current density during the anodization process.4. Anomalous diffusion of electrically neutral molecules in charged nanochannelsDiffusion flux of phenol, a neutral molecule which is usually used as a common indicator in the study of mass transport behavior in nanochannels, varies with the ionic strength in nanochannels of porous anodic alumina. This anomalous non-Fickian behavior can be attributed to the restricted diffusion of polar molecules in the electric double layer (EDL), a shielding layer that is naturally created near a charged surface upon contacting to an electrolyte. The interaction between the electric field and phenol molecules restricts the self-diffusion of these electric neutral polar molecules in EDL. Therefore, the diffusion of phenol molecules through the PAA nanochannels can be divided into two distinctively different contributions, one is the free diffusion region outside the EDL, and the other is the confined diffusion region inside the EDL. The total diffusive flux across the nanochannels is determined by the spatial distribution of the confined and free diffusive regions. The ionic strength of the electrolyte and the pore surface charge density play important roles in the diffusive flux variation. The contribution of the confined diffusive region to the total diffusion flux strongly depends on the pore size.5. Characterization and manipulation of the electroosmotic flow in porous anodic alumina membranesPorous anodic alumina membranes (PAAMs) have uniform and high-density nanopores, and the dimension and interval of the pores can be easily controlled by varying the anodization conditions. The application of PAAMs could widely impact the cost and efficiency of the liquid based nanoscale separations. We report here the property of electroosmotic flow in PAAMs, which plays a significant role in the mass transport across these membranes that have charged pore surfaces. By controlling the solution pH and the magnitude and sign of the applied current, the mass transport through these nanoporous membranes can be spatially and temporally manipulated. The effects of electrosurface properties and electrolyte ionic strength on electroosmotic flow were studied. The anion incorporation and adsorption cause the variation of the electrosurface properties of PAAMs, which in turn influence the rate and direction of the mass transport. As compared to the membrane with fixed surface charge, this diversity makes it possible for the PAAMs to be used in various conditions.6. An electrokinetic method for rapid synthesis of nanotubesAn electrokinetic method was employed for the rapid synthesis of nanotubes using porous anodic alumina as templates. When two halves of a U-shaped cell are separated by a PAA membrane, electromigration of ions in both cell halves occurs upon application of a transmembrane potential. If the anodic and cathodic cells contain reactants of positively and negatively charged species, respectively, the difference of the electrophoretic direction of these charged species through the nanochannels of the PAA membrane will result in their encounter, forming a reaction zone where nanoparticles form, which are deposited on the inner walls of the PAA membrane. The anions and cations depleted during the chemical reaction can be continuously replenished via electromigration. The electroosmotic flow in the nanochannels and the difference of electrophoresis rates between the cationic and anionic species causes the reaction zone to move along the pores, leading to the formation of tubular structures. By this method, Prussian blue nanotubes can be formed rapidly with the morphologies faithfully replicating the nanopores in the template. The diameter of the nanotubes can be tuned by the fabrication conditions such as the diameter of the nanopores, concentration of the reactants, transmembrane potential, and fabrication time. This approach is not specific to a certain material, so that a wide variety of nanotubes can be synthesized quickly.7. Highly stable nickel hexacyanoferrate nanotubes for electrically switched ion exchangeNickel hexacyanoferrate (NiHCF) nanotubes are fabricated by an electrokinetic method based on the distinct surface properties of porous anodic alumina. By this method, nanotubes can be formed rapidly with the morphologies faithfully replicating the nanopores in the template. The prepared nanotubes were carefully characterized using SEM and TEM. Results from IR, UV, EDX, and electrochemical measurements show that the NiHCF nanotubes exist only in the form of K2Ni[Fe(CN)6]. Because of this single composition and unique nanostructure, NiHCF nanotubes show excellently stable cesium-selective ion-exchange ability. The capacity for electrodes modified with NiHCF nanotubes after 500 potential cycles retains 95.3% of its initial value. Even after 1500 and 3000 cycles, the NiHCF nanotubes still retain 92.2%and 82.9%, respectively, of their ion-exchange capacity.8. Protein trapping in the hybrid of nanochannels and ion channelsThe existence of ion channels in the barrier layer was verified by electrokinetic measurement of the porous anodic alumina membrane with complete barrier layer. A novel hybrid of nanochannels and ion channels is established for trapping and enrichment of protein. The transmembrane potential can induce the electromigration of proteins into the nanochannels but impossible across the ultra-thin barrier layer due to the size-exclusive effect. As a result, the proteins can be trapped in the nanochannels and enriched.