Channels Of Anodic Alumina Membranes Research Articles

Enhanced gas separation factors of microporous polymer constrained in the channels of anodic alumina membranes.

New composite membranes based on porous anodic alumina films and polymer of intrinsic microporosity (PIM-1) have been prepared using a spin-coating technique. According to scanning electron microscopy, partial penetration of polymer into the pores of alumina supports takes place giving rise to selective polymeric layers with fiber-like microstructure. Geometric confinement of rigid PIM-1 in the channels of anodic alumina causes reduction of small-scale mobility in polymeric chains. As a result, transport of permanent gases, such as CH4, becomes significantly hindered across composite membranes. Contrary, the transport of condensable gases (CO2, С4H10), did not significantly suffer from the confinement due to high solubility in the polymer matrix. This strategy enables enhancement of selectivity towards CO2 and C4H10 without significant loss of the membrane performance and seems to be prospective for drain and sweetening of natural gas.

Fabrication and characterization of mesoporous silica nanochannels inside the channels of anodic alumina membrane

Brij type surfactants (CnEOx) that have different chemical structures were used to fabricate 3D Mesoporous Silica Nanochannels (MSN) inside the channels of Anodic Alumina Membrane (AAM) under acidic conditions. The fabricated 3D MSN were characterized using TEM-ED, SEM, Small angle XRD, and N2 isotherm. Results revealed that the synthesis of ordered 3D cubic Im3m mesostructures can be formed with tunable pore diameters varied from 4.0 to 4.9nm that are partially affected with the length of ethylene oxide (EO) group in the template surfactant.

Formation of hexagonal and cubic fluorescent periodic mesoporous organosilicas in the channels of anodic alumina membranes

The synthesis of periodic mesoporous organosilicas (PMOs) in the confinement of porous anodic alumina membranes (AAMs) was successfully achieved through a modified evaporation-induced self-assembly (EISA) process. 1,3,5-Tris(4-triethoxysilylstyryl)benzene, (a three-armed oligo(phenylenevinylene) organosilane compound, abbr. 3a-OPV), the precursor of the first reported charge-conducting PMO, was used as an organosilica source. Triblock-copolymers Pluronic F127 (EO106 PO70 EO106) or F108 (EO132 PO50 EO132) were used as structure directing agents. The block-copolymer F127 led to a 2D-hexagonal circular mesostructure within the AAM channels and the block copolymer Pluronic F108 resulted in a mesophase with a body-centered cubic (Imm) structure. Compared to the previously reported 3a-OPV-PMO film, the resulting hierarchical PMO/AAM systems have improved features, that is, the synthesized PMOs have a pore size of around 10 nm and the compounds are found to be stable against thermal treatment at temperatures of up to 200 °C and they are also stable in the electron beam of the electron microscope. Additionally, both of the resulting hierarchical mesoporous composites show fluorescence in the visible region due to the strongly interacting phenylenevinylene chromophores in the PMO frameworks.

Mesostructured Silica Containing Conjugated Polymers Formed within the Channels of Anodic Alumina Membranes from Tetrahydrofuran-Based Solution

The synthesis of mesostructured silica from a tetrahydrofuran (THF)-based sol gel was carried out in the channels of an anodic alumina membrane (AAM) using the evaporation-induced self-assembly (EISA) method. Two different nonionic surfactants were used as structure-directing agents, the triblock copolymer Pluronic P123 and the oligomer surfactant Brij56. The effect of the relative humidity and surfactant concentration on the type of mesophase and orientation of the in-channel mesostructures was studied using transmission electron microscopy (TEM) and grazing incidence small angel X-ray scattering (GISAXS). The in-channel structures obtained in this study were primarily of the 2D hexagonal phase with a circular orientation in which the hexagonally packed cylinders form a spiral-like shape from the channel wall inward. In addition, a columnar orientation of the hexagonal phase, in which the axes of the hexagonally packed cylinders are oriented parallel to the channel axes, was also observed. Finally, the use of the THF-based synthesis allowed the in situ incorporation of the highly hydrophobic yellow-emitting conjugated polymer poly[9,9-dioctylfluorene-co-benzothiadiazole] into the in-channel mesostructure upon its formation. The conjugated polymer was well distributed within the mesostructure and maintained its optical properties.

Fabrication of different silica nanotubes and examination of their catalytic activity in organic solvents

Silica nanotubes (SNTs) with different geometrical architectures and comparable pore sizes were fabricated inside the one-dimensional (1D) channels of anodic alumina membrane (AAM) using different nonionic Brij template surfactants. Experimental results revealed the formation of SNTs with ordered 1D, two-dimensional (2D), three-dimensional (3D), and wormlike mesostructures. Cytochrome C (Cyt C) was chosen as a hemoprotein to be immobilized inside the SNTs. Under the same starting Cyt C concentration, the amount immobilized was found to be dependent on the SNT mesostructure. Unlike native Cyt C, which lost its activity while interacting in organic solvents, Cyt C entrapped within silica nanotube hybrid membrane (SNM) showed remarkable catalytic conversion activity in organic solvents, with wormlike mesostructure being favorable over ordered mesostructures. This could be ascribed to facile access to active centers of Cyt C due to the random orientations of wormlike SNTs.

Hierarchically structured biphenylene-bridged periodic mesoporous organosilica

Novel composites of highly ordered and stable biphenyl-bridged periodic mesoporous organosilica (PMO) materials confined within the pores of anodic alumina membranes (AAM) were successfully synthesized by evaporation-induced self-assembly (EISA). 4,4′-Bis(triethoxysilyl)biphenyl (BTEBP) was used as a precursor in combination with the ionic surfactant cetyltrimethylammonium bromide (CTAB) or triblock-copolymer F127 as structure-directing agents. The resulting mesophases were characterized by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). With ionic CTAB as a structure directing agent, samples with a mixture of the 2D-hexagonal columnar and a lamellar mesophase were obtained within the AAM channels. When using the nonionic surfactant F127, mesophases with a 2D-hexagonal circular structure were formed in the AAM channels. Additionally, a cubic Imm phase could also be obtained with the same nonionic surfactant after the addition of lithium chloride to the precursor solution. The stability of both the circular and cubic biphenylene-bridged PMO against calcination temperatures of up to 250 °C was confirmed by NMR spectroscopy. Nitrogen sorption in the porous composite membrane shows typical type IV isotherms and narrow pore size distributions. All the biphenyl PMO/AAM composites show fluorescence due to the existence of biphenyl chromophores in the stable organosilica framework.

Mesoporous silica hybrid membranes for precise size-exclusive separation of silver nanoparticles

One-dimensional (1D) nanomaterials have unique applications due to their inherent physical properties. In this study, hexagonally ordered mesoporous silica hybrid anodic alumina membranes (AAM) were synthesized using template-guided synthesis with a number of nonionic n-alkyl-oligo(ethylene oxide), Brij-type (C x EO y ), which are surfactants that have different molecular sizes and characteristics. The hexagonal mesoporous silicas are vertically aligned in the AAM channels with a predominantly columnar orientation. The hollow mesostructured silicas had tunable pore diameters varying from 3.7 to 5.1 nm. In this synthesis protocol, the surfactant molecular natures (corona/core features) are important for the controlled generation of ordered structures throughout AAM channels. The development of ultrafiltration membranes composed of silica mesostructures could be used effectively in separating silver nanoparticles (Ag NPs) in both aqueous and organic solution phases. This would be relevant to the production of well-defined Ag NPs with unique properties. To create a size-exclusive separation system of Ag NPs, we grafted hydrophobic trimethylsilyl (TMS) groups onto the inner pores of the mesoporous silica hybrid AAM. The immobilization of the TMS groups allowed the columnar mesoporous silica inside AAM to retain this inner pore order without distortion during the separation of solution-phase Ag NPs in organic solvents that may cause tortuous-pore membranes. Mesoporous TMS-silicas inside 1D AAM channels were applicable as a size-exclusive separation system to isolate organic solution-phase Ag NPs of uniform morphology and size.

Time-resolved SAXS studies of periodic mesoporous organosilicas in anodic alumina membranes

A method for producing oriented periodic mesoporous organosilica filaments within the confined channels of anodic alumina membranes is presented. Deposition of the mesoporous filaments were performed under a variety of conditions, which favoured the evaporation induced self-assembly of the mesoporous material. The experimental conditions examined included different drying rates, over a range of humidity values, and sol compositions, e.g. varying the amount of the organosilica component. The deposition process was followed in situ by time resolved small angle X-ray scattering which was essential for evaluating the formation mechanism of the mesophase structures. Through careful control of the deposition environment, the structure and orientation of the mesoporous filaments could readily be varied.

Characterization of Sn–Co Nanowires Grown into Alumina Template

Nanowires of Sn-Co alloys were grown inside the channels of anodic alumina membrane by potentiostatic deposition. The scanning electron microscope images showed the formation of cylindrical nanowires whose height was increasing with deposition time. The X-ray patterns did not show significant diffraction peaks, suggesting the formation of amorphous phases. The higher content of Co in the nanowires, in comparison to the initial composition of the electrolytic bath, was attributed to a higher rate of Co electrodeposition. These nanowires seem to possess specific features suitable for innovative application in the field of Li-ion batteries due to their dimensional stability and high specific surface.

Mesoporous Carbon Nanofibers for Supercapacitor Application

Mesoporous carbon nanofibers have been prepared by a confined self-assembly of triblock copolymers with soluble low molecular weight phenolic resol within the channels of anodic alumina membranes. SEM and TEM observations showed that hexagonally arranged mesoporous channels were coiled concentrically around the longitudinal axis of the carbon nanofibers. These carbon nanofibers with BET surface area over 1424 m2 g−1 have been used as electrode materials for electrochemical capacitors tested in KOH and EC/DEC electrolyte solutions. Compared to mesoporous carbon prepared from the same precursor sol, these one-dimensional nanofibers could provide a shortened diffusion distance for electrolyte ions. The better performance of these mesoporous nanofibers greatly benefited from their high specific surface area, shortened diffusion distance, mesoporous openings on the outer surface, and well-controlled pore size. These mesoporous carbon nanofibers have been proved to be promising electrode materials for electroch...

Vertical Columnar Block-Copolymer-Templated Mesoporous Silica via Confined Phase Transformation

An efficient method is described for the preparation of phase-pure columnar mesoporous silica nanosystems within the channels of anodic alumina membranes (AAM) via evaporation-induced self-assembly (EISA). Upon the basis of a systematic investigation of the effects of interfacial interactions and different synthesis parameters on the resulting hierarchical mesophase, a salt-induced phase transformation was developed for efficient structural control. Samples with a columnar hexagonal 2D structure along the vertical channels of the AAM can be produced with ionic CTAB as template. However, when nonionic surfactants (Pluronic P123 and Brij 56) are used, samples with a circular hexagonal 2D structure perpendicular to the channels or phase mixtures of circular and columnar orientations are obtained. The behavior of ionic CTAB can be mimicked by adding inorganic salt to the nonionic template precursor solution, thus leading to a phase transformation toward columnar orientation. The distribution between the orientations was determined by means of small-angle X-ray scattering (SAXS) experiments. The effects of other synthesis parameters were also investigated, including temperature, surfactant: silica ratio, and salt composition. Strikingly, calcination-stable mesoporous materials with a columnar orientation exhibiting high mesoporosity and specific surface area were obtained for the first time with such structure directors. The salt-induced phase transformation is an efficient means for achieving a desired hierarchical mesostructure in the confined space of the AAM channels.

Formation Mechanism of Mesostructured Silica in Confined Space: An In Situ GISAXS Study

The structural evolution of periodic mesoporous material within the channels of anodic alumina membranes (AAMs) by evaporation-induced self-assembly (EISA) is investigated by a combination of in situ grazing-incidence small-angle X-ray scattering (GISAXS) with parallel detection of solvent evaporation and ex situ transmission electron microscopy (TEM). Kinetically controlled and equilibrium-controlled structural evolution can be distinguished for these EISA processes. A new mechanism for formation of mesostructures in the confined environment of AAMs is proposed. Data are presented for samples synthesized with nonionic surfactants at various surfactant:silica ratios and relative humidities. The formation of and transformations between circular or columnar 2D hexagonal and tubular lamellar structures are observed. The circular hexagonal phase is kinetically favored over the columnar hexagonal orientation. The TEM images provide evidence that phase transformations, depending on their type, either start preferentially at the channel wall or in the center of the mesostructured fibers.

Confined Growth and Crystallography of One-Dimensional Bi2S3, CdS, and SnSx Nanostructures within Channeled Substrates

The growth, composition, and structural properties of one-dimensional Bi2S3, CdS, and SnSx nanostructures, confined within the channels of anodic alumina membranes, have been comprehensively investigated by electron microscopy and spectroscopy. In particular, the morphology, loading factors, crystal structure and orientation, and potential growth mechanisms of the II−VI nanostructures confined within the alumina templates, with mean pore diameters of 80 and 20 nm, were investigated. A solventless pressure-injection method was used to form ordered arrays of these 1 D nanostructures, with controlled diameters (by the dimensions of the templating channels), controlled orientations (via the direction of the templating channels), and isolated from each other within the alumina templates. A variety of 1 D nanostructures ranging from well-faceted nanowires to nanotubes and nanobelts have been identified and characterized. For all of the materials investigated, single crystalline and oriented structures were observed having preferred growth directions within the alumina channels.

Growth of Ordered Arrangements of One-Dimensional Germanium Nanostructures with Controllable Crystallinities

Channelled templates such as anodic alumina membranes (AAMs) can be utilized to host, isolate, and guide the growth of one-dimensional (1D) nanostructures. In this study, we present a method for Au-seeded and confined growth of 1D Ge nanostructures with controlled crystallinities and electrical properties within the channels of AAMs. Our approach combines Au nanoparticle seeded growth of semiconductor nanowires by supercritical fluid−liquid–solid (SFLS) mechanism and the highly anisotropic structure of the aligned channels in AAMs. Au seeds with nanosized dimensions were prepositioned inside channelled substrates, followed by SFLS growth at temperatures slightly higher than the Au/Ge eutectic point. Microscopy and XRD measurements reveal that the 1D nanostructures can be obtained with tuneable and controllable crystallinity, grain size and domain boundaries, ranging from chains of Au-nanoparticles connected through semiconductor Ge nanocrystallities, to Au-seeded Ge single crystalline nanowires. Conditions that control the type of Ge nanostructures are (i) the distribution and size of the Au seeds across the alumina surfaces, (ii) the type of SCF deposition, e.g., batch versus flow-through deposition. Additionally, we present electrical data of the ordered arrays of 1D nanostructures, measured by conductive atomic force microscopy (c-AFM), and contrast the data to that previously obtained for similar systems.

Highly Ordered Arrays of Mesoporous Silica Nanorods with Tunable Aspect Ratios from Block Copolymer Thin Films

Mesoporous silica thin films have received much attention in applications as diverse as separation devices, sensors, and optoelectronic devices. The evaporation-induced self-assembly method (EISA) has been established as an efficient process for the rapid preparation of mesoporous silica thin films. Usually, beginning with a highly dilute homogenous solution of a soluble silica species and a surfactant in an ethanol/water mixture, preferential evaporation of ethanol concentrates the nonvolatile surfactant and silica species in water, thereby inducing the self-assembly of silica-surfactant micelles and their further organization into liquid-crystalline mesophases. Recently, this efficient EISA method has been applied to prepare ordered mesoporous materials within the confined channels of anodic alumina membranes (AAM), silicon membranes, and other resist molds. Compared to mesoporous powders and thin films, a hierarchically ordered mesoporous material is advantageous for self-assembly on a macroscopic scale and allows easy control over the morphology. A reduction in diameter below 50 nm to obtain mesoporous nanowires and -fibers is believed to offer even more benefits. However, studies of this type of materials have been limited so far due to the size limitation of the templates available. This shortcoming can be overcome by using the block copolymer lithography technique, which provides a powerful way to fabricate highly ordered arrays of inorganic nanoparticles with diameters of 5 – 50 nm. Both the diameter and interparticle distance of the resulting nanoparticle arrays can be tuned by adjusting the volume fractions of the block copolymer templates. Unfortunately, control of the aspect ratio (or the height) still remains a challenge. Herein, we report for the first time preparation of a highly ordered array of mesoporous silica nanorods with tunable aspect ratios, through an integrated strategy of block copolymer lithography and EISA. Block copolymer thin films with normal, cylindrical domains are used as external scaffolds to define the morphology of SiO2 nanorods. The surfactant cetyltrimethylammonium bromide (CTAB) is used as an internal template to form mesoporous structures inside the SiO2 nanorods. The control of the diameter, the center-to-center distance, and the height of SiO2 nanorods were studied systematically. Microphase-separated diblock copolymer films of amphiphilic PEOm-b-PMA(Az)n (m and n denote repeated units of the individual segments), consisting of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(methacrylate) with azobenzene mesogens (PMA(Az)) in the side chain, can be prepared by spin-coating 1∼ 3 wt % toluene or chloroform solutions thereof on silicon wafers substrates followed by annealing at 140 °C for 24 h in vacuum. Figure 1A–C show typical atomic force microscopy (AFM) height images of PEO114-b-PMA(Az)45, PEO272-b-PMA(Az)116, and PEO454b-PMA(Az)184 thin films, respectively, on a silicon wafer after annealing. Dark dots and bright surroundings in the images can be assigned to cylindrical PEO microdomains and PMA(Az) matrix, respectively. The insets showing fast Fourier transformed (FFT) images indicate a hexagonal or quasihexagonal arrangement of the PEO domains. The average diameter of PEO cylinders and their average center-to-center distance are (11.2± 1.6) nm and (22.1± 1.4) nm for PEO114-bPMA(Az)45, (18.6± 1.8) nm and (33.4± 1.9) nm for PEO272-bPMA(Az)116, and (23.7± 4.0) nm and (51.7± 3.1) nm for PEO454-b-PMA(Az)184, respectively. When the silicate sol was introduced into the PEOm-bPMA(Az)n thin film, the swollen PEO domains had been selectively doped, which was confirmed by TEM (See Supporting Information Figure S1). Through the sol-gel process driven by the solvent evaporation, the silicate sol will penetrate into the swollen PEO cylindrical domains to form one dimensional SiO2 nanoparticles. Figure 2A shows an AFM height image of the silicate sol hybridized with the PEO domains in a PEO272-b-PMA(Az)116 thin film (thickness: ∼ 150 nm) after immersion in the silicate sol for 3 h. A regular nanoparticle array protruded from the surface of the PEO272-bPMA(Az)116 thin film (height: ∼ 2 nm), which was not observed in cases with thicker PEOm-b-PMA(Az)n film templates or shorter immersion times. As shown by cross-sectional AFM image (Figure 2B) the cylindrical structure was still present after immersion, but most of the SiO2 nanorods appeared to lie flat on the substrate after calcination (Fig. S2). C O M M U N IC A IO N

Preparation of Oriented Mesoporous Carbon Nano-Filaments within the Pores of Anodic Alumina Membranes

Oriented mesoporous carbon nano-filaments were prepared as replicas of Fe-containing silica mesostructures deposited inside the channels of anodic alumina membranes (AAMs). Due to the confinement imposed by the channels of the AAMs, rather unusual mesophase structures showing "circular" or "columnar" mesoscopic channels were formed that were replicated as pure carbon analogues after removal of the templating hosts. These structures are assembled in the forms of macroscopic membranes and are hierarchically ordered first due to the arrangement of the channels of the AAMs and second due to the mesophase counterparts.

Tuning the Structure and Orientation of Hexagonally Ordered Mesoporous Channels in Anodic Alumina Membrane Hosts: A 2D Small‐Angle X‐ray Scattering Study

Circular or columnar orientations of mesoporous silica can be selectively prepared by tuning the silica-to-surfactant ratio and the humidity during the synthesis of the mesoporous material within the regular larger channels of anodic alumina membranes. The transmission electron microscopy image shows the circular orientation of these highly ordered hexagonal mesostructures.

Synthesis of polyaniline nanotubes in the channels of anodic alumina membrane

Syntheses of polyaniline tubes (or rods) inside the pores of anodic alumina membrane by electrochemical and chemical methods are reported. A comparison of polyaniline synthesized in polycarbonate track-etched membrane is made. FT-IR spectra show that the products synthesized inside the pores of templates are emeraldine salts. The morphologies of polyaniline tubes (or rods) synthesized electrochemically in anodic alumina membrane are better than those synthesized in polycarbonate track-etched membrane. ESR spectra reveal that the degree of delocalization of those tubes (or rods) synthesized electrochemically in the anodic alumina membranes is better. The tubes synthesized inside the pore of anodic alumina membrane electrochemically show an enhancement of the electronic conductivity compared to polyaniline bulk materials. The polyaniline tubes synthesized by electrochemical method show better properties (morphology, degree of delocalization and conductivity) compared to those tubes synthesized by chemical method in the same templates.

Chemical fabrication of Al 2O 3 nano-trilobes

The recent development of silicon-based microreactors requires conventional catalyst carriers to be trimmed down to the sub-micrometer regime while keeping their complex geometrical shapes unchanged. In this work, we use a “top-down” etching approach to fabricate Al 2O 3 nano-trilobe carriers from the inter-channel parts of pseudo-hexagonally arranged one-dimensional channels of anodic alumina membranes. With a dilute NaOH (0.04 M) etchant and prolonged etching time (1.5 h), highly regulated Al 2O 3 nanocarriers have been prepared into two major forms: well-aligned (bundles) and freestanding trilobes. It has been found that the Al 2O 3 nano-trilobes are generated only on the upward facing of the membranes owing to the support provided by less etched membrane underneath and a less vibrant region in fluid exchange. The etching process can be described as a nucleophilic reaction that involves an attachment of hydroxyl ions to Lewis acid sites of aluminum cations. The amorphous as-prepared nano-trilobes can be further heat-treated to any crystallographic phases. For example, phase pure polycrystalline θ-Al 2O 3 nano-trilobes have been prepared at 1000 °C in laboratory air. Future applications of these nanocarriers have also been discussed.