Porous Anodic Alumina Membranes Research Articles

Flow through polydisperse pores in an anodic alumina membrane: A new method to measure the mean pore diameter

We describe a novel technique for the characterization of the mean pore radius of self-ordered porous anodic alumina membranes. We have studied the flow of liquids through cylindrical membrane pores under conditions of very low Reynolds number (Re ≪ 1). We find that the flow data are well-described by Poiseuille's law. Characterization is achieved through the application of the Hägen-Poiseuille equation on flow measurements conducted in a simple and compact experimental setup. This setup allows for the monitoring of flow as a result of the pressure difference between the two sides of the membrane without the use of sophisticated and costly laboratory equipment. The proposed method has been tested by flow experiments using liquids of various viscosities. Good agreement is found between the results obtained from the flow data and the pore radii measured by scanning electron microscopy. Consistent results for the mean pore radius for the same membrane have been obtained in all cases.

Daytime passive radiative cooler using porous alumina

Passive radiative cooling is a promoting way for thermal energy management. High reflectance of solar light and high emittance in the atmospheric window (8–13 μm) are acquired for effective radiative cooling. In this work, we demonstrate a daytime passive radiative cooler using chemically fabricated porous anodic aluminum oxide (AAO) membranes. Effective medium theory (EMT) has been applied to analyzing the optical properties of the air-doped porous medium. The composite structure is specifically optimized so that it has a high absorbance (emittance) in the far-infrared atmospheric window and nearly no loss in the solar spectrum. The calculated emittance is well reproduced in the experiment by our AAO sample. The fabricated porous membrane shows a potential cooling power density of 64 W/m2 at ambient temperature (humidity ~70%) under direct sunlight irradiance (AM1.5). Experimentally, the sample is cooled by a 2.6 °C temperature reduction below the ambient air temperature in the sunlight. This performance shows little variance at night. The AAO approach proposed here may provide a promising way to produce low-cost and efficient radiative cooler in large scales for feasible energy conservation.

Optical and mechanical studies on free standing amorphous anodic porous alumina formed in oxalic and sulphuric acid

Anodic porous alumina (APA) membranes with a uniform pore arrangement typically serve as an ideal template for formation of different types of nanostructured materials. In the present work, APA membranes were synthesized using two-step anodization in 0.3 M of oxalic and 0.3 M of sulphuric acid under the anodization potential of 40 V and 20 V respectively, at 8 °C. Alumina nanowires (ANW) were synthesized by the chemical etching of the APA membranes using phosphoric acid solution. The optical absorbance and reflectance measurements of APA membranes were performed on a spectrometer in the wavelength range of 200–600 nm. The band-gap energy (3.7 and 4.3 eV for oxalic and sulphuric acid) of APA membranes was determined from UV–visible absorption data. The photoluminescence (PL) investigations have revealed the presence of F and F+ defect centers, which could be attributes to oxygen vacancy-related defect centers in oxalic (483 and 466 nm) and sulphuric (423 and 421 nm) alumina. The mechanical properties of amorphous APA membranes were investigated by micro- and nanoindentation techniques. The results indicate that highest hardness (7.70 GPA) and Young’s modulus (138.80 GPA) for sulphuric alumina compared with oxalic alumina. In sulphuric alumina, more number of ANW were observed compared with oxalic alumina.

A non-enzymatic uric acid sensor utilizing ion channels in the barrier layer of a porous anodic alumina membrane

Porous anodic alumina (PAA) membranes have attracted considerable interest in biosensor research because they are simple to prepare and the size of the nanochannels can be controlled. However, traditional methods only utilize the porous layer as the barrier layer is removed before use. In this study, we propose a new biosensor for the detection of uric acid based on ion channels in the barrier layer of a PAA membrane. This exhibits a wide linear range (1 nM–0.1 mM) and has excellent selectivity in the presence of interferents (d‑(+)‑glucose, l‑ascorbic acid or dopamine). It is hoped that this new biosensor will prove useful in biological research and clinical medicine.

A Comparative Study of Magnetic Properties of Large Diameter Co Nanowires and Nanotubes.

A comparative study of the magnetic properties of the arrays of Co nanowires and nanotubes with large external diameters (180 nm) has been carried out. The nanowires/nanotubes were grown by electrodeposition into the self-assembled pores of anodic alumina membranes. The experimental study of their magnetic behavior was focused on the angular dependence of hysteresis loops and their parameters. In both nanowire and nanotube arrays, from the analysis of experimental data, effective longitudinal magnetic anisotropy is concluded, which is stronger in the case of the nanotube array. In addition, the extremely small remanence observed for all loops indicates the important role played by magnetostatic interactions. Micromagnetic simulations were first performed considering intrinsic shape and magnetocrystalline anisotropy terms, together with an effective easy-plane anisotropy to account for those magnetostatic interactions. A qualitative agreement between experiments and simulations is found despite the complexity introduced by the intrinsic and extrinsic array properties (i.e., large diameters, grain structure, and array configuration). In addition, simulations were also carried out for individual nanowire/nanotube with a particular emphasis to understand their differences at the remanence, due to pure geometry contribution.

A mini review: Functional nanostructuring with perfectly-ordered anodic aluminum oxide template for energy conversion and storage

Nanostructures have drawn great attentions for functional device applications. Among the various techniques developed for fabricating arrayed nanostructures of functional materials, nanostructuring technique with porous anodic aluminum oxide (AAO) membrane as templates becomes more attractive owing to the superior geometrical characteristics and low-cost preparation process. In this mini review, we summarize our recent progress about functional nanostructuring based on perfectly-ordered AAO membrane to prepare perfectly-ordered nanostructure arrays of functional materials toward constructing high-performance energy conversion and storage devices. By employing the perfectly-ordered AAO membrane as templates, arrayed nanostructures in the form of nanodot, nanorod, nanotube and nanopore have been synthesized over a large area. These as-obtained nanostructure arrays have large specific surface area, high regularity, large-scale implementation, and tunable nanoscale features. All these advanced features enable them to be of great advantage for the performance improvement of energy conversion and storage devices, including photoelectrochemical water splitting cells, supercapacitors, and batteries, etc.

Effect of Cu-doping on the photoluminescence and photoconductivity of template synthesized CdS nanowires

Cu-doped CdS nanowires were synthesized within the pores of anodic alumina membrane as template via electrochemical deposition. Morphological, structural, optical and photoconductive properties of the synthesized nanowires were studied. Morphological and structural properties were studied by using SEM with EDS (point & line mapper mode) and X-Ray Diffractometer. Scanning electron microscopy with point EDS and line EDS mapping confirms the deposition of pure CdS and Cu-doped CdS throughout the template length respectively. XRD investigations shows the presence of polycrystalline phase and cubic structure of pristine CdS nanowires. Optical properties were studied by UV–visible absorption spectroscopy and time resolved photoluminescence. A gradual decrease in energy band gap from 2.38 eV to 2.10 eV is observed for Cu doped CdS nanowires. Time resolved photoluminescence studies shows the red shift in emission wavelength and increase of excited state lifetimes with Cu doping and also with increase in its concentration in CdS nanowires. Current-voltage characteristics of pure and doped CdS nanowires studied at varying incident light intensity show that the Cu doped nanowires show negative photoconductivity.

Synthesis and Characterization of Graphitic Carbon Nitride Nanotubes Using Porous Anodic Alumina Templates

In the present paper, we reported the preparation of Graphitic carbon nitride (GCN) nanotubes using porous anodic aluminium oxide membranes as templates through thermal condensation of cynamide at 500 °C. Structural analysis of synthesized nanotubes was systematically carried out by scanning electron microscope (SEM), X-ray powder diffraction (XRD) and results confirmed the growth of GCN inside the nanochannels of anodic alumina templates. Optical features of GCN were carried out using UV visible spectroscopy and photoluminescence. PL analysis showed that a strong emission peak occurs in the wavelength range of 350–600 nm in blue region. Thermal behaviour of GCN was studied using differential scanning calorimetry (DSC).

The effect of colloids on nanofluidic power generation

We discussed the influences of colloidal particles on ion transport in nanofluidic power generation system, which is an unavoidable issue for actual membrane-based power generation systems. By introducing negatively charged colloidal particles (∼100 nm) with different concentrations, power generation by reverse electrodialysis in a porous anodic alumina membrane was analyzed. The power was enhanced in a colloid concentration of (2.6–4.2)×10-5g/ml. The maximum power was acquired when the colloid (∼3.05×10-5g/ml) was on the same side as the salt solution (1 M KCl). Modifying the surface of the porous anodic alumina membrane by using an ultra-thin metal layer (10 nm of platinum) achieved almost the same maximum power, but the colloid concentration range of the power enhancement broadened. The electrokinetic interplay between colloidal particles and nanofluidics is complex. The appearance of colloidal particles in water bodies does not always result in a reduction in power generation, and their effects can be effectively regulated by engineering parameters.

Enhanced electrocatalytic activity and durability of Pt nanoparticles supported on ordered bimodal mesoporous carbon nanowires

Ordered bimodal mesoporous carbon nanowires (OMCWs) is prepared from dual templates that were fabricated by filling silica nanospheres in the channels of porous anodic aluminum oxide (AAO) membranes. The prepared OMCWs material exhibits a large surface area (876.7 m2 g−1), ordered large mesoporous (30 nm) nanostructure and small mesopores on the large mesopore walls. Pt nanoparticles are deposited on OMCWs to form Pt/OMCWs electrocatalyst for methanol oxidation reaction (MOR). In Pt/OMCWs, Pt nanoparticles (∼1.8 nm) are uniformly located on the OMCWs. The Pt/OMCWs catalyst possesses larger electrochemically active surface area (ECSA) (93.3 m2 g−1), higher activity for MOR and better durability compared with the commercial Pt/C catalyst. The good electrochemical performance of Pt/OMCWs is ascribed to the unique bimodal mesoporous structure that could facilitate the rapid mass transfer and enhanced the dispersion of Pt nanoparticles and the one-dimensional structure that could enhance the electron transport.

Transparent sapphire substrates with tunable optical properties by decorating with nanometric oxide on porous anodic aluminum oxide patterns

A method to achieve transparent sapphire substrates with tunable optical properties is proposed. After establishing a porous anodic aluminum oxide (AAO) membrane on sapphire, a thin Sn layer was deposited onto the AAO nano-patterned structure and then oxidized to tin monoxide (SnO). Typically, a thin SnO coating has two characteristic absorption peaks within the visible light region, but when deposited on the high-aspect-ratio AAO, more peaks were observed due to multiple interactions. This simple multilayered structure and its investigations exhibited a new potential application for sapphire in optical controls.

A smart membrane with antifouling capability and switchable oil wettability for high-efficiency oil/water emulsions separation

There are large amounts of oily wastewater produced by industrial production processes and daily life, and oily wastewater is one of the most widespread environmental issues. These effluents contain a large amount of emulsified oil/water mixtures. However, separation of stabilized oil/water emulsions is difficult and a global challenge. Oily wastewater is not only harmful to the environment but it also affects human health. Thus, development of materials that can effectively solve this problem is of great interest. Here, a smart surface with antifouling property and pH-controllable oil wettability in aqueous media is reported. The smart surface on a porous anodic aluminum oxide (PAAO) membrane (pore size < 1 µm) shows switchable superoleophilic/superoleophobic performance in different pH aqueous media. The smart surface was fabricated by modifying the PAAO membrane with the pH-responsive poly(4-vinylpyridine) and hydrophobic/oleophilic polystyrene block copolymer. The as-prepared PAAO membrane can effectively separate stabilized oil-in-water and water-in-oil emulsions by adjusting the pH of the aqueous medium (3.0 and 7.0). Furthermore, the as-prepared membrane with superior antifouling capacity and could be used at least 10 times with high separation efficiency (99.95%). The present study suggests that PAAO membranes have potential applications for separation of stabilized oil/water emulsions.

Kramers–Kronig method for determination of optical properties of PZT nanotubes fabricated by sol–gel method and porous anodic alumina with high aspect ratio

Ferroelectric Pb(ZrTi)O3 (PZT) nanotubes were prepared by sol–gel method and porous anodic alumina (PAA) membrane using spin-coating technique. This method is based on filling-pyrolysis-filling process and the use of one-stage alumina membranes. One of the advantages of this method is its rapidity, which takes only 1 h time before the calcination step. The effect of repeated pores filling was investigated to get the required size of nanotubes. The field emission scanning electron microscope (FE-SEM) images were shown that the PZT nanotubes have inner diameters in the range of 65–90 nm and length of about 50–60 [Formula: see text]m. This means that the samples have a significant aspect ratio (700–800). Also the FE-SEM image confirmed that the highly ordered, hexagonally distributed PAA membranes with the pore diameter about 140–150 nm were formed. The X-ray diffraction (XRD) results showed that the PZT nanotubes have a tetragonal structure. The metal oxide bands like ZrO6 and TiO6 of the final PZT nanotubes were detected by Fourier transform infrared (FT-IR) analysis and confirmed the formation of perovskite structure. By using FT-IR spectroscopy and Kramers–Kronig transformation method, the optical constants like real [Formula: see text]([Formula: see text]) and imaginary [Formula: see text]([Formula: see text]) parts of dielectric function, extinction coefficient k([Formula: see text]) and refractive index n([Formula: see text]) were determined. It was shown that the optical constants of PZT nanotubes are different from PZT nanoparticles.

Wettability of porous anodic aluminium oxide membranes with three-dimensional, layered nanostructures

The architecture-dependent wettability of three-dimensional (3D), porous anodic aluminium oxide (AAO) membranes with varying surface morphologies including hierarchical, mesh and honeycomb nanostructures is reported. The surface morphology and underlayer structure play different roles in regulating the wetting behaviour of the AAO membranes. For the mild AAO membranes, the wetting behaviour of the ultra-thin top layer is dominated by the surface morphology in which the water contact angles (WCAs) of the AAO membranes with hierarchical, mesh and honeycomb structures are approximately 113.7° ± 4.6°, 94.9° ± 0.7° and 98.8° ± 5.8°, respectively. The wetting behaviour of the 3D, layered AAO membranes is dominated by both the surface morphology and the underlayer structure. Notably, the WCA of the mild AAO membrane with a layered hierarchical structure increases in the second layer (increase in the hole density). The WCAs of the three kinds of layered hard AAO membranes decrease in the second layer (increase in the hole depth) and then decrease slowly or increase in the third transition layer (decrease in the hole density). The WCAs of all the AAO membranes decrease linearly at different rates with the formation of the ordered bottom layer. The above results can facilitate the engineering of nanostructures for controlling the surface wetting behaviour of materials and devices for applications in multiple fields.

A Mercury Ion Electrochemical Sensor Based on Porous Anodized Alumina Membrane Nanochannels Modified with DNA

A novel mercury ion (Hg2+) electrochemical sensor was established via chemical modification of porous anodic alumina (PAA) membrane nanochannels with DNA. PAA membrane was prepared by a two-step anodization. Amino groups were introduced to nanochannels by silanization. Special sequence DNA (along with amino group) was modified on PAA membrane nanochannels by the condensation reaction of amino and aldehyde groups. Electrochemical detector was built via sputtered Au nanoparticles on the surface of PAA nanochannels, which could be used as working electrode. The reduction potential of 0 V was applied, where the flux of Fe(CN)63− was determined by diffusion. The current response has a positive correlation with the concentration of Fe(CN)63−. After DNA immobilized, the negative charge and the steric hindrance of the nanochannels is increased, resulting in the decreasing of the flux of Fe(CN)63−. The detection current is reduced compared with pure PAA membrane nanochannels. When Hg2+ is present, a stable thymine-Hg2+-thymine structure is formed due to the specific reaction between thymine in DNA and Hg2+. In this case, the charge density and steric hindrance of the nanochannels are decreased, thereby increasing the flux of Fe(CN)63− and restoring current. The results show that the detection limit is about 0.08 pM with a good linear response (from 1 pM to 100 nM). Meanwhile, the sensor has a good selectivity. The current responses of other metal ions (Mn2+, Mg2+, Ba2+, Co2+ and Cd2+) are not obvious compared with Hg2+. Therefore, this new sensor could be applied for detecting Hg2+ in the environment.

Selective through-holing of anodic porous alumina membranes with large area.

Anodic porous alumina membranes with site controlled through-holes were prepared by the formation of a masking layer on the surface of anodic porous alumina and subsequent selective second anodization in concentrated sulfuric acid to form a readily soluble layer. After the anodization, the residual Al substrate was removed, and the highly soluble alumina layer formed in concentrated sulfuric acid was dissolved selectively by wet etching. An advantageous point of this process is the controllability of the pattern of through-holes, and the preparation of large samples with selective through-holes is possible. The pattern of through-holes was controlled by changing the mask pattern formed on the surface of anodic porous alumina. The alumina membranes obtained by this process are expected to be used for various applications that require porous alumina membranes with site controlled through-holes.

Influence of porewidening duration on the template assisted growth of graphitic carbon nitride nanostructures

Porous anodic aluminium oxide (AAO) membranes with a highly ordered pore arrangement are typically used as ideal templates for the synthesis of numerous nanostructured materials. Highly ordered templates gained significant attraction due to the fact that they are readily fabricated through self-organised simple anodization process. In this paper, the effect of different pore-widening treatments on the quality of the pores of the AAO templates prepared with different electrolytes were inspected. Results confirmed that, without altering the interpore distance different pore dimensions and diameters of the AAO templates can be easily achieved by chemical pore widening process at room temperature. Also, graphitic carbon nitride nanorods of different dimension have been fabricated from AAO template after porewidening process. These nanostructures are widely used in case of metal free visible light driven photo catalysis, photo degradation of organic pollutants, photo electric conversion and water splitting applications.

Spatial atomic layer deposition for coating flexible porous Li-ion battery electrodes

Ultrathin atomic layer deposition (ALD) coatings on the electrodes of Li-ion batteries can enhance the capacity stability of the Li-ion batteries. To commercialize ALD for Li-ion battery production, spatial ALD is needed to decrease coating times and provide a coating process compatible with continuous roll-to-roll (R2R) processing. The porous electrodes of Li-ion batteries provide a special challenge because higher reactant exposures are needed for spatial ALD in porous substrates. This work utilized a modular rotating cylinder spatial ALD reactor operating at rotation speeds up to 200 revolutions/min (RPM) and substrate speeds up to 200 m/min. The conditions for spatial ALD were adjusted to coat flexible porous substrates. The reactor was initially used to characterize spatial Al2O3 and ZnO ALD on flat, flexible metalized polyethylene terephthalate foils. These studies showed that slower rotation speeds and spacers between the precursor module and the two adjacent pumping modules could significantly increase the reactant exposure. The modular rotating cylinder reactor was then used to coat flexible, model porous anodic aluminum oxide (AAO) membranes. The uniformity of the ZnO ALD coatings on the porous AAO membranes was dependent on the aspect ratio of the pores and the reactant exposures. Larger reactant exposures led to better uniformity in the pores with higher aspect ratios. The reactant exposures were increased by adding spacers between the precursor module and the two adjacent pumping modules. The modular rotating cylinder reactor was also employed for Al2O3 ALD on porous LiCoO2 (LCO) battery electrodes. Uniform Al coverages were obtained using spacers between the precursor module and the two adjacent pumping modules at rotation speeds of 25 and 50 RPM. The LCO electrodes had a thickness of ∼49 μm and pores with aspect ratios of ∼12–25. Coin cells were then constructed using the ALD-coated LCO electrodes and were tested to determine their battery performance. The capacity of the Al2O3 ALD-coated LCO battery electrodes was measured versus the number of charge-discharge cycles. Both temporal and spatial ALD processing methods led to higher capacity stability compared with uncoated LCO battery electrodes. The results for improved battery performance were comparable for temporal and spatial ALD-coated electrodes. The next steps are also presented for scale-up to R2R spatial ALD using the modular rotating cylinder reactor.

Boosting the Photoluminescence of Monolayer MoS2 on High‐Density Nanodimer Arrays with Sub‐10 nm Gap

AbstractPatterned plasmonic nanodimers are fabricated exploiting an ultrathin porous anodic aluminum oxide membrane as a mask during angle‐resolved shadow deposition. The fabricated nanodimer arrays exhibit consistent sub‐10 nm gaps and a high particle density up to 1.0 × 1010 cm−2 over a large area. The ultrasmall dimer gaps provide highly confined electromagnetic fields, which strongly enhance the photoluminescence (PL) emission and Raman scattering from the surrounding monolayer molybdenum disulphide (MoS2). The ensemble PL intensity from MoS2/dimers is enhanced by up to a factor of ≈160 by resonant excitation of the dimer modes. Anisotropic polarization‐dependent characteristics of PL and Raman from the MoS2/dimers confirm that the dominant enhancement originates from the dimer configuration. These experiments demonstrate a facile approach for the fabrication of low‐cost high‐performance 2D material‐based optoelectronic devices.

Synthesis and photocatalytic properties of graphitic carbon nitride nanofibers using porous anodic alumina templates

In the present study, we describe an effective method for the synthesis of Graphitic carbon nitride (GCN) nanostructures using porous anodic alumina (AAO) membrane as template by simple thermal condensation of cyanamide. Synthesized nanostructure was fully analysed by various techniques to detect its crystalline nature, morphology, luminescent properties followed by the evaluation of its photocatalytic activity in the degradation of Methylene blue dye. Structural analysis of synthesized GCNNF was systematically carried out using x-ray powder diffraction (XRD) and scanning electron microscope (SEM), and. The results confirmed the growth of GCN inside the nanochannels of anodic alumina templates. Luminescent properties of GCNNF were studied using photoluminescence (PL) spectroscopy. PL analysis showed the presence of a strong emission peak in the wavelength range of 350–600 nm in blue region. GCNNF displays higher photocatalytic performance in the photodegradation of methylene blue compare to the bulk GCN.Highlights1. In the present paper, we report the synthesis of graphitic carbon nitride nanofibers (GCNNF) using porous anodic aluminium oxide membranes as templates through thermal condensation of cyanamide at 500 °C.2. The synthesis of Graphitic carbon nitride nanofibers using porous andic alumina template is the efficient approach for increasing crystallinity and surface area.3. The high surface area of graphitic carbon nitride nanofibers has a good impact on novel optical and photocatalytic properties of the bulkGCN.4. AAO templating of GCN is one of the versatile method to produce tailorable GCN nanostructures with higher surface area and less number of structural defects.5. Towards photocatalytic degradation of dyes, the tuning of physical properties is very essential thing hence we are succeeded in achieving better catalytic performance of GCN nanostructures by making use of AAO templates.