Porous Anodic Alumina Membranes Research Articles

Unleashing the Power of Osmotic Energy: Metal Hydroxide-Organic Framework Membranes for Efficient Conversion.

Osmotic energy, as a renewable clean energy with huge energy density and stable yield, has received widespread attention over the past decades. Reverse electrodialysis (RED) based on ion-exchange membranes is an important method of obtaining osmotic energy from salinity gradients. The preparation of ion-exchange membranes with both high ion selectivity and ion permeability is in constant exploration. In this work, metal hydroxide-organic framework (MHOF) membranes are successfully prepared onto porous anodic aluminum oxide (AAO) membranes by a facile hydrothermal method to form Ni2 (OH)2 @AAO composite membranes, used for osmotic energy conversion. The surface is negatively charged with cation selectivity, and the asymmetric structure and extreme hydrophilicity enhance the ionic flux for effective capture of osmotic energy. The maximum output power density of 5.65Wm-2 at a 50-fold KCl concentration gradient is achieved, which exceeds the commercial benchmark of 5Wm-2 . Meanwhile, the composite membrane can also show good performance in different electrolyte solutions and acid-base environments. This work provides a new avenue for the construction and application of MHOF membranes in efficient osmotic energy conversion.

Anodic Porous Alumina Membranes with Chemical Stability Improved by Atomic Layer Deposition Coating of TiO2

Anodic porous alumina (APA) membranes with a high density of uniformly sized pores are promising materials for microfiltration. However, such membranes obtained by anodizing Al are amorphous, chemically less stable, and cannot be used to filter acidic or basic solutions. The chemical stability of APA membranes can be improved by heat treatment at temperatures above 1000 °C, resulting in membrane filters with excellent chemical stabilities. However, such a high-temperature treatment makes APA membranes brittle owing to alumina crystallization, which reduces their mechanical strength and makes them less durable. In this study, a membrane filter with both chemical resistance and mechanical strength was fabricated by coating an APA membrane with a TiO2 layer by atomic layer deposition (ALD). The resulting membrane filters showed improved chemical stability in acidic and basic solutions because the TiO2 layer coated on the surface of the APA membrane protected the membrane against its dissolution. In addition, the resulting TiO2-coated APA membrane retained its high mechanical strength, as the membrane itself was not exposed to high-temperature conditions during TiO2 coating by ALD, and the crystallization of the alumina layer did not proceed. The obtained TiO2-coated APA membranes are promising as microfiltration membranes applicable to acidic and basic solutions.

Fabrication and magnetic properties of pulse electrodeposited FeSn nanowire arrays

When it comes to fabricating efficient nanomaterials for magnetic recording media and spintronic devices, Fe-based nanowires (NWs) containing nonmagnetic elements can represent themselves as one of ideal candidates. Here, a pulse electrodeposition method in porous anodic alumina membranes is employed to fabricate Fe100-xSnx (2 ≤ x ≤ 68) NW arrays with a diameter of about 35 nm. During the NW fabrication, Sn electrolyte concentration (CSn) and off-time (Toff) between pulses change in the range from 0.0025 to 0.02 M and 0–75 ms, respectively. As a general trend, magnetic properties including coercivity and squareness values decrease with increasing the Sn content. Meanwhile, first-order reversal curve diagrams show single domain and pseudo-single domain states of the FeSn NWs, depending on the Sn content in the structure dominated by Fe or FeSn alloy crystal phase. Magnetostatic interactions and coercive field distributions are also observed to significantly change by varying CSn and Toff, providing magnetically tunable FeSn NW arrays induced by the nonmagnetic element addition.

Confined amphipathic ionic-liquid regulated anodic aluminum oxide membranes with adjustable ion selectivity for improved osmotic energy conversion

To attain carbon neutrality and carbon peaking, there is an urgent need to convert the vast amount of blue energy present between seawater and river water into usable electricity. Reverse electrodialysis based on ion-exchange membranes is a promising way to efficiently achieve osmotic energy conversion. Anodic aluminum oxide (AAO) membranes are frequently used for osmotic energy harvesting because of their uniform nanopore channels, high flux, and excellent stability. However, the existing surface modification methods are complex and inefficient. In this study, an amphiphilic ionic liquid was selected to modify a porous anodic alumina membrane via simple capillary insertion. Due to the abundance of pH-dependent amphiphilic OH groups on the surface of AAO pore channels, the ionic liquids not only provide abundant surface charge but can also intelligently adjust its surface charge to different environments. In addition, it fills the AAO nanochannels to provide a continuous ion transport network. The modified hybrid membrane achieves efficient and stable osmotic energy conversion performance. This simple and feasible strategy paves the way for further improvements in commercial membranes.

Self‐Assembled Nanoporous Metal–Organic Framework Monolayer Film for Osmotic Energy Harvesting

AbstractOsmotic energy represents a promising energy resource because it is sustainable and environmentally benign. Subnanoscale channels are considered as a competitive platform for generating this blue energy due to their highly selective and ultrafast ion transport. However, fabricating functional subnanochannels capable of high energy output remains challenging. Here, a heterogeneous subnanochannel membrane formed by coating a functionalized self‐assembled metal−organic framework (MOF) monolayer (SAMM) film on a porous anodic aluminum oxide membrane, is reported. The SAMM film, with a thickness of ≈160 nm, is fabricated by self‐assembly of poly(methyl methacrylate‐co‐vinylimidazole)‐modified UiO‐66‐NH2 nanoparticles at the water−air interface. In the SAMM, imidazole and NH2 groups provide abundant positive charges, while the angstrom‐scale windows act as ionic filters for selective screening of anions with different hydration diameters. As a result, the heterogeneous membrane exhibits excellent capacity for anion‐selective transport, which contributes to an optimal osmotic power of 6.76 W m−2 under a 100‐fold NaCl gradient, as well as a high Cl−/SO42− selectivity of ≈42.2. Further, the output power is increased to 10.5 W m−2 by methylating imidazole moieties on the MOF surface. This work provides a facile and modular approach to fabricate subnanochannels for enabling highly selective and efficient osmotic energy conversion.

Selective Separation of Singly Charged Chloride and Dihydrogen Phosphate Anions by Electrobaromembrane Method with Nanoporous Membranes.

The entrance of even a small amount of phosphorus compounds into natural waters leads to global problems that require the use of modern purification technologies. This paper presents the results of testing a hybrid electrobaromembrane (EBM) method for the selective separation of Cl- (always present in phosphorus-containing waters) and H2PO4- anions. Separated ions of the same charge sign move in an electric field through the pores of a nanoporous membrane to the corresponding electrode, while a commensurate counter-convective flow in the pores is created by a pressure drop across the membrane. It has been shown that EBM technology provides high fluxes of ions being separated across the membrane as well as a high selectivity coefficient compared to other membrane methods. During the processing of solution containing 0.05 M NaCl and 0.05 M NaH2PO4, the flux of phosphates through a track-etched membrane can reach 0.29 mol/(m2×h). Another possibility for separation is the EBM extraction of chlorides from the solution. Its flux can reach 0.40 mol/(m2×h) through the track-etched membrane and 0.33 mol/(m2×h) through a porous aluminum membrane. The separation efficiency can be very high by using both the porous anodic alumina membrane with positive fixed charges and the track-etched membrane with negative fixed charges due to the possibility of directing the fluxes of separated ions in opposite sides.

Controlled self-cleaning aluminum surfaces mediated by anodic aluminum oxide.

Special porous anodic aluminum oxide (AAO) membranes are created on Al in a phosphonic acid electrolyte solution through one-step anodic oxidation and modified with polydimethysiloxane using a vapor deposition technique. In this context, the anodic oxidation time is tuned during its process. As such, the Al surface's wettability, and self-cleaning property are controlled by the tunable anodic oxidation time, for the anodic oxidation, time can regulate the structure of the AAO and the proportion of the air-liquid interface.

Templated Synthesis of Diamond Nanopillar Arrays Using Porous Anodic Aluminium Oxide (AAO) Membranes

Diamond nanostructures are mostly produced from bulk diamond (single- or polycrystalline) by using time-consuming and/or costly subtractive manufacturing methods. In this study, we report the bottom-up synthesis of ordered diamond nanopillar arrays by using porous anodic aluminium oxide (AAO). Commercial ultrathin AAO membranes were adopted as the growth template in a straightforward, three-step fabrication process involving chemical vapor deposition (CVD) and the transfer and removal of the alumina foils. Two types of AAO membranes with distinct nominal pore size were employed and transferred onto the nucleation side of CVD diamond sheets. Subsequently, diamond nanopillars were grown directly on these sheets. After removal of the AAO template by chemical etching, ordered arrays of submicron and nanoscale diamond pillars with ~325 nm and ~85 nm diameters were successfully released.

Electrodeposited MOFs Membrane with In Situ Incorporation of Charged Molecules for Osmotic Energy Harvesting.

Ion-selective membranes are considered as the promising candidates for osmotic energy harvesting. However, the fabrication of highly perm-selective membrane is the major challenge. Metal-organic frameworks (MOFs) with well-defined nanochannels along functional charged groups show great importance to tackle this problem. Here, a series of dense sodium polystyrene sulfonate (PSS) incorporated MOFs composite membranes (PSS@MOFs) on a porous anodic aluminum oxide (AAO) membrane via in situ anodic electrodeposition process are developed. Benefiting to the novel structural design of the confined Ag layer, PSS@MOFs dense composite membrane with less defects formed. The sulfonated nanochannels of the PSS@MOFs composite membrane provided rapid and selective transport of cations due to the enhanced electrostatic interaction between the permeating ions and MOFs. While osmotic energy conversion, 860 nm thick negatively charged PSS@MOFs composite membrane achieves an ultrahigh cation transfer number of 0.993 and energy conversion efficiency of 48.8% at a 100-fold salinity gradient. Moreover, a large output power of 2.90 µW has been achieved with an ultra-low internal resistance of 999 Ω, employing an effective area of 12.56 mm2 . This work presents a promising strategy to construct a high-performance MOFs-based osmotic energy harvesting system for practical applications.

Rapid change in polarization accompanying Fabry-Pérot resonance in anodic porous alumina coated with a gold thin film.

We experimentally investigate the polarization property of the specularly reflected light from an anodic porous alumina (APA) membrane coated with a gold (Au) thin film. As a result, we reveal a rapid change in the normalized Stokes parameter s 3 of the specularly reflected light around the angle of incidence θ A at which the resonance absorption of the incident light occurs. The rapid change in s 3 demonstrates that the specularly reflected light can rapidly be right- to left-elliptically polarized via linear polarization at the zero-crossing point θ Z of s 3. Moreover, θ Z is located close to θ A , and θ Z as well as θ A depend on the occurrence conditions of the resonance absorption. From numerical aspects based on the Maxwell Garnett effective medium approximation, we clarify that the rapid change in s 3 accompanies the Fabry-Pérot (FP) resonance in the Au-coated APA membrane. The numerical results also suggest that the change in the refractive index of the filling material into nanopores of the Au-coated APA membrane can be successfully estimated by using the rapid change in s 3.

Research on the gas migration trend and mechanism of the transition flow regime in coal based on MRT-LBM simulation

In order to reveal the process and mechanism of gas flow in a low-permeability coal seam, a new multiple-relaxation-time lattice Boltzmann method (MRT-LBM) model of gas migration in coal micro/nanopores based on Langmuir monolayer adsorption theory, the slip boundary scheme and Bosanquet effective viscosity was established. The software MATLAB was used to carry out the simulation study of uniform pore gas flow based on the MRT-LBM model, and the results were compared and verified with the porous anodic alumina membrane gas flow experimental results. On this basis, the gas flow in coal pores with different micro/nanopore sizes considering adsorption was simulated. The results show that the dimensionless permeability coefficient increases with decreasing pore size under the same pressure, which reflects the subsequent enhancement of the microboundary constraint effect and reveals that the pore system becomes the main controlling factor of coal seam permeability within the coal matrix in the middle and late stages of coal seam gas extraction, while the role of the microboundary constraint effect needs to be considered. The gas adsorption layer weakens the pore gas flow capacity, but for pores with a radius greater than 16 nm, the apparent change in permeability caused by the adsorption layer is less than 5%, and the adsorption effect can be ignored. N2, CH4, and CO2 enter the transition flow regime under different pressure conditions; with gas extraction, the gas pressure decreases, and the difference in the gas flow characteristics of the three gases increases.

Strongly Improving the Sensitivity of Phosphorescence-Based Optical Oxygen Sensors by Exploiting Nano-Porous Substrates.

Sensitivity is one of the crucial factors in determining the quality of a fluorescence/phosphorescence-based gas sensor, and is estimated from the measurement of responses (I0/I, where I0 and I refer to the measured optical intensity of a sensor in absence and presence of analyte molecules) at various concentrations of analytes. In this work, we demonstrate phosphorescence-based optical oxygen sensors fabricated on highly porous anodic aluminum oxide (AAO) membranes showing dramatically high response. These sensors exploit the enormous surface area of the AAO to facilitate the effective interaction between the sensing molecules and the analytes. We spin-coat an AAO membrane (200 nm pore diameter) with a platinum-based oxygen sensing porphyrin dye, platinum(II) meso-tetrakis (pentafluorophenyl) porphyrin (PtTFPP), to fabricate a sensor exhibiting I0/I ~400 at 100% oxygen atmosphere. To address the generality of the AAO membrane, we fabricate a separate sensor with another porphyrin dye, platinum octaethylporphyrin (PtOEP), which exhibits an even higher I0/I of ~500. Both of these sensors offer the highest responses as an optical oxygen sensor hitherto reported. SEM and EDS analysis are performed to realize the effect of the increased surface area of the AAO membrane on the enhanced sensitivity.

High-Performance pH Sensor Electrodes Based on a Hexagonal Pt Nanoparticle Array-Coated Nanoporous Alumina Membrane.

Porous anodic alumina membranes coated with Pt nanoparticles (PAAM/Pt) have been employed as pH sensor electrodes for H+ ion detection. The PAAM was designed using a two-step anodization process. Pt nanoparticles were then sputtered onto the membrane at different deposition times. The membrane’s morphological, chemical, and optical characteristics were carefully assessed following the fabrication stage using a variety of analytical techniques. The potential of the PAAM/Pt sensor electrode was investigated by measuring the potential using a simple potentiometric method. The effects of depositing Pt nanoparticles for 3–7 min on sensor electrode sensitivity were examined. The optimal potentiometric Nernstian response slope for the PAAM/Pt sensor electrode with 5 min Pt sputter coating is 56.31 mV/decade in the pH range of 3.0 to 10 at 293 K. Additionally, the PAAM/Pt sensor electrode’s stability and selectivity in various ions solutions were examined. The sensor electrode had a lifetime of more than six weeks and was kept in a normal air environment.

The fabrication of unwettable superhydrophobic surfaces by reactive wetting of AgCuTi into an AAO template

Multi-level hierarchical surface roughness is the key to natural superhydrophobic materials. In this work, rough surfaces of a novel superhydrophobic material are achieved by the reactive wetting technique. The molten AgCuTi alloy is applied to infiltrate into the porous anodic aluminum oxide (AAO) membrane, and wetting of molten AgCuTi is promoted by the reaction layer formed at the interface between AAO and AgCuTi. The AgCuTi infiltrates the AAO nanopores and forms the first-level surface roughness based on reactive wetting. To release the nanostructures embedded in the AAO pores, hydrothermal etching with NaOH aqueous solution was used to remove AAO. Meanwhile, TiO2 nanostructures are synthesized under hydrothermal conditions, resulting in the second-level surface roughness. After modification, the surface is found to exhibit excellent superhydrophobic properties, including a high water contact angle (WCA) and low roll-off angle (RA). The superhydrophobic surface also shows self-cleaning, mechanical durability and long-term stability. The as-prepared surface exhibits excellent repellence for water solutions with different pH values. This method could be applied to fabricate superhydrophobic surfaces on different kinds of materials, showing broad application prospects.

The Interrelation of Synthesis Conditions and Wettability Properties of the Porous Anodic Alumina Membranes.

The results of studies on the wettability properties and preparation of porous anodic alumina (PAA) membranes with a 3.3 ± 0.2 μm thickness and a variety of pore sizes are presented in this article. The wettability feature results, as well as the fabrication processing characteristics and morphology, are presented. The microstructure effect of these surfaces on wettability properties is analyzed in comparison to outer PAA surfaces. The interfacial contact angle was measured for amorphous PAA membranes as-fabricated and after a modification technique (pore widening), with pore sizes ranging from 20 to 130 nm. Different surface morphologies of such alumina can be obtained by adjusting synthesis conditions, which allows the surface properties to change from hydrophilic (contact angle is approximately 13°) to hydrophobic (contact angle is 100°). This research could propose a new method for designing functional surfaces with tunable wettability. The potential applications of ordinary alumina as multifunctional films are demonstrated.

Encrypted information reading technology at the micro/nano scale based on surface plasma-driven reactions

The plasma exciton induced photocatalytic reaction has considerable potential in terms of controllability and selectivity. In this paper, with the advantage of Raman fingerprinting, the localized photocatalytic reaction driven by surface plasmons is realized by the writing and reading process of encrypted information at the micro/nano scale. A layer of probe molecules (4-nitrobenzenethiol, 4-NBT) was assembled on a gold nanoporous array grown on porous anodic aluminium oxide (AAO) membranes. The focused Raman spot is manipulated in a two-dimensional micro/nano manipulation technique to control the movement of the spot at an excitation wavelength of 633 nm. Probe molecules within the spot trajectory will undergo a photocatalytic reaction to produce p,p'-dimercaptoazobenzene (DMAB) molecules, thereby writing the specific information required. The use of Raman mapping to image the characteristic peaks of formed DMAB under excitation light with a longer wavelength of 785 nm enables the readout of 2D micro/nano cryptograms. Combined with finite-difference time-domain (FDTD) simulations, it was found that the presence of a large number of regularly arranged hot spots on the surface of the array is the key to achieving the efficient photocatalytic reaction. This study enables real-time, lossless recording/reading of encrypted information with the aid of 2D Raman technology. This would be a very interesting research area with broad application in confidential information storage.

Effect of Surface Morphology Changes on Optical Properties of Silicon Nanowire Arrays.

The optical properties of silicon nanowire arrays (SiNWs) are closely related to surface morphology due to quantum effects and quantum confinement effects of the existing semiconductor nanocrystal. In order to explore the influence of the diameters and distribution density of nanowires on the light absorption in the visible to near infrared band, we report the highly efficient method of multiple replication of versatile homogeneous Au films from porous anodic aluminum oxide (AAO) membranes by ion sputtering as etching catalysts; the monocrystalline silicon is etched along the growth templates in a fixed proportion chemical solution to form homogeneous ordered arrays of different morphology and distributions on the surface. In this system, we demonstrate that the synthesized nanostructure arrays can be tuned to exhibit different optical characteristics in the test wavelength range by adjusting the structural parameters of AAO membranes.

Nanostructured Broadband Solar Absorber for Effective Photothermal Conversion and Electricity Generation

Photothermal conversion is an environmentally friendly process that harvests energy from the sun and has been attracting growing research interest in recent years. However, nanostructured strategies to improve light capture performance deserve further development, and the application of solar heating effects for clean energy needs to be explored. Herein, a multiscale nanomaterial was prepared by in situ polymerizing the polyaniline (PANI) nanoparticles into porous anodic aluminum oxide (AAO) membrane. As a result, the as-prepared PANI-AAO shows broadband solar absorption and provides a platform for efficient photothermal conversion. What is more, we introduced a typical thermoelectricity generator (TEG) with excellent output performance and combined it with PANI-AAO to prepare a solar thermoelectric generator (s-TEG). The s-TEG harvests solar energy and converts it into electricity, showing an outstanding power generation capability in outdoor conditions. Thus, the nanostructured broadband solar absorber and the integrated solar thermoelectric generator offer a promising candidate for a sustainable and green energy source in the future.

Engineering highly efficient Li+ responsive nanochannels via host–guest interaction and photochemistry regulation

Artificial nanochannels have seen great progress in energy storage, molecular filters, nanofluidic diodes, and biosensors. Precisely recognizing and transporting molecules or ions underpin the basis of artificial nanochannels. Here, we report highly efficient Li+ responsive nanochannels by design of functionalized porous anodic aluminum oxide (AAO) membranes. The 12-crown-4 unit was tethered to the synthesized spiropyran molecule, creating a smart Li+ responsive element, photochromic crowned spiropyran (CE-SP). The crown unit can specifically bind with Li+ while exempting the harassment of other metallic cations; and importantly, the spiropyran moiety possesses reversible photochemistry response, adding the flexibility to regulate ion transport behaviors. Upon the ultraviolet light stimulus, hydrophobic CE-SP molecules transform to positively charged crowned merocyanine (CE-MC), which can further convert to oxygen-negative ions in weak alkaline environment, thereby forming O--Li+ interaction. Such structural evolutions enable to deftly tune the surface charge and wettability of the nanochannels, and thus ingeniously manipulate Li+ recognition profiled by ionic current signal. Moreover, electrically-driven release of the captured Li+ can be realized in an amiable condition, showing controllable capture-to-release ability. Finite element simulation unveils the fundamental mechanisms of ion transport pertaining to Li+ recognition events in the nanoconfinement. This work thus paves a way to construct high-performance Li+-selective nanodevice, and holds promise for Li+-related electrochemistry energy research.

Magnetically controlled insertion of cobalt ferrite nanoparticles into a porous anodic aluminum oxide (AAO) membrane

Magnetically controlled insertion of cobalt ferrite nanoparticles into a porous anodic aluminum oxide (AAO) membrane