Porous Anodic Alumina Research Articles

Development of Y-Shaped Porous Anodic Alumina Humidity Sensor to Enhance Lower Detection Limit and Sensitivity

The amount of water vapours in the air can affect the productivity and safety of many manufacturing processes, such as agriculture, pharmaceuticals, and semiconductor industries, as well as human comfort. The humidity sensors based on porous materials, such as Porous Anodic Alumina (PAA), have persuaded much attention because of their high porosity and tuneable structure. In this work, we have developed a PAA capacitive sensor using phosphoric acid electrolyte by the Differential Pulse Voltammetry method of anodization. The PAA-based humidity sensor shows ultra-high sensitivity, fast response, and a wide detection range of relative humidity (RH). Response and recovery time of the sensor are ~ 16 s and ~ 4 s, respectively at 64 RH%, and the sensor is capable of sensing 1-100 RH%. These results advocate that PAA is a propitious applicant for various humidity sensing applications, especially where low humidity detection is required i.e., paper, lithium-ion battery, tea industries, etc.

A Study of Ta2O5 Nanopillars with Ni Tips Prepared by Porous Anodic Alumina Through-Mask Anodization.

The paper discusses the formation of Ta2O5 pillars with Ni tips during thin porous anodic alumina through-mask anodization on Si/SiO2 substrates. The tantalum nanopillars were formed through porous masks in electrolytes of phosphoric and oxalic acid. The Ni tips on the Ta2O5 pillars were formed via vacuum evaporation through the porous mask. The morphology, structure, and magnetic properties at 4.2 and 300 K of the Ta2O5 nanopillars with Ni tips have been studied using scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometry. The main mechanism of the formation of the Ta2O5 pillars during through-mask anodization was revealed. The superparamagnetic behavior of the magnetic hysteresis loop of the Ta2O5 nanopillars with Ni tips was observed. Such nanostructures can be used to develop novel functional nanomaterials for magnetic, electronic, biomedical, and optical nano-scale devices.

Boosting of Surface Anion Density on Porous Anodic Alumina Surface—A Key Factor to Enhance Moisture Sensor Operation to Critical Sub-RH Level

Humidity sensing at low RH%, its mechanism, and even lowering further minimum detection limit has no clear answer as of now. Porous anodic alumina (PAA), though an excellent hygroscopic material; still lacks a comprehensive theory on how to lower the minimum detection limit (LOD) to the sub-RH level. In this report, in addition to the pore morphology design, the effect of electrolyte driven anion concentration on PAA surface has been suitably fine-tuned by increasing the attachment of hydroxyl bond density. The organic solvent like glycerol mixed with the anodizing electrolyte has been chosen in proper concentration; leading to enhance the degree of ionic attachment to the PAA surface, hence increase in the sensitivity and the lower detection limit. A lower detection limit of 0.5 RH% is obtained with optimized glycerol concentration, examined through FTIR spectroscopy, which is a direct consequence of increased surface anion concentration that facilitates the adsorption of water vapor molecules on the surface. The developed sensor shows wide detection range 0.5-99 RH%, ultrahigh sensitivity of 275.4 pF/RH%, very fast response- and recovery time of 3 s and 15 s and negligible drift over cycling over 30 times for 0-11.4 RH%. The sensor provides a low-cost alternative solution specifically, to industries having low moisture monitoring requirements e.g., tea and lithium battery manufacturing industry.

Superhydrophobic Ag‐Decorated CuO Nanowire Arrays with Analyte‐Concentrating and Self‐Cleaning Binary Functions for Ultrasensitive and Recyclable Surface‐Enhanced Raman Scattering

AbstractSurface‐enhanced Raman scattering (SERS) is considered a promising analytical technique for the detection of analytes. Considering the practical SERS detection, it is necessary to develop low‐cost, highly sensitive, and recyclable SERS‐active substrates. Here, a simple and flexible approach is proposed to fabricate a superhydrophobic Ag‐decorated CuO (named as CuO@Ag) nanowire array substrate with analyte‐concentrating and self‐cleaning binary functions for ultrasensitive and recyclable SERS detection. During this process, a superhydrophobic CuO@Ag nanowire array is made via fabrication of a CuO nanowire array by oxidizing and calcinating an ordered porous anodic aluminum oxide template on Cu foil, and following with Ag sputtering deposition on CuO nanowire array. Remarkably, this SERS substrate exhibits ultrasensitive SERS detection ability attributed to superhydrophobic plasmonic metal nanostructured arrays with high analyte‐concentrating ability and excellent photocatalytic capability due to this metal‐semiconductor composite nanostructure. These advantageous properties allow for SERS detection of malachite green with a limit of detection of 6.73 × 10−13 m and self‐cleaning via photodegradation of adsorbed analytes with visible‐light illumination. Consequently, it achieves recyclable SERS detection. This research demonstrates a simple and flexible approach for fabricating recyclable SERS‐active substrate, which expands the practical SERS applications in chemical and biological analysis, food safety, and healthcare.

Light-Enhanced Osmotic Energy Harvester Using Photoactive Porphyrin Metal-Organic Framework Membranes.

High ion selectivity and permeability, as two contradictory aspects for the membrane design, highly hamper the development of osmotic energy harvesting technologies. Metal-organic frameworks (MOFs) with ultra-small and high-density pores and functional surface groups show great promise in tackling these problems. Here, we propose a facile and mild cathodic deposition method to directly prepare crack-free porphyrin MOF membranes on a porous anodic aluminum oxide for osmotic energy harvesting. The abundant carboxyl groups of the functionalized porphyrin ligands together with the nanoporous structure endows the MOF membrane with high cation selectivity and ion permeability, thus a large output power density of 6.26 W m-2 is achieved. The photoactive porphyrin ligands further lead to an improvement of the power density to 7.74 W m-2 upon light irradiation. This work provides a promising strategy for the design of high-performance osmotic energy harvesting systems.

Light‐Enhanced Osmotic Energy Harvester Using Photoactive Porphyrin Metal–Organic Framework Membranes

AbstractHigh ion selectivity and permeability, as two contradictory aspects for the membrane design, highly hamper the development of osmotic energy harvesting technologies. Metal–organic frameworks (MOFs) with ultra‐small and high‐density pores and functional surface groups show great promise in tackling these problems. Here, we propose a facile and mild cathodic deposition method to directly prepare crack‐free porphyrin MOF membranes on a porous anodic aluminum oxide for osmotic energy harvesting. The abundant carboxyl groups of the functionalized porphyrin ligands together with the nanoporous structure endows the MOF membrane with high cation selectivity and ion permeability, thus a large output power density of 6.26 W m−2 is achieved. The photoactive porphyrin ligands further lead to an improvement of the power density to 7.74 W m−2 upon light irradiation. This work provides a promising strategy for the design of high‐performance osmotic energy harvesting systems.

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.

Formation and characterisation of porous anodized aluminum oxide, synthesized electrochemically in the presence of graphene oxide

Formation and characterisation of porous anodized aluminum oxide, synthesized electrochemically in the presence of graphene oxide

Electric field manipulation of nonvolatile magnetization in MgO-based memory device with resistive switching effect

In the development of multifunctional data storage devices, controlling magnetic properties by electric field would be highly desirable. This work reported the relationship of magnetic properties of MgO film on porous anodic alumina (PAA) substrates to the different resistance states of Ag/MgO/PAA/Al(Ag/MP/Al) device at room temperature. We suggest that the fully deterministic magnetic switching in this composite film is based on the high sensitivity of oxygen vacancy defect-based conductive filaments to electric field. Our review offers a convenient way to control the magnetic properties using electric field, and shows great promises for developing novel multifunctional material in the future.

Light-Regulated Nanofluidic Ionic Diodes with Heterogeneous Channels Stemming from Asymmetric Growth of Metal–Organic Frameworks

Nanofluidic ionic diodes have attracted much attention, because of the unique property of asymmetric ion transport and promising applications in molecular sensing and biosensing. However, it remains a challenge to fabricate diode-like nanofluidic system with molecular-size pores. Herein, we report a new and facile approach to construct nanofluidic ionic diode by in situ asymmetric growth of metal-organic frameworks (MOFs) in nanochannels. We implement microwave-assisted strategy to obtain asymmetric distribution of MOFs in porous anodic aluminum oxide with barrier layer on one side. After etching the barrier layer and modifying with positively charged molecules, the nanofluidic device possesses asymmetric geometry and surface charge, performing the ionic current rectification (ICR) behavior in different electrolyte concentrations. Moreover, the ICR ratio is readily regulated with visible light illumination mainly due to the enhancement of surface charge of MOFs, which is further confirmed by finite element simulation. This study provides a reliable way to build the nanofluidic platform for investigating the asymmetric ion transport through the molecular-size pores, which is envisaged to be important for molecular sensing based on ICR with molecular-size pores.

Preparation of moth-eye structures on curved surfaces by nanoimprinting using anodic porous alumina molds

Al substrates with smooth curved surfaces were fabricated by peeling off Al foil sputtered on the surface of concaved substrates. Anodic porous alumina with tapered pores on the curved surface was obtained by repeated anodization and etching of the obtained Al substrate. The moth-eye structure was formed on the curved surface by nanoimprinting using the anodic porous alumina as a mold. From the reflectance measurement, it was shown that the reflection of the incident light was suppressed on the curved surface where the moth-eye structure was formed. This process can be used to fabricate moth-eye structures on the surface of substrates of various shapes by changing the substrate used for sputtering and is expected to expand the range of applications for moth-eye structures.

Directional controllable electrodeposition growth of homogeneous Au nano-rampart arrays and its reliable SERS applications

High-yield large-area uniform metal nanoarrays have shown attractive application potential in many fields. Electrodeposition growth has unique advantages in the construction of metal nanostructures due to its high efficiency. However, the uniformity and stability of electrodeposition are still remain challenging. Here, an efficient, stable and cost-effective directional electrodeposition route was proposed to construct large-scale uniform metal nanoarrays, which could be further used as reliable SERS substrates. Assisted with uniform porous anodized aluminum oxide (AAO) templates, a series of homogeneous Au nano-rampart arrays (Au-NRAs) were prepared controllably by directional electrodeposition growth technology. Using the prepared Au-NRAs as SERS substrates, the results showed that the optimal electrodeposition growth is 600 s. At this time, Au-NRAs showed highest SERS activity and the low relative standard deviation (RSD) value of Raman intensity at 619 cm−1 mode of R6G (with concentration of 10-5 M) was 2.08%, which indicated excellent SERS stability. This method integrated not only cost-effectiveness, but also good directional controllability and repeatability, showing great potential in constructing large-area uniform metal nanostructures.

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.

Galvanic-Replacement-Assisted Surface-Initiated Atom Transfer Radical Polymerization for Functional Polymer Brush Engineering.

Here we present a facile and robust strategy, namely, galvanic-replacement-assisted surface-initiated Cu(0)-mediated atom transfer radical polymerization (gr-SI-Cu0ATRP, or gr-SI-Cu0CRP) for polymer brush engineering under ambient conditions. In gr-SI-Cu0ATRP, highly active and nanostructured Cu(0) surfaces are obtained by a simple galvanic replacement on zinc/aluminum surfaces in dilute Cu2+ solution. Polymer brush growth rate is extremely high (up to ∼904 nm in 30 min polymerization); meanwhile, both nano Cu(0) surfaces and Cu2+ solution can be reused multiple times without losing grafting efficiency. We also demonstrate that the gr-SI-Cu0ATRP is advantageous for polymer brush engineering on arbitrary substrates, including flexible (polyethylene terephthalate), curved (polycarbonate), and porous (anodic aluminum oxide), and endow the substrates with various functionalities, for example, anti-icing, antifogging, and ion selectivity.

Light Absorption Enhancement and Laser-Induced Damage Ability Improvement of Aluminum Alloy 6061 with Non-Porous Alumina/CdSe@Al2O3/SiO2 Functional Gradient Films.

Numerical calculations of ultraviolet to near-infrared absorption spectra by cadmium selenide quantum dots (CdSe QDs) doped in anodic aluminum oxide pores were performed using a finite-difference time-domain model. The height, diameter, and periodic spacing of the pores were optimized. Light absorption by the dots was enhanced by increasing the height and decreasing the diameter of the pores. When the height was less than 1 μm, visible light absorption was enhanced as the spacing was reduced from 400 nm to 100 nm. No enhancement was observed for heights greater than 6 μm. Finally, the optical mode coupling of the aluminum oxide and the quantum dots was enhanced by decreasing the pore diameter and periodic spacing and increasing the height. Laser ablation verified light absorption enhancement by the CdSe QDs. The experiments verified the improvement in the laser-induced damage ability with a nanosecond laser at a wavelength of 355 nm after aluminum alloy 6061 was coated with functional films and fabricated based on numerical calculations.

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

An Ultrahigh-Flux Nanoporous Graphene Membrane for Sustainable Seawater Desalination using Low-Grade Heat.

Membrane distillation has attracted great attention in the development of sustainable desalination and zero-discharge processes because of its possibility of recovering 100% water and the potential for integration with low-grade heat, such as solar energy. However, the conventional membrane structures and materials afford limited flux thus obstructing its practical application. Here, ultrathin nanoporous graphene membranes are reported by selectively forming thin graphene layers on the top edges of a highly porous anodic alumina oxide support, which creates short and fast transport pathways for water vapor but not liquid. The process avoids the challenging pore-generation and substrate-transfer processes required to prepare regular graphene membranes. In the direct-contact membrane distillation mode under a mild temperature pair of 65/25°C,the nanoporous graphene membranes show an average water flux of 421.7 L m-2 h-1 with over 99.8% salt rejection, which is an order of magnitude higher than any reported polymeric membranes. The mechanism for high water flux is revealed by detailed characterizations and theoretical modeling. Outdoor field tests using water from the Red Sea heated under direct sunlight radiation show that the membranes have an average water flux of 86.3 L m-2 h-1 from 8 am to 8 pm, showing a great potential for real applications in seawater desalination.

Optical and Thermal Characteristics of Porous Anodic Aluminum Oxide for Photothermal Applications

Optical and Thermal Characteristics of Porous Anodic Aluminum Oxide for Photothermal Applications

High-temperature annealing of porous anodic aluminium oxide prepared in selenic acid electrolyte

Selenic acid electrolyte provides porous anodic aluminium oxide (AAO) films with low porosity and high optical transparency. Here, the high-temperature behaviour of AAO obtained in a selenic acid electrolyte is reported for the first time. The crystallization of as-prepared amorphous AAO at about 800 °C is accompanied by the elimination of electrolyte impurities, an increase in pore diameter from 15 to 21 nm, and an increase in specific surface area to 55 m2·g−1 due to the mesopore formation inside the cell walls. The second crystallization stage at about 1150 °C results in the formation of coarse-grained corundum (α-Al2O3) films with a mean grain size of about 11 μm. Each of the grains consists of regularly arranged 27-nm-diameter pores, which are stable up to 1300 °C. AAO physicochemical properties depend on the annealing temperature. In particular, chemical stability is up to three orders of magnitude higher after two-step crystallization. Optical properties and thermal expansion of selenic-acid-made porous AAO films with different phase composition are quantified. The crystallization of as-prepared AAO to low-temperature Al2O3 polymorphs has a minor influence on optical transmittance and effective refractive index of AAO, whereas the coarse-grained structure of corundum causes an order of magnitude decrease in average transmittance in the visible region. The coefficient of thermal expansion increases by 25% to 8.5 × 10−6 K−1 for α-Al2O3 porous films compared to as-prepared AAO.