Porous Anodic Alumina Research Articles

Chemical stability of porous anodic aluminum oxide in both acidic and alkaline solutions

A porous anodic aluminum oxide (AAO) possessing higher chemical resistance in both acidic and alkaline solutions was fabricated by anodizing Al in an alkaline sodium tetraborate solution. The 5 N Al plates were anodized in five types of major acidic solutions (sulfuric, oxalic, phosphoric, chromic, and etidronic acids) and in alkaline sodium tetraborate solution to form AAOs. The anodized specimens were then immersed in a 0.52 M phosphoric acid solution (pH = 1.3) and a 2.5 M sodium hydroxide solution (pH = 13.8). Subsequently, the electrochemical impedance measurements and morphological characterizations of the AAOs before and after immersion were investigated. The time required for the complete dissolution of the AAOs formed in typical acidic electrolytes, including sulfuric, oxalic, phosphoric, and etidronic acids, increased linearly with the anodizing voltage. The AAO formed in chromic acid exhibited a higher chemical resistance than this linear relationship owing to the formation of high-purity alumina without incorporated anions. Moreover, the AAO formed in sodium tetraborate exhibited the highest chemical resistance in both acidic and alkaline solutions – approximately two times higher than that formed in etidronic and chromic acids. This can be attributed to their higher anodizing ratio and the formation of a high-purity alumina layer.

Ppb‐Level ethanol gas sensor of porous anodic aluminum oxide at room temperature

AbstractExcessive ethanol in breath can be an indicator of alcoholism and a biomarker of liver disease. Herein, we present a novel porous aluminum anodic oxide (AAO)‐based chemi‐capacitive sensor that can detect ppb‐levels of ethanol gas at room temperature. The oxalic acid‐based porous AAO is an amorphous material contaminated with Al(OH)3 and C2O42− ions (anion‐contamination), which react with adsorbed ethanol gas. The optimal resonance frequency of ethanol was determined to be 500 Hz, and the response to 200 ppm ethanol at this frequency was 1.56%. Especially, the limit of detection of ppb‐level (16.49 ppb) was obtained. The detection range, reliability, and selectivity of the sensor, as well as its surface properties were comprehensively investigated. The proposed sensor is promising for the detection of liver disease as well as for commercial breath ethanol analyzers.

Vertically Integrated Nanowires on Si Wafers and Into Circuits

Vertically integrated copper (Cu) and nickel (Ni) nanowires (NWS) were fabricated on silicon using anodized aluminum oxide (AAO) thin film templates integrated onto silicon wafers. Both the AAO and NWs were mechanically robust and demonstrated to withstand further fabrication processes used for integrated circuits. The AAO pores were measured to be ~30 nm with size distributions narrowing from ±12 to ±6 nm after a second anodization. The NWs, therefore, had similar diameters and distributions inside these integrated AAO films. Magnetic hysteresis loops demonstrated the out-of-plane anisotropy of the vertically oriented Ni wires in the presence of pore outgrowth that had in-plane anisotropy. When used as vias and integrated with coplanar waveguides (CPWs), the Cu NWs had lower losses than standard vias above 60 GHz.

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.

Electrochemical Impedance Spectroscopy Study of Porous Anodic Alumina Films Fabricated at Low Temperature

A method of fabrication of AAO film in 0.3 mol/L H2C2O4 by adding 1,2-propanediol(PROH) and at sub-zero temperature has been proposed. The electrochemical characteristics of the barrier and porous layers of AAO films before and after pore-filling are examined using electrochemical impedance spectroscopy (EIS). It is shown that the high and medium frequency range corresponds to the barrier layer properties and the low frequency ranges reflect the sealed porous layer properties. The calculated thickness of the barrier layer is in the range of 1 ~ 18 nm. The resistance (Rb) and the thickness (δb) of the barrier layer increase and the capacitance of the barrier layer (CPEb) decrease with the volume percentage of PROH increasing from in electrolyte from 25 % to 75 %. The surface non-homogeneity of AAO film goes better by pore-filling, leading to a decrease in the capacitance of the porous layer (CPEp) and an increase in the porous layer thickness.

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.

Two-Level 3D Column-like Nanofilms with Hexagonally-Packed Tantalum Fabricated via Anodizing of Al/Nb and Al/Ta Layers-A Potential Nano-Optical Biosensor.

Reanodizing metal underlayers through porous anodic alumina has already been used extensively to fabricate ordered columns of different metal oxides. Here, we present similar 3D multilayered nanostructures with unprecedented complexity. Two-level 3D column-like nanofilms have been synthesized by anodizing an Al/Nb metal layer in aqueous oxalic acid for forming the first level, and an Al/Ta layer in aqueous tartaric acid for forming the second level of the structure. Both levels were then reanodized in aqueous boric acid. The Ta layer deposited on partially dissolved porous anodic alumina of the first level, with protruding tops of niobia columns, acquired a unique hexagonally-packed structure. The morphology of the first and second levels was determined using scanning electron microscopy. Prolonged etching for 24 h in a 50%wt aqueous phosphoric acid was used to remove the porous anodic alumina. The formation mechanism of aluminum phosphates on the second-level columns in the process of long-time cold etching is considered. The model for the growth of columns on a Ta hexagonally-packed structure of the second level is proposed and described. The described approach can be applied to create 3D two- or three-level column-like systems from various valve metals (Ta, Nb, W, Hf, V, Ti), their combinations and alloys, with adjustable column sizes and scaling. The results of optical simulation show a high sensitivity of two-level column-like 3D nanofilms to biomedical objects and liquids. Among potential applications of these two-level column-like 3D nanofilms are photonic crystals for full-color displays, chemical sensors and biosensor, solar cells and thermoresponsive shape memory polymers.

Nanoconfinement-Dependent Chain Orientation of Polymorphs in Poly(3-dodecylthiophene)s

Nanoconfinement of conjugated polymers (CPs) is an effective strategy to control the orientations and orderings of CPs, which determine their electrical properties. We investigate the chain orientations of two crystal polymorphs, Form I and Form II, in poly(3-dodecylthiophene)s (P3DDTs) using a porous anodic aluminum oxide (AAO) template. The control of regioregularity (RR) of P3DDTs results in well-defined Form II/Form I ratios of crystal polymorphs. Interestingly, Form I and Form II crystals show considerably different orientational changes depending on the pore diameter (Dpore) of the porous AAO template. As Dpore decreases from 100 to 30 nm, Form II crystals change their orientations from face-on to edge-on dominant morphologies, whereas Form I crystals consistently exhibit edge-on dominant morphologies. In addition, for a fixed Dpore of 100 nm, temperature-dependent orientational changes of Form I and Form II are investigated. While Form II crystals show a significant orientational change with increasing temperature, Form I crystals only experience chain reconfiguration. This finding provides useful guidelines for achieving the CPs with controlled intermolecular assembly and chain orientations.

Designing porous photonic crystals for MIR spectral region—a deeper insight into the anodic alumina layer thickness versus charge density relation

The mid-infrared region (MIR) is crucial for many applications in security and industry, in chemical and biomolecular sensing, since it contains strong characteristic vibrational transitions of many important molecules and gases (e.g. CO2, CH4, CO). Despite its great potential, the optical systems operating in this spectral domain are still under development. The situation is caused mainly by the lack of inexpensive and adequate optical materials which show no absorption in the MIR. In this work, we present an easy and affordable way to develop 1D photonic crystals (PCs) based on porous anodic alumina for MIR region. The porous PCs were produced by the pulse anodization of aluminum using charge-controlled mode. The first order photonic stopbands (λ 1) were located within ca. 3.5–6.5 μm. Annealing of the material at 1100 °C for an hour has allowed to recover the wavelength range from around 5.8 to 7.5 μm owing to the decomposition of the absorption centers (oxalate anions) present in the anodic oxide framework while maintaining the PC structural stability. The spectral position and the shape of the resonances were regulated by the charge passing under high (U H) and low (U L) voltage pulses, porosity of the corresponding d H and d L segments, and dura tion of the process (t tot). The thickness of the d H and d L layers was proportional to the charge passing under respective pulses, with the proportionality coefficient increasing with the applied voltage. Despite the constant charge (2500 mC cm−2) applied during the anodization, the thickness of anodic alumina (d) increased with applied voltage (10–60 V) and anodizing temperature (5 °C–30 °C). This behavior was ascribed to the different kinetics of the anodic alumina formation prompted by the variable electrochemical conditions. The photonic material can be used in portable nondispersive gas sensors as an enhancement layer operating up to around 9 μm.

Overview of Engineering Carbon Nanomaterials Such As Carbon Nanotubes (CNTs), Carbon Nanofibers (CNFs), Graphene and Nanodiamonds and Other Carbon Allotropes inside Porous Anodic Alumina (PAA) Templates

The fabrication and design of carbon-based hierarchical structures with tailored nano-architectures have attracted the enormous attention of the materials science community due to their exceptional chemical and physical properties. The collective control of nano-objects, in terms of their dimensionality, orientation and size, is of paramount importance to expand the implementation of carbon nanomaterials across a large variety of applications. In this context, porous anodic alumina (PAA) has become an attractive template where the pore morphologies can be straightforwardly modulated. The synthesis of diverse carbon nanomaterials can be performed using PAA templates, such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), and nanodiamonds, or can act as support for other carbon allotropes such as graphene and other carbon nanoforms. However, the successful growth of carbon nanomaterials within ordered PAA templates typically requires a series of stages involving the template fabrication, nanostructure growth and finally an etching or electrode metallization steps, which all encounter different challenges towards a nanodevice fabrication. The present review article describes the advantages and challenges associated with the fabrication of carbon materials in PAA based materials and aims to give a renewed momentum to this topic within the materials science community by providing an exhaustive overview of the current synthesis approaches and the most relevant applications based on PAA/Carbon nanostructures materials. Finally, the perspective and opportunities in the field are presented.

Preparation of Anodic Porous Alumina with Gradient Hole Size for Directional Droplet Transport.

A surface with a wettability gradient can be used as a surface where water droplets transport in one direction. In this report, a surface with a wettability gradient was formed by Al anodization and subsequent gradient etching. Since the surface wettability of anodic porous alumina formed by the Al anodization was dependent on hole size, anodic porous alumina with gradient hole size was prepared to form a surface with a wettability gradient. The surface obtained by this method showed continuously changing wettability, and it was possible to control the range and amount of change in the contact angle of the water droplets by adjusting the hole size. In addition, water droplets were observed to transport spontaneously from an area with a high contact angle to that with a low contact angle on the surface of anodic porous alumina with gradient hole size. The obtained surface with a wettability gradient can be used in various applications, such as microfluidic devices, water collection, and oil-water separation.

Preparation of size-controlled LiCoPO4 particles by membrane emulsification using anodic porous alumina and their application as cathode active materials for Li-ion secondary batteries.

Membrane emulsification using anodic porous alumina is an effective method for preparing monodisperse droplets with controlled sizes. In this study, membrane emulsification using anodic porous alumina was applied to the preparation of size-controlled particles composed of composite metal oxides. To obtain size-controlled composite metal oxide particles, membrane emulsification was performed using an aqueous solution containing a water-soluble monomer and metal salts as a dispersed phase. After the membrane emulsification, composite metal oxide particles were obtained by solidifying the droplets in a continuous phase and subsequent heat treatment. Here, as a demonstration of this process, the fabrication of size-controlled LiCoPO4 particles, which are considered high-potential cathode active materials for Li-ion secondary batteries (LIBs), was investigated. The application of the obtained LiCoPO4 particles as cathode active materials for LIBs was also investigated. The results of this study showed that LiCoPO4 particles with controlled sizes could be fabricated on the basis of this process and that their cathode properties could be improved by optimizing the heat treatment conditions and particle sizes. According to this process, size-controlled particles composed of various metal oxides can be fabricated by changing the metal salt in the dispersed phase, and the resulting size-controlled particles are expected to be applied not only as cathode active materials for LIBs but also as components of various functional devices.

A graphene oxide (GO)–porous anodic alumina (PAA) bilayer system: How GO dispersion regulates the lower RH detection limit to near zero in conjugation with PAA

In RH-humidity sensors, improving the lower detection limit (LOD) with high sensing responsiveness is an unsolved problem to date.

Continuous spinning of polymer nanofibers with uniform diameters using an anodic porous alumina spinneret with holes of different diameters

Polymer nanofibers prepared via continuous spinning using anodic porous alumina are reported.

Fabrication of nanoparticle assemblies with a controlled number of constituent nanoparticles by membrane emulsification using anodic porous alumina

Nanoparticle assemblies with a controlled number of nanoparticles prepared by membrane emulsification using anodic porous alumina.

Light-Programmed Bistate Colloidal Actuation Based on Photothermal Active Plasmonic Substrate.

Active particles have been regarded as the key models to mimic and understand the complex systems of nature. Although chemical and field-powered active particles have received wide attentions, light-programmed actuation with long-range interaction and high throughput remains elusive. Here, we utilize photothermal active plasmonic substrate made of porous anodic aluminum oxide filled with Au nanoparticles and poly(N-isopropylacrylamide) (PNIPAM) to optically oscillate silica beads with robust reversibility. The thermal gradient generated by the laser beam incurs the phase change of PNIPAM, producing gradient of surface forces and large volume changes within the complex system. The dynamic evolution of phase change and water diffusion in PNIPAM films result in bistate locomotion of silica beads, which can be programmed by modulating the laser beam. This light-programmed bistate colloidal actuation provides promising opportunity to control and mimic the natural complex systems.

Development of nano structures for solar driven desalination

Now-a-days there is shortage of pure water in several areas. So, the emerging technology solar driven desalination is one of the core practices that employ solar energy for fresh water making in sunny areas with lacking quality of water resources. In this work, the nano structure was developed using two step anodization procedure. Anodization was performed on pure aluminum foil (99.99 % purity) in 0.3 M of oxalic acid at constant voltage and effect of temperature on the morphology of formed porous anodic aluminum oxide (AAO) film was investigated. The temperature is varied between −5 to 25 °C. The characterization of the material is done using Scanning Electron Microscopy (SEM), Energy dispersive X-ray (EDAX), Ultraviolet-Differential reflectance spectroscopy (UVDRS). It is observed that, at −5 °C, micro indentations are formed on surface. At 5 °C, pores are randomly formed on the surface. At 15 °C, nano balls are formed on the surface. Whereas, at 25 °C, complete nano porous structure is formed. Hence, the formed nano structures were applied in solar driven desalination. In comparison to all other temperatures, we have observed that, the nano porous structure developed at 25 °C has 27 % of higher evaporation than the conventional.

Flat and roll-type translucent anodic porous alumina molds anodized in oxalic acid for UV nanoimprint lithography

Transparent anodic porous alumina mold for photo-nanoimprinting.

Pore Size Dependence of Optical Absorption Enhancement in Porous Anodic Aluminum Oxide

Three samples of high ordered AAO template were prepared via a two-step anodization procedure, the pore size was modified during the pore-widening process to tune the pore diameter to 50, 70, and 90 nm. Scanning electron microscopy (SEM) was adopted to gauge the pore diameter and the cell unit of the periodical hexagonal structure of the prepared AAO templates. In addition, the UV-vis spectrometer shows the variation of the absorbance spectrum for each pore size of the prepared AAO templates. To prove that the pore diameter (PD) and the intermediate layer (IL) could enhance the absorbance of the materials, a Lumerical FDTD solution was used by involving the exact experimental conditions of the AAO template. The resulting data show that a specific pore diameter with a specific intermediate layer can improve the absorbance spectrum of the materials. Thus, the results could serve the applications related to solar energy conversion (antireflective and photocatalyst) and photonics.

Алюмооксидные нанопористые мембраны как основа функциональных нанокомпозитных материалов

In the present study, we report about results of obtaining nanocomposite materials based on various types of nanoporous alumina membranes (NAM). The NAM were fabricated by two ways: (I) — using the technique of thinning the barrier layer of porous anodic alumina; (2) — by two-layer galvanostatic electrochemical anodization of aluminium samples. Using atomic force microscopy, it was found that a method of thermal decomposition of K2MnO4 leads to the formation of nanoparticles γ-MnO2 with size ~ 20 nm on the surface and in the pores of NAM, and the use of the method of photochemical synthesis of Ag nanoparticles with a size of ~ 30 nm.