Porous Anodic Alumina Research Articles

On the Scaleability of the Morphological Structure of Porous Anodic Aluminum Oxide Films

On the Scaleability of the Morphological Structure of Porous Anodic Aluminum Oxide Films

Anisotropic Plasmonic Property of Al Nanowires Obtained by AC Electrodeposition Using Anodic Porous Alumina Template

Anisotropic Plasmonic Property of Al Nanowires Obtained by AC Electrodeposition Using Anodic Porous Alumina Template

Influence of Induced Local Stress on The Morphology of Porous Anodic Alumina at The Initial Stage of Oxide Growth

A characteristic feature of the disordered pore growth at the initial stage of aluminum anodizing is the development of three large groups of pores: the major pores of larger diameter and two groups of minor pores of smaller diameter. The samples were obtained by the electrochemical oxidation of thin aluminum films (100 nm thick) on SiO2/Si substrates in a 0.3 M oxalic acid at 30 V at 5 °C–40 °C. According to SEM studies, the pore distribution by diameter for the films obtained at 20 and 40 °C has three distinct peaks at ca. 13.5, 17.2, and 20.3 nm. The ratio of the diameter of major pores to the diameter of minor pores of group 1 or group 2 is constant and approximately equal to 1.17 and 1.51, respectively. The generation of local compressive stress influences the development of porous morphology. The distribution of zones with high and low compressive stress levels inside hexagonal cells is shown, and their correlation with the porous morphology is confirmed. The generation of local stress and strains in the anodic alumina layer with a porous, cellular structure is associated with local areas with changes in the geometric properties on its surface.

Target-triggered cascade signal amplification in nanochannels: An ingenious strategy for ultrasensitive photoelectrochemical DNA bioanalysis

Artificial solid-state nanochannels have aroused intense interests in biosensors and bioelectronics because of their special architectures. Herein, we pioneered an ingenious approach of target-triggered cascade signal amplification in porous anodic aluminum oxide (AAO) nanochannels for ultrasensitive photoelectrochemical (PEC) DNA bioanalysis. In the design, AAO nanochannels were modified initially with capture DNA (cDNA) and then incorporated with a photoelectrode, yielding the desired architecture of highly ordered nanoarrays on top of the signal transducer. For target DNA (tDNA) probing, exonuclease III (Exo-III) mediated target recycling (ETR) was first activated to generate plenty of output DNA (oDNA) fragments. After oDNA and the conjugate of Au-labeled probe DNA (Au-pDNA) were anchored within the nanochannels via DNA hybridization, in-situ synthesis of Ag shells on tethered Au nanoparticles was conducted. The resulting large-sized Au@Ag core-shell nanostructure within the nanochannels would cause conspicuous blocking effect to hinder the transportation of electrons accessing the photoelectrode. Since the signal inhibition was directly related to tDNA concentration, an innovative nanochannels PEC DNA assay was exploited and qualified for ultrasensitive detection. The anti-interference ability of this platform was also emphasized by the split AAO membrane for biological incubation without participation of the photoelectrode. This featured nanochannels PEC strategy with cascade amplification launched a novel detecting platform for trace levels of DNA, and it could spark more inspiration for a follow-up exploration of other smart nanochannels PEC bioassays.

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.

Wearable electret based metal oxide nanostructure for self-powered respiration monitoring

Exhaled human breath contains information regarding metabolic and physiological data, and its monitoring can play a crucial role in the early clinical diagnosis and treatment of diseases. An energy-autonomous breath monitoring device was designed by anodizing Al sheet (Nose Bridge), present in existing commercial, medical, and industrial masks for monitoring breath. The electrochemically grown porous anodic alumina (PAA) film acts as the electret layer for generating and trapping charge carriers and has the dual function of energy harvesting and sensing. The device is based on the principle of a triboelectric nanogenerator (TENG), where PAA and Polydimethylsiloxane (PDMS) were used as tribo-electrodes, generating a peak-to-peak voltage of 225 V and a short circuit current of 1 μA with a power density of 110.75 mW/m2. PAA with small pore diameter, depth, and long-range ordering exhibit the best performance. The nanoporous structure of PAA provides an exceptionally high surface area of 14.31 m2/g, significantly enhancing the performance. The device exhibits an exceptionally high sensitivity of −15.84/RH% in the 25–90 RH% detection range, with a hysteresis of 2.2% at 65 RH%. This research paves the way for developing personalized healthcare monitoring systems by integrating anodized nose bridges in medical and industrial masks that provide mechanical support and can be used for energy harvesting and breath monitoring.

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.

Effect of Al Polishing Conditions on the Growth and Morphology of Porous Anodic Alumina Films.

The conditions applied during the electrochemical polishing of aluminum were found to be important parameters for the successive formation of nanoporous alumina films. First, a high-purity Al foil was electrochemically polished in an aqueous solution containing C2H5OH and HClO4 at various sets of conditions, such as applied potential (5-35 V), temperature (0-20 °C), and process duration (10-180 s). Extensive studies of the topography of Al after polishing by scanning electron microscopy and atomic force microscopy allow verification of the correlations between conditions applied during the substrate pretreatment and dimensions of the nanopatterns generated on the metal surface. Next, Al polished samples at two different sets of conditions were used as starting materials for anodization. Unpolished Al samples were also anodized for reference. It was confirmed that electropolishing conditions do not significantly affect the oxide growth rate during anodization and the efficiency of anodic film formation. On the contrary, it was proved that the dimensions of the surface texture formed during Al polishing significantly affect the morphology and pore order within the anodic film. Therefore, it can be stated that it is possible to tune to some extent the arrangement of nanochannels within anodic aluminum oxide films by simply changing conditions during the electropolishing procedure..

Magnetron sputtering process for deposition of multilayered thin diamond-like carbon films with silver nanoparticles for anti-reflective coatings and refractometric sensing

Multilayered film stacks serving optical applications usually require multiple target sources for deposition. In this work, we present a convenient method for the deposition of single- and multilayered thin films with varying diamond-like carbon (DLC) and silver content employing a single magnetron in the deposition system. For this, a reactive magnetron sputtering system was utilized and the silver target was poisoned based on desired structure: clean target and argon gas resulted in a pure silver film, completely poisoned target and acetylene and argon gas resulted in a DLC film, and anything in between resulted in a nanocomposite DLC:Ag film. The films were deposited on fused quartz, crystalline silicon, and porous anodized aluminum oxide substrates. We investigated the optical properties of these films and showed their applications for anti-reflective, black-appearing coatings and refractometric sensors, both experimentally and by simulation methods.

Optical Fiber Temperature and Humidity Dual Parameter Sensing Based on Fiber Bragg Gratings and Porous Film.

A porous anodic alumina film is proposed to construct an optical fiber temperature and humidity sensor. In the sensor structure, a fiber Bragg grating is used to detect the environment temperature, and the porous film is used to detect the environment humidity. The proposed porous anodic alumina film was fabricated by anodic oxidation reaction, and it is suitable for the use of humidity detection due to its porous structure. Experimental results show the temperature sensitivity of the proposed sensor was 10.4 pm/°C and the humidity sensitivity of the proposed sensor was 185 pm/%RH.

Analysis of Effect of Concentration Dependence of Exchange Current on the Metal Electrodeposition into Template Nanopores

Metal electrodeposition into the nanopores of template of porous anodic alumina type under the conditions of mixed kinetics is studied theoretically using analytical and numerical methods. Two main stages of the process are studied: the non-steady-state formation of diffusion layer in the template pores and much longer process of pore filling with metal. The effect of nonlinearity of the concentration dependence of exchange current density of metal electrodeposition on the current density of the diffusion layer formation and pore filling with metal is studied.

Studying the Interaction between Bendamustine and DNA Molecule with SERS Based on AuNPs/ZnCl2/NpAA Solid-State Substrate.

Bendamustine (BENDA) is a bifunctional alkylating agent with alkylating and purinergic antitumor activity, which exerts its anticancer effects by direct binding to DNA, but the detailed mechanism of BENDA-DNA interaction is poorly understood. In this paper, the interaction properties of the anticancer drug BENDA with calf thymus DNA (ctDNA) were systematically investigated based on surface-enhanced Raman spectroscopy (SERS) technique mainly using a novel homemade AuNPs/ZnCl2/NpAA (NpAA: nano porous anodic alumina) solid-state substrate and combined with ultraviolet-visible spectroscopy and molecular docking simulation to reveal the mechanism of their interactions. We experimentally compared and studied the SERS spectra of ctDNA, BENDA, and BENDA-ctDNA complexes with different molar concentrations (1:1, 2:1, 3:1), and summarized their important characteristic peak positions, their peak position differences, and hyperchromic/hypochromic effects. The results showed that the binding modes include covalent binding and hydrogen bonding, and the binding site of BENDA to DNA molecules is mainly the N7 atom of G base. The results of this study help to understand and elucidate the mechanism of BENDA at the single-molecule level, and provide guidance for the further development of effective new drugs with low toxicity and side effects.

Preparation of ordered nanoporous WO3 thin films and the mechanism of large room-temperature ferromagnetism

Undoped porous WO3 nanofilms with large surface area were fabricated on porous anodic alumina substrates by magnetron sputtering. The nanofilms exhibited significant room-temperature ferromagnetism, at least one order of magnitude stronger than other reported undoped WO3 films. By controlling the oxygen partial pressure and the sputtering time during the deposition process, the saturation magnetization of the samples could be tuned over a large range, from 0.1 to 21.7 emu/cm3. Obvious magnetic anisotropy was found in the porous samples, indicating that the magnetic behavior is related to the regular pore structure perpendicular to the surface. Moreover, the first theoretical evidence that the Curie temperature of the sample is higher than room temperature was provided. When annealed in argon, the WO3 nanofilms turned black and showed a combination of a narrowed bandgap and enhanced ferromagnetism. These interesting phenomena are explained based upon the concept of hydrogen-like impurity states associated with oxygen vacancies.

Suitability of Anodic Porous Alumina as a Passive Radiative Cooler: An In-Depth Study.

Passive radiative cooling technology has the potential to revolutionize the way of cooling buildings and devices, while also helping to reduce the carbon footprint and energy consumption. Pioneer works involving anodic aluminum oxide (AAO) nanostructures showed controversial results. In this work, we clarify how the morphological properties and chemical structure of AAO-Al samples affect their optical properties and their cooling performance. Changes in thickness, interpore distance, and porosity of the alumina layer, as well as the used counterions, significantly affect the cooling ability of the AAO-Al structure. We measure a maximum temperature reduction, ΔT, of 8.0 °C under direct sunlight on a summer day in Spain, coinciding with a calculated peak cooling power, P cool, of 175 W/m2, using an AAO-Al sample anodized in sulfuric acid, with 12 μm of AAO thickness and 10% of porosity. These results represent a significant improvement over previous studies, demonstrating the potential of AAO nanostructures to be used in thermal management applications.

Se-doped Nb2O5–Al2O3 composite-ceramic nanoarrays via the anodizing of Al/Nb bilayer in selenic acid

Novel arrays of Nb2O5-based ceramic nanostructures of various sizes (9–210 nm) and morphologies (dots, goblets, rods) aligned on substrates are fabricated via the anodizing of a thin Nb film through the initially formed porous anodic alumina (PAA) film in 1.5 M selenic acid (H2SeO4) – a new aqueous electrolyte generating extraordinarily thinner PAA pores than any other solutions. Accordingly, the nanostructures formed in the selenic acid are 1.3-fold thinner and better self-ordered than their counterparts formed from the same Al/Nb precursor bilayer in a reference oxalic-acid electrolyte. The nanostructures have a dual (core/shell) composition: the inner material (the core) is stoichiometric Nb2O5, whereas the outer layer (the shell) is a few nm-thick substoichiometric NbOx mixed with Al2O3. The composite-ceramic nanoarrays grow doped with selenium species such as selenate (SeO42−) and selenide (Se2−) anions originating from the electrolyte and migrating inward under the high electric field. The incorporated Se species do not contribute to photoluminescence emission nor hinder the Raman signal from the nanoarrays, which makes them highly promising as Nb2O5-based SERS biosensing substrates. The planar PAA-inbuilt Se-doped Nb2O5–Al2O3 nanostructured ceramic film performs like a high-k low-loss low-leakage-current dielectric promising for on-chip integration. More potential applications of the Se-doped ceramic nanoarrays developed here include biomedical antibacterial coatings, advanced superhydrophobic surfaces, gas-sensing, and catalytic layers.

Neuron-Inspired Nanofluidic Biosensors for Highly Sensitive and Selective Imidacloprid Detection.

Pesticides have caused concerns about food safety due to their residual effects in vegetables and fruits. Imidacloprid, as the frequently used neonicotinoid pesticide, could harm cardiovascular and respiratory function and cause reproductive toxicity in humans. Therefore, reliable methods for portable, selective, and rapid detection are desirable to develop. Herein, we report a neuron-inspired nanofluidic biosensor based on a tyrosine-modified artificial nanochannel for sensitively detecting imidacloprid. The functional tyrosine is modified on the outer surface of porous anodic aluminum oxide to rapidly capture imidacloprid through π-π interactions and hydrogen bonds. The integrated nanofluidic biosensor has a wide concentration range from 10-8 to 10-4 g/mL with an ultralow detection limit of 6.28 × 10-9 g/mL, which outperforms the state-of-the-art sensors. This work provides a new perspective on detecting imidacloprid residues as well as other hazardous pesticide residues in environmental and food samples.

Understanding the fast formation mechanism of hard nanoporous alumina films on aluminum in acidic solutions containing nitric acid

Nanoporous alumina films formed on aluminum via anodization are widely used for scientific and technical purposes owing to their various advantageous properties, including excellent corrosion and wear resistance, good thermal and electrical insulation, and facile producibility at low cost. Herein, we report a novel approach to fabricate hard porous anodic alumina (PAA) films by adding an appropriate amount of nitric acid to sulfuric-, oxalic-, and phosphoric-acid solutions at critical high current densities. The effects of the nitric additives on the anodizing behavior, film growth mechanism, microstructure, chemical composition, film thickness, microhardness, and appearance of the PAA films formed from various solutions were systematically investigated. The addition of nitric acid enhanced the formation of alumina and resulted in fast growth rates of 196 μm/h and 166 μm/h for sulfuric- and oxalic-based solutions, respectively, while maintaining high hardness of approximately 380 and 430 HV0.1, respectively. The surface hardness of the films decreased to some extent with the increase in nitric acid concentration and/or film thickness owing to the strong corrosive nature of nitric acid during prolonged anodization, although nitrogen inclusion in anodic alumina films was not detected through X-ray photo-electron spectroscopy, which differs from the other acidic anions. In particular, the barrier layer thickness and pore intervals of PAA films increased with the nitric concentration in the aforementioned solutions, leading to a fast growth rate and high hardness inside the films.

Study of black protective-decorative nanocomposite anodic coatings on the surface of AMg5 aluminum alloy

A widely used nanocomposite coating is a porous anodic alumina colored by particles of metals or their compounds deposited into the pores. The insertion of light-scattering nanosized particles into the pores changes the optical properties of the anodic oxide, whereas the immobilization of metal nanoparticles in pores ensures their corrosion resistance. We present the results of studying black protective and decorative coatings on the surface of AMg5 aluminum alloy. The surface morphology of the samples was analyzed using atomic force microscopy, and the electrophysical properties were monitored by electrochemical impedance spectroscopy (EIS). The growth kinetics of the anodic coating has been studied, and optimal conditions for the formation of a regularly porous oxide coating 10 – 12 μm thick with a regular pore diameter of 15 ± 5 nm on the alloy surface were determined. It is shown that subsequent electrochemical coloring for 15 min makes it possible to obtain a black color of the coatings due to the deposition of Cu and/or CuO nanoparticles into the pores. Simulation of electrical equivalent circuits makes it possible to separate and calculate the electrical parameters corresponding to different layers and elucidate their regular changes after coloring and hydrothermal treatment. The high corrosion resistance of electrochemically colored anodized alloy samples subjected to hydrothermal treatment has been revealed. The results obtained can be used in the application of protective and decorative anodic coatings for the manufacture, for example, of solar panels due to the high absorption and low reflectivity of black coatings.

Magnetic saturation enhancement of gold-capped nickel nanorods

Nickel nanorods (NRs) capped with gold (Au/Ni) were grown into porous anodic aluminum oxide templates and subsequently transferred onto Au/Si (100) substrates. A high dense 2D array of Ni and Au/Ni nanorods was analyzed by vibrating sample magnetometry; it was found that an increase in 14.8% of the magnetic moment following the deposition of Au caps. In order to further investigate this phenomenon, the magnetic distribution of Au/Ni nanorods was studied by off-axis electron holography. The magnetization and induction strengths were evaluated to be 4.7 × 105 A/m and 0.62 T, respectively, which is equivalent to magnetometry measurements of the Ni NR arrays. Remarkably, a vortex state configuration was imaged in the Au segment by the retrieved magnetic phase of the electron holograms under free lens conditions of the transmission electron microscope column. It was concluded that the magnetic distribution in the Au segment is associated with a ferromagnetic coupling with Ni and correlated with the magnetometry measurements.

From Self-Assembly of Colloidal Crystals toward Ordered Porous Layer Interferometry.

Interferometry-based, reflectometric, label-free biosensors have made significant progress in the analysis of molecular interactions after years of development. The design of interference substrates is a key research topic for these biosensors, and many studies have focused on porous films prepared by top-down methods such as porous silicon and anodic aluminum oxide. Lately, more research has been conducted on ordered porous layer interferometry (OPLI), which uses ordered porous colloidal crystal films as interference substrates. These films are made using self-assembly techniques, which is the bottom-up approach. They also offer several advantages for biosensing applications, such as budget cost, adjustable porosity, and high structural consistency. This review will briefly explain the fundamental components of self-assembled materials and thoroughly discuss various self-assembly techniques in depth. We will also summarize the latest studies that used the OPLI technique for label-free biosensing applications and divide them into several aspects for further discussion. Then, we will comprehensively evaluate the strengths and weaknesses of self-assembly techniques and discuss possible future research directions. Finally, we will outlook the upcoming challenges and opportunities for label-free biosensing using the OPLI technique.