Anodic Aluminum Oxide Structure Research Articles

Nano-patterned aluminum surface with oil-impregnation for improved antibacterial performance

Extensive research has been reported to improve the performance of existing antibacterial surfaces by reducing the extent of bacterial adhesion and biofilm formation. This study aimed to validate oil-impregnated slippery nanoporous anodic aluminum oxide (AAO) layers for the prevention of bacterial attachment on aluminum substrate. The oil-impregnation into the nanoporous structures of Teflon-coated hydrophobic AAO enhanced the repellency to the water with a low sliding angle as low as 2°, showing the omniphobicity, which is effective to reduce the contact of bacterial culture medium onto the surface of AAO. Bacterial attachment tests using Escherichia coli K-12 and Salmonella Typhimurium were conducted to evaluate the effect of antibacterial performance on the developed surfaces. Teflon-coated large pore AAO with oil-impregnation showed the highest bacterial reductions for both E. coli K-12 and S. Typhimurium among the treated samples. These results can lead to further optimization of nano-patterned AAO surfaces for antibacterial attachment.

NMR Spectroscopic Analysis of the Local Structure of Porous-Type Amorphous Alumina Prepared by Anodization

Alumina is classified as an intermediate oxide that cannot form a glass and is industrially used as a multifunctional material. Intensive studies on the atomistic structure of amorphous alumina have been conducted because a fundamental understanding of its structure can be meaningful to the design of new materials and devices. Here we focused on anodic alumina as a model material for clarifying the atomistic structure of amorphous alumina and prepared anion-free and anion-incorporated porous-type amorphous alumina by anodization using chromic acid electrolyte and typical electrolytes (e.g., sulfuric acid, oxalic acid, and phosphoric acid), respectively. The local structure around aluminum atoms in the anodic alumina was investigated by nuclear magnetic resonance spectroscopy. We found that the structure of anodic amorphous alumina comprises AlO4, AlO5, and AlO6 units with predominant fractions of AlO5. We also observed that the fraction of each unit was 37.7, 54.3, and 8.0%, with an average coordination n...

Studying Properties of Composite Nano-PAA-Au Array for the Optimal SERS Sensitivity

The composite structure of nano-porous anodic alumina (nano-PAA) coated with noble metal layer has been demonstrated to be one kind of high-sensitive surface-enhanced Raman scattering (SERS)-active substrate. However, the experimental investigations on influences of parameters were restricted by correlation between variables. And in relevant simulation studies, nano-tips at intersections of unit edges on the surface and the metal attaching to inside walls of nano-holes which are crucial for the enhancement of electric field were commonly idealized. To obtain the optimal structure of nano-PAA-Au array for SERS applications, numerical evaluations of the electric field distributions and intensities at hotspots were carried out on a proposed precise model. The influence of structure and excitation parameters which determined the electromagnetic enhancement have been systematically investigated. The numerical data shows that the intensity and distribution of surface electric field is evidently affected by both the characteristics of nano-tips and the metal attaching to the inside wall of holes. In addition, the designed substrate was synthesized accordingly and demonstrated to be higher sensitive with an achieved enhancement factor of 1.04 × 107. The study would be helpful in designing high-sensitive SERS-active substrates.

Structure and Dynamics of Nanopore-Confined Membrane Proteins are Affected by Bilayer Lipid Composition

Self-assembled nanotubular lipid bilayers confined within macroscopically aligned high-density homogeneous nanopore array structure of anodic aluminum oxide (AAO) membranes exhibit a high degree of macroscopic alignment that facilitates their studies by oriented sample solid state (OS SS) NMR and spin-labeling EPR methods. The principal advantages of the lipid nanotube arrays are in the applicability of this method to essentially any lipid composition and a tolerance of the macroscopic alignment to a wide range of environmental conditions such as, e.g., temperature, pH, and ionic strength. Here we describe 1) a recent progress in reconstructing small pore-forming and transmembrane peptides as well as large membrane protein complexes, such as photosynthetic reaction center (RC) protein in inorganic nanopores and 2) utilization of this technology to study lipid-induced changes in protein conformations by magnetic resonance methods. Specifically, by using OS SS NMR we have shown that the tilt and α-helix kink angles of Pf1 coat protein are affected by the bilayer composition on example of DOPC, POPC and DMPC bilayers. Effects of unsaturated lipids on Pf1 dynamics as evidenced by the changes in the linewidths in the Pf1 spectra are also reported. We relate these changes to the bilayer fluidity and its effects on the uniaxial rotational diffusion of the protein within the membrane. Further, we demonstrate the use of DEER spectroscopy to obtain long-range distance constraints for membrane protein systems incorporated into lipid nanotubes. The improved nanopore alignment technique described here provides a general method for studying lipid-induced structural conformations of membrane proteins under physiologically relevant conditions by magnetic resonance. Supported by U.S. DOE contract No. DE-FG02-02ER15354 to A.I.S. and NSF MRI 1229547 to A.A.N.

Wafer‐Scale Highly Ordered Anodic Aluminum Oxide by Soft Nanoimprinting Lithography for Optoelectronics Light Management

The porous anodic aluminum oxide (AAO) membranes have been intensively studied as nanoscale templates for material fabrication. However, the preparation of large‐area highly ordered AAO remains a challenge. In this work, the large‐area soft polydimethylsiloxane mold is employed to transfer the nanopatterns onto Al foil substrates by UV nanoimprinting lithography. It is found that the anodizing behaviors are closely related to the electric field profiles, which can be defined by the morphologies of nanoindentations. The first‐step anodization carried out on the chemical etched patterns leads to some sub‐pores randomly distributed in the main channels. After removal of the as‐formed oxide layer, the stepwise second anodization produces highly ordered inverted nanocone AAO structure. In order to explore the potential application as high‐performance light management nanostructure, electromagnetic simulations are performed on thin‐film perovskite (CH3NH3PbI3) photodetectors using the inverted nanocone AAOs as a substrate. The results indicate that the nanotextured substrate delivers a much higher absorption property than the flat counterpart, demonstrating a promising application for high‐performance optoelectronic devices.

Effect of Confinement on the Synthesis of PMMA in AAO Templates and Modeling of Free Radical Polymerization

The hierarchical surface structure of polymer materials leads to unique properties which are of both academic and commercial interest. In this work we explore the ability to replicate the well-defined surface structure of anodic aluminum oxide (AAO) templates by in situ polymerization of methyl methacrylate. When the reaction is conducted inside the templates, the rate of polymerization was significantly higher and molecular weight was reduced, as compared to the reaction in bulk. These observations were explained using a mathematical model to describe the polymerization based on free-volume theory and confinement effects. As the dimensions of the template are reduced, a catalytic effect on the rate of initiator decomposition leads to a faster polymerization rate in the early stages of polymerization. In addition, due to the confined reaction volume, the rate of termination is increased in the latter stages of the reaction. The two confinement effects combine to lead to lower molecular weight polymers of ...

Electrochemical growth of nanowires in anodic alumina templates: the role of pore branching

A comparative study of electrochemical growth of nanowires in the anodic alumina templates with various degree of porous structure ordering is performed. Scanning electron microscopy and coulometric analysis are used for experimental evaluation of the average filling of pores with metal. The theoretical model of metal growth inside anodic alumina templates is proposed. The model takes into account the presence of branched channels in the real structure of anodic alumina and operates with completeness of template filling achieved at the moment when metal reaches the external surface of the oxide film. In case of the diffusion-controlled regime the strong dependence of the pore filling factor on the thickness of porous film and the degree of its structure ordering is predicted theoretically and observed experimentally. The influence of the nature of limiting current on the homogeneity and completeness of template filling is discussed.

Growth Behavior of Anodic Alumina Nanofibers Fabricated By Pyrophosphoric Acid Anodizing and Their Hydrophilicity

Introduction Anodizing of aluminum is a simple electrochemical technique that has been widely investigated in the fields of surface science and engineering for corrosion protection, electronic devices, and optical materials. Anodic oxide films fabricated via anodizing can be typically classified into the following two groups: barrier type oxide films formed in neutral solutions and porous type oxide films (porous alumina) formed in acidic and alkaline solutions. Here, we have demonstrated the fabrication of novel anodic alumina nanofibers via a simple anodizing technique in a new electrolyte, pyrophosphoric acid (H4P2O7). Experimental High-purity aluminum specimens (99.99 wt%, 110–400 µm thick) were ultrasonically degreased in ethanol for 10 min. After drying, the specimens were electropolished in a 13.6 M CH3COOH/2.56 M HClO4 mixture (T = 280 K) at a constant voltage of 28 V for 1 min. The electropolished specimens were immersed in a concentrated pyrophosphoric acid solution (74.0-78.0 wt%, T = 293 K), and then were anodized at a constant voltage of 10 to 95 V using a PC-connected direct-current power supply. A platinum plate was used as the cathode for pyrophosphoric acid anodizing. The surface nanomorphology of the anodized specimens was examined by field emission scanning electron microscopy (FE-SEM) and scanning transmission electron microscopy (STEM). The water contact angles on the aluminum surface anodized via each anodizing condition were measured by an optical contact angle meter. For the wettability measurements, the volume of distilled water placed on the surface of the specimens was adjusted to a relatively large amount of 4 µL to evaluate the superhydrophilic behavior in the initial stage of the measurements. Results and discussion Figure 1 shows the SEM images of the aluminum specimen anodized in pyrophosphoric acid at 50 V and 293 K for 12 min. The thin barrier anodic oxide, which was several tens of nanometers in thickness, was observed on the aluminum substrate by SEM observation of the fracture cross-section (Fig. 1a). This thin barrier oxide was immediately formed on the aluminum substrate by applying a constant voltage for 30 s. Due to additional anodizing for 4 min, a honeycomb oxide structure with narrow hexagonal walls was formed on the aluminum surface (Fig. 1b). Although the nanomorphology of the oxide is very similar to the structure of anodic porous alumina, the nanopores in the honeycomb layer are considerably larger than those obtained by typical anodizing in other acidic solutions. Therefore, vigorous chemical dissolution of the anodic oxide occurs during anodizing. When the anodizing time increased to 12 min, fibrous nanostructures were formed on the honeycomb layer, and several nanofibers became tangled and bundled together due to bending under their own weight (Fig. 1c). The bottom honeycomb alumina contains phosphorus that is due to the incorporation of anions originating from the electrolyte, which is similar to typical porous alumina. However, phosphorus is not found in the alumina nanofibers. The anodic oxides without anions remain during long-term pyrophosphoric acid anodizing because of their slow dissolution rate, and numerous anodic alumina nanofibers cover the aluminum surface during anodizing. The water contact angle on the nanofiber-covered aluminum surface decreased with time after a 4-µL droplet was placed on the surface, and a superhydrophilic behavior with a contact angle measuring 2.2° was observed within 2 s; this contact angle is considerably lower than those observed for electropolished and porous alumina-covered aluminum surfaces. Figure 1

Micro and nanoindentation analysis of porous anodic alumina prepared in oxalic and sulphuric acid

In this article, the mechanical behavior of porous anodic alumina (PAA) structures obtained from two different electrolytes (oxalic and sulphuric acids) was investigated using micro and nanoindentation techniques. Regularity ratio (RR) of PAA structures was calculated using WSxM software and it was found that strength of the PAA structures varies with the RR of the pores. Micro hardness of the PAA structures was studied using 0.98, 9.8N loads and it was observed that surface ring cracks were generated for 9.8N load in oxalic acid. PAA structures formed in sulphuric acid exhibits an extremely high hardness of 7.5GPa and Young's modulus as 146.5GPa compared with oxalic acid due to low porosity. Results indicate that the indentation modulus and hardness of the PAA structures decrease with increasing pore size. Further the effect of porosity and regularity ratio of pores on mechanical properties of PAA structures was studied in detail.

온도와 전압 및 바닥면 형상에 따른 양극산화 알루미늄의 구조

Since the development of anodic aluminum oxide (AAO), extensive studies have been conducted ranging from fundamental research to the applications of AAO. Most of the research on AAO structures have focused on well-aligned nanoporous structures fabricated under specific conditions. This study investigated fabricable AAO structures with anodization performed with various temperatures, electrical potentials, and basal plane surfaces. As a result, nanoporous and nanofibrous structures were fabricated. The nanopores were formed at a relatively lower temperature and potential, and the nanofibers were formed at a relatively higher temperature and potential regardless of the basal plane surface. The shape of the base surface was found to influence the structural arrangement in nanoporous morphologies. These interesting findings relating to new morphologies have the potential to broaden the possible applications of AAO materials.

The influence of the porous structure of anodic alumina on the formation and properties of germanium nanostructures

The influence of the porous structure of anodic alumina matrices on the structural and morphological properties of germanium nanostructures, obtained by high vacuum thermal deposition on anodic alumina, is reported. Scanning electron microscopy micrographs reveal the formation of Ge nanostructures in the form of rods and tubes with diameters from 16–130 nm and arrangements specified by the geometry characteristics of the porous matrix structure. X-ray diffraction and Raman scattering studies show the formation of nanostructures with structural states specified by the temperature of the substrate: the amorphous state at 200 °C and 300 °C, the mixed amorphous–crystalline state at 400 °C and the crystalline state at 500 °C. Germanium nanostructures with ordered distributions in the dielectric matrices are desired for application in optoelectronic devices.

Controllable Fabrication and Microstructure Modulation of Unique AAO Structures Based on Patterned Aluminum Surface

In this paper, three types of unique anodic aluminum oxide (AAO) structure were fabricated based on patterned aluminum surface: (1) AAO with three-dimensional (3D) interconnected pores; (2) controllable self-assembled AAO nanowires; (3) supporting AAO skeleton structures. Patterned aluminum surface was obtained by using high voltage anodization processes (400–600 V) in ethanol-oxalic acid electrolyte and a following AAO removing method. The voltage/current-time curves during anodization were analyzed in detail. Moreover, the AAO microstructure, film thickness, and film growth rate have been studied based on smooth and also on patterned aluminum surface. In addition, the influences of anodization and etching conditions on AAO microstructure evolutions, e.g., 3D interconnected pores, controllable self-assembled nanowires and supporting skeleton structures have also been investigated. The present fabrication methods for obtaining unique AAO structures are facile, effective, and inexpensive, facilitating practical applications of AAO in diverse related fields.

Effects of Complex Structured Anodic Oxide Dielectric Layer Grown in Pore Matrix for Aluminum Capacitor.

Anodization of aluminum is generally divided up into two types of anodic aluminum oxide structures depending on electrolyte type. In this study, an anodization process was carried out in two steps to obtain high dielectric strength and break down voltage. In the first step, evaporated high purity Al on Si wafer was anodized in oxalic acidic aqueous solution at various times at a constant temperature of 5 degrees C. In the second step, citric acidic aqueous solution was used to obtain a thickly grown sub-barrier layer. During the second anodization process, the anodizing potential of various ranges was applied at room temperature. An increased thickness of the sub-barrier layer in the porous matrix was obtained according to the increment of the applied anodizing potential. The microstructures and the growth of the sub-barrier layer were then observed with an increasing anodizing potential of 40 to 300 V by using a scanning electron microscope (SEM). An impedance analyzer was used to observe the change of electrical properties, including the capacitance, dissipation factor, impedance, and equivalent series resistance (ESR) depending on the thickness increase of the sub-barrier layer. In addition, the breakdown voltage was measured. The results revealed that dielectric strength was improved with the increase of sub-barrier layer thickness.

(Invited) A Rational Design for Batteries at Nanoscale by Atomic Layer Deposition

A self-aligned nanostructured battery fully confined within a single nanopore presents a powerful platform to determine the rate performance and cyclability limits of nanostructured storage devices. Atomic layer deposition (ALD) has enabled us to create and evaluate such structures, comprised of nanotubular electrodes and electrolyte confined within anodic aluminum oxide (AAO) nanopores as “all-in-one” nanopore batteries.This nanopore battery displays exceptional power-energy performance and cyclability when tested as massively parallel devices (~2billion/cm2), each with ~1um3 volume (~1fL). [1]The unprecedented thickness and conformality control of ALD and the highly self-aligned nanoporous structure of anodic aluminum oxide (AAO) are essential to enable fabrication of precision, self-aligned, regular nanopore batteries. To realize such structure, we began with fine tuning of ALD process conformality inside high aspect ratio (300:1) AAO nanopores. Ru/Pt metal is optimized to be 15µm deep at both sides of 50µm long AAO pores in order to provide fast electron transport to overlying V2O5 at both anode and cathode sides, while keeping them spatially and electrically isolated. Crystalline V2O5 was deposited as active storage material inside the metal nanotubes using O3 as the oxidant. Then the V2O5 was electrochemically prelithiated at one end to serve as anode while pristine V2O5 without Li at the other end served as cathode, enabling the battery to be cycled between 0.2V and 1.8V. Capacity retention of this full cell is 95% at 5C rate and 46% at 150C, with more than 1000 charge/discharge cycles. Further increase of full cell output potential is also demonstrated for SnO2 and TiO2 anodes in asymmetric full cells with V2O5cathodes. These results demonstrate the promise of ultrasmall, self-aligned/regular, densely packed nanobattery structures as a test bed to study ionics and electrodics at nanoscale with a variety of geometrical modifications and as a building block for high performance energy storage systems. [1] Liu, C.; Gillette, E. I.; Chen, X.; Pearse, A. J.; Kozen, A. C.; Schroeder, M. A.; Gregorczyk, K. E.; Lee, S. B.; Rubloff, G. W., An all-in-one nanopore battery array. Nature Nanotechnology 2014, 9 (12), 1031-1039.

Porous and mesh alumina formed by anodization of high purity aluminum films at low anodizing voltage

Electrochemical oxidation of high-purity aluminum (Al) films under low anodizing voltages (1–10) V has been conducted to obtain anodic aluminum oxide (AAO) with ultra-small pore size and inter-pore distance. Different structures of AAO have been obtained e.g. nanoporous and mesh structures. Highly regular pore arrays with small pore size and inter-pore distance have been formed in oxalic or sulfuric acids at different temperatures (22–50°C). It is found that the pore diameter, inter-pore distance and the barrier layer thickness are independent of the anodizing parameters, which is very different from the rules of general AAO fabrication. The brand formation mechanism has been revealed by the scanning electron microscope study. Regular nanopores are formed under 10V at the beginning of the anodization and then serve as a template layer dominating the formation of ultra-small nanopores. Anodization that is performed at voltages less than 5V leads to mesh structured alumina. In addition, we have introduced a simple one-pot synthesis method to develop thin walls of oxide containing lithium (Li) ions that could be used for battery application based on anodization of Al films in a supersaturated mixture of lithium phosphate and phosphoric acid as matrix for Li-composite electrolyte.

Nanoporous anodic alumina platforms: engineered surface chemistry and structure for optical sensing applications.

Electrochemical anodization of pure aluminum enables the growth of highly ordered nanoporous anodic alumina (NAA) structures. This has made NAA one of the most popular nanomaterials with applications including molecular separation, catalysis, photonics, optoelectronics, sensing, drug delivery, and template synthesis. Over the past decades, the ability to engineer the structure and surface chemistry of NAA and its optical properties has led to the establishment of distinctive photonic structures that can be explored for developing low-cost, portable, rapid-response and highly sensitive sensing devices in combination with surface plasmon resonance (SPR) and reflective interference spectroscopy (RIfS) techniques. This review article highlights the recent advances on fabrication, surface modification and structural engineering of NAA and its application and performance as a platform for SPR- and RIfS-based sensing and biosensing devices.

1-D nanoporous anodic alumina rugate filters by means of small current variations for real-time sensing applications.

A rugate filter based on nanoporous anodic alumina was fabricated using an innovative sinusoidal current profile with small current variation. The resulting structure consisted of highly parallel pores with modulations of the pore diameter along the pore axis and with no branching. The effect of the period time and the pore widening post-treatment was studied. From reflectance measurements, it was seen that the position of the reflection band can be tuned by adjusting the period time and the width by pore-widening post-treatments. We tested one of the rugate filters by infiltrating the structure with EtOH and water in order to evaluate its sensing capabilities. This method allows the fabrication of complex in-depth modulated nanoporous anodic alumina structures that open up the possibility of new kinds of alumina-based optical sensing devices.

Electrochemical Impedance Characterization of Porous Aluminum Oxide after Different Stages of Two-Step Anodization

In this paper, porous anodic aluminum oxide (AAO) membrane was prepared with highly pure aluminum film in 0.3M oxalic acid under dc voltage of 40V at about 8°C by two-step anodization. The morphology of the specimens processed in different stages of two-step anodization was observed with a scanning electron microscope (SEM). The electrochemical behavior of the anodized aluminum after different stages of two-step anodization was studied by electrochemical impedance spectroscopy (EIS). Different equivalent circuits were established to fit EIS experiment data, based on morphologies and structures of AAO during processing. The electrical elements in the equivalent circuits were used to explain experimental parameters.

Growth of porous anodized alumina on the sputtered aluminum films with 2D–3D morphology for high specific surface area

Abstract The porous anodic aluminum oxide (AAO) with high-aspect-ratio pore channels is widely used as a template for fabricating nanowires or other one-dimensional (1D) nanostructures. The high specific surface area of AAO can also be applied to the super capacitor and the supporting substrate for catalysis. The rough surface could be helpful to enhance specific surface area but it generally results in electrical field concentration even to ruin AAO. In this article, the aluminum (Al) films with the varied 2D–3D morphology on Si substrates were prepared using magnetron sputtering at a power of 50 W–185 W for 1 h at a working pressure of 2.5 × 10−1 Pa. Then, AAO was fabricated from the different Al films by means of one-step hybrid pulse anodizing (HPA) between the positive 40 V and the negative −2 V (1 s:1 s) for 3 min in 0.3 M oxalic acid at a room temperature. The microstructure and morphology of Al films were characterized by X-ray diffraction, scanning electron microscope and atomic force microscope, respectively. Some hillocks formed at the high target power could be attributed to the grain texture growth in the normal orientation of Al(1 1 1). The 3D porous AAO structure which is different from the conventional 2D planar one has been successfully demonstrated using HPA on the film with greatly rough hillock-surface formed at the highest power of 185 W. It offers a potential application of the new 3D AAO to high specific surface area devices.

Impedance Spectroscopy of Highly Ordered Nano-Porous Electrodes Based on Au–AAO (Anodic Aluminum Oxide) Structure

Electrochemical measurements using the microelectrodes are increasingly utilized for the label-free detection of the small amount of biological materials such as DNA, protein, and cells. However, the interfacial electrode impedance increases and may hinder the detection of weak signals as the size of electrode decreases. To enhance the measurement sensitivity while reducing the electrode size, in this study, microelectrodes employing a nanoporous structure were fabricated and characterized by using electrical impedance spectroscopy. We made the highly ordered honeycomb nanoporous structure of Anodic Aluminum Oxide (AAO) by electrochemical anodizing and formed Au layer on the surface of AAO (Au/AAO) by electroless Au plating method. The electrical characteristics of the fabricated Au/AAO electrodes were evaluated by using de Levie's model derived for the pore electrodes. As a result, the interfacial electrode impedance of the fabricated Au/AAO electrodes was 2-3 order lower than the value of the planar electrodes at frequencies below 1 kHz. It implies this nanoporous electrode could be directly applied to label free detection of biomaterials.