Porous Anodic Alumina Films Research Articles

Variable Thickness Porous Anodic Alumina/Metal Film Bilayers for Optimization of Plasmonic Scattering by Nanoholes on Mirror.

Continuously variable thickness porous anodic aluminum oxide (PAAO) films were obtained using electrochemical oxidation of bulk aluminum sheet while both electrodes were simultaneously withdrawn from the electrolyte solution. The thickness gradient was controlled by the withdrawal rate (1–10 mm/min range) and thickness variation demonstrated from below 50 nm to above 1 micrometer. The thickness increased linearly with the sample lateral coordinate, whereas the nanopore structure (diameter and interpore distance) remained unchanged. Effects of the initial pore growth and capillary forces are discussed. The presented method can be used for tuning optimal PAAO thickness for optical and other applications as exemplified by finding maximum plasmonic scattering in structured Al–PAAO–Au multilayers. Enhanced scattering from porous gold film separated by a specific-thickness PAAO layer from aluminum mirror surface is demonstrated.

Preparation and characterization of anodic alumina films with high-saturation structural colors in a mixed organic electrolyte

Porous anodic alumina films with high-saturation and wide-color-gamut structural colors were successfully prepared in a mixed organic acidic electrolyte added with propylene glycol using a high voltage. The experiment result shows that the color of the anodic alumina films changes slowly, and thus the fine regulation of the color could be achieved just by shortening time intervals. Scanning electron microscope (SEM) measurement shows that the sample from the organic experimental group has a granular surface. This granular surface has a greater scattering of aluminum substrate reflected light, and this produces a higher-saturation structural color. It is also observed that underlying pores are not obvious on the surface of the organic experimental group sample. The mechanism behind this observation is in a good agreement with Acidic Field-assisted Dissolution Mechanism. Additionally, the samples from the organic experimental group appear a strong reflection in their red-spectrum area, which agrees with the samples' overall optical appearance of being reddish. The film with brilliant color may have potentials in color display, decoration and anti-counterfeiting technology.

Hafnium-Oxide 3-D Nanofilms via the Anodizing of Al/Hf Metal Layers

Hafnium-oxide films with self-organized nanostructured 3-D architectures and variable dimension (10 to 400 nm) are synthesized via the high-current anodizing of thin aluminum-on-hafnium layers in phosphoric, malonic, and oxalic acid electrolytes. In the approach, the self-organized growth of a porous anodic alumina (PAA) film is immediately followed by the fast PAA-assisted reanodizing of the hafnium underlayer. The PAA-dissolved films consist of arrays of upright-standing hafnium-oxide nanorods held on the substrate by the tiny needle-like “nanoroots” widespread over a continuous hafnium-oxide bottom layer. The roots are amorphous Hf2O3, while the rods are amorphous HfO2–Hf2O3–Al2O3 mixed oxides, the bottom layer being, however, highly textured nanocrystalline HfO2. The calculated transport numbers for O2– and Hf4+(3)+ ions are, respectively, ∼0.55 and ∼0.45, which is a unique situation for anodic hafnium oxide, which normally grows by O2– transport only. Annealing the films in air at 600 °C oxidizes the remaining Hf metal to polycrystalline HfO2, still leaving the roots and rods amorphous. The annealing in vacuum results in partial oxide reduction and crystallization of the roots and rods to stable orthorhombic and monoclinic HfO2 phases. A model of the anodic film growth and solid-state ionic transport is proposed and experimentally justified. Potential applications of the 3-D hafnium-oxide nanofilms are in advanced electronic, photonic, or magnetic micro- and nanodevices.

Enhancing tribological performance by anodizing micro-textured surfaces with nano-MoS2 coatings prepared on aluminum-silicon alloys

Abstract Aluminum-silicon alloys exhibit poor tribological performance and suffer from severe adhesive wear. The single surface treatment has demonstrated unsatisfactory anti-friction performances. In this paper, the synergistic effect of the surface micro-texturing and nano-solid lubricant improved the tribological performance of aluminum-silicon alloys effectively. The anodizing micro-textured surface with nano-MoS2 coatings was fabricated by micro-milling, anodizing and ultrasonic impregnation. Pin-on-disc tribological tests revealed that the friction coefficient of the anodizing micro-textured surface with nano-MoS2 coatings decreased by 22.6% compared with that of the untextured specimen. The anti-friction characteristic was attributed to the synergistic effect of the entrapping wear debris of dimple structure, the high hardness of porous anodic aluminum oxide films and the self-lubricating property of nano-MoS2 coatings.

Stress induced by incorporation of sulfate ions into aluminum oxide films

The geometry of self-ordered porous anodic aluminum oxide films depends strongly on the type of acid anion in the anodizing solution. Recent studies suggest that electric field-induced anion incorporation in the oxide may generate compressive surface stress that drives oxide flow responsible for formation of self-ordered arrays of pores. This communication reports direct evidence for sulfate ion incorporation-induced compressive stress in native aluminum oxide films from stress measurements during open-circuit Al dissolution in 0.4 M H2SO4, coupled with analysis of the film composition by X-ray photoelectron spectroscopy. The measured force per width (integrated in-plane stress) was compared to the calculated elastic force assuming that all detected sulfate ions are incorporated in the oxide, rather than adsorbed on the oxide surface. Quantitative agreement was demonstrated between the calculated and experimental force, showing that sulfate incorporation dominates over adsorption and induces greater than 1 GPa levels of compressive stress in the oxide. The present work and an earlier study of phosphate incorporation show that anions incorporate at small open-circuit exposure times, when the outer portion of the oxide can support a high electric field because it is not yet completely hydrated. Therefore the electric field in the oxide is the likely driving force for incorporation.

Rapid Fabrication of Porous Anodic Alumina Films with Large Interpore Distances in an Ethylene Glycol and Ammonium Dihydrogen Phosphate-Based Electrolyte

Porous anodic alumina (PAA) films have become popular nanotemplates due to the ordered hexagonal pore array structure. The fabrication of well-ordered PAA films with larger interpore distances is still a challenge to date. Here, we develop a unique electrolyte system based on the reaction products of ethylene glycol (EG) with ammonium dihydrogen phosphate (ADP). In the novel electrolytes, stable high-voltage anodization with extremely high current densities (the peak value is close to 300 mA cm−2) can be realized at 400 V or even higher without breakdown. In 5 wt% EG-ADP-based electrolyte containing phosphoric acid, a well-ordered PAA film with an interpore distance of ∼610 nm can be fabricated at 250 V at a high growth rate of 1.3 μm·min−1.

Fabrication of Crystallized Porous Anodic Aluminum Oxide under Ultra-High Anodization Voltage

Porous anodic aluminum oxide (PAAO) films have attracted increasing attention due to their widespread applications as porous templates. In fact, PAAO films fabricated using high-voltage anodization have apparent advantages which include large-range tunable interpore distance, less impurities, and diverse microstructures. Finding the proper methods to explore the maximum anodization voltage and to analyze related mechanisms is important. Here, crystallized PAAO films with novel microstructures have been fabricated under ultra-high anodization voltage of 700–1000 V in ethanol-oxalic electrolyte system. The unique microstructure and composition of the prepared PAAO films have been studied by using characterization methods of scanniong electron microscopy and X-ray diffraction. Some important phenomena become more obvious under extreme experimental conditions, thus a better understanding of related intrinsic mechanisms of aluminum anodization can be achieved.

Self-organization of porous anodic alumina films studied in situ by grazing-incidence transmission small-angle X-ray scattering.

Self-ordered porous anodic alumina (PAA) films are studied extensively due to a large number of possible applications in nanotechnology and low cost of production. Whereas empirical relationships between growth conditions and produced oxides have been established, fundamental aspects regarding pore formation and self-organization are still under debate. We present in situ structural studies of PAA films using grazing-incidence transmission small-angle X-ray scattering. We have considered the two most used recipes where the pores self-organize: 0.3 M H2SO4 at 25 V and 0.3 M C2H2O4 at 40 V. During anodization we have followed the evolution of the structural parameters: average interpore distance, length of ordered pores domains, and thickness of the porous oxide layer. Compared to the extensively used ex situ investigations, our approach gives an unprecedented temporal accuracy in determination of the parameters. By using of Al(100), Al(110) and Al(111) surfaces, the influence of surface orientation on the structural evolution was studied, and no significant differences in the interpore distance and domain length could be observed. However, the rate of oxide growth in 0.3 M C2H2O4 at 40 V was significantly influenced by the surface orientation, where the slowest growth occurs for Al(111). In 0.3 M H2SO4 at 25 V, the growth rates were higher, but the influence of surface orientation was not obvious. The structural evolution was also studied on pre-patterned aluminum surfaces. These studies show that although the initial structures of the oxides are governed by pre-patterning geometry, the final structures are dictated by the anodization conditions.

Model of Stress Generation in Anodic Aluminum Oxide Films: Part II. Surface Stress Accumulation Preceding Formation of Self-Organized Pore Arrays

Growth of self-organized porous anodic alumina films initiates after growth of the initial compact barrier oxide becomes unstable. The process of self-ordering is characterized by evolution of an irregular distribution of incipient pores into the characteristic steady-state pore array. We explore the roles of mechanical stress and oxide flow in self-ordering, through a modeling analysis of stress-driven viscous flow of oxide. To account for oxide topography evolution, the model domain is chosen to be a spherical shell of oxide representative of the film during the transition to the steady-state porous layer. The model is based on coupled viscous flow of oxide and stress- and electric field-driven migration of Al+3 and O−2 ions. The calculation results successfully depict the buildup of compressive stress within a layer of a few nanometers thickness at the oxide surface, as inferred previously from in situ stress measurements. Additionally, the model predictions are consistent with observations of tensile bulk stress in the oxide accompanying inward oxide flow during the transition to the steady-state pore array. This transition is due to competitive growth of the initially nonuniform distribution of incipient pores until a monodisperse steady-state distribution is attained.

Advances of the research evolution on aluminum electrochemical anodic oxidation technology

This article gives an overview on the development of aluminum anodization technique in terms of fundamental aspects and practical applications in the past decades. Besides, the formation mechanism and structural characteristics of anodic alumina films as well as the factors affected the formation of porous anodic alumina films are also discussed. Anodic aluminum oxide (AAO) prepared by the anodization method can be divided into two categories: dense anodic alumina (DAA) and porous anodic alumina (PAA). This article also summarizes the optical properties, magnetic properties, solar absorption properties, and catalytic properties of porous anodic alumina film and its applications in nanomaterials, optical materials, magnetic materials, biosensors, solar cells, and so on. In addition, future developmental trend of porous anodic alumina film is covered.

The effect of propylene glycol on the optical properties of iridescent porous anodic alumina films

Porous anodic alumina films with structural colors were successfully prepared in the mixed electrolyte which was the phosphoric acid of 0.6 M/L added with propylene glycol at 100 V and − 5 °C. In the experiment, by changing the electrolyte composition, the anodization rate of the film was controlled, and thus a precise adjustment to the structural colors of the films was achieved and, as a result, more intermediate colors were obtained. This study shows that the pore growth rate of the films during anodization can be greatly decreased by adding propylene glycol, and the reasons would be given in this paper. It was also discovered that the Au-coated thin layer helps to enhance the saturation of the film colors. These films show potential in many areas including decoration, display, anti-counterfeiting and so on.

Formation of Porous Anodic Alumina under Unstable Electroconvection Flow Regimes: A Case Study of Tartronic Acid Electrolyte

This work focuses on the systematic study of porous anodic alumina films grown in novel moderately concentrated tartronic (2-hydroxymalonic) acid electrolyte. The morphological analysis enabled us to reveal new details of the alumina pore formation mechanism and to discover the processes responsible for such frequently occurring defects as branched pores and multiple pore openings within a single cell. The qualitatively new model developed herein explains satellite nanochannel formation from a colloidal electroconvection standpoint—namely, by the temporary change of the existing nanoconvective flows, which give rise to the cooperative cell pattern in porous alumina, from laminar to turbulent mode (such flow regimes are generally termed “transitional” or in some recent literature also “laminar unstable”). Our theoretical discussion demonstrates that the morphological parameters of self-organized layers are strongly related to such properties of the liquid phase as the dielectric constant and the Debye leng...

(Invited) Role of Stress Induced By Anion Incorporation in Formation of Porous Anodic Alumina Films

Procedures to grow porous anodic alumina films with ordered and controllable arrangements of nanoscale pores have led to extensive research on PAA films as functional materials. Experiments and calculations show that transport in the oxide is assisted by plastic flow driven by mechanical stress (1-3). Pores initiate as the result of a morphological instability at the oxide-solution interface of the initially formed barrier oxide (4). Stress distribution measurements revealed high near-surface compressive stress levels, which may be associated with locally elevated concentrations of incorporated electrolyte anions. The instability leading to pore formation may be influenced by flow driven by anion incorporation-induced stress (5). Direct evidence for anion-induced stress in aluminum oxide was sought through stress measurements during open-circuit immersion of Al in phosphoric and sulfuric acids. The stress is compared with elastic anion incorporation-induced stress calculated from X-ray photoelectron spectroscopy (XPS) measurements of film composition. Al sheet samples of 99.998% purity were exposed at open circuit in either 0.4 M H3PO4 or 0.4 M H2SO4 solutions. In situ stress measurements during acid dissolution were carried out by phase-shifting curvature interferometry (6). According to the thin-film Stoney approximation, the detected sample curvature changes are directly related to the surface force change per sample width. The force per width is the biaxial in-plane stress integrated through the sample thickness. XPS was performed with a K-alpha instrument (Thermo Scientific) (7). The relative concentrations of P, S, Al, and O in the oxide were determined from areas of P2s, S2s, Al2p, O1s peaks. The oxide thickness and the anion concentration per area were determined from the relative areas of the Al2p peak contributions from oxide and metal. Fig. 1 shows that upon immersion of Al in both acids, force per width increased abruptly in the compressive direction to 2 min, and then relaxed slowly to constant levels. The initial compressive increases are not due to mechanical disturbances, as they significantly exceed the stress change in distilled water. The hypothetical anion incorporation-induced stress was calculated from the anion concentration per area by assuming that all the detected anions are absorbed within the oxide. The elastic force per width accompanying incorporation is then FwA = -(2/9) W A EoxNA /(1 - n ox ) where WA is the anion molar volume, Eox and n ox the elastic modulus and Poisson's ratio of alumina, and NA the per-area anion concentration from XPS. Fig. 1 shows that for dissolution in H2SO4, FwA is in excellent agreement with the measured force at all times. For H3PO4, agreement during the initial compressive increase is found but thereafter phosphate adsorption causes the calculation to diverge. Fig. 1 is strong evidence that anion incorporation in alumina films induces compressive stress. The stress of ~ 2 GPa inferred from the figure are comparable to near-surface stress levels measured earlier in anodic films (5). Additionally, we discuss evidence for anion-incorporated surface layers in anodic alumina from high-voltage cyclic voltammetry (CV) experiments (8). At high scan rates, the current-voltage dependence in the cathodic-direction scan reveals the conduction characteristics of the anodic film. By varying the anodic potential limit of CV, the dependences of conduction on both film thickness and electric field are assessed. High-resistivity surface layers are revealed in anodic films formed in phosphoric, sulfuric and oxalic acids. An instability mechanism for anodic alumina is described in which both the high resistivity and elevated stress level of the anion-containing near-surface layer play important roles. ACKNOWLEDGMENT Financial support was provided by JFE Steel Corporation and China Scholarship Council. REFERENCES 1. S. J. Garcia-Vergara, et al., Electrochim. Acta. 52, 681 (2006). 2. J. E. Houser, K. R. Hebert, Nature Mater. 8, 415 (2009). 3. J. Oh, C. V. Thompson, Electrochim. Acta, 56, 4044 (2011). 4. P. Mishra, K. R. Hebert, Electrochim. Acta, 222, 1186-1190 (2016). 5. Ö. Ö. Çapraz, et al., Electrochim. Acta 167, 404-411 (2015). 6. Ö. Ö. Çapraz, K. R. Hebert, P. Shrotriya, J. Electrochem. Soc., 160, D501-516 (2013). 7. Ö. Ö. Çapraz et al., Electrochim. Acta, 238, 368-374 (2017). 8. S. Ide, P. Mishra and K. R. Hebert, Electrochim. Acta, 221, 1-7 (2016). Figure 1

Formation Mechanism of Alumina Nanotubes By Anodization of Aluminum Alloy

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is used as a starting material of nanofabrication of functional devices besides a corrosion resistant surface oxide. In contrast to anodic titania nanotubes, anodic porous alumina is generally compact without spacing between the cell boundary. The tubular shape of anodic titania is resulted in enrichment of fluoride between cells which is originated form electrolyte and more soluble than anodic titania. In the case of aluminum, it is reported that enrichment of only sulfate or ethylene glycol originated form electrolytes produces alumina nanotubes by anodization. In the present paper, the formation mechanism of anodic alumina nanotubes will be discussed. STEM image of vertical cross section prepared by ion-milling of the anodic film, which was formed on 99.99 % pure aluminum in oxalic acid at 160V, indicated close-packed hexagonal cell arrays with small voids of 15-20 nm at the triple cell points. Dark field STEM image clearly revealed a bright particle in the each void suggesting the presence of a heavy metal because of Z-contrast. By EDX point analysis, Cu enriched particles were found in the voids at the triple points. Nevertheless, in the case of anodic film formed at the similar condition on AC8A cast alloy including 0.97 % of Cu, tubular cells with the space of approximately 50 nm between cell boundary were formed unlike the case of pure aluminum including 60 ppm of Cu. Numerous branching of pores were also observed. EDX mapping revealed that Si was enriched in the outer layer of the cells and Cu was enriched at the area near cell boundaries. Therefore, it is deduced that enrichment of Cu at the cell boundaries produces the tubular cells by accelerating oxide dissolution similar to the cases of enrichment of sulfate and ethylene glycol.

Effect of Electric Field Strength on Barrier Layer Thickness and Anion Incorporation of Anodic Porous Alumina

Anodic porous alumina films having a closely packed hexagonal array of cells have attracted much attention as a starting material in the fabrication of nano-devices because of the high controllability of porous structures (e.g., cell size, pore size, pore interval, and pore depth) on dimensions ranging from the submicron to nanometer range. In recent years, the mechanism of self-ordering of cell arrangement of anodic porous alumina has been intensely studied. We proposed anodizing under high electric field strength as a key factor for self-ordering of cell arrangement [1, 2]. However, only a few reports are found on the irregularities of barrier layer thickness and morphology of each cell of anodic porous alumina. In this study, we evaluated the influence of electrolytic factors (e.g., current density, applied voltage, and anodizing time) on cell morphology of anodic porous alumina. Especially, we focused on the effect of the electric field strength on the barrier layer thickness and anion incorporation behavior. High purity aluminum sheets were anodized in typical electrolytes of sulfuric acid, oxalic acid, and phosphoric acid at different voltage. After detachment of the porous alumina films from the substrate in saturated mercuric chloride solution, the specimens were immersed in phosphoric acid to dissolve the barrier layer from the backside. By SEM observation of structural change of the cells during chemical etching, the homogeneity of the barrier layer thickness could be evaluated. Although the thickness of the barrier layer of porous alumina formed at constant voltage has been believed to be uniform, we confirmed that the thickness of the barrier layer of each cell was not uniform; the thickness of the barrier layer of the smaller cells was thinner than that of the larger cells. Concerning anion incorporation behavior, we evaluated the depth of anion incorporation into the cell wall by the difference in the strength of contrast of the SEM image of rear surface of porous alumina films because the dissolution rate of the anion-free layer was slower than that of the anion-incorporated layer. The details of the thickness inhomogeneity of the barrier layer and anion incorporation behavior will be discussed from a standpoint of the high field theory.

Switchable Plasmonic Film Using Nanoconfined Liquid Crystals.

Structural coloration using plasmonic particles has received substantial attention due to its robust, permanent, and scalable characteristics across the full color range. In this study, a plasmonic structure based on a porous anodic aluminum oxide (AAO) film coated with a metallic film was fabricated. Colors were varied by changing the refractive index, which was achieved with a convolution with nanopores of AAO film and an infiltrated liquid crystal (LC) material. LC molecules confined in the porous AAO film were uniformly aligned, and they exhibited pore-size-dependent colors because of the specific refractive index. The thermal phase transition of the LC material in the nanopores changed the effective refractive index, switching the reflected colors, and the LC-infiltrated AAO remained stable over a month. We believe LC materials can extend the use of rigid conventional plasmonic structures from simple sensor applications to multifunctional uses such as color printing, writing pens, and displays.

Optical reflectance from anodized Al-0.5 wt % Cu thin films: Porosity and refractive index calculations

Porous anodic alumina (PAA) films with periodically arranged horizontal and vertical nanopores have been applied as templates or platforms upon which optical sensors can be based. While the porosity and the refractive index of these 3D PAA can be tuned by varying the anodization parameters, the determination of these parameters from reflectance data has not been explored so far. In this study, we estimated the porosity and the refractive index of thin anodized Al-0.5 wt% Cu from the alumina film thickness and the interference pattern composed of the incident and reflected light beam in the alumina film. The 3D PAA films were prepared by anodization of Al-0.5 wt% Cu thin film deposited on TiN/Si substrate. Anodization was performed in three electrolytes at different concentrations (1 M sulfuric acid, 0.3 M oxalic acid and 0.75 M phosphoric acid) within the voltage range of 10–90 V. The structural characteristics showed that both pore size and inter-pore distance increased with increased anodization voltage and time while the pore density decreased exponentially. The results from reflectance spectra clearly showed that the effective refractive index of the samples decreased with voltage and anodization time, while porosity (total, vertical and horizontal) increased. The density of the samples was found to be inversely proportional to porosity.

Role of Electrode Reaction Kinetics in Self-Ordering of Porous Anodic Alumina

Porous anodic aluminium oxide with long-range ordered structure is of high practical importance in modern materials science and nanotechnology owing to its wide applications as membranes or templates for various nanostructures. However, the fundamental reason of self-organized growth of honeycomb-like porous anodic alumina films solely within a narrow region of processing conditions remains unknown. Here we show that the formation of the long-range ordered porous structure occurs only when anodization rate is limited by migration in barrier layer, which separates metal and electrolyte, or by diffusion in pores, whereas the mixed control of anodic oxidation process leads to disordered porous structures. The universality of this conclusion is experimentally confirmed for anodization in oxalic and sulfuric acid solutions, which are the most versatile anodizing electrolytes. The predictive power of the suggested model is demonstrated for anodic oxidation of aluminium in 2M sulfuric acid electrolyte, in which mild and hard anodization regimes leading to the formation of highly ordered porous structures are developed. It allows one to intentionally choose proper anodization conditions instead of simple empirical search that has been used so far.

On hydrophilicity improvement of the porous anodic alumina film by hybrid nano/micro structuring

In both, laboratory and industry, tremendous attention is paid to discover an effective technique to produce uniform, controllable and (super) hydrophilic surfaces over large areas that are useful in a wide range of applications. In this investigation, by combing porous anodic alumina (PAA) film with nano-structures and microarray of aluminum, the hydrophilicity of hybrid nano-micro structure has been significantly improved. It is found some factors can affect the hydrophilicity of film, such as the size and aspect ratio of microarray, the thickness of nano-PAA film etc. Comparing with pure nano-PAA films and microarray, the hybrid nano-micro structure can provide uniform surface with significantly better hydrophilicity. The improvement can be up to 84%. Also, this technique exhibits good stability and repeatability for industrial production. By optimizing the thickness of nano-PAA film and aspect ratio of micro-structures, super-hydrophilicity can be reached. This study has obvious prospect in the fields of chemical industry, biomedical engineering and lab-on-a-chip applications.

Subtle vertical structures of porous anodic alumina films for use as waveguides

Since porous anodic alumina (PAA) is a frequently-used optical waveguide material, accurate characterization of its structure parameters and optical properties is in urgent need. To characterize PAA, spectroscopic ellipsometry is preferred due to its undamaged detection, no sample pretreatment, and having a coverage area relatively larger than that of scanning electron microscopy. For spectroscopic ellipsometric data fitting, previous studies usually adopted a four-layer model, which displays a large bias from the raw data. Here, a modified six-layer model is built in consideration of the more elaborate porous layer that is the dominating contributor for the optical property of a PAA film. By using this six-layer model, PAA films with different thicknesses and under different oxidation voltages were analyzed, and the disperse curves of the porous layer were provided. This study will be helpful for learning the subtle structure of PAA and widen its applications for optical purposes.