Anodic Alumina Films Research Articles

A set of empirical equations describing the observed colours of metal-anodic aluminium oxide-Al nanostructures.

Structural colours have received a lot of attention regarding the reproduction of the vivid colours found in nature. In this study, metal–anodic aluminium oxide (AAO)–Al nanostructures were deposited using a two-step anodization and sputtering process to produce self-ordered anodic aluminium oxide films and a metal layer (8 nm Cr and 25, 17.5 and 10 nm of Au), respectively. AAO films of different thickness were anodized and the Yxy values (Y is the luminance value, and x and y are the chromaticity values) were obtained via reflectance measurements. An empirical model based on the thickness and porosity of the nanostructures was determined, which describes a gamut of colours. The proposed mathematical model can be applied in different fields, such as wavelength absorbers, RGB (red, green, blue) display devices, as well as chemical or optical sensors.

On the Prediction of Well-Ordered Porous Anodic Alumina Films

Porous hexagonal ordered anodic alumina (PAOX) is a long-known porous oxide material. Despite its straightforward experimental approach, a precise understanding of the microscopic processes that le...

(Invited) Highly Durable Platelet-Type Carbon Nanofibers for Oer in Alkaline Electrolyte

Porous anodic alumina films formed by anodizing of aluminum in acid electrolytes have a highly ordered, nanopore channels, which have attracted increased attention as template for various nanomaterials. Utilizing this template, platelet-type carbon nanofibers (pCNFs) have been successfully prepared by simply heating a mixture of the template and polymer powders, such as polyvinyl chloride and polyvinyl alcohol, in argon atmosphere via liquefied intermediate of the polymers (1, 2). The graphitization degree of pCNFs increases with annealing temperature, and interestingly, the neighboring reactive carbon edge sites at the sidewall of pCNFs form loops of several carbon layers at above 2000ºC. Here, we demonstrate that such pCNFs with high graphitization degree and loop structure exhibit extremely high oxidation resistance under oxygen evolution reaction (OER) conditions in highly alkaline electrolyte for a zinc-air secondary battery application.During galvanostatic OER on the state-of-the-art highly active Ca2FeCoO5 electrode with a conventional carbon black conductive agent in 4 mol dm-3 KOH electrolyte consumed the carbon black almost completely within a few days, whereas that with pCNFs annealed at 2400ºC exhibited almost no consumption of the pCNFs even after one month. The carbon nanofibers with platelet structure and high graphitization degree are promising for the air electrode materials in a zinc-air battery. Acknowledgment This work was supported in part by the RISING2 (Research and Development Initiative for Science Innovation of New Generation Batteries 2) of the New Energy and Industrial Technology Development Organization (NEDO), Japan. 1. H. Konno, S. Sato, H. Habazaki and M. Inagaki, Carbon, 42, 2756 (2004).2. H. Habazaki, M. Kiriu, M. Hayashi and H. Konno, Mater. Chem. Phys., 105, 367 (2007).

Tailor-engineered structural and physico-chemical properties of anodic alumina nanotubes by pulse anodization: A step forward

A study on advanced structural engineering of anodic alumina nanotubes (AANTs) manufactured by pulse anodization is presented. In-depth analysis of the electrochemical parameters of pulse anodization was carried out, distinctive sections of the anodization were defined. Evolution of the profiles has been quantified and described with regards to different current density applied and pulse duration during hard anodization. A smart combination of different anodization conditions enables precise control over the nanotubes' geometric features and physiochemical properties. Nanotubes are produced with varying pulse period, current density level and various post-anodization treatments (different sonication time, temperature and thermal annealing of anodic alumina film before fragmentation) under galvanostatic mode in sulfuric acid electrolyte modified with 10% (v) ethanol. The average length of nanotubes is tailor-engineered from 424 ± 92 to 1010 ± 118 nm, with an average inner diameter that ranges from 37 to 48 nm. It is also demonstrated that the level of current density input during pulse anodization has a direct effect on the ζ-potential of nanotubes, which can be tuned between 25 and 8.5 mV. Additionally, it is possible to produce nanotubes with a negative ζ-potential of −6.3 mV upon post-annealing treatment. Separation of nanotubes from the original template is improved by sonication at low temperature, providing a new means of increasing the production yield of these nanostructures. Furthermore, nanotubes are found to increasingly degrade with the bath temperature during separation. To better understand the impact of the fabrication parameters over the physical and chemical properties of nanotubes is a key step to design and tailor-engineer these model 1D nanostructures for specific applications. AANTs provide many attractive features that could find broad applicability in disciplines such as catalysis, drug delivery, nanofabrication and sensing. • In-depth analysis of anodization profile evolution accordingly to process parameters. • Evaluation of process parameters on physiochemical properties of alumina nanotubes. • Improved separation/degradation at lower/higher sonication temperatures. • ζ -potential change with anodization current, pulse duration and thermal annealing. • Morphology and crystalline form can be affected with thermal annealing.

Nanoindentation study of the mechanical properties and deformation behavior of nanoporous alumina films

Over the past decade, anodic aluminium oxide (AAO) has become one of the most widely used materials as a platform for developing new types of devices in micro- and nanotechnology. Due to the potential use of highly ordered honeycomb porous AAO membranes in many engineering applications, considerable attention is being paid to the mechanical characterization of such thin films. In this study, the mechanical properties and deformation behavior of a nanoporous alumina film were investigated by nanoindentation. AAO films with an average pore diameter of 40 nm were fabricated electrochemically. The morphology and the mechanical properties of AAO were studied using scanning electron microscopy and nanoindentation, respectively. The force-displacement dependences obtained revealed that in the case of a freestanding AAO membrane the sample demonstrates extremely high elasticity. The indentation modulus and the hardness were found to decrease nonlinearly with an increase in the applied force.

Hydrothermal treatment of metallic-monolith catalyst support with self-growing porous anodic-alumina film

Metallic-monolith catalyst support with self-growing porous anodic alumina (PAA) film was prepared by anodizing Al plate. The effect of hydrothermal treatment (HTT) on the crystalline state and textural properties of PAA film was investigated by XRD, BET, SEM and TG. The HTT treatment above 50 °C and the subsequent calcination above 300 °C could convert the amorphous skeleton alumina into γ-alumina and increase the specific surface area ( S BET ). However, SEM images showed the HTT modification was a non-uniform process along the thickness of PAA film. The promotion effect of HTT on S BET was non-linear, and the slope of S BET gradually decreased with the HTT temperature or time increased. The limited HTT effect should be attributed to a changed pore structure caused by an unfavorable pore sealing limitation. Pore widening treatment (PWT) before HTT could break the pore sealing limitation, because of the reduced internal diffusion resistance of hydrothermal reaction. The synergistic combination of PWT and HTT developed a PAA support with a large S BET comparable to commercial γ-alumina. In the catalytic combustion of toluene, the Pt-based catalyst prepared by using the PWT and HTT co-modified PAA support gave higher Pt dispersion and more favorable catalytic activity than that treated by HTT alone. The presence of a bimodal pore structure was suggested to be a decisive reason.

Improvement of Plasma Resistance of Anodic Aluminum-Oxide Film in Sulfuric Acid Containing Cerium(IV) Ion

The parts of equipment in a process chamber for semiconductors are protected with an anodic aluminum-oxide (AAO) film to prevent plasma corrosion. We added cerium(IV) ions to sulfuric acid in the anodizing of an AAO film to improve the plasma corrosion resistance, and confirmed that the AAO film thickness increased by up to ~20% when using 3 mM cerium(IV) ions compared with general anodizing. The α-Al2O3 phase increased with increasing cerium(IV) ion concentration. The breakdown voltage and etching rate improved to ~35% and 40%, respectively. The film’s performance regarding the generation of contamination particles reduced by ~50%.

Evaluation of the influence of synthesis conditions on the morphology of nanostructured anodic aluminum oxide coatings on Al 1050

Currently, the use of nanostructured films of anodic aluminum oxide (AAO) arouses great interest for a variety of nanotechnological applications. Previous studies have shown that the most important parameters in the anodic synthesis of nanoporous AAO films are the pretreatment of the substrate, the anodization voltage, and the nature, concentration and temperature of the electrolyte. To advance the development of nanomaterials, which are increasingly specific according to their functionality, in a simple and inexpensive way, it is essential to study the combined effect of the synthesis variables on the morphological properties of the coatings. The objective of this work was to study and predict the combined effect of the concentration and temperature of oxalic acid and the anodization voltage in the ordering, the pore diameter, the interpore distance, the pore density and the thickness of nanostructured films of AAO synthesized at 20, 30, 40 and 60 V and 20, 30 and 40 °C in 0.3 and 0.9 M oxalic acid, from the Al 1050 alloy. For this, surface images were obtained by optical microscopy and scanning electron microscopy, the latter being processed through the Fast Fourier Transform, and non-linear regressions were performed with the data obtained.The results proved which anodic synthesis conditions have the greatest influence on the ordering and each of the morphological parameters of the AAO films. The optimum voltage and temperature of the self-ordering regime for both oxalic acid concentrations tested were 40 V and 20 °C. The combined effect of the synthesis variables allowed obtaining coatings with a pore diameter of 27–150 nm, an interpore distance of 51–160 nm, a pore density of 35 × 108–390 × 108 and a thickness of 10–130 μm.

Depth dependent X-ray diffraction of porous anodic alumina films filled with cubic YAlO3:Tb3+ matrix

The presented paper deals with the measurement methodologies of the structural properties of porous anodic alumina (PAA) films filled with YAlO3:Tb3+ composite using X-ray diffraction, atomic force microscopy and scanning electron microscopy. It shows that the deposited material does not uniformly fill the porous volume of the anodic alumina film and the part of it forms a thick layer on the PAA surface. The aim of this work is to show the differences in the XRD response obtained at different angles of incidence of the excitation beam for the PAA/YAlO3:Tb3+ system. Furthermore, this simple approach enables separation of the signal from both regions on the surface and inside the PAA pores, providing more accurate data interpretation. It reveals that the crystallization of the material on the PAA surface and within the pores is different.

Nanoparticles deposition effects on 1050A aluminum alloy anodic layers

The objective of this research is to investigate the electrochemical features of aluminum alloy owing to the deposition of SiO2 or TiO2 nanoparticles in an anodic aluminum oxide film (AAO) via electrophoretic deposition (EPD). The modified oxide layer shows potential in view of corrosion protection while retaining the adhesion promotion properties. The modified oxide layer with TiO2 nanoparticles was more effective in corrosion prevention.

Cover: Journal of the Chinese Chemical Society 1/2020

In this paper, anodic aluminum oxide films form globally self‐aligned and locally self‐organized order nanochannels with characteristic diameter/spacing and unmatched aspect ratio. The range of order can be increased tremendously by various lithographic guiding methods. Focused ion beam can selectively close individual nanochannels to fabricate arrays with custom‐designed geometry, which can be exploited to create arrays of nanochannels with different diameters after subsequent chemical etching. By controlling the anodization voltage to change the characteristic diameter/spacing, nanochannles can be bended and enlarged to change their longitudinal geometry, forming arrays with fascinating 3‐dimensional architecture. More details about this figure will be discussed by Prof. Yuh‐Lin Wang and his co‐workers on page 11~24 in this issue. image

The Confinement Effect of Angstrom-Sized Pores in Asymmetrical Membrane Constructed by Zeolitic Imidazolate Frameworks: Partially Dehydrated Ion Transport Performance.

The confinement effect in asymmetrical biological ion channels makes the state of molecules and ions differs from that in the external environment, and the mass transfer confined in the biological ion channels is in a single strand form. Herein, an asymmetrical membrane with angstrom-sized pores is constructed by growth of ZIF-90 membrane on the porous anodic aluminum oxide film. Due to the confinement effect of angstrom-sized pores of ZIF-90, ions transport through the pores of ZIF-90 suffer from multiple dehydration-hydration-dehydration process in the form of a single ionic chain. Molecular dynamics simulations imply that ions inside the pores of ZIF-90 are partially dehydrated. In alkaline condition, high rectification ratios of 237, 295, and 357 can be achieved in 10 × 10-3 m KCl, NaCl, and LiCl electrolyte, respectively. Besides, the strong electrostatic interaction between ions and the confined ZIF-90 pores makes the ions transport through the asymmetrical membrane suffer from an energy barrier, and the energy barrier is different with different ion species. This work helps to understand the ions transfer mechanism through angstrom-sized pores, which can provide guidance for the design of asymmetrical membrane and boost their applications.

Preparation of Nb2O5-Al2O3 Composite Anodic Oxide Film for an Aluminum Electrolytic Capacitor by Electrodeposition-Annealing and Anodization

Nb was electrodeposited on etched aluminum foils for an Al electrolytic capacitor. After annealing at 500°C in air, the foils were anodized in H3BO4 solution at 530 V to form Nb2O5-Al2O3 composite anodic oxide film as a dielectric layer. The voltage–time variations during the anodization process were monitored. The structure, composition, and electrical properties of the anodized foils were investigated by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and electrochemical impedance spectroscopy. The obtained foils were assembled into aluminum electrolytic capacitors, and the capacitor performance was tested according to the Japanese Nichicon standard. It was found that after electrodeposition and annealing, the slope of the voltage–time curve of the aluminum foil became steeper during the anodization process. The composite anodic oxide film showed a triple-layer structure consisting of Nb2O5/Al-NbOx/Al2O3 layers. The specific capacitance (C and Cox) of the composite anodic film was about 14% greater than that of the aluminum anodic oxide film. However, the leakage current (I) of the composite film was increased and its specific resistance (Rox) and withstanding voltage (Uw) decreased relative to the aluminum anodic oxide film, probably due to the greater number of intrinsic defects. During the load life and shelf life test, the composite anodic oxide film demonstrated capacitor performance similar to that of the aluminum anodic film, and can thus be used as a dielectric layer for capacitors to enhance specific capacitance.

Annealing induced structural and phase transitions in anodic aluminum oxide prepared in oxalic acid electrolyte

Anodic aluminum oxide (AAO) films with uniform cylindrical pores are widely used as templates and membranes. The annealing of initially amorphous AAO enhances its chemical stability, but simultaneously changes its morphology and phase composition. Here we study the effect of annealing in the temperature range up to 1150 °C on structural and phase transitions in porous AAO films prepared by Al anodization in oxalic acid. Full-profile analysis of high-resolution X-ray diffraction patterns using the Rietveld method reveals the sequence of phase transformations (amorphous alumina → a mixture of γ-Al2O3 and θ-Al2O3 → a mixture of γ-Al2O3, δ-Al2O3, and θ-Al2O3 → α-Al2O3) and is used for quantitative phase analysis. Decomposition of oxalate-impurities, induced by the AAO crystallization, is studied using combined Fourier-transform infrared spectroscopy and thermogravimetry analysis. The removal of impurities results in the formation of mesopores in the AAO cell walls with a consequence increase in the specific surface area up to 80 m2·g−1. The apparent activation energies of the removal of water (~35 kJ·mol−1) and of the decomposition of carbon-containing impurities (~350 kJ·mol−1) are calculated. Morphology-dependent kinetics of the impurities removal is described in the framework of the limited mass-transport of oxalate decomposition products through the cell walls.

Anodic aluminum oxide film fabricated with galvanostatic anodization for non-electrolytic dyeing

Galvanostatic anodization in H2SO4 solutions at high current density has been used to prepare porous anodic aluminum oxide (AAO) films for coloring using a non-electrolytic dyeing process. The effects of anodizing current, electrolyte concentration and time on the film morphology have been systematically investigated and compared with previous results. Trends in the film morphology suggest that the pore structure continues to evolve during anodization at high current density, and the film dissolution rate plays a prominent role in the development of the pore morphology. Coloring of the porous films shows that efficacy of non-electrolytic dyeing is dependent upon the porosity of the film and on the state of agglomeration of the suspension of dye particles.

Anodisation of aluminium alloy AA7075 – Influence of intermetallic particles on anodic oxide growth

Microstructure and Volta-potential analyses were conducted to characterise intermetallic-particles (IMPs) in AA7075-T5. EIS and AFM were applied under operando-conditions to investigate anodisation processes. IMPs have pronounced influence on the growth of anodic aluminium oxide (AAO) films resulting in low charge-transfer resistance. Cu-bearing constituents show cathodic-character, whereas Mg2Si and MgZn2 particles show anodic-character. During anodisation, Al7Cu2Fe remain stable with peripheral-dissolution around boundary. De-alloying of S-phase particles leads to the detachment. Mg2Si undergoes de-alloying at low potential, and re-passivation at high potential. MgZn2 dissolves entirely upon anodization. Localised-dissolution in large-IMPs boundaries or nanometre-sized IMPs facilitates bubble evolution, confirming local breakdown of barrier-layer.

Ultra-slow growth rate: Accurate control of the thickness of porous anodic aluminum oxide films

Porous anodic aluminum oxide (PAAO) films have attracted widespread attention because they can be used as porous templates for the preparation of various nanomaterials. However, there are still some challenges. For example, although fine control of the thickness of AAO films is generally quite easy to achieve, it is difficult to control the thickness of extremely thin films (e.g. <1 µm). This is important for practical applications where accurate control of film thickness is necessary. In the present work, PAAO films have been prepared in an electrolyte composed of a mixture of ethanol and oxalic acid under 40–50 V (at −20 to 0 °C), attaining a minimum growth rate (νa) of ~1.4 nm min−1 (at −20 °C). The influence of anodization conditions on νa, the regularity of structural cells, and the interpore distance has been studied in detail. The results show that νa can be reduced effectively by using an extremely low anodization temperature. Ultra-thin PAAO films with precisely controlled thicknesses have been fabricated using this approach.

Total effective surface area principle for enhancement of capacitive humidity sensor of thick-film nanoporous alumina

Total effective surface area is an important factor for environmentally sensing performance. The porous anodic aluminum oxide (AAO) film with a high density of nanopores leads to atremendous surface area for absorbing water molecules. But such an AAO humidity sensor formed in oxalic acid exhibits a low response of capacitance, especially under the low relative humidity (RH). Here, we demonstrate total effective surface area principle to greatly enhance the performance of AAO capacitive humidity sensor using small anodizing potential in oxalic acid. For pore-dependent surface area, the AAO pore wall would directly affect the absorbance of water molecules and the response of capacitive sensor. Decreasing the anodizing potential reduces both of the pore diameter and interpore distance proportionally but increases the surface area inversely. Therefore, the AAO sensor formed at small 20 V can greatly increase the amount of water molecules absorbed on the wall for enhancing 2–3 times response under low-to-high RH compared to those at 40 and 50 V. The good stability and reliable response/recovery time are also obtained for the AAO sensor synthesized at 20 V in oxalic acid.

Development of QCM humidity sensors using anodized alumina film

AbstractHumidity sensors based on QCM (quartz crystal microbalance) have been developed using an anodized alumina thin‐film as a sensing material. An aluminum thin‐film is deposited on a silver‐coated quartz crystal by electron beam deposition and is anodized in an acid solution. The resultant alumina film is porous. The resonant frequency gradually decreases from 5% to 94% relative humidity (RH) and steeply changes near 100% RH. This result shows that the sensor can be used as a dew‐point sensor as well as a humidity sensor. The response times to the humidity changes from 27% to 59% RH and from 59% to 27% RH are less than 5 s. The sensor shows reversible response to some gases such as ethanol, toluene, and acetone vapors in addition to airborne particles such as pollen.

Anodic Alumina Photonic Crystals as Refractive Index Sensors for Controlling the Composition of Liquid Mixtures

Photonic crystals based on anodic aluminum oxide films are examined as refractive index sensors for controlling the composition of water-alcohol liquid mixtures. The position of the reflectance maximum corresponding to the first photonic stop band is used as the analytical signal. Impregnation of a photonic crystal with water-ethanol and water-glycerol mixtures results in a redshift of the reflectance maximum. A fairly high refractive index sensitivity, sufficient to determine the composition of water-ethanol and water-glycerol mixtures with an accuracy of about 1 wt.%, is observed. The detailed dependencies of the analytical signal on the composition of mixtures are experimentally investigated and compared with numerical calculations. Prospects and limitations of the refractive index sensors based on anodic alumina photonic crystals are discussed.