Contact angle studies on anodic porous alumina

The preparation of nanostructures using porous anodic aluminum oxide (AAO) as templates involves the introduction of dissolved materials into the pores of the membranes; one way to determine which materials are preferred to fill the pores involves the measurement of the contact angles (theta) of different solvents or test liquids on the AAOs. Thus, we present measurements of contact angles of nine solvents on four different AAO sheets by tensiometric and goniometric methods. From the solvents tested, we found dimethyl sulfoxide (DMSO) and N,N(')-dimethylformamide (DMF) to interact with the AAOs, the polarity of the solvents and the surfaces being the driving force.     

Impurity-driven defect generation in porous anodic alumina

We report that the morphologies of porous anodic alumina films are affected significantly by the presence of small amounts of copper impurity in the aluminium substrate, resulting in generation of defects during anodizing in phosphoric acid over a wide range of conditions. The copper is accumulated at the metal/film interface and transported preferentially to triple points of the alumina cells, where oxygen gas is generated leading to development of gas-filled voids.     

An environment-friendly electrochemical detachment method for porous anodic alumina

This paper describes an improved one-step voltage pulse detachment method by using perchloric acid and ethanol mixture as detaching solution for the preparation of through-hole porous anodic alumina (PAA) membranes. The detachment of PAA from aluminum substrate and the dissolution of the barrier layer can be completed simultaneously in the detachment solution by applying a pulse voltage in situ after the anodization process. The influence of voltage pulse height and nature of the detachment solution on the efficiency of detachment have been systematically investigated. The present procedure is more environmental friendly and efficient as compared to the conventional electrochemical detachment methods and is promising for the preparation of freestanding PAA films with through-hole morphology which are important for nanomaterials synthesis and nanoscale separation.     

Palladium coated porous anodic alumina membranes for gas reforming processes

Nanostructured ceramic membranes with ultrathin coatings of palladium metal have been demonstrated to separate hydrogen gas from a gas mixture containing nitrogen with 10% carbon dioxide and 10% hydrogen at temperatures up to 550 °C. The mechanically robust and thermally durable membranes were fabricated using a combination of conventional and high-efficiency anodisation processes on high purity aluminium foils. A pH-neutral plating solution has also been developed to enable electroless deposition of palladium metal on templates which were normally prone to chemical corrosion in strong acid or base environment. Activation and thus seeding of palladium nuclei on the surface of the template were essential to ensure uniform and fast deposition, and the thickness of the metal film was controlled by time of deposition. The palladium coated membranes showed improved hydrogen selectivity with increased temperature as well as after prolonged exposure to hydrogen, demonstrating excellent potential for gas separation technologies.     

Oxygen evolution: the mechanism of formation of porous anodic alumina

Abstract  Aluminium anodization behavior in ammonium sebacate solution (w = 4%) in ethylene glycol, and in several H₃PO₄-containing electrolytes, has been investigated. A new mechanism is proposed for the formation of porous anodic films. The model emphasizes the close relationship between pore generation and oxygen evolution. PO₄ ³⁻ ions incorporated in the anodic films behave as the primary source of avalanche electrons. It is the avalanche electronic current through the barrier film that causes oxygen evolution during anodization. When growth of anodic oxide and oxygen evolution occur simultaneously at the aluminium anode, cavities or pores are formed in the resulting films. Accordingly, the mechanisms of growth of barrier and porous films are not very different in nature. These findings are a decisive new step towards full understanding of the nature of anodic alumina films. Graphical abstract        

Fabrication of ordered Ni nanocones using a porous anodic alumina template

Hexagonally ordered Ni nanocones have been produced using an electroless Ni deposition technique on a porous anodic alumina (PAA) template, where the pores are of a cone shape. The conical PAA film was found to exhibit hexagonal order with a period of 100 nm. The aspect ratio (cone height vs. the diameter of the base of the cone) of the conical pores on the PAA film was found to be one. The Ni nanocones and the surface morphology of the nano-conical film exhibit the same periodic structure of the template. A significant advantage of the fabrication process employed in this work is that it utilizes existing techniques.     

Organic modification and subsequent biofunctionalization of porous anodic alumina using terminal alkynes

Porous anodic alumina (PAA) is a well-defined material that has found many applications. The range of applications toward sensing and recognition can be greatly expanded if the alumina surface is covalently modified with an organic monolayer. Here, we present a new method for the organic modification of PAA based on the reaction of terminal alkynes with the alumina surface. The reaction results in the the formation of a monolayer within several hours at 80 °C and is dependent on both oxygen and light. Characterization with X-ray photoelectron spectroscopy and infrared spectroscopy indicates formation of a well-defined monolayer in which the adsorbed species is an oxidation product of the 1-alkyne, namely, its α-hydroxy carboxylate. The obtained monolayers are fairly stable in water and at elevated temperatures, as was shown by monitoring the water contact angle. Modification with 1,15-hexadecadiyne resulted in a surface that has alkyne end groups available for further reaction, as was demonstrated by the subsequent reaction of N-(11-azido-3,6,9-trioxaundecyl)trifluoroacetamide with the modified surface. Biofunctionalization was explored by coupling 11-azidoundecyl lactoside to the surface and studying the subsequent adsorption of the lectin peanut agglutinin (PNA) and the yeast Candida albicans, respectively. Selective and reversible binding of PNA to the lactosylated surfaces was demonstrated. Moreover, PNA adsorption was higher on surfaces that exposed the β-lactoside than on those that displayed the α anomer, which was attributed to surface-associated steric hindrance. Likewise, the lactosylated surfaces showed increased colonization of C. albicans compared to unmodified surfaces, presumably due to interactions involving the cell wall β-glucan. Thus, this study provides a new modification method for PAA surfaces and shows that it can be used to induce selective adsorption of proteins and microorganisms.     

Novel electrodeposition behavior of Ni on porous anodic alumina templates without a conductive interlayer

During template-assisted electrodeposition, single-crystalline metallic nanowires could be obtained only when the overpotential is low. However, an unusual electrodeposition behavior on the PAA/Si substrate without a conductive interlayer between the template and Si is described in the present study. Through the electrical breakdown of the template, Ni nanodots, nanowires and nanotubes could be obtained by only changing the electrodeposition voltage on the same substrate. The mechanisms leading to the formation of various nanostructures are described in detail and compared with those for the conventional template-assisted electrodeposition process. The electrodeposition first occurred on the pore wall instead of from the underlying substrate, leading to the formation of some Ni nanotubes at a more negative voltage. Besides, single-crystalline Ni nanowires could also be formed even when the electrodeposition voltage was as negative as -40 V, indicating that the formation of single-crystalline metallic nanowires under a large overpotential is possible.     

Roles of pH and acid type in the anodic growth of porous alumina

Several theoretical models have been formulated to explain the growth of porous structures in anodized alumina. Using some basic assumptions, these models predict the size and shape of the pores in the anodic porous alumina as functions of pH and voltage. Additionally, they address issues of stability in the pore growth. In this work, we have carried out a systematic experimental investigation to study the stability phase diagram as a function of pH and applied voltage. We also obtain the dependence of pore dimensions on the pH, voltage, and acid type. Based on our results, and insight gained from recent chemical analysis of the porous alumina anodization process, we conclude that the models must include an appropriate weighting factor to account for the oxidation and dissolution mechanism during the pore formation.     

Influence of current density on the distribution of tungsten tracer in porous anodic alumina films

Porous anodic alumina films have been much studied recently due to interest in the application of the self‐ordered porosity in nanotechnological systems. Experimental investigations have identified anodising regimes that generate pores with a relatively high degree of long‐range order. However, the growth mechanism of the films, and its relation to the ordering of pores, is only partially understood. In the present work, the growth processes are studied over a range of current densities for films formed in oxalic acid. The films are formed on sputtering‐deposited substrates containing tungsten nanolayers that provide W⁶⁺ tracer species in the films. The distributions of tracer species are observed by scanning and transmission electron microscopes and the amounts of tracer species quantified by Rutherford backscattering spectroscopy. It is shown that the tungsten tracer remains within an inner region of the cells, with a tungsten‐free region being present next to the pore walls during the growth of the anodic films. Further, the thickness of the anodic film relative to that of oxidised metal increases with increasing current density, which is associated with an increase in the efficiency of film formation. This behaviour is consistent with the formation of pores by flow of film material in the barrier layer to the pore wall regions. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of continuous magnetic field on the growth mechanism of nanoporous anodic alumina films on different substrates

The effects induced by an external homogeneous magnetic field on the oxide film growth on aluminum in aqueous solutions of oxalic and sulfuric acid and on surface morphology of the alumina films were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on SiO₂/Si and glass-ceramic substrates. The pore diameter for oxalic acid alumina films on the SiO₂/Si substrate decreased by 0.8 nm, the interpore distance by 5.9 nm, and cell diameter by 6.9 nm if a magnetic field of 0.5 T was applied. When aluminum was anodized in sulfuric acid on the same substrate, the significant changes in parameters of porous structure of alumina, which were similar to the ones in oxalic acid, are firstly observed in stronger magnetic fields (of 0.7 T). On the basis of data obtained in this study and of previous investigation on the negative space charge and thermally activated defects in anodic alumina, we concluded that the intensity of the magnetic field is associated with energy of electron traps and that the changes of cell diameter characterize the trap concentration. The energy of electron traps in oxalic acid alumina films was proved to be smaller than the one in films formed in sulfuric acid, but the concentration of traps was of the same order of magnitude. When the substrate was replaced with the glass-ceramic one, the pore diameter in oxalic acid alumina films increased to ca. 17.6 nm.     

Nanoscale membrane electrode assemblies based on porous anodic alumina for hydrogen–oxygen fuel cell

In this paper, we demonstrate that nanoscale membrane electrode assemblies, functioning in a H₂/O₂ fuel cell, can be fabricated by impregnation of anodic alumina porous membranes with Nafion® and phosphotungstic acid. Porous anodic alumina is potentially a promising material for thin-film micro power sources because of its ability to be manipulated in micro-machining operations. Alumina membranes (Whatman, 50 μm thick, and pore diameters of 200 nm) impregnated with the proton conductor were characterized by means of scanning electron microscopy, X-ray diffraction, and thermal analysis. The electrochemical characterization of the membrane electrode assemblies was carried out by recording the polarization curves of a hydrogen–oxygen 5 cm² fuel cell working at low temperatures (25?÷?80 °C) in humid atmosphere. Our assemblies realized with alumina membranes filled with phosphotungstic acid and Nafion® reach respectively the peak powers of 20 and 4 mW/cm² at room temperature using hydrogen and oxygen as fuel and oxidizer.     

Controlled way to prepare quasi-1D nanostructures with complex chemical composition in porous anodic alumina

Herein we propose a novel approach to the preparation of quasi-1D nanostructures with various chemical compositions based on infiltration of colloidal solution into the asymmetric anodic alumina membrane. The proposed technique was successfully applied for the preparation of ordered arrays of the magnetically hard anisotropic hexaferrite nanostructures.     

Adsorption of argon on mesoporous anodic alumina

We have studied the adsorption of Ar on regular, highly-ordered alumina membranes made by anodization. The straight, non-interconnected pores have nominal diameters of 31 and 83 nm, with a relative dispersion better than 5 % in the pore size. Adsorption isotherms taken on bare membranes with pores of 83 nm present two distinct hysteresis loops. This is found to be a consequence of the fabrication procedure that yields a central circular region formed by open pores surrounded by an outer ring with closed bottom pores of smaller size, about 40 nm. For the membrane with pores of 31 nm, the difference between these pores is much smaller, about 2 nm, and this explains why the isotherms on these membranes show a single hysteresis loop as expected. Detailed real space analysis of the membranes by electron microscopy confirms the adsorption conclusions.     

A new route for the formation of self-organized anodic porous alumina in neutral electrolytes

The present work reports the formation of regular porous aluminum oxide layers in neutral fluoride containing (NH₄)₂SO₄ electrolytes. For a fluoride free (NH₄)₂SO₄ electrolyte only irregular and thin porous alumina layers can be grown under these conditions. Upon addition of small amounts of fluorides highly regular, smooth high aspect ratio pore arrays can be produced. Pores with a typical diameter of approximately 50 nm and a length of several micrometers are formed. This finding significantly widens the spectrum of synthesis routes for ordered porous alumina structures and templates.     

The effects of electropolishing on the nanochannel ordering of the porous anodic alumina prepared in oxalic acid

A series of porous anodic alumina has been prepared by anodizing aluminum surface in 0.3 M oxalic acid at different voltages. Prior to anodizing, the surface was pretreated in two different electropolishing electrolytes. One was Brytal solution (15% Na₂CO₃ and 5% Na₃PO₄) at 80 °C in which the electropolishing was performed at 2 V. This resulted in about 100–150 nm apart random features of 4–5 nm height. The other was the commonly employed perchloric acid–alcohol solution (1:4 ratio by volume), in which the electropolishing was performed at 20 V. The resulting surface comprised nanostripes of 1–2 nm amplitude with a wavelength of about 50 nm. The former pretreatment proved better for self-ordering of the pores at the anodizing voltage of 50–60 V, while the latter pretreatment was found better at the anodizing voltage of 40 V. The improved pore ordering at a given voltage was attributed to the higher pore density as associated with greater repulsive interactions among the pores.     

Creating anodic alumina nanochannel arrays with custom-made geometry

Porous anodic alumina oxide (AAO) is one of the most commonly used nanotemplates for growing arrays of nanoparticles, nanowires, nanocomposites, and nanoarchitectures because its pores, which are of a very uniform size, can grow longitudinally into arrays of self-aligned nanochannels with an extremely high aspect ratio. Furthermore, under specific combinations of anodization voltage and electrolyte, the lateral positions of nanochannels can self-organize into arrays of two-dimensional hexagonally close-packed lattices with domain sizes on the order of few tens of lattice units. The domain size can be greatly increased by prepatterning the Al surface with custom-designed nanoconcaves prior to the anodization process. The concaves guide the growth fronts of nanochannels and lead to the formation of an ideally long-range ordered lattice of nanochannel array. Such concaves have been fabricated by many methods, such as stamp imprinting, grating imprinting, and focused ion beam direct writing. In this review, we summarize the development of various methods to create AAO nanochannel arrays with custom-made geometry and discuss the mechanism responsible for the guiding process.