Porous Anodic Oxide Films Research Articles

Irregularity and Defects of Porous Anodic Oxide Films Formed on Metals

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18O distributions in porous anodic alumina by plasma profiling time‐of‐flight mass spectrometry and nuclear reaction analysis

A comparison is made between plasma profiling time‐of‐flight mass spectrometry (PP‐TOFMS) and nuclear reaction analysis (NRA) for depth profiling of 18O tracer in porous anodic oxide films on aluminum. The films were formed galvanostatically, for a range of times, using phosphoric acid electrolytes that were either enriched in 18O or of the natural isotopic concentration. The morphologies of the films were determined by electron microscopy. The findings from PP‐TOFMS and NRA reveal a partitioning of the tracer between the surface regions and buried layers of the films. However, a relatively high background of 16O in PP‐TOFMS prevents a reliable quantification of the concentration of 18O. Copyright © 2012 John Wiley & Sons, Ltd.

Porous alumina thin films on conductive substrates for templated 1-dimensional nanostructuring

The growth of thin porous anodic aluminum oxide (AAO) films on silicon by anodizing Al on Ti/Au/Si and Ti/Pt/Si substrates in oxalic acid was demonstrated. Removal of the Al2O3 barrier layer was effected by selective chemical etching in H3PO4 and a reversed bias method in the anodizing solution. Ion transport and the influence of the Ti adhesion layer at the oxide–metal interface during the critical stages of anodization and pore opening were investigated. The AAO films may be exploited as templates in the creation of silicon-integrated nanostructured wire arrays. Electrodeposition of Pt into the AAO template yielded a nanowire array with superior methanol oxidation activity that can be integrated in a micro direct methanol fuel cell.

Optical characteristics of pore size on porous anodic aluminium oxide films with embedded silver nanoparticles

Dielectric porous anodic aluminum oxide films embedded with two-dimensional silver nanoparticles arrays (porous Ag/AAO) have been fabricated by electrochemical anodization and deposition, wherein the position and rate of the Ag nanoparticles within the nanopores can be properly controlled. These porous Ag/AAO films have photo-induced plasmonic coupling and can be applied toward photo-absorption sensors. The plasmonic absorption is clearly visible and its absorption maximum red-shifts with increasing particle diameters. An investigation of how the absorption spectrum is influenced by systematically changing the pore diameter and inter-pore spacing is laid out. This work provides an approach to precisely tune the peak wavelength and the bandwidth of plasmonic absorption of porous Ag/AAO films utilizing fine control of the pore diameters. The wavelength shift slope of absorption peak with the pore diameter is measured to be approximately 0.82nm/nm (peak wavelength/pore diameter). These tuning curves may be useful in the design of photon-response sensors and 2D tunable plasmonic-based nanostructure is expected to enable applications such as tunable photon-induced electrical conductivity.

<I>In-Situ</I> Fabrication of AAO Template without Oxide Barrier Layer and Its Applications

Porous anodic aluminum oxide (AAO) film is the most widely used template in combination with electrodeposition (ED) method to fabricate one-dimensional nanostructures such as nanowires, nanotubes and nanorods. However, the existing oxide barrier layer after the anodization blocks the application of AAO template in synthesis of nanostructures via direct electrodeposition. In this paper, AAO template without oxide barrier layer was successfully fabricated by stepwise voltage decrement; influence of two types of stepwise voltage decrement on the removal of oxide barrier layer was introduced. Field Emission Scanning Electron Microscopy (FESEM) images indicated that stepwise voltage decrement could make the oxide layer thin effectively. Meanwhile, highly ordered gold nanowire arrays were fabricated by using direct electrodeposition method based on AAO template with the second anodization process with stepwise voltage decrement of 1 V/min, FESEM image showed that as-prepared gold nanowires are uniform in diameter and the diameter is in accordance with the diameter of AAO template pores. XRD pattern revealed that gold nanowires were indexed as face-centered cubic phase.

Morphological Instability Leading to the Formation of Self-Ordered Porous Anodic Oxide Films

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Theoretical derivation of ionic current and electronic current and comparison between fitting curves and measured curves

The mechanism of metal anodization has been investigated for several decades, however the present models can only partially explain the experimental phenomena. It is known that the total current in the oxide mainly includes ionic current and electronic current. The formation of oxide film is determined by ionic current, and the electronic current gives rise to the evolution of oxygen gas. In this paper, the theoretical expressions for time dependent ionic current and electronic current are derived to deduce the anodizing process. The results indicate that the ionic current decreases exponentially with time, while the electronic current increases with time in a more complicated way. Both types of current will approach infinitely close to a constant value in the steady stage. Consequently, the current evolution under constant anodizing voltage are experimentally investigated for aluminum and titanium anodization. All of the current-transient plots, including that in the formation of both porous and barrier type films, can be well fitted by the proposed expression. The morphology transition between porous and barrier anodic oxide films is determined by the magnitude of electronic current within the oxide.

Morphological instability leading to formation of porous anodic oxide films

Electrochemical oxidation of metals, in solutions where the oxide is somewhat soluble, produces anodic oxides with highly regular arrangements of pores. Although porous aluminium and titanium oxides have found extensive use in functional nanostructures, pore initiation and self-ordering are not yet understood. Here we present an analysis that examines the roles of oxide dissolution and ionic conduction in the morphological stability of anodic films. We show that patterns of pores with a minimum spacing are possible only within a narrow range of the oxide formation efficiency (the fraction of oxidized metal atoms retained in the film), which should exist when the metal ion charge exceeds two. Experimentally measured efficiencies, over diverse anodizing conditions on both aluminium and titanium, lie within the different ranges predicted for each metal. On the basis of these results, the relationship between dissolution chemistry and the conditions for pore initiation can now be understood in quantitative terms.

The Kinetics and Mechanism of Long-Range Pore Ordering in Anodic Films on Aluminum

Anodic aluminum oxide has unique and highly attractive properties, including self-ordering of porous structure during anodization. Although anodization regimes leading to formation of highly ordered porous structures had been found experimentally, many aspects of the self-organization mechanism remain unsolved. Here, the detailed in situ small-angle X-ray diffraction study of the self-ordering in porous alumina films is reported. Structure evolution kinetics was deduced by a quantitative analysis of diffraction patterns combined with electron microscopy. The rate of pore ordering is shown to have maximal value at the initial anodization stage and rapidly decreases inversely proportional to t0.2. Self-organization is shown to occur via growth of domains possessing preferential in-plane orientation and “death” of other domains, similar to Ostwald ripening governed by difference in pore growth rates for domains of different orientations. The process is accompanied by pore death and splitting making a significant impact on anodic oxides utilization in any mass-transport issues. This finding opens a novel approach for growth of highly ordered porous anodic oxide films.

Influence of desiccation procedures on the surface wettability and corrosion resistance of porous aluminium anodic oxide films

A hydrophobic oxide film was formed on aluminium by anodizing followed by desiccation treatment. Films subjected to gradual heating and cooling exhibit larger water contact angles than samples exposed to fast heating and cooling at the same temperature. From SEM and Auger Electron Spectroscopic observations, the low wettability surface shows a regular porous morphology with no significant chemical composition differences due to the different treatments. The desiccation process improves the corrosion resistance, shown by immersion in NaCl. The change in morphology by the desiccation processes is considered a main reason to lower the wettability, which further affects the corrosion properties.

Luminescence of ZnS and ZnSe nanostructures in anodic aluminum oxide porous matrices

Investigations of the luminescent characteristics of ZnSe and ZnS nanostructures obtained by thermal deposition on porous anodic aluminum oxide films are presented. Luminescence associated with the interaction of semiconductors with the matrix material was detected.

Formation of porous SnO 2 by anodic oxidation and their optical properties

Porous anodic tin oxides were constructed via anodic oxidation of tin foils at room temperature. The optimal conditions for the porous anodic tin oxide growth has been explored based on the voltage-, oxalic acid concentration-, and time-dependent reactions. The formation mechanism of the porous anodic tin oxide film is also discussed. Raman spectra show an abnormal Raman peak at 693 cm −1 which is attributed to the IR-active A 2uLO mode for the disorder of tin oxide with nanochannels. A broad photoluminescence emission band centered at 600 nm is disclosed to originate from the electronic states determined by the oxygen vacancies.

Discontinuities in the porous anodic film formed on AA2099-T8 aluminium alloy

The anodizing behaviour of constituent particles (Al–Fe–Mn–Cu) and dispersoids (Al–Cu–Mn–Li and β′(Al 3Zr)) in AA2099-T8 has been investigated. Low-copper-containing Al–Fe–Mn–Cu particles anodized more slowly than the alloy matrix, forming a highly porous anodic oxide film. Medium- and high-copper-containing Al–Fe–Mn–Cu particles were rapidly dissolved, resulting in defects in the anodic film. The anodizing of Al–Cu–Mn–Li dispersoids is slightly slower than the alloy matrix, forming a less regular anodic oxide film. β′(Al 3Zr) dispersoids anodized at a similar rate to the alloy matrix. Further, the potential impact of the discontinuities in the resultant anodic films on the performance of the filmed alloy is discussed.

Structural coloring of aluminum

A porous anodic aluminum oxide film with a regular periodic variation of pore density normal to the surface is created by alternating the current density in current-controlled anodization. Regularity of the effective refractive index is achieved by altering the current pulse time to compensate for the etching of the layers in the electrolyte as anodization proceeds. The resulting photonic crystals display brilliant interference colors in reflected white light. These structural colors can be adjusted throughout the visible light region simply by altering the current pulse time. The method can be applied both to aluminum and its alloys.

Morphological Instability Leading to Formation of Porous Anodic Oxide Films

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Electrochemically fabricated self-aligned 2-D silver/alumina arrays as reliable SERS sensors

A novel SERS sensor for adenine molecules is fabricated electrochemically using an ordered two-dimensional array of self-aligned silver nanoparticles encapsulated by alumina. Silver is electro-deposited on the interior surfaces at the bottom of nano-channels in a porous anodic aluminum oxide (AAO) film. After etching aluminum, the back-end alumina serves as a SERS substrate. SERS enhancement factor greater than 10(6) is measured by 532 nm illumination. It exhibits robust chemical stability and emits reproducible Raman signals from repetitive uses for eight weeks. The inexpensive mass production process makes this reliable, durable and sensitive plasmon based optical device promising for many applications.

Area Selective Formation of Porous Type Aluminum Anodic Oxide Film by a Solution Flow-Type Micro Droplet Cell

A novel solution flow-type micro droplet cell with co-axial dual capillary tubes (Sf-MDC) was applied to form porous type aluminum anodic oxide film locally. This technique makes it possible to form porous type anodic oxide film on aluminum locally. When the droplet cell remains stationary, the oxide film was growing both vertically and laterally. The growth rate in the lateral direction can be controlled by moving the droplet cell. By moving the droplet cell, a porous type anodic oxide film line was successfully formed. The thickness of the oxide film increases with decreasing scanning speed of the cell. The oxide film growth rate was greatly affected by specimen temperature.

Morphology and growth of porous anodic oxide films on Ti-10V-2Fe-3Al in neutral tartrate solution

Porous anodic oxide films were fabricated galvanostatically on titanium alloy Ti-10V-2Fe-3Al in ammonium tartrate solution with different anodizing time. Scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) were used to investigate the morphology evolution of the anodic oxide film. It is shown that above the breakdown voltage, oxygen is generated with the occurrence of drums morphology. These drums grow and extrude, which yields the compression stress. Subsequently, microcracks are generated. With continuous anodizing, porous oxides form at the microcracks. Those oxides grow and connect to each other, finally replace the microcrack morphology. The depth profile of the anodic oxide film formed at 1 800 s was examined by Auger electron spectroscopy (AES). It is found that the film is divided into three layers according to the molar fractions of elements. The outer layer is incorporated by carbon, which may come from electrolyte solution. The thickness of the outer layer is approximately 0.2–0.3 μm. The molar fractions of elements in the intermediate layer are extraordinarily stable, while those in the inner layer vary significantly with sputtering depth. The thicknesses of the intermediate layer and the inner layer are 2 μm and 1.0–1.5 μm, respectively. Moreover, the growth mechanism of porous anodic oxide films in neutral tartrate solution was proposed.

Impact of the type of anodic film formed and deposition time on the characteristics of porous anodic aluminium oxide films containing Ni metal

AbstractPorous anodic films containing nickel were prepared by AC electro-deposition. The porosity of the films was controlled by using different working conditions (anodisation electrolyte, voltage, and time). Then nickel was electro-deposited using an alternating voltage. The impact of the anodic film on the current density waveforms and the metal content can largely be explained by the porosity differences, while changing the deposition time caused changes due to over-oxidation of the aluminium substrate, experimentally proved by TEM. Finally, the impact of deposition time on the deposited metal was successfully fitted using an Elovich type law over a large time-span (up to 1800 s), showing the ability to achieve precise control of the metal content.

Plasmon-induced optical switching of electrical conductivity in porous anodic aluminum oxide films encapsulated with silver nanoparticle arrays

We report on plasmon induced optical switching of electrical conductivity in two-dimensional (2D) arrays of silver (Ag) nanoparticles encapsulated inside nanochannels of porous anodic aluminum oxide (AAO) films. The reversible switching of photoconductivity greatly enhanced by an array of closely spaced Ag nanoparticles which are isolated from each other and from the ambient by thin aluminum oxide barrier layers are attributed to the improved electron transport due to the localized surface plasmon resonance and coupling among Ag nanoparticles. The photoconductivity is proportional to the power, and strongly dependent on the wavelength of light illumination. With Ag nanoparticles being isolated from the ambient environments by a thin layer of aluminum oxide barrier layer of controlled thickness in nanometers to tens of nanometers, deterioration of silver nanoparticles caused by environments is minimized. The electrochemically fabricated nanostructured Ag/AAO is inexpensive and promising for applications to integrated plasmonic circuits and sensors.