Porous Anodic Oxide Research Articles

Terminated nanotubes: Evidence against the dissolution equilibrium theory

Electrochemical anodizations of metals and alloys have attracted increasing attention as a method to fabricate highly ordered porous films. The popular formation mechanism of porous anodic oxides is dissolution equilibrium theory, which indicates that the equilibrium between oxide growth and dissolution via digging manner at the bottom of pores is the major cause of tubular structure. Here, terminated nanotubes with sealed bottoms were discovered while titanium foils were anodized under different conditions. In one terminated nanotube, oxide at the bottom does not grow whereas dissolution from electrolyte still exists. According to the estimated dissolution rate, it only takes approximately 20s for the bottom of the terminated nanotube to be penetrated absolutely based on dissolution equilibrium theory. Apparently, most anodizations in this work all last for more than 20s after the appearance of terminated nanotubes. Therefore, the bottom of the terminated nanotube is bound to be penetrated absolutely. Nevertheless, its bottom is sealed completely, namely the experimental results are not consistent with theoretical results based on dissolution equilibrium theory. Hence, terminated nanotubes are evidence against the dissolution equilibrium theory.

Nonaqueous Sol-Gel Synthesis of Anatase Nanoparticles and Their Electrophoretic Deposition in Porous Alumina.

Titanium dioxide (TiO2) nanoparticles were synthesized by nonaqueous sol-gel route using titanium tetrachloride and benzyl alcohol as the solvent. The obtained 4 nm-sized anatase nanocrystals were readily dispersible in various polar solvents allowing for simple preparation of colloidal dispersions in water, isopropyl alcohol, dimethyl sulfoxide, and ethanol. Results showed that dispersed nanoparticles have acidic properties and exhibit positive zeta-potential which is suitable for their deposition by cathodic electrophoresis. Aluminum substrates were anodized in phosphoric acid in order to produce porous anodic oxide layers with pores ranging from 160 to 320 nm. The resulting nanopores were then filled with TiO2 nanoparticles by electrophoretic deposition. The influence of the solvent, the electric field, and the morphological characteristics of the alumina layer (i.e., barrier layer and porosity) were studied.

Controlled synthesis of nanoporous tin oxide layers with various pore diameters and their photoelectrochemical properties

Porous anodic tin oxide films with various pore diameters were synthesized by one-step anodization of Sn foil in 1M NaOH followed by 2h of annealing at 200°C. When the potential of 2V was applied during anodization, oxide layers with ultra-small nanochannels having diameter slightly above 10nm were obtained, and an average distance between neighboring pores was ∼20nm. On the contrary, applying the potential of 4V resulted in the formation of anodic films with much wider channels (∼40nm in diameter) and larger pore spacing (∼55nm). The composition of as formed samples was examined by XRD and XPS measurements. The as obtained anodic films were amorphous even after annealing. However, the greater Sn2+ ion content was observed for the sample anodized at 4V. In consequence, the porous oxide layer grown at the higher potential exhibited a red-shifted absorption edge as well as the lower optical band-gap in comparison to the sample synthesized at 2V. This fact was ascribed to a higher content of Sn2+ defects which are mainly responsible for a significant enhancement of photoelectrochemical activity of the material in the visible range. However, photoelectrochemical activity in the visible range was noticeable for anodic tin oxide films independently of the anodizing potential. We believe that such kind of self-supported electrodes can be promising candidates for photocatalytic and photoelectrochemical applications.

Development of novel surface treatments for corrosion protection of aluminum: self-repairing coatings

Abstract Two types of self-repairing coatings for the protection of Al and its alloys are reviewed: (1) organic coatings with capsules containing repairing agent and (2) porous anodic oxide films with inhibitor solution stored in the pores of the oxide film. First, polyurethane microcapsules containing liquid surface-repairing agents were synthesized and polyurethane coating with the capsules was painted on Al alloy specimens. After mechanical damage to the coating, self-repairing occurred by the reaction of water vapor in the air with the repairing agents released from the capsules. Second, porous-type anodic oxide films were formed on Al alloys, and the pores of the anodic oxide films were filled with inhibitor solutions, followed by application of a covering polyurethane layer. Inhibitors released from the pores efficiently protected the Al alloy substrate from corrosion arising from induced mechanical damage.

Interface strength and degradation of adhesively bonded porous aluminum oxides

For more than six decades, chromic acid anodizing has been the main step in the surface treatment of aluminum for adhesively bonded aircraft structures. Soon this process, known for producing a readily adherent oxide with an excellent corrosion resistance, will be banned by strict international environmental and health regulations. Replacing this traditional process in a high-demanding and high-risk industry such as aircraft construction requires an in-depth understanding of the underlying adhesion and degradation mechanisms at the oxide/resin interface resulting from alternative processes. The relationship between the anodizing conditions in sulfuric and mixtures of sulfuric and phosphoric acid electrolytes and the formation and durability of bonding under various environmental conditions was investigated. Scanning electron microscopy was used to characterize the oxide features. Selected specimens were studied with transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy to measure resin concentration within structurally different porous anodic oxide layers as a function of depth. Results show that there are two critical morphological aspects for strong and durable bonding. First, a minimum pore size is pivotal for the formation of a stable interface, as reflected by the initial peel strengths. Second, the increased surface roughness of the oxide/resin interface caused by extended chemical dissolution at higher temperature and higher phosphoric acid concentration is crucial to assure bond durability under water ingress. There is, however, an upper limit to the beneficial amount of anodic dissolution above which bonds are prone for corrosive degradation. Morphology is, however, not the only prerequisite for good bonding and bond performance also depends on the oxides’ chemical composition.

Fabrication and characterization of Mg-M layered double hydroxide films on anodized magnesium alloy AZ31

Highly oriented Mg-M layered double hydroxide (LDH) films were firstly fabricated by a facile in-situ growth method on anodized magnesium alloy AZ31. It is proved to be an effective method to seal the porous anodic oxide film. M presents metal cations, such as Al3+, Cr3+ and Fe3+. The characteristic structure, morphology and composition of the LDH films were investigated via field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), fourier transform infrared spectrometer (FT-IR) and energy dispersive spectrometer (EDS) respectively. Besides, the corrosion resistance of the LDH films on magnesium alloy was investigated by using potentiodynamic polarization and electrochemical impedance spectroscopy. The result demonstrates that anodic oxide film can be sealed by the formation of LDHs with nano-container structures. The corrosion resistance of anodized magnesium alloy is remarkably improved by Mg-M LDHs, especially by Mg-Al LDHs. Finally, the protection mechanism of Mg-M LDHs was proposed.

Synthesis, structure, and optical properties of manganese phthalocyanine thin films and nanostructures

Manganese phthalocyanine (MnPc) nanostructures with different morphologies were prepared on porous anodic alumina oxide (AAO) at different substrate temperature (Ts=50℃, 80℃, 120℃, 180℃, 240℃) in an organic molecular beam deposition (OMBD) system. The nanostructures morphologies were studied using scanning electron microscopy (SEM) and the results showed that the nanostructures morphologies could be modulated by the control of Ts, as a result, the continuous film was obtained at 50℃, whereas the nanorods (NRs), nanoribbons (NBs), nanowires (NWs), nanosheets (NSs) and nanoparticles (NPs) were facilely generated as Ts increased. At the same time, the density and the uniformity of the nanostructures decreased. The results of X-ray diffraction (XRD) indicated that only the β-phase polymorph formed throughout the growth process irrelevant to the Ts. Additionally, the ultraviolet visible (UV–Vis) absorption spectra demonstrated that the main absorption bands of MnPc nanostructures showed a remarkable band broadening as the Ts was increased.

Image binarization to calculate porosity of porous anodic oxides and derivation of porosity vs current

Current methods to calculate porosity of tubular or porous structures are either inaccurate or complicated for operation. We introduce a facile method (M7) based on image binarization of top-view specimen images from scanning electron microscopy. M7 is both accurate and convenient, and shows high robustness regardless of external parameters like brightness or contrast. Relation between porosity and applied current during galvanostatic anodization is derived. The porosity-current relation differs in aqueous and organic solution, which can be explained as different conductivity of two electrolytes.

Oxide Microstructural Changes Accompanying Pore Formation During Anodic Oxidation of Aluminum

Porous anodic oxide formation results from a morphological instability of uniform barrier oxide growth leading to the establishment of pores. To gain insight into this process, evidence for microstructural changes in the oxide accompanying pore initiation in anodic alumina was sought through two types of experiments: potentiodynamic anodizing and stress measurements during dissolution of the anodic films. Cyclic voltammetry during anodizing in sulfuric acid shows that pore formation coincides with the appearance of localized ohmic- conducting regions close to the oxide-solution interface of individual pores. These defects induce large current increases which are apparently responsible for growth of concave scalloped features on the metal-oxide interface at the pore base. It is argued that interface defects are generated by surface forces in the oxide that accompany flow contributing to pore initiation. Stress measurements reveal that pore initiation introduces tensile residual stress into the oxide at the pore base, suggesting the formation of vacancy-type defects in the film. Such defects are explained by increases of local conduction current density due to the concentration of anodizing current in pores. Both sets of experiments reveal that pore formation generates locally defective oxide, which may help explain the phenomenon of burning during high-rate anodizing.

Magnetoimpedance Effect on Single NiFe Nanowire

In this study, Ni82Fe18 nanowires were deposited on Au-coated porous anodic alumina oxide (AAO) template by electrochemically. Scanning electron microscopy showed that wires have diameters of about 250–310 nm and length of 50–60 μm. NiFe nanowire arrays embedded in AAO were mechanically polished until nanowires appeared. Later, electrical contacts were made to a single nanowire using a focused ion beam (FIB) system. Magnetoimpedance (MI) properties of single nanowire were investigated, and nearly 3.5 % MI was observed at 4 GHz driving current frequency.

Constitution of Anodizing Current and Effects on the Growth of Porous Anodic Alumina

Field-assisted dissolution theory was the generally accepted mechanism for pore formation and development in porous anodic aluminum oxide (AAO). However, field-assisted dissolution rate or dissolution current has never been reported. The quantitative relationship between the length of AAO and total anodizing current Jtotal is unclear, because the constitution of Jtotal is hard to be determined in situ. In order to make clear of the constitution of Jtotal and effects on AAO growth, different anodizing processes in two electrolytes (0.3 M and 0.7 M H2C2O4) are explored under constant current rather than constant voltage. Quantitative relationship between AAO lengths (y) and total anodizing currents (x) were investigated. Two fitting equations in 0.3 M and 0.7 M H2C2O4 are y = 0.92(x−4.58) and y = 1.00(x−5.05), respectively. The interesting results indicate that the Jtotal is composed of ionic current Jion and electronic current Je, the pore development and the length of AAO are attributed to the Jion rather than the dissolution current, the corresponding Je = 4.58 and 5.05 mA/cm² contribute to the formation of oxygen bubbles which act as models for the pore formation in 0.3 M and 0.7 M H2C2O4, respectively. This is an efficient and simple method to determine the kinetics of porous anodic oxides.

Derivation of a Mathematical Model for the Growth of Anodic TiO2 Nanotubes under Constant Current Conditions

Due to certain limitations of traditional models, the growth mechanism of porous anodic TiO2 nanotubes has not been well determined currently. Herein, for the first time, a mathematical model of voltage-time transient curves under constant current conditions is derived theoretically, based on the conception of ionic and electronic currents and Ohm's law. The simulation results show high fidelity to the experimental curves, and illustrate the linear correlation between nanotube length and ionic current. Further, based on this model, the avalanche breakdown can be explained, which shows advantage over the former derivations on compact films. And this model indicates that the discrepancy between compact and porous oxide films lies on the magnitude of electronic current during anodization. Moreover, the proportion of the ionic and electronic current is then calculated during constant current anodization. It can be concluded that the ionic current contributes to the oxide growth while the electronic current gives rise to the oxygen bubble evolution which acts as the growth mould of the oxide. The present results promote the understanding of the growth kinetics of porous anodic oxides from qualitative interpretation to quantitative analyses.

Relationship between the Generation of Electronic Current and Film Morphology during Anodization of Ti

The formation mechanism of porous anodic oxides has been investigated for several decades. However, the current density-time curves for the growth of anodic TiO2 nanotubes (ATNTs) are still not well understood. It is known that the length of ATNTs increases with the anodizing current (Jtotal). However, shorter ATNTs are obtained at larger Jtotal while applying a high voltage, as Jtotal includes ionic current (Jion) and electronic current (Je). Though the Jtotal is larger at high voltages, its main part is Je in fact, which leads to oxygen evolution but makes no contribution to the growth of ATNTs. Further, compact oxides rather than nanotubes are obtained as the applied voltage decreases, which cannot be clarified by the field-assisted dissolution theory. Herein, we verify that Je generates at a critical thickness (dc) for the first time. ATNTs cannot form in fluoride-containing solutions at lower voltages, because the thickness of barrier layer cannot reach dc at lower voltages. The generation of Je at dc is the precondition to form nanotube embryos and the length of ATNTs is determined by the Jion instead of the Jtotal.

Hierarchically organized Li–Al-LDH nano-flakes: a low-temperature approach to seal porous anodic oxide on aluminum alloys

Li-LDH sealing is accounted for being highly competitive to standard hot-water sealing as referred to reduced treatment temperature and higher corrosion protection efficiency.

Flow Instability Mechanism for Formation of Self-Ordered Porous Anodic Oxide Films

Experimental studies have suggested that oxide flow plays an important role in the growth of porous anodic oxide films on reactive metals. Here a mathematical model for anodic oxide formation is presented that includes viscous flow of oxide, along with high-field ionic conduction and interfacial oxygen transfer and metal dissolution reactions. Flow is driven by near-surface compressive stress, as has been detected by measurements of residual stress distributions in anodic alumina. Linear stability analysis of the model shows that pattern formation at the solution interface is possible near the critical value of a parameter expressing competition between stabilizing electrochemical oxide formation and destabilizing flow. According to the critical parameter value, pattern formation requires that the Faradaic efficiency for oxygen transfer at the solution interface must be close to the transport number for electrical migration of oxygen ions across the film. This relationship between efficiency and transport number is indeed suggested by experimental data. A scaling ratio of pore separation to voltage, as characteristically found in anodic films, is predicted by the wavelength at the critical point.

A capacitor circuit model for theoretical derivation of anodizing current

The formation mechanism of porous anodic oxide has been investigated for several decades. However, the anodizing current density-time curves of anodic TiO2 nanotubes obtained under constant voltage are still not well-understood. In addition, existing theory cannot interpret the relationship between different parameters and anodizing current properly. Herein a capacitor circuit model is proposed to overcome these challenges. Based on the capacitor circuit model, theoretical expression of anodizing current is derived. And the influences of different anodizing parameters were quantified. The present results demonstrate that anodizing current density depends on the applied voltage, electrode distance, anodization temperature and electrolyte composition. The fitting curves are in accordance with the measured ones. The influence of the electrode distance on anodizing current was investigated for the first time, which provided evidences for the capacitor circuit model.

Transient Relaxations of Ionic Conductance during Growth of Porous Anodic Alumina Films: Electrochemical Impedance Spectroscopy and Current Step Experiments

Transient electrochemical relaxations during growth of anodic oxide films can provide insight into mechanisms of transport processes. Few such studies have focused on porous anodic oxides, which are of significant interest as functional materials. Conductance relaxations during anodizing of aluminum in acidic porous anodic oxide-forming solutions are investigated, using high voltage impedance spectroscopy and constant current reanodizing experiments. Both experiments follow phenomenology demonstrated in prior studies of anodic barrier films. Low-frequency inductive loops in impedance spectra exhibit time constants with inverse proportionality to current density and characteristic resistances nearly equal to those of high-frequency capacitive features. Potential decays during reanodizing depend on charge passed instead of time. Both sets of experiments are analyzed with a model including conductance relaxations due to electric field-dependent generation and removal of bulk charge-carrying defects. The model is found to be quantitatively consistent with nearly all aspects of experimental behavior; the inverse current density dependence of the inductive time constant and the charge parameterization of potential decays both arise because of exponential electric field dependence of ionic conduction and defect generation/removal rates. Oxide volume changes accompanying defect relaxations may help explain mechanical stress generation during anodizing, which is thought to drive pore formation.

Preparation and characterisation of single‐crystalline structure Sb/Bi 2 Te 3 superlattice nanowires

High figures of merit is important and necessary for thermoelectric materials in a particular engineering application of thermoelectric generation. Crystalline is an important factor for the high figures of merit of thermoelectric materials. Single-crystalline structure Sb/Bi2Te3 supperlattice nanowires (SLNWs) were synthesised in large quantity by pulsed electrodeposition technology using porous anodic alumina oxide as a template. The X-ray diffraction result indicates that the as-prepared compound consists of rhombohedral phase Bi2Te3 and monoclinic phase Sb. Transmission electron microscope images demonstrate that the segments of SLNWs are built by Bi2Te3 and Sb alternately and exhibit the distinguished interface between two segments. The average diameter and length of each segment are about 80 and 90 nm, respectively. The composition of neighbouring segments was investigated by selected area electron diffraction (SAED), further verifying that the neighbouring segments of SLNWs are comprised of Bi2Te3 and Sb. Meanwhile, the spot pattern of SAED indicates these segments are crystallised well.

(Invited) Stress-Assisted Formation of Self-Organized Porous Anodic Oxides

Porous anodic oxide (PAO) films are grown by electrochemical polarization of Al, Ti, and other metals in baths that dissolve the oxide. Procedures to grow films with highly ordered and controllable arrangements of nanoscale pores have led to the extensive use of PAO films in nanofunctional devices. Experiments and calculations show that transport in the oxide occurs by electrical migration coupled with plastic flow driven by mechanical stress in the oxide, and suggest that flow may play a role in formation of self-ordered PAO (1-3). Pores initiate by a morphological instability at the oxide-solution interface when the initially formed barrier oxide reaches a critical thickness (4). Herein is reported a combined experimental and modeling investigation of the formation and self-ordering of PAO. Experiments were carried out to identify stress generation mechanisms during galvanostatic formation of Al barrier films in 0.4 M H3PO­4. Stress distributions in these films were measured by monitoring stress-induced sample curvature changes during oxide formation followed by open circuit dissolution of the films (5). Elevated compressive stress reaching several GPa was found to build up within a few nanometers of the oxide-solution interface, coinciding with high phosphorus contamination in the same layer revealed by GDOES (6). The compressive surface stress is likely due to electric field-induced phosphate incorporation. The experimentally characterized surface stress was incorporated in our anodizing model based on transport by coupled viscous flow and electrical migration (2). Morphological stability analysis of the model reveals pattern formation consistent with the well-established characteristic pore spacing to voltage relationship of PAO (7). Pore formation is governed by competition between stabilizing oxide formation at the solution interface and destabilizing flow driven by gradients of anion incorporation-induced surface stress. The model successfully predicts the critical threshold electric field and the narrow ranges of anodizing efficiency characterizing PAO formation on many metals (3,4). A compelling analogy exists between the mechanisms of PAO ordering and formation of hexagonally-ordered Marangoni patterns in thin liquid films.

Self-Organized Porous Anodic Oxide Formation: Role of Oxide Flow Driven By Surface Stress

Porous anodic oxide (PAO) films are grown by electrochemical polarization of Al, Ti, and other metals in baths that dissolve the oxide. Procedures to grow films with highly ordered and controllable arrangements of nanoscale pores have led to the extensive use of PAO films in nanofunctional devices. Experiments and calculations show that transport in the oxide occurs by electrical migration coupled with plastic flow driven by mechanical stress in the oxide, and suggest that flow may play a role in formation of self-ordered PAO (1-3). Pores initiate by a morphological instability at the oxide-solution interface when the initially formed barrier oxide reaches a critical thickness (4). Results of a combined experimental and modeling investigation of the formation and self-ordering of PAO are discussed. Experiments were carried out to identify stress generation mechanisms during galvanostatic formation of Al barrier films in 0.4 M H3PO4. Stress distributions in these films were measured by monitoring stress-induced sample curvature changes during oxide formation followed by open circuit dissolution of the films (5). Elevated compressive stress reaching several GPa was found to build up within a few nanometers of the oxide-solution interface, coinciding with high phosphorus contamination in the same layer revealed by GDOES (6). The compressive surface stress is likely due to electric field-induced phosphate incorporation. The experimentally characterized surface stress was incorporated in an anodizing model based on transport by coupled viscous flow and electrical migration (2). Morphological stability analysis of the model revealed pattern formation consistent with the characteristic pore spacing to voltage relationship of PAO (7). Pore formation is governed by competition between stabilizing oxide formation at the solution interface and destabilizing flow driven by gradients of anion incorporation-induced surface stress. The model successfully predicts the narrow ranges of anodizing efficiency characterizing PAO formation on many metals (3,4). A compelling analogy exists between the mechanisms of PAO ordering and formation of hexagonally-ordered Marangoni patterns in thin liquid films. Electrochemical impedance spectroscopy of steady-state porous anodic alumina formed in sulfuric and oxalic acid solutions are presented. It is shown that the characteristic inductive loop of impedance spectra of anodic oxide can be understood quantitatively in terms of electric field-induced homogeneous formation and annihilation of oxygen vacancies. We argue that this reaction can produce oxide displacement in the presence of sufficiently large stress gradients. The possible explanation of viscous oxide flow by such a vacancy creep mechanism is discussed.