Self-ordering of anodic porous alumina formed in organic acid electrolytes

New self-ordering porous alumina films were fabricated in organic acid electrolytes. Highly ordered cell arrangements of porous alumina films were realized in malonic acid at 120 V and tartaric acid at 195 V having 300 nm and 500 nm pore intervals, respectively. Self-organization was achieved at the maximum voltage required to induce high-current-density anodization while preventing burning, i.e., an extremely high-current flow concentrated at local points. The cells of the film grown at a high field must be pressed against each other, so that the self-ordering proceeds with the porous layer growth. When the self-ordering of cell arrangement proceeds, the cells became smaller. To improve the regularity of the cell configuration, a low electrolyte temperature and a relatively high electrolyte concentration were effective for maintaining a high-current-density to prevent burning. Surface flatness was an essential factor for self-ordering, however, the surface oxide film produced by electropolishing an aluminum substrate prevented quick pore growth in the organic acids having a low dissociation constant. It is confirmed that electropolishing followed by alkaline treatment was most appropriate as the pre-treatment in preparing flat surfaces.     

The role of electric field in pore formation during aluminum anodization

Nanoporous anodic aluminum oxide (AAO) can be created with pores that self-assemble into ordered configurations. For more than 60 years it has been assumed that field-assisted dissolution of the oxide leads to pore formation, despite a lack of direct experimental evidence that confirms this expectation. In this work, we have developed a method for separately studying the onset of field induced growth instabilities and the instability that leads to pore formation. We find that field-assisted dissolution models are consistent with the observed dependence of the Al₂O₃ dissolution rate on the electric field, as well as the existence of a critical field for pore initiation. However, we further show that the well-known porous structure, which has a significantly different length scale, does not result from a field-induced instability, but is instead the result of a mechanical instability with forced plastic deformation and flow of the oxide during further anodization. Through interpretation of these results we develop a generalized mechanism for pore formation in AAO, and by analogy, for pore formation in other anodization processes.     

Kinetics of the electrolytic coloring process on anodized aluminum

The kinetics of tin electrodeposition during the electrolytic coloring of porous anodic oxide films on aluminum is studied as a function of the oxide properties, e.g., the thickness of the porous oxide layer, and the surface resistance offered by the barrier oxide layer. While the thickness of the porous oxide layer is controlled by the anodization time, the surface resistance is controlled by the anodization voltage, and the anodization bath temperature. Steady-state polarization measurements are employed to characterize the dependence of the coloring kinetics on the oxide properties. Measurements indicate that the kinetics of the electrolytic coloring process can be accelerated by: (i) reducing the surface resistance of the oxide film (primarily offered by the barrier oxide layer) by growing the oxide at a lower anodization voltage, and/or a higher bath temperature, or (ii) growing a thinner porous oxide layer by decreasing the anodization time. The electrochemical measurements are supported by gravimetric analysis of electrolytically colored alumina samples (using calibrated wavelength-dispersive X-ray fluorescence spectroscopy), and by optical spectrophotometry.     

Preparation of nanoporous tin oxide by electrochemical anodization in alkaline electrolytes

This paper reports the creation of a porous tin oxide structure by means of anodization of pure tin foil in alkaline NaOH solutions. The tin oxide film formed is polycrystalline and possesses an irregular nanoporous structure with a porosity of ∼50%. The average pore size is ∼37 nm with a size distribution from 10 to 60 nm irrespective of anodization conditions, including the applied voltage (5–15 V) and NaOH concentration (0.125–1 M). The BET specific surface area of this porous structure is 79.6 m²/g. Linear relationships are observed for the dependence of tin oxide layer thickness on anodization time, applied voltage, and NaOH concentration. A thermodynamic model is established to explain the pore growth mechanism in the anodization process.     

Structural Characteristics of p-Type Porous Silicon and their Relation to the Nucleation and Growth of Pores

The influence of the anodization time on some structural characteristics (dissolved mass, maximum porous layer thickness, porosity, crystallite size, etc.) of p-type porous silicon has been investigated. It is shown that chemical dissolution of the porous layer during the anodization must be taken into account in order to correct some of the experimental data. Measurements of the anode potential have allowed to distinguish an early stage of the porous layer formation due to nucleation of pores. Equations based on pore nucleation and growth processes explain well the evolution of the maximum layer thickness with anodization time.     

Porous WO3 formed by anodization in oxalic acid

This work reports the investigation of the anodic growth of porous tungsten oxide (WO₃) in oxalic acid. It was found that pore diameter increases from approximately 50 to 90 nm while the layer thickness of porous structures increases from approximately 100 to 400 nm as the voltage varies from 40 to 100 V. Extended anodization duration improves the homogeneity of the porous structure at 40 V but it has no effect on the pore layer thickness. During the pore formation process, nucleation of small pores in the existing pore is observed while several layers of pores with voids underneath the surface layer are formed especially at high anodization voltage. A possible growth mechanism of the pores is proposed.     

AC-anodising of aluminium: Contribution to electrical and efficiency study

AC-anodising of AA1050 aluminium alloy in sulphuric and phosphoric acid solutions has been examined, with the contributions of the anodic and cathodic processes evaluated from the electrical signal analysis. The porous anodic film morphology and the potential-time behaviour confirm the similarity with DC-anodising, although lower anodic charge and film formation efficiencies are evident from gravimetric measurements. The anodic charge loss is mainly related to the capacitive current required to re-establish (between successive charge cycles) the electric field strength for ionic migration across the pre-existing and healed barrier layer oxide. Cathodic hydrogen gas evolution is considered to proceed at flaw sites in the barrier layer oxide, leading to high localized current densities that result in high cathodic potentials. Hydrogen gas retention at flaws and pores of the anodic film also contributes to the high cathodic potentials.     

Ion migration in anodic barrier oxide films on iron in acidic phosphate solutions

A kinetic study was carried out of the ionic current in the anodic barrier oxide layer on iron at the steady state in the passive potential region in acidic phosphate solutions. It is found that the ionic current follows the classical high field ion conduction equation in which the current is expressed as an exponential function of the electric field in the barrier layer. From the kinetic constants appearing in this equation and their temperature dependence, the diffusion coefficient and the activation energy for diffusion of moving ions in the layer were estimated and compared with those of high temperature diffusion in iron oxides. It is suggested that the ionic current is carried by oxygen ions than by iron ions in the barrier oxide film.     

Controlled morphology and optical properties of n-type porous silicon: effect of magnetic field and electrode-assisted LEF

Fabrication of photoluminescent n-type porous silicon (nPS), using electrode-assisted lateral electric field accompanied with a perpendicular magnetic field, is reported. The results have been compared with the porous structures fabricated by means of conventional anodization and electrode-assisted lateral electric field without magnetic field. The lateral electric field (LEF) applied across the silicon substrate leads to the formation of structural gradient in terms of density, dimension, and depth of the etched pores. Apart from the pore shape tunability, the simultaneous application of LEF and magnetic field (MF) contributes to a reduction of the dimension of the pores and promotes relatively more defined pore tips as well as a decreased side-branching in the pore walls of the macroporous structure. Additionally, when using magnetic field-assisted etching, within a certain range of LEF, an enhancement of the photoluminescence (PL) response was obtained.     

铝高压阳极氧化膜微观形貌影响因素的研究

探讨了混合酸体系中电解质浓度与氧化参数对铝高压阳极氧化膜微观形貌的影响。结果表明,磷酸、草酸和成膜添加剂的浓度对氧化膜的孔径和孔的形貌有显著影响,随着电解质浓度的增加,氧化膜的孔径增大,变化范围为250~500 nm,孔形貌从圆形变为规则六边形结构;较高的峰值电流密度导致氧化膜的外边缘破裂;铝高压阳极氧化膜的纵截面照片表明,混合酸体系中的铝高压阳极氧化膜呈现双层结构;厚的多孔层和薄的致密层,孔道垂直于致密层和铝基底,且互相平行。     

A study on the formation of titania nanopillars during porous anodic alumina through-mask anodization of Ti substrates

The formation of anodic titania during porous anodic alumina (PAA) through-mask anodization has been analysed for varying anodization conditions on mechanically polished bulk Ti surfaces. Titania nanopillars were formed through the porous masks in both oxalic and phosphoric acid electrolytes. For applied potentials above 40 V the titania formed along narrow channels through the alumina pore bottoms resulting in root-like attachments of the titania pillars to the Ti substrate. We further demonstrated that high-field anodization can be used for PAA through-mask anodization. The formation of titania changed with increased current density which resulted in more efficient oxide growth through the alumina pores. When the Al/Ti samples were immersed in the electrolyte without exclusively exposing the Al surface to the electrolyte the titania formed solely on top of the alumina pore bottoms which resulted in that the titania structures were detached from the Ti substrates during selective removal of the PAA templates.     

Galvanostatic anodization of pure Al in some aqueous acid solutions Part I: Growth kinetics, composition and morphological structure of porous and barrier-type anodic alumina films

The growth kinetics of anodic films formed on the surface of high purity Al by anodization under galvanostatic conditions at current densities in the range 5–75 mA cm^–2 in thermostatically controlled and vigorously stirred solutions of chromic, sulfuric, phosphoric, citric, tartaric and oxalic acids at different temperatures, were studied. It has been shown that chromic acid solution produces a typical barrier type oxide growth at any given temperature, while the specific kinetic curve representing the combined barrier/porous type film growth is observed when the anodization process is carried out in a nonstirred chromic acid solution. The oxide growth in the rest of the anodizing solutions occurs in different ways depending on the bath temperature. Barrier oxide growth is observed at temperatures lower than 30 °C. Above this temperature, combined barrier/porous oxide growth is observed. In all cases, the slope of the linear part of the potential against time curves, and therefore the rate of barrier oxide growth, increases with increasing anodizing current density and acid concentration, while it decreases with increase in temperature. The composition and surface morphology of the anodic films have been studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM).     

Fabrication of self-ordered nanoporous alumina with 500–750 nm interpore distances using hard anodization in phosphoric/oxalic acid mixtures

A hard anodization (HA) technique is employed using different mixtures of phosphoric/oxalic acid for fast fabrication of alumina nanopore arrays in voltages higher than 200 V. The mixtures enable to avoid the breakdown of porous anodic alumina (PAA) in the high voltages. It is revealed for the first time that continuously tunable pore intervals (D_int) from 500 to 750 nm can be controlled by varying the concentrations of oxalic acid at anodization voltages (U_anod) from 230 to 360 V, far beyond the U_anod in the single electrolyte of phosphoric acid or oxalic acid. The ratios of interpore distance, pore diameter and barrier layer thickness to anodization voltage are in the range of conventional HA process for each acid mixture. In this approach, the PAA film growth rate is 26 µm/h, being 7 times larger than that in typical mild anodization.     

Anodization of nanoimprinted titanium: a comparison with formation of porous alumina

Electrochemically prepared ordered porous alumina has become one of the most important nanotemplate materials to date. Naturally the questions arises whether other valve metals such as Ti, Ta, Nb, V, Hf or W can also be used for fabricating ordered pore arrays. We investigate in detail the electrochemical anodization of titanium in different electrolytes and its potential and temperature dependence. It turns out that due to the semiconducting properties of titania, a mirror image of the behavior of the electrically insulating porous alumina seems to be impossible. So-called porous titania in literature corresponds to the pitting regime of aluminum where pores are created due to dielectric breakdown of titania or alumina, respectively. Below the breakdown potential of titania, only thick barrier layers can be obtained. However, by nanoimprint of titanium and successive anodization below the breakdown potential, monodomain porous titanium oxide with a pore depth of 60 nm on a cm²-scale can be prepared. We discuss in detail the growth mechanism of porous structures of titanium and compare it with that of porous alumina.     

Nature of the passive film on nickel

The passive film formed anodically on nickel in borate buffer solution in both the passive and transpassive regions is found to be p-type in electronic character, corresponding to a preponderance of metal vacancies (over oxygen vacancies and nickel interstitials) in the barrier layer. However, at high anodic potentials, some n-type character was detected by Mott-Schottky analysis, which is probably due to the presence of free charge carriers (electrons) from the evolution of oxygen and/or the oxidative ejection of Ni³⁺ at the barrier layer/outer layer interface. The p-type character of the film is consistent with the diagnostic criteria obtained from the Point Defect Model for a passive film, in which the majority defect in the NiO barrier layer is the metal vacancy. The transpassive state is postulated to comprise a thick, porous oxide film on the surface, with the current probably being due to the oxidative ejection of Ni³⁺ species from the barrier layer and oxygen evolution within the pores, or both.     

Effect of the anodization voltage on the pore-widening rate of nanoporous anodic alumina

ABSTRACT: A detailed study of the pore-widening rate of nanoporous anodic alumina layers as a function of the anodization voltage was carried out. The study focuses on samples produced under the same electrolyte and concentration but different anodization voltages within the self-ordering regime. By means of ellipsometry-based optical characterization, it is shown that in the pore-widening process, the porosity increases at a faster rate for lower anodization voltages. This opens the possibility of obtaining three-dimensional nanostructured nanoporous anodic alumina with controlled thickness and refractive index of each layer, and with a refractive index difference of up to 0.24 between layers, for samples produced with oxalic acid electrolytes.     

Fabrication of Porous Anodic Alumina with Ultrasmall Nanopores

Anodization of Al foil under low voltages of 1–10 V was conducted to obtain porous anodic aluminas (PAAs) with ultrasmall nanopores. Regular nanopore arrays with pore diameter 6–10 nm were realized in four different electrolytes under 0–30°C according to the AFM, FESEM, TEM images and current evolution curves. It is found that the pore diameter and interpore distance, as well as the barrier layer thickness, are not sensitive to the applied potentials and electrolytes, which is totally different from the rules of general PAA fabrication. The brand-new formation mechanism has been revealed by the AFM study on the samples anodized for very short durations of 2–60 s. It is discovered for the first time that the regular nanoparticles come into being under 1–10 V at the beginning of the anodization and then serve as a template layer dominating the formation of ultrasmall nanopores. Under higher potentials from 10 to 40 V, the surface nanoparticles will be less and less and nanopores transform into general PAAs.     

Well-defined meso- to macro-porous film of tin oxides formed by an anodization process

Anodically formed tin oxide typically displays a self-ordered porous structure with a large degree of cracking. In addition, its surface pores are frequently closed, especially in the case where the deposited tin film is anodized. Herein, we report a simple way of eliminating virtually all the inner cracks and ensuring that the surface pores are totally open, leading to well-defined one-dimensional anodic tin oxide. The current efficiency ratio of oxygen gas generation to tin oxide formation and the amount of charge allocated for pore initiation are suggested to be the key factors affecting the internal crack development and pore opening, respectively. Pulsed anodization proved to be quite an effective way to create a well-defined structure with few inner cracks and completely open pores.     

Effects of Energy Relaxation via Quantum Coupling Among Three-Dimensional Motion on the Tunneling Current of Graphene Field-Effect Transistors

We report theoretical study of the effects of energy relaxation on the tunneling current through the oxide layer of a two-dimensional graphene field-effect transistor. In the channel, when three-dimensional electron thermal motion is considered in the Schrödinger equation, the gate leakage current at a given oxide field largely increases with the channel electric field, electron mobility, and energy relaxation time of electrons. Such an increase can be especially significant when the channel electric field is larger than 1 kV/cm. Numerical calculations show that the relative increment of the tunneling current through the gate oxide will decrease with increasing the thickness of oxide layer when the oxide is a few nanometers thick. This highlights that energy relaxation effect needs to be considered in modeling graphene transistors.