Prolonged Anodization Research Articles

Understanding the fast formation mechanism of hard nanoporous alumina films on aluminum in acidic solutions containing nitric acid

Nanoporous alumina films formed on aluminum via anodization are widely used for scientific and technical purposes owing to their various advantageous properties, including excellent corrosion and wear resistance, good thermal and electrical insulation, and facile producibility at low cost. Herein, we report a novel approach to fabricate hard porous anodic alumina (PAA) films by adding an appropriate amount of nitric acid to sulfuric-, oxalic-, and phosphoric-acid solutions at critical high current densities. The effects of the nitric additives on the anodizing behavior, film growth mechanism, microstructure, chemical composition, film thickness, microhardness, and appearance of the PAA films formed from various solutions were systematically investigated. The addition of nitric acid enhanced the formation of alumina and resulted in fast growth rates of 196 μm/h and 166 μm/h for sulfuric- and oxalic-based solutions, respectively, while maintaining high hardness of approximately 380 and 430 HV0.1, respectively. The surface hardness of the films decreased to some extent with the increase in nitric acid concentration and/or film thickness owing to the strong corrosive nature of nitric acid during prolonged anodization, although nitrogen inclusion in anodic alumina films was not detected through X-ray photo-electron spectroscopy, which differs from the other acidic anions. In particular, the barrier layer thickness and pore intervals of PAA films increased with the nitric concentration in the aforementioned solutions, leading to a fast growth rate and high hardness inside the films.

Optimal condition for fabricating mechanically durable superhydrophobic titanium surface by rapid breakdown anodization: Self cleaning and bouncing characteristics

In this paper, we describe a simple, cost effective, fast, scalable and sustainable method for fabrication of mechanically durable superhydrophobic (SHP) surface of titanium by a rapid breakdown anodization (RBA) followed by molten stearic acid (MSA) chemical modification. In this new approach, the preparation time is only a minute. Instead of stearic acid- ethanol solution, molten stearic acid is the post surface modification, which made the preparation process rapid and simple. The anodized samples showed islands of TiO2 microclusters with complex hierarchical structures with nano pores uniformly distributed in the passive TiO2 matrix. Laser Raman spectroscopy (LRS) analysis revealed the TiO2 microclusters are polycrystalline in nature. The applied voltage, anodization time, immersion time and MSA bath temperature are optimized to obtain a robust mechanically durable SHP surface. FESEM images showed that both anodization and immersion parameters have significant effect on the surface morphology of microclusters. A maximum water contact angle (WCA) of 167.8° with a sliding angle of 6° was obtained for a Ti sample anodized at 50 V for 10 min after molten stearic acid immersion at 125 °C for 30 min. The prolonged anodization for 10 min promoted microcluster growth in all dimensions to form a highly hierarchical micro-nanostructure. The prepared samples showed vertical micro plate-like crystals of stearic acid on the micro clusters, which enhanced the very high WCA. The FTIR results revealed multiple bonding modes of stearic acid with TiO2 microclusters through surface hydroxyl groups. The prepared superhydrophobic samples showed excellent self-cleaning and abrasion resistance.

Hexavalent Chromium Removal via Photoreduction by Sunlight on Titanium–Dioxide Nanotubes Formed by Anodization with a Fluorinated Glycerol–Water Electrolyte

In this paper, titanium–dioxide (TiO2) nanotubes (TNTs) are formed by anodic oxidation with a fluorinated glycerol–water (85% and 15%, respectively) electrolyte to examine the effect of fluoride ion concentration, time, and applied voltage on TNT morphologies and dimensions. For fluoride ion concentration, the surface etching increases when the amount of ammonium fluoride added to the electrolyte solution increases, forming nanotube arrays with a clear pore structure. At a constant voltage of 20 V, TNTs with an average length of ~2 µm are obtained after anodization for 180 min. A prolonged anodization time only results in a marginal length increment. The TNT diameter is voltage dependent and increases from approximately 30 nm at 10 V to 310 nm at 60 V. At 80 V, the structure is destroyed. TNTs formed at 20 V for 180 min are annealed to induce the TiO2 anatase phase in either air or nitrogen. When ethylenediaminetetraacetic acid is added as a hole scavenger, 100% hexavalent chromium removal is obtained after 120 min of sunlight exposure for nitrogen-annealed TNTs.

Modification and photocatalytic activity of open channel TiO2 nanotubes array synthesized by anodization process

Titanium dioxide nanotubes (TNTs) with an open-top-tube morphology were synthesized by adjusting the anodization parameters. The main effective parameters were determined to be the multi-step anodization, alteration of anodization time, and agitation of the electrolyte. The simultaneous influence of agitation and prolonging the anodization time was studied on the obtained structure. The multi-step anodization process was shown to be a promising approach to obtain well-ordered TiO2 nanotubes. Accordingly, the entrance of the tubes came to be opened and the diameter of the surface pores increased from ~45 to 60 nm. By increasing the anodization time, chemical etching intensifies, especially on the upper part of the TNTs resulting in the formation of a nanograss structure. Applying agitation in prolonged anodization process showed to have destructive effects on the surface ordering and synthesis of nanograss free TNTs. However, it was found that subsequent agitation using ultrasonic pulses is an effective approach to obtain an open-top tube morphology. The DRS results indicated no difference in the band gap energy for different steps and anodization times. The photocatalytic activity of the open-top-tube TNTs was found to be promising in the degradation of methylene blue (MB) reaching 85% after 480 min irradiation.

RuO2-Doped Anodic TiO2 Nanotubes for Water Oxidation: Single-Step Anodization vs Potential Shock Method

Simultaneous single-step anodization (SSA) or potential shock (PS) were performed for ruthenium/ruthenium oxide doping into anodic TiO2 nanotubes, which can be used in an oxygen evolution reaction (OER) in KOH. Regardless of the electrolyte type, SSA allows for the formation of nanotube walls more than twice as thick as other methods, leading to increased stability in the electrolyte. As a result, formation of nanograss is suppressed even after prolonged anodization. However, the SSA method exhibits a high onset potential for OER, whereas a newly grown barrier oxide is formed in PS. As a higher PS voltage is applied, a greater amount of ruthenium/ruthenium oxide is doped into TiO2 and a thicker barrier oxide is formed. Thus, optimization of PS voltage is required for each sample. The amount of ruthenium in TiO2 depends on the electrolyte used rather than the doping method: a higher amount of Ru in TiO2 is generally obtained in aqueous electrolytes compared to non-aqueous electrolytes. Empirical equation for maximum OER current density was deduced in terms of the total surface area of nanotubes, concentration of ruthenium, and barrier oxide thickness.

The effect of electrolyte change on the morphology and degree of nanopore order of porous alumina formed by two-step anodization

The aim of this work was to verify if the quality, regularity and spacing of nanoconcaves formed on the Al surface during the first step of anodization (pre-texturing) have a significant effect on the morphology of AAO layers grown during the second anodization continued at the same potential but in different electrolytes. High purity (99.999%) aluminum substrates were anodized at 25V and 45V in sulfuric acid and oxalic acid, respectively. After chemical etching of initially formed oxide in a mixture of chromic and phosphoric acid, the samples were re-anodized under the same potentials, but in different electrolytes (sulfuric, oxalic, phosphoric acids). It was confirmed that the regular nanoporous structure formed on the Al surface during the first anodizing step is initially replicated at the beginning of the second anodization, independently of the electrolyte used. However, these initially formed oxide layers exhibit significantly different pore diameters but the same pore spacing, which is determined by the cell spacing on the Al surface. When phosphoric acid is used as an electrolyte, the prolonged anodization resulted in the formation of typical serrated nanochannels. However, we successfully obtained the almost ideally-ordered porous anodic alumina layer with a thickness of about 700nm, and the average diameter of nanochannels of ∼50nm after the second anodization carried out for 20min at 45V in 0.3M H3PO4, outside from the self-ordering regime for this electrolyte.

Photoelectrochemical Fabrication of Porous GaN and Their Applications in Ultraviolet and Ammonia Sensing

This article reports the studies of porous GaN fabricated using photo-electrochemical anodization method and their applications in UV and ammonia gas sensing. GaN was anodized under different durations and their fundamental aspects were investigated. Electron micrographs revealed that the porous surface was obtained, however prolonged anodization durations would lead to breakdown of porous layer, forming nanostructures. A plausible pore formation and breakdown mechanism were proposed based on current-transient profile. In Raman spectroscopy, forbidden bands such as E1(TO) and A1(TO) were noted in anodized samples. The intensity of these bands was dependent on the geometrical position of the nanostructures formed on porous surface. In UV and ammonia sensing studies, change in Schottky barrier height was the greatest for anodized samples. Porous GaN in UV sensing was 5 times more sensitive relative to as-grown GaN. In ammonia sensing, porous GaN displayed positive results even in the absence of catalytic metal, Pt.

Fast fabrication of self-ordered anodic porous alumina on oriented aluminum grains by high acid concentration and high temperature anodization

Anodic porous alumina, which exhibits a characteristic nanohoneycomb structure, has been used in a wide range of nanotechnology applications. The conventional fabrication method of mild anodization (MA) requires a prolonged anodization time which is impractical for batch processing, and self-ordered porous structures can only be formed within narrow processing windows so that the dimensions of the resultant structures are extremely limited. The alternative hard anodization (HA) may easily result in macroscopic defects on the alumina surface. In this work, by systematically varying the anodization conditions including the substrate grain orientation, electrolyte concentration, temperature, voltage, and time, a new oxalic acid based anodization method, called high acid concentration and high temperature anodization (HHA), is found, which can result in far better self-ordering of the porous structures at rates 7–26 times faster than MA, under a continuous voltage range of 30–60 V on (001) oriented Al grains. Unlike HA, no macroscopic defects appear under the optimum self-ordered conditions of HHA at 40 V, even for pore channels grown up to high aspect ratios of more than 3000. Compared to MA and HA, HHA provides more choices of self-ordered nano-porous structures with fast and mechanically stable formation features for practical applications.

Long-Length Titania Nanotubes Obtained by High-Voltage Anodization and High-Intensity Ultrasonication for Superior Capacity Electrode

To modify the morphology and electrochemical properties of the resulting titanium oxide layer, we have applied high-intensity ultrasonication during the potentiostatic anodization of metallic titanium, and the applied voltage and anodizing time has been changed. The influence of the imposed voltage, anodizing time, and ultrasonication on the nanotubes growth has been studied. Additional dissolution process takes place under ultrasonication, as is observed in the anodizing curves (current density vs time) that show values on the order of ca. 200 A/m2. After only 30 min of ultrasound-assisted anodization at 42 V, the resulting nanotubes length is ca. 4 μm and, in contrast, in the case of non ultrasound-assisted anodization, the length is only ca. 1 μm. Further prolonged anodization under ultrasound induced the complete dissolution of the titanium. After anodization at 60 V during 20 h (no ultrasounds), the observed length of the nanotubes is as long as ca. 45 μm. The nanotube TiO2 aspect ratio has been tailored between 40 and 320. The obtained nanotubes of TiO2 exhibit high areal capacity (up to ca. 2 mAh/cm2 and stabilized around 0.3 to 0.5 mAh/cm2) and good cycling behavior in lithium batteries. A nonlinear relationship between the nanotubes length and the resulting capacity has been revealed.

Matrix of rectangular pores obtained by AFM nanoindentation and electrolytic oxidation of Al

AFM nanoindentation is used for pretexturing of Al sample intended for anodic oxidation in porous film-forming electrolytes. The rectangular quadratic and non-quadratic matrices of nanoindents were made by means of an AFM with diamond tip. The period of the 2D array of nanoindented concaves ranged from ∼50 to ∼370 nm. The pitch of the holes ranged from 13 to 170 nm. After anodization in water solutions of three different acids (0.3 M sulfuric, 0.3 M oxalic and 0.3 M phosphoric) at voltages chosen according to matrix period (20, 30 and 160 V), the well-arranged matrices of rectangular pores were obtained. The rectangular symmetry of the pretextured Al surface almost ideally transferred on porous alumina matrix, maintaining the highly ordered arrangement of the pores and rectangular cross section trough the whole film thickness with high pore depth to aperture size aspect ratio. During prolonged anodization in phosphoric acid, standard cells on the boundary of nanoindented area disturbed the arrangement and shape of quadratic cells.

Electrochemical Nucleation of SiO<SUB><I>x</I></SUB> Nanoparticles into the Pore Bottoms of an Anodic Aluminum Oxide

Utilizing the wafer-scale anodization of a thermally evaporated Ti layer onto a Si substrate, SiOx nanoparticles could be electrochemically nucleated into the pore bottoms of an anodic aluminum oxide. The formation of a Si-containing Ti layer (Ti(1-x)Six, x < 0.1) was identified between the Al and the silicon substrate by thermal diffusion of Si during the evaporation. Upon prolonged anodization of approximately 1 h after alumina barrier layer touched the Si-containing Ti layer, pyramid-shaped TiOx nanopillars formed underneath the pore bottoms as a result of a curvature inversion of the barrier oxide with Ti migration. These TiOx nanopillars were observed to act as a diffusion route of silicon from the Si-containing Ti layer. Only one SiOx nanoparticle (approximately 8 +/- 5 nm) for each pore was generally precipitated without Ti contamination. This finding suggests a new route which can make SiOx nanoparticles confined within an anodic aluminum oxide template.

Experimental study of macropore formation in p-type silicon in a fluoride solution and the transition between macropore formation and electropolishing

Anodic dissolution of p-Si is studied in diluted fluoride solution (HF 0.05 M + NH 4F 0.05 M, pH 3), with special focus on the physico-chemical parameters which govern the morphology of pore formation (crystallographic orientation, applied potential, and etching time). The effect of potential has been investigated in the transition region between macropore formation and electropolishing. Upon increasing the anodization potential, the pore cross-section changes from circular to square shape, and the bottom of the pores changes from a rounded to a V-shaped profile. Prolonged etching of the contour of (1 1 0) p-Si disks in the regime of porous silicon formation allows for a comparison of the etching characteristics of the ( 1 1 ¯ x ) orientations. SEM observation indicates indeed different morphologies as a function of the crystal orientation, and the formation of fractal-like structures is obtained for some orientations. In the same geometry and at a potential just above the onset of the electropolishing regime, prolonged anodization allows for a direct measurement of the Si thickness removed as a function of the crystallographic orientation. We clearly observe the etching anisotropy, with etch depth τ(1 1 1) < τ(1 1 0) < τ(1 0 0). This sequence, similar to that observed for current density in more concentrated HF, differs from that observed for the chemical etching of Si in an alkaline solution.

Growth process of atomically flat anodic films on titanium under potentiostatical electrochemical treatment in H 2SO 4 solution

The as-deposited titanium film on silicon wafer was electrochemically treated in potential sweep and potential step modes in 0.1 M H 2SO 4 solution at 30 °C. Under the anodization conditions of potential sweep and properly modulated cyclic voltammetry (CV), nanoscale grains, step-terrace structure and atomic images were clearly observed on the surface of anodic oxide film on titanium. Under potential step conditions, if the anodization time was short (1 s), no grains could be found on the anodic oxide film surface, even though the potential was high up to 9000 mV. Moreover, whatever potential sweep or potential step mode was performed, sufficient time (low sweep rate means a prolonged anodization time) was needed for the formation of nanoscale grains, atomically flat surface and step-terrace structure on the anodized titanium film.

Growth characteristics of oxide during prolonged anodization of aluminum in preparing ordered nanopore arrays

In this study, one-step anodization was conducted on both mechanically polished and electropolished Al foils at different temperatures to examine their growth characteristics during prolonged anodization. Several unusual phenomena different from those reported previously were observed, especially for the preparation of ordered pore arrays on electropolished Al foils. First, under a constant voltage condition, a continuous increase of the oxidation current with fluctuation was observed in a prolonged period of anodization at high temperatures. Second, the ordered pore arrangement of the oxide initially obtained on the electropolished Al foil anodized at 25 °C was found to become disordered after being anodized for 72 h. Third, the total film thickness increased in the beginning and decreased in the prolonged period when the anodization was conducted at a high temperature. Thus, it is concluded that a highly ordered pore configuration with a large aspect ratio could only be obtained on the electropolished substrate by anodizing for a moderate period of time at low temperatures.

Anodization of electroformed nickel for the manufacture of compact disc stampers

Compact discs are manufactured using electroformed nickel stampers. The process of making stampers involves the anodization of the metal works in alkaline solutions. During the anodization process, a layer of nickel oxide is formed on the surface of the electroformed metal works that subsequently is removed by the next process. This process may be repeated several times for each metal work in the course of CD manufacturing. In the present work, the effect of the growth and removal of NiO film on the quality of the stampers has been investigated. This work shows that although the nickel oxide layer formed under normal conditions of the anodizing bath is extremely thin, the quality of the stampers can be affected, by repeating the standard anodizing process for a number of times or by prolonged and vigorous anodization. A model has been presented for NiO formation and safe conditions for anodization have been proposed. Compact disc (CD-CDROM-DVD) manufacture involves a complex series of processes including engraving the digital data onto a glass coated with photoresist using a laser beam, in the form of little pits and lands. The dimensions of these pits and lands are critical and should be kept within a very narrow range. Both pits and lands vary in length but their depths and widths remain constant. In fact, in a typical compact disk there are pits and lands with 9 different lengths, conventionally referred to as 3T to 11T. The depth of the pits are adjusted to be a quarter of the playback laser’s wavelength, which works out to be about 110 to 113 nm. The data image face of the glass master is then sputtered with a thin layer of nickel/vanadium allowing the subsequent electroforming process. The next process in CD manufacture is the transfer the image from the so-called “glass master” onto a metal work, which has a hardness that is suitable for usage as a stamper in molding machines. The stamper, now having the negative image of the data is placed in an injection molding machine, pressing a molten transparent polycarbonate against it, replicating the original positive image. The polycarbonate disc is then covered with lacquer and together with artwork it forms the familiar CD/CDROM/ DVD shape. The electroforming process is, however, far from a straightforward process, involving several intermediate stages, including the anodization process. There are two reasons for anodizing the Ni metal works:

Effect of nonaqueous anodic oxidation on the intensity of photoluminescence of porous silicon

Porous silicon (PS) was anodized for short periods in 0.02 m KNO3–ethylene glycol electrolyte to improve the maximum intensity of its photoluminescence (PL) by changing surface –Si–H bonds to –Si–OH or –Si–O-related compounds. A PS sample prepared in 1:1 (49% HF:99.5% EtOH) electrolyte gave 15-fold PL intensity as well as stabilized luminescence with 5 min anodization. Prolonged anodization, however, peeled off the nano-ordered silicon particles and resulted in a decrease in PL intensity. The PL intensity of the PS sample prepared in 1:2 electrolyte decreased with 1min anodization but increased with 30 s anodization. During anodization, the nano-ordered silicon particles reacted with water, an impurity in ethylene glycol, to give Si–OH and Si–O-related compounds. Ethylene glycol proved to be the best anodization solvent for nano-ordered silicon particles because of its high resistivity, high viscosity, and good electrochemical stability. However, ethylene glycol had to be removed completely from the PS surface by rinsing with pure water, because polyhydroxy alcohols such as ethylene glycol behaved as quenchers for excited electrons formed in Si–OH-related compounds on the nano-ordered silicon as a result of illumination.

Compositional profiles of anodized aluminum on InP

Abstract Previously it has been shown that aluminum films on InP can be converted to Al2O3 by wet anodization and that a native InP oxide can be grown beneath the Al2O3 layer if the anodization is continued after all of the aluminum has been oxidized. This paper presents an investigation of the location of this oxide layer and the distribution of substrate elements within the Al2O3. The native oxide is found to be localized at the interface although oxidized and unoxidized indium is distributed throughout the Al2O3 after prolonged anodization. The results reported here differ from those previously reported for plasma anodization of aluminum films on InP.

ChemInform Abstract: THE ANODIC EVOLUTION OF OXYGEN ON PLATINUM OXIDE ELECTRODE IN ALKALINE SOLUTIONS

Abstract The anodic evolution of oxygen was investigated on the platinum oxide electrode, prepared by a thermal decomposition method, in alkaline solutions; the overvoltage data were reproducible on this electrode. On the basis of reliable mechanistic observations, the most probable path under Langmuir conditions was proposed, which is the same as that suggested by Krasil'shchikov for a nickel electrode in alkaline solution. The rate-determining step changes from the 2nd to the 1st step, depending on the overpotentials. This was also supported by the corresponding change in the values of activation energy. The similarity of this platinum oxide to oxide films produced by prolonged anodization of platinum in the anodic polarization characteristics was suggested.

The anodic evolution of oxygen on platinum oxide electrode in alkaline solutions

The anodic evolution of oxygen was investigated on the platinum oxide electrode, prepared by a thermal decomposition method, in alkaline solutions; the overvoltage data were reproducible on this electrode. On the basis of reliable mechanistic observations, the most probable path under Langmuir conditions was proposed, which is the same as that suggested by Krasil'shchikov for a nickel electrode in alkaline solution. The rate-determining step changes from the 2nd to the 1st step, depending on the overpotentials. This was also supported by the corresponding change in the values of activation energy. The similarity of this platinum oxide to oxide films produced by prolonged anodization of platinum in the anodic polarization characteristics was suggested.

Morphology of PbO2 in the Positive Plates of Lead Acid Cells

Several preparations of including positive active material from lead antimony and lead calcium cells were examined by electron microscopy. The morphology of the particles is believed to have a bearing on positive paste retention and plate performance in the lead acid cell. The anodic coating of formed on pure lead sheet in was found to be prismatic when first formed, but lost its prismatic character on prolonged anodization and on cycling.