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The properties of anodic porous alumina films, which are generally formed by the anodization of aluminum in an acid electrolyte, are well known to be affected by the dimensions of their structure (e.g., pore diameter, pore density, and film thickness). For instance, to increase the hardness of anodic films on aluminum, the anodization is industrially carried out at low-temperature to avoid chemical dissolution of the formed oxide film during anodization. Anodization in an electrolyte containing organic compounds is also one approach for improving the performance of anodic oxide film not only for aluminum but also for other light metals such as titanium and magnesium (1). One such initiative is anodization of aluminum at a relatively high current density or high voltage using a common acidic electrolyte (e.g., sulfuric acid, oxalic acid, or phosphoric acid) with an alcohol added. In such studies, the effects of adding an alcohol are summarized as follows: i) the alcohol functions as an anti-freeze agent in the electrolyte, enabling the anodization temperature to be lowered; ii) the alcohol acts as a cooling agent—specifically, the vaporization of alcohol at the bottom of the pore channels removes the heat generated at the aluminum/oxide interface; and iii) nonuniform growth of the anodic film (i.e., burning) is effectively prevented, even during high-field anodization. However, these effects are observed not only for monohydric alcohols with a boiling point less than 100 °C (e.g., methanol and ethanol) but also for polyhydric alcohols with a boiling point greater than 100 °C (e.g., ethylene glycol and glycerol). Therefore, the effect of alcohol addition cannot be reasonably explained solely on basis of the boiling point of the alcohol used as an additive. Understanding the effects of alcohol addition necessitates a systematic investigation comparing types of alcohols, in addition to further fundamental studies. In this study, monohydric alcohols or polyhydric alcohols were added to the sulfuric-acid-based electrolyte to examine their effects on the anodization behavior of aluminum, the coating ratio, the porosity, the maximum attainable film thickness, and the resulting etching ability. Anodization was conducted in sulfuric acid solutions containing various alcohols with different physical properties under mild anodization conditions with a constant current of 100 A·m−2 at 20 °C (2, 3). The influence of the type and concentration of alcohol on the coulombic efficiency of film formation and the structure of the porous alumina under a constant electric charge was also investigated. In terms of basic research, the potential impact of alcohol addition in electrolytes will be discussed. H. Asoh, K. Asakura, H. Hashimoto, RSC Advances, 10, 9026-9036 (2020). H. Asoh, M. Matsumoto, H. Hashimoto, Surface and Coatings Technology, 378, 124947 (2019). M. Matsumoto, H. Hashimoto, H. Asoh, Journal of The Electrochemical Society, 167, 041504 (2020).

In recent years, there has been renewed interest in bipolar electrochemistry, which deals with reactions occurring on unconnected conductive objects placed between outer electrodes, due to its potential applications in various fields ranging from sensing to the fabrication of functional materials. In most of the studies regarding bipolar electrochemistry, a direct current (DC) electric field is applied between the driving electrodes, and an inhomogeneous reaction field on the bipolar electrode is used. Based on the polarization of both sides of a bipolar electrode under a DC electric field, the electrochemical redox reactions on each part of a bipolar electrode can be controlled. In our previous study, we investigated the application of an alternating current (AC) electric field for surface coating of aluminum and successfully formed porous alumina films on unconnected aluminum by indirect oxidation under an AC electric field without a direct electrical connection (“AC-bipolar anodization”) 1, 2). A common strategy applied in bipolar anodization is to use the interfacial potential differences between the driving electrodes under a DC electric field, resulting in the asymmetrical formation of oxide films (e.g., gradients in the thickness of the porous layer and pore diameter) on a metal substrate used as a bipolar electrode. On the other hand, we focused on bipolar anodization under an AC electric field as uniform surface treatment for aluminum. With the above background, we continued our preliminary work and tried to fabricate Pt/alumina composite on an unconnected aluminum substrate using one-pot electrochemical synthesis in a single electrolyte 3). This synthetic approach overcomes the drawbacks of conventional methods in terms of the operation efficiency and will lead to the development of various functional composites using one-pot synthesis. In terms of basic research, potential applications of our developed method will be also discussed. H. Asoh, M. Ishino, H. Hashimoto, RSC Adv., 6, 90318 (2016). H. Asoh, M. Ishino, H. Hashimoto, J. Electrochem. Soc., 165, C295 (2018). H. Asoh, S. Miura, H. Hashimoto, ACS Appl . Nano Mater., in press (2019).

III-V compound semiconductors have attracted attention as next-generation materials and potential alternatives to silicon-based semiconductors because of their excellent properties including superior carrier mobility and direct band gap. In a previous study, we fabricated microbump arrays of InP [1] and line patterns and pillar arrays of GaAs using metal-assisted chemical etching [2]. However, the dimensions of the resultant patterns ranged from several micrometers to several tens of micrometers. To the best of our knowledge, no study has reported the formation of ordered GaAs nanostructures with submicron-scale or smaller periodicity using metal-assisted chemical etching. Thus, we attempted to clarify the etching mechanism of GaAs on the nanometer scale. In this study, we investigated the effects of etchant composition and temperature on the morphology of the etched GaAs during metal-assisted chemical etching using Au nanodot arrays as a catalyst. A through-hole porous alumina mask with an ordered array of openings was set on an n-type GaAs. Subsequently, a 20-30 nm thick Au layer was evaporated through the alumina mask using a mixed vacuum deposition system. The periodicity of Au nanodot arrays, which were formed on GaAs substrate, was 100 nm and the diameter of Au dot was 50-60 nm corresponding to the dimensions of anodic alumina mask. After the formation of Au dot arrays on GaAs substrates, the specimens were immersed in a mixed solution of HF and KMnO4. When the specimen was etched in a HF containing relatively low concentration of KMnO4 at high temperature, Au dots sank into the GaAs substrate. The positive holes (h+), which were generated by reduction of oxidant (i.e., KMnO4) at Au surface, are thought to be consumed immediately at the Au/GaAs interface. As a result, Au dot arrays sank into the substrate, resulting in the formation of pore arrays. On the other hand, in the case of high concentration of KMnO4, h+ diffused into the surrounding Au-coated GaAs; thus, site-selective etching occurred on the exposed GaAs surface. Attempts to clarify the effects of the concentration of oxidizing agent on the generation of h+ and morphology of etched are currently underway. [1] H. Asoh, T. Yokoyama and S. Ono, Jpn. J. Appl. Phys., 49, 046505 (2010). [2] H. Asoh, Y. Suzuki and S. Ono, Electrochim. Acta, 183, 8-14 (2015).

Anodic porous alumina films having a closely packed hexagonal array of cells have attracted much attention as a starting material in the fabrication of nano-devices because of the high controllability of porous structures (e.g., cell size, pore size, pore interval, and pore depth) on dimensions ranging from the submicron to nanometer range. In recent years, the mechanism of self-ordering of cell arrangement of anodic porous alumina has been intensely studied. We proposed anodizing under high electric field strength as a key factor for self-ordering of cell arrangement [1, 2]. However, only a few reports are found on the irregularities of barrier layer thickness and morphology of each cell of anodic porous alumina. In this study, we evaluated the influence of electrolytic factors (e.g., current density, applied voltage, and anodizing time) on cell morphology of anodic porous alumina. Especially, we focused on the effect of the electric field strength on the barrier layer thickness and anion incorporation behavior. High purity aluminum sheets were anodized in typical electrolytes of sulfuric acid, oxalic acid, and phosphoric acid at different voltage. After detachment of the porous alumina films from the substrate in saturated mercuric chloride solution, the specimens were immersed in phosphoric acid to dissolve the barrier layer from the backside. By SEM observation of structural change of the cells during chemical etching, the homogeneity of the barrier layer thickness could be evaluated. Although the thickness of the barrier layer of porous alumina formed at constant voltage has been believed to be uniform, we confirmed that the thickness of the barrier layer of each cell was not uniform; the thickness of the barrier layer of the smaller cells was thinner than that of the larger cells. Concerning anion incorporation behavior, we evaluated the depth of anion incorporation into the cell wall by the difference in the strength of contrast of the SEM image of rear surface of porous alumina films because the dissolution rate of the anion-free layer was slower than that of the anion-incorporated layer. The details of the thickness inhomogeneity of the barrier layer and anion incorporation behavior will be discussed from a standpoint of the high field theory. References: [1] S. Ono, M. Saito, M. Ishiguro and H. Asoh; J.Electrochem. Soc., 151, B473 (2004). [2] S. Ono, M. Saito and H. Asoh; Electroch im. Acta, 51, 827 (2005).

Anodization of aluminum in electrolytes containing alcohols as an additive has been studied for improvement of coating properties such as corrosion and wear resistance, and hardness. The influence of the addition of alcohols on the pore arrangement in anodic alumina was also investigated. However, the essential understanding of the effect of alcohol addition is still insufficient. In this study, we used the sulfuric acid-based electrolyte containing various alcohols (e.g., methanol, ethanol, glycerol, and ethylene glycol) as an additive and investigated the effects of types and concentrations of alcohols on the efficiency for the oxide formation and the structure of the porous alumina. High purity aluminum sheets were used as a starting material and anodized at constant current of 100 A m-2 in 1.5 mol dm-3 sulfuric acid with alcohol concentration in the range of 0~50 vol%. The formation of porous alumina film on the aluminum substrate was investigated by measuring voltage transient. In the case of ethanol concentration less than 10 vol%, voltage-time curve shows the steady-state voltage of approximately 15.7 V. When ethanol concentration exceeded 20 vol%, the steady-state voltage obviously increased with increasing ethanol concentration. In sulfuric acid without ethanol addition, the surface layer of oxide film was excessively dissolved after anodization for 220 min. The film thickness was approximately 65 μm. While using sulfuric acid with 50 vol% ethanol, chemical dissolution of the outer surface was significantly suppressed and the film thickness increased from 65 μm to 70 μm at the same anodization time, i.e., the same electric charge. Even in other type of alcohols, chemical dissolution of oxide was suppressed, resulting in improvement of coulombic efficiency for the oxide formation. The variation of dissociation constant of the electrolyte by addition of alcohols might affect the chemical dissolution and formation efficiency of anodic film. K. Kuroda, M. Ozawa, J. Met. Finish. Soc. Jpn., 15, 3 (1965). Y. Li, M. Zheng, L. Ma, W. Shen, Nanotechnology, 17, 5101 (2006).

Porous-type anodic aluminum oxide films have attracted considerable attention as a key material in the fabrication of several types of devices owing to their potential technological applications. However, conventional anodic alumina films have low chemical resistance because the anodic oxide films obtained by typical anodizing processes are amorphous. Expanding the application of alumina films necessitates enhancement of the chemical resistance of alumina. Previously, we identified the optimized anodizing conditions and the detachment method for suppressing thermal deformation, such as curving and cracking, during heat treatment of the alumina membrane (1-4). In our previous study, anodizing was conducted under an applied voltage of 25 V–185 V, which resulted in the formation of an anodic porous alumina film with pore diameters tunable over a wide range of approximately 30-350 nm. When oxalic acid was used as an electrolyte, the fabrication of a crack-free and flat α-alumina membrane with a diameter of 25 mm, a thickness of 50 μm and a pore diameter of approximately 60 nm was fabricated using the optimized anodizing conditions in oxalic acid at 40 V, followed by subsequent detachment from the substrate and heat treatment (2). In the case of phosphoric acid, the fabrication of α-alumina membrane with a pore diameter of 350 nm was achieved through optimization of various conditions (3). In the present study, the chemical composition and morphological changes of the alumina membrane formed in phosphoric acid during heat treatment were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA). The results of analyses clearly showed that the alumina membrane formed in phosphoric acid has unique characteristics such as coexistence of crystalline alumina and aluminum phosphate at high temperature region above around 1100 oC. Therefore, we focused on the effect of heat treatment conditions (e.g., temperature, heating rate and holding time) on the morphological changes of alumina cell wall in comparison with alumina films formed in sulfuric acid and oxalic acid. By controlling of heating conditions, alumina membrane with hierarchically porous structures consisted of main macropores and smaller textural mesopores in the cell walls could be fabricated. Through-hole structure of heated alumina membrane was maintained even at high temperature, and additional mesopores were formed by the selective removal of aluminum phosphate from alumina cell wall. [1] F. Rashidi, T. Masuda, H. Asoh, S. Ono, Surface and Interface Analysis, 45, 1490-1496 (2013) [2] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014) [3] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015) [4] H. Asoh, T. Masuda, S. Ono, ECS Transactions, 69, 225-233 (2015)

In general, industrial anodization based on direct current or alternating current (AC) electrolysis can be applied to the surface treatment of large-sized and long aluminum products such as building materials. However, conventional anodization is unsuitable for the simultaneous surface treatment of a large number of small components including fine particles because direct conduction between the aluminum anode and the cathode is essential for completing the electrical circuit. Recently, we proposed new surface treatment method, i.e., indirect oxidation and investigated the influences of electrolysis conditions such as voltage, frequency, reaction time, concentration of electrolyte, and temperature on the growth of porous oxide film [1]. In our previous study, high-purity (99.99 %) aluminum sheets with a working area of 10 cm2 and aluminum particles (99.5 %) of 3 mm diameter were used as processing objects. In the case of AC electrolysis in 8 mmol dm−3 oxalic acid at 20 °C, 60 VM, and 150 Hz, which were the base condition in this study, the current density was approximately 100 A m−2. When AC electrolysis was conducted at higher temperatures and voltages using a high concentration of oxalic acid compared with that in the base condition, a high current density in the 120–200 A m−2range was observed. Frequency had relatively small effect on current density. Even in the indirect oxidation, we observed that the relationship between the film thickness and the AC electrolysis time was approximately linear. We also used aluminum particles as a processed material to demonstrate the versatility of the indirect-oxidation-based surface treatment [1]. After indirect oxidation for 240 min under the base condition, the treated aluminum particles were immersed in dye solution. The dye was absorbed in porous alumina films over the entire surface of the aluminum particles. This result indicates that the porous films were uniformly and simultaneously formed on aluminum particles, i.e., the present technique can accommodate a wide variety of particle shapes and sizes with high throughput and low costs. In this study, we carry out a preliminary study to investigate the effect of frequency on the structure of porous alumina formed by indirect oxidation. We mainly focus on the difference between the proposed method and conventional AC anodization. The correlation between the cell dimensions and applied voltage will be also discussed. [1] H. Asoh, M. Ishino, H. Hashimoto, RSC Adv., 6, 90318 (2016).

Micro-/nanostructures formed on semiconductor substrates have been applied in various devices such as electronic devices and biosensors. In particular, III-V semiconductors (e.g., GaAs, GaN, and InP) have received a great attention for realizing future generation optoelectronic devices owing to their excellent optical and electrical properties including high electron mobility and a direct band gap. Recently, we have reported the fabrication of ordered pore arrays, pillar arrays, and nanowire arrays with a high-aspect-ratio in n-type GaAs (111) B substrate by a combination of anodic etching and subsequent chemical etching [1–3]. The controlled GaAs micro-/nanostructures have potential technological and scientific applications in optoelectronics and chemical sensors that require ordered patterns, high device densities, and high surface-to-volume ratios. In this study, we investigated the effects of etchant composition on the orientation-dependent surface morphology of GaAs during anisotropic chemical etching using various types of etchants. p-type GaAs (111) B was mainly used as a substrate. To control the reactive sites during chemical etching, the photoresist layer with the honeycomb-like openings of 3 μm periodicity was applied as a mask, as described in our previous report [1]. The GaAs substrates coated with a photoresist mask were immersed in various types of etchants consisted of acid (H2SO4, H3PO4, HF) and oxidant (KMnO4 , H2O2) at 20°C. When a mixture of 2 mol dm−3 H2SO4 and 0.05 mol dm−3 KMnO4 was used as an etchant, etch pits having curved bottom and hexagonal aperture were formed (Fig. 1a). The diameter and depth of pits were 1.7 µm and 0.6 µm, respectively. Although pore diameter increased with increasing etching time from 30 s to 150 s, the profile shape of etch pits was not clear (Fig. 1b). However, when KMnO4 concentration increased to 0.2 mol dm−3, anisotropic etching proceeded obviously, resulting in the formation of pit arrays with equilateral-triangular aperture depending on crystal plane of {100} (Fig. 1c). On the other hand, shallow etch pits with hexagonal aperture were formed by chemical etching for 30 s using 0.05 mol dm−3 H2O2 as an oxidant instead of KMnO4 (Fig. 1d). With further chemical etching for 300 s, the diameter of etch pit increased causing lateral etching (Fig. 1e). However, the obtained etch pits were shallower than that of pits formed using KMnO4 of the same concentration despite longtime etching. These results indicate that etching rates for each crystal plane are strongly affected by the type and concentration of oxidants. Correlation between the etchant composition and orientation-dependent surface morphology of GaAs after chemical etching will be discussed in detail. [1] H. Asoh, S. Kotaka and S. Ono, Electrochem. Commun., 13, 458–461 (2011). [2] S. Ono, S. Kotaka and H. Asoh, Electrochim. Acta, 110, 393–401 (2013). [3] H. Asoh, S. Kotaka and S. Ono, Mater. Res. Express, 1 045002 (2014). Figure 1

Porous anodic oxide film, which is formed by anodizing of aluminum, is a typical self-ordered nanoporous material. Over the past several decades, studies have been carried out on the protection or design of aluminum surfaces for commercial applications. Recently, anodic alumina films have attracted considerable attention as a key material due to their potential technological applications in area such as filters, catalyst supports, biological applications, electronic, magnetic and optical devices. However, commercially available anodic porous alumina membranes have insufficient pore arrangement and low chemical resistance because the porous alumina films obtained by typical anodizing processes are amorphous. To expand the application of anodic porous alumina with ordered pore arrays, it is necessary to control the cell dimensions (e.g., pore interval, pore diameter, and pore depth) and to enhance the chemical resistance of alumina membranes in extreme environments such as high temperature, high vapor pressure, and high-concentration acid/base solutions. Several fundamental studies have been reported on the fabrication of crystalline anodic porous alumina aimed at improving the chemical properties of α-alumina membranes using a combination of anodizing and heat treatment processes. Since the 2000s, the crystallization of free-standing alumina membranes to the α-phase has also been attempted; however, curling and cracking as a result of thermal deformation caused by the change in density and decomposition and desorption of electrolyte anions from the outer anion-incorporated layer in the cell wall were unavoidable during the transition to crystalline alumina in addition to the desorption of bound water at lower temperatures. In this study, we investigated the optimum conditions for the fabrication of α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm by the anodizing of aluminum and subsequent heat treatment. The morphological and structural change of the porous alumina membrane during heat treatment was also evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), and thermogravimetry-mass spectrometry (TG-MS). Because the pore diameter could be adjusted by changing the anodizing conditions including the electrolyte species and formation voltage, anodizing of high-purity aluminum sheets was conducted in sulfuric acid, oxalic acid and phosphoric acid at each specific voltage. Here, we applied two-step anodizing process to improve the regularity of the pore arrangement of the anodic film. As a result, the deformation of alumina membrane such as warp and cracking during heat treatment for crystallization was effectively suppressed [1, 2]. By optimizing the conditions for anodizing, subsequent detachment, and heat treatment, nanoporous and single phase α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm were successfully fabricated. Even in the case of anodic porous film with short pore interval of approximately 60 nm, which was formed at 25 V in sulfuric acid, α-alumina membrane that maintained straight channels was obtained. The α-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure [2]. These results open a new route to technological and scientific applications. [1] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014) [2] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015)

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is used as a starting material of nanofabrication of functional devices besides a corrosion resistant surface oxide. In contrast to anodic titania nanotubes, anodic porous alumina is generally compact without spacing between the cell boundary. The tubular shape of anodic titania is suggested to be resulted in enrichment of fluoride between cells which is originated from electrolyte and more soluble than anodic titania. In the case of aluminum, it is reported that enrichment of only sulfate or ethylene glycol from electrolytes produces alumina nanotubes by anodization. In the present paper, the formation mechanisms of alumina nanotubes by anodization of Al cast alloy caused except the electrolyte origin will be discussed. STEM image of vertical cross section prepared by ion-milling of the anodic film, which was formed on 99.99 % pure aluminum in oxalic acid at 160V, indicated close-packed hexagonal cell arrays with small voids of 15-20 nm at the triple cell points. Dark field STEM image clearly revealed a bright particle in the each void suggesting the presence of a heavy metal because of Z-contrast. By EDX point analysis, Cu enriched particles were found in the voids at the triple cell points. Nevertheless, in the case of anodic film formed at the similar condition on AC8A cast alloy including 0.97 % of Cu, tubular cells with the space of approximately 50 nm between cell boundary were formed unlike the case of pure aluminum including 60 ppm of Cu. Numerous branching of pores were also observed. EDX mapping revealed that Si was enriched in the outer layer of the cells and Cu was enriched at the area near the cell boundaries. Therefore, it is deduced that enrichment of Cu at the cell boundaries produces the tubular cells by accelerating oxide dissolution in addition to the electric breakdown accompanied by severe gas evolution that resulted in void formation at the triple cell points.

Wrinkle structures are often observed at the surface and/or interface of materials and have a role for gaining surface area. Recently, we have found that wrinkle structure at the adhesive interface of gels strengthen the adhesion strength. Then we hypothesize that adhesion can be controlled by formation and deformation of wrinkle structures. We fabricated a stimuli-responsive wrinkling films by using thremoresponsive polymers. Adhesion and detachment of hydrogels can be regulated by formation/deformation of wrinkle structure in response to temperatures.

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is a promising candidate for starting materials of nanofabrication of various devices. Investigations concerning the cell dimensions of porous anodic alumina films have been extensively performed after the confirmation of the classical cell model proposed by Keller et al (1). It was suggested in these studies that cell dimensions, such as pore and cell diameters and barrier layer thickness, primarily depended on the formation voltage, and that they increased linearly with increasing voltage (2). Nevertheless, the importance of current density as a controlling factor of the cell geometry was sometime claimed (3, 4). According to the classical theory of ionic conduction at the high field strength for the anodic barrier film grown on various metals (5, 6), the film thickness of each metal is inversely proportional to the logarithm of ionic current when the film is formed up to the same voltage. Thus, it is indicated that the log of current density I is proportional to the electric field strength E, i.e., the formation voltage / film thickness ratio at the barrier layer, as we reported previously (7). Concerning the anion incorporation into the film, we found that anion content was also proportional to the electric field strength E, i.e., the log of current density i (7). This principal is suggested to be applicable to porous anodic films. The purpose of the present study is the confirmation of the effect of electric field strength as a controlling factor of the cell morphology of porous anodic alumina such as pore diameter / cell diameter ratio that affects to the radius of curvature of cell base pattern and porosity. In addition, effect of electric field strength on incorporation behavior of electrolyte anions into cell wall has been studied. Obtained knowledge could help us to get further insight into self-ordering mechanism of anodic porous alumina. Although pore and cell sizes of the films formed in oxalic acid were larger than those formed in sulfuric acid even at the same voltages, the ratio of cell diameter to pore diameter (dcell / d pore) of both films is in a linear relation with the logarithm of current density (log i). The electric field strength across the barrier layer of porous films varies linearly with the logarithm of current density as in the case of barrier type films. Therefore, dcell / d pore ratio is regarded to be proportional to the electric field strength. From these results, it is deduced that pore diameter is proportional to voltage and inversely propor­tional to the square of the electric field strength, whereas barrier layer thickness and cell diameter are proportional to voltage and inversely proportional to the electric field strength. Accordingly, pore diameter is more strongly affected by current density than other cell parameters are. The content of incorporated anion in the films increases linearly with increasing log i, hence electric field strength. Therefore, it is evident that the anion incorporation is exactly governed by the electric field strength. [1] F.Keller, M.S.Hunter and D.L.Robinson, This Journal, 100, 411 (1953). [2] J.P.O'Sullivan and G.C.Wood, Proc. Roy. Soc. Lond. A.317, 511 (1970). [3] K.V.Heber, Electrochim. Acta, 23, 127 (1978). [4] S. Ono, M. Saito, M. Ishiguro and H. Asoh, J. Electrochem. Soc ., 151, B473 (2004). [5] N. Cabrera and N. F. Mott, Ret. Pror. Phys., 12, 163 (1948). [6] C. J. Dell’Oca and L. Young, J. Electrochem. Soc., 117, 1548 (1970). [7] S. Ono, F. Mizutani, M. Ue, and N. Masuko, PV 2001-22, p.1129, The Electrochemical Society Proceedings Series, Pennington, NJ (2001).

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is used as a starting material of nanofabrication of functional devices besides a corrosion resistant surface oxide. In contrast to anodic titania nanotubes, anodic porous alumina is generally compact without spacing between the cell boundary. The tubular shape of anodic titania is resulted in enrichment of fluoride between cells which is originated form electrolyte and more soluble than anodic titania. In the case of aluminum, it is reported that enrichment of only sulfate or ethylene glycol originated form electrolytes produces alumina nanotubes by anodization. In the present paper, the formation mechanism of anodic alumina nanotubes will be discussed. STEM image of vertical cross section prepared by ion-milling of the anodic film, which was formed on 99.99 % pure aluminum in oxalic acid at 160V, indicated close-packed hexagonal cell arrays with small voids of 15-20 nm at the triple cell points. Dark field STEM image clearly revealed a bright particle in the each void suggesting the presence of a heavy metal because of Z-contrast. By EDX point analysis, Cu enriched particles were found in the voids at the triple points. Nevertheless, in the case of anodic film formed at the similar condition on AC8A cast alloy including 0.97 % of Cu, tubular cells with the space of approximately 50 nm between cell boundary were formed unlike the case of pure aluminum including 60 ppm of Cu. Numerous branching of pores were also observed. EDX mapping revealed that Si was enriched in the outer layer of the cells and Cu was enriched at the area near cell boundaries. Therefore, it is deduced that enrichment of Cu at the cell boundaries produces the tubular cells by accelerating oxide dissolution similar to the cases of enrichment of sulfate and ethylene glycol.

Porous anodic oxide films formed on various metals, especially anodic alumina, have attracted huge attention as a starting material in the fabrication of nano-devices because of the high controllability of porous structures in nanometer scale such as pore size, pore interval, as well as pore depth by changing electrolysis condition in addition to their high regularity of pore arrangement. In recent years, mechanism of self-ordering of anodic alumina films has been intensely studied. We proposed anodizing under high electric field strength as a key factor for self-ordering of pore arrangement (1). However, only a few reports are found on the mechanism of irregularity or defects formation of porous anodic oxide films. For example, an regular deposition of metals into all anodic pores are often difficult because of irregularity of the barrier layer. In the case of anodic films formed on aluminum in chromic acid solution, for example, sever pore blanching appears especially at the higher voltage. Anodic alumina films formed in phosphoric acid and Ematal bath also indicates numerous pore branching and caved pores in the cell wall (2, 3) where anodizing conditions such as high formation voltage and relatively low dissolution ability of electrolytes to anodic oxides are used. These irregularities were caused by defects/holes induced by electric breakdown between pore bottom and protrusion at triple points of cell boundary. Impurity elements in metals and alloys also bring defects and irregularity into porous structures. Furthermore, anodic films formed at some specific condition, especially on titanium, yielded tubular structure having interspaces at cell boundaries. All of these irregularities should be created only at the barrier layer grown under the anodic field by specific reactions other than stable anodic oxide growth. This inhomogeneity of barrier layer such as unevenness of the thickness and voids that induce irregularity in porous oxide seems to be playing an important role at an application for industry both in a positive and negative way. In this paper, we report the specific futures of irregularity appeared in porous anodic oxides films formed on aluminum and evaluated to get unified formation mechanism of such imperfections to be induced in the oxides grown on various substrates. [1] S. Ono, M. Saito, M. Ishiguro and H. Asoh, J.Electrochem. Soc., 151, B473 (2004). [2] S. Ono, H.Ichinose and N. Masuko, J.Electrochem.Soc., 138, 3705 (1991) [3] S. Ono and N. Masuko, J. Jpn Inst. Light Metal, 43, 453 (1993).

Anodic porous alumina film, which is formed by anodizing of aluminum in an appropriate acidic solution, has been studied on the protection or design of aluminum surfaces owing to commercial applications. The appearance and surface quality of aluminum products (e.g., architectural materials, electronic devices) are key points as with surface properties such as a corrosion resistance, hardness and wear resistance. Although dyeing, painting and electrolytic coloring are well known as industrial methods to provide color variation for aluminum surface, integral coloring, which produces color by anodizing step itself without use any pigment or metallic salt, is also applied in terms of the simplicity and production cost. In our previous work, it was reported that alumina films with irregular porous structure formed in Ematal bath or chromic acid showed opaque white appearance [1, 2]. Irregular porous structure in the direction of pore depth, which contained branching, curving and distortion of pores in anodic alumina, produced the roughening of the metal-oxide interface and made the appearance of the film more opaque [1, 2]. However, Ematal bath containing expensive potassium titanium oxalate deteriorates easily. In terms of reduction of environmental burdens, it is required to avoid using harmful chemical compound such as chromic acid. Under such background, we studied a low-cost alternative anodizing method to form opaque white oxide film. In addition, we also investigated in detail the relation between the appearance and porous structure of oxide film. Anodizing was conducted using high purity (99.99 %) aluminum sheets in various electrolytes (phosphoric acid, sulfuric acid, critic acid, and Ematal bath). Whiteness of the obtained samples was evaluated by spectrophotometry (KONICA MINOLTA, CM-5) based on standard of American Society for Testing Materials (ASTM E313-73). When anodizing was carried out in phosphoric acid at 60 °C at a constant current density of 200 Am−2, anodized specimen indicated whiteness value of 39. Surface appearance was gray in color. The aluminum specimen anodized in Ematal bath showed whitish appearance compared with that treated in phosphoric acid. Whiteness value of aluminum specimens anodized in Ematal bath increased from 35 to 47 with increasing final arrival voltage from 150 V to 350 V during constant current anodizing. On the other hand, whiteness value increased effectively using two-step anodizing; long first anodizing and short second anodizing were conducted in phosphoric acid and subsequently Ematal bath. Under optimized conditions, whiteness value increased to 58. The more increase of the thickness of total oxide film and barrier layer and the degree of branching, the more white appearance indicated. Thus, it is confirmed that the appearance of anodized specimen was strongly affected by these factors. [1] S. Ono, S. Chiaki, T. Sato; J. Met. Finish. Soc. Jpn., 26, (10), 456-460 (1975). [2] S. Ono, T. Sato; J. Met. Finish. Soc. Jpn., 31, (3), 134-139 (1980).

Wettability is one of the important properties of solid surfaces from both fundamental and practical aspects. Among various factors, surface energy and roughness are the dominant factors for the wettability. Static wettability, that is, water contact angle (WCA) has been commonly used as a criterion for the evaluation of hydrophobicity of solid surface. In recent years, WCA can be designed easily by controlling surface energy and roughness based on Young’s, Wenzel’s, and Cassie’s equations. On the other hand, dynamic wettabilities such as sliding property on the hydrophobic surface are poorly undersutood. Although water sliding angle (WSA: the critical angle where a water droplet with a certain weight begins to slide down the inclined plate) and contact angle hysteresis (the cosine of receding contact angle minus that of advancing contact angle) are often measured to evaluate the water-repellent property of the hydrophobic surface, comprehensive understanding of the water-repellent properties is still limited because the relationships between the WSA and the WCA are not clear [1-2]. In this study, we used nanoporous silicon surfaces with different surface morphologies, which were formed by metal-assisted chemical etching using Ag nanoparticles, as a solid surface. Static and dynamic wettabilities of their surfaces were investigated with focusing on the influences of pore size and surface roughness on wettability of porous silicon surfaces. p-type silicon (100) substrates were immersed in a mixed solution of 2×10-2 mol dm-3 AgNO3 and 5 mol dm-3 HF to deposit Ag particles on the entire surface of silicon substrates. After deposition of Ag particles, silicon substrates were immersed in a mixed solution of 10 mol dm-3 HF and 1 mol dm-3 H2O2at room temperature at different time. SEM images of obtained porous silicon after etching with different surface roughness are shown in Fig. 1. WCAs (approximately 135°) of etched silicon were higher than that of flat silicon (75°), presumably due to decrease of contact area between porous structure and droplet based on Cassie’s model. Although surface roughness increased with increasing etching time, however, WCAs of etched silicon were about the same (135°) despite the difference in surface morphologies (Fig. 1a-c). On the other hand, WSAs increased with increasing surface roughness. The changes in WSAs were thought to be caused by the pinning effect generated by increase of pore diameter and surface roughness during chemical etching. These results indicate that WSA is affected more significantly by physical inhomogeneity such as surface roughness of the etched surface than WCA. [1] M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir, 16, 5754-5760 (2000). [2] S. Suzuki, A. Nakajima, Y. Kameshima, K. Okada, Surface Science Letters, 557, L163-L168 (2004). Figure 1

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Microarrays of n-GaAs were fabricated for both (100) and (111) by colloidal crystal templating, ion sputtering and chemical etching using nanosized Au particles as the etching catalyst. Since self-organized polystyrene spheres were used as a mask, Au particles were selectively deposited at sites resulting in the formation of Au honeycomb pattern on GaAs. Microsized GaAs column arrays were achieved by chemical etching of GaAs, where the honeycomb-patterned Au metals were deposited. Although ordered column structures were obtained for both planes, different anisotropic etching patterns were observed by Au-assisted chemical etching between n-GaAs (100) and (111). The crystal-face orientation strongly affects the etching morphology and the rate of metal-assisted chemical etching.

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The fabrication of nano-/microsized columns and hole arrays on an n-GaAs substrate was carried out using a combination of colloidal crystal templating, electroless plating/two-step catalyzation, and subsequent metal-assisted chemical etching. Using self-assembled polystyrene spheres as a mask, metal particles, specifically Ag and Pd, were selectively deposited at the interspaces between the polystyrene spheres, resulting in the formation of metal honeycomb patterns on GaAs. Ordered GaAs column arrays were then fabricated by the chemical etching of the GaAs originating from the honeycomb-patterned Ag and Pd metals, which acted as etching catalysts. Each metal catalyst resulted in the formation of a characteristic etched structure and etching depth. Pd-etched column arrays exhibited conical structures, while Ag-etched specimens showed slender column structures owing to the anisotropic etching of the GaAs. Moreover, patterned-GaAs slender grooves exhibiting triangular cross sections were fabricated by Ag-assisted chemical etching. To obtain the groove structures, honeycomb-structural polystyrene spheres were used as an etching mask. After the chemical etching of the GaAs, patterned slender grooves were formed.