Anodic alumina membranes (AAMs) with ordered porous structure have been investigated as possible ionic conducting membranes for thin film (= 50μm) fuel cells (TFFC) and as templates for the production of a variety of nanostructure arrays. The aims of this work are to present some of our recent results pertaining to the functionalisation of AAM with proton conductors for the production of membranes to be used in hydrogen/oxygen TFFC, at low or intermediate temperature (25°C ≦ T ≦ 250°C), and to the fabrication by template technique of nanowires/nanotubes arrays of oxide and hydroxides for possible future technological application.
An attempt was made to tailor porous aluminum films with an isolated columnar structure by magnetron sputtering for potential application to medium-voltage and high-voltage electrolytic capacitors. The aluminum film was deposited at a relatively high Ar pressure of 3.3 Pa on rough aluminum substrate with a cellular texture. Shadowing effects, which were enhanced by the random incident angle of aluminum atoms at the high Ar pressure and the use of a rough substrate, produced the isolated columnar structure of the deposited film. However, gaps separating neighboring columns were not high enough to maintain high surface roughness after anodic oxide formation, due to the large Pilling-Bedworth ratio for the Al/Al2O3 system. The gaps became filled with anodic oxide during anodic film formation. Slight alkaline etching of the deposited film the increased the gaps such that high surface area was maintained to high formation voltages.
The formation of a self assembled monolayer (SAM) on anodic titanium dioxide was monitored by impedance spectroscopy and compared to results from X-ray photoelectron spectroscopy (XPS). Results provide information on the formation kinetics of an n-octadecylphosphonic acid monolayer and its stability against UV-irradiation. The results reveal typical adsorbtion characteristics of the monolayer depending on the immersion time and the immersion concentration, respectively. The XPS results show a strong stability of the bond between the phosphonic acid anchor group and the TiO2 surface under UV-irradiation. The phosphate rest remains on the surface while the carbonic rest of the molecule is decomposed.
Dielectric barrier discharge (DBD), properly combined with porous ceramics and/or catalysts, is a promising technology for diesel exhaust gas treatment. In this study, anodic porous alumina, which has numerous pores with nanometer-order diameter and a high aspect ratio, has been examined for use as a barrier in a DBD reactor. The anodic porous alumina greatly improves the DBD reactor. Nevertheless, few reports describe DBD using the anodic porous alumina barrier. As described herein, the DBD reactor’s basic characteristics and NOx removal in atmospheric pressure air were studied to support its future development. Results were compared with those of a typical DBD to clarify the influence of anodic porous alumina on those characteristics. The electropolished 99.99% pure Al substrate was anodized in 1.0 M sulfuric acid of 10°C for 5 h at dc 24 V. The Al substrate and anodic porous alumina respectively act as the electrode and dielectric barrier of the DBD. The dry gas mixture of NO (several hundred ppm) / O2 (20%) / N2 was supplied into the DBD reactor at 1.0 L/min. The resultant voltage and current waveforms resembled those of a typical DBD. Results revealed that NO was mostly oxidized to NO2 ; about 20 ppm N2O was detected. Different from a typical DBD the NO2 increased characteristically as a function of operation time.
The impedance properties of Ta capacitors fabricated with the conductive polymer poly-3,4-ethylenedioxythiophene (PEDOT), which was polymerized photoelectrochemically (e-PEDOT) or chemically (c-PEDOT) using the oxidizer Fe(III) p-toluenesulfonate, were evaluated. The photoelectrochemically prepared capacitor exhibited higher capacitance than the chemically prepared one, which might be explained by the relative absence of insulating materials near the oxide/e-PEDOT interface compared with the c-PEDOT interface. Moreover, both chemically and photoelectrochemically prepared Ta/PEDOT capacitors have more ideal capacitor characteristics than Ta/liquid electrolyte capacitors do: the real impedance component of both solid-state capacitors is substantially less than that of the Ta/liquid electrolyte capacitors. For c-PEDOT, our data suggest that the real component of the oxide itself was reduced because the Ta anodic oxide film was subjected to heat treatment during chemical polymerization, thereby reducing the real component of the entire Ta/c-PEDOT system. A discrepancy was found in the case of e-PEDOT: the real component of the entire system was smaller, although the real component of the oxide itself was markedly greater during the photoelectrochemical process.
A porous anodic oxide film formed on Al was developed as a new pH-sensitive film. A porous anodic oxide film was immersed in an aqueous solution containing a pH-indicator reagent and boiled to seal it in the film. Litmus, bromthymol blue (BTB), and Congo red were used as pH-indicator reagents. The film, containing litmus, changed color reversibly within 1 s as a function of the test solution pH. The film showed superior durability in the solution at pH of 1−12 because of a stable boehmite layer covering the surface within that range. From the response of color change as a function of sealing time, it was inferred that the color change occurred mostly at the film’s surface region. For example, the film containing BTB prepared with long sealing time appeared to be green in an alkaline solution because of mixing of blue at the surface and yellow inside the film. Optimization of film thickness, the choice of pH-indicator reagent, and sealing conditions are important to prepare a highly pH-sensitive film.
Organic hydride is anticipated for use as a future hydrogen carrier because it has both high gravimetric and volumetric hydrogen density for storage and transportation. Improving the performance of the dehydrogenation catalyst is necessary to miniaturize the reactor, which generates H2 from organic hydride in an endothermic reaction. The porous anodic oxide film on aluminum is widely known for its application to dehydrogenation catalysts. Increasing the specific surface area of a porous anodic oxide film improves the catalytic performance. For this study, we obtained a catalyst with high specific surface area by investigating the anodic oxidation temperature and boehmite process time. The specific surface area of a porous anodic oxide film produced at higher temperature was larger than that at a lower temperature. It increased with boehmite processing time. However, when the specific surface area became too large, the micropores were too small for Pt particles to enter. Consequently, at 50°C anodic oxidation temperature and at about 5 h boehmite processing time, the catalyst had the most suitable specific surface area to generate H2 from methylcyclohexane (MCH) because the smallest Pt particles were supported on the porous anodic oxide film. Furthermore, a microreactor using the proposed catalyst had twice the MCH conversion as that using a conventional catalyst.
Dielectric properties of anodic oxide films formed on niobium in oxalic acid or succinic acid solution with addition of ammonium hydroxide were investigated with particular attention to the incorporated electrolyte species. The films’ capacitance increased considerably when the electrolyte pH was adjusted to more than 8 and the nitrogen content in the films increased concomitantly with increasing electrolyte pH. The anodic film formed in the oxalic acid solution showed the highest capacitance and the relative permittivity of the film was assumed to be 256. Impedance measurements indicated a monolayer in films formed in acidic electrolytes, whereas films formed in alkaline electrolytes were composed of two layers having different resistance. The outer layer thickness, accounting for approximately 65% of the film, was related to that of the layer containing nitrogen. Therefore, nitrogen incorporation was inferred as a cause of double layers in films formed in alkaline electrolytes, as reported in the case of alkaline phosphoric acid. The leakage current of the films was independent of the electrolyte species when measurements were conducted in the dark. However, under the room lighting, the leakage current of films formed in an electrolyte of pH 10 was approximately 10 times higher than that of films formed in acidic electrolytes. The photoreactivity is attributed to the semiconductor property of a niobium oxide film induced by nitrogen incorporation.
Formation of anodic porous zinc oxide (ZnO) films with high specific surface area was investigated in various electrolytes such as oxalic acid, ammonium borate, and sodium hydroxide solutions. Composition of the anodic film formed in oxalic acid, which was composed of an outer rock-like structure and an inner granular structure, was mainly Zn(OH)2 containing a small amount of ZnO. In contract, crystalline porous ZnO film with a straight cellular structure was formed in a sodium hydroxide solution. At low anodizing voltage of 9 V, formation of thicker films of more than 30 μm was achieved through suppression of extensive gas evolution. Photocatalytic activity of the ZnO films assessed using photodegradation of methylene blue under UV irradiation was comparable with that of TiO2 nanoparticle film (P25).
Palladium-activation-based techniques for area-selective microscale metallization on porous oxide film of aluminum and complete metallization of the oxide/ hydroxide surface of aluminum are demonstrated. For area-selective metallization, a porous anodic oxide film of aluminum was colored in an organic dye solution containing small amounts of palladium acetate. It was then hydrothermally sealed, thereby trapping the palladium acetate underneath the newly formed crystalline hydroxide layer. Removal of the surface hydroxide layer using a laser beam, exposes the palladium acetate, which also undergoes photothermal decomposition and reduction to metallic palladium and acts as a catalytic center for subsequent electroless plating. Similarly, complete metallization of the porous oxide and hydroxide of aluminum were performed by adsorbing palladium acetate onto the surface, followed by electroless nickel plating of the specimen. Here, palladium acetate (on the surface) was first reduced to metallic palladium by the hypophosphite that is present in electroless plating solution, providing a catalytic center subsequent electroless nickel deposition.