Networked three-dimensional (3D) nanoporous alumina nanostructures-consisting of vertical cylindrical stem pore arrays (170-310 nm in diameter) and periodical transverse branched pores (20-80 nm in diameter) interspaced regularly by 190-220 nm across the vertical pore walls-were controllably fabricated from low purity Al materials (99.0%, 99.3%, and 99.56%) by anodization in a phosphoric acidic solution at 110-190 V. The formation of transverse pores depended predominantly on the purity of the Al base materials and the corrosion resistance of the anodic porous alumina films, which can be mainly attributed to anodic and/or chemical dissolution of impurities such as Fe, Cu, Zn, Mg, and Mn incorporated in the Al base materials. Moreover, the effects of the purity of Al materials on the growth and morphology of porous alumina films in oxalic and sulfuric acid solutions were also investigated by a two-step anodization of Al sheets with different purities, namely 99.0%, 99.3%, 99.56%, and 99.999% Al. The alumina films grew more slowly in all electrolytes with decreasing Al purity, which can be attributed to the lower corrosion resistance of the PAA films containing small quantities of Fe oxides and/or hydroxides produced during anodization. Porous anodic alumina (PAA) films with ordered parallel nanopore arrays obtained from Al anodization [1][2][3][4][5][6] have been widely used as nanotemplates in the past decade, to fabricate various one-dimensional (1D) nanostructures such as nanowires and nanotubes for many applications, including in photocatalysts, 7-10 perpendicular magnetic recording media, 11-14 and electrode catalysts, 15,16 and for solar energy conversion. 17 However, most studies so far have used Al with a purity greater than 99.99% as starting material to produce highly ordered porous structures. From the viewpoint of practical applications, using low-purity Al or Al alloy materials to fabricate nanoporous alumina nanostructures is highly desirable in terms of lowering costs and gaining satisfied mechanical strength.Recently, much effort has been dedicated to fabricating tailored nanoporous alumina films with varying pore diameters gradually or step-wise from high-purity Al foils (e.g., 99.997% Al), by means of changing the anodizing voltages or current densities, the electrolytes, temperature [18][19][20] or by using oscillatory currents, followed by chemical etching.21 However, to the best of our knowledge, there has been no report so far on the fabrication of 3D networked nanoporous alumina films with both vertical and transverse pores, whether from high-or low-purity Al materials. Moreover, Skeldon et al. recently reported the generation of defects or branched pores on the pore walls of PAA films by anodizing a commercial Al sheet (A1050-99.5%Al) and a Al-0.05%Cu sputtered film in phosphoric electrolytes, and proposed a mechanism that the formation of branched pores was related to the Cu component in Al base materials.22,23 Zaraska et al. reported the effects of anode surface area on the oxide formati...

Conventional nanoporous anodic alumina films are composed of one-tiered pore arrays that can only render to one-dimensional (1D) nano-structures if they are used as nano-templates. Hence, three-dimensional (3D) nanoporous alumina films are desirable to dramatically increase the variety of nanostructures by template-fabrication for various potentially enhanced performances. Herein we report a facile approach to fabricate a variety of ordered multi-tiered 3D porous alumina nanostructures with multiple integer (m = 2, 3, 4, 5, 7, 9, and 10) and fractional ratios (n = 1/2 and 1/3) of the pore interval and pore diameter among different tiers on ITO/glass and Al sheets. The multi-tiered nanostructures and the pore interval ratios of the neighboring tiers can be accurately and independently controlled by a tailored multiple stepwise anodization, successively in appropriate acidic electrolytes of sulfuric, oxalic, and/or phosphoric solutions at designed voltages of 15-180 V, corresponding to the multiple or fractional ratios of pore interspacing. Moreover, the multi-tiered hierarchical nanoporous alumina films on ITO/glass substrates exhibited a lower transmittance compared with that of the single-tiered films with ordered cylindrical pores, which can be attributed to the diffusive reflection from the tier interface with different pore intervals. In past decades, porous anodic alumina (PAA) films have attracted considerable attention in both scientific and technical applications owing to the unique geometric structure with ordered hexagonal arrays of cylindrical nano-pores, similar to a nano-scaled honeycomb.1-4 In recent years, PAA nanostructures have been extensively used as a core nanotemplate to fabricate various one-dimensional (1D) functional nanostructures, such as nanowires 5-9 and nanotubes, 10-14 which can be used in a wide variety of applications including DNA and gas sensors, electro-chromic devices, light-emitting diodes, field emitters, super-capacitors, nano-electronic devices, and nano-generators. In order to facilitate the functional integration of nanostructures, template synthesis is evolving towards the fabrication of nanomaterials with increasing geometric complexity, i.e., going from two-dimensional (2D) to three-dimensional (3D) nanostructures. However, a major challenge in obtaining 3D nanostructures is the prior fabrication of ordered 3D nanoporous template with independently controllable size, spacing, position, and shape of the pore arrays as required for various applications.Many studies so far have attempted to fabricate various porous alumina nanostructures with variable pore diameters using tailored anodization methods assisted by chemical dissolution at different operating temperatures. Lee at al. reported first on the fabrication of ordered PAA films with variable pore diameters from 40 to 59 nm by combining mild anodization (MA) with hard anodization (HA) in oxalic acidic electrolytes. 15 This approach was further developed to achieve 3D PAA films with shaped pore geometries in periodic...

Three-dimensional networked porous anodic alumina (PAA) nanostructures, which consisted of perpendicular cylindrical stem pore arrays (170–310 nm) and periodical transverse branched pores in 20 – 80 nm across the pore walls, were successfully fabricated from industrial pure Al materials of Al-1100 (99%), Al–1N30 (99.3%), and Al-1050 (99.56%) by constant-potential anodizing in phosphoric acidic solutions at 110–190 V. The formation of transverse branched pores was predominantly dependent on the purity of Al materials, and irrespective of the anodizing voltages and solution concentrations. The formation of the transverse branched pores can be mainly ascribed to the anodic and/or the chemical dissolution of impurities like Cu, Fe, Zn, Mg, and Mn components included in the Al base materials, and the corrosive nature of phosphoric electrolytes driven by the high electrical field during the anodization.

Introduction. Direct ethanol fuel cells (DEFCs) have received a growing interest in recent years as promising candidates for portable power sources, electric vehicles and transportation applications owing to a high energy density of ethanol (8 kWh kg-1), less toxicity, and ease in handling and transportation. On the anode electrode, ethanol molecules are completely oxidized into CO2 involving the release of electrons and the cleavage of the C-C bond. The ORR reaction is kinetically more facile in alkaline medium than in acid medium. It has been proved that Pd possesses a higher activity than Pt for ethanol oxidation in alkaline medium and that the catalytic activity of Pd can been further increased by the addition of a second metal or metal oxide promoter. Among various Pd-based alloys or composites catalysts, the binary PdNi alloys are regarded as the most attractive electrocatalyst for ethanol oxidation in alkaline medium. This is because nickel is non-noble and low-cost metal electrocatalyst with high corrosion resistance in alkaline medium. The present study is aimed at fabricating a novel nanoporous Ni-Pd/Al2O3composite film on Al sheets by successive anodizing and electroless-plating processes, to achieve a large surface area and an enhanced activity for ethanol oxidation as anode materials for DEFCs.     Experimental. Al sheets (99.56%, 20 × 50 × 0.5 mm) were anodized in phosphoric o and citric acidic solutions at 160 and 320 V for 30 min – 2 h, to obtained nanoporous alumina films with different pores sizes and pore densities. Then, then nanoporous alumina films after pore-widening were used as templates in an electroless-plating to achieve nanoporous Ni-Pd alloy films. The electroless deposition was performed in a plating bath mainly containing NiSO4 and DMAB at 333 K for 15 – 120 sec, after active treatment by successive immersing in SnCl2 and PdCl2 solutions. Moreover, in some experiments, the nanoporous Ni-Pd/Al2O3composite films were heated at 673 K for 4 h. The morphology, chemical composition, chemical state, and crystalline structure of the specimens obtained at each step were investigated by FE-SEM, EDX, TEM, XRD, and XPS.     Various nanoporous Ni-Pd/Al2O3composite films with and without annealing were used as working electrodes ( 10×10 mm) in cyclic votamentremic tests. A common three-electrode electrochemical cell system was used for the measurements. The counter and reference electrodes were a platinum wire and an Ag/AgCl (saturated KCl, 0.199 V vs. SHE) electrode, respectively. The electrochemical measurements were carried out at ambient temperature using an electrochemical workstation (Ivium Compact stat, Hokudo Denko Co. Ltd).     Results and Discussion. Fig. 1a-b shows the surface FE-SEM images of (a) a representative nanoporous Ni-Pd/Al2O3 composite film on Al, which was fabricated by Ni electroless- plating on a nanoporous anodic alumina film formed in a phosphoric acidic solution, and (b) a flat Ni-Pd film on Al. The Ni-Pd/Al2O3composite film reserved the nanoporous structure of anodic alumina film with diameter of 150 – 200 nm and pore interval of around 350 nm. The Ni-Pd alloy film composed of nanoparticles of 50 – 150 nm, thus leading to a large surface area that is much higher than that of a flat film (Fig.1b). The nanoporous morphologies, or the pore size and pore density of porous alumina films can be readily controlled by the anodizing voltage and electrolytes, and the film thickness by anodizing time. The composition of Ni-Pd alloy films varied within 1–60 at%Pd upon the periods of active treatment and electroless deposition. According to XPS analysis, the as-deposited Ni-Pd films were covered with a thin NiO film less than 1 nm thick, due to the highly active nature of the Ni nanoparticles in air.     Figure 1c shows the cyclic voltammograms of (i, iii) a nanoporous Ni-Pd/Al2O3 composite film and (b) a flat Ni-Pd alloy film in 0.1 M NaOH solutions with (i, ii) 0.1 M ethanol and (iii) without ethanol, respectively. In case of 0.1 M NaOH solution, two small peaks appeared at 0.3 – 0.5 V, which can be ascribed to the redox reaction of NiOOH or NiO on the nanoporous Ni-Pd film. Whereas in case of 0.1 M NaOH + 0.1 M EtOH solution, two well-defined oxidation peaks, at -0.14 V in the anodic sweep curve and -0.39 V in the cathodic sweep curve, were observed, which are typically characterized as the ethanol oxidation. The net current densities of the nanoporous Ni-Pd/Al2O3 composite film and flat Ni-Pd films at -0.14 V are around 17 and 2 mA cm-2, respectively, indicating an enhanced activity on ethanol oxidation for the nanoporous films with larger surface area. Figure 1