Nickel deposition into ordered alumina pores

ABSTRACT

Аnodic aluminum oxide (AAO) tubular membranes were fabricated from aluminum alloy tubes in sulfuric and oxalic acid electrolytes using a two-step anodization process. The membranes were investigated for characteristics such as pore size, interpore distance and thickness by varying applied voltage and electrolyte concentration. Morphology of the membranes was examined using light optical and scanning electron microscopy and characterized using Image software. The pore sizes were ranging from 10 to 100 nm and the wall thicknesses 60 μm.

АННОТАЦИЯ

Трубчатые мембраны из анодного оксида алюминия (AAO) были изготовлены из трубок из алюминиевого сплава в серных и щавелевых электролитах с использованием двухэтапного процесса анодирования. Мембраны были исследованы на предмет таких характеристик, как размер пор, расстояние между отверстиями и толщина, путем изменения приложенного напряжения и концентрации электролита. Морфологию мембран исследовали с помощью световой оптической и сканирующей электронной микроскопии и характеризовали с помощью программного обеспечения Image. Размеры пор составляли от 10 до 100 нм, а толщина стенки 60 мкм.

Keywords: Anodic aluminum oxide (AAO), carbon nanotube (CNT), tubular membrane, thickness, anodising, film.

Ключевые слова: Анодный оксид алюминия (AAO), углеродные нанотрубки (УНT), трубчатая мембрана, толщина, анодирование, пленка.

Anodization is an electrochemical oxidation process employed to increase the thickness of the native oxide layer on the surface of metals (e.g., Al, Ti, Hf, W, Nb, Sn, Zr, etc.) or semiconductors (e.g., Si, InP, GaAs, etc.) [1]. Nanoporous anodic aluminum oxide (AAO) has become a commonly used material with potential applications in a wide range of areas, such as catalysis, electronics, photonics, and sensing. Owing to their regular structures and narrow size distributions of pore diameters and interpore spacings, porous alumina membranes are used in the fabrication of nanometer-scale composites. The nanoporous AAO sheet membrane has been investigated for a potential application in hemodialysis by measuring the hydraulic conductivity and comparing it to those of hollow fiber polymer dialysis membranes. It is known that carefully controlled anodization of aluminum in an acidic electrolyte produces a thin layer of dense aluminum oxide, followed by an ordered array of smaller-sized nanopores [2,3]. The main disadvantage of the catalysts based on nickel nanoparticles is the rapid deactivation of the catalyst due to various factors, for example, coke deposits. To prevent agglomeration and sintering, the active phase is applied to various inorganic substrates - Al₂O₃, SiO₂, and CaCO₃ [4]. The use of films, for example, Al₂O₃ as a matrix, allows combining the flexibility of the electrochemical method for obtaining catalysts and stabilizing nanoparticles in the inert matrix of porous aluminum oxide. Catalysts on substrates have several advantages, but they have low thermal conductivity. In this regard, it is relevant to create and study of nickel catalysts on metal substrates, to identify patterns of influence of synthesis conditions on the surface structure and to establish the relationship among synthesis conditions, surface structure, and such catalytic properties as an activity, stability of activity and resistance to carbonization of catalysts in the hydrogenation of olefins. Also, it is very important to have a developed surface for all metal catalysts, i.e. search the ways to increase their specific surface area.

Indeed, the establishment of their structure and nature of interaction with reagent molecules provides valuable information on the mechanisms of catalytic processes. It is the substrate that largely determines the size, shape, and the electronic state of the deposited particle [5,6]. Anodic aluminum oxide ~ AAO, which is usually prepared by the anodic oxidation of Al in an acid solution usually sulfuric H₂SO₄, oxalic H₂C₂O₄, or phosphoric H₃PO₄ acids [6], is one of the typical self-organized structures with a nanochannel array. Thus AAO has been commonly used to fabricate nanometer-sized structures via the template-mediated process because of its relatively low cost and ease of fabrication compared with conventional lithography-processed materials [7]. Recently, many researches have been focused on the nanostructured materials due to some of their significant physical properties. Although several techniques like photolithography, etching, or gas phase synthesis can produce nanowires or nanotubes, a template-assisted growing approach of nanoporous AAO is considered as one of the most prominent methods due to the advantages of a controllable diameter, high aspect ratio, and economical way in producing [8].

In our experiment a high purity aluminum foil (99.9% purity, 0.5 mm thickness) was used as a starting material. Prior to anodizing the aluminum foil was annealed at 500 °C in air for 2 h and degreased in acetone. Then the samples were electropolished at room temperature in 1:4 volume mixture of HClO₄ and ethanol at constant current density at 25 mA/cm² for 5 min. The hard anodization technique was applied using oxalic acid containing 0.3 M as electrolyte. During the anodization the electrolyte temperature of all samples was kept constant at 20-25 °C. This anodising process was done at 40 V with temperature for 120 minutes.

Table 1.

The conditions of anodic oxidation depending on the selected electrolyte

The preanodization step for 10 min was applied to produce a thin porous oxide layer (about 500 nm thick) to create a protective layer against burning at high voltages [9]. As we know in each electrolyte mixture the appropriate voltage for pre-anodization is related to the current and we choose this voltage such that the current density to be in the range of 1.5 mA/cm² < J < 2.5 mA/cm². After preanodization (in voltage lower than 40 V), the anodization voltage was increased to a final constant value by a suitable rate that is exactly related to the concentration of sulfuric acid [10]. To investigate themorphology and self ordering degree of nanopores, the SEM images were taken from the Al surface (imprint barrier layer) after selective etching of the porous alumina film by a mixture of 6 wt.% H₃PO₄ and 1.8% H₂SO₄ at the end of each process. The morphology and structure of porous aluminium oxide film obtained were characterised by Scanning electron microscope (SEM) model NVision 40-38-50 SEM.

The synthesis of nickel nanoparticles into a matrix is carried out by the method of periodic dipping with sequential drying. The aluminum oxide porous plate is alternately dipped in aqueous solutions of Ni(NO₃)₂, then it is washed in water, dried and again immersed in the solution. After a certain number of impregnation cycles, the plate is annealed in the air at 500°С. Figure 1a shows images of the surface of the film of Ni/Al₂O₃ on a scanning electron microscope. This figure represents the formation of a spongy structure with filament sizes with-in few nanometers on the surface of the plate. The as-prepared powders were calcined for 4h and then analyzed by XRD patterns are depicted in Fig. 1б.

Figure 1. SEM and XRD analysis diffraction of aluminium substrate

The equation is a simple one where an Al substrate is used with a particular acid to achieve nano-pores with an empirical derived set of electrochemical conditions.

Al + acid + voltage = nano-pores

Initially, the quality of the Al substrate, its surface structure and/or any surface pre-treatments will have a significant impact on the morphology and the resulting nano-structures formed on the substrate surface during the anodization process. To begin with, the Al substrate will have a pre-existing oxide layer over its surface, which is normally produced by the ambient oxygen in the atmosphere. In addition, the substrate could also have a pre-existing surface structure produced by a mechanical, thermal, chemical and electrochemical process. All of these surface treatments prior to anodization can have a significant impact on the self-ordering of the pore structures that form on the surface of the substrate during the anodization process.

Morphology and pore size of porous film:

Figure 2 shows the morphology of the film anodised in temperatures, from 20 °C to 25 °C respectively.

Figure 2: SEM images of aluminium oxide films formed at 20-25 °C.

The average pore diameter anodised in 20-25 °C was ~100 nm. The pores were more irregular and smaller. The effect of temperature on the formation of oxide is well-defined because it affects the rate of ion transport across the barrier layer, the oxide dissolution from the pore wall as well as the oxide surface and heat transport rates within the pore and the bulk electrolyte.

Fig. 3 shows the cross-sectional SEM images of AAO films formed under at 20-25°C. These images display the nanostructures of the local AAO films near the aluminum alloy substrates, which were taken at the same magnification.

Figure 3: SEM image of cross sectional film anodized

In summary, the nanostructured catalysts based on 3d metals (nickel) with a matrix of porous aluminum oxide was obtained. The morphological studies by SEM-EDX and XRD demonstrates a spongy structure with filament sizes of few nanometers is formed on the surface of the aluminum oxide matrix and this structure contains nanoparticles of nickel.

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