Cobalt and Nickel Nanopillars on Aluminium Substrates by Direct Current Electrodeposition Process
© to the authors 2009
Received: 26 March 2009
Accepted: 14 May 2009
Published: 31 May 2009
A fast and cost-effective technique is applied for fabricating cobalt and nickel nanopillars on aluminium substrates. By applying an electrochemical process, the aluminium oxide barrier layer is removed from the pore bottom tips of nanoporous anodic alumina templates. So, cobalt and nickel nanopillars are fabricated into these templates by DC electrodeposition. The resulting nanostructure remains on the aluminium substrate. In this way, this method could be used to fabricate a wide range of nanostructures which could be integrated in new nanodevices.
KeywordsNanoporous anodic alumina membranes Transfer mask Metallic nanopillars Electrodeposition
The template synthesis of nanostructures has attracted scientists’ attention in the last years owing to their possible application in fabricating high-density magnetic storage memories  and nanoelectrodes for electrochemical processes in nanometric range . In addition, this kind of nanostructures could be integrated in smaller and smaller devices such as filters  or sensors . In terms of nanostructure fabrication, choosing a suitable template is one of the most crucial factors in the synthesis process, because any defect in the template structure could be transferred to the resulting nanostructure by replication. So far, several materials have been used as template for synthesizing nanowires or nanotubes. Nanoporous anodic alumina membranes (NAAMs) have become widely used for the following reasons: first, in contrast to other membranes as polycarbonate membranes, NAAMs present a higher pore density and a narrower diameter pore distribution . Secondly, both the pore diameter and their interpore distance are rather controllable, because they can be adjusted by varying the anodization voltage or changing the electrolyte . Thirdly, by means of a two-step anodization process , we can fabricate NAAMs with a self-ordered hexagonally and periodic pore arrangement in a more inexpensive way than with other methods like electron beam lithography . Recently, electrochemical deposition from an electrolyte has been used [9, 10], since it is a fast and well-controlled way of fabricating nanowires and nanotubes by filling porous templates. Nonetheless, as-produced NAAMS have certain disadvantages to be used as template when an electrochemical deposition is desirable. The main disadvantage is that there is an aluminium oxide (Al2O3) barrier layer between the pore bottom and the aluminium (Al) substrate. This barrier layer electrically isolates the metallic aluminium substrate from the inner side of the pores. For this reason, when an electrodeposition of a metallic or semiconducting material is carried out by direct current (DC) in an as-produced NAAM, it is rather unstable and there is no uniform filling of the pores. Moreover, high electrodeposition potentials are needed for tunnelling the electrons throughout the oxide barrier layer of the pore bottom. Other deposition techniques like electroless deposition , chemical vapour deposition (CVD)  or sol–gel  can avoid this drawback, since the growth of nanowires or nanotubes does not start at the pore tips, but from the pore walls. So far, several methods have been developed for carrying an electrochemical deposition using NAAMs as template. The most commonly used are two. In the first one [9, 10], the nanoporous alumina membrane must be detached from the aluminium substrate by the dissolution of the Aluminium in a saturated solution of cupric chloride and hydrochloric acid (HCl·CuCl2)  or in a saturated solution of mercury (II) chloride (HgCl2) . Subsequently, the aluminium oxide barrier layer is removed from the pore bottoms by a chemical etching process in a solution of phosphoric acid (H3PO4). Finally, an electrical contact is sputtered on one side of the free-standing NAAM. The second one is the pulsed electrodeposition (PED) method , in which the NAAM remains on the aluminium substrate. By means of this method, magnetic nanowire arrays of nickel and cobalt have been fabricated [16, 17]. Nevertheless, only free-standing metallic nanowires can be fabricated using this method.
In this work, we present an innovative method for fabricating cobalt (Co) and nickel (Ni) nanopillars (NPs) on aluminium substrates. In contrast to previous works [16, 17], the metallic nanowires remain on the aluminium substrate after removing the alumina template. Recently, we have used a technique, previously developed by ourselves, for dissolving in situ the aluminium oxide barrier layer on the pore bottom tips of NAAMs . We describe the experimental procedure to fabricate Co and Ni nanopillars as follows: first, we explain the technique used to achieve the aluminium oxide barrier layer dissolution. Secondly, we describe the DC electrochemical deposition process. Thirdly, we show and discuss the results of the template synthesis method presented, and finally we present our conclusions.
Fabrication of Nanoporous Anodic Alumina Membrane
Hexagonally ordered home-made NAAMs were prepared using direct anodization of aluminium substrates, which is described in detail somewhere else [19, 20]. First, commercial aluminium substrates (high-purity aluminium [99.999%] foils from Goodfellow Cambridge Ltd) were pre-treated. The aluminium foils were annealed in nitrogen (N2) environment at 400 °C for 3 h. In this way, both their crystalline phase and grain size were homogenized. Subsequently, samples were electropolished in a mixture of ethanol (EtOH) and perchloric acid (HClO4) 4:1 (v:v) to reduce their surface roughness. Finally, the samples were washed with deionized water, dried under a draught and stored in a dry environment to prevent the formation of oxide thin films because of environmental humidity. Once the aluminium foils were pre-treated, the anodization process was carried out following an innovative electrochemical approach for dissolving in situ the aluminium oxide barrier layer on the pore bottom tips of the NAAMs . The two-step hard anodization (HA) procedure was performed on the aluminium surface using an oxalic acid (H2C2O4) solution (0.3 M) at 0 °C in order to prevent the oxide film burning by catastrophic electric current flow. The first stage of the anodization process was started under constant voltage at 40 V for 5 min. So, a protective thin layer about 0.5 μm thick was formed on the aluminium surface. This layer suppresses breakdown effects due to high temperature and enables uniform oxide film growth at high voltage. Subsequently, the voltage was slowly increased to the HA anodization voltage (120 V) at a constant rate of 0.8 V s−1. The voltage was then maintained constant for 20 min in order to achieve a suitable hexagonal arrangement of the pores. When the first anodization stage finished, the aluminium oxide film was removed from the aluminium substrate by wet chemical etching in a mixture of phosphoric acid (H3PO4) (0.4 M) and chromic acid (H2Cr2O7) (0.2 M) at 70 °C during the same time of the first anodization stage (about 30 min). In this way, we produced a pre-pattern on aluminium surface. Afterwards, the second stage of the anodization process consisted of directly applying an anodizing voltage of 120 V in the same electrolyte in which the first stage was carried out. The anodization voltage was maintained until the desired pore depth had been reached (around 10 min). Previous studies have found that the rate of film growth is nonlinear , being approximately between 50 and 70 μm h−1. The third stage of the anodization process is initiated, applying a stepwise current-limited re-anodization procedure under a galvanostatic regime in the same electrolyte. In this way, the aluminium oxide barrier layer of the pore bottom tips of the NAAMs was penetrated. In this step, the previous value of the current density was halved, and the sample was re-anodized. Then, the voltage fell until it reached a quasi-steady value. When this almost steady state had been reached, the current density was again halved and the voltage decreased again. So, the thickness of the oxide barrier layer was reduced several tens of nanometres in each re-anodization step. By means of consecutive repetitions of this procedure, the oxide barrier layer was penetrated without the NAAM detachment from the aluminium substrate. Finally, since the aluminium oxide barrier layer is not uniform in the whole aluminium-alumina interface; the electrolyte temperature was increased to 30 °C for 30 min to uniformly remove the rest of the oxide barrier layer from the pore bottom. In this way, we made sure that the remains of the aluminium oxide barrier layer were completely removed from the pore bottom tips.
Electrodeposition of Co and Ni Nanopillars
Characteristics of the electrolyte solutions employed for Ni and Co electrodeposition
Concentration (g L−1)
The morphology and structure of the Ni and Co nanopillars were characterized by an environmental scanning electron microscope (ESEM FEI Quanta 600). Elemental qualitative analysis of prepared Ni and Co nanopillars was carried out using energy dispersive X-ray spectroscopy (EDXS) coupled with the ESEM equipment. The crystal phases of Ni NPs were analysed by μ-XRD measurements, which were made using a Bruker-AXS D8-Discover diffractometer, and the crystal phases of Co NPs were analysed by conventional XRD measurements, which were made using a Siemens D5000 diffractometer.
Results and Discussion
In summary, we have reported a simple and innovative electrochemical approach to fabricate cobalt and nickel nanopillar arrays on aluminium substrates. This technique improves other methods previously proposed, because the number of stages in the fabrication process is smaller. For this reason, it is faster and more cost-effective than previous works. This advantage is due mainly to the fact that the removal of aluminium oxide from the pore bottom tips in the NAAM template takes place in the same electrolyte in which the anodization is carried out. Another main feature of this process is that the Co and Ni nanopillars remain on the aluminium substrate after removing the NAAM template. In addition, the technique presented here can be applied to NAAMs produced by both the MA and HA techniques with different acids, which opens a wide range of nanopillar morphologies. The nanopillar diameter and weight and the interpillar distance can be established beforehand by modifying the anodization parameters (anodization voltage, acid and concentration mainly).
By applying this technique with other methods for fabricating this kind of nanostructures, it is expected that the present method can be used to produce novel nanostructures such as nanotube arrays. This is a promising technique for future applications and a means for fabricating new nanodevices. One example of a future application of the resulting structure presented in this work could be using the metallic nanopillars as nanoelectrodes for the direct deposition of nanoparticles from a gas draught. This nanostructure would act as an electrostatic precipitator by applying a high-voltage field.
This work was supported by the Spanish Ministry of Education and Science (MEC) under grant number TEC2006-06531 and CONSOLIDER HOPE project CSD2007-00007.
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