Fabrication of cobalt-nickel binary nanowires in a highly ordered alumina template via AC electrodeposition
© Ali and Maqbool; licensee Springer. 2013
Received: 22 July 2013
Accepted: 5 August 2013
Published: 14 August 2013
Cobalt-nickel (Co-Ni) binary alloy nanowires of different compositions were co-deposited in the nanopores of highly ordered anodic aluminum oxide (AAO) templates from a single sulfate bath using alternating current (AC) electrodeposition. AC electrodeposition was accomplished without modifying or removing the barrier layer. Field emission scanning electron microscope was used to study the morphology of templates and alloy nanowires. Energy-dispersive X-ray analysis confirmed the deposition of Co-Ni alloy nanowires in the AAO templates. Average diameter of the alloy nanowires was approximately 40 nm which is equal to the diameter of nanopore. X-ray diffraction analysis showed that the alloy nanowires consisted of both hexagonal close-packed and face-centered cubic phases. Magnetic measurements showed that the easy x-axis of magnetization is parallel to the nanowires with coercivity of approximately 706 Oe. AC electrodeposition is very simple, fast, and is useful for the homogenous deposition of various secondary nanostuctured materials into the nanopores of AAO.
Porous anodic aluminum oxide (AAO) attracted a remarkable interest due to the pioneer work of Masuda and Fukuda . Self-organized nanoporous structure with hexagonal ordered morphology can be obtained on a highly pure Al surface via electrochemical anodization in acidic medium [1, 2]. AAO is extensively applied in the fields of biosensor  and biofiltration  and as a nanotemplate [5, 6] for the fabrication of secondary nanostructured materials. AAO templates have many advantages over the polycarbonate membranes like high pore density, thermal stability, cost effectiveness and versatility. Pore diameter, length, inter-pore spacing, and pore ordering can be easily tailored by tuning the anodizing parameters such as voltage, time, electrolytes, pH value, and temperature.
One-dimensional (1D) nanostructured materials such as nanowires, nanorods, and nanotubes play a special role in the field of nanoscience and nanotechnology due to their high aspect ratio (length/diameter) and large surface area. Ferromagnetic (Fe, Co, Ni) nanowires gain a lot of attention of scientific community in the last few decades due to their potential application in the fields of ultra-high density magnetic storage , magnetio-electronics , high sensitive giant magnetoresistance (GMR) sensors [9, 10]. Co–Ni is an important type of binary ferromagnetic alloys having high mechanical strength , good wear resistance , anti-corrosive performance , and electrocatalytic activity [14, 15]. Moreover, the standard electrochemical potentials of Co2+ and Ni2+ almost have the same value of −0.28 and −0.23 V, respectively, so Co–Ni binary alloy nanowires can be easily fabricated in the nanopores of AAO template by co-electrodeposition. Information technology made much progress especially in the last few years, which reflects the interest of the researchers and investment of companies in this field. A decade ago, the limit of areal density was about few 10 gigabits (GB)/in.2. Today, the limit reached to several hundred GB/in.2. Terabit (TB) hard disk is already available commercially, and a number of companies are in competition to increase the capacity and decrease the size of the hard disk . The areal density has been increased using nanomagnet, in which 1 bit of information corresponds to a single-domain nanosized particle. One simple and economical way of achieving nanomagnetic arrays over a large area is based on highly ordered AAO templates . Up till now, several methods have been applied to fill the pores of AAO template with metallic or magnetic nanowires like sol–gel , chemical vapor deposition , electroless deposition , and electrochemical deposition . Electrochemical deposition is the most simple, efficient, versatile, and cost effective technique. It is well known that anodization of metals is always associated with an insulting barrier layer between the metal substrate and metal oxide film . Thickness of the barrier layer mainly depends upon the anodizing voltage with a growth rate of 1 nm/V . It has been reported that the insulting properties of the barrier layer significantly affect the uniformity and quality of the depositing material . Therefore, handling of the barrier layer during deposition of secondary material in the nanopores of AAO is very essential and important. Until now, three different kinds of electrochemical deposition methods are applied for filling the pores of AAO template: direct current (DC) electrodeposition , pulse electrodeposition (PED) , and alternating current (AC) electrodeposition . Filling of AAO pores with metallic or magnetic nanowires via direct current (DC) electrodeposition is a tedious process and requires many steps. For instance, first AAO template has to be isolated from Al substrate, and this is achieved by dissolving the Al substrate in a toxic saturated solution of HgCl2. Subsequently, the barrier layer has to be etched away using chemical etching which often leads to the non-uniform widening of pores at the bottom. This process produces AAO template with different pore diameters at the top and the bottom surface; resulting in non-uniform-diameter nanowires which is undesirable in device fabrication. Finally, a thin metallic contact is sputtered on one side of AAO which act as a cathode during electrodeposition. These steps are time consuming, and additionally, the handling of a fragile AAO template during the whole process is a very difficult task. Furthermore, electrodeposition via direct current in the pores of AAO without modification of barrier layer is generally unstable and leads to a non-uniform filling of the AAO nanopores due to the cathodic side reaction . PED method is also widely used for the fabrication of metallic or magnetic nanowires in the nanopores of AAO templates. Ni [16, 25] and Co [27, 28] nanowires have been fabricated in the nanopores of AAO applying this method. Although the uniformity and pore-filling efficiency increased many folds compared to DC electrodeposition; however this method also needs modification of the barrier layer [16, 25–28]. In contrast, AC electrodeposition is a very powerful technique and it does not need the detachment of AAO template from the Al-substrate or modification of the barrier layer. Moreover, the Al-substrate is used as cathode during electrodeposition.
To the best of the author knowledge, Co-Ni binary alloy nanowire electrodeposition in the AAO template without modification of the barrier layer has not been reported to date. In this study, the fabrication of dense Co-Ni binary alloy nanowires within the nanopores of AAO templates via AC electrodeposition has been reported. Co-Ni binary alloy nanowires with different composition were co-deposited into the nanopores of AAO templates from a single sulfate bath of Co and Ni without modifying the barrier layer at room temperature.
Aluminum foils (Goodfellow, 0.1-mm thickness, 99.999% purity), sulfuric acid (H2SO4, Sigma-Aldrich, 99.999%, St. Louis, MO, USA), cobalt (II) sulfate heptahydrate (CoSO4·7H2O, Sigma-Aldrich, ≥99%), nickel (II) sulfate hexahydrate (NiSO4·6H2O, Sigma-Aldrich, 99%), boric acid (H3BO3, Sigma-Aldrich, ≥99.5%) were used in their as-received forms without further treatment. The electrolyte was prepared with deionized (DI) water.
Preparation of AAO templates
For all experiments, Al foils were cut into 4.5 × 4.5 cm2 pieces. Before anodization, Al foils were annealed at 500°C for 5 h in air to remove the mechanical stresses. Subsequently, the foils were etched in 1.0 M NaOH at room temperature until bubbles over the surface of the foils were observed, followed by a rinse in DI water many times and dried by air at high pressure. Al foils were used for anodization without any pre-treatment of electro-polishing. A simple, homemade, two-electrode system, with Al foil as a working electrode and a Pt foil as a counter electrode, was used for an electrochemical anodization. A circular shape surface of the Al foil was exposed to the electrolyte. Anodization was conducted in 0.4 M aqueous H2SO4 electrolyte at constant voltage of 26 V for 23 h using a DC power source at 0°C. The anodization induced highly ordered nanopores with hexagonal morphology over the exposed surface of Al foil to the electrolyte. The templates were washed with DI water and dried using air at high pressure before deposition of Co-Ni binary alloy nanowires.
Deposition of Co-Ni binary nanowires
Concentration ratio of Co(II) to Ni(II) in the bath solution
Concentration ratio of the metallic species in the bath
Amount of powder in the reaction solution (g/L)
Final solution (mL)
The structural morphology of AAO templates and Co-Ni binary nanowires was studied with the help of field emission scanning electron microscope (FESEM, Magellan, FEI, USA). The cross-sectional SEM images were taken from mechanically cracked samples. Elemental analysis was done using an energy dispersive X-ray analyzer (EDX) analyzer attached onto the SEM. The crystallographic structure of the nanowires were determined by a high-power X-ray generator (18 kW) Rigaku D/MAX-2500 X-ray diffractometer (Shibuya-ku, Japan) with Cu Kα radiation (λ = 1.54056 Å). The magnetic properties were measured with the help of vibrating sample magnetometer (Lake Shore 7407, Westerville, OH, USA) at room temperature.
Results and discussions
In summary, dense Co-Ni binary alloy nanowires were deposited into highly hexagonal ordered nanopores of AAO template via AC electrodeposition at room temperature without barrier layer modification. Hexagonal ordered AAO templates were synthesized in 0.4 M H2SO4 at 26 V in 0°C environment via single-step anodization. Co-Ni binary alloy nanowires were homogenously co-deposited within the nanopres of AAO template from a single sulfate bath. FESEM results showed that the nanowires have uniform lengths and diameters. Diameters of the nanowires were approximately 40 nm which is equal to the nanopore diameter. XRD analysis confirmed the fabrication of Co-Ni binary alloy nanowires with hcp and fcc phases. EDX analysis confirms the fabrication of Co-Ni binary alloy nanowires in the AAO template. Magnetic measurement showed that easy x-axis of magnetization is along the parallel direction of the nanowires with coercivity of approximately 706 Oe. The AC deposition method is very simple, fast, and cost effective as it does not need the complex process of barrier layer removal or modification for the deposition of secondary nanostructured materials in the nanopres of AAO.
One of the authors (GA) is very thankful to the National Commission on Nanoscience and Technology (NCNST) of Pakistan for providing the financial support. The author (GA) is also very thankful to Professor S. G. Yang of the National Laboratory of Solid State Microstructures and Physics Department, Nanjing University, Nanjing, China for providing all the experimental facilities. Help from Mr. Hamid Saeed Raza is also acknowledged.
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