Synthesis of Arrayed Aluminum Oxide Templates by Hard Anodization and DC Electrodeposition of Co Nanowires

In recent years particular attention has been paid to one dimensional nanostructures for example nanotubes, nanodots and more important nanowires. Reason of these attention is application of these materials in nanodevices [1, 2], sensors and magnetic devices with high capacity especially [3,4]. Electrodeposition into ordered aluminum oxide templates is one of applicable methods to synthesis of these nanostructures. In addition to the electrodeposition parameters, structure of templates effect on properties of them directly and indirectly. Innovation of porous alumina return to 1950 that was only one step anodization but synthesis ordered aluminum oxide template is introduced by Masuda at 1995 [5]. This mild anodization almost do in sulfuric, oxalic or phosphoric acid at certain voltage 25, 40 and 195 volt respectively. Chu, et al [6] could anodize at same mild anodization condition in sulfuric acid at 70 volt as hard anodization to produce template [7]. Also Woo Lee et al, did hard anodization in oxalic acid at voltages 3 time more than normal voltage and introduced diameter modulated templates that have large potential to use nanotechnology. High purity Al foils (99.999 wt. %) were used substrates to fabricate highly ordered AAO templates using anodization process. The Al foil was cleaned and electropolished and anodization was done under a constant cell potential in a 0.3 M oxalic acid electrolyte at 0° C. However Al layer is removed to reach template and then pore widening was done in Phosphoric acid 5% at 32°C. As last step, the Gold layer was coated on the bottom of template. Co nanowires were then DC electrodeposited at room temperature into the nanoporous AAO templates in a threeelectrode cell at 1.1 V, where a platinum plate acts as a counter-electrode and Ag/AgCl3 as 373 reference electrod. The electrolyte consisted of 0.1 M CoSO4 • 7H2O, and 30 g/L boric acid for Ni nanowires. Scanning electron microscopy (SEM) were used to investigate the morphology. Magnetic loops were measured by vibrating sample magnetometer (VSM). Wave form figure 1a includes 80 V for 600 s and an 130 V for1200 s. As presented in figure 1a, first current increase then decrease and increase again and to be constant finally. Firstly anodizing includes two solution and oxidation reactions that cause to growth of oxide layer. This layer is stable and integrated as barrier layer that causes to increase resistance and decrease current to minimum amount. Gradually cracks are created and the oxide layer growth by those reactions as stable condition. This is a general growth behavior for the formation of porous-type aluminum oxide films meaning that the oxide film comprises two layers: (1) a thin barrier or compact film on aluminum and (2) porous aluminum oxide film. Current density increases at the beginning exhibiting the formation of the barrier layer until a peak value is achieved where the pore initiation takes place. At the end, a plateau is achieved because the barrier layer thickness becomes constant at the bottom of the pores. To interpret this behavior, one has to consider the formation and dissolution of oxide layer in respect to the applied electrical field or anodizing potential. Under the anodizing conditions of the experiments, the growth rate of the oxide layer exceeds its dissolution rate, leading to the formation of a barrier type layer right on the substrate at the bottom and a porous-type layer at the top. The process of the development of pores requires the interaction between the electrolyte and surface and is currently understood using the so-called field-assisted dissolution (FAD) hypothesis which describes the initiation and propagation of pores at the barrier layer. Current increase by increasing voltage at 1.3 V/s to 80 volt and barrier layer has resistance in return of this increasing at primary times but after some minutes resistance is broken by increasing of crack and slope of current is increased. Barrier layer restores itself at constant 80 V gradually and in result current decrease to a constant current. This behavior is repeated when voltage increase to 130 V at 0.04 V/s. 374 0 1000 2000 3000 4000 5000 6000 -20 0 20 40 60 80 100 120 140 t (s) V (V) 0.00 0.05 0.10 0.15 0.20 0.25 I (mA) 0 1000 2000 3000 4000 5000 6000 1.0 1.5 2.0 2.5 3.0 3.5 I (mA) Q (C) t (s) I (mA) -0.5 0.0 0.5 1.0 1.5 2.0 2.5 Q (C) Fig 1. Hard anodization V-t and I-t curves (a), electrodeposition I-t and Q-t curves (b) and SEM image of nanowires in template. Figure 1b shows I-t curves of electrodeposition of cobalt nanowires into templates that was explained. Figure 1c is SEM image of these nanowires into template that shows almost of pores are filled. It can be seen diameter of nanowires as modeled on template is about 90 nm and length is 44 μm. Also distance between pores is about 100 nm. Figure 2 shows magnetic hysteresis loops for Co nanowire into template at perpendicular (90°) and parallel (0°) between magnetic field and nanowire axis. In parallel state, coercively and squareness are more than perpendicular sate. It shows that easy axis for magnetization in this nanowire is perpendicular with axis of nanowires. -10000 0 10000 -1 0 1 90 0 M/Ms H (Oe) Hc (Oe) Squ % 0 371 17.9 90 97 2.3 Fig 2. Magnetic hysteresis loops measured by VSM