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

In recent years particular attention has been paid to one dimensional nanostructures for examplenanotubes, nanodots and more important nanowires. Reason of these attention is application ofthese 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 tosynthesis of these nanostructures. In addition to the electrodeposition parameters, structure oftemplates effect on properties of them directly and indirectly. Innovation of porous aluminareturn to 1950 that was only one step anodization but synthesis ordered aluminum oxide templateis introduced by Masuda at 1995 [5]. This mild anodization almost do in sulfuric, oxalic orphosphoric acid at certain voltage 25, 40 and 195 volt respectively. Chu, et al [6] could anodize atsame mild anodization condition in sulfuric acid at 70 volt as hard anodization to producetemplate [7]. Also Woo Lee et al, did hard anodization in oxalic acid at voltages 3 time more thannormal voltage and introduced diameter modulated templates that have large potential to usenanotechnology.High purity Al foils (99.999 wt. %) were used substrates to fabricate highly ordered AAOtemplates using anodization process. The Al foil was cleaned and electropolished and anodizationwas done under a constant cell potential in a 0.3 M oxalic acid electrolyte at 0° C. However Allayer is removed to reach template and then pore widening was done in Phosphoric acid 5% at32°C. As last step, the Gold layer was coated on the bottom of template. Co nanowires were thenDC electrodeposited at room temperature into the nanoporous AAO templates in a threeelectrodecell at 1.1 V, where a platinum plate acts as a counter-electrode and Ag/AgCl3 as reference electrod. The electrolyte consisted of 0.1 M CoSO4 • 7H2O, and 30 g/L boric acid forNi 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 anodizingincludes two solution and oxidation reactions that cause to growth of oxide layer. This layer isstable and integrated as barrier layer that causes to increase resistance and decrease current tominimum amount. Gradually cracks are created and the oxide layer growth by those reactions asstable condition. This is a general growth behavior for the formation of porous-type aluminumoxide films meaning that the oxide film comprises two layers: (1) a thin barrier or compact filmon aluminum and (2) porous aluminum oxide film. Current density increases at the beginningexhibiting the formation of the barrier layer until a peak value is achieved where the poreinitiation takes place. At the end, a plateau is achieved because the barrier layer thicknessbecomes constant at the bottom of the pores. To interpret this behavior, one has to consider theformation and dissolution of oxide layer in respect to the applied electrical field or anodizingpotential. Under the anodizing conditions of the experiments, the growth rate of the oxide layerexceeds its dissolution rate, leading to the formation of a barrier type layer right on the substrateat the bottom and a porous-type layer at the top. The process of the development of pores requiresthe interaction between the electrolyte and surface and is currently understood using the so-calledfield-assisted dissolution (FAD) hypothesis which describes the initiation and propagation ofpores at the barrier layer. Current increase by increasing voltage at 1.3 V/s to 80 volt and barrierlayer has resistance in return of this increasing at primary times but after some minutes resistanceis broken by increasing of crack and slope of current is increased. Barrier layer restores itself atconstant 80 V gradually and in result current decrease to a constant current. This behavior isrepeated when voltage increase to 130 V at 0.04 V/s.

کلیدواژه ها:

Arrayed Aluminum Oxide Templates ، Hard Anodization ، Electrodeposition ، Nickel Nanowireand Magnetic propertie