Study On Abnormal Codeposition Mechanism Of Fe-Ni Alloy Nanowires

Fe-Ni alloy nanowires have various potential applications due to their excellent magnetic and mechanical properties. The functional properties of Fe-Ni alloys depend on greatly on the composition of Fe-Ni alloys. The anomalous codeposition is that the compositions in deposited alloys are different from the normalized concentrations of metal ions in the electrolyte. Understanding the mechanism of anomalous codeposition can be usefull for controlling the composition of the deposited alloy. In this thesis, iron nanowires, nickel nanowires and iron-nickel alloy nanowires were prepared by electrodeposition onto the porous alumina template. The deposited nanowires of the samples was characterized by XRD, SEM and EDS. The anomalous codeposition behavious of Fe-Ni nanowires has been elucidated by studying the crystal structure and composition of the Fe - Ni alloy nanowires.The method of preparation of porous alumina template was described in detail, and the influence of gold sputtered time on deposited nanowires was discussed.Deposition of Fe, Ni and Fe-Ni nanowires were performed in a three-electrode cell,and their XRD, SEM and EDS were characterized. The change of the current during the growth process of nanowires was studied and the polarization curves of the Fe and Ni nanowires were discussed.We deposited iron-nickel alloy nanowires at four different voltages (-0.95 V, -1.2 V,-1.8 V, -2.5 V) using potentiostatic electrodeposition. It was found that the composition of Fe and Ni atoms in the alloy nanowires is significantly different from the concentrations of Fe and Ni ions in the solution (40% Fe2+ and 60% Ni2+ )indicating that Anomalous codeposition occurrs, that is, the metal with more negative potential (Fe:-0.44 V, Ni: -0.25 V) is preferentially deposited. With the decrease of deposition voltage,the anomalous codeposition phenomenon is weakened at more negative potentials.Normal codeposition occurs at -2.5 V. We propose that hindrance of crystallization of Ni atoms can be the cause of anomalous codeposition. The bcc alloy phase and fcc alloy phase are formed during the deposition of Fe-Ni alloy nanowires. According to the Fe-Ni phase diagram, the solid solubility of Ni atoms in the bcc phase is about 5%, and Fe and Ni exhibit complete solid solubility in the fcc phase. When the nickel ions in the electrolytic solution diffuse to bcc phase, they are partly prevented from depositing into the bcc lattice due to the solid solubility limit, resulting in anomalous codeposition.When the nickel ions diffuse to fcc phase, hindrance of crystallization of Ni cannot occur due to complete solid solubility of Fe and Ni atoms in fcc phase. Thus, the degree of anomalous codeposition depends mainly on the amount of bcc phase formed.