Template Fe-Ni-Co Nanowire Electrodeposition with H2 Evolving Side Reactions
Authors: Li Deyang , Podlaha Elizabeth J 
Published: February 2020

Introduction

Template-based electrodeposition is of interest for the fabrication of nanowires due to its low cost, ease of implementation, and for 1D dimensional control at the nanoscale. Two types of templates are commonly used: (1) polymer track etched membranes, and (2) porous alumina (AAO) membranes.1-2
There is a challenge in electrodepositing metals and alloys when the side reaction is appreciable, due to the generation of hydrogen gas bubbles, and the accompanying local pH change. One strategy to circumvent these problems is to apply a pulsating current or potential. Previous work in our lab showed successful deposition of Super Invar (64 wt% Fe, 31 wt% Ni, and 5 wt% Co) nanowires by using a pulse strategy.3 Disadvantages of pulsed techniques include the formation of layers with dissimilar composition, when a uniform composition is desired, and that the process is more time intensive. This research focusses on the use of direct current (DC) deposition with the membrane choice being a key factor when the hydrogen side reaction is substantial.

Experimental

Fe-Ni-Co nanowires were fabricated galvanostatically with a three-electrode cell. The electrolyte contained 0.72 M nickel sulfate, 0.155 M ferrous sulfate, 0.005 M cobalt sulfate, 0.5 M boric acid, 0.001 M sodium lauryl sulfate, 0.011 M ascorbic acid, at pH = 2 and the temperature of 40 °C with and without surfactants.  Both AAO and polycarbonate (PC) membranes were used. A gold conductive film was sputtered on one side of membrane, serving as the cathode. The electrodeposition process was terminated when the nanowire reached the top of the membrane. Nanowires were removed from templates by being dissolved in dichloromethane (for PC) or 1 M NaOH solution (for AAO). The structures of nanowires were inspected by TEM (JEOL JEM-1010). 

Results and Discussion
 

Figure 1 shows TEM images of nanowires in (a) AAO and (b) PC with DC current. Long, unbroken nanowires are observed in AAO while in contrast, fragmented nanowires are observed in PC membranes. An explanation for the different structures observed is based on the hydrogen permeability into the membrane materials. It is expected that inorganic membranes have a higher hydrogen permeability than polymeric membranes.4 The lower hydrogen permeability of PC membranes lead to higher hydrogen concentration inside the pore region of the  membranes, creating a higher driving force for its permeability into the metal, and thus producing more fragile nanowires than in AAO.

Conclusion

Fabrication of FeNiCo nanowires in both AAO and PC membranes via DC electrodeposition was achieved. More robust nanowires are found with AAO due to differences in hydrogen permeability between AAO and PC.

Acknowledgement

The authors thank Roche Diagnostics for support of this project. The author also would like to thank Bill Fowle and Wentao Liang for microscopy help. 

Reference

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Figure 1