A challenge in depositing iron-group elements into nanoporous membranes from aqueous electrolytes is the avoidance of gas bubble generation that can impede deposition. One solution is to use pulse deposition. A number of studies have reported successful deposition of Ni,^{1, 2} Co,³ Fe,⁴ and Ni-Fe⁵ nanowires with a pulse potential or current. Also, Fe-Ni-Co nanowires using nanoporous alumina⁶ or polycarbonate membranes⁷ have been reported from our lab. Since the local pH is expected to increase within nanopores with gas evolving side reactions, an applied anodic potential step can be used to create a stable oxide layer in the same electrolyte that is used for deposition.  In this paper we use this idea to (1) create a selective area for subsequent etching for the fabrication of novel nanotips and to (2) better understand the pulse response during deposition by examining the influence of oxide surfaces that may be generated during the off time in a pulse cycle.

Experimental

A rotation cylinder electrode (RCE) was used to characterize the deposition and etching behavior of Fe-Ni-Co alloy thin films. These results were then used to determine the conditions for depositing the nanowires. The nanowires were electrodeposited into a polycarbonate template. The first layer of Fe-Ni-Co was pulse deposited (-50 mA/cm² for 2 s, 0 mA/cm²for 2 s). The surface of the nanowires was then treated with an applied potential. The potential was stepped to 0 V vs OCP for 10 min, or to an anodic potential for 10 s, before a second layer of Fe-Ni-Co was deposited under the same condition. After deposition, the nanowires were released from the membrane by dissolving the membrane in dichloromethane, followed by etching in a pH 5 citrate-acid solution. The resulting structure was characterized with Field Emission Scanning Electron Microscope (FESEM) and High-resolution transmission electron microscopy (HRTEM).

Results and Discussion

RCE results show that the current efficiency decreases as the rotation rate increases, while there is no change in composition. Thus, in the nanoporous template a decreasing current efficiency in the direction of growth can occur, as the proton boundary layer size changes, during Fe-Ni-Co nanowire deposition. The influence of the intermediate oxide layer upon exposure in the sodium citrate solution resulting in different morphological changes. If the intermediate anodic potential in the middle layer was at its open circuit potential, then upon etching the nanowires formed a ~ 10 nm nanogap. However, as the applied potential in this second step was adjusted to values more positive than +0.2 V vs SCE, nanotips were observed after etching.

The pulse transients in the deposition of the metal in the third layer was helpful in evaluating the reduction of the oxide. When a cathodic current is applied the “on” potential reaches -0.8 V vs SCE, and this region is extended in time with larger overpotentials, which contributes to the increasing amount of iron oxide produced in the second step (Fig. 1). The reduction of this oxide layer creates gradient region in structure in between two pulsed deposited Fe-Ni-Co layers.

Conclusion

In this study, a novel approach for creating nanotips at the interfacial region of two Fe-Ni-Co layers was introduced via a combination of pulse electrodeposition and an intermediate potential reverse procedure. A sacrificial region was created in the middle of the nanowires as the result of a combination of oxide reduction and alloy deposition, which was the fundamental reason for the formation of nanotips after etching.

Acknowledgement

The authors acknowledge and thank Roche Diagnostics for support of this project.

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