Formation of Nanoporous Nickel by Selective Anodic Etching of the Nobler Copper Component from Electrodeposited Nickel−Copper Alloys

Abstract

The preparation of nanoporous nickel films by electrochemical deposition of Ni-Cu alloy followed by the selective anodic etching of the less-active component (Cu) from the alloy was studied in an aqueous solution containing Cu(II) and Ni(II) at room temperature. Constant potential electrodeposition produced columnar Ni-Cu alloys, in which the Ni content increased as the deposition potential became more negative. X-ray diffraction and Auger mapping results indicate the presence of separated Cu-rich and Ni-rich phases in the alloys, with the Cu-rich phase being more concentrated in the middle of the column and surrounded by the Ni-rich phase. Cyclic voltammetric data indicates that anodic dissolution of nickel is retarded by passivation. By taking advantage of nickel passivation, selective anodic etching of copper from the Ni-Cu alloy produces nanohollow nickel tubes on indium-tin-oxide-coated glass substrates. The nanohollow tube structure obtained in this study is different from the interconnected bicontinuous nanopores that are usually obtained by dealloying the less noble component from a homogeneous solid solution alloy. The nanohollow tubes may have resulted from the fact that multiple phases columnar alloy deposits were produced by the electrodeposition step and from the limited mobility of nickel during the anodic etching step.

Dealloying Behavior of NiCo and NiCoCu Thin Films

“…Material is removed from NiCo films as a result of the dealloying procedure, as indicated by SEM images and increased capacitance measurements; however this occurs only at relatively high linear sweep potentials, and in these cases, Ni and Co are removed in approximately equal amounts. These results are distinct from those for NiCu films where "reverse" dealloying occurs and Cu is selectively removed [17,18]. Additionally, the higher the Ni percentage in the film, the higher the linear sweep potential that the sample can experience without becoming structurally unsound.…”

“…The reverse process can occur if the more thermodynamically active component of an alloy is kinetically stabilized allowing the more noble component to be selectively removed instead. This atypical "reverse" dealloying behavior has been shown in NiCu alloys, where nickel is passivated and copper is removed [17,18]. The porous material produced in this way has been studied for a number of applications, including as a substrate for the subsequent deposition of a pseudocapacitor oxide material [19][20][21][22][23][24][25][26], as a scaffolding for the addition of small amounts of Pd for methanol and ethanol oxidation catalysis [27], and directly as a catalyst for the hydrogen evolution reaction [28,29].…”

“…In contrast, the passiviation that allows the "reverse" dealloying to occur should also hinder any significant atomic movement of the component that remains. In fact, this has been observed in the NiCu system, 2 International Journal of Electrochemistry where a phase-separated core-shell structure in the asdeposited film led to a tube-like morphology in the dealloyed material [18,30]. Thus, "reverse" dealloying of this type has the potential to produce a variety of final architectures based on the particular characteristics of the starting material.…”

Nanoporous Nickel by Electrochemical Dealloying

“…This alone however does not forbid the synthesis of NP Ni; the formation of NP Pt in fact, with an even smaller value of D s , has been reported [12]; (ii) Ni exhibits a strong interaction with water and tends to easily form surface oxides or hydroxides, which may further hinder the surface diffusion of Ni adatoms during the dealloying process; (iii) most potential precursor alloys tend to form multiple phases or metastable nanostructured alloys, limiting the possibility to start the dealloying process from an uniform solid solution [9,13].…”

“…Few examples have demonstrated NP Ni formation from Cu-Ni precursors [9,10] under conditions of passivity for Ni and therefore cannot be considered truly nanoporous structures. Despite this, the reported morphology closely resembles the conventional nanoporous morphology.…”