Abstract
A novel electrochemical methodology for the growth of arrays of Ni and Co nanowires (NWs) with linear and non-linear varying micro-height gradient profiles (μHGPs), has been developed. The growth mechanism of these microstructures consists of a three-dimensional growth originating from the allowed electrical contact between the electrolyte and the edges of the cathode at the bottom side of porous alumina membranes. It has been shown that the morphology of these microstructures strongly depends on electrodeposition parameters like the cation material and concentration and the reduction potential. At constant reduction potentials, linear Ni μHGPs with trapezoid-like geometry are obtained, whereas deviations from this simple morphology are observed for Co μHGPs. In this regime, the μHGPs average inclination angle decreases for more negative reduction potential values, leading as a result to more laterally extended microstructures. Besides, more complex morphologies have been obtained by varying the reduction potential using a simple power function of time. Using this strategy allows us to accelerate or decelerate the reduction potential in order to change the μHGPs morphology, so to obtain convex- or concave-like profiles. This methodology is a novel and reliable strategy to synthesize μHGPs into porous alumina membranes with controlled and well-defined morphologies. Furthermore, the synthesized low dimensional asymmetrically loaded nanowired substrates with μHGPs are interesting for their application in micro-antennas for localized electromagnetic radiation, magnetic stray field gradients in microfluidic systems, non-reciprocal microwave absorption, and super-capacitive devices for which a very large surface area and controlled morphology are key requirements.
Highlights
The shi from two to three dimensions (3D) in nanoarchitectures is a promising avenue to develop a new generation of physicochemical and biological multifunctional device applications
We have shown that both the material and concentration of metallic cations in solution signi cantly in uence the morphology of micro-height gradient profiles (mHGPs) grown at constant reduction potential
In the second stage at time t1, the NWs begin their nucleation inside the anodic aluminum oxide (AAO) membrane pores, which is accompanied by a thin metallic layer that grows from the edges of the EGaIn cathode along the in-plane direction
Summary
The shi from two to three dimensions (3D) in nanoarchitectures is a promising avenue to develop a new generation of physicochemical and biological multifunctional device applications. Different methods for the fabrication of gradients have been developed depending on the technological application being aimed at, as for instance compositionally gradient electrodes fabricated by selective potential-pulse electrodeposition,[6] polymer brush gradients synthesized by catalyst diffusion,[7] gradient polymer nanocomposites by magnetophoresis and capillary electrophoresis,[8,9] patterned inverse opals by selective photolysis modi cation process,[10] gradient plasmonic nanostructures by physical vapor deposition on curved nanomasks,[11] wettability gradients made by electrochemical polymerization of pyrrole arrays,[12] silicon geometric gradients by colloidal lithography,[13] functionality gradients with thickness graded pro les by dipcoating process[14] and gradient nanoclusters prepared by wireless electro-functionalization.[15] Gradients for the study of biological systems are receiving special interest, for instance, cellculturing materials[16] and regulation of cytosolic pH.[17] Besides these approaches, electrochemical methods have been widely used because of its low cost, reliability and ease of 25892 | RSC Adv., 2021, 11, 25892–25900. The morphology of the proposed mHGPs may be interesting for energy conversion,[44] optoelectrical,[45] and thermoelectric energy conversion[46] applications that take advantage of NWs length-dependent properties
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