Nanoporous anodic alumina-based iontronics: fundamentals and applications

Abstract

Synthetic iontronic systems harness ions as elementary carriers to perform functions that mimic those of biological systems; such as information transduction, sensing, and energy generation. The underlying principle of synthetic...

Harnessing the Intrinsic Ionic Rectification Properties of Blind‐Hole Nanoporous Anodic Alumina for Osmotic Energy Generation

“…During this process, counter‐ions within the BOL undergo enrichment at one polarity and depletion at the opposite polarity. [ 8b ] Analysis of the ionic conductance of the blind‐hole NAA membrane system shows a deviation from the linear conductance of the bulk solution at a concentration of electrolyte of [KCl] ≤ 1×10 −2 m . This behavior denotes that ionic transport across the BOL is governed by space charges (Figure 3d).…”

“…[ 7 ] Of all these systems, the most widespread solid‐state membrane architecture incorporates a hybrid Janus structure composed of: i) a thick meso/macroporous membrane (i.e., pore diameter ≥ 2 nm), the function of which is to provide mechanical support and favor transmembrane ionic flux; and ii) a thin functional microporous membrane (i.e., pore diameter < 2 nm) with active surface chemistry to achieve highly selective ionic flux. [ 8 ] Meso/macroporous membranes based on anodic aluminum oxide—henceforth “nanoporous anodic alumina” (NAA) membranes—produced by electrochemical oxidation (anodization) of aluminum have been the structural platform of choice in many of these emerging systems due to their unique properties. [ 9 ] These include mechanical strength (i.e., 2.5–5 GPa of microhardness), [ 10 ] chemically stable surface, [ 11 ] and tailorable structure composed of hexagonally arranged, self‐organized, straight cylindrical nanopores with homogeneous size distribution, the geometric features of which (i.e., length, diameter, and shape) can be judiciously engineered through anodization.…”

“…[ 9a ] This layer of anodic oxide was thought to be an ionic barrier and, as such, the through‐hole version of NAA membranes with BOL removed by wet or dry chemical etching has been used as a mechanical support to develop iontronic systems for osmotic energy generation. [ 8 ] However, recent advances in NAA technology by Kim and colleagues have unveiled the intrinsic ionic rectification properties of NAA's native BOL, where the flow of ionic species across this barrier of anodic oxide under a voltage bias is dependent on the pH of the electrolyte and the direction of the external electric field bias (Figure 1b). [ 13 ] The BOL of as‐produced NAA membranes can be approximated to a solid‐state ionic semiconductor, where the p‐ and n‐type regions are associated with oxygen ( V O 2+ ) and aluminum vacancies ( V Al 3− ) generated by the inward and outward migration of O 2− and Al 3+ across the BOL during the anodization process, respectively.…”

Negative Differential Resistance in Conical Nanopore Iontronic Memristors

“…Iontronics and circuit elements with memory effects, e.g., memristors, have attracted significant research interest recently as they could emulate biological neural systems and have the potential to outperform traditional solid-state-element-based systems in applications such as neuromorphic computation. − Iontronics refers to the electronics hardware and logic operations based on the emerging properties of solution ion transport confined in nanometer-scale interfaces. − Its development is inspired by the information transduction and processing abilities of the central nervous systems, which can express over 20 dynamical behaviors in response to electrochemical stimulation. , Compared to the solid-state materials and devices that constitute majority of recent advances in memory devices, iontronics itself and ionic memristors are in infancy and fast growing. − Memristors, among other circuit elements with memory effect, are resistors with memory that dynamically adjust their conductivity based on past states under external stimuli. − With their unique capability of integrated data storage and information processing, a single memristor can effectively replace multiple transistors on a chip, mitigating the needs of data exchange between logic process and information storage and thus enhancing energy efficiency and computing capacity. − Significant challenges remain to be addressed, for example, the first-order complexity problem of the consistent accessibility of multiple states, e.g., the short-term and long-term memory, which in biological systems arise from the adaptation of internal states responding to the environment and history. More advanced functions require controllable dynamics directly coupled with those states, referred to as second- and higher-order complexity problems.…”

Optical Modification of a Nanoporous Alumina Structure Associated with Surface Coverage by the Ionic Liquid AliquatCl

“…Initially, these nanoporous structures were used as templates for the fabrication of nanotubes, nanodots, or nanowires [ 2 , 3 ], but due to their high thermal and chemical stability, as well as their non-toxic and biocompatibility properties, they were also considered (after removing the bottom aluminium surface) for environmental, biotechnological, or biomedical applications (protein filtration, water desalination, diffusion controlling systems, virus detection, biomolecules recognition, or separation processes [ 4 , 5 , 6 , 7 , 8 , 9 ]). Moreover, the electropositive character of the NPASs also provides them with a certain ion selection character (anion exchanger) in the case of samples with a low pore size and porosity [ 10 , 11 , 12 ]. On the other hand, NPASs with modulated diameters or branched channels, that is, with an asymmetric structure, can also be obtained [ 13 , 14 ].…”