Understanding and morphology control of pore modulations in nanoporous anodic alumina by discontinuous anodization

Abstract

Here, we report on an electrochemical approach so‐called discontinuous anodization (DA) for controlling the generation of pore modulations in nanoporous anodic alumina (NAA) under self‐ordering conditions. In this method, the anodization of aluminium is performed by voltage pulses under mild anodization regime and thermal‐acidic conditions. Furthermore, we perform an exhaustive and systematic study about the relationship between the pore modulation morphology and the main anodization parameters (i.e. acid electrolyte temperature and relax time between consecutive voltage pulses). The generation mechanism of the pore modulations is associated with the formation of a gel‐like layer under these thermal‐acidic conditions and its subsequent detachment during the beginning of each voltage pulse. All this makes it possible to control the pore modulation morphology in NAA at will, what is crucial for developing new nanostructures with accurately controlled geometry for such purposes as nanofluidics, magnetic nanostructures, optical biosensors, templates, selective molecular separators and so forth. A three‐dimensional network based on NAA with controlled geometry: The DA by voltage pulses under hydrothermal conditions yields an accurate control over the pore modulation morphology. Therefore, the structure of the resulting 3D‐NAA network can be designed at will for later applications.

Rational engineering of nanoporous anodic alumina optical bandpass filters

“…[22][23][24][25][26][27][28] Recently, some of these approaches have been used to produce numerous NAA-based photonic structures, including rugate filters, optical microcavities, FabryPerót interferometers and distributed Bragg reflectors. [29][30][31][32][33][34][35][36][37][38][39] The development of optical platforms based on NAA has spread the use of this nanomaterial, typically used as a nanoporous template to grow other nanostructures such as nanowires and nanotubes 15 , towards more relevant applications such as optical sensing systems, drug delivery chips and photonic tags. [40][41][42][43][44][45][46][47][48][49][50][51] In this study, we present an innovative and rationally designed pulse anodisation approach, so called pseudo-stepwise pulse anodisation (PSPA), which enables the fabrication of nanoporous anodic alumina optical bandpass filters (NAABPFs) with advanced optical properties (Fig.…”

Structural Engineering of Porous Anodic Aluminum Oxide (AAO) and Applications

“…The modulated pores of the resulting AAOs changed from a slender node-like, ellipsoidal, and asymmetric serrated morphology depending on the relax period. It was proposed that the generation of the pore modulations is associated with the formation of a gel-like layer (e.g., Al(OH) 3 ) during the period of relax and its subsequent detachment during the beginning of each potential pulse [169]. The proposed mechanism appears to explain the experimental observations.…”

“…An approach for pore modulations without combination of MA and HA processes has been reported by Santos et al [169]. The process, so-called "discontinuous anodization (DA)", uses potential pulses under MA regime and thermally activated oxide dissolution condition [169].…”

“…Gel-like layer on the pore wall surface may be formed by re-deposition of dissoluted oxide species within the pores, which typically occurs upon drying of an improperly rinsed porous AAO. Santos et al [169] attributed the sharp increase in anodizing current (j) at the beginning of each potential pulse to the detachment of the gel-like layer from the pore base. But, an alternative interpretation can be made for this observation as follow: During the period of relax, acidic dissolution of pore wall oxide would occur due to the high electrolyte temperature, thus pores are enlarged.…”

“…The process, so-called "discontinuous anodization (DA)", uses potential pulses under MA regime and thermally activated oxide dissolution condition [169]. In this method, self-ordered porous AAO was first grown by anodizing a textured aluminum in 0.3 M H 3 PO 4 (1°C) at 170 V. After 6 min, anodizing was stopped and the electrolyte temperature was increased to 35°C.…”