Nanoporous Anodic Alumina Layers Research Articles

Nanoporous anodic alumina layers obtained from novel deep eutectic solvent formulations

ABSTRACT The possible fabrication of porous anodic oxide films on aluminium in ionic liquids based on choline dihydrogen citrate eutectic mixtures both with oxalic acid and isopropyl alcohol and ethylene glycol, has been investigated. The anodisation has been carried out in either potentiostatic or galvanostatic regime, at temperatures of 45–80°C, for different process durations. Quite compact, uniform anodic alumina layers have been obtained. Based on AFM and SEM investigations, pore diameters between 50 and 80 nm and interpore distances in the range of 160–200 nm have been estimated, with values influenced by the electrolyte type and anodisation conditions. The highest anodisation rate of about 0.4 µm min−1 has been determined by applying operation temperatures of 60°C. The recorded EIS spectra showed a pure capacitive behaviour and high anodic oxide resistances of 106–107 Ω cm2 order.

Tuning intrinsic photoluminescence from light-emitting multispectral nanoporous anodic alumina photonic crystals

To control and harness the intrinsic photoluminescence of solid-state, light-emitting materials produced by self-organization approaches remain challenging. This study demonstrates how the intrinsic broadband photoluminescence emission from nanoporous anodic alumina (NAA) produced by anodization of aluminum in oxalic acid electrolyte can be precisely tuned by engineering its structure in the form of photonic crystals (PCs). A combination of pulse and constant anodization in distinct acid electrolytes makes it possible to engineer a novel heterogeneous optical structure consisting of two layers: (i) a non-emitting, light-filtering layer in the form of multi-spectral nanoporous anodic alumina photonic crystals (MS–NAA–PCs) on its top (i.e., 58 µm thick and average pore diameter of 17 nm) and (ii) an intrinsically light-emitting layer of NAA at its bottom (i.e., 50 µm thick an average pore diameter of 40 nm). MS–NAA–PCs are engineered to feature three intense, well-resolved photonic stopbands (PSBs), the positions of which are spaced at specific regions of the visible spectrum from ∼380 to 560 nm. It is demonstrated that the PSBs of the non-emitting MS–NAA–PCs on top of the heterogeneous optical structure act as a light-filtering component, which makes it possible to narrow and tune the characteristically broad, Gaussian-like photoluminescence emission from the underlying light-emitting NAA layer. This structural design makes it possible to narrow the width of photoluminescence emission up to ∼50 nm and blue shift its position for ∼15 nm. Our advances pave the way for novel designs of intrinsic, light-emitting NAA-based PC structures, which could find broad applicability across light technologies, such as sensing and biosensing, photodetection, and solar light harvesting.

Titanium dioxide-coated nanoporous anodic alumina optical properties

Abstract Ordered nanoporous anodic alumina (NAA) samples are fabricated by the two-step anodization process. The surface of NAA is modified by means of the spray pyrolysis of titanium dioxide (TiO2). The morphology and optical properties of the samples are investigated. UV–Vis-NIR reflectance of NAA and NAA coated with TiO2 has been both measured and simulated. The influence of pore diameter and thickness of both NAA and TiO2 in reflectance is analyzed. Pore diameters between 30 nm and 80 nm and thicknesses from 3 to 14 μm are evaluated for the NAA. TiO2 coating thicknesses from 87 nm to 438 nm are determined by the number of sprayed TiO2 layers. Samples up to 40 nm TiO2 coating exhibit an increase in the intensity of the oscillations produced by NAA. Whereas, thicker TiO2 coatings generate a modulated oscillating behavior in the UV–Vis-NIR region. Numerical simulation based on the Transfer Matrix Method has also been carried out to analyze the measured reflectance spectra. The porosity of films has been modeled using an effective medium approximation. It is found the importance of including some absorption in the UV–Vis zone of the NAA layers to obtain accurate predictions of the optical behavior of NAA.

High-Density Plasmonic Nanoparticle Arrays Deposited on Nanoporous Anodic Alumina Templates for Optical Sensor Applications.

This study demonstrates a new, robust, and accessible deposition technique of metal nanoparticle arrays (NPAs), which uses nanoporous anodic alumina (NAA) as a template for capillary force-assisted convective colloid (40, 60, and 80 nm diameter Au) assembly. The NPA density and nanoparticle size can be independently tuned by the anodization conditions and colloid synthesis protocols. This enables production of non-touching variable-density NPAs with controllable gaps in the 20–60 nm range. The NPA nearest neighbor center distance in the present study was fixed to 100 nm by the choice of anodization protocol. The obtained Au NPAs have the resonant scattering maxima in the visible spectral range, with a refractometric sensitivity, which can be tuned by the variation of the array density. The thickness of the NAA layer in an Aluminum-NAA-NPA multilayer system enables further tuning of the resonance frequency and optimization for use with specific molecules, e.g., to avoid absorption bands. Applicability of the mentioned multilayers for colorimetric refractive index (RI) sensing is demonstrated. Their use as Surface-Enhanced Raman Scattering (SERS) substrates is tested using hemoglobin as a biological probe molecule.

Fabrication and characterization of nanoporous anodic alumina membrane using commercial pure aluminium to remove coliform bacteria from wastewater

Nanoporous anodic alumina (NAA) membranes were produced by anodizing process using commercially pure aluminium (1050 alloy). The optimization of processing parameters, such as applied potential, temperature and time in two different electrolyte solutions, has been performed to produce the ordered alumina nano-membrane. Nanoporous anodic alumina membrane (NAAM) was obtained by removing the aluminium substrate and opening the bottom of the pores. The microstructure of samples was studied and analysed by field emission scanning electron microscopy (FESEM). Image processing of FESEM pictures was performed with MATLAB. The results showed that at the optimum conditions of anodizing, the percentage of porosity and ordering of pores in the samples were similar to the productions of the high purity aluminium base. NAA layers that formed in sulphuric acid solution in comparison with oxalic acid had lower pore diameter (20 nm), lower inter-pore distance (45-50 nm) and higher hardness (325HV). The optimized membranes which have been produced with the same thickness were used for separation of Coliform bacteria from the secondary clarified wastewater. The results showed that these membranes can be used as selective filters because of their desirable physical properties.

Engineered therapeutic-releasing nanoporous anodic alumina-aluminum wires with extended release of therapeutics.

In this study, we present a nanoengineered therapeutic-releasing system based on aluminum wires featuring nanoporous anodic alumina layers and chitosan coatings. Nanoporous anodic alumina layers are produced on the surface of aluminum wires by electrochemical anodization. These nanoporous layers with precisely engineered nanopore geometry are used as nanocontainers for bovine serum albumin molecules labeled with fluorescein isothiocyanate (BSA-FITC), which is selected as a model drug. The surface of these therapeutic-releasing implants is coated with a biocompatible and biodegradable polymer, chitosan, in order to achieve a sustained release of protein over extended periods of time. The performance of this therapeutic-releasing device is systematically assessed through a series of experiments under static and dynamic flow conditions. In these experiments, the effect of such parameters as the number of layers of chitosan coating and the temperature and pH of the eluting medium is established. The obtained results reveal that the proposed therapeutic-releasing system based on nanoporous aluminum wires can be engineered with sustained release performance for up to 6.5 weeks, which is a critical factor for medical treatments using sensitive therapeutics such as proteins and genes when a localized delivery is desired.

Fast Response Hydrogen Microsensor Based on Semiconductor Niobium-oxide Nanostructures via Smart Anodizing of Al/Nb Metal Layers

Nanostructured niobium oxide semiconductor is gaining increasing attention as electro-optic and gas sensing material. However, the preparation of stable niobium oxide nanofilm with reproducible morphology and behaviour remains a challenge. Here we describe a rapid and well-controlled approach to synthesize a niobium oxide film with the columnlike nanostructured morphology via anodic processing of Al/Nb metal layers sputtered onto an oxide-coated Si wafer. The film is developed due to the growth of a nanoporous anodic alumina layer followed by pore-directed oxidation of the Nb underlayer. The post-anodizing treatment results in the controlled formation of Nb2O5 crystal phase, which causes the transformation from dielectric to n-type semiconductor behavior of the film. A laboratory gas sensor fabricated by uniting the anodizing approach developed here with standard micromachining technologies shows superior characteristics for hydrogen gas detection, the response-recovery time being among best ever reported.

Formation–structure–properties of niobium-oxide nanocolumn arrays via self-organized anodization of sputter-deposited aluminum-on-niobium layers

Nanostructured niobium oxide (NO) semiconductors are gaining increasing attention as electronic, optical, and electro-optic materials. However, the preparation of stable NO nanofilms with reproducible morphology and behavior remains a challenge. Here we show a rapid, well-controlled, and efficient way to synthesize NO films with self-organized columnlike nanostructured morphologies and advanced functional properties. The films are developed via the growth of a nanoporous anodic alumina layer, followed by the pore-directed anodization of the Nb underlayer. The columns may grow 30–150 nm wide, up to 900 nm long, with an aspect ratio of up to 20, being anchored to a thin continuous oxide layer that separates the columns from the substrate. The as-anodized films have a graded chemical composition changing from amorphous Nb2O5 mixed with Al2O3, Si-, and P-containing species in the surface region to NbO2 in the lower film layer. The post-anodization treatments result in the controlled formation of Nb2O5, NbO2, and NbO crystal phases, accompanied by transformation from nearly perfect dielectric to n-type semiconductor behavior of the films. The approach allows for the smooth film growth without early dielectric breakdown, stress-generated defects, or destructive dissolution at the respective interfaces, which is a unique situation in the oxide films on niobium. The functional properties of the NO films, revealed to date, allow for potential applications as nanocomposite capacitor dielectrics and active layers for semiconductor gas microsensors with the sensitivity to ethanol and the response to hydrogen being among best ever reported.

Morphological zeta-potential variation of nanoporous anodic alumina layers and cell adherence

Nanoscale surface modification of biomedical implant materials offers enhanced biological activity concerning protein adsorption and cell adherence. Nanoporous anodic alumina oxide (AAO) layers were prepared by electrochemical oxidation of thin Al-seed layers in 0.22M C2H2O4, applying anodization voltages of 20–60V. The AAO layers are characterized by a mean pore diameter varying from 15 to 40nm, a mean pore distance of 40–130nm, a total porosity of ∼10% and a thickness of 560±40nm. Zeta potential and isoelectric point (iep) were derived from streaming potential measurements and correlated to the topology variation of the nanoporous AAO layers. With decreasing pore diameter a shift of iep from ∼7.9 (pore diameter 40nm) to ∼6.7 (pore diameter 15nm) was observed. Plain alumina layers, however, possess an iep of ∼9. Compared to the plain alumina surface an enhanced adherence and activity of hFOB cells was observed on the nanoporous AAO after 24h culture with a maximum at a pore size of 40nm. The topology-induced change of the electrochemical surface state may have a strong impact on protein adsorption as well as on cell adhesion, which offers a high potential for the development of bioactive AAO coatings on various biomaterial substrates.

Effect of the anodization voltage on the pore-widening rate of nanoporous anodic alumina

A detailed study of the pore-widening rate of nanoporous anodic alumina layers as a function of the anodization voltage was carried out. The study focuses on samples produced under the same electrolyte and concentration but different anodization voltages within the self-ordering regime. By means of ellipsometry-based optical characterization, it is shown that in the pore-widening process, the porosity increases at a faster rate for lower anodization voltages. This opens the possibility of obtaining three-dimensional nanostructured nanoporous anodic alumina with controlled thickness and refractive index of each layer, and with a refractive index difference of up to 0.24 between layers, for samples produced with oxalic acid electrolytes.

Fabrication of WO 3 nanodot-based microsensors highly sensitive to hydrogen

A novel approach that involves electrochemical anodising for fabricating WO 3 nanodot-based microsensors is presented. The formation of gas sensing film is assisted by a nanoporous anodic alumina layer grown from a thin layer of aluminium sputtering-deposited over a thin layer of tungsten (Al/W bilayer). The anodic film annealed at 500 °C consists of a self-ordered array of WO 3 nanodots (∼50 nm), each composed of few nanocrystallites (∼9 nm) having thermostable tetragonal structure. The processes for forming the WO 3 nanodot active films are fully compatible with standard technology for fabrication of sensor chips on a micromachined silicon wafer. Characterization of the WO 3 nanodot film was performed by field-emission scanning electron microscopy, atomic force microscopy and X-ray diffraction analysis. A study of the ability of the WO 3 nanodot-based microsensors to detect H 2, CO, ethanol and relative humidity was presented for the first time. High sensitivity to hydrogen, with linear response characteristic was obtained in the range of low H 2 concentrations (5–20 ppm). The gas sensing mechanism of the WO 3 nanodot-based sensors was discussed.