Nanoporous Alumina Substrates Research Articles

Two-axis beamsteering using a wideband Butler matrix and series-fed arrays for millimeter wave frequencies fabricated on metallic-nanowire-membrane platform

This work presents a two-axis beamsteering system designed for WLAN millimeter-wave frequencies. The system comprises a wideband Butler matrix and series-fed arrays. The Butler matrix, implemented with microstrip lines, features a wideband crossover with low phase imbalance, enabling four discrete beamsteering directions on the radiation H-plane. The series-fed arrays enable frequency-dependent beamsteering on the E-plane. The entire system was fabricated on a nanoporous alumina substrate using metallic-nanowire-membrane technology. Measurement results indicate an impedance bandwidth of 17.71%, which accommodates typical requirements for 60 GHz WLAN applications. Additionally, beamsteering in the desired directions was achieved. The presented Butler matrix structure stands out as one of the most compact found in the reviewed literature. The produced device successfully balances wideband behavior, compactness, and fabrication simplicity.

Hydrophobic-oleophilic surfaces based on chemical modification of nanoporous alumina

Hydrophobic/oleophilic nanoporous anodic alumina substrates were fabricated by two-step anodization method following of chemical modification with 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (FOTS). Surface morphology was characterized using a scanning electron microscopy. The influence of the topography and the pore size of the surface in the contact angle measurements with different solvents have been studied. The water repellence capacity of the nanoporous anodic alumina structure as fabricated depends on pore diameter but, there is a critical value of pore size beyond which the water contact angle decrease. After chemical modification of the surface, this effect is not observed. The porous topography of the material is not an advantage for the oleophibicity. The CA values for the non-porous surface using different solvents are lower than the structured surface.The results obtained in this study can be used to desire wettability properties. Modified NAA exhibit hydrophobicity and at the same time permeability for solvents with low surface tension.

Surface Modification of Nanoporous Anodic Alumina during Self-Catalytic Atomic Layer Deposition of Silicon Dioxide from (3-Aminopropyl)Triethoxysilane.

Changes associated to atomic layer deposition (ALD) of SiO2 from 3-aminopropyl triethoxysilane (APTES) and O3, on a nanoporous alumina structure, obtained by two-step electrochemical anodization in oxalic acid electrolyte (Ox sample) are analysed. A reduction of 16% in pore size for the Ox sample, used as support, was determined by SEM analysis after its coverage by a SiO2 layer (Ox+SiO2 sample), independently of APTES or O3 modification (Ox+SiO2/APTES and Ox+SiO2/APTES/O3 samples). Chemical surface modification was determined by X-ray photoelectron spectroscopy (XPS) technique during the different stages of the ALD process, and differences induced at the surface level on the Ox nanoporous alumina substrate seem to affect interfacial effects of both samples when they are in contact with an electrolyte solution according to electrochemical impedance spectroscopy (EIS) measurements, or their refraction index as determined by spectroscopic ellipsometry (SE) technique. However, no substantial differences in properties related to the nanoporous structure of anodic alumina (photoluminescent (PL) character or geometrical parameters) were observed between Ox+SiO2/APTES and Ox+SiO2/APTES/O3 samples.

Tuning dendritic structure at nanoporous alumina/aluminium interface for uniform electrodeposition of metals for nanocomposite coatings

Highly ordered nanoporous structures developed by anodising aluminium and its alloys have potential applications in various fields, including wear-resistant coatings in tribology. Filling nanopores with different metals and solid lubricants provide a nanocomposite coating which has a higher hardness. A major hurdle in developing the nanocomposite coatings is that a thick non-conducting barrier layer at the interface between the nanoporous alumina and aluminium substrate. Creating a conducting path at the interface between nanoporous alumina and aluminium without weakening the interfacial strength are expected to have significant advantages for developing tribological coatings. In this work, we systematically modified the interface by the voltage reduction process at different rates and created a dendritic structure at the interface. The dendritic structures growth at the interface is monitored by measuring the capacitance using an LCR meter. The capacitance exponentially decreased with increase in voltage reduction rate. The cross-sectional view of scanning electron microscopy images interface shows that the dendritic structure thickness linearly increases with a decrease in voltage reduction rate. After interface modification, the copper into the nanopores using pulse electrodeposition method. The scanning electron microscopy images show that the capacitance of 0.8–1.2 µF and ∼1 V/15 s voltage reduction rate gives nearly uniform filling of nanopores over a large area.

Nano-PAA-CuCl2 Composite as Fenton-Like Reusable Catalyst to Enhanced Degrade Organic Pollutant MB/MO

The treatment of organic dye contaminants in wastewaters has now becoming more imperative. Fenton-like degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution was investigated by using a nanostructure that a layer of CuCl2 nanoflake film grown on the top surface of nanoporus anodic alumina substrate (nano-PAA-CuCl2) as catalyst. The new nano-PAA-CuCl2 composite was fabricated with self-assembly approach, that is, a network porous structure film composed of CuCl2 nanoflake grown on the upper surface of nanoporous anodic alumina substrate, and the physical and chemical properties are characterized systematically with the X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), and high-resolution transmission electron microscopy (HRTEM), Energy Dispersive Spectrometer (EDS), X-ray photoelectron spectroscopy (XPS). The experimental results showed that the nano-PAA-CuCl2 catalyst presented excellent properties for the degradation of two typical organic pollutants such as MB and MO, which were almost completely degraded with 8 × 10−4mol/L nano-PAA-CuCl2 catalyst after 46 min and 60 min at reaction conditions of H2O2 18 mM and 23 mM, respectively. The effects of different reaction parameters such as initial pH, H2O2 concentration, catalyst morphology and temperature were attentively studied. And more, the stability and reusability of nano-PAA-CuCl2 were examined. Finally, the mechanism of MB and MO degradation by the nano-PAA-CuCl2/H2O2 system was proposed, based on the experimental data of the BCA and the temperature-programmed reduction (H2-TPR) and theoretical analysis, the reaction kinetics belonged to the pseudo-first-order equation. This new nanoporous composite material and preparation technology, as well as its application in Fenton-like reaction, provide an effective alternative method with practical application significance for wastewater treatment.

Immobilization of Photocatalytic ZnO Nanopowders Using Anodized Nanoporous Alumina Substrates

Nanoporous Al₂O₃ substrates with an average pore size of about 150 nm were prepared via anodization of Aluminum plates. Depending on the anodization condition, the surface area of the anodized Al₂O₃ was increased more than six-fold. Solution-combusted ZnO nanopowders were prepared as a function of fuel/oxidant ratios. At a fuel/oxidant ratio of 0.8, ZnO powder showed excellent powder characteristics such as average particle sizes of 30 nm and spherical shape. Electrical properties of SCM ZnO nanopowders with different fuel/oxidant ratios were investigated by Hall measurement. The carrier concentration of SCM ZnO nanopowders at the fuel/oxidant ratio of 0.8, was the highest, three-fold higher than that of any commercial ZnO powders. Using spray coating, these nanopowders were coated onto Al₂O₃ substrates for immobilization. To evaluate the photo-catalytic effect, Ag ions were removed from the wastewater via photocatalysis. The photocatalytic efficiency with the SCM ZnO nanopowders on nanoporous Al₂O₃ substrates was 2.5-fold higher than that with the SCM ZnO nanopowders on normal Al₂O₃ substrates. However, commercial zinc oxide powders did not show any photocatalytic phenomena. The large difference in photocatalytic efficiency was probably attributed to the characteristics of SCM ZnO nanopowder and the large surface area of anodized Al₂O₃.

Facile one-step fabrication of super-repellent nanoporous anodic alumina using a non-fluorinated approach

Inspired by lotus-effect, superhydrophobicity has attracted considerable interest in various areas such as self-cleaning, antifouling and liquid transportation, and so on. Superhydrophobic surfaces can be prepared mostly by altering the surface with creating micro/nanoscale structures or with chemically modifying by materials of low surface energy. In this study, the electrochemically fabricated nanoporous alumina is deposited with poly(dimethylsiloxane) via a one-step thermal treatment to achieve ultra-water repellent or superhydrophobic surfaces. Nanoporous anodic alumina substrates with varying pore diameters were used for the poly(dimethylsiloxane) deposition, and their morphological characterization is carried out using the field emission scanning electron microscope. Energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy inferred the chemical composition, while contact angle measurements using a commercial contact angle measurement system is exploited to probe the water spreading behaviour on the fabricated substrates. The substrates also exhibit high repellence behaviour towards ethylene glycol. The superhydrophobic behaviour of the fabricated substrates is investigated while the substrate is immersed in organic solvents like decane, hexane and toluene. In addition, the super repellence behaviour of the substrates against the corrosive chemicals such as aqua regia and saturated sodium hydroxide is investigated and the substrate was found to retain its super repellence behaviour against corrosive solutions. Further, the self-cleaning properties and antifouling behaviour of the fabricated substrates were also demonstrated.

Effect of substrate topography for graphene-based humidity sensors

Chemical vapor deposition graphene (CVDG) and electrochemical exfoliated graphene (ECG) were transferred and spin-coated on the nanoporous alumina substrate (NS) and flat alumina substrate (FS) to investigate the effect of substrate topography for graphene-based humidity sensors. The electrical resistance change of two graphene types on different substrate topography could be measured to estimate the environmental relative humidity. The relationship between graphene types, substrate topography, wettability, and humidity sensing performance was studied and discussed. The results of the water contact angle of CVDG on NS/FS and ECG on NS/FS were 77.0 ± 5.0°, 88.8 ± 4.9°, 55.9 ± 7.0°, and 59.8 ± 3.2°, respectively. The response, linearity and hysteresis characteristics of four graphene-based humidity sensors were measured and estimated. The ECG on NS humidity sensor shows the best performance in regards to the response, linearity and hysteresis.

Culture of dental pulp stem cells on nanoporous alumina substrates modified by carbon nanotubes.

PurposeAlumina substrates are one of the commonly used scaffolds applied in cell culture, but in order to prevent formation of biofilm on the alumina substrate, these substrates are modified with carbon nanotube.MethodsThe alumina substrate was made by a two-step anodization method and was then modified with carbon nanotubes by simple chemical reaction. The substrates were characterized with FTIR, SEM, EDX, 3D laser scanning digital microscope, contact angle (CA) and surface free energy (SFE). To determine how this modification influences the reduction of biofilm, biofilm of two various bacteria, Escherichia coli (E.coli) and Staphylococcus aureus (S. aureus), were investigated.ResultsThe biofilm on the modified substrate decreased due to the presence of carbon nanotubes and increased antibacterial properties. Dental pulp stem cells (DPSCs) were cultured onto flat alumina (FA) and nanoporous alumina-multiwalled carbon nanotubes (NAMC) substrates to examine how the chemical modification and surface topography affects growth of DPSCs.ConclusionCell attachment and proliferation were investigated with SEM and Presto Blue assay, and the findings show that the NAMC substrates are suitable for cell culture.

An efficient and low-cost photoanode for backside illuminated dye-sensitized solar cell using 3D porous alumina

This paper presents a new strategy for developing back-side illuminated dye-sensitized solar cells (DSSCs) using nanoporous alumina substrate containing Cu impurities. The presence of a traces of Cu in the aluminum leads to the formation of 3D nanoporous anodic aluminum oxide (AAO) structure which consisted of an array of cylindrical pores interconnected by horizontal holes. Under different anodizing conditions, four different AAO substrates were fabricated and applied for developing DSSCs. The current-voltage characteristics of the DSSCs were measured and compared. Results obtained indicated that the performance (photocurrent density, open-circuit photovoltage and efficiency) of DSSCs based on 3D AAO was significantly enhanced compared to those of FTO/TiO2. The efficiencies of DSSCs based on alumina P90A, P90B and OX60 were improved by 172%, 51%, and 274%, respectively. The enhancement in photocurrent and efficiency is attributed to the reduction in the carrier recombination, uniform distribution of the dye molecules inside the vertical tubes, horizontal interconnections of pores as well as a reduction in pore size. This indicates that AAO based DSSCs can contribute significantly to energy harvesting from the sun.

Ring gyroscope sensitive element based on nanoporous alumina

PurposeThis paper aims to present new technological approaches of manufacturing of micromechanical gyroscope ring-sensitive element based on the nanoporous anodic alumina instead of traditional silicon technology. Simulation and the operation analyses of such elements have been performed.Design/methodology/approachThe design of gyroscope represents a sensitive element on a glass substrate; in the center of a ring, there is a permanent magnet in a steel box. The sensitive element is made of profiled nanoporous anodic alumina consisting of an octagonal frame which is connected to a ring in the center with eight N-shaped spokes. The technology of the sensitive element fabrication involves the electrochemical formation of nanoporous anodic alumina substrate given the thickness and porosity and its chemical etching on the element topology. The basic parameters and the operation principle of the nanoporous alumina-sensitive element have been defined by finite element simulation.FindingsIt is shown that the resonance frequencies of the sensitive element change as functions of the alumina porosity. The main parameters of the nanoporous alumina-sensitive element have been compared with parameters of a silicon-sensitive element. Calculations have shown that the mechanical deformations of the von Mises are approximately lower by two times in the nanoporous alumina-sensitive element.Practical implicationsHigh-precision angular rate measurement will be achieved by reducing mechanical and electrical noises practically to zero through careful designing of a ring magnetoelectric gyroscopeOriginality/valueThe ring resonator made of nanoporous anodic alumina will allow to increase the threshold of sensitivity and stability of micromechanical gyroscope characteristics owing to the high precision of geometric dimensions, the stability of the elastic properties and the quality factor.

Fabrication of high-aspect-ratio aluminum oxyhydroxide nanopillars for anti-reflection surfaces

This study presents the anti-reflection characteristics of high-aspect-ratio aluminum oxyhydroxide nanopillars prepared by the hydrothermal synthesis method from a nanoporous alumina substrate. Three different alumina-based nanostructures, including nanolens, short-nanopillar, and long-nanopillar were prepared, and the center-to-center distance of the nanostructures was fixed as 380 ± 50 nm. The diameter of the nanopillars was 140 ± 30 nm, and the aspect ratios of the short and long nanopillars were 1.5 and 4.0, respectively. From visual investigation, the surfaces of fabricated aluminum oxyhydroxide nanopillars appear black, although they did not have any black color. The anti-reflection characteristics of high aspect ratio aluminum oxyhydroxide nanopillars were compared with those of an alumina nanolens array. According to the experimental results, the high-aspect-ratio aluminum oxyhydroxide nanopillar array demonstrated remarkable anti-reflection characteristics, with 0.18% reflectance at an incident angle of 15°.

Nanostructured substrate effects on diamond-like Carbon films properties grown by pulsed laser deposition

A study on the effect of using a nanostructured substrate in the growth of a diamond-like carbon (DLC) film by pulsed laser deposition (PLD) has been carried out. It was found that the deposition on a nanoporous substrate gives origin to a film with a lower ratio of the sp3 to sp2 hybridizations of carbon atoms bondings as compared to a film deposited on a flat substrate under the same conditions, namely flat Silicon (Si) or Aluminum (Al) foil. This could be a consequence of a local stress relaxation during the growing process induced by the nanoporous structure of the substrate, suggesting that it might not be possible to obtain a high content of tetrahedral amorphous carbon when that kind of substrates is used. Motivated by previous investigations of nanodots growth using a nanoporous alumina substrate, a low pressure Argon background was used during the deposition process, in order to achieve different values of the sp3 content.

Morphological and optical properties of ultra‐thin nanostructured Cu films deposited by RF sputtering on nanoporous anodic alumina substrate

Nanoporous materials have attracted considerable technological interest due to their wide range of applications. In this study, highly ordered nanoporous anodic alumina substrates (NAAS) were fabricated by two-steps anodisation. Radio-frequency (RF) magnetron sputtering was used to deposit ultrathin Cu layers of different morphologies on the top surface of NAAS. From the field emission-scanning electron microscope images, the Cu/NAAS morphologies were tuned from nanoporous ultra-thin Cu layer, to nanoporous-grains of Cu aggregates, then to a continues layer of Cu nanostones as the deposition time increased from 1 to 4 min. The reflection spectra of the Cu/NAAS samples were dominated with localised surface plasmon modes; longitudinal, transverse and higher-order modes. The surface plasmon resonances of the fundamental and higher-order longitudinal modes were shifted to longer wavelengths as the deposition time increased. In addition, the transverse surface plasmon resonances were shifted to longer wavelengths for deposition time ≤3 min (nanoporous Cu film) and suddenly shifted to shorter wavelengths for deposition time ≥4 min (continuous film). The correlations between the structural parameters and the shift of surface plasmon resonance modes were discussed. The strong red-shifted of the surface plasmon modes to the NIR region suggests a unique opportunity for the design of biomedical sensor based on the proposed structures.

Micro-patterning of porous alumina layers with aligned nanopores

Arrays of nanoporous microfeatures of alumina are demonstrated via isotropic chemical etching of commercially available nanoporous alumina substrates. Etch rates are established for four morphologies of aligned nanopores having pore sizes in the range 20–220 nm and porosities from 30–60%. Parametrically varying etch conditions yields a process for creating microfeatures over large areas which is used to create a 384 porous tip array at a pitch of 70 µm with the 200 nm porosity.

Structural and optical characterization of nanostructured ZnO grown on alumina templates

The growth of nanostructured ZnO thin films on nanoporous anodic alumina substrates (NAAF) by dc reactive magnetron sputtering using a pure Zn metal target is reported. ZnO nanostructures reproduce the pore arrays of the NAAF substrates used as templates mimicking their hexagonal long range order. Stoichiometric nanostructured ZnO samples were grown with wurtzite type structure, highly textured and oriented in the (002) direction. The study of the NAAF pore size effect in the final morphology and optical properties of the nanostructured ZnO is presented for different ZnO thicknesses. The pore size of the nanostructured ZnO films was controlled in the range of 15–65 nm by choosing appropriate NAAF and the sputtering deposition conditions. The broad emission band observed in PL spectra of the samples should be associated with color centers transitions (F and H centers) appearing in the alumina templates.

Surface functionalization of nanoporous alumina with bone morphogenetic protein 2 for inducing osteogenic differentiation of mesenchymal stem cells

Many studies have demonstrated the possibility to regulate cellular behavior by manipulating the specific characteristics of biomaterials including the physical features and chemical properties. To investigate the synergistic effect of chemical factors and surface topography on the growth behavior of mesenchymal stem cells (MSCs), bone morphorgenic protein 2 (BMP2) was immobilized onto porous alumina substrates with different pore sizes. The BMP2-immobilized alumina substrates were characterized with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Growth behavior and osteogenic differentiation of MSCs cultured on the different substrates were investigated. Cell adhesion and morphological changes were observed with SEM, and the results showed that the BMP2-immobilized alumina substrate was able to promote adhesion and spreading of MSCs. MTT assay and immunofluorescence staining of integrin β1 revealed that the BMP2-immobilized alumina substrates were favorable for cell growth. To evaluate the differentiation of MSCs, osteoblastic differentiation markers, such as alkaline phosphatase (ALP) activity and mineralization, were investigated. Compared with those of untreated alumina substrates, significantly higher ALP activities and mineralization were detected in cells cultured on BMP2-immobilized alumina substrates. The results suggested that surface functionalization of nanoporous alumina substrates with BMP2 was beneficial for cell growth and osteogenic differentiation. With the approach of immobilizing growth factors onto material substrates, it provided a new insight to exploit novel biofunctional materials for tissue engineering.

Adhesion and Proliferation of Osteoblast-Like Cells on Anodic Porous Alumina Substrates With Different Morphology

We have fabricated nanoporous alumina surfaces by means of anodization in oxalic acid in different conditions and used them as the substrates for the growth of cells from a human osteoblast-like cell line. The rough nanoporous alumina substrates have been compared both with smooth standard Petri dishes used as the control and with commercial substrates of similar material. The viability of the cells has been assessed at different culture times of 4, 11, 18, and 25 days in vitro. It turned out that the porous side of the galvanostatically fabricated alumina performed similar to the control and better than the commercial porous alumina, whereas the potentiostatically fabricated porous alumina performed better than all the other substrates at all times, and in particular at the two shortest time periods of 4 and 11 days in vitro. The best performance of the substrates is associated with intermediate surface roughness and feature spacing.

Wetting dynamics and evaporation of sessile droplets on nano-porous alumina surfaces

An experimental investigation of wetting and evaporation of sessile droplets is presented on nano-porous alumina substrates having different pore distribution (uniform, random and linearly arranged) morphologies and pore sizes (70–120nm). Firstly, the behavior of a droplet as it spreads and wets a surface is captured using high speed videography and benchmarked with the correlation given by the Tanner's law. The Cassie–Wenzel state transition of a droplet on nano-porous surfaces is also scrutinized with 3D-Laser Confocal Microscopy which clearly reveals the sub-stages of the transition process. During wetting transition, entrapped air bubbles are seen coming out of the nano-porous substrate, which lead to micro-convection. In this study, sessile droplet evaporation on the nano-textured surfaces is also investigated. Evaporation can be considered as a quasi-steady-state process, such that the vapor concentration distribution above the droplet satisfies the Laplace equation, but with a time-varying droplet surface. For benchmarking, the evaporation of sessile water and ethanol droplets on standard borosilicate glass and Teflon surfaces is examined, and results are compared with available diffusion models. Contact angle variation with time is recorded along with rate of evaporation in a controlled environment with specified humidity and temperature boundary condition. The coupling between wettability and eventual rate of droplet evaporation is established. The results clearly show that nano-structuring is an effective tool to control wettability as well as the diffusive evaporation process. In addition, the physical morphology and pore distribution affects wettability as well as evaporation rates.

Effect of the nanostructure of porous alumina on growth behavior of MG63 osteoblast-like cells

It is well known that cellular responses to materials, in terms of adhesion, migration and proliferation, are highly affected by the surface characteristics of the materials. The investigation of the effect of material surface topography on cell behaviors is of great importance for the development of implanted biomaterials in tissue engineering. Alumina is one of the most popular implant materials used in orthopedics, but few data are available concerning the potential cellular responses of MG63 to nanoporous alumina. The present study investigated the size effect of nanoporous alumina substrates on MG63 cell behaviors in terms of cell viability, expression of integrin β1, alkaline phosphatase (ALP) activity and changes of cell morphology, respectively. Cell viability was measured by means of MTT assay and integrin β1 expression was detected by immunofluorescence staining and real-time PCR. Scanning electron microscopy (SEM) was used to observe cell morphology. Cell function was evaluated by detecting the ALP activity and mineralization. Results showed that cell viability and expression of integrin β1 were decreased with the increasing pore size, however, the increasing pore size of the alumina resulted in elongated cell morphology, enhanced ALP activity and mineralization. This study showed that the surface topography of nanoporous alumina plays an important role in regulating the behaviors of MG63 osteoblast-like cells and porous alumina can be regarded as useful substrate in tissue engineering.