Regularity of nanopores in anodic alumina formed on orientated aluminium single-crystals

On aluminium single crystals with (1 1 1), (1 1 0) and (1 0 0) orientation, nanoporous alumina layers were formed in a two-step anodization process within sulphuric acid. The pore ordering within the hexagonal arrangement of the nanopores was documented by scanning electron microscopy (SEM), described on the basis of defect thermology and analyzed quantitatively by image evaluation. The best ordering was obtained in nanoporous alumina on (1 0 0) aluminium. We supposed that this is caused by the interface energy term within the driving force for the formation of the nanoporous alumina, since – in contrast to (1 1 1) and (1 1 0) aluminium as substrate – in the case of (1 0 0) aluminium the interface energy is minimised in the waved interface between aluminium and hexagonally arranged nanoporous alumina.     

Fabrication and transfer of nanoporous alumina thin films for templating applications: Metal dots array deposition and porous ZnO film growth

An approach to fabricate nanoporous alumina thin films on aluminum foils followed by their transfer to desired substrates via chemical lift-off and Van der Waals bonding is presented. By employing a black wax technique, we demonstrate a crack free (within the area of 2 × 2 cm²) lift-off and bonding of a nanoporous alumina film with thickness of as small as ~ 300 nm onto a substrate. The liftoff-and-bonding process changes neither the morphology nor the structure of the nanopores in the alumina film. Templating applications of the bonded alumina thin films are demonstrated and discussed. The results reveal that the alumina nanopores, combined with the liftoff-and-bonding technique, have great potential for templating applications in both nano-sized dots array deposition and functional materials growth.     

Fabrication of a Au nanoporous film by self-organization of networked ultrathin nanowires and its application as a surface-enhanced Raman scattering substrate for single-molecule detection

Due to its demonstrated usefulness in fields such as trace analysis, biodiagnosis, and in vivo study, surface-enhanced Raman scattering (SERS) has received renewed interest in recent years. Development of SERS substrates is of great importance as the SERS intensity and reproducibility depend strongly on the SERS substrates. In this paper we report the fabrication of Au nanoporous film (NPFs) by self-organization of networked ultrathin Au nanowires for use as SERS substrates. The acquired Au NPFs display controllable thickness, low relative density, and considerable specific surface area. Furthermore, this self-organization of nanowires not only provides abundant junctions between nanowires, 5-20 nm nanopores, and three-dimensional nanowells, but also makes nanopores/nanogaps down to 1-2 nm. These nanoscale characteristics result in a high spatial density of hotspots with Raman enhancement factors up to 10(9). Combined with the uniformity and high purity, our Au NPF provides high-quality substrates for SERS sensing.     

Electrochemical Fabrication of Metallic Nanowires and Metal Oxide Nanopores

A nuclear track etched polycarbonate membrane filter with numerous cylindrical nanopores was applied as a nanoporous template for growing metallic nanowires. Nickel, cobalt, and iron nanowires were electrodeposited into the cylindrical nanopores. Cathodic polarization curves were measured to determine an optimum condition for growing nanowires. The shape of nanowires was observed using scanning electron microscope (SEM) and the crystal structure was analyzed using transmission electron microscopy (TEM). Diameter and length of nanowires corresponded to those of nanopores and each nanowire was composed of a single crystal. Anodized aluminum oxide films were also fabricated as a novel nanoporous template. The pore length and diameter was controlled changing anodizing conditions. Ordering behavior of nanopores array in an anodized aluminum oxide film was also investigated to make a novel nanoporous template with a highly ordered honeycomb array of nanopores.     

两步阳极氧化法制备多孔阳极氧化铝膜

直流恒压下,在酸性溶液中对铝实施两步阳极氧化制备了多孔氧化铝膜。采用扫描电镜(SEM)、原子力显微镜(AFM)对制备的多孔氧化铝膜进行形貌分析,孔径在纳米级且孔分布具有高度均匀性。采用SEM对试样进行观察,分析了工艺对多孔氧化铝膜形貌的影响。利用阳极氧化初期电流密度的变化并结合阳极氧化过程中的试样的SEM照片,分析了多孔氧化铝膜的形成机理。     

Role of alumina nanoporosity in acute cell response

This work studied the effect of nanoporous alumina in acute cellular response in an in vivo model. Nanoporous alumina membranes, with pore size diameters of 20 and 200 nm, were fabricated by anodic oxidation of aluminium. The membranes were thereafter characterized in terms of pore size distribution and chemical composition. To evaluate acute inflammatory response, the membranes were implanted in the peritoneal cavity of mice. Cell recruitment to the implant site was determined by fluorescence activated cell sorting (FACS) analysis. Cell adhesion to material surfaces was studied in terms of cell number, type, and morphology using scanning electron microscopy (SEM) and immunocytochemical staining followed by fluorescence microscopy. The fabricated nanoporous alumina membranes were found to have narrow pore size distribution. The in vivo study showed that 200 nm alumina membranes induced stronger inflammatory response than 20 nm membranes. This was reflected by the number of implant-associated phagocytes and the number of cells recruited to the implantation site. Since both pore-size membranes possess similar chemical composition, we believe that the observed difference in cell recruitment and adhesion is an effect of the material nanotopography. Our results suggest that nanotopography can be used to subtly control the recruitment and adherence of phagocytic cells during the acute inflammatory response to alumina membranes.     

A facile template-free approach to metal oxide spheres with well-defined nanopore structures

Nanoporous Al₂O₃ with well-defined pore structure, crystallized framework and spherical morphology has been prepared by a facile template-free approach, which involves the preparation via homogeneous precipitation and subsequent decomposition of spherical basic aluminium sulphate particles. The particle size of the spheres can be tuned by controlling the holding time from the beginning of precipitation, and a proper decomposition temperature is important to get high surface area, high pore volume and well-defined pore structures. By the similar way, nanoporous ZrO₂ and TiO₂ spherical particles can also be prepared. These nanoporous oxides all have moderately high surface area (50–70 m²/g) and well-defined nanopores of around 4–12 nm with very narrow pore size distribution. The frameworks of these oxide spheres consist of many small nanocrystallites, between which the nanopores exist. Compared with the soft and hard template routes, this decomposition strategy of sulphates for nanoporous oxides has the advantages of simplicity and low cost.     

Nanoporosity of Al2O3 coatings obtained by impulse plasma deposition

Investigations of the size distribution of nanopores were carried out for alumina coatings deposited by the impulse plasma method. The single-phase (metastable and stable correspondingly) alumina coatings were deposited under different conditions of impulse plasma processes. The investigation of nanopore distribution was carried out using small-angle X-ray scattering. Despite the different phase composition of the coatings obtained, the most probable value of nanopores for both alumina coating materials were practically the same and equal to 5 nm. It appears that a coating porosity of the order of nanometres is characteristic for all coatings deposited by the impulse plasma method, because previously similar dimensions of nanopores were found for diamond, TiN and BN. It seems that during the impulse plasma deposition the coating grows on the substrate surface by condensation of ultra-small particles nucleated in the impulse plasma.     

Synthesis of carbon nanotubes within Pt nanoparticles-decorated AAO template

Template-based synthesis of Pt-doped carbon nanotubes has been conducted with a corona discharge enhanced chemical vapor deposition. In this paper, nanoporous anodic alumina template was firstly decorated with Pt nanoparticles upon its interior walls. As the fabrication of carbon nanotubes within the nanopores, Pt nanoparticles were simultaneously embedded into the carbon nanotubes. HRTEM indicated that the grain size of these Pt nanoparticles is 5 nm and they are homogeneously exposed on the outer surface of carbon nanotubes.     

Heat Capacity of Liquid 4He Confined in a Nanoporous Glass

We report a preliminary study of heat capacities of ⁴He confined in a nanoporous Gelsil glass that has nanopores of 2.5 nm in diameter. The heat capacity has a broad peak at a temperature far above the superfluid transition temperature obtained by torsional oscillator technique. The heat-capacity peak is attributed to formation of localized Bose-Einstein Condensates in the nanopores, in which the long-range superfluid coherence is destroyed by pore size distribution or random potential inherent to the porous glass.      

Nanoporous titania-coated alumina membranes: sol-gel synthesis and characterisation

In this work, nanoporous titania top layers were deposited by dip-coating process on microporous alpha-alumina substrates using the sol-gel process. The alumina substrates were synthesized by slip casting method using Taguchi optimising approach. The microporous substrate was then used to coat nanoporous titania layers by the sol-gel method. The thickness, pore size, structure and permeability of the membranes were characterised using SEM, XRD, STA and Hg-Porosimetry. The process conditions to achieve defect-free nanoporous titania layers with the average pore size of about 4 nm coated on the microporous alumina substrates with the average pore size of about 270 nm were determined.     

Instabilities in nanoporous media

Rayleigh instabilities in thin polymer films confined within nanoporous alumina membranes were studied. Thin films of poly(methyl methacrylate) (PMMA) were prepared by filling cylindrical nanopores in an anodic aluminum oxide (AAO) membrane with a PMMA solution in chloroform followed by solvent evaporation. When the PMMA nanotubes were annealed above the glass transition temperature (Tg), undulations in the film thickness were observed that were induced by a Rayleigh instability. The amplitude of the undulations increased with time and eventually bridged across the cylindrical nanopore in the AAO membrane, resulting in the formation of polymer nanorods with periodic encapsulated holes. A similar behavior was observed when PMMA films were confined within carbon nanotubes (CNTs). The Rayleigh instabilities in these confined geometries offer a novel means of controlling and fabricating the polymer nanostructures. These compartmentalized nanorods may have potential applications as delivery devices.     

Towards nanoporous polymer thin film-based drug delivery systems

A nanoporous polymer thin film has been developed as a potential platform for drug delivery. The film was fabricated by a light-induced polymerization process in which non-reactive solvent was first separated from photopolymer (dipentaerythritol penta-/hexa-acrylate as the monomer) and then removed from polymer via evaporation, yielding pores with diameters between 20 and 40 nm. Loading and release of Rhodamine B (drug model molecules) on both porous and non-porous thin films proved that nanopores enhanced the film's effectiveness in encapsulating and releasing the drug model molecules, which was attributed to the high surface-to-volume ratio of nanoporous film. Ultrasound-enhanced cumulative and pulsatile release revealed the advantages of ultrasound in controlled drug delivery.     

Perpendicular orientation of cylindrical domains upon solvent annealing thin films of polystyrene-b-polylactide

Polystyrene-b-polylactide (PS-PLA) was employed as a precursor to nanoporous thin films containing perpendicular cylindrical channels. Cylinder-forming PS-PLA was spin coated onto Si substrate and solvent annealed using acetone, chlorobenzene and tetrahydrofuran (THF) for different durations. By atomic force microscopy, three types of final morphology were observed at the free surface of the films (PLA surface layer, perpendicular cylinders and parallel cylinders) depending on the type of solvent and annealing time. Well-organized perpendicular domains were obtained by annealing in THF. From this oriented PS-PLA annealed thin films, a mild hydrolysis led to a highly ordered array of perpendicularly-oriented cylindrical nanopores arranged on a hexagonal lattice, rendering the resulting nanoporous mask useful for nanopattern transfer processes. The weak resistance of the film/substrate interface during PLA etching was overcome by UV light exposure prior hydrolysis.     

Highly sensitive thermoluminescent carbon doped nanoporous aluminium oxide detectors

In this work we present the synthesis, characterisation and the thermoluminescence (TL) response of nanoporous carbon doped aluminium oxide Al2O3:C produced by anodic oxidation of aluminium in organic and inorganic solvents. The X-ray and scanning electron microscopy (SEM) measurements reveal that the synthesised samples are amorphous and present highly ordered structures with uniform pore distribution with diameter of the order 50 nm. The photoluminescence and spectroscopic analysis in the visible and infrared regions show that the luminescence properties presented by the samples prepared in organic acid are due to carboxylate species, incorporated in anodic alumina films during the synthesis process. After an annealing treatment, part of the incorporated species decomposes and is incorporated into the structure of the aluminium oxide yielding a highly thermoluminescent detector (TL) . The results for X-ray irradiation in the range from 21 to 80 keV indicate a linear TL response with the dose in the range from 5 mGy to 1 Gy, suggesting that nanoporous aluminium oxide produced in the present route of synthesis is a suitable detector for radiation measurements.     

Gas adsorption and capillary condensation in nanoporous alumina films

Gas adsorption and capillary condensation of organic vapors are studied by optical interferometry, using anodized nanoporous alumina films with controlled geometry (cylindrical pores with diameters in the range of 10-60?nm). The optical response of the film is optimized with respect to the geometric parameters of the pores, for potential performance as a gas sensor device. The average thickness of the adsorbed film at low relative pressures is not affected by the pore size. Capillary evaporation of the liquid from the nanopores occurs at the liquid-vapor equilibrium described by the classical Kelvin equation with a hemispherical meniscus. Due to the almost complete wetting, we can quantitatively describe the condensation for isopropanol using the Cohan model with a cylindrical meniscus in the Kelvin equation. This model describes the observed hysteresis and allows us to use the adsorption branch of the isotherm to calculate the pore size distribution of the sample in good agreement with independent structural measurements. The condensation for toluene lacks reproducibility due to incomplete surface wetting. This exemplifies the relevant role of the fluid-solid (van der Waals) interactions in the hysteretic behavior of capillary condensation.     

IR Scattering by Optically Inhomogeneous Nanoporous Anodic Alumina Films

This paper presents results of thermographic studies of nanoporous alumina films having different geometric parameters of their porous layer (thickness and average pore diameter), which were exposed to thermal radiation. The films have been shown to shield thermal radiation. The present results suggest that nanoporous alumina membranes can be used as heat shields for reducing the thermal contrast of an object and the surrounding background in the IR spectral region.     

Hierarchical nanoporous metals as a path toward the ultimate three-dimensional functionality

Abstract

Nanoporous metals prepared via dealloying or selective leaching of solid solution alloys and compounds represent an emerging class of materials. They possess a three-dimensional (3D) structure of randomly interpenetrating ligaments/nanopores with sizes between 5 nm and several tens of micrometers, which can be tuned by varying their preparation conditions (such as dealloying time and temperature) or additional thermal coarsening. As compared to other nanostructured materials, nanoporous metals have many advantages, including their bicontinuous structure, tunable pore sizes, bulk form, good electrical conductivity, and high structural stability. Therefore, nanoporous metals represent ideal 3D materials with versatile functionality, which can be utilized in various fields. In this review, we describe the recent applications of nanoporous metals in molecular detection, catalysis, 3D graphene synthesis, hierarchical pore formation, and additive manufacturing (3D printing) together with our own achievements in these areas. Finally, we discuss possible ways of realizing the ultimate 3D functionality beyond the scope of nanoporous metals.     

Small angle neutron scattering from helium bubbles in metals

Small angle neutron scattering (SANS) has been widely used in investigating defects in metals, and in particular, to characterize the helium bubble population in implanted materials. The main advantage in using SANS is the non-destructive feature of the tests and the quantitative results obtained by averaging over a large sample volume. SANS is a powerful technique, very sensitive to microstructural changes and its use was of fundamental importance to show the bimodal distribution of the bubble population: in the vicinity of grain boundaries and free surface and inside grains, respectively. Here the most important applications of the SANS technique to the study of the helium bubbles in implanted materials are reviewed. Most of the work has been done on nickel samples, but also a ternary alloy Fe-Ni 15%wt-Cr 15%wt and a steel (MANET) with a more complicated structure have been successfully investigated. Different annealing treatments, isothermal and isochronal, were investigated in order to determine the active mechanisms of the bubble coarsening and their activation energies. From the SANS data the bubble size distributions have been determined, from which parameters such as mean radius and density of the bubble population have been calculated. The gas pressure inside the bubbles was also determined by the contrast variation technique in SANS and by a computational procedure, and an excellent agreement was found between the results. These results show a marked overpressure inside the bubbles as compared to thermodynamical equilibrium values of about 3 GPa. A comparison with results obtained by other techniques confirms the validity of SANS, which has to be considered as a complementary technique for its indirect image of the sample.