Nanoporous Alumina Films Research Articles

Acoustic microscopy characterization of highly-ordered anodized nanoporous alumina films for nanotechnology applications

The acoustic properties of a well-aligned nanoporous alumina (NP Al2O3) film synthesized through a two-step anodization process have been characterized nondestructively using scanning acoustic microscopy (SAM). A novel method is proposed to extract the material properties of thin nanoporous film such as Elastic modulus, Poison's ratio, thickness, and porosity. The proposed technique to obtain the material properties is based on leaky surface-acoustic waves. These investigated material properties also agree well with their corresponding standard values obtained by other methods. An additional experimental investigation is conducted to detect non-uniformity in NP Al2O3 films based on the V(z) curve. It is found that the Rayleigh velocity for various scanning areas shows variation corresponding to non-uniformity in the film. This non-uniform surface morphology of nanoporous Al2O3 is also validated by polarized-resolved terahertz time-domain spectroscopy (THz TDS). A further thickness measurement using THz TDS coupled with acoustic microscopy provides a complete characterization of the NP Al2O3 film.

Simple Self-Powered Sensor for the Detection of D2O and Other Isotopologues of Liquid Water.

Distinguishing between heavy water and regular water has been a continuing challenge since these isotopologues of water have very similar physical and chemical properties. We report the development and evaluation of a simple, inexpensive sensor capable of detecting liquid D2O and other isotopologues of liquid water through the measurement of electrical signals generated from a nanoporous alumina film. This electrical output, consisting of a sharp voltage pulse followed by a separate broad voltage pulse, is present during the application of microliter volumes of liquid. The amplitude and temporal characteristics of these pulses have been combined to enable four diagnostic parameters for sensing D2O and H218O. The sensing mechanism is based on different modification effects on the alumina surface by H2O and D2O, spatially localized variations in the surface potential of alumina induced by isotopically substituted water molecules, combined with the effect of isotopic composition on charge transfer. As a proof-of-concept demonstration, a sensing system has been developed that provides real-time detection of liquid D2O in a stand-alone system.

Effect of Al Polishing Conditions on the Growth and Morphology of Porous Anodic Alumina Films.

The conditions applied during the electrochemical polishing of aluminum were found to be important parameters for the successive formation of nanoporous alumina films. First, a high-purity Al foil was electrochemically polished in an aqueous solution containing C2H5OH and HClO4 at various sets of conditions, such as applied potential (5-35 V), temperature (0-20 °C), and process duration (10-180 s). Extensive studies of the topography of Al after polishing by scanning electron microscopy and atomic force microscopy allow verification of the correlations between conditions applied during the substrate pretreatment and dimensions of the nanopatterns generated on the metal surface. Next, Al polished samples at two different sets of conditions were used as starting materials for anodization. Unpolished Al samples were also anodized for reference. It was confirmed that electropolishing conditions do not significantly affect the oxide growth rate during anodization and the efficiency of anodic film formation. On the contrary, it was proved that the dimensions of the surface texture formed during Al polishing significantly affect the morphology and pore order within the anodic film. Therefore, it can be stated that it is possible to tune to some extent the arrangement of nanochannels within anodic aluminum oxide films by simply changing conditions during the electropolishing procedure..

Understanding the fast formation mechanism of hard nanoporous alumina films on aluminum in acidic solutions containing nitric acid

Nanoporous alumina films formed on aluminum via anodization are widely used for scientific and technical purposes owing to their various advantageous properties, including excellent corrosion and wear resistance, good thermal and electrical insulation, and facile producibility at low cost. Herein, we report a novel approach to fabricate hard porous anodic alumina (PAA) films by adding an appropriate amount of nitric acid to sulfuric-, oxalic-, and phosphoric-acid solutions at critical high current densities. The effects of the nitric additives on the anodizing behavior, film growth mechanism, microstructure, chemical composition, film thickness, microhardness, and appearance of the PAA films formed from various solutions were systematically investigated. The addition of nitric acid enhanced the formation of alumina and resulted in fast growth rates of 196 μm/h and 166 μm/h for sulfuric- and oxalic-based solutions, respectively, while maintaining high hardness of approximately 380 and 430 HV0.1, respectively. The surface hardness of the films decreased to some extent with the increase in nitric acid concentration and/or film thickness owing to the strong corrosive nature of nitric acid during prolonged anodization, although nitrogen inclusion in anodic alumina films was not detected through X-ray photo-electron spectroscopy, which differs from the other acidic anions. In particular, the barrier layer thickness and pore intervals of PAA films increased with the nitric concentration in the aforementioned solutions, leading to a fast growth rate and high hardness inside the films.

Human Papilloma Virus DNA Detection in Clinical Samples Using Fluorogenic Probes Based on Oligonucleotide Gated Nanoporous Anodic Alumina Films.

In this work, fluorogenic probes based on oligonucleotide capped nanoporous anodic alumina films are developed for specific and sensitive detection of human papilloma virus (HPV) DNA. The probe consists of anodic alumina nanoporous films loaded with the fluorophore rhodamine B (RhB) and capped with oligonucleotides bearing specific base sequences complementary to genetic material of different high-risk (hr) HPV types. Synthesis protocol is optimized for scale up production of sensors with high reproducibility. The sensors' surfaces are characterized by scanning electron microscopy (HR-FESEM) and atomic force microscopy (AFM) and their atomic composition is determined by energy dispersive X-ray spectroscopy (EDXS). Oligonucleotide molecules onto nanoporous films block the pores and avoid diffusion of RhB to the liquid phase. Pore opening is produced when specific DNA of HPV is present in the medium, resulting in RhB delivery, that is detected by fluorescence measurements. The sensing assay is optimized for reliable fluorescence signal reading. Nine different sensors are synthesized for specific detection of 14 different hr-HPV types in clinical samples with very high sensitivity (100%) and high selectivity (93-100%), allowing rapid screening of virus infections with very high negative predictive values (100%).

Optical Anisotropy of Nanoporous Alumina Films as the Basis for Creation of Achromatic Phase Plates with a Variable Phase Difference of the Orthogonal Polarized Components of Transmitted Radiation

Optical Anisotropy of Nanoporous Alumina Films as the Basis for Creation of Achromatic Phase Plates with a Variable Phase Difference of the Orthogonal Polarized Components of Transmitted Radiation

Low-Cost Nanostructured Coating of Anodic Aluminium Oxide Synthesized in Sulphuric Acid as Electrolyte

The anodic oxidation of aluminium is an electrochemical technique that allows obtaining nanostructures with easily adjustable morphology depending on the synthesis variables, for its application in medicine, engineering, biotechnology, electronics, etc. In this work, low-cost aluminium oxide nanostructured films were synthesized and morphologically characterized using two anodization steps in sulphuric acid, varying the concentration and temperature of the electrolyte and anodization voltage. The order of the porous matrix, pore diameter, interpore distance, pore density, thickness, and porosity were measured and statistically analyzed. The results showed that under the proposed conditions it is possible to synthesize low-cost nanoporous aluminium oxide films, with a short-range ordering, being the best ordering conditions 10 °C and 0.3 M sulphuric acid at 20 V and 5 °C and 2 M sulphuric acid at 15 V. Furthermore, it was determined that the pore diameter and the interpore distance vary proportionally with the voltage, that the pore density decreases with the voltage and increases with the concentration of the electrolyte, and that the thickness of the oxide film increases with electrolyte concentration, temperature, and anodization voltage.

Thin nanoporous anodic alumina film on aluminium for passive radiative cooling

We demonstrate a simple, low-cost, and passive radiative cooler based on a monolithic design consisting of thin nanoporous anodic alumina (NAA) films grown on aluminium sheets. The NAA/Al structure maintains a high broadband reflectivity close to 98 $$\%$$ within the solar spectrum (0.4–2.2 $$\mu $$ m) and simultaneously exhibits a high average emissivity of 88 $$\%$$ within the atmospheric infrared (IR) transmission window of 8–13 $$\mu $$ m with the peak IR emission approaching 99 $$\%$$ at a wavelength of 10 $$\mu $$ m. Optical modelling of the system using optical parameters of the materials confirms that the high solar reflectance arises due to the transparent nature of NAA and high reflectivity of bottom Al, while the large thermal IR emissivity arises from the interference effects of the NAA film and the high absorption of IR light due to phonon resonances in alumina at wavelength larger than 10 $$\mu $$ m. Further, we estimate the average cooling power of NAA/Al to be about 136 W $$\hbox {m}^{-2}$$ at ambient temperature even after including the contribution to heat input from external non-radiative processes. This robust and lightweight NAA/Al can be projected as an excellent alternative to optical solar reflectors used in spacecraft for thermal heat management and rooftop cooling green technologies.

Microstructure and photoluminescence properties of anodized aluminum oxide films treated by argon ion

Nano-porous anodized aluminum oxide (AAO) films were prepared by secondary anodization, and these AAO films were subjected to argon ion treatment with different sputtering durations by radio frequency (RF) sputtering. FE-SEM, XRD, and ATR-FTIR were used to study the morphology and structural characteristics of the AAO films. The photoluminescence (PL) spectra of the AAO films were studied with variable wavelengths of excitation light, the results show that the argon ion treatment has a significant effect on the microstructure and the PL properties of the AAO films. The influence of argon ion treatment on the microstructure and the relationship between the PL properties is discussed.

Nondestructive characterization of nanoporous alumina films using terahertz scattering imaging

Ordered nanoporous (NP) alumina (Al 2 O 3 ) films were grown on Al substrates through a two-step electrochemical anodization process. The morphology as well as the roughness are characterized by terahertz (THz) reflectometry and scattering imaging. In particular, we show that before the second anodization, irregular surface morphology of the NP Al 2 O 3 films leads to significant THz scattering, whereas the far more homogeneous films following the second anodization show far less scattering. The regularity of the surface morphology is not readily ascertained using visible-light optical microscopy. The THz results are corroborated by field-emission scanning electron microscopy (SEM) and atomic-force microscopy (AFM), both of which are time-consuming, not easy to operate, and can only provide the local characterization of NP Al 2 O 3 films on Al substrate. THz-based techniques allow for the nondestructive characterization of the surface morphology of the entire NP Al 2 O 3 films on the 10–100 μm length scale important for a range of applications while providing information over macroscopic scan areas, and obviate the need for sample preparation required by other common surface-morphology characterization techniques. • Terahertz scattering imaging is applied to characterize the homogeneity of nanoporous alumina films after anodization. • Irregular surface morphology of nanoporous alumina films observed in terahertz images is corroborated by FE-SEM and AFM. • The stratigraphy and thickness of nanoporous alumina films are obtained nondestructively based on terahertz images. • Terahertz imaging may provide a reliable nondestructive and contactless modality to evaluate the quality of nanoporous films.

A fluorogenic capped mesoporous aptasensor for gluten detection

Celiac disease is a complex and autoimmune disorder caused by the ingestion of gluten affecting almost 1% of global population. Nowadays an effective treatment does not exist, and the only way to manage the disease is the removal of gluten from the diet. Owing the key role played by gluten, clear and regulated labelling of foodstuff and smart methods for gluten detection are needed to fight frauds on food industry and to avoid the involuntary ingestion of this protein by celiac patients. On that scope, the development of a novel detection system of gluten is here presented. The sensor consists of nanoporous anodic alumina films loaded with a fluorescent dye and capped with an aptamer that recognizes gliadin (gluten’s soluble proteins). In the presence of gliadin, aptamer sequences displace from the surface of anodic alumina resulting in pore opening and dye delivery. The dispositive shows a limit of detection (LOD) of 100 μg kg−1 of gliadin, good selectivity and a detection time of approximately 60 min. Moreover, the sensor is validated in real food samples. This novel probe allows fast gluten detection through a simple signalling process with potential use for food control.

Анизотропные алюмооксидные нанопористые мембраны.

In the present study, we report the development of a new type of anodic aluminium oxide membranes. Nanoporous alumina films have been fabricated by galvanostatic two-step anodizing technique. For the 1stgroup of samples at both stages was used water solution of oxalic acid as an electrolyte; and for the 2ndgroup at the 2ndstage was used a special multicomponent electrolyte. Using atomic force microscopy, it was found that a method of barrier layer thinning “from above” in combination with chemical removal of metal enables permeable anisotropic alumina membranes fabrication.

Mechanical and chemical robustness of the aluminum oxide-infiltrated block copolymer films and the resulting aluminum oxide coatings

Block copolymer (BCP) templates have been widely used for the design of functional organic-ceramic composites and ceramic structures. In this study, we explore the mechanical characteristics of such structures designed using sequential infiltration synthesis (SIS). We demonstrate that SIS significantly reinforces the BCP template and decreases their sensitivity to temperature and chemical environment. We also probe tribological characteristics of nanoporous alumina films obtained from the organic-ceramic composites after the BCP template removal. We determined with atomic force microscopy (AFM) that the lateral forces between the tip and the substrate are highly affected by the surface reactivity, surface structure, area of contact, environment, and porosity accessibility and interconnectivity. As confirmed by the quartz crystal microbalance (QCM) analysis, the high accessibility of the pores in the alumina film to the water penetration leads to a decrease in the friction for the ~4 μm in size colloidal tip sliding against the nanoporous alumina in contrast to the bulk alumina. The effect diminishes once the size of the AFM probe becomes comparable to the size of the pores, which is attributed to the limited water flow under the tip to support the applied during sliding stresses. Understanding the surface structure and its effect on the mechanical and frictional characteristics of materials is important for the predictive design of coatings that can sustain thermomechanical stresses and can be used to enhance the lubrication of fluids in various mechanical systems.

Renovate Tribological Properties of Nanostructured Alumina Templates by Diverse Electrolytes

Friction and wear properties of NAAO templates were calculated in affinity to pore dimensions and applied load. Homogeneously uniformly decorative synthesized by anodization of nanoporous aluminium oxide films having 65–95 μm thick and pores of 143.5, 105, 84.4 nm diameter. A tribological competency of the material checked out with loads and 250 rpm on the pin on a dry wear disc. The anodized NAAO sample has wear resistance increased by 25% as compared to the non-anodized sample. The pore density little bit impressed the frictional characters of NAAO template. We counsel that these course templates basically contribute to the reduction of friction distrait the pore structure by proving energy-dispersive spectroscopy (EDS).

Nanoindentation study of the mechanical properties and deformation behavior of nanoporous alumina films

Over the past decade, anodic aluminium oxide (AAO) has become one of the most widely used materials as a platform for developing new types of devices in micro- and nanotechnology. Due to the potential use of highly ordered honeycomb porous AAO membranes in many engineering applications, considerable attention is being paid to the mechanical characterization of such thin films. In this study, the mechanical properties and deformation behavior of a nanoporous alumina film were investigated by nanoindentation. AAO films with an average pore diameter of 40 nm were fabricated electrochemically. The morphology and the mechanical properties of AAO were studied using scanning electron microscopy and nanoindentation, respectively. The force-displacement dependences obtained revealed that in the case of a freestanding AAO membrane the sample demonstrates extremely high elasticity. The indentation modulus and the hardness were found to decrease nonlinearly with an increase in the applied force.

Experimental determination of the role of increased surface area in pool boiling from nanostructured surfaces

The use of nanostructured surfaces to enhance pool boiling heat transfer performance has previously been demonstrated for a variety of outwardly-projecting nanostructures such as nanowires and nanotubes. Such enhancement has been attributed to a variety of factors, including greater surface area, improved wickability, and superior nucleation site density as compared to unmodified surfaces. However, since these three phenomena are inherently interlinked with the presence of the nanostructures, isolating each one for independent study to truly understand its relative importance in enhancing pool boiling heat transfer has remained a challenge. In this work, nanoporous anodized aluminum oxide (AAO) films on metallic aluminum (Al) substrates are used to serve as an inverse representation of an aligned nanowire array with similar increase in surface area but without inter-nanowire/nanotube wicking action or significant change in observed static wetting behavior relative to Al control samples with a solid native oxide film. Further, it is shown via a combination of experiment and analytical modeling that the AAO-covered Al samples studied here do not represent a significant increase in nucleation site density relative to the control samples. In this way, the influence of enhanced surface area of a nanostructured sample on pool boiling performance was isolated and quantitatively determined. Pool boiling performance for Al samples with solid native oxides and AAO films was measured using a custom-built test setup, with the commercial, low surface tension, dielectric coolant Novec™ HFE-7100 as the working fluid. Results were interpreted via a nanopore wetting model along with imaging analysis of bubble size, which collectively point to wickability and nucleation site density playing a greater role than increased surface area in nanostructure-based pool boiling enhancement.

(Invited) Formation, Characterization and Applications of Nanowire Films Formed By Anodizing of Metals

Anodizing of metals and alloys is a simple electrochemical process to form various nano-structured oxide films, including self-ordered nanoporous and nanotubular films. Typical examples are nanoporous alumina films with honeycomb morphology and nanotubular array of anodic titanium oxide. Recently, several nanowire-type anodic films have been prepared on metals, including aluminum, copper and zinc. The formation mechanism of of the nanowire films may be different from those of the nanoporous and nanotubular anodic films. Here, our recent studies on the formation, characterization and applications of the anodic nanowire films on metals are presented. When zinc plate was anodized at a constant voltage in KHCO3 aqueous electrolytes, nanowires were covered on the entire zinc surface. From the SEM observations, it was found that pits were initially formed on zinc surface. The low-voltage SEM observations disclosed that some small pits were covered with a thin passive film, and in such pits nanowires were preferentially nucleated probably because of the increased concentration of dissolved Zn2+ ions in the pits. Then, the nanowires were grown radially over the pits and covered the entire zinc surface. The nanowires were not composed of ZnO. XPS and FT-IR analyses of the nanowire films indicated the presence of carbonate and hydroxide species; the nanowire consisted of basic zinc carbonate. TEM observations confirmed the single crystalline nature of the as-formed nanowires. The basic zinc carbonate was converted to ZnO without changing the nanowire morphology by heat treatment in air, although single crystalline nanowires changed to polycrystalline one. In addition, slit-type nanopores were introduced into the nanowires so that the specific surface area of the nanowire films was highly enhanced by the heat treatment. The nanowire films exhibits unique wettability to liquids. The as formed nanowire films showed superhydrophilicity with a water contact angle close to 0o and after organic coating the surface changed to superhydrophobic with a water contact angle greater than 150o. Such surface may have many potential applications.

Label-free detecting oligonucleotide hybridization melting temperature in real-time with a reflectometric interference spectroscopy–based nanosensor system

The accurate determination of melting temperature (Tm) of oligonucleotide hybridization is the primary for various biosensors based on the technique of using surface-tethered nucleic acids as probes. In this paper, a new type of nanosensor based on the reflectometric interference spectroscopy (RIFS) was proposed to determine the Tm in real time. A composite solid nanostructure, i.e., a nanoporous anodic alumina film with a 15 nm top layer of gold sputtered on surface, was employed as the substrate of the RIFS-based nanosensor, and a 30 bp oligonucleotide molecule was bond to the inner wall of pores and hybridized, and then its Tm was detected with raising temperature. The experimental data showed that there were obvious signs of dsDNA molecules denatured and melted into ssDNA molecules at 95 ℃, which was highly consistent with the theoretical analysis. The cost-effective RIFS nanosensor has excellent thermal and physicochemical stabilities, higher determination accuracy, and great potential in the field of biomolecule research.

Synthesis and Morphological Characterization of Nanoporous Aluminum Oxide Films by Using a Single Anodization Step

Nanoporous anodic aluminum oxide (AAO) films play an important role in nanotechnology due to their easily adjustable morphological properties and wide range of applications. Thus, a deep and systematic characterization of the morphological properties of these coatings is essential. The most important variables in the synthesis of nanoporous AAO films include the anodization voltage, nature, concentration and temperature of the electrolyte, which, combined, result in pores of different sizes and geometries. In the present work, AA 1050 alloy was used to synthesize AAO films, using 0.3 and 0.9 M oxalic acid as the electrolyte and combining different electrolyte temperatures (20, 30 and 40 °C) and anodizing voltages (30, 40 and 60 V), with the aim to correlate the morphological properties of the coatings with the synthesis parameters of a single anodization step. The coatings obtained were characterized by optical microscopy and scanning electron microscopy, determining pore diameter, interpore distance, pore density and coating thickness. The results showed that, by varying the anodic synthesis conditions, it is possible to obtain coatings with a pore diameter between 21 and 97 nm, an interpore distance between 59 and 138 nm, pore density between 2.8 × 1010 and 5.4 × 109 pores/cm2 and thicknesses between 15 and 145 µm. In this way, the right combination of synthesis variables allows synthesizing AAO coatings with morphological characteristics best suited to each particular application.

Enhancement in sustained friction and wear resistance of nanoporous aluminum oxide films obtained by controlled electrochemical oxidation process.

The primary purpose of this study is to investigate the effect of the electrochemical parameters required in the anodic oxidation process on the friction and wear resistance of the obtained nanoporous aluminum oxide films. The wear resistance of the aluminum oxide films and the electrochemically polished Al1050 alloy were tested using a ball-on-disc tribometer having a corundum ball as counterbody in dry conditions. The friction coefficient graphs were recorded during wear tests at a 5 N normal force simulating the application of nanoporous aluminum oxide films in industrial areas requesting moderate wear resistance. The wear tracks formed on the tested surfaces were analyzed ex situ both qualitatively and quantitatively. Higher wear resistance is showed by nanoporous aluminum oxide films as compared with electro polished Al1050 alloy substrate.