Two-step Anodization Procedure Research Articles

Pore Size Dependence of Optical Absorption Enhancement in Porous Anodic Aluminum Oxide

Three samples of high ordered AAO template were prepared via a two-step anodization procedure, the pore size was modified during the pore-widening process to tune the pore diameter to 50, 70, and 90 nm. Scanning electron microscopy (SEM) was adopted to gauge the pore diameter and the cell unit of the periodical hexagonal structure of the prepared AAO templates. In addition, the UV-vis spectrometer shows the variation of the absorbance spectrum for each pore size of the prepared AAO templates. To prove that the pore diameter (PD) and the intermediate layer (IL) could enhance the absorbance of the materials, a Lumerical FDTD solution was used by involving the exact experimental conditions of the AAO template. The resulting data show that a specific pore diameter with a specific intermediate layer can improve the absorbance spectrum of the materials. Thus, the results could serve the applications related to solar energy conversion (antireflective and photocatalyst) and photonics.

Preparation and characterization of a high-efficiency photoelectric detector composed of hexagonal Al2O3/TiO2/TiN/Au nanoporous array

The light sensitivity of a novel TiO2/TiN-nanotubes-based photodetector is improved by coating it with Au nanoparticles. Using a porous aluminum oxide template (PAOT), TiO2/TiN nanotubes were produced. A two-step anodization procedure was used to produce the PAOT using the Ni-imprinting method. Atomic layer deposition and magnetron sputtering were used to deposit the TiO2/TiN layers. X-ray diffraction, scanning electron microscopy, and reflection/absorption optical spectroscopy were used to characterize the PAOT, PAOT/TiO2/TiN, and PAOT/TiO2/TiN/Au. With a pore diameter of 320 nm and an interpore distance of 61 nm, the PAOT exhibits a high-ordered hexagonal nanoporous array. Following the deposition of TiN on the PAOT/TiO2, the bandgap of PAOT/TiO2 reduced from 3.1 to 2.2 eV, indicating increased visible light absorption. The photoresponsivity values for PAOT/TiO2/TiN and PAOT/TiO2/TiN/Au photodetectors are almost the same (453 mA W−1) at 390 nm. Without and with Au coating, the photoresponsivity values at 636 nm are 392 and 438 mA W−1, suggesting a considerable increase in the Vis/NIR range. Moreover, the detectivity values for the photodetector are enhanced after Au coating, in which the detectivity value for photodetectors are 8.74 × 109 and 8.75 × 1012 Jones at 636 nm before and after Au coating, respectively. While the detectivity values are 8.86 × 1012 Jones for both photodetectors at 390 nm. As a result of the hot electrons from the TiN and Au layers, the constructed photodetector has a very high photoelectrical response, making it extremely promising as a broadband photodetector.

Growth of less than 20 nm SnO Nanowire Using AAO Template for Gas Sensing

Large-scale stannous oxide (SnO) nanowires were synthesized via a template and catalyst-free thermal oxidation process. After annealing Sn nanowires embedded AAO template in atmosphere, we observed a large scale of SnO nanowires. SnO nanowires were first prepared via the electrochemical deposition and an oxidization method based on an AAO template. The preparation of SnO nanowires use aluminum sheet (purity 99.999%) and then two-step anodization procedure to obtain raw alumina mold. Finally, transparent alumina mold (AAO template) were obtained by the reaming, soaking with phosphoric acid for 20 minutes and a stripping process. We get a pore size of < 20 nm transparent alumina mold. In order to electroplating needs, we produce platinum film on the bottom surface of AAO template by using sputtering method as the electrode of electroplating deposition. The structure was characterized by X-ray diffraction (XRD). High resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM) with x-ray energy dispersive spectrometer (EDS) was used to observe the morphology. The EDS spectrum showed that components of the materials are Sn and O. FE-SEM results show the synthesized SnO nanowires to have an approximate length of ~ 10 - 20 μm with a highly aspect ratio > 500. SnO nanowires with an Sn/O atomic ratio of ~ 1 : 1 were observed from EDS. The crystal structure of SnO nanowires showed that all the peaks within the spectra can be indexed to SnO with a tetragonal structure. This studies may lead to the use of the 1D structure nanowires into electronic nanodevices and/or sensors, thus leading to nanobased functional structures.

Growth of Less than 20 nm SnO Nanowires Using an Anodic Aluminum Oxide Template for Gas Sensing.

Stannous oxide (SnO) nanowires were synthesized by a template and catalyst-free thermal oxidation process. After annealing a Sn nanowires-embedded anodic aluminum oxide (AAO) template in air, we obtained a large amount of SnO nanowires. SnO nanowires were first prepared by electrochemical deposition and an oxidization method based on an AAO template. The preparation of SnO nanowires used aluminum sheet (purity 99.999%) and then a two-step anodization procedure to obtain a raw alumina mold. Finally, transparent alumina molds (AAO template) were obtained by reaming, soaking with phosphoric acid for 20 min, and a stripping process. We got a pore size of < 20 nm on the transparent alumina mold. In order to meet electroplating needs, we produced a platinum film on the bottom surface of the AAO template by using a sputtering method as the electrode of electroplating deposition. The structure was characterized by X-ray diffraction (XRD). High resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM) with X-ray energy dispersive spectrometer (EDS) were used to observe the morphology. The EDS spectrum showed that components of the materials were Sn and O. FE-SEM results showed the synthesized SnO nanowires have an approximate length of ~10–20 μm with a highly aspect ratio of > 500. SnO nanowires with a Sn/O atomic ratio of ~1:1 were observed from EDS. The crystal structure of SnO nanowires showed that all the peaks within the spectrum lead to SnO with a tetragonal structure. This study may lead to the use of the 1D structure nanowires into electronic nanodevices and/or sensors, thus leading to nano-based functional structures.

Improved corrosion resistance of commercially pure magnesium after its modification by plasma electrolytic oxidation with organic additives.

The optimal mechanical properties render magnesium widely used in industrial and biomedical applications. However, magnesium is highly reactive and unstable in aqueous solutions, which can be modulated to increase stability of reactive metals that include the use of alloys or by altering the surface with coatings. Plasma electrolytic oxidation is an efficient and tuneable method to apply a surface coating. By varying the plasma electrolytic oxidation parameters voltage, current density, time and (additives in the) electrolytic solution, the morphology, composition and surface energy of surface coatings are set. In the present study, we evaluated the influence on surface coatings of two solute additives, i.e. hexamethylenetetramine and mannitol, to base solutes silicate and potassium hydroxide. Results from in vitro studies in NaCl demonstrated an improvement in the corrosion resistance. In addition, coatings were obtained by a two-step anodization procedure, firstly anodizing in an electrolyte solution containing sodium fluoride and secondly in an electrolyte solution with hexamethylenetetramine and mannitol, respectively. Results showed that the first layer acts as a protective layer which improves the corrosion resistance in comparison with the samples with a single anodizing step. In conclusion, these coatings are promising candidates to be used in biomedical applications in particular because the components are non-toxic for the body and the rate of degradation of the surface coating is lower than that of pure magnesium.

Ordered nanopore arrays with large interpore distances via one-step anodization.

Preparation of pre-patterned alumina substrates using bottom-up techniques compatible with nanotechnology applications is still a challenge. We present a novel methodology to achieve superior order in 'anodic' alumina with large interpore distances by a convenient one-step anodization process. The use of transparent insulators renders such anodic layers applicable as templates for nanostructured photovoltaic or photoelectrochemical devices.

Design of alveolate Se-inserted TiO2 and its effect on osteosarcoma cells and osteoblasts

Osteosarcoma is a common clinical disease with high incidence, relapse rate and lethality. Thus it is urgently necessary to fabricate bone implants with anti-tumor functions to improve the prognosis of bone cancer. In this study, a novel double-layer TiO2 nano-grid with an outer diameter of 100 nm and an inner diameter of 35 nm is prepared by a simple two-step anodization procedure. In the outer pore of the double-layer TiO2 nano-grid, we precisely embed selenium (Se) nanoparticles using electrodeposition, and this process can be readily tailored by adjusting the time. Biological assays indicate that the prepared Se-embedded TiO2 nano-grid arrays show a dose-dependent anti-tumor effect by inducing apoptosis in MG63 cells. Compared with the Ti substrate, the Se-embedded TiO2 which has a double-layer honeycomb structure facilitates healthy cell (MC3T3-E1) growth when the electrodeposition time is less than 1 h. Thus the novel bone implants have a balanced anti-tumor effect and biocompatibility and are therefore potential candidates for the treatment of osteosarcoma.

Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties

Black TiO2 nanotube arrays were synthesized via a two-step anodization procedure by controlling the duration of anodization and annealing in an argon atmosphere. Oxygen vacancies were introduced in black TiO2 nanotube arrays during the anodization and annealing process due to an insufficient oxygen atomosphere, as confirmed by EPR, XRD, SEM, TEM, XPS, and Raman analysis. Black TiO2 nanotube arrays prepared in 6 h (denoted as Ar TNT-6h) exhibited excellent photoelectrocatalytic performance and stable photoelectrochemical performance due to a considerable amount of oxygen vacancies, which ensured charge separation efficiency and strong visible light absorption. The photoelectrocatalytic activities of the samples were monitored by the decomposition of Rhodamine B (RhB). Notably, the black TiO2 nanotube arrays exhibited lower charge-transfer resistance in electrochemical impendence spectroscopy (EIS). The photocurrent density of Ar TNT-6h (1.2 mA/cm2) was nearly twelve times higher than that of air-TNTs (pristine TiO2 nanotube arrays annealed in air, 0.1 mA/cm2). The formation of oxygen vacancies in black TiO2 nanotube arrays was influenced by the duration of anodization and the annealing atmosphere. A considerable amount of oxygen vacancies significantly improved the separation efficiency of photogenerated electrons and holes, thus enhancing the photoelectrocatalytic efficiency.

On the anodic aluminium oxide refractive index of nanoporous templates

In the present study, we have determined the intrinsic refractive index of anodic aluminium oxide, which is originated by the formation of nanoporous alumina templates. Different templates have been fabricated by the conventional two-step anodization procedure in oxalic acid. Their porosities were modified by chemical wet etching allowing the tuning of their effective refractive indexes (air-filled nanopores + anodic aluminium oxide). By standard spectroscopic light transmission measurements, the effective refractive index for each different template was extracted in the VIS–NIR region. The determination of the intrinsic anodic aluminium oxide refractive index was performed by using the Maxwell–Garnett homogenization theory. The results are coincident for all the fabricated samples. The obtained refractive index (~1.55) is quite lower (~22%) than the commonly used Al2O3 handbook value (~1.75), showing that the amorphous nature of the anodic oxide structure strongly conditions its optical properties. This difference is critical for the correct design and modeling of optical plasmonic metamaterials based on anodic aluminium oxide nanoporous templates.

Effect of pore diameter in nanoporous anodic alumina optical biosensors.

The influence of pore diameter over the optical response of nanoporous anodic alumina (NAA) films is analyzed by reflectance interference spectroscopy. NAA films manufactured by a two-step anodization procedure in oxalic acid exhibiting three well-defined pore diameter distributions with pores of 32 ± 4, 50 ± 3, and 73 ± 2 nm are studied. The optical detection of biomolecules is investigated by serially dosing protein A, human IgG and anti-human IgG into a nanoporous matrix using a custom-made flow cell. The results demonstrate that the transduction signal, the variation of effective optical thickness upon IgG binding to protein A (ΔEOT), depends on the nanopore diameter: for small pore diameter (32 nm) no significant differences in signals are observed for different protein concentrations whereas for larger pore diameters (50 nm and 73 nm) the signals increase for increasing concentrations from 10 to 100 μg mL(-1). Our experiments also show that this signal can be further enhanced by amplification with anti-human IgG due to the multiple binding events between the antigen and the antibody. These results will enable the development of more sensitive interferometric biosensors based on NAA.

Performance of through-hole anodic aluminum oxide membrane as a separator for lithium-ion battery

A high porosity through-hole anodic aluminum oxide (AAO) membrane has been prepared by a two-step anodization procedure. The as-prepared AAO membrane is around 60μm thick with a uniform pore size of about 80nm. Moreover, we also found that the AAO membrane not only has a good mechanical strength but also shows better performances on electrolyte infiltration and retention compared with the commercial polymer separator. The LiFePO4/graphite battery using the AAO separator exhibits better cycling stability, rate capability and low-temperature performance than those using the polymer separators. The excellent electrochemical performances can be attributed to good hydrophilic characteristic, high porosity and through-pore structure of the AAO separators used. Therefore, the AAO separator is very promising to be applied in lithium-ion batteries.

Pushing the Size Limits in the Replication of Nanopores in Anodized Aluminum Oxide via the Layer-by-Layer Deposition of Polyelectrolytes

We report on the successful replication of the smallest pores in anodized aluminum oxide (AAO) via the layer-by-layer (LBL) deposition of polyelectrolytes to date to yield free-standing, open nanotubes with inner and outer diameters (±2σ) down to 37 ± 4 and 52 ± 19 nm, respectively. This work is based on the fabrication of defined arrays of highly regular nanopores by anodic oxidation of aluminum. Pores with pore diameters between 53 ± 9 and 356 ± 14 nm and interpore distances between 110 ± 3 and 500 ± 17 nm were obtained using an optimized two-step anodization procedure. 3-(Ethoxydimethylsilyl)propylamine-coated pores were replicated by alternating LBL deposition of poly(styrenesulfonate) and poly(allylamine). The detrimental adsorption of polyelectrolyte on the top surface of the template that typically results in partial pore blocking was eliminated by controlling the surface energy of the top surface via deposition of an ultrathin gold layer. The thickness of the deposited LBL multilayer assembly at the pore orifice agreed to within the experimental error with the thicknesses measured by variable angle spectroscopic ellipsometry and atomic force microscopy (AFM) for layers assembled on flat substrates. The selective dissolution of the alumina template afforded free-standing, open polymer nanotubes that were stable without any cross-linking procedure. The nanotubes thus obtained possessed mean outer diameters as small as 52 nm, limited by the size of the AAO template.

Anodic Aluminum Oxide (AAO) Membranes for Neurite Outgrowth

ABSTRACTAnodic aluminum oxide (AAO) membranes were fabricated in a mild two-step anodization procedure. The voltage was varied during both anodization steps to control the pore size and morphology of the AAO membranes. Pore sizes ranged from 34 nm to 117 nm. Characterization of the pore structure was performed by scanning electron microscopy (SEM). To assess the potential of the AAO membranes as a neuronal differentiation platform, C17.2 neural stem cells (NSCs), an immortalized and multipotent cell line, were used. The NSCs were forced to differentiate via serum-withdrawal. Cellular growth was characterized by immunocytochemistry (ICC) and SEM. ImageJ software was used to obtain phenotypic cell counts and neurite outgrowth lengths. Results indicate a highly tunable correlation between AAO nanopore sizes and differentiated cell populations. By selecting AAO membranes with specific pore size ranges, control of neuronal network density and neurite outgrowth length was achieved.

Nanostructured anodic iron oxide film as photoanode for water oxidation

Two different configurations of photoanodes based on anodic iron oxide were investigated for photoelectrochemical water oxidation. A self-ordered and vertically oriented array of iron oxide nanotubes was obtained by anodization of pure iron substrate in an ethylene glycol based electrolyte containing 0.1M NH4F + 3 vol% water (EGWF solution) at 50 V for 15 min. Annealing of the oxide nanotubes in a hydrogen environment at 500 °C for 1 h resulted in a predominantly hematite phase. The second type of photoanode was obtained by a two-step anodization procedure. This process resulted in a two-layered oxide structure, a top layer of nano-dendrite morphology and a bottom layer of nanoporous morphology. This electrode configuration combined the better photocatalytic properties of the nano-dendritic iron oxide and better electron transportation behaviour of vertically oriented nano-channels. Annealing of these double anodized samples in an acetylene environment at 550 °C for 10 min resulted in a mixture of maghemite and hematite phases. Photocurrent densities of 0.74 mA cm−2 at 0.2 VAg/AgCl and 1.8 mA cm−2 at 0.5 VAg/AgCl were obtained under AM 1.5 illumination in 1M KOH solution. The double anodized samples showed high photoconductivity and more negative flat band potential (−0.8 VAg/AgCl), which are the properties required for promising photoanode materials.

Amphiphilic TiO2Nanotube Arrays: An Actively Controllable Drug Delivery System

Amphiphilic TiO(2) nanotube arrays are fabricated by a two-step anodization procedure combined with hydrophobic monolayer modification after the first step. These tubes can be used as biomolecular carriers, where the outer hydrophobic barrier provides an efficient cap against drug leaching to the environment. By utilizing the photocatalytic ability of TiO(2), a precisely controlled removal of the cap and a highly controlled release of the hydrophilic payload (drug) can be achieved.

Simple fabrication of hexagonally well-ordered AAO template on silicon substrate in two dimensions

In this study, thin anodic aluminum oxide (AAO) templates both on silicon substrates (AAO template/SiO2/Si) and Ti-coated silicon substrates (AAO template/Ti/SiO2/Si) were developed for design of magnetic, electronic and optoelectronic devices, chemical sensors and chip-scale lithium-ion rechargeable microbatteries. Two types of AAO template were prepared by using a two-step anodization procedure. The templates were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The obtained thin AAO templates were approximately 50 nm in diameter and 700 nm in length with 80-nm interpore distances in a relatively large area of 6 cm2. A barrier layer of the AAO templates was removed by a cathodic polarization method in KCl solution for several seconds. The current–time transient during removing the alumina barrier layer of the thin AAO template and the mechanism of electrochemical dissolution of the barrier layer are given in detail.

Fabrication and properties of anodic alumina humidity sensor with through-hole structure

Through-hole structural humidity sensor was fabricated by radio-frequency magnetron sputtering deposition of gold electrodes on two sides of anodic aluminum oxide (AAO) membranes which were prepared by two-step anodization procedure at 0–5°C and 40 V in 0.5 mol/L oxalic acid electrolyte. The investigation on the impedance at various humid conditions showed a linear relationship between impedance and relative humidity over the range of 12%–97% RH. Other excellent properties such as rapid response and good reproducibility were also obtained.

Mechanisms for Formation of a One-Dimensional Horizontal Anodic Aluminum Oxide Nanopore Array on a Si Substrate

A simple theoretical model based on electric-field-assisted growth and dissolution of the barrier oxide has been proposed to understand the mechanisms for formation of a one-dimensional (1D) anodic aluminum oxide (AAO) nanopore array. It is shown that formation of a one-dimensional horizontal nanopore array is the natural result of the self-adjusting effect of anodic oxidation. Because the electric field for anodization is in parallel with an aluminum film, the porous structure is formed on the side of aluminum. When the thickness of an aluminum film approaches the size of a single cell, a single array of nanopores evolves after anodization in a steady state for sufficiently long time. Our experimental results support the theoretical model. A well-ordered one-dimensional nanopore array was obtained at in oxalic acid solution using the modified two-step anodization procedure.

Nanoporous Alumina Capsules for Cellular Macroencapsulation: Transport and Biocompatibility

Currently, the most widely used treatment for diabetes is the daily subcutaneous injection of recombinant human insulin. Daily injections, however, cannot match the physiological biphasic behavior of normal insulin release, nor can they precisely meet the demands of food intake, exercise, and stress. Cellular encapsulation, or immunoisolation, is a possible solution to this problem. This cell-based therapy allows patients to receive the benefits of more physiological insulin and blood glucose regulation, without the need for immunosuppressants that are associated with organ or cell transplantation. Immunoisolation capsules were fabricated out of aluminum and aluminum oxide using a two-step anodization procedure. The diffusion behavior of glucose, immunoglobulin G (IgG), and insulin were measured. Furthermore, the functionality and viability of encapsulated MIN6 cells were tested. Finally, live cells were stained and imaged using confocal microscopy. Data indicated that this device is effective in allowing the transport of relevant molecules such as glucose and insulin, while at the same time significantly impeding the transport of IgG, suggesting that it would be efficacious in protecting cell grafts in vivo. Furthermore, encapsulated cells were able to respond dynamically to glucose input signals. Finally, cell staining showed that the viability of encapsulated cells is maintained after 24 h, although the cells appear to be more heavily concentrated at the area that is closest to the membrane. This study has shown that nanoporous alumina membranes, with well-controlled pore sizes, can be used for the encapsulation of therapeutic cells and may provide an alternative encapsulation strategy for the treatment of diabetes.

Fabrication of Anisotropic Super Hydrophobic/Hydrophilic Nanoporous Membranes by Plasma Polymerization of C 4 F 8 on Anodic Aluminum Oxide

Freestanding anodic aluminum oxide (AAO) membranes were fabricated using a two-step anodization procedure, voltage reduction method, and inductively coupled plasma polymerization. The fabrication process was monitored with electrochemical and spectroscopic ellipsometry measurements, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy. Static contact angle measurements demonstrated that the two surfaces of the fabricated AAO membranes have dramatically different degrees of hydrophobicity. The contact angle of water on the side modified with the fluorocarbon film was 150°, whereas the contact angle on the hydrophilic side, which was initially attached to the aluminum substrate, was less than 5°. The fabricated freestanding AAO membranes with only one hydrophobic surface are expected to find applications in the design of new systems for separation and filtration and as gas and liquid transport devices such as gas-diffusion electrodes for miniature power sources and sensors. © 2004 The Electrochemical Society. All rights reserved.