Nanoporous Anodic Aluminum Oxide Templates Research Articles

Fe-Co and Fe-Ni Nanocluster Wires by Hydrogen Reduction in Nanoporous Alumina Templates

This paper reports the fabrication, formation mechanism, nanostructure and magnetic properties of Fe50Co50, Fe65Co35 and Fe50Ni50 (at.%) nanocluster wires (NCWs) with different aspect ratios fabricated by thermal decomposition of metal nitrates and subsequent hydrogen reduction in nanoporous anodized aluminum oxide (AAO) templates. The as-synthesized NCWs have diameters in the range of 80-360 nm, and lengths in the range of 0.5-10 μm. The NCWs are composed of roughly round-shaped nanoclusters in the range of about 5-50 nm in size and a weighted average size of 14 nm with a mixture of single-crystal and poly-crystalline structures. The Fe-Co NCWs have a bcc structure while the Fe50Ni50 NCWs have an fcc structure. An intrinsic coercivity at room temperature iHc of 1.15 kOe was obtained for the Fe65Co35 NCWs. The NCWs exhibit the easy magnetization direction along the length of the nanowires as a result of the shape anisotropy.

Fabrication of carbon nanostructures from polymeric precursor by using anodic aluminum oxide (AAO) nanotemplates

Anodic aluminum oxide (AAO) nano-templates are used in many fields of nanotechnology, particularly for use in creation of nanowires and nanotubes. In this research, the method for fabricating AAO nano-templates in two different electrolyte solutions (sulfuric acid and oxalic acid) via two step anodization procedure is presented. The influence of parameters related to both anodization steps such as the electrolyte, solution temperature, voltage and time on the pore size, porous distance and pore density was investigated. Scanning electron microscopy (SEM) images of the nano-templates also pointed out the effectiveness of this anodization method. The synthesis of carbon nanostructures from a polymeric precursor such as epoxy via fully filling the nanoporous AAO templates is reported. The prepared nanowires and nanotubes have been characterized by transmission electron microscopy (TEM), Raman spectroscopy and SEM. The results show the typical morphology and properties of multiwall carbon nanotubes and other nanostructures.

Enhancing the performance of free-standing TiO2 nanotube arrays based dye-sensitized solar cells via ultraprecise control of the nanotube wall thickness

Free standing TiO2 nanotube (TNT) array film was fabricated via template-assisted Atomic Layer Deposition (ALD) in nanoporous anodic alumina templates followed by alumina dissolution. Effect of TiO2 nanotube wall thickness on the photovoltaic performance was studied on the dye sensitized solar cells with the TNT wall thickness precisely controlled by ALD between 250 and 550 cycles. The results show that the photovoltaic performance can be improved by optimizing the TNT wall thickness. A thick enough tube wall is crucial for forming space charge layer to allow efficient charge separation, but too thick wall thickness will reduce dye loading relatively. A photovoltaic energy conversion efficiency of 4.65% was obtained on the device with 450 ALD cycles of TNT wall thickness, which is about 1.8 times of that obtained with 250 cycles thickness.

Hysteresis measurement of individual multilayered Fe-Ga/Cu nanowires using magnetic force microscopy

We have investigated nano-scale magnetic structures to learn how magnetic behaviors differ from what is observed in bulk materials. In this study, we have measured the magnetic hysteresis of individual multilayered nanowire using a magnetic force microscopy (MFM). The nanowires were composed of low-aspect ratio segments of iron-gallium (Galfenol, Fe80Ga20) and copper layers. Individual wires were released from nano-porous anodized aluminum oxide (AAO) templates and separated from other nanowires to avoid significant interactions between nearby nanowires. In this experiment, structures with aspects ratios c/a of ∼3 were studied. Different MFM phase magnitudes in response to the interaction of magnetic film coated MFM tips and Fe-Ga/Cu nanowire were observed as a function of applied external magnetic field (−850 Oe < H < 850 Oe). Amplitude differences between bright and dark responses in MFM images were used to demonstrate hysteresis. Coercivity measured was about 125 Oe, which is almost same as the coercivity value (∼150 Oe) of a nanowire array measured with a vibrating sample magnetometer (VSM). When fields are applied perpendicular to the nanowires, the hysteresis loops sheared, indicating that the easy axis is along the nanowire axis. Remanence magnetizations of an individual nanowire and a nanowire array were compared to demonstrate inter-wire magnetostatic dipolar interaction.

Optimal Processing for Hydrophobic Nanopillar Polymer Surfaces Using Nanoporous Alumina Template

Nanoporous anodic aluminum oxide (AAO) templates are fabricated using an anodization method. The mean diameters of nanoporous anodic aluminum oxide templates are 100 nm and 200 nm by various processing parameters of the anodization method. A molded plastic thin film nanostructure is fabricated by nanoimprinting using the AAO template as a mold. The surface properties of the molded plastic thin film are discussed using various nanoimprinting process parameters. Contact angles of the molded plastic thin film with the nanostructure exceed those without the nanostructure. The molded plastic thin films with a nanostructure and a hydrophobic surface are formed, and their contact angles exceed 90°.

Magnetization Reversal of High Aspect Ratio Iron Nanowires Grown by Electrodeposition

Smooth and high aspect ratio Fe nanowire arrays with different diameters were prepared by a low cost electrodeposition method in nanoporous anodic aluminum oxide templates. The structural and magnetic properties of these nanowire arrays have been investigated. The nanowires have smooth surface and uniform diameters. Because of the very large length-to-diameter ratio ( ~ 250), the effective magnetic anisotropy is dominated by the shape anisotropy and the easy magnetization direction is oriented along the nanowire. Coercivity as a function of diameter indicates that the magnetization reversal takes place by curling mode. Whereas, the angular dependence of the coercivity doesn't follow what is predicted by the curling mechanism, which may indicate that the magnetic reversal is also influenced by magnetic interactions between nanowires. The larger length and the smaller interpore distance induced stronger magnetic coupling between Fe nanowires. This dipolar interaction also reduces the coercivity and remanance squareness of the Fe nanowires.

Dual-Band Noise Suppressors Based on Co/Au Multilayered Magnetic Nanowires

Co/Au multilayered magnetic nanowires with a diameter of 200 nm were electrodeposited into nanoporous anodic aluminum oxide (AAO) templates. Coplanar waveguide transmission lines were patterned on the AAO templates that were filled with the Co/Au nanowires. Noise suppressors based on pure Co and Au nanowires were also fabricated as references. The transmission coefficient S21 of the device based on Co nanowires showed an absorption peak due to the loss of ferromagnetic resonance; S21 of the device based on Au nanowires decreased linearly with increasing frequency due to the loss of eddy current. S21 of the noise suppressors based on Co/Au multilayered nanowires showed a dual-band absorption phenomenon, because of the ferromagnetic resonance of the nanowires and the periodicity of the magnetic/nonmagnetic nanostructure. The frequencies of the two absorption peaks were 24-25 GHz and 36-38 GHz, which indicated that the working frequency of the microwave devices could be beyond 20 GHz.

Stability of iron (Fe) nanowires

The iron nanowires were obtained by DC electrochemical method in nanoporous anodic alumina templates (AAO). For stability tests these materials were placed in different solutions: white wine, citric acid, 0.9% NaCl, distilled water and ethanol for a 1–3week time period. The results of such treatment (solubility or possible changes on the surface) of the Fe nanowires were measured by IR (infrared spectroscopy), ASA (atomic absorption spectroscopy), TEM (transmission electron microscopy), XRD (X-ray diffraction) and DSC (differential scanning calorimetry) methods.

Angular selective backreflector for semitransparent photovoltaics

A nanoporous anodized aluminum oxide template is used as an incident-angle selective backreflector to lift the transparency vs. light absorption tradeoff in semi-transparent photovoltaics. Light incident at angles greater-than 45° from the template normal is backscattered at greater-than 35% intensity to a semitransparent photovoltaic increasing absorption and photocurrent. Meanwhile, light incident orthogonal to the template is transmitted with minimum attenuation (>85% at 600 nm), limited only by a single-pass absorption within the active layer. Using a vertical semitransparent photovoltaic proposed here, efficient capture of direct sunlight is possible while maintaining transparency at viewing angles close to the substrate normal.

Synthesis of Highly Ordered Silver Nanowire Arrays with SERS Activity

The highly ordered silver nanowire arrays were fabricated successfully by an easily produced and handled approach with the assistance of nanoporous anodized aluminum oxide(AAO) template. The silver nanowires with uniform diameters are highly ordered and parallel to each other. A broad UV-visible absorption band with a center at 401nm of silver nanowire arrays were detected. Optimized surface enhanced Raman scattering(SERS) signals of crystal violet molecules(CV) were recorded. It was found that the substrate exhibits strong enhancement properties due to the surface plasma resonance(SPR) of highly ordered silver nanowire arrays. The enhancement factor was calculated as 3.5×106. In addition, the results offered a simple technique in preparing highly ordered nanowire arrays which can be used as negative-refractive-index material as well as excellent SERS substrate.

Calculation of Nanowire Growth Activation Energy

Two-step preparation technology was used to prepare anodic aluminum oxide (AAO) templates. Then deposit Ni nanowire arrays in nanopores of AAO templates by direct current deposition. TEM spectra of nanowires show that the length of nanowires is uniform and that the shape of nanowires is the same with that of nanopores. Finally, reaction activation energy of Ni growing in nanopores was calculated by experimental data. Results show that Ni growing in the smaller nanopores is much easier than in the bigger nanopores.

Adhesive modification of indium–tin-oxide surface for template attachment for deposition of highly ordered nanostructure arrays

Polyvinyl alcohol (PVA), a very cheap polymer with one hydroxyl group in each repeating unit, was spun coated on the surface of an indium–tin-oxide (ITO) substrate to improve the adhesion between the substrate and a nanoporous anodic aluminum oxide (AAO) template layer for a template-directed fabrication of nanostructures. Compared with dihydroxy-terminated polystyrene (PS-dOH) and a silane coupling agent (KH550), PVA was a superior binder because of its abundant hydroxyl groups for adhesion enhancement and its low cost for applications. As an example, a highly ordered CdSe nanorod array free standing on the ITO substrate was electrochemically deposited by using an ultrathin AAO layer as the template on the PVA modified surface. It was demonstrated that the PVA modified ITO can be reliably used for the template-directed fabrication of nanostructures.

The use of PEEK nanorod arrays for the fabrication of nanoporous surfaces under high temperature: SiNx example

Large area silicon nitride (SiN(x)) nanoporous surfaces are fabricated using poly(ether-ether-ketone) (PEEK) nanorod arrays as a template. The procedure involves manipulation of nanoporous anodic aluminum oxide (AAO) templates in order to form an ordered array of PEEK nanopillars with high temperature resistant characteristics. In this context, self-ordered AAO templates are infiltrated with PEEK melts via the "precursor film" method. Once the melts have been crystallized in the porous structure of AAO, the basis alumina layer is removed, yielding an ordered array of PEEK nanopillars. The resulting structure is a high temperature and chemical resistant polymeric nanomold, which can be utilized in the synthesis of nanoporous materials under aggressive conditions. Such conditions are high temperatures (up to 320 °C), vacuum, or extreme pH. For example, SiN(x) nanopore arrays have been grown by plasma enhanced chemical vapor deposition at 300 °C, which can be of interest as mold for nanoimprint lithography, due to its hardness and low surface energy. The SiN(x) nanopore array portrays the same characteristics as the original AAO template: 120 nm diameter pores and an interpore distance of 430 nm. Furthermore, the aspect ratio of the SiN(x) nanopores can be tuned by selecting an AAO template with appropriate conditions. The use of PEEK as a nanotemplate extends the applicability of polymeric nanopatterns into a temperature regime up to now not accessible and opens up the simple fabrication of novel nanoporous inorganic surfaces.

Amperometric biosensor for hydrogen peroxide based on the co-electrodeposition of horseradish peroxidase and methylene blue on an ITO electrode modified with an anodic aluminum oxide template

We report on a nano-array sensor for hydrogen peroxide (H2O2) that is based on a nanoporous anodic aluminum oxide template. This was used as a matrix for the co-immobilization of horseradish peroxidase (HRP) and methylene blue (MB) on the surface of an indium tin oxide electrode. The immobilized HRP retained its natural activity and MB is capable of efficiently shuttle electrons between HRP and the electrode. The new electrode was characterized by SEM and electrochemical methods. It exhibits fast response, long-term stability, high sensitivity and good selectivity to H2O2. Under optimized conditions, it linearly responds to H2O2 in the concentration range from 1.0 μM to 26 mM, with a detection limit of 0.21 μM (at S/N = 3).

A comparative study on the hydrogen absorption of thin films at room temperature deposited on non-porous glass substrate and nano-porous anodic aluminum oxide (AAO) template

The influence of different morphology of the thin films deposited on the non-porous glass and nano-porous anodic aluminum oxide (AAO) substrates on the hydrogen absorption at room temperature was studied. A well-known sandwich-like Pd/Mg/Pd film was investigated. It is observed that the film deposited on the porous AAO template demonstrates a better hydrogen absorption in the H 2 pressure range 10–600 mbar with respect to the same film supported by the non-porous glass substrate. Moreover, the layer grown on the AAO, owing to its specific morphology inherited from the nano-porous substrate, has revealed its resistance toward stress accumulation caused by the lattice expansion, showing no buckle-to-crack network formation upon the hydrogen uptake. This interesting feature is expected to improve the cycling properties and structural stability of the system, and may help to investigate better the interaction of the H 2 with metal.

High‐Performance Micro‐Solid Oxide Fuel Cells Fabricated on Nanoporous Anodic Aluminum Oxide Templates

AbstractMicro‐solid oxide fuel cells (μ‐SOFCs) are fabricated on nanoporous anodic aluminum oxide (AAO) templates with a cell structure composed of a 600‐nm‐thick AAO free‐standing membrane embedded on a Si substrate, sputter‐deposited Pt electrodes (cathode and anode) and an yttria‐stabilized zirconia (YSZ) electrolyte deposited by pulsed laser deposition (PLD). Initially, the open circuit voltages (OCVs) of the AAO‐supported μ‐SOFCs are in the range of 0.05 V to 0.78 V, which is much lower than the ideal value, depending on the average pore size of the AAO template and the thickness of the YSZ electrolyte. Transmission electron microscopy (TEM) analysis reveals the formation of pinholes in the electrolyte layer that originate from the porous nature of the underlying AAO membrane. In order to clog these pinholes, a 20‐nm thick Al2O3 layer is deposited by atomic layer deposition (ALD) on top of the 300‐nm thick YSZ layer and another 600‐nm thick YSZ layer is deposited after removing the top intermittent Al2O3 layer. Fuel cell devices fabricated in this way manifest OCVs of 1.02 V, and a maximum power density of 350 mW cm−2 at 500 °C.

FITC-modified PPy nanotubes embedded in nanoporous AAO membrane can detect trace PCB20 via fluorescence ratiometric measurement

A highly sensitive and selective fluorescence ratiometric sensor membrane for 2,3,3'-trichlorobiphenyl has been achieved, via depositing polypyrrole nanotubes (PPyNTs, the fluorescence indicator) in nano-porous anodic aluminium oxide (NPAAO) template and subsequently immobilizing fluorescein isothiocyanate (as an internal reference) onto the inner walls of the PPyNTs embedded in the NPAAO.

Hierarchical nanoporous anodic alumina templates by asymmetric two‐step anodization

AbstractAn asymmetric two‐step anodization process is used to fabricate hierarchical nanoporous anodic alumina templates (HNAATs) with different geometric characteristics. This nanostructure consists of an outer hexagonal lattice of concavities in the interior of which pores grow. The anodization conditions (anodization voltage, and type and concentration of the acid electrolyte) between the first and the second step are considerably varied to obtain different types of templates. Two growth regimes (mild and hard), three types of acid electrolytes (phosphoric, oxalic and sulphuric) and three acid electrolyte concentrations (0.3, 0.2 and 0.1 M) are combined. In addition, the voltage ratio between the second and the first step of the anodization process is ranged from 0.11 to 0.71. By modifying these parameters, it is possible to control the fabrication process and obtain HNAATs with a wide range of morphologies.

Synthesis of nanoporous activated iridium oxide films by anodized aluminum oxide templated atomic layer deposition

Iridium oxide (IrOx) has been widely studied due to its applications in electrochromic devices, pH sensing, and neural stimulation. Previous work has demonstrated that both Ir and IrOx films with porous morphologies prepared by sputtering exhibit significantly enhanced charge storage capacities. However, sputtering provides only limited control over film porosity. In this work, we demonstrate an alternative scheme for synthesizing nanoporous Ir and activated IrOx films (AIROFs). This scheme utilizes atomic layer deposition to deposit a thin conformal Ir film within a nanoporous anodized aluminum oxide template. The Ir film is then activated by potential cycling in 0.1 M H 2SO 4 to form a nanoporous AIROF. The morphologies and electrochemical properties of the films are characterized by scanning electron microscopy and cyclic voltammetry, respectively. The resulting nanoporous AIROFs exhibit a nanoporous morphology and enhanced cathodal charge storage capacities as large as 311 mC/cm 2.

Fabrication of SERS substrate using nanoporous anodic alumina template decorated by silver nanoparticles

SERS substrate is fabricated using nanoporous anodic alumina membrane decorated by silver nanoparticles, which are self assembled on the surface as well as inside the pores to form a network structure of aligned cylindrical vertical wells creating artificial roughness on the surface of the membrane. When electromagnetic radiation falls on the rough surface, a strong electric field is created due to plasmon excitation on sharp edges of the network structure or at interparticle spacing known as ‘hot spots’. This enhanced electric field produces remarkable SERS effect from Raman active R6G molecules attached to silver nanoparticles when excited by a laser source.