Cubic and hexagonal mesoporous carbon in the pores of anodic alumina membranes

Cubic and circular hexagonal mesoporous carbon phases in the confined environment of the pores of anodic alumina membranes (AAM) were obtained by organic-organic self-assembly of a preformed oligomeric resol precursor and the triblock copolymer templates Pluronic F127 or P123, respectively. Casting and solvent evaporation were followed by self-assembly and the formation of a condensed wall material by thermopolymerization of the precursor oligomers, thus resulting in mesostructured phenolic resin phases. Subsequent thermal decomposition of the surfactant and carbonization were achieved through thermal treatment at temperatures up to 1000 °C under an inert atmosphere. The resulting hierarchical mesoporous composite materials were characterized by small-angle X-ray scattering and nitrogen-sorption measurements. The structural features were directly imaged in TEM cross-sections of the composite membranes. For both structures, the AAM pores were completely filled and no shrinkage was observed due to strong adhesion of the carbon-wall material to the AAM pore walls. As a consequence, the pore size of the mesophase system stays almost constant even after thermal treatment at 1000 °C.     

Preparation of oriented mesoporous carbon nano-filaments within the pores of anodic alumina membranes

Oriented mesoporous carbon nano-filaments were prepared as replicas of Fe-containing silica mesostructures deposited inside the channels of anodic alumina membranes (AAMs). Due to the confinement imposed by the channels of the AAMs, rather unusual mesophase structures showing "circular" or "columnar" mesoscopic channels were formed that were replicated as pure carbon analogues after removal of the templating hosts. These structures are assembled in the forms of macroscopic membranes and are hierarchically ordered first due to the arrangement of the channels of the AAMs and second due to the mesophase counterparts.     

In situ GISAXS study of the formation of mesostructured phases within the pores of anodic alumina membranes

The formation and subsequent transformations of mesostructured silica within the confined tubular environment of anodic alumina membrane (AAM) channels [porous alumina membrane (PAM) channels] were investigated for the first time in situ with grazing incidence small-angle X-ray scattering (GISAXS) techniques, in combination with ex situ transmission electron microscopy (TEM) of the same samples. A better understanding of the mesostructure formation mechanism within the confined space of the AAM pores is a direct result of this study. Three different surfactants were used as the structure-directing agents in acid-catalyzed silica synthesis solutions. With ionic cetyltrimethylammonium bromide acting as the structure-directing agent, a columnar hexagonal structure with mesopores oriented parallel to the AAM channels was observed to form directly from the beginning of the synthesis. In samples synthesized with the nonionic surfactants Brij 56 and Pluronic P123, a circular hexagonal structure was found to form first; here, the mesopores are aligned around the circumference of the AAM channels. The circular structure subsequently transforms directly into a columnar hexagonal (P123 surfactant), or a mixture of columnar hexagonal and a new curved lamellar phase with lamellae oriented parallel to the walls of the AAM channels (Brij 56 surfactant). These transformations occur after complete solvent evaporation and therefore differ from a simple evaporation-induced phase formation. The existence of a previously postulated lamellar phase could be proven by GISAXS and TEM investigations.     

Growth and characterization of ZnO nanowire arrays electrodeposited into anodic alumina templates in DMSO solution

ZnO nanowire arrays were grown by potentiostatic cathodic electrodeposition on aluminum anodic oxide template (AAO) from dimethyl sulfoxide (DMSO) solutions containing zinc chloride and molecular oxygen as precursors. The nanowires presented high aspect ratio and exhibited a very high crystallinity with a wurtzite crystal structure with preferential orientation along the (0001) crystallographic axis. Chronoamperometric experiments were performed on gold bulk electrodes in order to model this preferential mode growth of ZnO nanowires, which has not been previously reported for similar precursors in DMSO solution. The analysis of the corresponding chronoamperograms revealed that chloride ions influence the oxide nucleation and growth mechanism. It was found that in the absence of KCl as a supporting electrolyte, the data fitted an instantaneous three-dimensional diffusion-controlled (IN-3D)_diff nucleation and growth mechanism (NGM). The presence of KCl, instead favored a progressive three-dimensional (PN-3D)_diff NGM. With these results, a model for the more complex nanowire’s growth inside the pores of the AAO template is proposed.     

Color properties of gold-silver alternate nanowires electrochemically grown in the pores of aluminum anodic oxidation film

Repeated electrodeposition of gold and silver on anodized aluminum plate yielded Au-Ag alternate nanowires in the geometrically anisotropic pores of the anodic oxidation films with mean diameter 10 nm and depth 10 mum, respectively. The absorption maximum wavelengths in the visible region resulting from the transverse surface plasmon resonances of Au and Ag nanorods are invariant with their growth. Consequently, the color of the anodized aluminum was simply tuned in a wide range, which is impossible to attain in each monometallic system.     

Porosity of anodic alumina membranes from electrochemical measurements

A procedure based on the high-field mechanism of the growth of anodic oxides was developed in order to evaluate the morphological features of porous layers. Since the thickness of the barrier film, separating the porous layer from the metal, does not change during the steady-state growth of an anodic porous layer, the rate of displacement of the metal-oxide interface to the metal direction must be equal to the rate of displacement of the pore base to the oxide direction. As a consequence, porosity can be expressed in terms of the ratio i _diss/i _ion, where i _diss is the dissolution current density at the pore base, and i _ion is the ionic current density at the metal-oxide interface. Pore diameter can be determined from geometrical considerations, while average pore population can be obtained from the ratio of porosity to the average surface area of a single pore. This procedure was checked by comparison with experimental results relative to membranes prepared in various conditions. The satisfactory agreement between theoretical and experimental findings indicates that porosity can be evaluated by current density data and vice-versa. Therefore, anodic alumina membranes may be tailored for different applications by choosing operative conditions giving the desired value of i _diss/i _ion.     

Structural, magnetic, and magnetoresistive properties of electrodeposited Ni5Zn21 alloy nanowires

Ni5Zn21 alloy nanowires were fabricated through template-assisted electrochemical deposition method. The morphology and microstructures of as-deposited nanowires were determined by field-emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), electron diffraction (ED), and electron probe microanalysis (EPMA). The accurate composition was measured via induced coupling plasma atomic emission spectroscopy. SEM results show that Ni5Zn21 nanowires are deposited in most of the nanopores of the template, and they are continuous and dense throughout the whole length. The XRD result demonstrates that the nanowires are mainly composed of a cubic gamma phase Ni5Zn21 alloy, but there also exists a trace of Zn-rich eta phase. HRTEM and ED reveal that the alloy nanowires are polycrystalline with the crystallite size of several tens of nanometers. EPMA of a single nanowire illustrates that there exist Ni-rich microzones in as-deposited nanowires. Subsequent magnetic measurements of the array also confirmed the existence of them. In addition, it can be further inferred that the shape of Ni-rich microzones is probably barlike or disklike, from the anisotropy of zero field cooling/field cooling (ZFC/FC) curves as well as the vortex magnetization behavior of the Ni5Zn21 nanowire array. The low-temperature magnetoresistance of the Ni5Zn21 nanowire array was also measured. Giant magnetoresistance instead of anisotropic magnetoresistance is suggested to be responsible for contributing to the magnetoresistance.     

Template electrosynthesis of La(OH)3 and Nd(OH)3 nanowires using porous anodic alumina membranes

High quality arrays of Ln(OH)₃ (Ln = La, Nd) nanowires have been successfully fabricated for the first time by an electrochemical process using anodic alumina membrane templates. A physico-chemical characterisation of electrodeposited hydroxides has been carried out by different techniques (XRD, SEM and EDX). The results show that the synthesized nanostructures are crystalline, dense, continuous, well aligned, and with high aspect ratio, suggesting further development of possible applications in the lanthanide family species.     

Fabrication and characterization of single layer and multi-layer anodic alumina membranes

Porous alumina films containing parallel capillary pores of uniform size were fabricated by anodically oxidizing high purity aluminum films in phosphoric acid and sulfuric acid solutions. These films were formed into membranes by post-oxidation processing that removes unoxidized aluminum as well as a barrier layer of alumina from the base of the pores. Symmetric membranes were made by oxidizing at constant current density conditions. Two layer composite membranes were made by changing current density during the oxidation process. The thickness, pore density and porosity of each membrane were predicted from the relationships between structural characteristics and processing conditions that were developed in previously reported kinetic studies of anodic oxidation of aluminum.Each membrane was then characterized using permeability measurements. The hydraulic permeability of membranes formed in phosphoric acid and the diffusive permeability of membranes formed in sulfuric acid were measured. A comparison of the measured permeability values to those predicted using the structural characteristics calculated using relationships developed in the kinetic studies shows excellent agreement. These results illustrate that porous alumina membranes can be fabricated with transport characteristics that can be predicted from the processing conditions used during membrane formation.     

Growth of single-crystalline Ni and Co nanowires via electrochemical deposition and their magnetic properties

Single-crystalline Ni nanowires have been successfully fabricated with anodic aluminum oxide as template by electrodeposition. Structural characterization (X-ray diffraction, XRD, and high-resolution transmission electron microscopy, HRTEM) shows that the single-crystalline Ni nanowire has a preferred orientation along the [220] direction. The effects of electrochemical deposition conditions on the structure of Ni nanowires are systematically studied to investigate the growth mechanism. Possible reasons for the growth of the single-crystalline Ni nanowires were discussed on the basis of electrochemistry and thermodynamics. These single-crystalline Ni nanowires have exhibited excellent magnetic properties (large anisotropy, large coercivity, and high remanence). By a similar process, single-crystalline Co nanowires with hexagonal close-packed (hcp) structure were achieved, also having large anisotropy, large coercivity (1.8 kOe), and high remanence ratio (80.8%).     

Melting of indium,tin, and zinc nanowires embedded in the pores of anodic aluminum oxide

The melting temperature of metal nanostructures embedded in the matrix is an essential thermodynamic characteristic and a key parameter of the processes of their transformation into semiconductor structures. In this work, great attention is paid to the investigation of the behavior of one-dimensional metal nanocrystals near the melting point. For this purpose, the arrays of In, Sn, and Zn nanowires with different diameters have been electrochemically grown in the pores of anodic aluminum oxide (AAO), which is confirmed by the results of the microscopy and the phase X-ray diffraction analysis. The melting of nanowire arrays with different diameters has been investigated by means of differential scanning calorimetry (DSC). Aside from the expected melting temperature decrease, with decreasing the diameter of nanowires, it has been established that the melting peaks of nanostructure arrays have a complex shape that requires detailed elaboration in order to more accurately define the melting temperature. It is shown that the signal waveform while melting depends on geometric parameters of the structure, and the peak being mapped onto the DSC curve is the result of superposition of the melting peaks of nanowires with several characteristic dimensions. For the arrays of In, Sn, and Zn nanowires in AAO, there have been defined the melting temperature values according to the methodology offered, and there has been presented the dependence of the melting temperature decrease on the nanowires' diameter.     

Electrodeposition of tubular-rod structure gold nanowires using nanoporous anodic alumina oxide as template

One-dimensional tubular-rod structure gold nanowires have been prepared using electrodeposition method at constant current mode with confined nanochannels of porous anodic aluminum oxide template. The reduction mechanism of gold ions and formation process of tubular-rod structure gold nanowires are studied. Electron microscopy results show that the tubular-rod structure gold nanowires transform to solid nanorods when the electrodeposition time is long enough. The tubular-rod structure gold nanowires have an average diameter of 180 nm, which coincide with the diameter of the template used. X-ray diffraction results confirm that the tubular-rod structure gold nanowires are crystalline structure.     

Correlation between magnetic properties of electrodeposited Co(P) and NH4Cl concentrations in the electrolyte

The effect of ammonium chloride on the electrodeposition process and resulting magnetic properties of Co(P) alloy films was investigated. It was observed that the magnetic properties of the films varied extensively with the ammonium chloride contents in the solution. Cyclic voltammetry was employed as an electroanalytical technique to characterize the variation of interfacial polarization at different electrolyte compositions. In addition, microstructural characterization of electrodeposited Co(P) thin films by transmission electron microscopy revealed a significant increase in both grain size and preferred orientation of Co(P) grains with ammonium chloride contents. Results of characterizations indicated that the addition of ammonium chloride controlled the kinetics of electrocrystallization of Co(P) by modifying the charge transfer process and thus gave rise to increased grain size and preferred orientation in the Co(P) thin films. These microstructural features provide reasonable explanation on the variation of the magnetic properties of electrodeposited Co(P) films with the amount of ammonium chloride in the solution.     

Temperature and pH influences on the structural and the emission properties of electrodeposited CdSe nanowires

Cadmium selenite (CdSe) nanowires have been electrodeposited by potentiostatic method using polycarbonate membranes as template. For the same potential value, the pH modification of the solution or the temperature variation of the substrate leads to drastic differences of the CdSe composition. A meticulous study of the influence of both temperature and pH value on the stoichiometry of electrodeposited CdSe nanowires has been done and several combinations of temperature/pH leading to the 50–50 CdSe composition have been found. The as-electrodeposited CdSe nanowires under these specific conditions reveal a good crystallinity with a <111> preferred growth orientation exhibiting a luminescence band in the visible range corresponding to the CdSe gap.     

Structural characterisation of heat-treated anodic alumina membranes prepared using a simplified fabrication process

A facile method for forming porous anodic alumina membranes based on one-step anodising in sulphuric acid is reported. A flat and well-ordered basal surface incorporating uniformly sized pores was obtained without the need for electrolytic polishing. Excess metallic aluminium was removed from the film using a saturated solution of iodine in methanol. The high-temperature properties of the oxide ceramic membranes were investigated using thermal analysis, mass spectrometry, X-ray diffraction and solid-state nuclear magnetic resonance. At 970 °C the amorphous alumina crystallises to γ-Al₂O₃ with the release of SO₂ and O₂. Finally at 1228 °C the alumina converts into the thermodynamically preferred phase, corundum. The pore structure of the oxide membrane was found to be very stable at elevated temperatures, suggesting applications in materials synthesis, catalysis and gas separation.     

Removal of trace Cr(VI) from water using chitosan-iron nanowires in porous anodic alumina

Chitosan-iron nanowires in porous anodic alumina (PAA) have been successfully prepared under ambient conditions as an adsorbent. The adsorbent was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and N₂-BET surface area. The results showed that PAA can disperse and protect Fe⁰ nanorods from oxidation. The adsorption characteristics of trace Cr(VI) onto adsorbent have been examined at different initial Cr(VI) concentrations with pH 5. Batch adsorption studies show that the removal percentage of adsorbent for the removal of trace Cr(VI) is strongly dependent on the initial Cr(VI) concentrations. Langmuir and Freundlich isotherm models were used to analyze the experiment data. The adsorption of trace Cr(VI) by adsorbent is well modeled by the Langmuir isotherm and the maximum adsorption capacity of Cr(VI) is calculated as 123.95 mg/g which is very closed to the experiment results. Intraparticle diffusion study shows that the intraparticle diffusion of adsorbent is not the sole rate-controlling step. The negative value of Gibbs free energy change, ΔG^o, indicated that the process of Cr(VI) onto adsorbent was spontaneous. This work has demonstrated that chitosan-iron nanowires in porous anodic alumina as an adsorbent has promising potential for heavy metal removal at trace level.     

Preparation and magnetic properties of Cu-ferrite nanorods and nanowires

An aqueous solution method has been developed for preparing Cu-ferrite nanorods (NRs) array and nanowires (NWs) on Cu substrate. The Cu-ferrite NRs exhibit a clear uniaxial anisotropy with the easy axis along rods. The decrease of the reduced remanence from 0.24 in the parallel magnetic field to almost zero in the perpendicular field shows a strong effect of the demagnetizing field perpendicular to the rod axis. The weaker uniaxial anisotropy makes NWs the lower saturation magnetization.     

Mesostructured silica containing conjugated polymers formed within the channels of anodic alumina membranes from tetrahydrofuran-based solution

The synthesis of mesostructured silica from a tetrahydrofuran (THF)-based sol gel was carried out in the channels of an anodic alumina membrane (AAM) using the evaporation-induced self-assembly (EISA) method. Two different nonionic surfactants were used as structure-directing agents, the triblock copolymer Pluronic P123 and the oligomer surfactant Brij56. The effect of the relative humidity and surfactant concentration on the type of mesophase and orientation of the in-channel mesostructures was studied using transmission electron microscopy (TEM) and grazing incidence small angel X-ray scattering (GISAXS). The in-channel structures obtained in this study were primarily of the 2D hexagonal phase with a circular orientation in which the hexagonally packed cylinders form a spiral-like shape from the channel wall inward. In addition, a columnar orientation of the hexagonal phase, in which the axes of the hexagonally packed cylinders are oriented parallel to the channel axes, was also observed. Finally, the use of the THF-based synthesis allowed the in situ incorporation of the highly hydrophobic yellow-emitting conjugated polymer poly[9,9-dioctylfluorene-co-benzothiadiazole] into the in-channel mesostructure upon its formation. The conjugated polymer was well distributed within the mesostructure and maintained its optical properties.     

Effects of sintering on properties of alumina microfiltration membranes

The transformation of membrane channels during the sintering process conforms to the Rhines' topological decay model of intermediate sintering stage. The pore size of the membranes enlarges with the increase of sintering temperature. The pore size increment caused by the increase of sintering temperature is more obvious for thin membranes than for thick membranes. With the increase of sintering temperature, the water permeance of membranes increases at first and then decreases after a turning point of sintering temperature.