Fabrication of nanoporous TiO2 by electrochemical anodization

The formation of self-organized porous titania is achieved by electrochemical anodization under a potentiostatic regime. Anodic titanium oxide (ATO) was fabricated by a three-step self-organized anodization of the Ti foil in an ethylene glycol electrolyte containing 0.38 wt% of NH₄F and 1.79 wt% of H₂O. Anodizing was carried out at the constant cell potential ranging from 30 to 70 V at the temperature of 20 °C. It was found that nanoporous TiO₂ arrays can be obtain only after a short duration of the third step (10 min). The influence of anodizing potential on the structural parameters of porous anodic titania including pore diameter, interpore distance, wall thickness, porosity and pore density was extensively studied. The linear dependencies between interpore distance, pore diameter and wall thickness upon the anodizing potential were found. The regularity of pore arrangement was monitored qualitatively by fast Fourier transforms (FFTs) of top-view FE-SEM images. It was found that the best arrangement of nanopores is observed at 40 V. This finding was confirmed by the analysis of pore circularity. The highest circularity of pores was observed once again at 40 V.     

Structural features of self-organized nanopore arrays formed by anodization of aluminum in oxalic acid at relatively high temperatures

Anodic aluminum oxide (AAO) membranes with a highly ordered nanopore arrangement typically serve as ideal templates for the formation of various nanostructured materials. A typical procedure of the template preparation is based on a two-step self-organized anodization of aluminum carried out at the temperature of about 1-3 °C. In the current study, AAO templates were fabricated in 0.3 M oxalic acid under the anodizing potential range of 30-65 V at a relatively high electrolyte temperature ranging from 20 to 30 °C. Due to a high rate of porous oxide growth, about 5-10-fold higher than in low-temperature anodizing, the process of the template fabrication can be shorten significantly. Similarly to the low-temperature anodization, the best hexagonal pore arrangement is observed for samples anodized at 40 V. With a prolonged duration of the first anodizing step the order degree of triangular nanoporous lattice, observed after the second anodization, improves considerably. The effects of the anodizing potential and the process duration on the structural features of porous anodic alumina such as: pore diameter (D_p), interpore distance (D_c), porosity (P), pore density (n) and anodizing ratio (B_U) were investigated in details at various temperatures. The obtained results were compared with theoretical predictions and data reported in the literature.     

Through-hole membranes of nanoporous alumina formed by anodizing in oxalic acid and their applications in fabrication of nanowire arrays

The nanopore arrays were fabricated by two-step self-organized anodization of aluminum carried out in 0.3 M oxalic acid at the temperature of 20 °C. This relatively high temperature shortens significantly the anodizing time and allows to fabricate quickly thick through-hole membranes without the additional operating cost of a cooling circuit. The structural features of anodic porous alumina such as pore diameter, interpore distance, porosity, pore density and pore circularity were investigated at various durations of pore opening/widening process carried out in 5% H₃PO₄. An excellent agreement of AAO structural features measured in FE-SEM images of the studied samples with results from software calculations was observed. The pore shape can be monitored qualitatively by fast Fourier transforms (FFTs) and quantitatively by calculation the percentage of pore circularity. Additionally, the regularity of the hexagonal arrangement of nanopores in through-hole AAO membranes was compared for various opening/widening time ranging from 40 to 100 min. It was shown that three-dimensional (3D) representations of FE-SEM images and their surface-height distribution diagrams provide interesting information about the surface roughness evolution during the pore opening/widening process. A template-assisted fabrication of Ag and Sn nanowire arrays by electrochemical deposition into the pores of the prepared AAO templates was also successfully demonstrated.     

Porous anodic alumina formed by anodization of aluminum alloy (AA1050) and high purity aluminum

A two-step anodization process performed at 0 °C was used to prepare highly ordered porous anodic alumina on the AA1050 alloy and high purity aluminum foil. The anodizing of both substrates was carried out in 0.3 M sulfuric acid and 0.3 M oxalic acid baths at 25 V and 40 V, respectively. The effect of the extended duration of the second anodizing step on pore order degree and structural features of AAO membranes was studied. The presence of alloying elements affects not only the rate of oxide growth but also the microstructure of the anodic film. It was found that pore circularity and regularity of pore arrangement in AAO membranes formed on the AA1050 alloy were always worse than those observed on the pure Al substrate. The structural features, such as pore diameter, interpore distance, wall thickness, barrier layer thickness, porosity and pore density of porous anodic alumina formed on AA1050 are a little different from those obtained for high purity Al. The extended time of the second anodizing step, up to 16 h does not affect significantly the regularity of pore order and all structural features of AAO membranes, independently of the anodizing electrolyte.     

Fabrication of highly ordered pore array in anodic aluminum oxide

Highly ordered pore array in anodic aluminum oxide was fabricated by anodizing pure aluminum. The order of a pore array was affected by anodizing voltage, electrolyte temperature, and first anodizing time. A regular pore array with mean diameter of 24 nm and interpore distance of 109 nm could be formed by two-step anodization at 40 V., oxalic acid concentration of 0.3 M and electrolyte temperature of 15 ‡C. The measured interpore distance showed linearity with anodizing voltage. The diameter of pores was adjusted by pore widening treatment in a 5 wt% phosphoric acid solution at 30 ‡C after two step anodization. The mechanism of self-arrangement of pores could be explained by the repulsive interaction between the pore walls.     

Highly ordered anodic alumina nanotemplate with about 14 nm diameter

A novel method for the fabrication of highly ordered nanopore arrays with very small diameter of 14 nm was demonstrated by using low-temperature anodization. Two-step anodization was carried out at 25 V, sulfuric acid concentration of 0.3 M, and electrolyte temperature of −15 °C. After anodization, a regular pore array with mean diameter of 14 nm and interpore distance of 65 nm was formed. The pore diameter and regular arrangement were confirmed by scanning electron microscopy (SEM) and fast Fourier transformation (FFT), respectively. The present results strongly suggest that the diameter of pores in a highly ordered alumina template can be reduced by lowering the anodization temperature.     

Gradient and alternating diameter nanopore templates by focused ion beam guided anodization

Ordered arrays of anodic alumina nanopores with uniform pore diameters have been fabricated by self-organized anodization of aluminum. However, gradient or alternating diameter nanopore arrays with designed interpore distances have not been possible. In this study, focused ion beam lithography is used to fabricate hexagonally arranged concaves with different diameters in designed arrangements on aluminum surfaces. The patterns are then used to guide the further growth of alumina nanopores in the subsequent oxalic acid anodization. Gradient and alternating nanopore arrangements have been attained by FIB patterning guided oxalic acid anodization. The fundamental understanding of the process is discussed.     

Adjusting the texture and nitrogen content of ordered mesoporous nitrogen-doped carbon materials prepared using SBA-15 silica as a template

Mesoporous nitrogen-doped carbon (N-MC) with highly ordered two dimensional hexagonal structures has been synthesized using diaminobenzene (DAB) as carbon and nitrogen sources, ammonium peroxydisulfate (APDS) as an oxidant, and SBA-15 as a hard template. The effect of synthesis temperatures and precursors on the textures and nitrogen content of the N-MC is investigated. By adjusting the synthesis temperatures in a range of 70-100 °C, the pore diameter of the N-MC materials can be tuned from 3.43 to 4.15 nm, while the specific surface area of the N-MC with a nitrogen content of 26.5 wt.%, can be tuned from 281.8 to 535.2 m²/g. The C/N molar ratio of the N-MC can be tuned in a range of 3.25-3.65 by adjusting the mole ratio of DAB/APDS precursors at a synthesis temperature of 80 °C, while the pore diameter of the N-MC be tuned in a range of 4.12-3.66 nm. With the nitrogen doping content increasing, the lattice constant of N-MC materials decreases. Investigation indicates that well ordered N-MC materials with large specific surface area, high total pore volume and high nitrogen content can be fabricated under a synthesis temperature of 80 °C and a molar ratio of 1.5 for DAB/APDS precursors.     

Fabrication of Porous Anodic Alumina with Ultrasmall Nanopores

Anodization of Al foil under low voltages of 1–10 V was conducted to obtain porous anodic aluminas (PAAs) with ultrasmall nanopores. Regular nanopore arrays with pore diameter 6–10 nm were realized in four different electrolytes under 0–30°C according to the AFM, FESEM, TEM images and current evolution curves. It is found that the pore diameter and interpore distance, as well as the barrier layer thickness, are not sensitive to the applied potentials and electrolytes, which is totally different from the rules of general PAA fabrication. The brand-new formation mechanism has been revealed by the AFM study on the samples anodized for very short durations of 2–60 s. It is discovered for the first time that the regular nanoparticles come into being under 1–10 V at the beginning of the anodization and then serve as a template layer dominating the formation of ultrasmall nanopores. Under higher potentials from 10 to 40 V, the surface nanoparticles will be less and less and nanopores transform into general PAAs.     

Self-ordering of anodic porous alumina formed in organic acid electrolytes

New self-ordering porous alumina films were fabricated in organic acid electrolytes. Highly ordered cell arrangements of porous alumina films were realized in malonic acid at 120 V and tartaric acid at 195 V having 300 nm and 500 nm pore intervals, respectively. Self-organization was achieved at the maximum voltage required to induce high-current-density anodization while preventing burning, i.e., an extremely high-current flow concentrated at local points. The cells of the film grown at a high field must be pressed against each other, so that the self-ordering proceeds with the porous layer growth. When the self-ordering of cell arrangement proceeds, the cells became smaller. To improve the regularity of the cell configuration, a low electrolyte temperature and a relatively high electrolyte concentration were effective for maintaining a high-current-density to prevent burning. Surface flatness was an essential factor for self-ordering, however, the surface oxide film produced by electropolishing an aluminum substrate prevented quick pore growth in the organic acids having a low dissociation constant. It is confirmed that electropolishing followed by alkaline treatment was most appropriate as the pre-treatment in preparing flat surfaces.     

Anodization of nanoporous alumina on impurity-induced hemisphere curved surface of aluminum at room temperature

Nanoporous alumina which was produced by a conventional direct current anodization [DCA] process at low temperatures has received much attention in various applications such as nanomaterial synthesis, sensors, and photonics. In this article, we employed a newly developed hybrid pulse anodization [HPA] method to fabricate the nanoporous alumina on a flat and curved surface of an aluminum [Al] foil at room temperature [RT]. We fabricate the nanopores to grow on a hemisphere curved surface and characterize their behavior along the normal vectors of the hemisphere curve. In a conventional DCA approach, the structures of branched nanopores were grown on a photolithography-and-etched low-curvature curved surface with large interpore distances. However, a high-curvature hemisphere curved surface can be obtained by the HPA technique. Such a curved surface by HPA is intrinsically induced by the high-resistivity impurities in the aluminum foil and leads to branching and bending of nanopore growth via the electric field mechanism rather than the interpore distance in conventional approaches. It is noted that by the HPA technique, the Joule heat during the RT process has been significantly suppressed globally on the material, and nanopores have been grown along the normal vectors of a hemisphere curve. The curvature is much larger than that in other literatures due to different fabrication methods. In theory, the number of nanopores on the hemisphere surface is two times of the conventional flat plane, which is potentially useful for photocatalyst or other applications.PACS: 81.05.Rm; 81.07.-b; 82.45.Cc.     

Galvanostatic anodization of pure Al in some aqueous acid solutions Part I: Growth kinetics, composition and morphological structure of porous and barrier-type anodic alumina films

The growth kinetics of anodic films formed on the surface of high purity Al by anodization under galvanostatic conditions at current densities in the range 5–75 mA cm^–2 in thermostatically controlled and vigorously stirred solutions of chromic, sulfuric, phosphoric, citric, tartaric and oxalic acids at different temperatures, were studied. It has been shown that chromic acid solution produces a typical barrier type oxide growth at any given temperature, while the specific kinetic curve representing the combined barrier/porous type film growth is observed when the anodization process is carried out in a nonstirred chromic acid solution. The oxide growth in the rest of the anodizing solutions occurs in different ways depending on the bath temperature. Barrier oxide growth is observed at temperatures lower than 30 °C. Above this temperature, combined barrier/porous oxide growth is observed. In all cases, the slope of the linear part of the potential against time curves, and therefore the rate of barrier oxide growth, increases with increasing anodizing current density and acid concentration, while it decreases with increase in temperature. The composition and surface morphology of the anodic films have been studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM).     

纳米有序多孔阳极氧化铝制备方法的研究进展

多孔阳极氧化铝(PAA)模板以六角形元胞紧密排列,孔径大小可调,且化学稳定性好,近年来在催化、传感、过滤和仿生等领域受到了越来越多的关注。PAA模板的制备一直以来都是研究的热点,因为模板的结构和性质直接影响其应用的效果。本文在简要介绍了自组织有序多孔阳极氧化铝的特点及影响因素的基础上,较为全面地综述了制备自组织PAA模板不同方法的研究进展,包括温和阳极氧化法、强烈阳极氧化法、脉冲式阳极氧化法和周期性阳极氧化法。具体分析了不同阳极氧化方法的特点以及各自得到的氧化铝模板不同的特点和应用范围,说明了氧化电压、氧化温度和电解液种类在制备PAA模板时对其孔洞尺寸的重要作用,最后对阳极氧化铝膜的发展前景进行了展望。     

Embedded space charge in porous alumina films formed in phosphoric acid

The embedded charge in the barrier layers of porous alumina, formed potentiostatically in phosphoric acid was studied as a function of anodizing voltage (30-57 V) and bath temperature (18 and 21 °C). For that, the polarization measurements of as-grown and annealed alumina/Al samples were conducted in the same anodizing bath by anodic potential sweep at a scan rate of 2.6 V/min. The plane capacitor model was used for the assessment of the charge density in the barrier layers of as-grown porous alumina. For the barrier layers of films formed at 18 °C this value equals to 0.747 μC cm⁻² and does not depend on the anodizing voltage. Increase in electrolyte temperature rises the embedded charge density. Polarization measurements carried out in this paper clearly present that the barriers of phosphoric acid films grown at the anodizing voltages lower than 39 V contain a layer of virtual cathode while at higher voltages this layer disappers. The obtained results allow speaking about promising opportunities of potentiodynamic polarization measurements of alumina films in the same anodizing solution before and after annealing for the studies of charges embedded within the alumina barriers and for the regularities of ion transport.     

Surfactant-assisted growth of anodic nanoporous niobium oxide with a grained surface

Nanoporous niobium oxide film with a maximum thickness of 520 nm was prepared by anodizing niobium in a mixture of 1 wt% HF, 1 M H₃PO₄, and a small amount of Sodium Dodecyl Sulfate (SDS) surfactant. The porosity of the anodic niobium oxide prepared without SDS is irregular with the surface of the oxide suggesting a grained surface pattern rather than an ordered porous structure. A proper amount of SDS addition can prepare a pore arrangement with stripe patterns. The pore depth and surface pattern were strongly affected by the concentration of SDS and bath temperature. We found that the addition of SDS surfactant facilitated improvement in the chemical resistance of niobium oxide, leading to the formation of pores with a longer length compared to those prepared without a SDS surfactant. This can be in part ascribed to the protection of the surface by the physical adsorption of SDS on the surface due to a charge-charge interaction and be in part attributed to the formation of NbO bonding on the outermost oxide layer by SDS. When anodization was carried out for 4 h, the surface dissolution of niobium oxide was observed, which means that the maximum tolerance time against chemical dissolution was less than 4 h.     

Three-dimensional alumina nanotemplate

Cylindrically and pentagonally shaped three-dimensional (3-D) alumina nanotemplates were fabricated by electrochemical anodization of high purity aluminum. Crack-free 3-D nanotemplates were achieved by anodizing various aluminum pre-formed structures through their concave surfaces. A critical parameter for obtaining crack-free 3-D nanotemplates was manipulating the stress at the aluminum/aluminum-oxide interface, because tensile stress at this interface can cause radial cracks. Two different cathode configurations were used to produce 3-D nanotemplates: inner and outer cathode configuration. The outer cathode configuration allowed fabrication of 3-D alumina templates with smaller diameters (<500 μm) and the inner cathode configuration allowed fabrication of 3-D alumina templates with larger diameters (>500 μm). A larger pore density was observed at the inner surface of 3-D template than at the outer surface due to the curvature of the template. The pore diameter of alumina nanotemplate was strongly depended on the applied current density. No defect was observed independent of anodizing solutions and applied current density. This cost-effective and manufacturable method can be utilized for preparation of hierarchical nanostructures.     

In situ WAXD study of structure changes during uniaxial deformation of ethylene-based semicrystalline ethylene-propylene copolymer

In situ strain-induced structure changes during uniaxial deformation of an ethylene-propylene copolymer, containing 78 wt% (or 85 mol%) of ethylene moiety, were studied by synchrotron wide-angle X-ray diffraction (WAXD). The chosen sample could crystallize into orthorhombic, pseudo hexagonal or a mixed form, depending on the annealing conditions. Crystallization at high temperatures (e.g. 50 °C) favored the formation of orthorhombic form, while crystallization at low temperatures (e.g. 20 °C) favored the formation of pseudo hexagonal form. Under deformation, the transition from orthorhombic to pseudo hexagonal form was observed at relatively low strains (e.g. 0.12). At higher strains, WAXD data indicated the occurrence of direction-dependent crystal destruction at strains <0.25 and subsequent re-crystallization with extended-chain conformation at high strains (>1.0) all of the pseudo hexagonal form. The drastic changes in the crystalline structures (orthorhombic to pseudo hexagonal) and phase transitions (crystal destruction and re-crystallization) at modest strains can be attributed to the high mobility of the amorphous ethylene-propylene segments at room temperature.     

Ordered mesoporous carbons: Implication of surface chemistry, pore structure and adsorption of methyl mercaptan

Three ordered mesoporous carbon materials (OMC) were prepared by pyrolysis of sucrose filled in the mesoporous channels of SBA-15 at three different temperatures of 600 °C (OMC-600), 850 °C (OMC-850) and 1100 °C (OMC-1100), and followed by dissolution of the silica matrix in hydrofluoric acid. The pore structures and surface characteristics of the OMC materials were evaluated using XRD, nitrogen adsorption, FTIR spectroscopy, pH of carbon surface, and the amount of acidic surface groups from Boehm titration, respectively. The increase of the pyrolysis temperature resulted in an increase in surface pH, but a decrease in the amount of acidic surface groups. The surface area and micro- and mesopore volumes increased by increasing the pyrolysis temperature from 600 °C to 850 °C, but there were no significant changes in these properties above 850 °C. In this paper, adsorption characteristics of methyl mercaptan on the OMC materials were studied using a dynamic adsorption method in a fixed bed. The results showed that the adsorption of methyl mercaptan was strongly influenced by pore structure and surface chemistry of the OMC.     

A transmission electron microscopy study of hard anodic oxide layers on AlSi(Cu) alloys

Transmission electron microscopy (TEM) has been employed to examine anodic oxide film formation on 99.8 wt.% aluminium, Al-10 wt.%Si and Al-10 wt.%Si-3 wt.%Cu alloys under conditions relevant to hard anodizing. In particular, anodic oxidation of silicon particles proceeded at a significantly reduced rate compared with that of the adjacent aluminium matrix. This gave rise to alumina film encroachment beneath the particles with development of tortuous porosity and, eventually, occlusion of partially anodized particle in the anodic film. Additional effects included the presence of gas-filled cavities above the silicon particles, associated with oxygen generation above the anodizing particle. The presence of such particles and the corresponding gas-filled voids across the anodic film thickness and at the alloy/film interface is considered responsible for the continuous voltage rise during anodizing of the Al-10 wt.%Si alloy, effectively blocking electrolyte access to the pore base and providing local region of high resistance at the alloy/film interface. A direct consequence of the voltage rise was a thickening of the barrier layer at the base of the porous anodic film. For the ternary alloy, with the additional presence of copper and the CuAl₂ particles, the latter appear to have undergone complete oxidation, with copper detected in local film regions.     

Effects of heat treatment on the physical properties of lightweight aggregate from water reservoir sediment

Lightweight aggregates (LWAs) were produced from water reservoir sediment with added calcium oxide by employing four heat treatments, respectively, at temperatures in the range of 1170-1230 °C. The results show that LWAs produced at temperatures above 1200 °C meet European Union regulation EN-13055-1, which states that the unit weight of LWAs should be lower than 2000 kg/m³. The bulk density was easily lowered by extending the soaking time and increasing the heating rate. The ratio of strength to unit weight of the LWA produced at 1230 °C with a short soaking time and a fast heating rate was near that of a commercial product. The level of water adsorption was below 4%, which increased initially and then decreased due to pore connections and pore sealing. The formation of a glassy phase made the LWAs treated at higher temperature rough and sealed small pores (<0.1 mm) connected to the walls of large pores (>0.2 mm). The mineral phases of the LWAs were quartz, anorthite, and hematite.