Anodic alumina membranes with defined pore diameters and thicknesses obtained by adjusting the anodizing duration and pore opening/widening time

The through-hole porous anodic aluminum oxide (AAO) membranes were fabricated by a simple two-step anodization of aluminum in 0.3 M oxalic acid, 0.3 M sulfuric acid, and 2 wt.% phosphoric acid solutions under different operating conditions followed by the removal of the remaining Al substrate and the pore opening/widening process. The effect of duration of the second anodizing step on the thickness of the porous oxide layer and the influence of other anodizing conditions such as applied voltage, type of electrolyte, and purity of the substrate on the rate of porous oxide growth were discussed in detail. The pore opening procedure for all synthesized membranes was optimized, and the influence of the duration of chemical etching on structural features of AAO membranes, especially pore diameter, was studied. The rate of pore widening was established for AAO membranes formed in various anodizing electrolytes and for different temperatures of 5 wt.% H₃PO₄ used for alumina dissolution.

     

Formation of gold-capped silicon nanocolumns on silicon substrate

Gold-capped silicon nanocolumns regularly distributed over silicon substrate were obtained. The columns length was roughly 100 nm; their deviation from perpendicular axis was less than 2°. The diameter of the columns was of the order of 10 nm or below of that. The proposed procedure of nanostructuring included the following main steps: deposition of aluminum thin layer (100–500 nm) by magnetron sputtering on (100) oriented Si wafers; formation of porous self-ordered alumina structures by electrochemical anodizing of the Al film in oxalic acid; electroless inversion of Au in alumina pores; and reactive ion etching. The obtained Si–Au structures are of importance as the platforms for biosensing applications, while the gold-free structures are of interest in photovoltaics.

     

Formation of gold-capped silicon nanocolumns on silicon substrate

Gold-capped silicon nanocolumns regularly distributed over silicon substrate were obtained. The columns length was roughly 100?nm; their deviation from perpendicular axis was less than 2°. The diameter of the columns was of the order of 10?nm or below of that. The proposed procedure of nanostructuring included the following main steps: deposition of aluminum thin layer (100?C500?nm) by magnetron sputtering on (100) oriented Si wafers; formation of porous self-ordered alumina structures by electrochemical anodizing of the Al film in oxalic acid; electroless inversion of Au in alumina pores; and reactive ion etching. The obtained Si?CAu structures are of importance as the platforms for biosensing applications, while the gold-free structures are of interest in photovoltaics.     

Anodic alumina membranes with defined pore diameters and thicknesses obtained by adjusting the anodizing duration and pore opening/widening time

The through-hole porous anodic aluminum oxide (AAO) membranes were fabricated by a simple two-step anodization of aluminum in 0.3?M oxalic acid, 0.3?M sulfuric acid, and 2?wt.% phosphoric acid solutions under different operating conditions followed by the removal of the remaining Al substrate and the pore opening/widening process. The effect of duration of the second anodizing step on the thickness of the porous oxide layer and the influence of other anodizing conditions such as applied voltage, type of electrolyte, and purity of the substrate on the rate of porous oxide growth were discussed in detail. The pore opening procedure for all synthesized membranes was optimized, and the influence of the duration of chemical etching on structural features of AAO membranes, especially pore diameter, was studied. The rate of pore widening was established for AAO membranes formed in various anodizing electrolytes and for different temperatures of 5?wt.% H₃PO₄ used for alumina dissolution.     

Fabrication of AAO films with controllable nanopore size by changing electrolytes and electrolytic parameters

In this paper, we fabricate two kinds of anodic aluminum oxide (AAO) films with controllable nanopore size by changing electrolytes and electrolytic parameters. The first AAO film with a four-layer structure was fabricated by sequential anodization of aluminum in aqueous solution of H₂SO₄, H₂C₂O₄, malonic acid, and tartaric acid at different anodic oxidation voltages. The average pore diameter of the as-prepared AAO film is 25 nm in the first layer, 54 nm in the second layer, 68 nm in the third layer, and 88 nm in the fourth layer, respectively. The pore densities of each layer decrease downwards to Al substrate, which are 300?×?10⁸, 100?×?10⁸, 21?×?10⁸, and 6.9?×?10⁸ cm^?2, respectively. Furthermore, another AAO film with periodically changed pore diameter was fabricated by alternating anodization of aluminum in aqueous solution of H₃PO₄ and tartaric acid under galvanostatic mode. The anodization processes present approximately identical best ordering voltage (195 V) in H₃PO₄ and tartaric acid under galvanostatic mode. The pore diameter with periodic change can be enlarged through a pore-widening treatment. Both AAO films with special nanopore structures can be used not only as templates for preparing nano-array materials whose pore diameter presents periodic change or gradual increase, but also as nanofilters to separate materials in some special media.     

The effect of anode surface area on nanoporous oxide formation during anodizing of low purity aluminum (AA1050 alloy)

Porous anodic alumina layers were obtained by a simple two-step anodization of low purity aluminum (99.5 % Al, AA1050 alloy) in a 0.3 M oxalic acid electrolyte at 45 V and 20 °C. The effect of anode surface area on structural features of nanoporous oxide and process of oxide formation was investigated. An ordered structure composed of nanostripes or nanopores was formed on the Al surface during electrochemical polishing in a mixture of perchloric acid and ethanol. This nanopattern is then replicated during the anodic oxide formation. It was found that the pore diameter, interpore distance, and porosity increase slightly with increasing surface area of the aluminum sample exposed to the anodizing electrolyte. On the other hand, a slight decrease in pore density and cell wall thickness was observed with increasing surface area of the sample. The detailed inspection of current density vs. time curves was also performed. The obtained results revealed that the higher surface area of the anode, the local current density minimum, was reached faster during first step of anodization and the increase in current density corresponding to the pore rearrangement process was observed earlier. Finally, a dense array of Pd nanowires (～90 nm in diameter) was synthesized by simple electrodeposition of metal inside the channels of through-hole nanoporous anodic alumina templates with relatively large surface areas (4 cm²).     

Sol–gel preparation of mesoporous Al2O3–SiO2 glasses: structural evolution monitored by solid state NMR

Glasses along the composition line 0.5Al₂O₃–xSiO₂ (1 ≤ x ≤ 6) were prepared via a novel sol–gel route using tetraethylorthosilicate and aluminum lactate as precursors. The structural evolution from solution to gel to glass is monitored by standard ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopies, revealing important insights about molecular level mechanisms occurring at the various stages of glass formation. Under the experimental conditions reported, silica and alumina precursors undergo homoatomic condensation processes when the gel is heat treated at about 100 and 300 °C, respectively, and only little heteroatomic co-condensation occurs in this temperature range. The latter is promoted only upon elimination of the residual lactate and water ligands upon annealing the gels above 300 °C. Following calcination at 650 °C, mesoporous glasses are obtained, having average pore diameter of about 3 nm and a surface areas near 500 m²/g. Si–O–Al connectivities are detected by ²⁹Si magic angle spinning (MAS)-NMR. ²⁷Al MAS-NMR spectra reveal aluminum in four-, five- and six-coordination. The spectra differ significantly from those of other sol–gel derived Al₂O₃–SiO₂ materials prepared from different precursor routes, suggesting that the lactate route results in a higher degree of compositional homogeneity.     

Structure and magnetic properties of La0.67Sr0.33MnO3 thin films prepared by sol–gel method

La_1?xSr_xMnO₃ (x = 0.33) (LSMO) thin films have been fabricated successfully by sol–gel method on two different types of substrates, Si (111) and SrTiO₃ (STO) (001). Microstructure and magnetic properties of LSMO thin films have been investigated. The X-ray diffraction studies of the films confirm the pure phase of the LSMO thin films. In contrast with LSMO thin films on Si substrate, the performances of LSMO on STO substrate are superior both from structural and magnetic properties. For the samples deposited on STO substrate, highly preferred orientation as well as less strain and grain defects was found; in other aspect, the magnetization, the residual and saturation moment value, tended greater while a decreased coercive field required merely (saturation moment value was about five times and coercive field was only about 13 % of those on Si substrate). The Curie temperature of LSMO thin films on Si and STO substrates is estimated to be about 349.7 and 359 K, respectively.     

Synthesis of Continuous Mesoporous Alumina Films with Large‐Sized Cage‐Type Mesopores by Using Diblock Copolymers

Mesoporous alumina films with large‐sized cage‐type mesopores were prepared by using commercially available diblock copolymer (PS‐b‐PEO) and economic inorganic salt (AlCl₃) as aluminum source. The obtained mesopore sizes drastically expand from 35 nm to 80 nm when the amount of ethanol in the precursor solutions were controlled. More interestingly, under an optimized amount of ethanol as co‐solvent, there was no significant change of micelle morphology on the substrate, even though the relative amount of PS‐b‐PEO to alumina source was dramatically varied. When the amount of alumina precursor was decreased, the pore walls gradually became thinner, thereby improving pore connectivity. The ordered mesoporous alumina films obtained in this study exhibit high thermal stability up to 1000 °C, and their frameworks are successfully crystallized to γ‐alumina phase. This technique could also be applicable for creating other metal oxide thin films with large mesopores.     

硅基超薄多孔氧化铝膜的制备

将二次阳极氧化法应用于硅基铝膜的制备, 在草酸溶液中得到了厚度可控的硅基超薄多孔氧化铝膜(PAM), 厚度小于100 nm. 实验中记录了氧化电流随时间的实时变化曲线, 发现硅衬底的氧化电流在大幅下降前有一小幅波动. 对应于Al/Si界面的氧化过程中, 孔洞底部之间的残留铝岛被优先氧化, 可将此作为终止铝氧化的标志. 扫描电镜(SEM)观察表明, 二次氧化提高了孔洞分布的均匀性, 使得孔在一定的区域内呈现有序六角分布.这种模板可进一步用于硅基纳米器件和纳米结构的制备.     

Behavior of acid species during heat treatment and re-anodizing of porous alumina films formed in malonic acid

In the present work infrared spectroscopy, photoluminescence spectral measurements and the potenthiodynamic technique for studying the effect of treatment temperature on compositional and electronic properties of malonic acid alumina films were used. In the course of our studies, it has been proven that heat treatment of malonic acid films at temperatures from 250 up to 400 °C leads to considerable changes in the photoluminescence properties and voltammetric response during their potentiodynamic re-anodizing. We suggest that defects, such as electron traps, in this type of porous anodic films are caused by the atoms of hydrogen (one or two) escaping from the CH₂ groups of the malonic acid species as a result of the heat treatment. The sites of such defects provide pathways for easy electron migration under a high electric field increasing electroconductivity of anodic alumina films. On the contrary, no structural defects responsible for enhanced electroconductivity are observed during thermal splitting of oxalate groups in the oxalic acid alumina films.     

Dielectric properties of Al–Si composite oxide films formed on electropolished and DC-etched aluminum by electrophoretic sol-gel coating and anodizing

Highly pure aluminum specimens (99.99%) after electropolishing and DC-etching were covered with SiO₂ films by electrophoretic sol-gel coating and were anodized in neutral boric acid/borate solutions. Time-variations in cell voltage during electrophoretic sol-gel coating and in anode potential during anodizing were monitored. Structure and dielectric properties of the anodic oxide films were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), and electrochemical impedance spectroscopy (EIS). It was found that electrophoretic sol-gel coating forms uniform SiO₂ films on the surface of both electropolished and DC-etched specimens. Anodizing of specimens after electrophoretic coating lead to the formation of anodic oxide films consisting of two layers: an inner alumina layer and an outer Al–Si composite oxide layer. The anodic oxide films formed, thus, had slightly higher capacitances than those formed on aluminum without any coating. Higher heating temperatures after electrophoretic deposition caused the increase in capacitance of anodic oxide films more effectively. Anodizing in a boric acid solution after SiO₂ coating on DC-etched foil allowed the anode potential to reach a value higher than 1,000 V, resulting in 39% higher capacitances than those on specimens without SiO₂ film. Dedicated to Professor Su-Il Pyun on the occasion of his 65th birthday.     

Effect of substrate on phase formation and surface morphology of sol-gel lead-free KNbO3, NaNbO3, and K0.5Na0.5NbO3 thin films

Environmentally acceptable lead-free ferroelectric KNbO₃ (KN) or NaNbO₃ (NN) and K_0.5Na_0.5NbO₃ (KNN) thin films were prepared using a modified sol-gel method by mixing potassium acetate or sodium acetate or both with the Nb-tartrate complex, deposited on the Pt/Al₂O₃ and Pt/SiO₂/Si substrates by a spin-coating method and sintered at 650°C. X-ray diffraction (XRD) analysis indicated that the NN and KNN films on the Pt/SiO₂/Si substrate possessed a single perovskite phase, while NN and KNN films on the Pt/Al₂O₃ substrate contained a small amount of secondary pyrochlore phase, as did KN films on both substrates. Scanning electron microscopic (SEM) and atomic force microscopic (AFM) analyses confirmed that roughness R _q of the thin KNN/Pt/SiO₂/Si film (?? 7.4 nm) was significantly lower than that of the KNN/Pt/Al₂O₃ film (?? 15 nm). The heterogeneous microstructure composed of small spherical and larger needle-like or cuboidal particles were observed in the KN and NN films on both substrates. The homogeneous microstructure of the KNN thin film on the Pt/SiO₂/Si substrate was smoother and contained finer spherical particles (?? 50 nm) than on Pt/Al₂O₃ substrates (?? 100 nm). The effect of different substrates on the surface morphology of thin films was confirmed.     

Preparation of super-hydrophobic white carbon black from nano-rice husk ash

Nano-rice husk ashes were prepared by burning rice husk with a self-propagating method. The white carbon black with high purity was prepared by an alkali dissolving–acid reaction method from nano-husk ash. The super-hydrophobic SiO₂ films were prepared by the sol–gel method using hexamethyldisilazane as a modifier. The effects of the pH and reaction time in the acid reaction process on the purity of the white carbon black, and the effect of the modifier on the hydrophobic property of SiO₂ films were studied. The performances were characterized by XRD, BET, SEM, IR, and contact angle analyzer. The results showed that the purity of white carbon black reached 98.48 % when the NaOH solution with the rice husk ash was heated for 2 h at 90 °C, then the pH of the solution was adjusted by sulfuric acid to 3, and the acid reaction time was 2 h. The contact angle of SiO₂ films was more than 160° when volume ratio of the modifier to silica–sodium hydroxide mixed solution was 0.15. The mechanism of the modifier on SiO₂ surfaces is a graft copolymerization. The hydrophobic groups in the modifier replace the hydroxy groups on SiO₂ surfaces and make SiO₂ surfaces present super-hydrophobicity.     

Alumina nanofibers obtained via electrospinning of pseudo-boehmite sol/PVP solution

Alumina nanofibers were fabricated by calcination of the polyvinylpyrrolidone (PVP)/pseudo-boehmite nanocomposite precursor fibers formed by electrospinning PVP/ethanol solution of dispersed pseudo-boehmite nanoparticles with and without additive of silica. The evolution of the phase, mechanical property and morphological features of the calcined fibers were studied and the effect of adding SiO₂ on the phase transformation of alumina was discussed. Adding SiO₂ can retard the phase transformation of γ-Al₂O₃ to α-Al₂O₃ and therefore inhibit the growth of alumina grains during calcination. Upon calcining the precursor fibers with 4 wt% SiO₂ additive at 1,300 °C, continuous alumina nanofibers with diameter ranging from 300 to 800 nm were obtained. These continuous nanofibers exhibited good flexibility and could be very promising for applications in filtration and catalyst support.     

Electrochemical behavior of CoSb<Subscript>3</Subscript> in sulfuric and oxalic acids over the potential range 0 to 40 V

The electrochemical behavior of pure Co, pure Sb, and CoSb₃ has been investigated over a large anodic potential range (0 to 40 V) in two acids, i.e., oxalic acid or sulfuric acid at different concentrations (0.01 to 1 M). Potentiodynamic polarizations performed on CoSb₃ plates reveal the possible formation of a passive layer between 1 and 3 V (vs SCE), on a passivation plateau. The oxidation of CoSb₃ at 2 V in sulfuric and oxalic acids results in the growth of an anodic conversion layer. This coating is mainly made of a porous layer of amorphous antimony oxides due to dealloying of CoSb₃. In the specific case of oxalic acid, rods of crystallized oxalates are tangled between the oxide sheets.     

Influence of Nanoporosity and Roughness on the Thickness-Dependent Coercivity of Iron Nanonetworks

Summary.  We have studied the coercivity of magnetic nanonetworks as a function of thickness, nominal pore diameter, and surface/interface roughness in the thickness range of approximately 2 to 45 nm where a Néel-type domain wall has been theoretically predicted. Such magnetic nanonetworks have been prepared by sputtering iron on the walls of commercially available porous nano-channel alumina (NCA) membranes. The thickness dependence of coercivity has also been studied on films deposited on surface-oxidized Si and glass subtrates. These substrates are essentially non-porous and much smoother than NCAs. Our investigation shows that the coercivity of films deposited on Si and glass depends on the spatial fluctuation of thickness which arises from the roughness of the apparently smooth substrates. On the other hand, NCAs are found to be inherently quite rough, and films on NCAs show a complex thickness dependence which arises from the interplay between surface/interface roughness, domain pinning due to porosity, surface anisotropy, surface torques, and oxidation of the iron films. It was found that the growing films on NCA substrates led to partial filling up of the pore entrance, thereby reducing its effective diameter. The film growth also affects the roughness of the film, which in turn affects its coercivity. We propose a model for the thickness dependence of coercivity based on the pore fill-up geometry considering the effective pore diameter and the critical thickness at which the pore will be completely filled up. Experimental results on coercivity with thickness variation of iron network deposited on NCA generally agree with the suggested model. Received October 16, 2001. Accepted (revised) January 11, 2002     

Well defined thermostable cellulose nanocrystals via two-step ionic liquid swelling-hydrolysis extraction

Cellulose nanocrystals were prepared from cotton fibers by a two-stage method involving ionic liquid swelling treatment followed by hydrolysis under mild acid conditions. Controlled swelling of cellulosic fibers was achieved in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) at 80 °C, while avoiding extensive dissolution of crystalline regions. Since the accessibility of the substrate was considerably enhanced, the hydrolysis occurred even under mild conditions, using up to 60 times less sulfuric acid than the traditional extraction methods based on concentrated sulfuric acid. The effects of process parameters on nanoparticle morphology, composition and stability were investigated. The individual rod-like nanocrystals, observed under field emission gun scanning electron microscopy, exhibited an average diameter of around 20 nm and a length ranging from 150 to 350 nm. According to X-ray photoelectron spectroscopy and thermogravimetric analysis, the surface of the so-extracted nanoparticles proved to be deprived of contaminating sulfate groups leading to significantly higher thermal stability with respect to cellulose nanocrystals extracted by traditional method in concentrated sulfuric acid.     

Fabrication of hollow SiO2 and Au (core)–SiO2 (shell) nanostructures of different shapes by CdS template dissolution

Core–shell silica (SiO₂) coated CdS nanorods (NR) and nanospheres (NS) were prepared (SiO₂@CdS) by deposition of a Si–O–Si amorphous layer over the CdS surface through the hydrolysis of 3-mercaptopropyltrimethoxysilane and tetraethylorthosilicate. Nanoporous SiO₂ matrix (NPSM), hollow SiO₂ nanotubes (HSNT) and nanospheres (HSNS) useful for efficient adsorption and catalytic processes were prepared by chemical dissolution of CdS–NS (size: 9–10 nm) and CdS–NR (length: 116–128 nm and width: 6–11 nm) template from SiO₂@CdS with 2 M HNO₃. These SiO₂ nanostructures were characterized by optical absorption, TEM, EDX, SAED and BET surface area analysis. TEM images revealed the fabrication of slightly distorted HSNS (size: 9–12 nm) and closed HSNT (length: 30–45 nm and diameter: 9–14 nm) of shorter dimensions than the CdS–NR template used. The BET surface area (112–134 m² g^?1) of NPSM and HSNS is found to be larger than the surface area (29–51 m² g^?1) of SiO₂@CdS composites indicating hollow SiO₂ morphology. Silica coated Au (SiO₂@Au) composites formed by CdS dissolution from Au (2 wt%) deposited CdS–NR core-encapsulated into SiO₂ shell (SiO₂@Au–CdS–NR) exhibited a surface plasmon band at 550 nm and displayed high catalytic activity for 4-nitrophenol reduction by Au nanoparticle.     

Structural and nanomechanical properties of sol–gel prepared (K,Na)NbO3 thin films

Perovskite (K, Na)NbO₃ (KNN) thin films (~100 nm) were prepared by sol–gel/spin coating process on Pt/SiO₂/Si substrates and annealed at 650 °C. The structural properties of KNN films were confirmed by X‐ray diffraction analysis (XRD), Raman spectroscopy and scanning electron, transmission electron and atomic force microscopy (SEM, TEM and AFM) analysis. Pure perovskite phase of K_0.65Na_0.35NbO₃ in nonstoichiometric composition with monoclinic symmetry in film was revealed. Uniform homogeneous microstructure of KNN film with the roughness (~6.9 nm) contained spherical particles (~50–90 nm). Nanoindentation technique was used to characterize the mechanical properties of KNN films. Elastic modulus and hardness of Pt, SiO₂ and KNN thin films were calculated from their composite values of KNN/Pt/SiO₂/Si film/substrate system. The modulus and hardness of KNN film (71 and 4.5 GPa) were lower in comparison with SiO₂ (100 and 7.5 GPa). Pt film (~30 nm) did not influence the composite modulus, but had effect on hardness of KNN film. Copyright © 2015 John Wiley & Sons, Ltd.