Micro and nano-texturization of intermetallic oxide alloys by a single anodization step: preparation of artificial self-cleaning surfaces

Micro- and nanostructures of Ti-γCu (γ = 0, 30, 50, 70, and 100 wt %) intermetallic alloys were produced through a single anodization step. It was found that the original alloy composition influences the final oxide morphology obtained after anodization which presented formation of a microstructure with nanotubes, nanoparticles or nanopillars on the surface. Pure Ti and Cu oxide metals and their alloys presented hydrophilic or superhydrophilic properties immediately after anodization. When the anodized pure metal and/or Ti-γCu surfaces were functionalized with trimethoxypropylsilane (TPMSi), by dipping and coating with a thin perfluorinated layer, the treated substrates became in all cases superhydrophobic (water contact angles in the range of 152-166°), showing excellent self-cleaning properties with hysteresis below 3°. These results can be explained by a combination of nanomicro morphologies with low surface energy compounds in the topmost monolayers. The decrease in hysteresis was associated with a higher M-OH bond concentration on the anodized surfaces, which allowed for more complete TMPSi coating coverage. This study also indicates that easy and effective fabrication of superhydrophobic surfaces in pure metals and alloys is possible without involving traditional multistep processes.     

Ellipsometric analysis of porous anodized aluminum oxide films

We performed an ellipsometric study of porous anodized aluminum oxide (AAO) films on Si substrates. Regular cylindrical porous AAO films with flat bottom structure were formed by chemical etching and anodization. The data showed typical interference oscillations as a result of the transparent characteristics of the film throughout the visible spectral range. We applied a combined effective medium approximation model with anisotropic model to obtain optical properties of the films, which can be used as basic information applicable for more complex structures.     

Analysis of characteristics of silicon metal/insulator/semiconductor tunnel diodes with d.c.-plasma-grown oxide

In this work the I–V and C-V characteristics of silicon metal/insulator/semiconductor (MIS) tunnel diodes with d.c.-plasma-grown oxide are analysed in order to investigate further some basic relationships between the oxide properties and the plasma anodization conditions. It is shown that once such relationships are established, the device-grade ultrathin (2–5 nm) oxide can be grown on silicon by use of the plasma anodization technique. The MIS tunnel diodes formed in this way exhibit properties comparable with those containing thermally grown oxide.     

Anodization: a promising nano-modification technique of titanium implants for orthopedic applications

Anodization is a well-established surface modification technique that produces protective oxide layers on valve metals such as titanium. Many studies have used anodization to produce micro-porous titanium oxide films on implant surfaces for orthopedic applications. An additional hydrothermal treatment has also been used in conjunction with anodization to deposit hydroxyapatite on titanium surfaces; this is in contrast to using traditional plasma spray deposition techniques. Recently, the ability to create nanometer surface structures (e.g., nano-tubular) via anodization of titanium implants in fluorine solutions have intrigued investigators to fabricate nano-scale surface features that mimic the natural bone environment. This paper will present an overview of anodization techniques used to produce micro-porous titanium oxide structures and nano-tubular oxide structures, subsequent properties of these anodized titanium surfaces, and ultimately their in vitro as well as in vivo biological responses pertinent for orthopedic applications. Lastly, this review will emphasize why anodized titanium structures that have nanometer surface features enhance bone forming cell functions.     

Catalytic plasma anodization of single-crystal 6H-SiC structures

The use of catalytic plasma anodization to obtain oxides on the surface of SiC is an original method, hitherto unused. This method can significantly reduce the process temperature, increase the oxide growth rate, and improve the properties of the structures, giving a low density of surface states and a small fixed charge in the oxide. Pis’ma Zh. Tekh. Fiz. 25, 5–7 (August 12, 1999)     

Single-step direct fabrication of pillar-on-pore hybrid nanostructures in anodizing aluminum for superior superhydrophobic efficiency

Conventional electrochemical anodizing processes of metals such as aluminum typically produce planar and homogeneous nanopore structures. If hydrophobically treated, such 2D planar and interconnected pore structures typically result in lower contact angle and larger contact angle hysteresis than 3D disconnected pillar structures and, hence, exhibit inferior superhydrophobic efficiency. In this study, we demonstrate for the first time that the anodizing parameters can be engineered to design novel pillar-on-pore (POP) hybrid nanostructures directly in a simple one-step fabrication process so that superior surface superhydrophobicity can also be realized effectively from the electrochemical anodization process. On the basis of the characteristic of forming a self-ordered porous morphology in a hexagonal array, the modulation of anodizing voltage and duration enabled the formulation of the hybrid-type nanostructures having controlled pillar morphology on top of a porous layer in both mild and hard anodization modes. The hybrid nanostructures of the anodized metal oxide layer initially enhanced the surface hydrophilicity significantly (i.e., superhydrophilic). However, after a hydrophobic monolayer coating, such hybrid nanostructures then showed superior superhydrophobic nonwetting properties not attainable by the plain nanoporous surfaces produced by conventional anodization conditions. The well-regulated anodization process suggests that electrochemical anodizing can expand its usefulness and efficacy to render various metallic substrates with great superhydrophilicity or -hydrophobicity by directly realizing pillar-like structures on top of a self-ordered nanoporous array through a simple one-step fabrication procedure.     

Supercapacitance of ruthenium oxide deposited on titania and titanium substrates

Titanium planar sheet formed by a chemical polishing process and titania nanotube array formed by an electrochemical anodization process are used as electrode substrates, on which electroactive ruthenium oxides are deposited by an electroreduction and electrooxidation process for supercapacitor applications. Morphological characterization and electrochemical properties of the electrode substrates and ruthenium oxide electrodes have been investigated. Crystalline titania nanotube array shows a much higher electric double layer capacitance than titanium planar sheet due to its high surface area of nanotube walls. Additionally, the well-defined ruthenium oxide–titania/titanium nanotube array electrode exhibits a much higher redox supercapacitance and a lower capacitance decay than ruthenium oxide/titanium planar film electrode. Such a superior energy-storage performance of ruthenium oxide–titania/titanium is ascribed to highly accessible nanotube channels for the reversible redox reaction of ruthenium oxide. The modification strategy of ruthenium oxide electrode by introducing highly ordered nanotube array structure instead of planar film structure can significantly improve specific capacitance as well as cyclic charge-discharge stability.     

Nanopatterned silicon emitters of a solar cell fabricated by anodic aluminum oxide masks

We investigated the nanopattern transferring process by a template of anodic aluminum oxide and the formation of a nanoporous aluminum oxide layer on a Si solar cell by the anodization process of Al thin films. The anodization process provided a template to transfer the nanopattern onto the Si surface. The small-sized nanoporous alumina template was attached to be covered on the textured surface and played the role of etching mask in the F-based dry etching process. Furthermore, we deposited an Al thin film onto the Si surface and the subsequent anodization process was performed. The alumina formulated on the deposited Al thin film did not show the array of nanoporous structure and no nanopatterns were transferred onto the surface. The large-areal alumina deposited on the Si surface showed enhanced photo-absorption in the ultraviolet spectral region of 243 nm, but increased the photo-reflectance in the visible and infrared spectral regions when compared to the Si-bare sample.     

In Metalloberflächen verankerte Polymere zur Herstellung ultrahydrophober Metalloberflächen

Anchoring polymers in metal surface to produce superhydrophobic metal surfaces A two‐stage process was developed to endow metal or metal oxide surfaces with superhydrophobic and self‐cleaning properties. In the first step sheets of aluminium were roughened by anodic oxidation under intensive conditions. In the porous oxide layer, which was formed during the anodization, chitosan was cathodically deposited. By the chitosan’s jointing the incorporated amino groups endowed the composite surface with a high reactivity towards electrophilic agents. This fundamental synthetic route offers a new possibility to functionalize metal oxide surfaces with properties as wanted or required. In our work the amino groups were reacted with carbonyl compounds. Self‐assembling processes of their long alkyl rests caused to superhydrophobic surface properties.     

Highly Polar Linkers (Urea,Carbamate, Thiocarbamate) for Superoleophobic/Superhydrophobic or Oleophobic/Hydrophilic Properties

Due to the much lower surface tension of oils in comparison to water, it is extremely difficult to obtain superoleophobic properties and also both oleophobic and hydrophilic properties. While the obtaining of superoleophobic properties needs extremely complex surface structures such as reentrant structures to impede the oil wetting, the obtaining of both oleophobic and hydrophilic properties needs the use of both oleophobic materials (fluorinated materials) and hydrophilic materials (charged or polar species). Here, by electropolymerization of original 3,4‐ethylenedioxythiophene derivatives containing both fluorinated chains (C₈F₁₇, C₆F₁₃, or C₄F₉) and highly polar linkers (thiocarbamate SCONH, carbamate OCONH, and urea NHCONH), the possibility to obtain superoleophobic properties and also both oleophobic and hydrophilic properties is reported for the first time. More precisely superoleophobic properties are obtained with different fluorinated chain lengths and linkers while the obtaining of both oleophobic and hydrophilic properties is possible only with the most polar urea NHCONH linkers.     

Titanium-oxide interface structures formed by degassing and anodization processes

High-resolution electron microscopy was used to investigate two types of titanium-oxide interface structures. The first type was generated by thermal oxidation during the degassing process, which is one step in the process of porcelain-fused-to-metal systems. The thermal oxidation was performed for 1 min at a temperature of 1073 K in a porcelain furnace under a reduced pressure at 27 hPa. Columnar and granular rutile oxide formed on the titanium, and the surface oxide layer was almost 1 μ m thick. On an atomic scale, the crystal size of the rutile adjacent to the interface decreased about 10 nm. In addition, a very thin transitional layer 2–3 nm thick formed at the titanium-oxide interface. The crystal structure of the thin layer seemed to be the T{i}O phase with a NaCl-type structure. The interface between the hcp titanium and T{i}O phases was coherent through the close-packed planes ((0 0 0 2)_hcp and (1 1 1)_TiO). Partial coherency was observed at the interface between the T{i}O and T{i}O₂ phases. The second type of titanium-oxide interface was generated by anodization on a screw-type titanium implant. The morphology of the surface suggested that the titanium implant had been treated by spark anodization. The surface oxide, which was estimated to be about 10 μ m thick, was a mixture of the anatase-type T{i}O₂ phase and the amorphous phase. The crystal size of the anatase varied from less than 10 nm to more than 100 nm. A phosphate anion was concentrated in the amorphous phase. Between the surface oxide and the titanium base, macroscopic defects like cracks and voids were observed. Microscopic observation could not confirm the formation of a clear interface and lattice coherency between the titanium and oxide as a result of the degassing process.     

Oxide thickness measurements by infrared ellipsometry

Current interest' in anodized aluminum surfaces as substrates for adhesive bonding has created a need to measure the properties of the oxide layer in a non-contacting manner. Visible light ellipsometry is a very sensitive non-contacting technique for measuring the thickness of very thin films on smooth surfaces whose optical constants are known. However, the method is limited to film thicknesses which are generally less than 2000 Å and validity is lost when there is appreciable scattering caused by the roughness of the substrate and the structure of the oxide itself. These objections become much less severe if the operating wavelength is in the infrared region. Such an infrared ellipsometer has been developed to measure the thickness of oxides produced by anodization of aluminum with production-finished surfaces. The instrument operates with a 10.6 μm beam from a low power CO₂ laser and uses a 6328 Å beam from a He-Ne laser for alignment and location of the measurement region. The oxides were formed on unclad 7075 aluminum by anodization in an ammonium pentaborate solution at constant current to termination voltages of from 25 to 275 V in 25 V increments. The measured ellipsometric quantities Δ and ψ were used to compute the corresponding metal oxide film thicknesses using a complex refractive index N = 2.39?i41.36 for the substrate. The results for a film refractive index of 1.50 were in close agreement with those measured with a scanning electron microscope. Elemental concentration profiles for each surface were made by Auger electron spectroscopy.     

Nanopore gradients on porous aluminum oxide generated by nonuniform anodization of aluminum

A method for surface engineering of structural gradients with nanopore topography using the self-ordering process based on electrochemical anodization of aluminum is described. A distinct anodization condition with an asymmetrically distributed electric field at the electrolyte/aluminum interface is created by nonparallel arrangement between electrodes (tilted by 45°) in an electrochemical cell. The anodic aluminum oxide (AAO) porous surfaces with ordered nanopore structures with gradual and continuous change of pore diameters from 80 to 300 nm across an area of 0.5-1 cm were fabricated by this anodization using two common electrolytes, oxalic acid (0.3 M) and phosphoric acid (0.3 M). The formation of pore gradients of AAO is explained by asymmetric and gradual distribution of the current density and temperature variation generated on the surface of Al during the anodization process. Optical and wetting gradients of prepared pore structures were confirmed by reflective interferometric spectroscopy and contact angle measurements showing the ability of this method to generate porous surfaces with multifunctional gradients (structural, optical, wetting). The study of influence of pore structures on cell growth using the culture of neuroblastoma cells reveals biological relevance of nanopore gradients and the potential to be applied as the platform for spatially controllable cell growth and cell differentiation.     

Electrochemical fabrication of complex copper oxide nanoarchitectures via copper anodization in aqueous and non-aqueous electrolytes

Described is the synthesis of various copper oxide nanostructured thin films by anodization of Cu foil in aqueous and ethylene glycol electrolytes containing hydroxide, chloride and/or fluoride ions at room temperature. The nanostructure topology was found to depend on the pH of the anodization electrolyte, KOH concentration, applied voltage and the presence of chloride and fluoride ions. Our results demonstrate the opportunity to grow complex copper oxide nanostructured films possessing sub-micron thick layers by a simple and straightforward electrochemical route. Although no film was observed on the Cu surface when the anodization was carried out at 10 V in KOH solutions with pH ≤ 10, various nanoarchitectures were formed upon increasing the electrolyte pH in the presence of chloride ions. Replacing chloride ions with fluoride ions resulted in the formation of highly porous nanoarchitectures. A simple mechanism for the formation of such porous structures is proposed. Anodizing in ethylene glycol-based electrolytes resulted in the formation of leaf-like nanoarchitectures up to 500 nm in thickness. XPS analysis was performed to study the composition of the formed nanoarchitectures. Vacuum annealing of the material at 280 °C resulted in the formation of porous Cu₂O nanoarchitectures.     

Anodizing of etched aluminum foil coated with modified hydrous oxide film for aluminum electrolytic capacitor

Prior to galvanostatical anodization in boric acid solution, aluminum capacitor foil with a tunnel etch structure is treated in a two-step process in which a non-dense hydrous oxide film is first formed on foil in neutral boiling water for 10 min [namely, conventional hydration (CH)] and the hydrous oxide is then modified in a 80 °C weakly acidic solution containing trace amount of citric acid for 3 min [namely, modified conventional hydration (MCH)]. After modification, the hydrous oxide film becomes dense and thin. Time variations in the anode potential during anodizing were monitored, and the structure and dielectric properties of the anodic oxide films were examined by transmission electron microscopy, X-ray diffraction and electrochemical impedance spectroscopy measurements. It was found that the MCH-induced hydrous oxide film results in a decreased power consumption during anodization and an increased crystallized anodic oxide film, which has a high specific capacitance and a low specific resistance, comparing with the CH-induced hydrous oxide film.     

Dynamic wettability study on the functionalized PEGylated layer on a polylactide surface constructed by the coating of aldehyde-ended poly(ethylene glycol) (PEG)/polylactide (PLA) block copolymer

A block copolymer of α-acetal-poly(ethylene glycol) and polylactide(PEG/PLA) was prepared and utilized as a surface modifier to construct a functionalized PEG layer on a PLA substrate by simple coating. An active functional group, aldehyde, was readily prepared by the derivatization of an α-acetal group at the tethered PEG-chain end and can be further utilized to link bioactive functionality molecules such as sugars and proteins. The PEGylated surface thus prepared was characterized by the method of dynamic wetting using the Wilhelmy plate technique and by the surface/interfacial free energy calculation. The results of the dynamic wetting study suggested that the interactions of PEG on the surface with water induce the reorientation of the hydrophilic PEG component to accommodate itself optimally with the water phase, resulting in a significant reduction in the interfacial free energy. By employing the extended Fowkes’ equation, the polar interactions are accurately estimated in the free energy calculation of PEGylated surface; the polar components in surface free energy may be divided into two contributions with distinctive characteristics; one is the γ_s^h component reflecting water-interactive Lewis sites and the other is the γ_s^p component reflecting the dipole moment on the surface. The γ_s^p component, related to the PEG conformation via the dipole moment on the substrate, was suggested to be a determinant factor in the protein adsorption. Because these surfaces have both non-fouling and ligand-binding properties, they might support the selective binding and the growth of particular cell populations, and eventually, they are expected to have a high utility in biomedical fields including the field of tissue engineering.     

Voltage-dependent fabrication of nanoporous alumina and the applications to nanocapacitors

We investigated the fabrication of anodized aluminum oxide by anodization processes under DC and AC voltage biases. A two-step anodization process was used to fabricate the anodized aluminum oxide dielectrics in order to regulate the ordered nanopore array at the surface. AC samples showed the distorted nanoporous structure instead of a straight nanopore array in DC samples. As increasing the frequency of AC bias the nanovoid or nanocavity structure was formulated with the increased density of nanovoids. Nanoporous alumina was used for the fabrication of Ni-insulator-Al capacitors. The DC sample shows the tunneling process of an increase in leakage current and breakdown. When a negative voltage is applied to the capacitor device, the small current of 2 nA flows at a voltage of 0 V, indicating the existance of residual leakage current. The AC sample had very low leakage currents of the AAO dielectrics and the AAO hexagonal unit cell formed the nanocapacitor with a capacitance of 1-2 aF.     

Study of self-assembled triethoxysilane thin films made by casting neat reagents in ambient atmosphere

We studied four trialkoxysilane thin films, fabricated via self-assembly by casting neat silane reagents onto hydrophilic SiO_x/Si substrates in the ambient. This drop-casting method is simple, yet rarely studied for the production of silane self-assembled monolayers (SAMs). Various ex-situ techniques were utilized to systematically characterize the growth process: Ellipsometry measurements can monitor the evolution of film thickness with silanization time; water droplet contact angle measurements reveal the wettability; the change of surface morphology was followed by Atomic Force Microscopy; the chemical identity of the films was verified by Infrared-Visible Sum Frequency Generation spectroscopy. We show that the shorter carbon chain (propyl-) or branched (2-(diphenylphosphino)ethyl-) silane SAMs exhibit poor ordering. In contrast, longer carbon chain (octadecyl and decyl) silanes form relatively ordered monolayers. The growth of the latter two cases shows Langmuir-like kinetics and a transition process from lying-down to standing-up geometry with increasing coverage.     

Charge migration on hydrophobic and hydrophilic silicon dioxide

Experimental results are presented which shed light on the properties of thin discontinuous metal films as chemical sensing elements. They demonstrate further some interesting differences between hydrophilic and hydrophobic sensing structures. Two different methods are used to study the migration of charges out onto the oxide surface outside the metal gate of metal–oxide–semiconductor capacitors after treatment of the surface with hydrochloric acid. The charge migration is observed either as a time dependent increase of the inversion capacitance or as a possibility to generate a photocapacitive current by a chopped light beam hitting the oxide surface at a distance from the contact. It is concluded that the charge migration occurs only if the surface was hydrophilic before the HCl-treatment. For a hydrophobic surface neither the inversion capacitance nor the photocapacitive current changed upon ion-treatment. An explanation for an observed increase of the capacitance level of hydrophilic structures immersed in electrolytes is also given.     

Electrostatic Properties of Ideal and Non‐ideal Polar Organic Monolayers: Implications for Electronic Devices

Molecules in (or as) electronic devices are attractive because the variety and flexibility inherent in organic chemistry can be harnessed towards a systematic design of electrical properties. Specifically, monolayers of polar molecules introduce a net dipole, which controls surface and interface barriers and enables chemical sensing via dipole modification. Due to the long range of electrostatic phenomena, polar monolayer properties are determined not only by the type of molecules and/or bonding configuration to the substrate, but also by size, (dis‐)order, and adsorption patterns within the monolayer. Thus, a comprehensive understanding of polar monolayer characteristics and their influence on electronic devices requires an approach that transcends typical chemical designs, i.e., one that incorporates long‐range effects, in addition to short‐range effects due to local chemistry. We review and explain the main uses of polar organic monolayers in shaping electronic device properties, with an emphasis on long‐range cooperative effects and on the differences between electrical properties of uniform and non‐uniform monolayers.