Influence of water content on nanotubular anodic titania formed in fluoride/glycerol electrolytes

Peculiar Porous Aluminum Oxide Films Produced via Electrochemical Anodizing in Malonic Acid Solution with Arsenazo-I Additive

The influence of arsenazo-I additive on electrochemical anodizing of pure aluminum foil in malonic acid was studied. Aluminum dissolution increased with increasing arsenazo-I concentration. The addition of arsenazo-I also led to an increase in the volume expansion factor up to 2.3 due to the incorporation of organic compounds and an increased number of hydroxyl groups in the porous aluminum oxide film. At a current density of 15 mA·cm⁻² and an arsenazo-I concentration 3.5 g·L⁻¹, the carbon content in the anodic alumina of 49 at. % was achieved. An increase in the current density and concentration of arsenazo-I caused the formation of an arsenic-containing compound with the formula Na_1,5Al₂(OH)_4,5(AsO₄)₃·7H₂O in the porous aluminum oxide film phase. These film modifications cause a higher number of defects and, thus, increase the ionic conductivity, leading to a reduced electric field in galvanostatic anodizing tests. A self-adjusting growth mechanism, which leads to a higher degree of self-ordering in the arsenazo-free electrolyte, is not operative under the same conditions when arsenazo-I is added. Instead, a dielectric breakdown mechanism was observed, which caused the disordered porous aluminum oxide film structure.

Crystallinity of TiO2 nanotubes and its effects on fibroblast viability, adhesion, and proliferation

Titanium and titanium alloys are widely used as a biomaterial due to their mechanical strength, corrosion resistance, low elastic modulus, and excellent biocompatibility. TiO₂ nanotubes have excellent bioactivity, stimulating the adhesion, proliferation of fibroblasts and adipose-derived stem cells, production of alkaline phosphatase by osteoblasts, platelets activation, growth of neural cells and adhesion, spreading, growth, and differentiation of rat bone marrow mesenchymal stem cells. In this study, we investigated the functionality of fibroblast on titania nanotube layers annealed at different temperatures. The titania nanotube layer was fabricated by potentiostatic anodization of titanium, then annealed at 300, 530, and 630 °C for 5 h. The resulting nanotube layer was characterized using SEM (Scanning Electron Microscopy), TF-XRD (Thin-film X-ray diffraction), and contact angle goniometry. Fibroblasts viability was determined by the CellTiter-Blue method and cytotoxicity by Lactate Dehydrogenase test, and the cell morphology was analyzed by scanning electron microscopy. Also, cell adherence, proliferation, and morphology were analyzed by fluorescence microscopy. The results indicate that the modification in nanotube crystallinity may provide a favorable surface fibroblast growth, especially on substrates annealed at 530 and 630 °C, indicating that these properties provide a favorable template for biomedical implants.

Influence of Two-Stage Anodization on Properties of the Oxide Coatings on the Ti–13Nb–13Zr Alloy

The increasing demand for titanium and its alloys used for implants results in the need for innovative surface treatments that may both increase corrosion resistance and biocompatibility and demonstrate antibacterial protection at no cytotoxicity. The purpose of this research was to characterize the effect of two-stage anodization—performed for 30 min in phosphoric acid—in the presence of hydrofluoric acid in the second stage. Scanning electron microscopy, atomic force microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, glow discharge optical emission spectroscopy, nanoindentation and nano-scratch tests, potentiodynamic corrosion studies, and water contact angle measurements were performed to characterize microstructure, mechanical, chemical and physical properties. The biologic examinations were carried out to determine the cytotoxicity and antibacterial effects of oxide coatings. The research results demonstrate that two-stage oxidation affects several features and, in particular, improves mechanical and chemical behavior. The processes influencing the formation and properties of the oxide coating are discussed.

Electrochemically Reduced Titania Nanotube Synthesized from Glycerol-Based Electrolyte as Supercapacitor Electrode

In this paper the synthesis of self-organized Titania nanotubes (TNTs) by a facile potentiostatic anodization in a glycerol-based electrolyte is reported. The optimized TNTs were subsequently reduced through a cathodic reduction process to enhance its capacitive performance. FESEM and XRD were used to characterize the morphology and crystal structure of the synthesized samples. XPS analysis confirmed the reduction of Ti4+ to Ti3+ ions in the reduced Titania nanotubes (R-TNTs). The tube diameter and separation between the tubes were greatly influenced by the applied voltage. TNTs synthesized at voltage of 30 V for 60 min exhibited 86 nm and 1.1 µm of tube diameter and length, respectively and showed high specific capacitance of 0.33 mF cm−2 at current density of 0.02 mA cm−2. After reduction at 5 V for 30 s, the specific capacitance increased by about seven times (2.28 mF cm−2) at 0.5 mA cm−2 and recorded about 86% capacitance retention after 1000 continuous cycling at 0.2 mA cm−2, as compared to TNTs, retained about 61% at 0.01 mA cm−2. The charge transfer resistance drastically reduced from 6.2 Ω for TNTs to 0.55 Ω for R-TNTs, indicating an improvement in the transfer of electrons and ions across the electrode–electrolyte interface.

Enhanced interfacial adhesion and osseointegration of anodic TiO2 nanotube arrays on ultra-fine-grained titanium and underlying mechanisms

The poor adhesion of anodic TiO₂ nanotubes (TNTs) arrays on titanium (Ti) substrates adversely affects applications in many fields especially biomedical engineering. Herein, an efficient strategy is described to improve the adhesion strength of TNTs by performing grain refinement in the underlying Ti substrate via high-pressure torsion processing, as a larger number of grain boundaries can provide more interfacial mechanical anchorage. This process also improves the biocompatibility and osseointegration of TNTs by increasing the surface elastic modulus. The TNTs in length of 0.4 µm have significantly larger adhesion strength than the 2.0 µm long ones because the shorter TNTs experience less interfacial internal stress. However, post-anodization annealing reduces the fluorine concentration in TNTs and adhesion strength due to the formation of interfacial cavities during crystallization. The interfacial structure of TNTs/Ti system and the mechanism of adhesion failures are further investigated and discussed. STATEMENT OF SIGNIFICANCE: Self-assembled TiO₂ nanotubes (TNTs) prepared by electrochemical anodization have a distinct morphology and superior properties, which are commonly used in photocatalytic systems, electronic devices, solar cells, sensors, as well as biomedical implants. However, the poor adhesion between the TNTs and Ti substrate has hampered wider applications. Here in this study, we describe an efficient strategy to improve the adhesion strength of TNTs by performing grain refinement in the underlying Ti substrate via high-pressure torsion (HPT) processing. The interfacial structure of TNTs/Ti system and the mechanism of adhesion failure are systematically studied and discussed. Our findings not only develop the knowledge of TNTs/Ti system, but also provide new insights into the design of Ti-based implants for orthopedic applications.

A Novel Methodology for Economical Scale-Up of TiO2 Nanotubes Fabricated on Ti and Ti Alloys

The prospective use of nanotechnology for medical devices is increasing. While the impact of material surface nanopatterning on the biological response is convincing, creating a large surface area with such nanotechnology remains an unmet challenge. In this paper, we describe, for the first time, a reproducible scale-up manufacturing technique for creating controlled nanotubes on the surfaces of Ti and Ti alloys. We describe an average of approximately 7.5-fold increase in cost and time efficiency with regards to the generation of 20, 50, and 100 nm diameter nanotubes using an anodisation technique. These novel materials have great potential in the medical field through their influence on cellular activity, in particular, protein absorption, focal adhesion, and osteoinduction. In this paper, we provide a step-by-step guide to optimise an anodisation system, starting with design rationale, proof of concept, device upscaling, consistency, and reproducibility check, followed by cost and efficiency analysis. We show that the optimised device can produce a high number of anodised specimens with customisable specimen shape at reduced cost and time, without compromising the repeatability and consistency. The device can fabricate highly uniform and vertically oriented TiO2 nanotube layer with desired pore diameters.

Transparent titanium dioxide nanotubes: Processing, characterization, and application in establishing cellular response mechanisms

The therapeutic applications of titanium dioxide nanotubes as osteogenic surface treatments for titanium-based implants are largely due to the finely tunable physical characteristics of these nanostructures. As these characteristics change, so does the cellular response, yet the exact mechanisms for this relationship remains largely undefined. We present a novel TiO₂ NT imaging platform that is suitable for use with live-cell imaging techniques, thereby enabling, for the first time, dynamic investigation of those mechanisms. In this work, fabrication methods for producing transparent TiO₂ NTs with diameters of 56 ± 6 nm, 75 ± 7 nm, 92 ± 9 nm, and 116 ± 10 nm are described. To demonstrate the diagnostic potential of these TiO₂ NT imaging platforms, the focal adhesion protein vinculin and actin cytoskeletal filaments were fluorescently tagged in osteoblasts and real-time, high-resolution fluorescent microscopy of live-cell interactions with TiO₂ NT substrates were observed. The scope of such a platform is expected to extend far beyond the current proof-of-concept, with great potential for addressing the dynamic response of cells interacting with nanostructured substrates.

STATEMENT OF SIGNIFICANCE

Titanium dioxide (TiO₂) nanotubes are known to strongly enhance bone/mesenchymal stem cell behavior and, consequently, have gained attention as potential osteogenic surface treatments for titanium-bone implants. The exact mechanism by which TiO₂ nanotubes influence cellular function remains controversial, partly due to limitations in existing cellular imaging methods with opaque substrates. This work identifies fabrication conditions for the successful production of transparent TiO₂ nanotube arrays with tailorable diameters, as well as their functionality with pre-osteoblast mouse cells (MC3T3-E1) transfected with fluorescent focal adhesion protein vinculin and cytoskeletal filament actin. We demonstrate a means of recording live-cell, cell-substrate interaction mechanisms via high-resolution fluorescent microscopy and customizable, transparent TiO₂ nanotubes to begin defining the relationship between TiO₂ nanotube features and cell function.

Understanding and augmenting the stability of therapeutic nanotubes on anodized titanium implants

Titanium is an ideal material choice for orthopaedic and dental implants, and hence a significant amount of research has been focused towards augmenting the therapeutic efficacy of titanium surfaces. More recently the focus has shifted to nano-engineered implants fabricated via anodization to generate self-ordered nanotubular structures composed of titania (TiO₂). These structures (titania nanotubes/TNTs) enable local drug delivery and tailorable cellular modulation towards achieving desirable effects like enhanced osseointegration and antibacterial action. However, the mechanical stability of such modifications is often ignored and remains underexplored, and any delamination or breakage in the TNTs modification can initiate toxicity and lead to severe immuno-inflammatory reactions. This review details and critically evaluates the progress made in relation to this aspect of TNT based implants, with a focus on understanding the interface between TNTs and the implant surface, treatments aimed at augmenting mechanical stability and strategies for advanced mechanical testing within the bone micro-environment ex vivo and in vivo. This review article extends the existing knowledge in this domain of TNTs implant technology and will enable improved understanding of the underlying parameters that contribute towards mechanically robust nano-engineered implants that can withstand the forces associated with implant surgical placement and the load bearing experienced at the bone/implant interface.

Fluoride doped SrTiO3/TiO2 nanotube arrays with a double layer walled structure for enhanced photocatalytic properties and bioactivity

Fluoride doped double layer walled SrTiO₃/TiO₂ nanotube arrays were obtained and demonstrated enhanced photocatalytic properties and bioactivity.

Aligned metal oxide nanotube arrays: key-aspects of anodic TiO2 nanotube formation and properties

Over the past ten years, self-aligned TiO₂ nanotubes have attracted tremendous scientific and technological interest due to their anticipated impact on energy conversion, environment remediation and biocompatibility. In the present manuscript, we review fundamental principles that govern the self-organized initiation of anodic TiO₂ nanotubes. We start with the fundamental question: why is self-organization taking place? We illustrate the inherent key mechanistic aspects that lead to tube growth in various different morphologies, such as ripple-walled tubes, smooth tubes, stacks and bamboo-type tubes, and importantly the formation of double-walled TiO₂ nanotubes versus single-walled tubes, and the drastic difference in their physical and chemical properties. We show how both double- and single-walled tube layers can be detached from the metallic substrate and exploited for the preparation of robust self-standing membranes. Finally, we show how by selecting specific growth approaches to TiO₂ nanotubes desired functional features can be significantly improved, e.g., enhanced electron mobility, intrinsic doping, or crystallization into pure anatase at high temperatures can be achieved. Finally, we briefly outline the impact of property, modifications and morphology on functional uses of self-organized nanotubes for most important applications.

Fast Growth of Highly Ordered TiO2 Nanotube Arrays on Si Substrate under High-Field Anodization

ABSTRACT

Highly ordered TiO₂ nanotube arrays (NTAs) on Si substrate possess broad applications due to its high surface-to-volume ratio and novel functionalities, however, there are still some challenges on facile synthesis. Here, we report a simple and cost-effective high-field (90-180 V) anodization method to grow highly ordered TiO₂ NTAs on Si substrate, and investigate the effect of anodization time, voltage, and fluoride content on the formation of TiO₂ NTAs. The current density-time curves, recorded during anodization processes, can be used to determine the optimum anodization time. It is found that the growth rate of TiO₂ NTAs is improved significantly under high field, which is nearly 8 times faster than that under low fields (40-60 V). The length and growth rate of the nanotubes are further increased with the increase of fluoride content in the electrolyte.

GRAPHICAL ABSTRACT

Highly ordered TiO₂ nanotube arrays (NTAs) on Si substrate have been fabricated by high-field anodization method. A high voltage (90-180 V) leads to a high growth rate of TiO₂ NTAs (35-47 nm s^-1), which is nearly 8 times faster than the growth rate under low fields (40-60 V). Furthermore, the current density-time curves recorded during the anodization provide a facial method to determine the optimal anodization parameters, leading to an easy obtaining of the desired nanotubes.

ZnO/ZnS heterostructures for hydrogen production by photoelectrochemical water splitting

ZnO/ZnS heterostructures anodized under stirring conditions and in glycerol/water electrolyte succeeded as being a photococatalyst for photoelectrochemical water splitting.

Controlled Fabrication of Nanoporous Oxide Layers on Zircaloy by Anodization

We have presented a mechanism to explain why the resulting oxide morphology becomes a porous or a tubular nanostructure when a zircaloy is electrochemically anodized. A porous zirconium oxide nanostructure is always formed at an initial anodization stage, but the degree of interpore dissolution determines whether the final morphology is nanoporous or nanotubular. The interpore dissolution rate can be tuned by changing the anodization parameters such as anodization time and water content in an electrolyte. Consequently, porous or tubular oxide nanostructures can be selectively fabricated on a zircaloy surface by controlling the parameters. Based on this mechanism, zirconium oxide layers with completely nanoporous, completely nanotubular, and intermediate morphologies between a nanoporous and a nanotubular structure were controllably fabricated.

Controlled morphology modulation of anodic TiO2 nanotubes via changing the composition of organic electrolytes

Titanium dioxide (TiO2) nanotubes are prepared by electrochemical anodization using Ti metal foils under a DC bias of 30 V for 20 h. The electrolyte is a mixture of formamide (FA) and ethylene glycol (EG), which contains NH4F (0.3 wt%) and H2O (2.0 v%). The diameter and wall thickness of the nanotubes decrease with the increase of EG content, while the length first decreases with the increase of EG content and then increases again. An O-ring-like pattern is formed on the outer surface of TiO2 nanotubes upon the introduction of FA into the EG electrolyte, upon which the surface becomes rougher and rougher with increasing FA content. This is caused by the breaking and re-establishment of a double layer at the interface. All of the observed phenomena are closely related to the conductivity and viscosity of the electrolyte as well as the formation of hydrogen bond in the system. The proposed mechanism is confirmed by introducing hydroxyl ions into the pure EG electrolyte.

Titanium nanostructures for biomedical applications

Titanium and titanium alloys exhibit a unique combination of strength and biocompatibility, which enables their use in medical applications and accounts for their extensive use as implant materials in the last 50 years. Currently, a large amount of research is being carried out in order to determine the optimal surface topography for use in bioapplications, and thus the emphasis is on nanotechnology for biomedical applications. It was recently shown that titanium implants with rough surface topography and free energy increase osteoblast adhesion, maturation and subsequent bone formation. Furthermore, the adhesion of different cell lines to the surface of titanium implants is influenced by the surface characteristics of titanium; namely topography, charge distribution and chemistry. The present review article focuses on the specific nanotopography of titanium, i.e. titanium dioxide (TiO2) nanotubes, using a simple electrochemical anodisation method of the metallic substrate and other processes such as the hydrothermal or sol-gel template. One key advantage of using TiO2 nanotubes in cell interactions is based on the fact that TiO2 nanotube morphology is correlated with cell adhesion, spreading, growth and differentiation of mesenchymal stem cells, which were shown to be maximally induced on smaller diameter nanotubes (15 nm), but hindered on larger diameter (100 nm) tubes, leading to cell death and apoptosis. Research has supported the significance of nanotopography (TiO2 nanotube diameter) in cell adhesion and cell growth, and suggests that the mechanics of focal adhesion formation are similar among different cell types. As such, the present review will focus on perhaps the most spectacular and surprising one-dimensional structures and their unique biomedical applications for increased osseointegration, protein interaction and antibacterial properties.

Cladding layer on well-defined double-wall TiO2 nanotubes

Highly ordered double-wall TiO2 nanotube arrays were obtained by a two-step anodization method in a fluoride-containing glycerol based electrolyte. The low water and fluoride content and high viscosity of the electrolyte support a partly undissolved fluoride-rich layer, and its hydrolyzed products remain on the tube walls. The double-wall structure and a cladding layer originating from the fluoride-rich layer were clearly observed after annealing. The morphology and crystal structure of the cladding layer were investigated. The study of the cladding layer gives a fundamental insight into the wall structure design of the anodic TiO2 nanotubes in the glycerol-based electrolyte.

Near-band-gap luminescence from TiO2 nanograss–nanotube hierarchical membranes

Freestanding TiO2 nanograss–nanotube hierarchical membranes are synthesized from a Ti metal foil by electrochemical anodization. It is found that the two nanostructures exhibit different luminescence properties, which are also affected by the crystal phases upon phase transformation. An unusual near-UV luminescence is observed from the amorphous nanograss, which is found to be excitation element specific. The amorphous nanotube shows no luminescence. Upon calcination, both nanograss and nanotubes are crystalized into the anatase phase with some rutile phase present, and both structures emit visible green luminescence at slightly different energies. The luminescence mechanism is explored using UV–vis spectroscopy, X-ray absorption near-edge structures (XANES), and X-ray excited optical luminescence (XEOL), and its implications are presented.

Observation of anatase nanograins crystallizing from anodic amorphous TiO2 nanotubes

Water content in an anodic electrolyte affects the crystallization route of anodic TiO₂ nanotube arrays during annealing, which determines the crystallographic orientation of the nanotubes.

Double-wall TiO2 nanotube arrays: enhanced photocatalytic activity and in situ TEM observations at high temperature

By decreasing the water content in an NH4F and glycerol-water electrolyte, the transition from single-wall to double-wall TiO2 nanotube arrays was successfully achieved using an anodization method. The double-wall TiO2 nanotube structures exhibited better photocatalytic activity than the typical single-wall structures. After modification with platinum nanoparticles, the photocatalytic activity of both the single- and double-wall TiO2 nanotubes was improved further. In situ observations at the annealing temperature of the TiO2 nanotubes were performed using a transmission electron microscopy (TEM) system. A slower structural failure of the nanotubes was obtained with the introduction of oxygen gas into the TEM column compared with the structural changes observed under high-vacuum conditions without the introduction of oxygen gas. These behaviors suggest that oxygen injection is an important factor in stabilizing the TiO2 nanotubes during the in situ TEM annealing process. The high-magnification TEM images of the double-wall TiO2 nanotubes revealed that the sintering of the inner wall can draw a clear distinction between the inner and outer walls.

Large-diameter titanium dioxide nanotube arrays as a scattering layer for high-efficiency dye-sensitized solar cell

Large-sized titanium dioxide (TiO2) nanotube arrays with an outer diameter of approximately 500 nm have been successfully synthesized by potentiostatic anodization at 180 V in a used electrolyte with the addition of 1.5 M lactic acid. It is found that the synthesized large-diameter TiO2 nanotube array shows a superior light scattering ability, which can be used as a light scattering layer to significantly enhance the efficiency of TiO2 nanoparticle-based dye-sensitized solar cells from 5.18% to 6.15%. The remarkable light scattering ability makes the large-diameter TiO2 nanotube array a promising candidate for light management in dye-sensitized solar cells (DSSCs).

Facile method to enhance the adhesion of TiO₂ nanotube arrays to Ti substrate

The weak adhesion of anodic TiO2 nanotube arrays (TNTAs) to the underlying Ti substrate compromises many promising applications. In this work, a compact oxide layer between TNTAs and Ti substrate is introduced by employing an additional anodization in a fluoride-free electrolyte. The additional anodization results in an about 200 nm thick compact layer near the nanotube bottoms. Scratch test demonstrates that the critical load of TNTAs with the compact oxide layer is a more than threefold increase in comparison with those without the compact layer. Moreover, this facile method can also improve the photoactivity and supercapacitor performances of TNTAs markedly.

Titanium dioxide nanotube films: Preparation, characterization and electrochemical biosensitivity towards alkaline phosphatase

Titania nanotubes (TNTs) were prepared by anodization on different substrates (titanium, Ti6Al4V and Ti6Al7Nb alloys) in ethylene glycol and glycerol. The influence of the applied potential and processing time on the nanotube diameter and length is analyzed. The as-formed nanotube layers are amorphous but they become crystalline when subjected to subsequent thermal treatment in air at 550°C; TNT layers grown on titanium and Ti6Al4V alloy substrates consist of anatase and rutile, while those grown on Ti6Al7Nb alloy consist only of anatase. The nanotube layers grown on Ti6Al7Nb alloy are less homogeneous, with supplementary islands of smaller diameter nanotubes, spread across the surface. Better adhesion and proliferation of osteoblasts was found for the nanotubes grown on all three substrates by comparison to an unprocessed titanium plate. The sensitivity towards bovine alkaline phosphatase was investigated mainly by electrochemical impedance spectroscopy in relation to the crystallinity, the diameter and the nature of the anodization electrolyte of the TNT/Ti samples. The measuring capacity of the annealed nanotubes of 50nm diameter grown in glycerol was demonstrated and the corresponding calibration curve was built for the concentration range of 0.005-0.1mg/mL.

Microstructure of the epitaxial film of anatase nanotubes obtained at high voltage and the mechanism of its electrochemical reaction with sodium

Anatase nanotubes showed preferred orientation and high capacityversussodium after many electrochemical cycles.