Self-organized Anodic TiO2 Nanotube Layers: Influence of the Ti substrate on Nanotube Growth and Dimensions

Ultrathin TiO2 Coatings via Atomic Layer Deposition Strongly Improve Cellular Interactions on Planar and Nanotubular Biomedical Ti Substrates

This work aims to investigate the chemical and/or structural modification of Ti and Ti-6Al-4V (TiAlV) alloy surfaces to possess even more favorable properties toward cell growth. These modifications were achieved by (i) growing TiO2 nanotube layers on these substrates by anodization, (ii) surface coating by ultrathin TiO2 atomic layer deposition (ALD), or (iii) by the combination of both. In particular, an ultrathin TiO2 coating, achieved by 1 cycle of TiO2 ALD, was intended to shade the impurities of F- and V-based species in tested materials while preserving the original structure and morphology. The cell growth on TiO2-coated and uncoated TiO2 nanotube layers, Ti foils, and TiAlV alloy foils were compared after incubation for up to 72 h. For evaluation of the biocompatibility of tested materials, cell lines of different tissue origin, including predominantly MG-63 osteoblastic cells, were used. For all tested nanomaterials, adding an ultrathin TiO2 coating improved the growth of MG-63 cells and other cell lines compared with the non-TiO2-coated counterparts. Here, the presented approach of ultrathin TiO2 coating could be used potentially for improving implants, especially in terms of shading problematic F- and V-based species in TiO2 nanotube layers.

Nanostructured photocatalysts for the abatement of contaminants by photocatalysis and photocatalytic ozonation: An overview

The water scarcity, the presence of different contaminants in the worldwide waters and wastewaters and their impacts should motivate their good elimination and water management. With this, photocatalysis and photocatalytic ozonation are strong solutions to obtain good quality reclaimed water, for different applications. Nanostructured supported photo-active catalysts, such as the TiO₂, WO₃ or ZnO can positively affect the performance of such technologies. Therefore, different semiconductors materials have been aroused the interest of the scientific community, mainly due to its functional properties as well as characteristics imposed by the different nanostructures. With this, this work overviews different works and perspective on the TiO₂ nanotubes and other semiconductors nanostructures, with the analysis of different works from 2001 to 2022. Aspects as the substrate effect, electrolyte nature, aspect ratio, electrolyte aging, and annealing treatment but also the effect of morphology, anodization time, applied voltage, temperature and viscosity are discussed. Modification of TiO₂ nanotubes is also presented in this paper. The main objective of this work is to present and discuss the key parameters and their effects on the anodization of different semiconductors, as well as the results obtained until today on the degradation of different contaminants by photocatalysis and photocatalytic ozonation, as well as their use on the treatment of real wastewater. TiO₂ nanotubes present unique properties and highly ordered configuration, which motivate their use on photo-driven technologies for the pollutant's abatement, even when compared to other nanostructures. However, photocatalysts with activity on the visible range and solar radiation, such as the WO₃, can present higher performance and can decrease operational costs, and must be an important source and a key to find efficient and cost-friendly solutions.

A review: research progress on the formation mechanism of porous anodic oxides

Owing to the great development potential of porous anodic oxides (PAO) in many fields, research on their formation mechanisms, fabrication processes and applications has a history of more than ten years. Although compared with research on the fabrication processes and applications of PAO, research on their formation mechanisms started late, several mainstream theories have been formed in the academic community, including the field-assisted dissolution (FAD) theory, the field-assisted ejection (FAE) theory, the self-organization theory, the ionic and electronic current theory and the oxygen bubble mould effect. This review will focus on summarizing the core views of the mainstream mechanisms mentioned above and comparing the explanations for some of their classical experimental phenomena.

Anodic TiO2 Nanotubes on 3D-Printed Titanium Meshes for Photocatalytic Applications

In this work, large 3D Ti meshes fabricated by direct ink writing were wirelessly anodized for the first time to prepare highly photocatalytically active TiO₂ nanotube (TNT) layers. The use of bipolar electrochemistry enabled the fabrication of TNT layers within the 3D Ti meshes without the establishment of an electrical contact between Ti meshes and the potentiostat, confirming its unique ability and advantage for the synthesis of anodic structures on metallic substrates with a complex geometry. TNT layers with nanotube diameters of up to 110 nm and thicknesses of up to 3.3 μm were formed. The TNT-layer-modified 3D Ti meshes showed a superior performance for the photocatalytic degradation of methylene blue in comparison to TiO₂-nanoparticle-decorated and nonanodized Ti meshes (with a thermal oxide layer), resulting in multiple increases in the dye degradation rate. The results presented here open new horizons for the employment of anodized 3D Ti meshes in various flow-through (photo)catalytic reactors.

Toward a Flexible and Efficient TiO2 Photocatalyst Immobilized on a Titanium Foil

Titanium foils of different thicknesses were anodized, and the photocatalytic activity of the resulting TiO₂ nanotube (NT) layers was determined. All of the titanium foils were anodized simultaneously under identical experimental conditions to avoid the influence of the aging of the anodizing electrolyte and other anodization parameters, such as voltage, time, and temperature. To characterize the microstructures of the titanium foils, we used electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), and stylus profilometry analyses. The adhesion was tested with a Scotch tape test and the morphology of the TiO₂ NTs was studied in detail using the SEM technique, while the surface areas of the TiO₂ NTs were determined using a three-dimensional (3D) optical interference profilometer. With X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the chemical composition and structure of TiO₂ oxide were established. The degradation of caffeine under UV irradiation was measured with a high-precision UV-vis-IR spectrophotometer, and the photoluminescence method was used to confirm the photocatalytic behavior of the TiO₂ NT layers. The influence of the intrinsic properties, including twinning and the grain boundaries of the starting titanium foils with similar chemical compositions, was determined and explained. Finally, we identified the main characteristics that define a highly effective and flexible photocatalyst.

The Influence of a Surface Treatment of Metallic Titanium on the Photocatalytic Properties of TiO2 Nanotubes Grown by Anodic Oxidation

Titanium dioxide (TiO2) nanotubes obtained by the anodic oxidation of titanium metal foils can be used for the photocatalytic degradation of organic pollutants. The aim of our study was to determine the influence of the titanium foil’s surface treatment on the final morphology of the TiO2 nanotubes and their photocatalytic activity. In our experiments, we used two different titanium foils that were electropolished or untreated prior to the anodic oxidation. The morphologies of the starting titanium foils and the resulting TiO2 nanotube layers were investigated and the photocatalytic activities measured by the decomposition of caffeine under UV irradiation. Our results showed that electropolishing of the starting foils produced a more uniform and smoother TiO2 nanotubes surface. In contrast, the TiO2 nanotube surfaces from untreated titanium foils mimic the initial surface roughness of the titanium foil. A comparison of the photocatalytic properties of the TiO2 nanotube layers obtained from the untreated and electropolished titanium foils showed that electropolishing does not necessarily improve the photocatalytic properties of the resulting TiO2 nanotube layer. It was found that the determining factors influencing the photocatalytic activity are the chemical impurities (Ti-nitride) on the surface of the titanium foils and the surface roughness of the TiO2 nanotube layer. The highest photocatalytic activity was achieved with the anodized untreated foil with the minimal presence of Ti-nitride and a relatively high roughness of the TiO2 nanotubes.

Less known facts and findings about TiO2 nanotubes

Anodic TiO₂ nanotubes that are grown on Ti substrates by a simple anodization in various types of fluoride containing electrolytes have attracted scientific and technological interest due to their wide potential applications, and therefore, numerous research efforts have been dedicated to these self-ordered oxide structures in the past decade. The present mini-review highlights less known but important aspects, such as the formation of spaced nanotubes with adjustable interspacing that is achieved in a few specific organic electrolytes, and strong effects of the metallic Ti substrate which significantly affect the growth of the tubes. We discuss the formation of oxide nanotubes grown from adequately alloyed substrates and noble metal nanoparticle decoration of tubes. We describe how specific heat-treatment can introduce a nanotwinned boundary in the oxide tube walls of single-walled nanotubes obtained by a decoring process. All the facts and findings were studied in recent years and TiO₂ nanotubes can be upgraded with more optimized functionalities for their applications.

Amorphous TiO2 Nanotubes as a Platform for Highly Selective Phosphopeptide Enrichment

This work reports highly selective phosphopeptide enrichment using amorphous TiO₂ nanotubes (TiO₂NTs) and the same material decorated with superparamagnetic Fe₃O₄ nanoparticles (TiO₂NTs@Fe₃O₄NPs). TiO₂NTs and TiO₂NTs@Fe₃O₄NPs materials were applied for phosphopeptide enrichment both from a simple peptide mixture (tryptic digest of bovine serum albumin and α-casein) and from a complex peptide mixture (tryptic digest of Jurkat T cell lysate). The obtained enrichment efficiency and selectivity for phosphopeptides of TiO₂NTs and TiO₂NTs@Fe₃O₄NPs were increased to 28.7 and 25.3%, respectively, as compared to those of the well-established TiO₂ microspheres. The enrichment protocol was extended for a second elution step facilitating the identification of additional phosphopeptides. It further turned out that both types of amorphous TiO₂ nanotubes provide qualitatively new physicochemical features that are clearly advantageous for highly selective phosphopeptide enrichment. This has been confirmed experimentally resulting in substantial reduction of non-phosphorylated peptides in the enriched samples. In addition, TiO₂NTs@Fe₃O₄NPs combine high selectivity and ease of handling due to the superparamagnetic character of the material. The presented materials and performances are further promising for applications toward a whole range of other types of biomolecules to be treated in a similar fashion.

A Review on the Electrochemically Self-organized Titania Nanotube Arrays: Synthesis, Modifications, and Biomedical Applications

Titania nanotubes grown by anodic oxidation have intrigued the material science community by its many unique and potential properties, and the synthesis of technology is merging to its mature stage. The present review will focus on TiO2 nanotubes grown by self-organized electrochemical anodization from Ti metal substrate, which critically highlights the synthesis of this type of self-organized titania nanotube layers and the means to influence the size, shape, the degree of order, and crystallized phases via adjusting the anodization parameters and the subsequent thermal annealing. The relationship between dimensions and properties of the anodic TiO2 nanotube arrays will be presented. The latest progress and significance of the research on formation mechanism of anodic TiO2 nanotubes are briefly discussed. Besides, we will show the most promising applications reported recently in biomedical directions and modifications carried out by doping, surface modification, and thermal annealing toward improving the properties of anodically formed TiO2 nanotubes. At last, some unsolved issues and possible future directions of this field are indicated.

Nonstoichiometric titanium dioxide nanotubes with enhanced catalytical activity under visible light

The catalytic activity of nanotubular titanium dioxide films formed during the oxidation of acetone to carbon dioxide under the action of visible light with a wavelength of 450 nm was found to be approximately 2 times higher compared to standard titanium dioxide (Degussa P25). The nanotubular films were grown by the anodization of titanium foil using an original technique. Diffuse reflectance spectra of the films are attributed to enhanced activity in the visible spectrum by the nonstoichiometry of titanium dioxide near the interface between the nanotubular film and the titanium foil substrate.

Metal TiO2 Nanotube Layers for the Treatment of Dental Implant Infections

Titanium oxide nanotube layers with silver and zinc nanoparticles are attracting increasing attention in the design of bone and dental implants due to their antimicrobial potential and their ability to control host cell adhesion, growth, and differentiation. However, recent reports indicate that the etiology of dental infections is more complex than has been previously considered. Therefore, the antimicrobial potential of dental implants should be evaluated against at least several different microorganisms cooperating in human mouth colonization. In this study, Ag and Zn nanoparticles incorporated into titanium oxide nanotubular layers were studied with regard to how they affect Candida albicans, Candida parapsilosis, and Streptococcus mutans. Layers of titanium oxide nanotubes with an average diameter of 110 nm were fabricated by electrochemical anodization, annealed at 650 °C, and modified with approx. 5 wt % Ag or Zn nanoparticles. The surfaces were examined with the scanning electron microscopy-energy dispersive X-ray analysis, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy techniques and subjected to evaluation of microbial-killing and microbial adhesion-inhibiting potency. In a 1.5 h long adhesion test, the samples were found more effective toward yeast strains than toward S. mutans. In a release-killing test, the microorganisms were almost completely eliminated by the samples, either within 3 h of contact (for S. mutans) or 24 h of contact (for both yeast strains). Although further improvement is advisable, it seems that Ag and Zn nanoparticles incorporated into TiO₂ nanotubular surfaces provide a powerful tool for reducing the incidence of bone implant infections. Their high bidirectional activity (against both Candida species and S. mutans) makes the layers tested particularly promising for the design of dental implants.

Ideally Hexagonally Ordered TiO2 Nanotube Arrays

Ideally hexagonally ordered TiO₂ nanotube layers were produced through the optimized anodization of Ti substrates. The Ti substrates were firstly covered with a TiN protecting layer prepared through atomic layer deposition (ALD). Pre‐texturing of the TiN‐protected Ti substrate on an area of 20×20 μm² was carried out by focused ion beam (FIB) milling, yielding uniform nanoholes with a hexagonal arrangement throughout the TiN layer with three different interpore distances. The subsequent anodic nanotube growth using ethylene‐glycol‐based electrolyte followed the pre‐textured nanoholes, resulting in perfectly ordered nanotube layers (resembling honeycomb porous anodic alumina) without any point defects and with a thickness of approximately 2 μm over the whole area of the pattern.

Atomic Layer Deposition Al2O3 Coatings Significantly Improve Thermal, Chemical, and Mechanical Stability of Anodic TiO2 Nanotube Layers

We report on a very significant enhancement of the thermal, chemical, and mechanical stability of self-organized TiO₂ nanotubes layers, provided by thin Al₂O₃ coatings of different thicknesses prepared by atomic layer deposition (ALD). TiO₂ nanotube layers coated with Al₂O₃ coatings exhibit significantly improved thermal stability as illustrated by the preservation of the nanotubular structure upon annealing treatment at high temperatures (870 °C). In addition, a high anatase content is preserved in the nanotube layers against expectation of the total rutile conversion at such a high temperature. Hardness of the resulting nanotube layers is investigated by nanoindentation measurements and shows strongly improved values compared to uncoated counterparts. Finally, it is demonstrated that Al₂O₃ coatings guarantee unprecedented chemical stability of TiO₂ nanotube layers in harsh environments of concentrated H₃PO₄ solutions.

Self-organized double-wall oxide nanotube layers on glass-forming Ti-Zr-Si(-Nb) alloys

In this work, we report for the first time on the use of melt spun glass-forming alloys - Ti₇₅Zr₁₀Si₁₅ (TZS) and Ti₆₀Zr₁₀Si₁₅Nb₁₅ (TZSN) - as substrates for the growth of anodic oxide nanotube layers. Upon their anodization in ethylene glycol based electrolytes, highly ordered nanotube layers were achieved. In comparison to TiO₂ nanotube layers grown on Ti foils, under the same conditions for reference, smaller diameter nanotubes (~116nm for TZS and ~90nm for TZSN) and shorter nanotubes (~11.5μm and ~6.5μm for TZS and TZSN, respectively) were obtained for both amorphous alloys. Furthermore, TEM and STEM studies, coupled with EDX analysis, revealed a double-wall structure of the as-grown amorphous oxide nanotubes with Ti species being enriched in the inner wall, and Si species in the outer wall, whereby Zr and Nb species were homogeneously distributed.

Atomic Layer Deposition for Coating of High Aspect Ratio TiO2 Nanotube Layers

We present an optimized approach for the deposition of Al₂O₃ (as a model secondary material) coating into high aspect ratio (≈180) anodic TiO₂ nanotube layers using the atomic layer deposition (ALD) process. In order to study the influence of the diffusion of the Al₂O₃ precursors on the resulting coating thickness, ALD processes with different exposure times (i.e., 0.5, 2, 5, and 10 s) of the trimethylaluminum (TMA) precursor were performed. Uniform coating of the nanotube interiors was achieved with longer exposure times (5 and 10 s), as verified by detailed scanning electron microscopy analysis. Quartz crystal microbalance measurements were used to monitor the deposition process and its particular features due to the tube diameter gradient. Finally, theoretical calculations were performed to calculate the minimum precursor exposure time to attain uniform coating. Theoretical values on the diffusion regime matched with the experimental results and helped to obtain valuable information for further optimization of ALD coating processes. The presented approach provides a straightforward solution toward the development of many novel devices, based on a high surface area interface between TiO₂ nanotubes and a secondary material (such as Al₂O₃).