Anodic Titania Nanotube Research Articles

Rapid growth of titanium oxide nanotubes under the critical breakdown voltage: Evidence against the dissolution reaction of fluoride ions

Rapid fabrication of anodic titanium oxide nanotubes is limited to its growth mechanism, because the growth mechanism is still dominated by the dissolution reaction of fluoride ions. Here, the growth rates of the nanotubes anodized at 50 V in four electrolytes of NH4F (0.2, 0.3, 0.4 and 0.5 wt%) increased slowly with the increasing concentration of NH4F. This phenomenon seems to be consistent with the dissolution reaction of fluoride ions. However, when titanium was anodized at the critical breakdown voltages, the growth rates of nanotubes obtained in 0.2, 0.3, 0.4 and 0.5 wt% NH4F electrolytes were 2.191, 0.869, 0.538 and 0.497 μm min−1, respectively. Therefore, the growth rates of nanotubes decreased with the increasing concentration of NH4F. Compared with the growth rate (0.140 μm min−1) under non-breakdown conditions (50 V), the growth rate (2.191 μm min−1) of nanotubes under the critical breakdown voltage was greatly increased. Obviously, the phenomenon of rapid growth of nanotubes in low concentration of NH4F electrolyte is contrary evidence of the dissolution reaction of fluoride ions and the dissolution equilibrium theory.

Intracellular Drug Delivery with Anodic Titanium Dioxide Nanotubes and Nanocylinders.

Titanium dioxide (TiO2) holds remarkable promises for developing current theranostic strategies. Anodic TiO2 nanostructures as a porous scaffold have offered a broad range of useful theranostic properties; however, previous attempts to generate single and uniform TiO2 one-dimensional nanocarriers from anodic nanotube arrays have resulted in a broad cluster size distribution of arbitrarily broken tubes that are unsuitable for therapeutic delivery systems due to poor biodistribution and the risk of introducing tissue inflammation. Here, we achieve well-separated, uniformly shaped anodic TiO2 nanotubes and nanocylinders through a time-varying electrochemical anodization protocol that leads to the generation of planar sheets of weakly connected nanotubes with a defined fracture point near the base. Subsequent sonication cleanly detaches the nanotubes from the base. Depending on the position of the fracture point, we can fabricate single-anodic nanocylinders that are open on both ends and nanotubes that are closed on one end. We proceed to show that anodic nanotubes and nanocylinders are nontoxic at therapeutic concentrations. When conjugated with the anticancer drug doxorubicin using a pH-responsive linker, they are readily internalized by cells and subsequently release their drug cargo into acidic intracellular compartments. Our results demonstrate that uniformly sized anodic TiO2 nanotubes and nanocylinders are suitable for subcellular delivery of therapeutic agents in cancer therapy.

Nanostructured surface coatings for titanium alloy implants

Abstract

On the Interfacial Adhesion between TiO2 Nanotube Array Layer and Ti Substrate.

Anodic titania nanotube arrays (TNTAs) with higher aspect ratio are observed to be liable to spontaneous curling or delamination from the underlying titanium (Ti) metal once dried because of the poor interfacial adhesion of the TNTA layer to the underlying Ti, especially when a thin Ti sheet is used. The interfacial adhesion strength was shown to decrease with increasing thickness of the TNTA layer. In this work, although the preparation of TNTAs in a frequently used fluoride-containing solution was completed, different anodization processes were further performed at lower current densities or at lower voltages for a short time in the same electrolyte to increase the adhesion. The mechanical test demonstrated that better adhesion properties have been achieved by applying these anodization posttreatment processes. It is believed that during the fabrication of TNTAs, a large residual stress at the interface of the nanotube layer and the underlying Ti is created. It is the residual stress that leads to the weak interfacial adhesion. The anodization posttreatment processes can reduce or eliminate the residual stress, thereby improving the interfacial adhesion. Further, these processes can also boost the performances of TNTAs for supercapacitors. When the anodization posttreatment processes are implemented at 1 mA cm-2 or at 10 V for 5 min, considerable improvements in the interfacial adhesion are observed. Particularly, both posttreatment processes are also applicable to the very thin Ti sheet (∼18 μm). The realization of robust and adherent TNTAs grown on very thin Ti sheets can not only significantly improve the volumetric capacitances of TNTAs but also make TNTAs an attractive material in flexible supercapacitor applications.

Stress-generating electrochemical reactions during the initial growth of anodic titanium dioxide nanotube layers

Titanium dioxide nanotube layers are widely investigated for diverse applications as functional materials. Previously, we demonstrated the importance of mechanical stress-generating open-circuit electrochemical reactions or chemical reactions in the initiation and growth of nanotube layers. These reactions were investigated here through analysis of stress and potential transients after interrupted anodization followed by open-circuit oxide dissolution. Large tensile stress increases after complete dissolution occur at potentials near the TiH2/Ti2O3 equilibrium potential, and are attributed to cathodic hydride formation at exposed metal surfaces. Tensile increases accompanying initiation of nanotubes during anodization itself were also explained by the hydride mechanism. The observation of tensile stress changes after both complete oxide dissolution and nanotube formation supports the view that nanotube initiation involves detachment of oxide at the metal interface. Tensile stress changes during anodization were followed by compressive transients that were also found during open-circuit dissolution, but not associated with a specific range of open-circuit potentials. The compressive stress change was explained by chemical oxidation of Ti+3 ions in nanotube walls by H+ ions, along with absorption of charge-compensating anionic species. Hydrogen ion absorption is promoted by large decreases of surface pH that accompany anodization in organic-based solutions.

Enhanced In Vitro Angiogenic Behavior of Selective Laser Melting Titanium Modified by Anodized Titanium Dioxide Nanotubes and Calcium Phosphate Nanoparticles

Enhanced<i> In Vitro</i> Angiogenic Behavior of Selective Laser Melting Titanium Modified by Anodized Titanium Dioxide Nanotubes and Calcium Phosphate Nanoparticles

Roles of mechanical stress and lower-valent oxide in the formation of anodic titanium dioxide nanotube layers

Self-organized TiO2 nanotube arrays are characterized by gaps that separate the layers of oxide surrounding cylindrical pores. Nanotube layer growth is thought to involve oxide flow induced by mechanical stress associated with anodization. The mechanism of TiO2 nanotube initiation was studied with in situ stress measurements during anodization in ethylene glycol-water-fluoride solutions. Compressive stress changes due to initial growth of barrier oxide films were detected, along with significantly larger tensile and compressive stress changes that followed the appearance of gaps between nanotubes at the metal-oxide interface. Interrupted anodizing experiments revealed that the latter stress changes are due to chemical processes or open-circuit electrochemical reactions. Further evidence for such reactions was deduced from the calculation of apparent Ti valences below four from measurements of Ti metal consumption during anodization. The gaps between nanotubes are thought to result from local oxide delamination at the metal interface induced by oxide flow, and the large compressive stress change is attributed to chemical oxidation of Ti+3 ions in the nanotube walls. This previously unrecognized chemical oxidation reaction may suggests new avenues to control nanotube layer growth and properties.

Template synthesis of methylammonium lead iodide in the matrix of anodic titanium dioxide via the direct conversion of electrodeposited elemental lead

A new processing technique is proposed for composites based on the hybrid organic–inorganic perovskite MeNH3PbI3 and anodic titania nanotube arrays, which includes a direct gasphase conversion of metal lead nanorods electrodeposited inside the pores of anodic titanium dioxide. This technique may be useful for a design of solar cells and optoelectronics applications

(Invited) Alumina Nanotubes Formed By Anodization of Aluminum Cast Alloy

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is used as a starting material of nanofabrication of functional devices besides a corrosion resistant surface oxide. In contrast to anodic titania nanotubes, anodic porous alumina is generally compact without spacing between the cell boundary. The tubular shape of anodic titania is suggested to be resulted in enrichment of fluoride between cells which is originated from electrolyte and more soluble than anodic titania. In the case of aluminum, it is reported that enrichment of only sulfate or ethylene glycol from electrolytes produces alumina nanotubes by anodization. In the present paper, the formation mechanisms of alumina nanotubes by anodization of Al cast alloy caused except the electrolyte origin will be discussed. STEM image of vertical cross section prepared by ion-milling of the anodic film, which was formed on 99.99 % pure aluminum in oxalic acid at 160V, indicated close-packed hexagonal cell arrays with small voids of 15-20 nm at the triple cell points. Dark field STEM image clearly revealed a bright particle in the each void suggesting the presence of a heavy metal because of Z-contrast. By EDX point analysis, Cu enriched particles were found in the voids at the triple cell points. Nevertheless, in the case of anodic film formed at the similar condition on AC8A cast alloy including 0.97 % of Cu, tubular cells with the space of approximately 50 nm between cell boundary were formed unlike the case of pure aluminum including 60 ppm of Cu. Numerous branching of pores were also observed. EDX mapping revealed that Si was enriched in the outer layer of the cells and Cu was enriched at the area near the cell boundaries. Therefore, it is deduced that enrichment of Cu at the cell boundaries produces the tubular cells by accelerating oxide dissolution in addition to the electric breakdown accompanied by severe gas evolution that resulted in void formation at the triple cell points.

Reduction in the band gap of anodic TiO2 nanotube arrays by H2 plasma treatment

Abstract Influence of the hydrogen (H2) plasma treatment on the anodic titanium dioxide (TiO2) nanotubes physical properties was studied. The Titanium substrates were anodized in an ethylene-glycol-based electrolyte solution containing 0.3 wt% ammonium fluoride (NH4F) and 3 vol% deionized (DI) water at a potential of 50 V for 1 h at room temperature. The TiO2 nanotubes arrays hold a great potential as the electrode materials for high-performance electrochemical applications. However, TiO2 nanotubes poor electronic conductivity limits their applications. Here, TiO2 nanotubes were treated with H2 plasma to improve the optical properties of the TiO2 nanotubes. The morphology, crystal structure, and optical band gap of the anodic TiO2 nanotubes were investigated. The morphology of the TiO2 nanotubes was studied by using field-emission scanning electron microscopy (FESEM). The roughness of the treated TiO2 nanotubes walls were enhanced compared to non-treated TiO2 nanotubes and also diameter of the H2 plasma treated nanotubes decreased. UV–Vis diffuse reflectance spectroscopy (DRS) and Photoluminescence (PL) measurements revealed that the H2 plasma treated TiO2 nanotubes have slightly lower band gap energy compare to the non-treated TiO2 nanotubes.

Photoelectrochemical ultraviolet photodetector by anodic titanium dioxide nanotube layers

A prototype ultraviolet (UV) photodetector based on photoelectrochemical (PEC) cell from material synthesis up to assembly was demonstrated in this work. Self-organized titanium dioxide (TiO2) nanotube layers (1 and 5 μm thicknesses) prepared by electrochemical anodization were applied as the sensing layer in the photodetector. A printed circuit board (PCB) platform with physical size of 2 cm × 2 cm was designed to mount the sensing layer. The photodetector system comprised of a sandwich structure can be described as glass/ITO/electrolyte/TiO2 nanotube layer/Ti/PCB platform. For this system, a low applied voltage of 0.5–1 V was required to drive the photodetector. TiO2 nanotube layers have shown positive photoresponse in the entire UV spectral region (λ = 250–400 nm, classified as UV-A, B and C). In particular, the highest response with >850 sensitivity, and responsivity of ≈740 mA/W had been achieved by the 5 μm TiO2 nanotube layer for detection in the UV-A region, and the respective rise and decay time were 0.88 and 1.28 s. This work shows that the sandwich structure PEC cell UV photodetector with TiO2 nanotube layer as sensing layer is stable, highly sensitive and provides rapid response. Such platform can be potentially applied for other sensing materials as an efficient photodetector.

A mathematical model for initiation and growth of anodic titania nanotube embryos under compact oxide layer

The kinetics for initiation and growth of anodic titania nanotubes is not clear yet, because nanotubes are usually formed rapidly and initiation of nanotube embryos cannot be observed. By decoupling oxide growth and dissolution, nanotube embryos are fabricated beneath a pre-formed compact layer. Based on theories of ionic and electronic current, as well as the oxygen bubble mould effect, a mathematical model is developed for embryo initiation. The model exhibits high fidelity to measured current-time transients. Based on this model, the mechanism of nanotube embryo growth under different thickness of pre-formed layers is quantitatively explained. The embryos initiate without exposure to electrolyte, which is evidence against the traditional field-assisted dissolution theory.

Anodic titanium oxide photonic crystals prepared by novel cyclic anodizing with voltage versus charge modulation

Photonic crystals based on titania are of great practical importance in modern photonics owing to a variety of possibilities for their applications in optoelectronics, sensorics, and solar photovoltaics. However, the reproducible, scalable, and low-cost method of the preparation of titania photonic crystals with desired optical characteristics is still absent. Here the novel anodizing regime with voltage versus electric charge modulation, U(Q), is suggested for the precise morphology control of anodic porous films of valve metals oxides. The potential of the suggested approach is demonstrated on the preparation of high-quality one-dimensional titania photonic crystals. A new type of anodic titania nanotubes possessing periodic change of inner diameter and constant outer diameter is prepared using sine-wave modulation of applied voltage with electric charge. The possibility to tune the position of photonic band gap within the whole visible spectrum is shown.

Effects of hydrogenated TiO2 nanotube arrays on protein adsorption and compatibility with osteoblast-like cells.

BackgroundModified titanium (Ti) substrates with titanium dioxide (TiO2) nanotubes have broad usage as implant surface treatments and as drug delivery systems.MethodsTo improve drug-loading capacity and accelerate bone integration with titanium, in this study, we hydrogenated anodized titanium dioxide nanotubes (TNTs) by a thermal treatment. Three groups were examined, namely: hydrogenated TNTs (H2-TNTs, test), unmodified TNTs (air-TNTs, control), and Ti substrates (Ti, control).ResultsOur results showed that oxygen vacancies were present in all the nanotubes. The quantity of -OH groups greatly increased after hydrogenation. Furthermore, the protein adsorption and loading capacity of the H2-TNTs were considerably enhanced as compared with the properties of the air-TNTs (P<0.05). Additionally, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to investigate the interactions of TNTs with proteins. During the protein-loading process, the H2-TNTs not only enabled rapid protein adsorption, but also decreased the rate of protein elution compared with that of the air-TNTs. We found that the H2-TNTs exhibited better biocompatibility than the air-TNT and Ti groups. Both cell adhesion activity and alkaline phosphatase activity were significantly improved toward MG-63 human osteoblast-like cells as compared with the control groups (P<0.05).ConclusionWe conclude that hydrogenated TNTs could greatly improve the loading capacity of bioactive molecules and MG-63 cell proliferation.

Dynamic Diffraction Studies on the Crystallization, Phase Transformation, and Activation Energies in Anodized Titania Nanotubes.

The influence of calcination time on the phase transformation and crystallization kinetics of anodized titania nanotube arrays was studied using in-situ isothermal and non-isothermal synchrotron radiation diffraction from room temperature to 900 °C. Anatase first crystallized at 400 °C, while rutile crystallized at 550 °C. Isothermal heating of the anodized titania nanotubes by an increase in the calcination time at 400, 450, 500, 550, 600, and 650 °C resulted in a slight reduction in anatase abundance, but an increase in the abundance of rutile because of an anatase-to-rutile transformation. The Avrami equation was used to model the titania crystallization mechanism and the Arrhenius equation was used to estimate the activation energies of the titania phase transformation. Activation energies of 22 (10) kJ/mol for the titanium-to-anatase transformation, and 207 (17) kJ/mol for the anatase-to-rutile transformation were estimated.

Photoelectrocatalytic Synthesis of Hydrogen Peroxide by Molecular Copper-Porphyrin Supported on Titanium Dioxide Nanotubes.

We report on a self‐assembled system comprising a molecular copper‐porphyrin photoelectrocatalyst, 5‐(4‐carboxy‐phenyl)‐10,15,20‐triphenylporphyrinatocopper(II) (CuTPP‐COOH), covalently bound to self‐organized, anodic titania nanotube arrays (TiO2 NTs) for photoelectrochemical reduction of oxygen. Visible light irradiation of the porphyrin‐covered TiO2 NTs under cathodic polarization up to −0.3 V vs. Normal hydrogen electrode (NHE) photocatalytically produces H2O2 in pH neutral electrolyte, at room temperature and without need of sacrificial electron donors. The formation of H2O2 upon irradiation is proven and quantified by direct colorimetric detection using 4‐nitrophenyl boronic acid (p‐NPBA) as a reactant. This simple approach for the attachment of a small molecular catalyst to TiO2 NTs may ultimately allow for the preparation of a low‐cost H2O2 evolving cathode for efficient photoelectrochemical energy storage under ambient conditions.

Ethanol-based electrolyte for nanotubular anodic TiO2 formation

We report fabrication of anodic titanium oxide nanotubes with rippled sidewalls using anodization of titanium foil in an ethanol-based electrolyte containing ammonium fluoride and water. The electrolyte was tested at 40 °C and at potential in the 30–60 V range. The internal diameter of nanotubes grows linearly from 88 nm for 30 V up to 124 nm for 50 V and next drops to 105 nm for 60 V. The thickness of anodic titanium oxide layer exceeds 1 μm. Presence of NH4F crystals in electrolyte is favorable for formation of nanotubes. The unusual shape of current curves during anodization was observed.

Enhanced silver loaded antibacterial titanium implant coating with novel hierarchical effect.

In this study, we present a novel strategy for hierarchical antibacterial implant coating by controlling structural and componential features as regulators of surface bactericidal property. Anodized titanium dioxide nanotubes and self-polymerized polydopamine were both used as preliminary antibacterial agents with a significant positive effect on surface bioactivity. At the same time, the storage capacity of nanotubes and the in situ reduction activity of polydopamine can introduce large amounts of strong attached silver nanoparticles for enhanced stable antibacterial performance. The surface morphology, chemical composition, and hydrophilicity had been thoroughly characterized. The sustained silver release performances were continuously monitored. The successively in vitro inhibition on Staphylococcus aureus growth of titanium dioxide nanotube, polydopamine layer and silver nanoparticles demonstrated the hierarchical antibacterial property of the final silver nanoparticles-incorporated polydopamine-modified titanium dioxide nanotube coating (silver/polydopamine/nanotube). Moreover, the bioactivity investigation indicated the vital role of polydopamine-modified titanium dioxide nanotube coating on preserving healthy osteoblast activity at the implant interface. The unique hierarchical coating for titanium implant may be a promising method to maximize antibacterial capacity and maintain good cellular activity at the same time.

Surface modification of titania nanotube arrays with crystalline manganese-oxide nanostructures and fabrication of hybrid electrochemical electrode for high-performance supercapacitors

A hybrid electrochemical microelectrode is fabricated with a simple and cost-effective sono-chemical method, which consists of anodized titania nanotube arrays covered with manganese oxide nanostructures (nanorods+nanoparticles mixed morphology). The modification of the surface of the highly porous titania nanotube arrays with high-surface-area manganese oxide nanomaterials leads to considerable increment in the surface roughness of the composite electrode, which manifests high active surface sites of the electrode, and hence, leads to excellent electrochemical properties of the hybrid samples. The cyclic voltammetry and galvanostatic charge–discharge characterizations depict considerably improved electrochemical performance with high areal capacitance values. The areal capacitance of the composite electrode is obtained around 65mFcm−1 @ 1.0mVs−1 scan rate, which is more than 160 times higher than the control electrode (TNT, 0.4mFcm−1 @ 1.0mVs−1 scan rate). The composite electrode also depicts high capacity retention with only 4% decrement in the capacitance value over 2500 cycles. Also the composite electrode reveals almost 65 times increment in the power density for a mere 2 times decrement in the energy density. This high cyclic stability along with excellent energy-power performance indicates very good applicability in practical charge storage devices. The electrochemical impedance spectroscopic studies showed near-ideal capacitive performance with very low charge transfer resistance. This superior supercapacitive performance of the hybrid electrode is due to the combinatorial effect of electric double-layer capacitance of TNT and pseudocapacitance of MO as well as high active surface sites of the electrode for higher utilization of the active material. Therefore, this simple and low-cost technique to fabricate hybrid microelectrode with superior electrochemical properties can be very useful for high-performance supercapacitors.

Formation Mechanism of Alumina Nanotubes By Anodization of Aluminum Alloy

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is used as a starting material of nanofabrication of functional devices besides a corrosion resistant surface oxide. In contrast to anodic titania nanotubes, anodic porous alumina is generally compact without spacing between the cell boundary. The tubular shape of anodic titania is resulted in enrichment of fluoride between cells which is originated form electrolyte and more soluble than anodic titania. In the case of aluminum, it is reported that enrichment of only sulfate or ethylene glycol originated form electrolytes produces alumina nanotubes by anodization. In the present paper, the formation mechanism of anodic alumina nanotubes will be discussed. STEM image of vertical cross section prepared by ion-milling of the anodic film, which was formed on 99.99 % pure aluminum in oxalic acid at 160V, indicated close-packed hexagonal cell arrays with small voids of 15-20 nm at the triple cell points. Dark field STEM image clearly revealed a bright particle in the each void suggesting the presence of a heavy metal because of Z-contrast. By EDX point analysis, Cu enriched particles were found in the voids at the triple points. Nevertheless, in the case of anodic film formed at the similar condition on AC8A cast alloy including 0.97 % of Cu, tubular cells with the space of approximately 50 nm between cell boundary were formed unlike the case of pure aluminum including 60 ppm of Cu. Numerous branching of pores were also observed. EDX mapping revealed that Si was enriched in the outer layer of the cells and Cu was enriched at the area near cell boundaries. Therefore, it is deduced that enrichment of Cu at the cell boundaries produces the tubular cells by accelerating oxide dissolution similar to the cases of enrichment of sulfate and ethylene glycol.