Oxide Nanotube Arrays Research Articles

(Invited) Novel Anodic Oxide Nanotube Array Architectures for Solar Photoelectrochemical Fuel Generation

Electrochemical anodization has progressed from its level as a low-cost commercial technique to fabricate corrosion and abrasion resistant compact oxide coatings to the status of an effective method for growing a variety of oxide nano-architectures, particularly nanorods, nanowires, nanotubes, nanoporous films, and nanoflowers. Among these structures, self-organized vertically aligned nanotubes of oxide semiconductors have been demonstrated to possess interesting electrical, optical and mechanical properties. As in the case of most low dimensional materials, these structures also possess high surface area. Nevertheless, their unique properties applicable to solar energy conversion processes such as photoelectrochemical (PEC) fuel generation arise primarily from their vertical orientation, their ability to keep the charge carriers always in the vicinity of the surface and the pathways they offer for facile charge transport. They can form heterostructures with other low dimensional materials and function synergistically to enhance the efficiency of the PEC processes. Anodic nanotube arrays of only a few oxide semiconductors have been discovered so far; however, expanding the list further has been proven difficult. Recently, we have developed a few novel nanotubular semiconductors promising for PEC fuel generation. This presentation provides the details of their fabrication process and performance as photocatalysts for solar fuel generation.

Improvement of photoelectrochemical water splitting performance using photoanodes based on reduced graphene oxide and ZnO nanotubes

Increasing the visible light efficiency, effective charge separation, and reducing the rate of charge recombination are essential factors in improving the performance of photoelectrochemical (PEC) water splitting. To enforce these, herein zinc oxide nanotube arrays hybridized with reduced graphene oxide (rGO/ZnO NTs) are synthesized on the fluorine-doped tin oxide substrates by using an electrodeposition technique for surveying PEC water splitting efficiency under visible light. The synthesized photoanodes were characterized by field emission scanning electron microscope, energy-dispersive X-ray spectroscopy, X-ray diffraction, and UV–visible absorption spectroscopy. The linear sweep voltammetry, electrochemical impedance spectroscopy, and Mott-Schottky analysis were used to evaluate the PEC performance. The rGO/ZnO NT photoanode shows a high photocurrent density of 0.441 mA/cm2 at 1.8 V vs. RHE compared to other samples. The improved PEC performance of rGO/ZnO NTs is attributed to the prominent role of rGO in more efficient charge separation, band gap reduction, and charge recombination.

Photocurrent response loss of dye sensitized solar cells owing to top surface nanograss growth and bundling of anodic TiO2 nanotubes

In this research, the effect of anodization time on the length of the titanium oxide nanotube arrays (TNAs) and photovoltaic parameters of back-side illuminated dye-sensitized solar cells (DSSCs) were investigated. The TNAs were characterized using X-ray diffraction (X-ray) or (XRD), and scanning electron microscopy (SEM). Anodic TNAs having tube lengths from 7.9 to 20.17 μm were produced in ethylene glycol containing ammonium fluoride-NH4F by increasing the anodizing time from 20 min to 6 h. Based on I–V curves, the power conversion efficiency (PCE) of back-side illuminated dye sensitized solar cells (DSSCs) increased for TNAs grown for up to 120 min, but decreased afterward. Using electrochemical impedance spectroscopy (EIS), we understand that the resistance of the TNAs decreased from 94.82 Ω cm−2 for TNAs anodized for 20 min down to 50.43 Ω cm−2 for those TNAs anodized for 120 min, however, it increases for TNAs anodized for longer periods of time. Furthermore, the short circuit current density (Jsc) increased from 3.14 to 5.67 mA cm−2 during 2 h anodic oxidation for TNAs, and leading to enhanced efficiency of about 200 % (from 1.19 % to 2.45 %). We interpret this behaviour with the top surface morphology evolution of TNAs as a function of anodization time which is associated with the formation of top surface nanograss and bundling the tubes for specific durations.

Fe-NC nanozymes-loaded TiO2 nanotube arrays endow titanium implants with excellent antioxidant capacity for inflammation inhibition and soft tissue integration

Titanium (Ti) and its alloys have been widely used as percutaneous and subcutaneous implants due to their excellent mechanical properties and biocompatibility. However, the aggregation of reactive oxygen species (ROS) and persistent inflammatory responses at the implant site severely affect the soft tissue integration of Ti implants, causing a series of biological complications. To address this issue, in this study, Fe-nitrogen-doped carbon single-atom nanozymes (Fe-NC nanozymes) loaded titanium oxide nanotube arrays (Fe-NC@TNT) were constructed by anodic oxidation and solvothermal method on medical titanium surfaces. The surface morphology, physical composition, enzyme-like catalytic activity, inflammatory response, and soft tissue compatibility of Fe-NC@TNT were investigated. The unique nanotube array fully exposes the catalytic active sites of Fe-NC nanozymes and significantly enhances their enzyme-like catalytic performance to eliminate superoxide anion, hydrogen peroxide, and more toxic hydroxyl radicals, which could effectively reduce the intracellular ROS levels of macrophages and fibroblasts, thereby inhibiting the inflammatory responses of macrophages and promoting the functional expression of fibroblasts. In vivo animal experiments have further demonstrated that Fe-NC@TNT can effectively regulate the immune response and promote the integration between the implant and the surrounding soft tissues.

Solar-light-sensitive applications of recycled iron oxyhydroxide as photoelectrolytic heterostructure by encapsulating in reduced graphene oxide and titanium oxide nanotube array (FeOxrGO/TiNTA)

FeOxrGO/Ti photoanodes by decorating TiO2 nanotube array (TiNTA) with waste iron oxide and reduced graphene oxide (rGO) were synthesized via wet impregnation and calcination. Photoelectrocatalytic (PEC) degradation of azo dye pollutants, accompanied by hydrogen gas evolution, was demonstrated in a dual-chamber divided cell. Characterization showed that the iron oxide recovered from the wastewater treatment plant were crystalline clusters consisting of mineral goethite nanoparticles (αFeOOH NPs) with a visible-light responsive bandgap. Furthermore, the incorporation of rGO led to an enhancement in the electrochemical surface area (ECSA, cm2) of the anode due to its larger capacitive current compared to TiNTA. This improvement facilitated the transport of charge carriers (e- and h+) within a type-II heterostructure of αFeOOH and TiO2. The applied bias photon-to-current conversion efficiency and transient photocurrent density, depending on the composition of iron oxide in FeOxrGO/Ti, substantially increased. 2%wt of iron oxide in FeO2.0rGO/Ti has been optimized in terms of azo dye decolorization, TOC removal, and H2 generation rates (on Pt/G cathode) in 0.1 M Na2SO4 at a working potential of +1.0 V (vs. Ag/AgCl). This study explored the reusability and valorization of Fenton fluidized-bed waste iron oxide with TiO2 for PEC applications.

Structure and oxygen vacancy engineered CuCo-layered double oxide nanotube arrays as advanced bifunctional electrocatalysts for overall water splitting.

In recent years, as a green renewable energy production technology, electrochemical water splitting has demonstrated high development potential. Many materials have been reported as successful catalysts in the water-splitting field. However, it is still a huge challenge to produce bifunctional electrocatalysts for the efficient and sustainable generation of hydrogen and oxygen simultaneously. Herein, we successfully developed oxygen vacancies abundant CuCo layered double oxide (Ov-CuCo-LDO) hollow nanotube arrays (HNTAs) loaded on nickel foam as advanced electrocatalysts for total water splitting. When the current density was 10 mA cm-2, the Ov-CuCo-LDO HNTAs exhibited outstanding onset overpotentials of 53.9 and 72.5 mV for the hydrogen evolution and oxygen evolution reactions (HER and OER) in alkaline medium, respectively, because of the bimetallic synergistic effect between the cobalt and copper and the unique hollow porous structure. In addition, an as-assembled Ov-CuCo-LDO||Ov-CuCo-LDO electrolytic cell showed a small potential of 1.55 V to deliver a current density of 10 mA cm-2. Moreover, it also showed remarkable durability after long-term overall water splitting for more than 20 h. The research results in this paper are of great interest to practical applications of the water decomposition process, providing clear and in-depth insights into preliminary robust and efficient multifunctional electrocatalysts for overall water splitting.

Influences of Synthetic Parameters on Morphology and Growth of High Entropy Oxide Nanotube Arrays

Nanoscale and nanostructured materials have drawn great attention owing to their outstanding and unique properties. Enlightened by the study of “entropy-stabilized oxides”, nanotubes consisting of multi-component mixed metal oxides are developed, which formed on equi-atomic TiZrHfNbTa high-entropy alloy (HEA). However, the growth mechanism and how the oxidation conditions influence the nanotube growth and morphology remains unknown. In the present study, by controlling the anodization parameters (applied voltages and time) and bath compositions (fluoride concentration and water content), scanning electron microscope and transmission electron microscopy are conducted to reveal the morphological evolution. The present work uncovers how the synthetic parameters influence the tube growth and morphology formed on equi-atomic TiZrHfNbTa HEA, therefore gaining insight into the growth mechanism and the feasibility of controlling the morphology of multi-component oxide nanotubes.

Efficient “on–off” photoelectrochemical sensing platform with layer-by-layer assembly of titanium dioxide nanotube arrays and silver@zinc sulfide nanoparticles for unbiased and accurate monitoring of clonazepam

A high-performance self-powered photoelectrochemical platform has been constructed for clonazepam assay with “turn-on” signal output by synergistically using silver doped zinc sulfide decorated on titanium oxide nanotube array (Ag@ZnS/TiO2 NTs). Ag@ZnS was first organized and then deposited on TiO2 NTs that was made from electrophoretic deposition, to yield Ag@ZnS/TiO2 NTs nanohybrid. The constructed analysis platform displays two linear concentrations ranging from 0.016 to 6.4 and 6.4–76 µM with a limit of detection of 1.19 nM and high sensitivity of 4.13 µA µM−1 (in the range of 0.016–6.4 µM) for clonazepam. What's more, the as-constructed senor represents excellent selectivity, good stability, and prominent reproducibility. Besides, the application feasibility of the as-prepared sensing platform with Ag@ZnS/TiO2 NTs could be attested by detecting clonazepam in tablet and biological fluids. The “On-Off” photoelectrochemical platform may enable new opinions for the application of intelligent nanohybrids.

Discharged Titanium Oxide Nanotube Arrays Coated with Ni as a High‐Performance Lithium Battery Electrode Material

Transition metal oxides are in high demand as electrode materials for alkali metal‐ion batteries due to high theoretical capacity. However, low electron transfer kinetics and slow ion migration lead to poor cycle stability and rate performance of the transition metal oxide electrode. Herein, TiO2 nanotube arrays coated with Ni (Ni/TiO2NTs) are fabricated by using predischarge‐electrodeposition method and subsequent calcination under H2/N2. In the Ni/TiO2NTs composite, TiO2NTs and Ni nanoparticles play the role of framework and coating layer, respectively. The as‐prepared Ni/TiO2NTs composites have a high specific surface area and a unique tubular structure, which facilitates the quick penetration of the electrolyte into the tubes and reduces the lithium ions’ diffusion distance. The incorporation of Ti3+ and metallic nickel in the hydrogenated TiO2NTs also increases conductivity. Besides, the spin‐polarized surface capacitance of Ni0 nanoparticles causes the magnetic changes that generate an extra lithium‐ion storage capacity. The Ni/TiO2NTs composite electrode maintains a high capacity of 507 mA h g−1 after 100 cycles at 0.2 A g−1. Here, the predischarge‐electrodeposition provides a new route for the preparation of composite materials.

Temperature-Modulated Selective Detection of Part-per-Trillion NO2 Using Platinum Nanocluster Sensitized 3D Metal Oxide Nanotube Arrays.

Semiconductor chemiresistive gas sensors play critical roles in a smart and sustainable city where a safe and healthy environment is the foundation. However, the poor limits of detection and selectivity are the two bottleneck issues limiting their broad applications. Herein, a unique sensor design with a 3D tin oxide (SnO2 ) nanotube array as the sensing layer and platinum (Pt) nanocluster decoration as the catalytic layer, is demonstrated. The Pt/SnO2 sensor significantly enhances the sensitivity and selectivity of NO2 detection by strengthening the adsorption energy and lowering the activation energy toward NO2 . It not only leads to ultrahigh sensitivity to NO2 with a record limit of detection of 107 parts per trillion, but also enables selective NO2 sensing while suppressing the responses to interfering gases. Furthermore, a wireless sensor system integrated with sensors, a microcontroller, and a Bluetooth unit is developed for the practical indoor and on-road NO2 detection applications. The rational design of the sensors and their successful demonstration pave the way for future real-time gas monitoring in smart home and smart city applications.

Hybrid TiNb oxides/nitrides nanotube arrays as active catalyst supports

In order to improve the thermal stability of anodized TiO2 nanotube arrays, this paper proposed to fabricate the hybrid TiNb oxide nanotube arrays (TiNb ONA) on β-phase TiNb alloy plates by anodic oxidation. Then, TiNb ONA precursors were transformed into conductive nitride nanotube arrays (TiNb NNA) by nitridation at 800 °C. Introducing Nb contributes to the higher structural stability and easier nitridation of TiNb ONA compared with TiO2 nanotube arrays. After that, the spinel CuFe2O4 and nanosized Re-Ni alloy were respectively loaded on TiNb ONA and TiNb NNA to form CuFe2O4/TiNb ONA and Re-Ni/ TiNb NNA composite electrodes, and their performances in the photoelectrochemical reduction of CO2 and hydrogen evolution reactions were assessed respectively. Results showed that the well-maintained TiNb ONA improved the photo-response performance of the thermally-synthesized CuFe2O4, and TiNb NNA support also boosted the hydrogen evolution reaction because of the conductivity and catalytic activity of TiNb nitrides.

Synthesis of Titanium Oxide Nanotubes for Photo Electrochemical Cells and its Characterizations

Nano scale materials have many peculiar properties than the bulk ones and have led to break through in various fields of science and technology. Titanium oxide is one of the excellent and most demanded materials because of its various applications and properties such as such as large surface area. In this paper, Titanium oxide (TiO2) nanotube arrays were synthesized by electrochemical anodization of titanium foil. Dimension of nanotubes and elemental composition were detected by Field Emission-Scanning Electron Microscope with Energy Dispersive X-Ray Analysis (FESEM- EDAX). From X- Ray Diffraction (XRD) the intensity of peaks, d- spacing values and phase of TiO2 nanotubes were analyzed. The Energy Bandgap (Eg) of the formed TiO2 nanotubes was determined by Diffuse Reflectance Spectroscopy from Kubelka-Monk Plot. From the obtained results, the synthesized TiO2 nanotubes are suitable for Photo Electrochemical Cell applications.

Formation of self-organized ZrO2–TiO2 and ZrTiO4–TiO2 nanotube arrays by anodization of Ti–40Zr foil for Cr(VI) removal

In this work, anodic oxidation of Ti-40 wt.% Zr alloy in EG/NH4F/H2O electrolyte was done to produce anodic self-aligned oxide nanotube arrays. The nanotubes were amorphous but upon annealing at 400° C they were crystallized into ZrO2–TiO2 nanotubes and ZrTiO4–TiO2 nanotubes at 600 °C. The nanotubes were used to reduce 60 ppm Cr(VI) ions under sunlight or UV irradiation. 100% removal of Cr(VI) was achieved on ZrTiO4–TiO2 nanotubes after 5 h of exposure to UV light but only 53% removal was obtained on ZrO2–TiO2 nanotubes. The higher photocatalytic activity of the ZrTiO4–TiO2 nanotubes can be related to the presence of the ZrTiO4 phase which has good acid–base properties as well as the junction formation between ZrTiO4 and TiO2 that can further enhance the reduction process.

Construction of Mussel-Inspired Dopamine-Zn2+ Coating on Titanium Oxide Nanotubes to Improve Hemocompatibility, Cytocompatibility, and Antibacterial Activity.

Zinc ions (Zn2+) are a highly potent bioactive factor with a broad spectrum of physiological functions. In situ continuous and controllable release of Zn2+ from the biomaterials can effectively improve the biocompatibility and antibacterial activity. In the present study, inspired by the adhesion and protein cross-linking in the mussel byssus, with the aim of improving the biocompatibility of titanium, a cost-effective one-step metal–catecholamine assembly strategy was developed to prepare a biomimetic dopamine–Zn2+ (DA-Zn2+) coating by immersing the titanium oxide nanotube (TNT) arrays on the titanium surface prepared by anodic oxidation into an aqueous solution containing dopamine (DA) and zinc ions (Zn2+). The DA-Zn2+ coatings with the different zinc contents exhibited excellent hydrophilicity. Due to the continuous release of zinc ions from the DA-Zn2+ coating, the coated titanium oxide nanotubes displayed excellent hemocompatibility characterized by platelet adhesion and activation and hemolysis assay. Moreover, the DA-Zn2+-coated samples exhibited an excellent ability to enhance endothelial cell (EC) adhesion and proliferation. In addition, the DA-Zn2+ coating can also enhance the antibacterial activity of the nanotubes. Therefore, long-term in situ Zn2+-releasing coating of the present study could serve as the bio-surfaces for long-term prevention of thrombosis, improvement of cytocompatibility to endothelial cells, and antibacterial activity. Due to the easy operation and strong binding ability of the polydopamine on various complicated shapes, the method of the present study can be further applied to other blood contact biomaterials or implantable medical devices to improve the biocompatibility.

Potassium Titanate Assembled Titanium Dioxide Nanotube Arrays Endow Titanium Implants Excellent Osseointegration Performance and Nerve Formation Potential.

Titanium based materials have been widely applied in bone-tissue engineering. However, inefficient bone repair remains to be solved due to the lack of neural network reconstruction at the bone-implant interface. Herein, we propose a functional surface modification approach to promote neurogenesis. Using an electrochemical technique and a hydrothermal approach, a potassium titanate nanorod-decorated titanium oxide (K2Ti6O13-TiO2) nanotube array is constructed on the surface of titanium implants. The K2Ti6O13-TiO2 hybrid nanotube array on titanium implants can enhance the osteogenic differentiation of mesenchymal stem cells due to the special nanostructures of titanium oxide nanorods. Meanwhile, the release of potassium ions is able to accelerate the neural differentiation of neural stem cells. This study provides a new approach to promote neuralization on the surface of implants, which is promising for future applications in constructing a fully functional interface in bone repair.

Design and Fabrication of Glucose Biosensors Based on Immobilization of Glucose Oxidase on Titanium Oxide Nanotube Arrays.

An electrochemical biosensor for the detection of glucose is realized by immobilizing glucose oxidase (GOx) enzyme onto titanium dioxide nanotube arrays by a coupling encapsulation process. We present details of a robust fabrication technique that results in a durable and reproducible sensor characteristics. The TiO₂ nanotube arrays are grown directly on a titanium substrate by a potentiostatic anodization process in a water and ethylene-glycol mixture solution, which contains ammonium fluoride. An electropolymerization process was also performed to enhance interfacial adhesion between GOx and TiO₂ nanotubes. Detection of glucose concentrations was achieved with a linear response in the range of 0.01 to 0.2 mM. Investigation of enhanced sensitivity by increasing the count, the length, and the cross-section of the nanotubes was also carried out. Surface morphologies of Ti substrate were examined by scanning electron microscopy to optimize the anodization process and thus the TiO₂/Ti nanotube dimensions. We utilized a time-based amperometric response for the quantitative determination of hydrogen peroxide concentration through electro-reduction reaction with a bare TiO₂/Ti nanotube-array electrodes, thus providing a reference for the determination of glucose levels with a GOx-coated TiO₂/Ti nanotube array electrodes. Detection levels down to 5.2 μM were recorded.

The prominent role of fully-controlled surface co-modification procedure using titanium nanotubes and silk fibroin nanofibers in the performance enhancement of Ti6Al4V implants

Modification of orthopedic implant surfaces through advanced nanoscale coating methods has made a major breakthrough in maximizing implantation success. Adjustable drug release and biocompatibility are among the most momentous features since they can significantly prevent the implantation failure. In this study, the potential of silk fibroin (SF) nanofibers fabricated via electrospinning, along with titanium oxide nanotube arrays (TNTs) formed through anodization, were exploited to produce a cyto-biocompatible, well-controlled drug delivery system. Highly-ordered TNTs were formed in an organic electrolyte solution within 2 h at the voltage of 60 V under temperature controlling (16 °C). Vancomycin was loaded using a vacuum system designed for this purpose. A two-level full factorial design of experiments was carried out to obtain the highly suitable, optimized conditions to create an electrospun fibrous network with aligned uniform nanofibers, which revealed the dominant role of polymeric solution concentration in the generation of nanofibers. The topography and surface roughness results determined by atomic force microscopy (AFM), showed a considerable boost in the surface roughness of the coated samples, which resulted in enhanced cell adhesion, viability, and osseointegration, especially in the co-modified one (TNT/SF). Drug release outcomes demonstrated a conspicuous decrease in the burst release percentage (from 88% for bare to 20% for TNT/SF sample), and a noticeable overall release time extension (from 10 days for bare to 40 days for TNT/SF sample) at a near zero-order release rate caused by the surface co-modification.

Synthesized and characterization of open-ended Ta3N5 nanotube arrays for the preparation of growth template

In this article, a two-step anodization method was applied to tantalum foils to grow compact open-ended tantalum oxide nanotube arrays, which were subsequently subject to high temperature nitridation. The nitride nanotube arrays were characterized with SEM, XRD, and electrochemical tests including I-V and Mott-Schottky analysis. The phases in the nitrided nanotube arrays may consist of Ta3N5, TaN, TaN0.5, N-Ta2O, or the amorphous oxide. The duration of the second anodization step was found to play a key role on the microstructure and the carrier density of the nitride nanotube arrays. It is found that the duration of the second anodization has to be more than 20 min to obtain a specimen with a top layer of Ta3N5, which can work as a photo electrode or a template for the growth of photo catalyst nanomaterials.

A long-term stable aqueous aluminum battery electrode based on one-dimensional molybdenum-tantalum oxide nanotube arrays.

Aluminum ion aqueous batteries (AIBs) are among the most promising candidates for high energy density devices due to the multivalent redox processes associated with Al3+ ion intercalation. However, only a few stable AIB electrode materials have been reported so far. MoO3 is a very promising electrode material due to its octahedral layered crystal structure which can accommodate multivalent cation by intercalation. However, the poor electrochemical stability of MoO3 and the sluggish intercalation kinetics of Al3+ ion in Mo oxides electrodes limit its practical application. In this work, we propose a strategy to overcome such shortcomings of MoO3 by fabricating electrodes composed of self-ordered one-dimensional (1D) MoTaOx nanotubes synthesized via electrochemical anodization of Mo-Ta alloy substrates. We show that this approach allows for direct incorporation of Ta in the Mo oxide nanotubes. The resulting MoTaOx nanotubes, composed of octahedral MoO3 and rhombohedral Mo2Ta2O11 phases, exhibit remarkable electrochemical stability and Al-ion storage properties in aqueous electrolytes that are superior to that of pristine Mo oxide or other most efficient electrode materials reported to date. Such MoTaOx nanotube-based electrodes can achieve a specific capacity of 1180 mA h cm-3 (337 mA h g-1, 141 μA h cm-2) at 1.25 A cm-3 (∼0.35 A g-1, 0.15 mA cm-2). More importantly, the capacity retention of such nanotube array electrodes remains above 83% of the initial capacity after 3000 cycles.

Comparison of Ordering Characteristics of Anodicformed Nanostructured Aluminum and Titanium Oxides Coatings

This article is concerned with the analysis of ordering the arrays of TiO2 and Al2O3 nanotubes using the correlation-spectral methods. As the tools, the spatial Fourier spectrum and one-dimensional autocorrelation function of SEM-image have served. It was shown that the arrays of the aluminum oxide nanotubes can have a nearly ideal ordering on a small scale at the expense of two-stage anodizing. It this case, the degree of order depends also on the purity of initial aluminum and sample preparation method. The introduced characteristics can serve as the measures of the structure order-disorder sensitive to both type and degree of order as a whole and to configuration of structural elements themselves.