Oxide Nanotube Arrays Research Articles

Ultrastable metal oxide nanotube arrays achieved by entropy-stabilization engineering

Metal oxide nanotube arrays have recently attracted extensive attention due to their widespread use in the field of catalysis and energy storage. However, the low structural stability of the currently available nanotube arrays severely limits their efficiency and high-temperature applications. By applying the entropy-stabilization engineering strategy, we developed ultrastable metal oxide nanotube arrays consisting of multiple oxide constituents by anodic oxidation of high-entropy alloy precursors, which can sustain 800°C heat-treatment without crystallization and resist 1000°C calcination without collapse. The superior thermal stability endows the high-entropy oxide nanotube arrays a promising candidate for high-temperature applications.

Defect engineering in 1D Ti-W oxide nanotube arrays and their correlated photoelectrochemical performance.

Understanding the nature of interfacial defects of materials is a critical undertaking for the design of high-performance hybrid electrodes for photocatalysis applications. Theoretical and computational endeavors to achieve this have touched boundaries far ahead of their experimental counterparts. However, to achieve any industrial benefit out of such studies, experimental validation needs to be systematically undertaken. In this sense, we present herein experimental insights into the synergistic relationship between the lattice position and oxidation state of tungsten ions inside a TiO2 lattice, and the respective nature of the created defect states. Consequently, a roadmap to tune the defect states in anodically-fabricated, ultrathin-walled W-doped TiO2 nanotubes is proposed. Annealing the nanotubes in different gas streams enabled the engineering of defects in such structures, as confirmed by XRD and XPS measurements. While annealing under hydrogen stream resulted in the formation of abundant Wn+ (n < 6) ions at the interstitial sites of the TiO2 lattice, oxygen- and air-annealing induced W6+ ions at substitutional sites. EIS and Mott-Schottky analyses indicated the formation of deep-natured trap states in the hydrogen-annealed samples, and predominantly shallow donating defect states in the oxygen- and air-annealed samples. Consequently, the photocatalytic performance of the latter was significantly higher than those of the hydrogen-annealed counterparts. Upon increasing the W content, photoelectrochemical performance deteriorated due to the formation of WO3 crystallites that hindered charge transfer through the photoanode, as evident from the structural and chemical characterization. To this end, this study validates the previous theoretical predictions on the detrimental effect of interstitial W ions. In addition, it sheds light on the importance of defect states and their nature for tuning the photoelectrochemical performance of the investigated materials.

High photocatalytic activities of zinc oxide nanotube arrays modified with tungsten trioxide nanoparticles

Well-aligned zinc oxide (ZnO) nanotube arrays loaded with tungsten trioxide (WO3) nanoparticles were synthesized by a process involving chemical bath deposition in combination with pyrolysis. The prepared ZnO–WO3 composites were characterized by X-ray diffraction, energy dispersive spectrometer, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, Fourier transform infrared spectroscopy and UV–vis diffuse reflectance spectroscopy. The photocatalytic activities of the ZnO–WO3 composite photocatalysts with different WO3 contents for the degradation of the herbicide chlorinated phenoxyacetic acid (MCPA-Na) under simulated sunlight irradiation were systematically evaluated. It was found that the WO3 content had a great effect on the photocatalytic activity of the ZnO–WO3 composites. The composite with 3% WO3 showed the highest photocatalytic activity, with a degradation rate of chlorinated phenoxyacetic acid of 98.5% after 200 min with 20 mg of photocatalyst. This photodegradation rate was about twice that of the pristine ZnO nanotube array. The recombination of photogenerated electrons and holes was increasingly suppressed with the addition of WO3 to ZnO. The high relative content of defects on the surface of the ZnO–WO3 composites was beneficial to their photocatalytic activity in the degradation of chlorinated phenoxyacetic acid.

Biodegradable nanostructures: Degradation process and biocompatibility of iron oxide nanostructured arrays

Surface nanostructures like titanium oxide nanotubes and nanopores have revealed excellent functionalization performance and good corrosion resistance. In certain cases, biodegradability is, however, an advantageous feature for implant materials. Recently iron-based materials have been studied as promising degradable biomaterials. In order to functionalize biodegradable iron-based materials, oxide nanostructured arrays have been used as a surface functionalization approach. In this study, two different iron oxide nanostructures, nanopores and nanotube arrays with different thicknesses, were fabricated on pure iron by an anodization method. The influence of anodization conditions on the morphology of the novel nanostructures was investigated. The degradation behavior is essential to the biomedical application of iron oxide nanostructured arrays. Hence electrochemical and immersion tests in simulated body fluid (SBF) solution were conducted to reveal details and the mechanism of the degradation process. Surface composition and morphology of immersed iron oxide nanostructures were investigated. Furthermore, indirect contact cell experiments were carried out by employing extracts from immersion solution of iron oxide nanostructures on human osteoblast-like MG-63 cells. Results of the in vitro tests indicated no cytotoxicity caused by the extracts of the nanoarrays on pure iron. The current understanding of the degradation process and cytocompatibility of iron oxide nanostructures gained from this work is essential for their further study and for considering biomedical application of such nanostructures.

Photocatalytic decomposition of gaseous methanol over anodized iron oxide nanotube arrays in high vacuum

Real-time monitoring of photocatalytic methanol decomposition over anodized iron oxide nanotube arrays (FNAs) in high vacuum was performed. Switching phenomena in the partial pressures of H2, CO and H2O were observed simultaneously according to the ON/OFF sequence of visible and ultraviolet light illuminations over Pt-loaded FNAs prepared with photodeposition, while no change in these gas species appeared for bare FNAs and iron foils. The absence of formaldehyde (CH2O) in the intermediates during methanol photodecomposition suggests that Pt-loaded FNAs can possibly work as a catalyst for oxidation of CH2O. The photoreforming of methanol to hydrogen without any external bias in Pt-loaded FNAs presumably indicate some differences in photocatalytic reaction mechanisms between in liquid and in high vacuum. The dependence of the Pt photodepositon time on gas phase photocatalysis suggests that the selective deposition of Pt cocatalyst particles on the aperture parts of the nanotubes is necessary for efficient gas phase photoreforming.

A novel local drug delivery system: Superhydrophobic titanium oxide nanotube arrays serve as the drug reservoir and ultrasonication functions as the drug release trigger

A local drug delivery system consisting of superhydrophobic titanium oxide nanotube (S-TNTs) arrays and ultrasonic-controlled release trigger was developed in this work. Hydrophilic TNTs arrays are converted into superhydrophobic after being treated by 1H,1H,2H,2H- perfluorooctyl-triethoxysilane (POTS). S-TNTs arrays serving as a drug-carrying vehicle require no extra sealing treatment due to the excellent isolation effect from the trapped air layer on the surface. Different amounts of drugs could be loaded into S-TNTs arrays by control of the structure of arrays (including length and diameter of tubes) and the original amount of drug in the drug-loading solution. The relation between surface morphology of TNTs arrays and superhydrophobicity (isolation effect) was thoroughly investigated. To achieve a stimulus-responsive drug delivery system, ultrasonication was employed as an efficient drug release trigger. Trapped air layer could be selectively removed by ultrasonication, and therefore the loaded drug could be released in a multiple and controlled manner. Any drugs that can dissolve in nonpolar solutions are expected to be suitable for this local drug delivery system.

Toward new gas-analytical multisensor chips based on titanium oxide nanotube array

Reliable environmental monitoring requires cost effective but highly sensitive and selective gas sensors. While the sensitivity of the sensors is improved by reducing the characteristic dimensions of the gas-sensing material, the selectivity is often approached by combining the sensors into multisensor arrays. The development of scalable methods to manufacture such arrays based on low-dimensional structures offers new perspectives for gas sensing applications. Here we examine an approach to produce multisensor array chips based on the TiOx nanotube layers segmented by multiple Pt strip electrodes. We study the sensitivity and selectivity of the developed chip at operating temperatures up to 400 °C towards organic vapors in the ppm range. The results indicate that the titania nanotubes are a promising material platform for novel cost-effective and powerful gas-analytical multisensor units.

(Physical and Analytical Electrochemistry Division David S. Grahame Award Address) Nanoscale Templates and Scaffolds for Electrochemical Device Applications

This talk will review some of our recent fuel cell-related research efforts, which have had a primary focus on increasing the lifetime and performance of both anode and cathode catalyst layers. In our recent work on PEM fuel cells, a new class of ordered, mesoporous carbon materials (both powders and free-standing scaffolds), with nano-engineered pore diameters and lengths, have been developed as support materials to better distribute and stabilize catalytic nanoparticles that are attached to their surface. These carbons have also been surface modified with a range of functional groups, showing that this can significantly alter their wettability, enhance their resistance to corrosion, better anchor catalytic nanoparticles, and are also beneficial in redox flow battery electrochemistry. These reproducibly ordered carbon scaffolds are also proving to be ideal for the investigation of the interactions of Nafion with Pt/carbon in relation to performance, especially as a function of carbon pore diameter, depth and surface hydrophilicity. In parallel research related to catalyst supports, we have constructed ordered metal oxide nanotubular arrays, then converting them to conducting oxy-nitride forms, primarily to replace carbon. These metal oxy-nitrides undergo interesting redox transitions that will be shown to correlate with the activity of these materials (after deposition of Pt nanoparticles) towards the oxygen reduction reaction. Ordered surface arrays of Zr oxide nanotubes are also very promising for use in novel, nano-structured, high temperature solid oxide fuel cells.

Improved performance of anodic titanium oxide nanotube arrays synthesized by sonoelectrochemical anodization method for dye-sensitized solar cells

With rising demand for using solar cell systems for energy and environmental applications, considerable interest in anode materials of these cells has dramatically emerged. In this work, TiO2 nanotubular electrodes of varying thicknesses as anode were fabricated using effective process in a short time by sonoelectrochemical anodizing of Ti using an organic electrolyte, containing Na2[H2EDTA]. Long TiO2 nanotubes about 30–40 μm thick containing ordered hexagonal TiO2 were achieved through manipulating anodization parameters. Dye-sensitized solar cells (DSSCs) with different TiO2 electrode morphologies of varying thicknesses were compared to DSSCs based on TiO2 Nanoparticle electrodes. The effect of controlling parameters of the sonoelectrochemical process including surface preparation and anodizing time was investigated. This report brings to attention the desirable properties of the structurally oriented TiO2 for dye-sensitized solar cell applications. It found that the best cell performance was achieved about 3.14% in the case of using TiO2 NT layers that were treated by TiCl4 treatment.

Enhancement of electrochemical properties of micro/nano electrodes based on TiO2 nanotube arrays

Titanium oxide nanotube (TiO2 nanotube) arrays were produced by anodizing titanium foils in two different electrolytes. The first electrolyte consisted of ethylene glycol containing 0.5 wt% NH4F and 4 vol% of distilled water to produce pure TiO2 nanotube arrays and the second consisted of HF aqueous solution (0.5 wt%) containing 0.5% polyvinylalcohol to produce carbon doped TiO2 nanotube arrays. The fabricated TiO2 nanotube arrays were subsequently annealed in the atmosphere of nitrogen. The morphology and crystal structure of fabricated arrays were characterized by means of scanning electron microscopy and X-ray diffraction. The electrical conductivity and capacitance of TiO2 nanotube arrays were investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Water contact angle and biocompatibility of fabricated nanotube arrays were investigated. The results showed that carbon doped TiO2 nanotube arrays annealed in the atmosphere of nitrogen have higher conductivity and capacitance than those of pure arrays annealed in the same atmosphere. Doping with carbon enhances the biocompatibility and wettability of TiO2 nanotube arrays. It has also noted that electrical conductivity and capacitance of TiO2 nanotube arrays were directly proportional to the tube wall thickness.

Tailoring UV emission from a regular array of ZnO nanotubes by the geometrical parameters of the array and Al2O3 coating

Self-aligned and equal-spaced zinc oxide (ZnO) nanotube arrays were fabricated with anodic aluminum oxide (AAO)-assisted growth and the ALD technique. The near band-edge (NBE) emission was strongly affected by the nanotube's geometrical parameters, such as a packing density and thickness of the nanotube walls. The NBE emission was further enhanced with Al2O3 coating. The effect was analyzed by X-ray photoelectron spectroscopy (XPS) and ascribed to the surface defect passivation and a ZnAl2O4 spinel formation. The NBE emission enhancement was greater in ZnO nanotubes with thicker walls. A smaller UV enhancement factor was explained by less uniform and integral Al2O3 coverage of the ZnO nanotubes with thinner walls; this, possibly induced a variation of the Al2O3 refractive index along the nanotubes. As a result, the optical conditions at the ZnO/Al2O3/air interfaces was changed and the light extraction efficiency was reduced in the latter samples.

Polydopamine coatings embedded with silver nanoparticles on nanostructured titania for long-lasting antibacterial effect

Although silver (Ag)-containing biomaterials have been considered as an effective way to prevent bacteria-induced implant infection, the sustaining and long-lasting release of Ag modified on the coating layer remains a problem for the clinical application. Herein, we designed a polydopamine coating with silver nanoparticles on TiO2 nanotube arrays (Ag-PDA-TiO2) by a series of treatment of electrochemical anodization, self-polymerization of dopamine, and the following reduction reaction between PDA and Ag ions. In the system, Ag nanoparticles were synthesized in situ through the reduction action of polydopamine on the surface of anodized titanium oxide nanotube array and PDA was used as reduction and adhesive agent simultaneously. The substrate of PDA coating with silver nanoparticles on titanium surface (Ag-PDA-Ti) and silver nanoparticles prepared by ultraviolet irradiation on TiO2 nanotube array (Ag-TiO2 (UV)) were used as control samples. The morphology, component and wettability of the coating were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and water contact angle analysis. The release rate of Ag of Ag-PDA-TiO2 was slow down due to the adhesion strength of PDA to Ag nanoparticles and the storage capacity of nanotubes. The antibacterial effect of Ag-PDA-TiO2 to E. coli lasted for 14days which is obviously longer than Ag-PDA-Ti and Ag-TiO2 (UV). The method used here could be an effective strategy to prevent implant-associated infection showing significant meaning in clinical application.

Aligned MoOx/MoS2 Core–Shell Nanotubular Structures with a High Density of Reactive Sites Based on Self‐Ordered Anodic Molybdenum Oxide Nanotubes

AbstractThe present work demonstrates the self‐organized formation of anodic molybdenum oxide nanotube arrays. The amorphous tubes can be crystallized to MoO2 or MoO3 and be converted fully or partially into molybdenum sulfide. Vertically aligned MoOx/MoS2 nanotubes can be formed when, under optimized conditions, defined MoS2 sheets form in a layer by layer arrangement that provide a high density of reactive stacking misalignments (defects). These core–shell nanotube arrays consist of a conductive suboxide core and a functional high defect density MoS2 coating. Such structures are highly promising for applications in electrocatalysis (hydrogen evolution) or ion insertion devices.

Aligned MoOx /MoS2 Core-Shell Nanotubular Structures with a High Density of Reactive Sites Based on Self-Ordered Anodic Molybdenum Oxide Nanotubes.

The present work demonstrates the self-organized formation of anodic molybdenum oxide nanotube arrays. The amorphous tubes can be crystallized to MoO2 or MoO3 and be converted fully or partially into molybdenum sulfide. Vertically aligned MoOx /MoS2 nanotubes can be formed when, under optimized conditions, defined MoS2 sheets form in a layer by layer arrangement that provide a high density of reactive stacking misalignments (defects). These core-shell nanotube arrays consist of a conductive suboxide core and a functional high defect density MoS2 coating. Such structures are highly promising for applications in electrocatalysis (hydrogen evolution) or ion insertion devices.

Effectively Improving Capacitive Performance of Three‐Dimensional Iron(III) Oxide Nanotube Arrays by Rationally Filling Mesopores with Polypyrrole

AbstractFe2O3 is a promising electrode material for electrochemical capacitors, but challenges remain in enhancing its capacitive performance. Herein, we effectively improved the capacitive properties of 3D Fe2O3 nanotube arrays by partly filling their mesopores with polypyrrole (PPy) through vapor‐phase polymerization. Although the filled PPy was only 9.7 wt %, the resulting hybrid Fe2O3/PPy arrays tripled the specific capacitance to 367 F g−1 at 1.0 A g−1, doubled their high rate capability (106 F g−1 at 10 A g−1), and exhibited good cycling stability at high current density. The capacitive performances of the hybrid arrays are among the best for Fe2O3‐based electrodes. It is believed that the partly filled mesopores not only increase the electronic conductivity and electroactivity of each hybrid nanotube but also avoid blocking the ion‐diffusion path. All the features allow the arrays to exhibit synergetic pseudocapacitive properties and to maintain stable and fast electrochemical kinetics. Our findings provide an efficient strategy to boost the electrochemical performances of transition‐metal oxide nanomaterials by rationally filling their mesopores with a small amount of a conductive polymer.

Free-Standing Oxide Nanotube Array Electrodes for High Energy Density and Power Density Li-Ion Batteries

During the past few years, with the increasing power demand generated by applications from portable devices to electric vehicles, more and more emphasis is put on manufacturing high energy and power density Li-ion batteries, i.e., to maximize the capacities while retaining a high rate capability. So far, the studies have been mainly dedicated to the development of powder type electrode materials and relatively little attention has been paid to studies of other electrode architectures. While composite electrodes containing a mixture of the active material powders, binders and conductive additives still are commonly used, such electrodes often yield poor material utilization, undefined material/component arrangements and a lot of complex interfaces. In the present work, we demonstrate that various highly ordered, free-standing oxide nanotube array electrodes, fabricated by electrochemical anodization approaches, can be used either for high energy density and power density Li-ion microbattery applications or as model monolithic electrodes for electrode engineering studies. By using anatase TiO2 nanotube electrodes (as an example of an intercalation type material), an areal capacity of 0.37 mAh cm-2 (i.e., 40 mAh g-1) at a rate of 10C (using a (dis-)charge current density of 9 mA cm-2), and 1 mAh cm-2 (i.e., 91 mAh g-1) at a rate of C/5), can be achieved. [1] Well-defined monolithic anatase TiO2 nanotube electrodes with fine-tuned nanotube size gradients (including tube length, diameter and wall thickness) can also be manufactured using a bipolar electrochemistry approach. [2] The gradient nanotube electrodes can provide excellent rate performance, with capacities from of 0.16 mAh cm-2 or 169 mAh g-1 at a rate of C/5 to 0.04 mAh cm-2 or 42 mAh g-1 at a rate of 50C. In addition, free-standing Nb2O5 nanotube electrodes, which can be cycled for 10000 cycles with only a 20% loss of initial capacities, can provide unprecedented high-rate battery performances, i.e. the capacities from of 0.1 mAh cm-2 or 110 mAh g-1 at a rate of C/5 to 0.04 mAh cm-2 or 44 mAh g-1at a rate of 100C.[3] Some recent work carried out to investigate highly ordered Fe3O4 nanotubular/nanoporous electrodes as a prototype free-standing conversion electrode will also be described.

Controlled Phase and Tunable Magnetism in Ordered Iron Oxide Nanotube Arrays Prepared by Atomic Layer Deposition.

Highly-ordered and conformal iron oxide nanotube arrays on an atomic scale are successfully prepared by atomic layer deposition (ALD) with controlled oxidization states and tunable magnetic properties between superparamagnetism and ferrimagnetism. Non-magnetic α-Fe2O3 and superparamagnetic Fe3O4 with a blocking temperature of 120 K are in-situ obtained by finely controlling the oxidation reaction. Both of them exhibit a very small grain size of only several nanometers due to the nature of atom-by-atom growth of the ALD technique. Post-annealing α-Fe2O3 in a reducing atmosphere leads to the formation of the spinel Fe3O4 phase which displays a distinct ferrimagnetic anisotropy and the Verwey metal-insulator transition that usually takes place only in single crystal magnetite or thick epitaxial films at low temperatures. The ALD deposition of iron oxide with well-controlled phase and tunable magnetism demonstrated in this work provides a promising opportunity for the fabrication of 3D nano-devices to be used in catalysis, spintronics, microelectronics, data storages and bio-applications.

Titanium oxide nanotube arrays for high light extraction efficiency of GaN-based vertical light-emitting diodes.

TiO2 nanotube (NT) arrays were fabricated on the surface of n-GaN through a liquid-phase conversion process using ZnO nanorods (NRs) as a template for high-efficiency InGaN/GaN multiple quantum well (MQW) vertical light-emitting diodes (VLEDs). The optical output power of the VLEDs with TiO2 NTs was remarkably enhanced by 23% and 189% at an injection current of 350 mA compared to those of VLEDs with ZnO NRs and planar VLEDs, respectively. The large enhancement in optical output is attributed to a synergistic effect of efficient light injection from the n-GaN layer of the VLED to TiO2 NTs because of the well-matched refractive indices and superior light extraction into air at the end of the TiO2 NTs. Light propagation along various configurations of TiO2 NTs on the VLEDs was investigated using finite-difference time domain simulations and the results indicated that the wall thickness of the TiO2 NTs should be maintained close to 20 nm for superior light extraction from the VLEDs.

Graphitic carbon nitride (g-C3N4) coated titanium oxide nanotube arrays with enhanced photo-electrochemical performance.

Herein, we report the successful formation of graphitic carbon nitride coated titanium oxide nanotube array thin films (g-C3N4/TiO2) via the facile thermal treatment of anodized Ti sheets over melamine. The proportion of C3N4 and TiO2 in the composite can be adjusted by changing the initial addition mass of melamine. The as-prepared samples are characterized by several techniques in order to understand the morphological, structural, compositional and optical properties. UV-vis absorption studies exhibit a remarkable red shift for the g-C3N4/TiO2 thin films as compared to the pristine TiO2 nanotubes. Importantly, the prepared composites exhibit an enhanced photocurrent and photo-potential under both UV-vis and visible light irradiation. Moreover, the observed maximum photo-conversion efficiency of the prepared composites is 1.59 times higher than that of the pristine TiO2 nanotubes. The optical and electrochemical impedance spectra analysis reveals that the better photo-electrochemical performance of the g-C3N4/TiO2 nanotubes is mainly due to the wider light absorption and reduced impedance compared to the bare TiO2 nanotube electrode. The presented work demonstrates a facile and simple method to fabricate g-C3N4/TiO2 nanotubes and clearly revealed that the introduction of g-C3N4 is a new and innovative approach to improve the photocurrent and photo-potential efficiencies of TiO2.

Optimized fabrication and characterization of TiO2–Nb2O5–Al2O3mixed oxide nanotube arrays on Ti–6Al–7Nb

The TiO2–Nb2O5–Al2O3mixed oxide nanotube arrays on Ti67 are the promising nanostructured materials for metallic orthopaedic implants.