Anodic TiO2 Nanotube Arrays Research Articles

Anodic TiO2 Nanotube Arrays as Fixed Photocatalyst

In the present study, self-organized TiO2 nanotube arrays were prepared by anodic oxidation in a viscous electrolyte of ethylene glycol containing NH4F with a concentration of 1,25% wt. the anodized samples expose single-walled nanotube arrays with an average pore diameter and wall thickness of about 80 and 15 nm, respectively. The photocatalytic efficiency of the nanotube arrays has been investigated and compared to commercial P25 nanopowder by measuring the degradation of methyl orange. The results showed that the nanotube arrays exhibit higher activity towards dye degradation as compared to commercial P25 nanoparticles with a degradation efficiency reaching up to 99.45% after an irradiation time of 60 min. The high surface to volume ratio and the mechanical adhesion makes them suitable as multiple use photocatalysts.

Facile fabrication of organic/inorganic nanotube heterojunction arrays for enhanced photoelectrochemical water splitting.

Organic/inorganic heterojunction photoanodes are appealing for making concurrent use of the highly photoactive organic semiconductors, and the efficient dielectric screening provided by their inorganic counterparts. In the present work, organic/inorganic nanotube heterojunction arrays composed of TiO2 nanotube arrays and a semiconducting N,N-(dicyclohexyl) perylene-3,4,9,10-tetracarboxylic diimide (PDi) layer were fabricated for photoelectrochemical water splitting. In this arrayed architecture, a PDi layer with a tunable thickness was coated on anodic TiO2 nanotube arrays by physical vapor deposition, which is advantageous for the formation of a uniform layer and an adequate interface contact between PDi and TiO2. The obtained PDi/TiO2 junction exhibited broadened visible light absorption, and an effective interface for enhanced photogenerated electron-hole separation, which is supported by the reduced charge transfer resistance and prolonged excitation lifetime via impedance spectroscopy analysis and fluorescence emission decay investigations. Consequently, such a heterojunction photoanode was photoresponsive to a wide visible light region of 400-600 nm, and thus demonstrated a highly enhanced photocurrent density at 1.23 V vs. a reversible hydrogen electrode. Additionally, the durability of such a photoanode can be guaranteed after long-time illumination because of the geometrical restraint imposed by the PDi aggregates. These results pave the way to discover new organic/inorganic assemblies for high-performance photoelectric applications and device integration.

Study on the Morphology Stability of TiO2 Nanotube Arrays towards Temperature as a Potential Toxic Gas Sensor

TiO2 nanotube arrays were prepared by electrochemical anodization of titanium foil in mixed electrolyte solution of NH4F, water, and glycerol. The anodized TiO2 nanotube arrays were calcined at various temperatures and characterized using scanning electron microscope to study the morphology transformation. The results show that at low calcination temperature (300̊C) highly ordered TiO2 nanotube arrays are observed and remained up to 500̊C. At 700̊C, TiO2 nanotube arrays are completely destroyed and transformed to irregular shaped particle.

Study of cell behaviors on anodized TiO2 nanotube arrays with different structures

Study of cell behaviors on anodized TiO2 nanotube arrays with different structures

Templated dewetting: designing entirely self-organized platforms for photocatalysis.

Formation and dispersion of metal nanoparticles on oxide surfaces in site-specific or even arrayed configuration are key in various technological processes such as catalysis, photonics, electrochemistry and for fabricating electrodes, sensors, memory devices, and magnetic, optical, and plasmonic platforms. A crucial aspect towards an efficient performance of many of these metal/metal oxide arrangements is a reliable fabrication approach. Since the early works on graphoepitaxy in the 70s, solid state dewetting of metal films on patterned surfaces has been much explored and regarded as a most effective tool to form defined arrays of ordered metal particles on a desired substrate. While templated dewetting has been studied in detail, particularly from a mechanistic perspective on lithographically patterned Si surfaces, the resulting outstanding potential of its applications on metal oxide semiconductors, such as titania, has received only limited attention. In this perspective we illustrate how dewetting and particularly templated dewetting can be used to fabricate highly efficient metal/TiO2 photocatalyst assemblies e.g. for green hydrogen evolution. A remarkable advantage is that the synthesis of such photocatalysts is completely based on self-ordering principles: anodic self-organized TiO2 nanotube arrays that self-align to a highest degree of hexagonal ordering are an ideal topographical substrate for a second self-ordering process, that is, templated-dewetting of sputter-deposited metal thin films. The controllable metal/semiconductor coupling delivers intriguing features and functionalities. We review concepts inherent to dewetting and particularly templated dewetting, and outline a series of effective tools that can be synergistically interlaced to reach fine control with nanoscopic precision over the resulting metal/TiO2 structures (in terms of e.g. high ordering, size distribution, site specific placement, alloy formation) to maximize their photocatalytic efficiency. These processes are easy to scale up and have a high throughput and great potential to be applied to fabricate not only (photo)catalytic materials but also a large palette of other functional nanostructured elements and devices.

Low-temperature synthesis of high-ordered anatase TiO2 nanotube array films coated with exposed {001} nanofacets

High-ordered anatase TiO2 nanotube array films coated with exposed high-reactive {001} nanofacets were fabricated by a modified hydrothermal method using amorphous anodic TiO2 nanotube arrays (ATONAs) as starting materials. It was found that the reaction between gas phase HF and solid ATONAs played a key role in the transformation process from amorphous to anatase TiO2, and the TiO2 tubular structure kept unchanged during the surface modification with an exposed {001} facets up to 76.5%, which could be attributed to the low reaction temperature of 130 °C. Our study provided a novel route for the facile preparation of {001} facets exposed anatase TiO2.

Study of Cell Behaviors on Anodized TiO2 Nanotube Arrays with Coexisting Multi-Size Diameters.

It has been revealed that the different morphologies of anodized TiO2 nanotubes, especially nanotube diameters, triggered different cell behaviors. However, the influence of TiO2 nanotubes with coexisting multi-size diameters on cell behaviors is seldom reported. In this work, coexisting four-diameter TiO2 nanotube samples, namely, one single substrate with the integration of four different nanotube diameters (60, 150, 250, and 350 nm), were prepared by repeated anodization. The boundaries between two different diameter regions show well-organized structure without obvious difference in height. The adhesion behaviors of MC3T3-E1 cells on the coexisting four-diameter TiO2 nanotube arrays were investigated. The results exhibit a significant difference of cell density between smaller diameters (60 and 150 nm) and larger diameters (250 and 350 nm) within 24 h incubation with the coexistence of different diameters, which is totally different from that on the single-diameter TiO2 nanotube arrays. The coexistence of four different diameters does not change greatly the cell morphologies compared with the single-diameter nanotubes. The findings in this work are expected to offer further understanding of the interaction between cells and materials.

The Morphological Characterization of Anodic TiO2 Nanotube Arrays.

The Morphological Characterization of Anodic TiO2 Nanotube Arrays.

Obtaining Enhanced Photocurrent by Beneficial Surface Treatment of Self-aligned TiO2 Nanotubes for Dye-sensitized Solar Cells

Abstract Electrochemically anodic TiO2 nanotube (TONT) arrays were prepared in ethylene glycol solvent containing 0.25 wt % NH4F and 2 wt % water under constant voltage (60 V) for 1 h. As-grown TONT arrays exhibited an amorphous phase, and thermal treatment was performed at 450 °C for 3 h under air gas to improve the crystallinity. In addition, to develop water-treated TONT arrays, as-grown TONT arrays were directly placed in water for 3 days. After removing the sample from water, thermal treatment was performed under the same conditions. Further, to prepare the TiCl4-treated TONT arrays (denoted as TiCl4/TONT), the annealed TONT arrays were placed in 0.02 M TiCl4 aqueous solution in a 70 °C electric oven for 30 min. Three types of TONT arrays were prepared and characterized: TONT, TiCl4/TONT, and water/TONT. In the case of TiCl4/TONT arrays, the TiCl4 treatment has a well-known ability to create TiO2 nanoparticles on the TiO2 surface, increasing the surface area for dye uptake and beneficial charge-transport properties. Here, the TiCl4/TONT arrays showed high photocurrent due to the high dye uptake, resulting in 18% improvement of photoelectric conversion efficiency (η). Meanwhile, water/TONT arrays provide the largest surface area due to the small nanoparticles composed of the surface in the nanotubular structure. The beneficial electron-transport properties of water/TONT system through c axis led to an enhancement of up to 40%, approaching η of 2.65%, compared to 1.87% for TONT-based dye-sensitized solar cells.

Effective approach to strengthening TiO2 nanotube arrays by using double or triple reinforcements

Porous anodic TiO2 nanotube arrays (TNTAs) are fragile and also susceptible to be damaged during physical manipulation. Few studies have involved the improvement of the poor interfacial adhesion of TNTAs to the Ti substrate. Here, the poor adhesion of TNTAs was dramatically improved by appending an additional compact layer (ACL) formed at the interface between TNTAs and the Ti substrate. The adhesion of TNTAs with single-ACL increased with the increase of the ACL thickness. Furthermore, the reinforced TNTAs with double-ACL and triple-ACL have been successfully developed for the first time. The experimental results indicated that the critical load of the TNTAs with triple-ACL is roughly 5.8 times higher than that of the untreated TNTAs. The present results may be helpful to assemble less brittle and large area TNTAs for extensive applications.

Observation of anatase nanograins crystallizing from anodic amorphous TiO2 nanotubes

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

Cr2O3 nanoparticles modified TiO2 nanotubes for enhancing visible photoelectrochemical performance.

TiO2 nanotube arrays modified by nanoparticles Cr2O3 with high sensibility in the visible spectrum were prepared by annealing the anodic TiO2 nanotube arrays pre-loaded with Cr(NO3)3 solution which was uniformly clung to the TiO2 nanotube arrays. The influence of the dipping time on the microstructure of the Cr2O3/TiO2-nanotubes was investigated. The microstructure and the elemental analysis were characterized by scanning electron microscope (SEM) and Energy dispersive X-ray (EDX). The photoelectrochemical performances of the as-prepared composite nanotubes were determined by measuring the photogenerated current and voltage under illumination of ultraviolet-visible (UV-vis)/visible light. The TiO2 nanotube arrays modified by Cr2O3 showed higher photocurrent values than those of unmodified TiO2 nanotube arrays. The enhanced photoelectrochemical behaviors can be attributed to the modified Cr2O3 which increases the probability of charge-carrier separation and extends the range of the TiO2 photoresponse from UV to visible region due to the low band gap of 2.3 eV of Cr2O3.

Electricity generation and pollutant degradation using a novel biocathode coupled photoelectrochemical cell.

The photoelectrochemical cell (PEC) is a promising tool for the degradation of organic pollutants and simultaneous electricity recovery, however, current cathode catalysts suffer from high costs and short service lives. Herein, we present a novel biocathode coupled PEC (Bio-PEC) integrating the advantages of photocatalytic anode and biocathode. Electrochemical anodized TiO2 nanotube arrays fabricated on Ti substrate were used as Bio-PEC anodes. Field-emission scanning electron microscope images revealed that the well-aligned TiO2 nanotubes had inner diameters of 60-100 nm and wall-thicknesses of about 5 nm. Linear sweep voltammetry presented the pronounced photocurrent output (325 μA/cm(2)) under xenon illumination, compared with that under dark conditions. Comparing studies were carried out between the Bio-PEC and PECs with Pt/C cathodes. The results showed that the performance of Pt/C cathodes was closely related with the structure and Pt/C loading amounts of cathodes, while the Bio-PEC achieved similar methyl orange (MO) decoloration rate (0.0120 min(-1)) and maximum power density (211.32 mW/m(2)) to the brush cathode PEC with 50 mg Pt/C loading (Brush-PEC, 50 mg). The fill factors of Bio-PEC and Brush-PEC (50 mg) were 39.87% and 43.06%, respectively. The charge transfer resistance of biocathode was 13.10 Ω, larger than the brush cathode with 50 mg Pt/C (10.68 Ω), but smaller than the brush cathode with 35 mg Pt/C (18.35 Ω), indicating the comparable catalytic activity with Pt/C catalyst. The biocathode was more dependent on the nutrient diffusion, such as nitrogen and inorganic carbon, thus resulting in relatively higher diffusion resistance compared to the brush cathode with 50 mg Pt/C loading that yielded similar MO removal and power output. Considering the performance and cost of PEC system, the biocathode was a promising alternative for the Pt/C catalyst.

Photoelectrochemical properties of TiO2 Nanotube Arrays Modified with BiOCl nanosheets

BiOCl nanosheets were deposited on anodized TiO2 nanotube arrays (NTAs) by sequential chemical bath deposition method to get BiOCl/TiO2 NTAs for photoelectrochemical detection of organic compounds (represented by glucose). The structures, elemental components and morphologies of TiO2 and BiOCl/TiO2 NTAs were characterized by using X-ray diffraction diffractometer, scanning electron microscope and transmission electron microscope. The photoelectrochemical behaviors of TiO2 and BiOCl/TiO2 NTAs in the buffer and glucose solutions were measured by cyclic votammetry and amperometry with different optical powers. The modification of BiOCl nanosheets on TiO2 NTAs decreases the photocurrents of TiO2 NTAs in the buffer solution and increases the current response to glucose. Both of the background photocurrent decrease and current response increase are benefit for photoelectrochemical detection of organic compounds. When glucose was used as the target organic compound, the optimized BiOCl/TiO2 NTAs sensor achieved a sensitivity of 0.327μA/μM (0.417μA·cm−2·μM−1), linear range from 0 to 1300μM and calculated detection limit of 5.7μM. Mechanisms of BiOCl modification were studied by measuring the optical absorption and hydroxyl radical HO· productivity. The transfer of holes from TiO2 to BiOCl and the direct oxidation of organic compounds on BiOCl nanosheets led to the decrease of background photocurrent (lower reaction rate of water splitting on BiOCl nanosheets) and the increase of current response to organic compounds (higher reaction rate of direct oxidation of organic compounds).

Interference Colors of TiO<sub>2</sub> Nanotube Arrays Grown by Anodic Oxidation

The study describes the interference color of anodic TiO2 nanotube arrays on titanium foil. TiO2 nanotube arrays are prepared by oxidizing titanium foil in an aqueous electrolyte solution of hydrofluoric acid (HF, 0.5 wt.%). The morphologies of the oxide film are characterized by FESEM. The empirical color properties are obtained using the L*a*b* system. The different interference colors of oxidized Ti samples are obtained depending on the anodization time. The anodization time clearly indicates a strong effect on the reflectance at the visible wavelength region, thus confirming the observed color differences. Consequently, the interference colors can be controlled by the anodization time. By observing the coloration of Ti during the anodization, the interference colors can be utilized to identify the thickness of oxide layer and the formation of nanotubes on Ti sheet.

Development of electrophoretically deposited hydroxyapatite coatings on anodized nanotubular TiO2 structures: Corrosion and sintering temperature

Hydroxyapatite (HA) coatings in and onto anodized TiO2 nanotube arrays were presented and prepared by electrophoretic deposition technique (EPD). Coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). EPD proved to be an innovative and versatile technique to coat HA on and into nanotubular structures of TiO2 with enhanced adhesion between nanotubes and HA particles provided by mechanical interlocking. After EPD of HA on TiO2 layer, samples were sintered at 400°C, 600°C and 800°C for 2h in an Ar atmosphere. Effect of EPD processing parameters on thickness of the deposits and rate of deposition was elucidated for HA coatings on the nanotubular TiO2 structures. It was shown that higher applied voltages increase deposition rate and thickness of the coatings. Potentiodynamic polarization measurements proved corrosion protection caused by both HA coating and nanotubular TiO2 structure in simulated body fluid (SBF). Effect of sintering temperature on adhesion strength of HA coatings on TiO2 nanotubes and their composition were also studied.

An Efficient Photocatalyst: Anodized TiO2 Nanotube Arrays Codoped with Gd–La

Gd–La codoped TiO2 nanotube arrays with diameter of 80–100 nm were successfully synthesized via a hydrothermal method and ion exchange by using anodized titanium oxide as a base reagent. The photocatalytic efficiency of the Gd–La codoped TiO2 nanotube arrays evaluated by photocatalytic degradation of organic dye methyl orange (MO) has been significantly enhanced by twice in comparison to that of the undoped TiO2. Abundant oxygen vacancies and surface defects created by the substitution of Ti4+ for La3+ and Gd3+ in their lattices can accelerate electron trapping and dye adsorption, therefore speeding the separation of photogenerated electron–hole pairs and accelerating the MO photodegradation. The results display the obtained Gd–La codoped TiO2 nanotube arrays will be acted as a very promising photocatalysts for enhancing the photocatalytic activity.

Construction of titania–ceria nanostructured composites with tailored heterojunction for photocurrent enhancement

Titania–ceria (TiO2–CeO2) nanostructured composites based on the design of coating the surfaces of anodized TiO2 nanotube arrays with small band gap CeO2 nanoparticles have been constructed and characterized to demonstrate the effectiveness of the TiO2–CeO2 semiconductor heterojunction in enhancing the photocurrent response of TiO2-based photoelectrodes. The TiO2–CeO2 heterojunction was confirmed to possess conduction and valence band offsets (0.81 and 1.59 eV, respectively) which promote the separation of photoinduced electron–hole pairs. The photocurrent densities of the TiO2–CeO2 composites prepared with low annealing temperatures were about 25–40% larger than that of the anatase TiO2 nanotube arrays. When the nanoparticle-on-nanotube architecture of the TiO2–CeO2 heterostructure was maintained under specific processing conditions, the benefits of having a high specific surface area, a small band gap component capable of absorbing visible light, and a favorable heterojunction were achieved together for photocurrent enhancement.

Effects of amorphous and crystalline MoO3coatings on the Li-ion insertion behavior of a TiO2nanotube anode for lithium ion batteries

Amorphous and crystalline MoO3 coatings are synthesized on anodic TiO2 nanotube arrays by electrodeposition, as a self-standing, binder-free anode material in Li-ion batteries for enhancing the Li-ion insertion performance. The amorphous MoO3 layer is uniform and conformal with a thickness of 10 nm, and is converted into crystalline nanoparticles via thermal treatment. Our results show that both the coated TiO2 nanotubes deliver much higher areal capacities than bare nanotubes or a dense crystalline α-MoO3 film, while the crystalline α-MoO3 coating greatly increases the areal capacity of TiO2 nanotubes compared to the amorphous. The results are obtained at 1340 μA h cm−2 initial capacity for nanotubes with a crystalline coating, 977 μA h cm−2 for those with an amorphous coating and 342 μA h cm−2 for the bare ones. The significant enhancement is due to a combination of MoO3 with high specific capacity and TiO2 nanotube arrays with large surface area allowing uniform MoO3 deposition and rapid ionic transfer. Crystalline α-MoO3 is better than amorphous MoO3 and the coating medium is discussed in terms of chemical state, crystal defects, capacitive contributions and the charge–discharge kinetics in coated TiO2 nanotube electrodes.

High-performance and renewable supercapacitors based on TiO2 nanotube array electrodes treated by an electrochemical doping approach

Although one-dimensional anodic TiO2 nanotube arrays have shown promise as supercapacitor electrode materials, their poor electronic conductivity embarrasses the practical applications. Here, we develop a simple electrochemical doping method to significantly improve the electronic conductivity and the electrochemical performances of TiO2 nanotube electrodes. These TiO2 nanotube electrodes treated by the electrochemical hydrogenation doping (TiO2-H) exhibit a very high average specific capacitance of 20.08 mF cm−2 at a current density of 0.05mAcm−2, ∼20 times more than the pristine TiO2 nanotube electrodes. The improved electrochemical performances can be attributed to ultrahigh conductivity of TiO2-H due to the introduction of interstitial hydrogen ions and oxygen vacancies by the doping. The supercapacitor device assembled by the doped electrodes delivers a specific capacitance of 5.42 mF cm−2 and power density of 27.66 mW cm−2, on average, at the current density of 0.05mAcm−2. The device also shows an outstanding rate capability with 60% specific capacitance retained when the current density increases from 0.05 to 4.00mAcm−2. More interestingly, the electrochemical performances of the supercapacitor after cycling can be recovered by the same doping process. This strategy boosts the performances of the supercapacitor, especially cycling stability.