Anatase TiO2 Nanotubes Research Articles

Topotactic conversion route to ultrafine crystalline TiO2 nanotubes with optimizable electrochemical performance

Anatase TiO2 nanotubes have been successfully synthesized via a topotactic chemical transformation from H2Ti3O7 nanotubes, in which the primary H2Ti3O7 nanotube frameworks can be preserved. The TiO2 nanotubes have an average outer diameter of around 8 nm and the inner diameter around 3–4 nm with 2 micrometers in length. Importantly, the nanotubes' shells are mainly formed by (113) planes perpendicular to the long axial of nanotubes, which is different from the structures found in the literature. The electrochemical properties are investigated by constant current discharge/charge measurements. There are potential plateaus at 1.75 and 1.93V in the process of Li insertion and extraction of the TiO2 nanotubes, and the initial Li insertion/extraction capacity is 590 and 298 mA h g−1 at 30 mA g−1, respectively. In the charge–discharge capacities of TiO2 nanotubes (calcination at 500 °C) at different current rates, the reversible capacity still remains at about 165 and 139 mA h g−1 at current density of 480 and 960 mA g−1, respectively with excellent coulombic efficiency of approximately 100%.

Preparation and Electrochemical Properties of Graphene-modified TiO2 (Anatase) Nanotubes Composite for Lithium-ion Batteries

TiO2 nanotubes decorated with graphene hybrid material was prepared via a green route, namely, both synthesis of nano tubular TiO2 and reduction of GO were simultaneously carried out in alkali solution under solvothermal conditions. The as-prepared nanocomposite was used for investigation of Li-ion insertion properties. TEM analysis highlights the excellent crystallinity of the anatase TiO2 nanotubes, 5∼7 nm in external diameter and several hundred nanometers in length, loaded on graphene sheets. The composite exhibits a high capacity and about 0.8 Li per TiO2 can be delivered in the first discharge and charge at a current density of 0.1 C, larger than the values for the TNs (0.6 Li per TiO2). The electrochemical performance of the composite is superior to that of the bare TNs anode, indicating a positive synergistic effect of nanotubular TiO2 and graphene.

Confined crystallization of anatase TiO2 nanotubes and their implications on transport properties

Nanotubes of TiO2 (anatase) and their ordered arrays are emerging, promising candidates as efficient host materials in many applications such as photovoltaic cells, batteries, sensors and catalysts/catalytic supports, but the interplay between these structures and their transport properties has been reported only rarely. Monodisperse, stoichiometric TiO2 nanotubes with smooth morphology and controlled wall thickness were fabricated by template-directed low-temperature atomic layer deposition (ALD), followed by annealing at elevated temperatures. We present a study on the wall thickness-dependent crystallization behaviors due to physical and/or self-confinement, as well as on the corresponding electrical properties. Over certain wall thicknesses, unexpectedly, our TiO2 nanotubes were found to be a new type of mesoporous wide gap semiconductor in which they possess similar porosity, but in terms of conductivity differ from previously known mesoporous photoanodes (i.e., anodized surfaces of Ti films and sintered films consisting of TiO2 nanoparticles). These results were ascribed to the large, elongated anatase domains (by a factor of up to 15–40 wall thicknesses) that developed via boosted crystal growth on porous alumina templates (physical confinement) as well as to the highly curved tubular shape (self-confinement). Indeed, nanotube arrays with walls thicker than 10 nm exhibited an enhancement in conductivity, by more than three orders of magnitude, compared to sintered, mesoporous TiO2 (anatase) particles, approaching the bulk value. The nearly single-crystalline TiO2 nanotubes presented here should allow for a good model system to study TiO2-based surface chemistry and have potential for many applications in photovoltaic and/or catalytic systems.

N-Doped lepidocrocite nanotubular arrays: hydrothermal formation from anodic TiO2nanotubes and enhanced visible light photoresponse

New Cs-lepidocrocite nanotubular arrays were successfully prepared from anodically grown TiO2 nanotubes by hydrothermal conversion using CsOH in a water–ethanol mixed solvent. The obtained open nanotubular structure consists of plate-like nano-crystals, reflecting a preferred facet growth of the lepidocrocite with a 2-dimensional layered crystal structure. N-Doping of the Cs-lepidocrocite nanotubular layer was carried out by annealing in NH3 gas. The N-doped sample showed an incident photon to current conversion efficiency (IPCE) of 1.43% at 450 nm, which is 57% higher than conventional N-doped anatase TiO2 nanotubes.

Controlled synthesis of anatase TiO2 nanotube and nanowire arrays via AAO template-based hydrolysis

A novel and facile method was successfully developed to fabricate nanotube (NT) and nanowire (NW) arrays through an anodic aluminum oxide (AAO) template-based Ti(OC4H9)4 hydrolysis process. The wall thickness of the TiO2 NTs can be controlled by adjusting the concentration of Ti(OC4H9)4 solutions. A continuous series from thin-walled NTs to completely filled NWs were synthesized using Ti(OC4H9)4 solutions of different concentrations from 0.1 to 2.0 M. The TiO2 NTs and NWs in the AAO membrane transformed from amorphous to nanocrystalline anatase completely after annealing at 650 °C, which is higher than the phase transformation temperature of TiO2 powder. Optical spectra of the synthesized TiO2 NT and NW arrays/AAO composite nanostructures were characterized and showed a blue shift of the optical absorption edge with the decrease of NT wall thickness, due to the nanoscale effect and shape dependent transformation.

Fabrication of Highly Ordered Arrays of TiO<sub>2</sub> Nanotubes on Ti Wires

Highly Ordered arrays of TiO2 nanotubes on Ti wire were prepared by anodic oxidation with ethylene glycol solution of NH4F electrolyte. The phase compositions of the samples were characterized using X-ray diffraction (XRD) analysis using Cu-Kα radiation. The microstructure was observed using scanning electron microscopy (SEM). Experimental results show that at the anode condition of 95V, 50°C, and then themal treatment at 550°C, the highly ordered anatase TiO2 nanotubes arrays arranged on the surface of Ti wire. The length of the nanotube achieved was about 31μm, the internal diameter and outer diameter of the nanotube was 127nm and 170nm, respectively.

Magnetically separable γ-Fe2O3/TiO2 nanotubes for photodegradation of aqueous methyl orange

Magnetically separable γ-Fe2O3/TiO2 nanotubes catalyst was successfully prepared by the confined reaction of liquid Fe(CO)5 in/around TiO2 nanotubes. The hybrids were systematically characterized and showed strong absorptions in the visible region. Under simulated sunlight and the same experimental conditions, 91% of methyl orange (MO) can be degraded by the hybrids, whereas only 49%, 59% and 81% of the MO can be degraded in the presence of anatase TiO2 powder, commercial P25, and TiO2 nanotubes. Importantly, the γ-Fe2O3/TiO2 nanotubes catalyst can be recycled by external magnetic field for repeated use without obvious loss of its photocatalytic efficiency.

Iron-doped TiO2 nanotubes with high photocatalytic activity under visible light synthesized by an ultrasonic-assisted sol-hydrothermal method

Abstract A series of iron-doped anatase TiO2 nanotubes (Fe/TiO2 NTs) catalysts with iron concentrations ranging from 0.88 to 7.00 wt% were prepared by an ultrasonic-assisted sol-hydrothermal process. The structures and the properties of the fabricated Fe/TiO2 NTs were characterized in detail and photocatalytic activity was examined using a reactive brilliant red X-3B aqueous solution as pollutant under visible light. The lengths of the NTs were determined to range from 20 nm to 100 nm. The incorporation of the iron ions (Fe3+) into the TiO2 nanotubes shifted the photon absorbing zone from the ultraviolet (UV) to the visible wavelengths, reducing the band gap energy from 3.2 to 2.75 eV. The photocatalytic activity of the Fe/TiO2 NTs was 2–4 times higher than the values measured for the pure TiO2 nanotubes.

A DFT study of gas molecules adsorption on the anatase (0 0 1) nanotube arrays

First-principles density functional theory (DFT) calculations have been performed on the structural and electronic properties of anatase (001) TiO2 nanotube arrays (TNTAs) without and with the adsorption of various gas molecules including O2, H2, CO, NO, H2O, CO2, NO2, N2O, NH3 and CH4 in details. Our study shows that the (12,0) and (18,0) TNTAs are semiconductors with indirect band gaps higher and lower than that of the bulk TiO2, respectively. The adsorption of the same gas molecule on (18,0) TNTAs is relatively stronger than that on (12,0) TNTAs due to the surface curvature effect. H2O and NH3 are chemisorbed while other gases are physisorbed. In addition, all the studied gases are charge donors to the TNTAs. The electronic and transport properties of both TNTAs are sensitive to the adsorption of certain gases such as O2, NO and NO2. In contrast, other gases have little effect on modifying the electronic structures of TNTAs.

Carbon-doped anatase TiO2 nanotube array/glass and its enhanced photocatalytic activity under solar light

To enhance the photocatalytic activity under solar light, highly ordered TiO2 nanotube arrays (TNAs) film with anatase phase was fabricated on glass and successfully doped with carbon at various temperatures of 450–550 °C. The characterization results indicate that, after carbon doping, the TNAs still remained nanotubular structure with anatase phase. But their optical response shifted from UV to the visible light region and the recombination of photogenerated carriers was suppressed effectively. It is more important that the carbon-doped TNAs/glass (C-TNAs) samples exhibited high solar light photocatalytic activity, and 68%, 61% and 56% MO was photodegraded in 150 min by the C-TNAs calcined at 550, 500 and 450 °C, respectively. Especially, the apparent reaction rate constant of C-TNAs calcined at 550 (k, 0.065 min−1) with the highest activity is 3.6 times that of pristine anatase TNAs (k, 0.018 min−1). It is clear that carbon doping enhanced the photocatalytic activity under sunlight at optimized annealing temperature. The efficient activity could be attributed to the synergetic effects of strong visible light absorption, good crystallization, large surface, and enhanced separation of photoinduced carriers.

Ab initio modeling of sulphur doped TiO2 nanotubular photocatalyst for water-splitting hydrogen generation

In order to construct an efficient visible-light-driven TiO2 photocatalyst for water splitting applications, one has to perform improvements of its electronic structure. In this theoretical study we consider single-walled anatase TiO2 nanotubes having following morphologies: (101) 3-layered wall with chirality indexes (n,0) and (n,n), (101) 6-layered wall with (n,0) and (0,n), (001) 6-layered wall with (n,0) and (0,n), and (001) 9-layered wall with (n,0) and (0,n). The latter configuration occurs to be the most energetically stable, due to possessing negative strain energy. In our study the most stable 9-layered anatase (001) (0,n) nanotube has been doped with sulphur. According to obtained results sulphur dopant creates the mid-gap states making the TiO2 nanotube to be a good candidate for efficient photocatalyst working under day light irradiation.

Fabrication of ZnxCd1–xSe Nanocrystal-Sensitized TiO2 Nanotube Arrays and Their Photoelectrochemical Properties

ZnxCd1–xSe (0 ≤ x ≤ 1) nanocrystals were pulse electrodeposited onto the surface of anatase TiO2 nanotube arrays for the first time. The pulse-electrodeposition duty cycle plays a determinative role during the nuclei formation and growth process of the ZnxCd1–xSe nanocrystals. The composite ZnxCd1–xSe/TiO2 nanotube arrays exhibit high absorption in the visible light region due to the narrow band gap of ZnxCd1–xSe and demonstrate a sensitive photoelectrochemical response under visible light illumination with an optimal response achieved for Zn0.8Cd0.2Se/TiO2 electrodes. In photoelectrochemical testing, 9-anthracene carboxylic acid acts as a hole donor resulting in a decrease of the measured photocurrent response. In contrast, 2-anthramine acts as an electron donor, resulting in an increase of the photocurrent response.

Synthesis and characterization of anatase TiO2 nanotubes with controllable crystal size by a simple MWCNT template method

Highly crystalline phase-pure anatase TiO2 nanotubes were produced by performing a simple calcinations process in air of the TiO2 coated MWCNTs composite material prepared by an in situ sol–gel method. A homogeneous thin coating layer of TiO2 on the surface of MWCNTs is firstly obtained. Calcination in ambient air at appropriate temperatures is performed to transform phase of TiO2 into anatase and remove MWCNTs simultaneously, resulting in pure anatase TiO2 nanotubes. Both end tips of the nanotubes are observed to be closed, probably due to covering up MWCNTs with TiO2 particles. The crystallization of anatase phase was formed upon 350°C, and MWCNTs are completely oxidized between 500 and 650°C, leaving anatase TiO2 nanotubes with an average crystal size increasing from about 8 to 24nm as the temperature rises. Moreover, the tubular structure was found to collapse after calcinations at 700°C.

The Effect of Bath Temperature on the Adhesive Strength and Bioactivity of TiO<sub>2</sub> Nanotube Arrays

The effect of bath temperature on the morphologies, crystallographic structure and adhesive strength of TiO2 nanotube arrays fabricated by electrochemical anodization of titanium in dimethyl sulfoxide containing hydrofluoric acid electrolyte were studied in this paper. The results show that the crystalline anatase TiO2 nanotube arrays can be directly fabricated at the bath temperature above 50oC without further annealing. The crystallinity of anatase at the top of nanotubes is higher than that at the bottom of nanotubes. The bath temperature has obviously impact on the surface morphology of the titanium substrate underneath the nanotube arrays. The adhesive strength of TiO2 nanotube arrays increases with the rise of bath temperature increasing, and reaches 11.3 MPa when TiO2 nanotube arrays are fabricated at 60 oC.

Pseudocapacitive Lithium-Ion Storage in Oriented Anatase TiO2 Nanotube Arrays

We report on the synthesis and electrochemical properties of oriented anatase TiO2 nanotube (NT) arrays as electrodes for Li-ion batteries. The TiO2 NT electrodes displayed both pseudocapacitive Li+ storage associated with the NT surface and the Li+ storage within the bulk material. The relative contribution of the pseudocapacitive and bulk storages depends strongly on the scan rate. While the charges are stored primarily in the bulk at low scan rates (≪1 mV/s), the surface storage dominates the total storage capacity at higher scan rates (>1 mV/s). The storage capacity of the NT electrodes as a function of charge/discharge rates showed no dependence on the NT film thickness, suggesting that the Li+ insertion/extraction processes occur homogeneously across the entire length of NT arrays. These results indicated that the electron conduction along the NT walls and the ion conduction within the electrolyte do not cause significant hindering of the charge/discharge kinetics for NT electrode architectures. As ...

Processing-Induced Modification of Photo- and Cathodoluminescence Spectra of TiO2 Nanotubes

Cathodoluminescence and photoluminecsence are investigated in titanium dioxide nanotubes (TiO2 NTs) produced in an ethylene glycol based electrolyte with the variation of the temperature of the electrolyte which allows the control of the inner diameter of nanotubes. The crystal structure of the produced nanotubes is controlled by the post-anodization thermal treatment. A new fast and cost-effective method for the fabrication of anatase TiO2 NTs is proposed. The cathodoluminescence spectra and panchromatic images from the produced TiO2 nanotubes suggest the formation of resonator modes in clusters of nanotubes. The enhancement of the ultraviolet near-bandgap or the green photoluminescence is demonstrated by means of coating the anatase TiO2 nanotubes with thin Ag or Au films, respectively.

Adsorption of water on single-walled TiO2 nanotube: A DFT investigation

We have performed a density functional theory (DFT) study based on the periodic method to provide a detailed description of the electronic and adsorption properties of single-walled TiO2 anatase nanotubes. Both (n, 0) and (m, 0) TiO2 nanotubes, with diameters ranging from 9.8Å to 23Å, are considered for the study of structure and adsorption processes. Our calculated results show that the molecular and dissociative adsorptions of water on TiO2 nanotubes are exothermic. However, molecular adsorption is energetically favorable. Molecular adsorption of the outer surface of (n, 0) tube is energetically favorable and molecular adsorption of the inner surface of (0, m) tube is energetically favorable. Furthermore, we found that the surface curvature of TiO2NTs plays a major role in water splitting and a minor role in the molecular adsorption of water on the surface of the tubes. These results are interpreted in terms of the peculiar electronic properties of the TiO2 nanotube during the examination of the interaction among water molecules on the TiO2 nanotubes surface.

Anatase Nanotubes as an Electrode Material for Lithium-Ion Batteries

Anatase TiO2 nanotubes were synthesized via the hydrothermal method followed by annealing at 500 °C in argon for 1 h. The phase structure, morphology, and composition were investigated in detail by means of X-ray diffraction, scanning and high-resolution transmission electron microscopy, infrared spectroscopy, and thermal analysis. The material consists of nanotubes with diameter of 10–15 nm and lengths of several hundred nanometers. The electrochemical properties were investigated by cyclic voltammetry and galvanostatic cycling. The data imply a first cycle irreversible capacity of 385 mAh/g, and capacities of 307 and 265 mAh/g after the second and 50th cycle, respectively, at C/10. The Coulombic efficiency of about 99% after cycle 50 implies excellent cycling stability. Hence anatase TiO2 nanotubes evidence great potential for usage in high-power lithium-ion batteries.

Analysis of the Electron Transport Properties in Dye-Sensitized Solar Cells Using Highly Ordered TiO<SUB>2</SUB> Nanotubes and TiO<SUB>2</SUB> Nanoparticles

This study uses TiO2 nanoparticles and highly ordered anatase TiO2 nanotubes (AOTnt) as thin film photoanodes for dye-sensitized solar cells (DSSCs). DSSCs are assembled by single-layer and double-layer films of photoanodes and their electron transfer performance is compared. TiO2 nanoparticles were fabricated by the sol-gel method, and AOTnts were grown on titanium foil. This study uses TiO2 nanoparticles or AOTnts to prepare single-layer photoanodes and TiO2 nanoparticles coated on an AOTnt film to fabricate double-layer photoanodes. These three different photoanodes are soaked in dye and assembled into DSSCs, and their open-loop voltage recession, electrochemical impedance, lifetime, life cycle, and effective diffusion coefficient are measured. Electron transfer efficiency of the photoanodes and light harvesting efficiency are further analyzed. The results show that the electron transfer efficiency, open-loop voltage recession, lifetime, life cycle, and effective diffusion coefficient of the DSSCs assembled using double-layer photoanodes (AOTnt-TiO2) are superior to those of single-layer photoanodes (TiO2 or AOTnt).

Removal of Uranium(VI), Lead(II) at the Surface of TiO2 Nanotubes Studied by X-Ray Photoelectron Spectroscopy

A thin film of well-ordered anatase TiO2 nanotubes prepared by anodic oxidation of titanium metal were synthesised and used as adsorbent medium for the purification of water from aqueous uranium and lead. The amount of subtracted metal ions was quantified by using X-ray photoelectron spectroscopy at the surface of the reacted TiO2 surface. Batch experiments for the sorption of U and Pb at the surface of the titania substrate were carried out in separated solution equilibrated with air of uranyl acetate and lead nitrate, in the pH range 3–9. For uranium, the experiments were also repeated in anoxic (N2) atmosphere. The amount of metal ions adsorbed onto the titania medium was quantified by measurements of the surface coverage expressed in atomic percent, by recording high-resolution XPS spectra in the Ti2p, U4f and Pb4f photoelectron regions. Adsorption of the uranyl species in air atmosphere as a function of pH showed an adsorption edge near pH 4 with a maximum at pH 7. At higher pH the presence of very stable uranyl–carbonate complexes prevented any further adsorption. Further adsorption increased until pH 8.5 was obtained when the uranyl solution was purged from dissolved CO2. Lead ion showed a sorption edge at pH 6, with a maximum uptake at pH 8. The results showed that the uptake of uranium and lead on the selected titania medium is remarkably sensitive to the solution pH. This study demonstrates the reliability of this type of material for treating water polluted with heavy metals as well as leachates from radioactive nuclear wastes.