Field emission from the structure of well-aligned TiO2/Ti nanotube arrays

Well-aligned TiO₂/Ti nanotube arrays were synthesized by anodic oxidation of titanium foil in 0.5 wt.% HF in various anodization voltages. The images of filed emission scanning electron microscopy indicate that the nanotubes structure parameters, such as diameter, wall thickness and density, can be controlled by adjusting the anodization voltage. The peaks at 25.3° and 48.0° of X-ray diffraction pattern illuminate that the TiO₂ nanotube arrays annealed at 500 °C are mainly in anatase phase. The filed emission (FE) properties of the samples were investigated. A turn-on electric field 7.8 V/µm, a field enhancement factors approximately 870 and a highest FE current density 3.4 mA/cm² were obtained. The emission current (2.3 mA/cm² at 18.8 V/µm) was quite stable within 480 min. The results show that the FE properties of TiO₂/Ti have much relation to the structure parameters.     

A facile template-free method for preparing bi-phase TiO2 nanowire arrays with high photocatalytic activity

Ordered bi-phase TiO₂ nanowire arrays were simply obtained by heat treating TiO₂ nanotube arrays prepared by a two-step anodization method. The nanowire arrays are composed of anatase and rutile phases with uniform diameters around 50 nm. The photocatalysis activities of TiO₂ nanowire arrays were characterized by quantifying the degradation of methyl orange solution. And the results indicated that the bi-phase nanowire arrays, especially obtained at 700 °C, showed much higher activity than that of P25 film or anatase TiO₂ nanotube array.     

Influence of secondary anodization voltage on free-standing crystallized TiO2 nanotube array membrane

Vertically aligned, free-standing crystallized TiO₂ nanotube arrays with a length of 32 μm have been fabricated by a two-step anodization method. The TiO₂ nanotube membrane can be detached from the Ti substrate through the secondary anodization process. The influence of the secondary anodization voltage on the morphology, crystalline phase and photovoltaic performance of the as-fabricated samples has been investigated. Results show that the side wall of TiO₂ nanotubes becomes obviously thin as the secondary anodization voltage increases and leads to crack when the voltage reaches 25 V. The mass fraction of the anatase reduces by the increase of the voltage. Furthermore, the dye-sensitized solar cells (DSSCs) based on TiO₂ nanotube arrays have been assembled. The energy conversion efficiency decreases with the increase of secondary anodization voltage, and a highest energy conversion efficiency of 10.6 % under UV illumination (368.1 nm) is obtained from the cell with TiO₂ nanotube membrane re-anodizad at 15 V.     

Fabrication of TiO2 nanotube arrays by electrochemical anodization in an NH4F/H3PO4 electrolyte

Highly ordered TiO₂ nanotube arrays were fabricated by electrochemical anodization of titanium in an NH₄F/H₃PO₄ electrolyte. A TiO₂ crystal phase was identified by X-ray diffraction, and the morphology, length and pore diameter of the TiO₂ nanotube arrays were determined by field-emission scanning electron microscopy (FE-SEM). The anodization parameters including the rate of magnetic stirring, F⁻ concentration, calcination temperature, anodization voltage and anodization time were investigated in detail. The results show that the as-prepared TiO₂ nanotube arrays possessed good uniformity, a well-aligned morphology with a length of 750 nm and an average pore diameter of 62 nm at a 150 rpm rate of magnetic stirring for 120 min at 20 V in an electrolyte mixture of 0.2 M H₃PO₄ and 0.3 M NH₄F with a 500 °C calcination to obtain 100% anatase phase. The adsorption of N-719 dye at different tube lengths was determined by UV-vis analysis and found to increase with increasing tube length. We also discuss the formation mechanism of the TiO₂ nanotube arrays. The findings indicate that the formation of the TiO₂ nanotube arrays proceeds by the combined action of the electrochemical etching and chemical dissolution.     

Fabrication of TiO2 nanotubes by anodization of Ti thin films for VOC sensing

Ti thin films were anodized in aqueous HF (0.5 wt.%) and in polar organic (0.5 wt.% NH₄F + ethylene glycol) electrolytes to form TiO₂ nanotube arrays. Ti thin films were deposited on microscope glass substrates and then anodized. Anodization was performed at potentials ranging from 5 V to 20 V for the aqueous HF and from 20 V to 60 V for the polar organic electrolytes over the temperatures range from 0 to 20 °C. The TiO₂ nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). It has been observed that anodization of the deposited Ti thin films with aqueous HF solution at 0 °C resulted in nanotube-type structures with diameters in the range of 30-80 nm for an applied voltage of 10 V. In addition, the nanotube-type structure is observed for polar organic electrolyte at room temperature at the anodization voltage higher than 40 V. The volatile organic compound (VOC) sensing properties of TiO₂ nanotubes fabricated using different electrolytes were investigated at 200 °C. The maximum sensor response is obtained for carbon tetrachloride. The sensor response is dependent on porosity of TiO₂. The highest sensor response is observed for TiO₂ nanotubes which are synthesized using aqueous HF electrolyte and have very high porosity.     

Photocatalytic characteristics of TiO2 nanotubes with different microstructures prepared under different pulse anodizations

In this work, different positive voltages of a pulsed waveform, time intervals and electrolyte stirring were used to prepare by anodization of pure titanium plates, a series of TiO₂ thin films based on nanotubes with different morphologies and dimensions. Electrolyte stirring results in large size TiO₂ nanotubes with uneven surface morphology and less oriented directions. The titanium plates containing the TiO₂ thin films were further annealed at 450 °C for 30 min under air. It was found that only crystalline anatase was formed under this condition. Both methylene blue degradation and antibacterial tests against Escherichia coli were performed to evaluate the photocatalytic performance of these TiO₂ films. Better methylene blue degradation ability was achieved by TiO₂ nanotubes prepared under electrolyte stirring. However, the antibacterial ability of the annealed TiO₂ nanotubes was affected by their inner diameter rather than their length. It is also concluded that the anodized TiO₂ nanotube arrays fabricated in this work are promising for photo-induced methylene blue degradation and bacteria killing applications.     

A Two-step anodization to grow high-aspect-ratio TiO2 nanotubes

High-aspect-ratio TiO₂ nanotubes with small diameter are favored in dye-sensitized solar cells for large dye loading provided by high surface areas. However, long TiO₂ nanotubes with small diameter are difficult to grow under usual anodizing conditions due to unavoidable chemical dissolution of the top portion of the as-fashioned tubes. In the present work, two kinds of double-layered TiO₂ nanotube arrays were prepared by changing voltage from high to low (i.e., from 30 V to 15 V) or from low to high (i.e., from 15 V to 30 V). It is found that the top layer can serve as a sacrificial layer to protect the continuous growth of the bottom layer from chemical dissolution. Accordingly, the two-step anodization from high voltage to low voltage is proposed to produce high-aspect-ratio TiO₂ nanotubes with small diameter underneath a sacrificial top layer.     

Photocatalytic reduction of methylene blue by TiO2 nanotube arrays: effects of TiO2 crystalline phase

TiO₂ nanotube arrays were synthesized by anodization of Ti metal sheets followed by thermal annealing at elevated temperatures from 400 to 600 °C. Scanning electron microscopic measurements showed that dense arrays of nanotubes were produced with the inner diameter about 100 nm, wall thickness 35 nm, and length about 10 μm. X-ray diffraction measurements showed that the as-prepared nanotubes were largely amorphous, whereas thermal annealing led to the formation of well-defined anatase crystalline phase. More interestingly, at 470 °C, the brookite crystalline phase also started to emerge, which became better defined at 500 °C and disappeared eventually at higher temperatures, a phenomenon that has not been observed previously in TiO₂ nanotube arrays prepared by anodization. The impacts of the TiO₂ nanocrystalline structure on the photocatalytic activity were then examined by using the reduction of methylene blue in water as an illustrating example. Upon exposure to UV lights, the visible absorption profiles of methylene blue exhibited apparent diminishment. Based on these spectrophotometric measurements, the corresponding pseudo-first-order rate constant was estimated, and the sample thermally annealed at 500 °C was found to exhibit the highest activity. The strong correlation between the TiO₂ crystalline characteristics and photocatalytic performance suggests that the synergistic coupling of the anatase and brookite crystalline domains led to effective charge separation upon photoirradiation and hence improved photocatalytic activity, most probably as a consequence of the vectorial displacement at the nanoscale junctions between these crystalline grains that impeded the dynamics of electron–hole recombination. These results demonstrate the significance of nanoscale engineering in the manipulation of oxide photocatalytic performance.     

Synthesis and characterization of anodized titanium-oxide nanotube arrays

Anodized titanium-oxide containing highly ordered, vertically oriented TiO₂ nanotube arrays is a nanomaterial architecture that shows promise for diverse applications. In this paper, an anodization synthesis using HF-free aqueous solution is described. The anodized TiO₂ film samples (amorphous, anatase, and rutile) on titanium foils were characterized with scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. Additional characterization in terms of photocurrent generated by an anode consisting of a titanium foil coated by TiO₂ nanotubes was performed using an electrochemical cell. A platinum cathode was used in the electrochemical cell. Results were analyzed in terms of the efficiency of the current generated, defined as the ratio of the difference between the electrical energy output and the electrical energy input divided by the input radiation energy, with the goal of determining which phase of TiO₂ nanotubes leads to more efficient hydrogen production. It was determined that the anatase crystalline structure converts light into current more efficiently and is therefore a better photocatalytic material for hydrogen production via photoelectrochemical splitting of water.     

Nanostructured surface changes of Ti-35Ta-xZr alloys with changes in anodization factors

The purpose of this study was to investigate the changes of the nanostructured surface of Ti-35Ta-xZr alloys for dental application resulting from changes in anodization factors. TiO₂ nanotubes were formed on Ti-35Ta-xZr alloys by anodization in H₃PO₄-containing NaF solutions. Anodization was carried out using a scanning potentiostat. Microstructures of the alloys were examined by optical microscopy (OM), field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD). Microstructures of the Ti-35Ta-xZr alloys were changed from α" phase to β phase, and morphologies changed from a needle-like to an equiaxed structure, with increasing Zr content. As the Zr content increased from 3 to 7 to 15 wt.%, the average thickness of the TiO₂ nanotubes increased from 4.5 μm to 6.1 μm to 9.0 μm. When the anodizing potential was increased from 3 V to 10 V, the thickness of the nanotube layers increased from about 0.5 μm to 9.5 μm. As the anodization time increased from 30 min to 180 min at 10 V, the nanotube thickness increased from 4 μm to 9.5 μm. The amorphous oxide phase in the nanotubes transformed to anatase and rutile phases of TiO₂ by heat treatment above 300 °C.     

Preparation of free–standing transparent titania nanotube array membranes

在由乙二醇、水、氟化铵组成的电解液中添加钼酸钠调节阳极附近的离子浓度, 制备出厚度大约为10微米的透明二氧化钛纳米管阵列薄膜. 所得二氧化钛是无定型结构, 在120℃水热处理可以将其转化成锐钛矿结构, 并保持薄膜的结构完整性. 该薄膜的透射率与其表面结构和晶体结构有关. 这种透明二氧化钛纳米管阵列薄膜可望应用于染料敏化太阳能电池.     

Structure induced optical properties of anodized TiO2 nanotubes

TiO₂ nanotubes were synthesized by means of anodization and investigated for their structure dependent optical properties. The anodization was conducted at operating voltages between 5 and 30 V for 3 h in a neutral, organic electrolyte consisting of 0.3 wt% NH₄F + 2 wt% H₂O + ethylene glycol and the resulting nanotubes were annealed at 450 °C for 2 h in air at atmospheric pressure. It is shown that an increase in the applied anodization voltage yielded an increase in the wall thickness, diameter and length of the nanotubes and that these varying morphologies have a direct influence on the crystallite size of the nanotubes during annealing. Photoluminescence spectra indicated that the optical bandgap of the TiO₂ nanotube film decreased with the increase in the anodization voltage, whereas supplementary Raman spectra showed a decrease in the confinement of the optical phonon modes as the crystallite sizes increased, in coherence with the phonon confinement model. These results present significant insights into the size-dependent properties of these novel nanostructured forms of TiO₂ and play an important role in their implementation in photovoltaic devices, such as the dye-sensitized solar cell.     

Fabrication and characterization of self-organized mixed oxide nanotube arrays by electrochemical anodization of Ti-6Al-4V alloy

Self-organized mixed oxide nanotube arrays were fabricated by anodization of Ti-6Al-4V alloy in H₃PO₄/NH₄F aqueous solution. The nanotubes of 90-180 nm in diameter and 10-20 nm in wall thicknesses could be tuned by changing anodization voltages. Whereas, the as-prepared nanotube arrays were amorphous; to induce crystallinity, the products were annealed at 400 °C, 500 °C and 600 °C, respectively. The UV-Vis spectra of samples annealed at 600 °C gives the maximum absorption in the visible spectra range. Various characterization techniques (viz., FESEM, XPS, XRD, and UV-Vis) were used to study the morphology, composition, phase and band gap of the films.     

Synthesis and electrophoretic deposition of hydrothermally synthesized multilayer TiO2 nanotubes on conductive filters

TiO₂ nanotubes were synthesized by the decomposition of titanium isopropoxide in water and the calcination at 450 °C for 2 h to form TiO₂ nanoparticles. The synthesized TiO₂ in anatase form nanoparticles were processed hydrothermally in 10 M NaOH solution at 130 °C for 24 h to obtain multilayer TiO₂ nanotubes. TEM analysis revealed that the diameters of the tubes were around 10 nm and they are in the length of 100 nm. Subsequently, colloidal suspensions containing 1% wt. Of TiO2 nanotubes were prepared with TEA and butanol and electrophoretic deposition (EPD) experiments were conducted in order to obtain coatings on Ni and carbon filters using a deposition time of 10 min. and an applied voltage of 65 V. It is also shown that multilayer TiO₂ nanotubes having outer diameter around 10 nm and inner diameters of 4.3 nm can be produced using the described technique. EPD is also shown to be an effective technique to coat three dimensional components, such as Ni and C filters for various applications including water and air purification systems.     

Nanoscale investigation on large crystallites in TiO2 nanotube arrays and implications for high-quality hybrid photodiodes

Anodized TiO₂ nanotube arrays fabricated on a TiO₂ thin film on conducting glass substrates can be readily implemented in diverse applications like hybrid solar cells. In this study, we concentrate on morphologies with inner tube diameter being around 30 nm which is in dimension of the exciton diffusion length of common organic hole conductors. Cross-sectional preparation of the intact tube array in correlation with transmission electron microscopy has been performed to get local information on the TiO₂ nanotubes and their arrangements, depending on anodization voltage. Crystallites have been found to be anatase and in size of several hundred nanometers along tube walls with increasing length for increasing anodization voltages. Inter-tube connections with similar crystal orientations of adjacent tubes are found. These give rise to large areas of uniform orientation. Thus, the number of grain boundaries within the film is low compared to the reported values for different TiO₂-polymer material systems. Using the arrays, hybrid TiO₂ solar cells were fabricated, which show high fill factors indicating good electron transport. The results suggest high electron mobility and are encouraging for a utilization of the nanotube arrays in next generation photovoltaics.     

Annealing study of titanium oxide nanotube arrays

Titanium dioxide nanotube arrays fabricated by anodization of titanium foil and annealed at different temperatures were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and positron annihilation spectroscopy (PAS). The crystallization process and morphological changes of the nanotubes have been discussed. It was found that anatase (1 0 1) only appeared on the walls of the nanotubes. The atomic concentration of fluoride and the ratio of Ti/O decreased when the annealing temperature increased. Vacancy type defects were found to diffuse toward the surface when the samples were annealed at 200 °C and 400 °C and healing of vacancies occurred at 600 °C. In addition, fluoride may form some complexes with vacancies on the surface hence lowering the value of S parameter.     

Hydrothermal synthesis of 1D TiO2 nanostructures for dye sensitized solar cells

Mono-dimensional titanium oxide nanostructures (multi-walled nanotubes and nanorods) were synthesized by the hydrothermal method and applied to the construction of dye sensitized solar cells (DSCs). First, nanotubes (TiNTs) and nanotubes loaded with titanium oxide nanoparticles (TiNT/NPs) were synthesized with specific surface areas of 253 m²/g and 304 m²/g, respectively. After that, thermal treatment of the nanotubes at 500 °C resulted in their transformation into the corresponding anatase nanorods (TiNT-Δ and TiNT/NPs-Δ samples). X-ray diffraction and Raman spectroscopy data indicated that titanium oxide in the pristine TiNT and TiNT/NP samples was converted into anatase phase TiO₂ during the heating. Additionally, specific surface areas and water adsorption capacities decreased after the heat treatment due to the sample agglomeration and the collapse of the inner nanotube channels. DSCs were fabricated with the nanotube TiNT and TiNT/NP samples and with the anatase nanorod TiNT-Δ and TiNT/NPs-Δ samples as well. The highest power conversion efficiency of η = 3.12% was obtained for the TiNT sample, despite its lower specific surface compared with the corresponding nanoparticle-loaded sample (TiNT/NP).     

Photoelectrochemical properties of TiO2 nanotube arrays: effect of electrolyte pH and annealing temperature

The effect of electrolyte pH and annealing temperature on the formation of TiO₂ nanotube arrays in connection with the photoelectrochemical response was investigated in this article. Well-aligned TiO₂ nanotube arrays were fabricated by anodisation of Ti foil in an electrolyte consisting of 1?M of glycerol (85?wt% of glycerol and 15?wt% of water) with 0.5?wt% of NH₄F at 30?V for 30?min. The pH of the electrolyte was varied from pH 1 to 7. With the increase of electrolyte pH to neutral condition, the length of the nanotube arrays was increased from ～320 to 1100?nm. As-anodised TiO₂ nanotube arrays were amorphous in nature. However, anatase phase was observed after annealing at 400°C and polycrystalline anatase and rutile phase could be observed by heating up to 500°C in air atmosphere. Based on the results obtained, the length and crystalline phases of TiO₂ nanotube arrays affect the performance of photoelectrochemical response and photoconversion efficiency significantly.     

Preparation of near micrometer-sized TiO2 nanotube arrays by high voltage anodization

Highly ordered TiO₂ nanotube arrays with large diameter of 680–750 nm have been prepared by high voltage anodization in an electrolyte containing ethylene glycol at room temperature. To effectively suppress dielectric breakdown due to high voltage, pre-anodized TiO₂ film was formed prior to the main anodizing process. Vertically aligned, large sized TiO₂ nanotubes with double-wall structure have been demonstrated by SEM in detail under various anodizing voltages up to 225 V. The interface between the inner and outer walls in the double-wall configuration is porous. Surface topography of the large diameter TiO₂ nanotube array is substantially improved and effective control of the growth of large diameter TiO₂ nanotube array is achieved. Interestingly, the hemispherical barrier layer located at the bottom of TiO₂ nanotubes formed in this work has crinkles analogous to the morphology of the brain cortex. These structures are potentially useful for orthopedic implants, storage of biological agents for controlled release, and solar cell applications.