Fabrication of open-ended TiO2 nanotube arrays by anodizing a thermally evaporated Ti/Au bilayer film

Fabrication of TiO₂ nanotube arrays (TNAs) with through-hole morphology is practical significance to enhance the photocatalytic activity of TNAs, as well as expanding their applications. In present work, open-ended TNAs are synthesized on a conductive Au layer by anodizing a thermally evaporated Ti/Au bilayer film. In the anodizing process, the upper Ti layer is transformed into well-aligned TNAs. The barrier layer under the growing TNAs ultimately touches the Au layer and is completely dissolved by the electrochemical etching. In order to avoid the bubble disruption of TNAs caused by the water electrolysis after the Au layer is exposed to the electrolyte, a specific non-aqueous electrolyte is used. The XRD results reveal that the as-formed open-ended TNAs are amorphous and can be transformed into anatase by annealing at 350 °C.     

Applying the statistical experimental method to evaluate the process conditions of TiO2 nanotube arrays by anodization method

Vertically oriented TiO₂ nanotube arrays were successfully produced by the anodization technique in NH₄F/H₃PO₄ electrolyte. The structure and morphology were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). It is found that TiO₂ nanotube arrays annealed at 500 °C containing 100% anatase phase and entirely converted into rutile at 800 °C. The response surface methodology (RSM) and Box-Behnken design were applied to find the optimal factor conditions in production of TiO₂ nanotube arrays. Based on the results in preliminary experiments, we selected anodization time, anodization voltage and NH₄F concentration as the key factors to investigate their effects on responses. The regression models were built by fitting the experimental results with a second-order polynomial. By using the regression models, the optimal factor conditions were obtained as follows: anodization time of 300 min; anodization voltage of 15.39 V; NH₄F concentration of 0.50 M. Corresponding to the optimal factor conditions, the predicted average length and diameter of nanotube array were 1429 nm and 33 nm, respectively. Confirmation experiments using the optimized conditions were performed: TiO₂ nanotube arrays were obtained with an average tube length of 1420 nm and average tube diameter of 36 nm. The experimental results are in good agreement with the predicted results.     

Effects of impurities containing phosphorus on the surface properties and catalytic activity of TiO2 nanotube arrays

TiO₂ nanotube arrays were prepared by titanium anodic oxidation with either HF or H₃PO₄/NH₄F aqueous electrolyte solutions. The samples were characterized by means of X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscope, photoluminescence spectra (PL) and photocurrent response. Aqueous solutions of methylene blue or Cr(VI) ions were used as the target pollutants to compare catalytic activities of the two nanotube array types. The amorphous impurities containing phosphorus were confirmed by XRD and IR, for the sample synthesized with H₃PO₄/NH₄F electrolytes. They closed a portion of the active sites, acted as recombination centers of photo-generated charges, and were also involved in the negative reactions of competing photo-generated holes or OH radicals. The TiO₂ nanotube arrays formed in the H₃PO₄/NH₄F electrolytes exhibited a stronger fluorescence spectrum, a weaker photocurrent and a lower catalytic activity than the sample fabricated with HF electrolyte without phosphorus impurities.     

Thermal stability of HfO2 nanotube arrays

Thermal stability of highly ordered hafnium oxide (HfO₂) nanotube arrays prepared through an electrochemical anodization method in the presence of ammonium fluoride is investigated in a temperature range of room temperature to 900 °C in flowing argon atmosphere. The formation of the HfO₂ nanotube arrays was monitored by current density transient characteristics during anodization of hafnium metal foil. Morphologies of the as-grown and post-annealed HfO₂ nanotube arrays were analyzed by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Although monoclinic HfO₂ is thermally stable up to 2000 K in bulk, the morphology of HfO₂ nanotube arrays degraded at 900 °C. A detailed X-ray photoelectron spectroscopy (XPS) study revealed that the thermal treatment significantly impacted the composition and the chemical environment of the core elements (Hf and O), as well as F content coming from the electrolyte. Possible reasons for the degradation of the nanotube at high temperature were discussed based on XPS study and possible future improvements have also been suggested. Moreover, dielectric measurements were carried out on both the as-grown amorphous film and 500 °C post-annealed crystalline film. This study will help us to understand the temperature impact on the morphology of nanotube arrays, which is important to its further applications at elevated temperatures.     

Sequence of structural and magnetic transitions in Ni48Co2Mn39Sn11 shape memory alloy

The structural and magnetic transitions in Ni₄₈Co₂Mn₃₉Sn₁₁ shape memory alloy were systematically investigated. During cooling, the paramagnetic austenite transforms into paramagnetic martensite at T_M∼375 K, followed by a gradual transition from paramagnetic to superparamagnetic martensite around T_S∼320 K. Upon further cooling through T_P∼100 K, the superparamagnetic clusters collectively freeze into a superspin glass state as corroborated by aging, rejuvenation, and memory effects. Consequently, the unique transition sequence of paramagnetic austenite→paramagnetic martensite→superparamagnetic martensite→superspin-glass martensite is disclosed.     

Influence of anodization parameters on the morphology of TiO2 nanotube arrays

TiO₂ nanotube arrays can be fabricated by electrochemical anodization in organic and inorganic electrolytes. Morphology of these nanotube arrays changes when anodization parameters such as applied voltage, type of electrolyte, time and temperature are varied. Nanotube arrays fabricated by anodization of commercial titanium in electrolytes containing NH₄F solution and either sulfuric or phosphoric acid were studied at room temperature; time of anodization was kept constant. Applied voltage, fluoride ion concentration, and acid concentrations were varied and their influences on TiO₂ nanotubes were investigated. The current density of anodizing was recorded by computer controlled digital multimeter. The surface morphology (top-view) of nanotube arrays were observed by SEM. The nanotube arrays in this study have inner diameters in range of 40-80 nm.     

A strain-fiber actuator by use of shape memory alloy spring

A strain-fiber actuator by use of shape memory alloy spring is proposed and experimentally investigated. The shape memory alloy actuator with a diameter of 0.3 mm and length of 2 mm was driven by a DC power supply of 7 V to persist for 5 s. The deformation and the strain ratio were found to be 60 μm and 3%, respectively. To apply the actuator to strain a fiber Bragg grating, we obtained a tunable spectrum width of 50 nm around the wavelength of 1550 nm for optical communication.     

Photoelectrochemical property and photocatalytic activity of N-doped TiO2 nanotube arrays

N-doped TiO₂ nanotube arrays (NTN) were prepared by anodization and dip-calcination method. Hydrazine hydrate was used as nitrogen source. The surface morphology of samples was characterized by SEM. It showed that the mean size of inner diameter was 65 nm and wall thickness was 15 nm for NTN. The ordered TiO₂ nanotube arrays on Ti substrate can sustain the impact of doping process and post-heat treatment. The atomic ratio of N/Ti was 8/25, which was calculated by EDX. Photoelectrochemical property of NTN was examined by anodic photocurrent response. Results indicated the photocurrent of NTN was nearly twice as that of non-doped TiO₂ nanotube arrays (TN). Photocatalytic activity of NTN was investigated by degrading dye X-3B under visible light. As a result, 99% of X-3B was decomposed by NTN in 105 min, while that of TN was 59%.     

Crystalline orientation preference for TiO2 nanotube arrays with efficient photoelectrochemical properties

Novel TiO₂ nanotube arrays (TiO₂ NTAs) with preferred oriented crystalline structure were firstly achieved by a facile method through anodizing Ti sheets with textured microstructure. Highly oriented crystalline structure of TiO₂ NTAs, with its (004) lattice plane of anatase TiO₂ paralleling to the top surface of TiO₂ NTAs was identified by the results of XRD, which was further confirmed by the regular diffraction spots and lattice fringes with uniform direction in HRTEM images. In comparison with normal randomly oriented TiO₂ NTAs, the quasi-single crystal structure of highly oriented TiO₂ NTAs can suppress the recombination of photogenerated electron–hole pairs and enhance the transfer of photocarriers, as was proved by the photoelectrochemical measurement conducted on both normal TiO₂ and oriented TiO₂ NTAs. The results of this study provide a promising avenue to obtain high photoelectrochemical performance TiO₂ NTAs-based hybrid nanomaterials.     

Optical and electrical characterization of TiO2 nanotube arrays on titanium substrate

Novel oriented aligned TiO₂ nanotube (TN) arrays were fabricated by anodizing titanium foil in 0.5% HF electrolyte solution. It is indicated that the sizes of the TNs greatly depended on the applied voltages to some extent. The electrical properties of the TN arrays were characterized by current-voltage (I-V) measurements. It exhibits a nonlinear, asymmetric I-V characterization, which can be explained that there exists an n-type semiconductor/metal Schottky barrier diode between TN arrays and titanium substrate interface. The absorption edges shift towards shorter wavelengths with the decrease of the anodizing voltages, which is attributed to the quantum size effects. At room temperature, a novel wide PL band consisting of four overlapped peaks was observed in the photoluminescence (PL) measurements of the TN arrays. Such peaks were proposed to be resulted from the direct transition X₁ → X₂/X₁, indirect transition Γ₁ → X₂/X₁, self-trapped excitons and oxygen vacancies, respectively.     

A study on photo-generated charges property in highly ordered TiO2 nanotube arrays

In this study TiO₂ nanotube arrays were fabricated by potentiostatic anodization of titanium sheet. The X-ray diffraction (XRD) pattern and field emission scanning electron microscopy (FE-SEM) image indicated the TiO₂ nanotube arrays were of pure anatase form and highly ordered. The properties of the photo-generated charges in the nanotube arrays were investigated by transient photovoltage (TPV) technique and surface photovoltage (SPV) technique based on lock-in amplifier with dc bias, in comparison with the commercial powder derived film. The separation processes of the photo-induced charges in the system of TiO₂ nanotubes on Ti have been demonstrated to be correlated with the incident light intensity, surface trapping states, and the interfacial electric field between Ti and TiO₂. The results also show that the highly ordered nanotube film could generate much stronger SPV responses under external electric field than the commercial powder derived film.     

Bi3.25La0.75Ti3O12 (BLT) nanotube capacitors for semiconductor memories

We report results of fabrication and examination of Bi_3.25La_0.75Ti₃O₁₂ (BLT) ferroelectric nanotubes. BLT nanotubes are suggested for developing 3D ferroelectric nanotube capacitors which could be used in high-density memory applications. BLT nanotubes were prepared by template-wetting process using polymeric sources where anodic aluminum oxide had been used as a template. After annealing, tubular BLT structures were crystallized inside the pores of the template. By selective etching of the template, released BLT nanotubes have been obtained. Crystallization and nucleation of the nanotubes were analyzed by XRD and FE-SEM techniques.     

由纳米管和纳米颗粒构成的TiO2膜的制备与机理研究

采用聚苯乙烯小球修饰Ti片表面,并进行阳极氧化,制备出一种由纳米颗粒和纳米管构成的TiO₂膜．通过数值模拟,分析了氧化表面附近的局部电场分布对TiO₂膜形貌的影响．结果表明,覆盖物增强了局部电场,从而加快了O^2-与Ti的反应速率,有利于TiO₂的生长;与此同时,[TiF₆]^6-的扩散受到阻碍,使得TiO₂的溶解速率减慢．可见,覆盖物打破了TiO₂纳米管形成的平衡条件,导致纳米颗粒的生成．此外,通过X射线衍射和Raman光谱的测试分析发现,所制备的TiO₂为锐钛矿结构.     

Synthesis and field emission of diamond-like carbon nanorods on TiO2/Ti nanotube arrays

Highly ordered TiO₂/Ti nanotube arrays were fabricated by anodic oxidation method in 0.5 wt% HF. Using prepared TiO₂/Ti nanotube arrays deposited Ni nanoparticles as substrate, high quality diamond-like carbon nanorods (DLCNRs) were synthesized by a conventional method of chemical vapor deposition at 750 °C in nitrogen atmosphere. DLCNRs were analyzed by filed emission scanning electron microscopy and Raman spectrometer. It is very interesting that DLCNRs possess pagoda shape with the length of 3–10 μm. Raman spectra show two strong peaks about 1332 cm⁻¹ and 1598 cm⁻¹, indicating the formation of diamond-like carbon. The field emission measurements suggest that DLCNRs/TiO₂/Ti has excellent field emission properties, a low turn-on field about 3.0 V/μm, no evident decay at 3.4 mA/cm² in 480 min.     

Composite SiO2/TiO2 and amine polymer/TiO2 nanoparticles produced using plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition was used to conformally coat commercial TiO₂ nanoparticles to create nanocomposite materials. Hexamethyldisiloxane (HMDSO)/O₂ plasmas were used to deposit SiO₂ or SiO_xC_yH_z films, depending on the oxidant concentration; and hexylamine (HexAm) plasmas were used to deposit amorphous amine-containing polymeric films on the TiO₂ nanoparticles. The composite materials were analyzed using Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). These analyses reveal film composition on the nanoparticles was virtually identical to that deposited on flat substrates and that the films deposit a conformal coating on the nanoparticles. The performance of the nanocomposite materials was evaluated using UV-vis spectroscopy to determine the dispersion characteristics of both SiO_x and HexAm coated TiO₂ materials. Notably, the coated materials stay suspended longer in distilled water than the uncoated materials for all deposited films.     

Highly ordered anodic TiO2 nanotube arrays and their stabilities as photo(electro)catalysts

Highly ordered TiO₂ nanotube arrays with an average diameter of 230 nm, a wall thickness of 30 nm and a length of 1.8 μm were fabricated within a large domain by electrochemically anodizing of a titanium foil in a mixed solution of glycerol and NH₄F aqueous electrolyte. The TiO₂ nanotubes exhibit an anatase structure after annealing at 450 °C in air for 3 h. The direct photolysis (DP), photocatalytic (PC), electrocatalytic (EC) and photoelectrocatalytic (PEC) activities of the TiO₂ nanotube arrays were investigated using methyl orange (MO) as the model pollutant. The degradation of MO in PC process is faster than that in DP process, which confirms the photocatalysis of TiO₂ nanotube arrays. The degradation rate in PEC process is much higher than those in EC and PC processes, which demonstrates the synergetic effect between PC and EC processes. The synergetic factor is 4.1, which suggests that the synergetic effect is strong. Moreover, the stabilities of morphology, structure and photo(electro)catalytic degradation performance of the TiO₂ nanotube arrays were studied in order to evaluate their applicability as photo(electro)catalysts. The photo(electro)catalytic experiments bring neither morphological nor structural modifications to the nanotube arrays. The photo(electro)catalytic degradation rates of the TiO₂ nanotube arrays maintain stable in 10 cycles, which indicates that the TiO₂ nanotube arrays are appropriate to be applied as photo(electro)catalysts.     

Surface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloy

NiTi alloys are attractive materials that are used for medicine, however, Ni-release may cause allergic reactions in an organism. The Ni-release rate is strongly affected by the surface state of the NiTi alloy that is mainly determined by its processing route. In this study, a NiTi shape memory alloy (50.9 at.% Ni) was heat-treated by several regimes simulating the shape setting procedure, the last step in the manufacture of implants. Heating temperatures were between 500 and 550 °C and durations from 5 to 10 min. Heat treatments were performed in air at normal and low pressure and in a salt bath. The purpose of the treatments was to obtain and compare different surface states of the Ni-Ti alloy. The surface state and chemistry of heat-treated samples were investigated by electron microscopy, X-ray photoelectron spectroscopy and Raman spectrometry. The amount of nickel released into a model physiological solution of pH 2 and into concentrated HCl was taken as a measure of the corrosion rate. It was found that the heat treatments produced surface TiO₂ layers measuring 15-50 nm in thickness that were depleted in nickel. The sample covered by the 15-nm thick oxide that was treated at 500 °C/5 min in a low pressure air showed the best corrosion performance in terms of Ni-release. As the oxide thickness increased, due to either temperature or oxygen activity change, Ni-release into the physiological solution accelerated. This finding is discussed in relation to the internal structure of the oxide layers.     

有序TiO2纳米管阵列结构的可控生长及其物相研究

分别在HF水溶液、含NH₄F和H₂O的乙二醇有机溶液中对Ti箔进行阳极氧化,得到TiO₂纳米管阵列结构.该结构高度有序、分布均匀、垂直取向,且通过阳极氧化工艺条件(如阳极氧化电压、电解液的选择与配比以及氧化时间等)可实现对其结构参数(如管径、管壁厚度、管密度、管长等)的有效控制.利用XRD研究了TiO₂纳米管阵列的物相结构.结果表明：退火前的TiO₂纳米管阵列为无定形结构;分别在真空和氧气氛中50 关键词： 2纳米管阵列')" href="#">TiO₂纳米管阵列 阳极氧化 可控生长     

Laser welding of thin foil nickel-titanium shape memory alloy

In this study, two L27 Taguchi experiments were carried out to study the effect of fibre laser welding parameters and their interactions upon the weld bead aspect ratio of nickel-titanium thin foil. The optimum parameters to produce full penetrated weld with the largest aspect ratio and desirable microstructure were successfully obtained by the Taguchi experimental design. The corrosion property of the optimized NiTi weld in Hank’s solution at 37.5 °C was studied and compared with the as-received NiTi. To improve the corrosion properties of the weld, the effect of post-weld-heat-treatments ranging from 573 to 1173 K was investigated. The corrosion properties, surface morphology, microstructure and Ti/Ni ratio of the heat-treated NiTi weld were analysed. It was found that a post-weld heat treatment at 573 K for 1 h provided the best pitting corrosion resistance at the weld zone.     

Adsorption and interaction of organosilanes on TiO2 nanoparticles

Organosilanes with different organic functional groups are precursors of corresponding organosilanol which can be attached to the surface of oxide nanoparticles by silyation. In this work, surface of commercial TiO₂ nanoparticles was modified by 3-aminopropyltrimethoxysilane (APS) and phenyltrimethoxysilane (PTMS) through an aqueous process. The amount of adsorbed organosilane was evaluated by energy dispersive X-ray spectroscopy and was found to be 3 times higher on PTMS treated sample than on APS treated sample. The orientation and bonding of the molecules on particle surface was analyzed using Fourier transform infrared spectroscopy and time-of-flight secondary ion mass spectrometry. The obtained data confirmed that bonding of organosilanols on particle surface was realized through Si-O-Ti bonds and organic functional groups were extended away from particle surface on both APS and PTMS modified particles. It was found that phenylsilanol molecules are cross-linked to each other through Si-O-Si bonds, while such bonds are very little to none between aminosilanol molecules. A model of adsorption is proposed to explain these observations.