Application of ultrasonic wave to clean the surface of the TiO2 nanotubes prepared by the electrochemical anodization

In this study, the TiO₂ nanotubes were fabricated by electrochemical anodization in a NH₄F/Na₂SO₄/PEG400/H₂O electrolyte system. Ultrasonic wave (80 W, 40 kHz) was used to clean the surface of TiO₂ nanotube arrays in the medium of water after the completion of the anodization. Surface morphology (FESEM) and X-ray diffraction spectrum of the nanotubes treated by sonication at 0 min, 9 min, 40 min and 60 min were compared. The experimental results showed that the precipitate on the surface of the nanotube arrays could be removed by the ultrasonic wave. The treating time had an influence on the precipitate removal and 9 min (corresponding to 12 Wh) is the suitable time for surface cleaning of the TiO₂ nanotubes on this experimental condition.     

A novel approach towards high-performance composite photocatalyst of TiO2 deposited on activated carbon

TiO₂ photocatalysts deposited on activated carbon (TiO₂/AC) were prepared by dip-hydrothermal method at 180 °C using peroxotitanate as a precursor, then calcinated at 300-800 °C. The samples were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and the nitrogen absorption. Their photocatalytic activity was evaluated by degradation of methyl orange (MO). The results showed that TiO₂ particles of anatase type were well deposited on the activated carbon surface. TiO₂/AC calcinated at 600 °C exhibited the best photocatalytic performance. For the comparison, the same photocatalysis experiment was carried out for two mixtures of commercial TiO₂ (Degussa P25) with AC and synthetic TiO₂ with AC. It was found that the composite catalyst TiO₂/AC was better than the two mixtures. Besides, different from fine powdered TiO₂, the granular TiO₂/AC photocatalysts could be easily separated from the bulk solution and reused; indeed, its photocatalytic ability was hardly decreased after a five-cycle for MO degradation. The kinetics of the MO degradation fitted well the Langmuir-Hinshelwood model.     

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%.     

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.     

Preparation, characterization and visible-light photocatalytic activity of AgI/AgCl/TiO2

In this paper, a novel composite photocatalyst AgI/AgCl/TiO₂ was prepared by ion exchange method and characterized by XRD, SEM and UV-Vis spectrometry. The as-prepared AgI/AgCl/TiO₂ composites show much higher photocatalytic activity than AgCl/TiO₂ and AgI/TiO₂ under visible-light irradiation (λ > 400 nm) in the process of methyl orange (MO) degradation. When the molar percentage of AgI to initial AgCl is 20% (sample SE-20%), the maximal degradation efficiency of MO has reached 85.8% after irradiation for 120 min. The enhancement of photocatalytic activity of the composite photocatalyst AgI/AgCl/TiO₂ will be attributed to its good absorption in the visible-light region, especially low recombination rate of the electron-hole pairs based on the photoluminescence (PL) spectra investigation of AgI/AgCl/TiO₂ and the matching band structures of AgI, AgCl and TiO₂. The detection of reactive species by radical scavengers displays that O₂⁻ and H₂O₂ are the main reactive species for the degradation of MO under visible-light irradiation. Moreover, PL analysis by using terephthalic acid (TA) as a probe molecule further reveals that OH can be negligible for the degradation of MO.     

Synthesis and characterization of TiO2/Fe2O3 core-shell nanocomposition film and their photoelectrochemical property

TiO₂/Fe₂O₃ core-shell nanocomposition film has been fabricated via two-step method. TiO₂ nanorod arrays are synthesized by a facile hydrothermal method, and followed by Fe₂O₃ nanoparticles deposited on TiO₂ nanorod arrays through an ordinary chemical bath deposition. The phase structures, morphologies, particle size, chemical compositions of the composites have been characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and ultraviolet-visible (UV-vis) spectrophotometer. The results confirm that Fe₂O₃ nanoparticles of mean size ca. 10 nm coated on the surface of TiO2 NRs. After depositing Fe₂O₃, UV-vis absorption property is induces the shift to the visible-light range, the annealing temperature of 600 °C is the best condition for UV-vis absorption property of TiO₂/Fe₂O₃ nanocomposite film, and increasing Fe content, optical activity are enhanced one by one. The photoelectrochemical (PEC) performances of the as-prepared composite nanorods are determined by measuring the photo-generated currents under illumination of UV-vis light. The TiO₂ NRs modified by Fe₂O₃ show the photocurrent value of 1.36 mA/cm² at 0 V vs Ag/AgCl, which is higher than those of unmodified TiO₂ NRs.     

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.     

Transparent titania nanotubes of micrometer length prepared by anodization of titanium thin film deposited on ITO

Transparent TiO₂ nanotube arrays of micrometer lengths were prepared by anodization of titanium thin film RF sputtered on indium tin oxide (ITO) which was coated on glass substrate. The sputtering process took place at elevated temperature of 500 °C. The structures of the films were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD) while the optical properties of the films were investigated using UV-visible spectroscopy. Two types of electrolytes were used in this work: an aqueous mixture of acetic acid and HF solution and a mixture of NH₄F and water dissolved in ethylene glycol. The concentration of NH₄F, voltage and the thickness of the sputtered titanium film were varied to study their effect on the formation of TiO₂ nanotube arrays. It is demonstrated in this work that the nanoporous layer is formed on top of the ordered array of TiO₂ nanotubes. Furthermore, the optical transmittance of TiO₂ nanotubes annealed at 450 °C is much lower than the non annealed TiO₂ nanotubes in the visible wavelength region.     

Synthesis and bioactivity of highly ordered TiO2 nanotube arrays

The highly ordered TiO₂ nanotube arrays were fabricated by potentiostatic anodization of Ti foils in fluorinated dimethyl sulfoxide (DMSO). TiO₂ nanotube arrays are formed using a 40 V anodization potential for 24 h, with a length of 12 μm, diameter of 170 nm and aspect ration of about 70. The as-prepared nanotubes are amorphous, but can be crystallized as the heat treatment temperature increases. Anatase phase appears at a temperature of about 300 °C, then transforms to rutile phase at about 600 °C. After heat treatment at 500 °C and soaking in SBF for 14d, a thick apatite layer of about 13 μm covers the whole surface of TiO₂ nanotube arrays, indicating their excellent in vitro bioactivity, which is mainly attributed to their high specific surface area and the anatase phase.     

Photooxidation of different organic dyes (RB, MO, TB, and BG) using Fe(III)-doped TiO2 nanophotocatalyst prepared by novel chemical method

The nano-structured Fe(III)-doped TiO₂ photocatalysts with anatase phase have been developed for the oxidation of non-biodegradable different organic dyes like methyl orange (MO), rhodamine B (RB), thymol blue (TB) and bromocresol green (BG) using UV-Hg-lamp. The different compositions of Fe_xTi_1−xO₂ (x = 0.005, 0.01, 0.05, and 0.1) nanocatalysts synthesized by chemical method (CM), have been characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectra, specific surface area (BET), transmission electronic microscopy (TEM) analysis, XPS, ESR and zeta potential. From XRD analysis, the results indicate that all the compositions of Fe(III) doped in TiO₂ catalysts gives only anatase phase not rutile phase. For complete degradation of all the solutions of the dyes (MO, RB, TB, and BG), the composition with x = 0.005 is more photoactive compared all other compositions of Fe_xTi_1−xO₂, and degussa P25. The decolorization rate of different dyes decreases as Fe(III) concentration in TiO₂ increases. The energy band gap of Fe(III)-doped TiO₂ is found to be 2.38 eV. The oxidation state of iron has been found to be 3+ from XPS and ESR show that Fe³⁺ is in low spin state.     

Effect of calcination temperature on the morphology and surface properties of TiO2 nanotube arrays

Well-ordered TiO₂ nanotube arrays were prepared by electrochemical anodization of titanium in aqueous electrolyte solution of H₃PO₄ + NH₄F at a constant voltage of 20 V for 3 h, followed by calcined at various temperatures. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) were used to characterize the samples. The results showed that the as-prepared nanotube arrays before being calcined were amorphous and could transform to anatase phase at a heat treatment temperature higher than 400 °C. As the calcination temperatures increased, crystallization of anatase phase enhanced and rutile phase appeared at 600 °C. However, further increasing the calcination temperature would cause the collapse of nanotube arrays. PL intensity of the nanotube arrays annealed at 500 °C was the lowest, which was probably ascribed to better crystallization together with fewer surface defects of the nanotube arrays.     

Synthesis and gas sensing characteristic based on metal oxide modification multi wall carbon nanotube composites

A new type of gas sensing material based on metal oxide modification multi wall carbon nanotube (MO/MWCNT) composites is presented since the interface between the composites enhance the carrier density so as to improve the gas sensitivity. Three kinds of MO/MWCNT composite materials, such as ZnO/MWCNT, SnO₂/MWCNT and TiO₂/MWCNT, have been acquired in situ growth using catalytic pyrolysis method. The MO nano particles have decorated on side of MWCNTs, whereas the introduction of SnO₂ nano particles makes part of MWCNT showing two-dimensional form of carbon nano-wall structure. Among four kinds of cathode of ZnO/MWCNTs, SnO₂/MWCNTs, TiO₂/MWCNTs and pure MWCNT composite film, TiO₂/MWCNTs composite has the lowest threshold electric field required to draw current of 12 μA has been found to be ∼1.2 V/μm, and also TiO₂/MWCNTs composite has the highest sensitivity of 16% to ethanol. The TiO₂/MWCNTs composite is superior to the others both in vacuum electron transportation and gas sensitivity.     

Synthesis, characterization and effect of calcination temperature on phase transformation and photocatalytic activity of Cu,S-codoped TiO2 nanoparticles

A novel copper and sulfur codoped TiO₂ photocatalyst was synthesized by modified sol-gel method using titanium(IV) isopropoxide, CuCl₂·2H₂O and thiourea as precursors. The samples were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), scanning electron microscopy equipped with energy dispersive X-ray micro-analysis (SEM-EDX), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) analysis. The XRD results showed undoped and Cu,S-codoped TiO₂ nanoparticles only include anatase phase. Effect of calcination temperature showed rutile phase appears in 650 and 700 °C for undoped and 0.1% Cu,S-codoped TiO₂, respectively. The SEM analysis revealed the doping of Cu and S does not leave any change in morphology of the catalyst surface. The increase of copper doping enhanced “red-shift” in the UV-vis absorption spectra. The TEM images confirmed the dopants suppressed the growth of TiO₂ grains. The photocatalytic activity of samples was tested for degradation of methyl orange (MO) solutions. The results showed photocatalytic activity of the catalysts with 0.05% Cu,0.05% S and 0.1% Cu,0.05% S were higher than that of other catalysts under ultraviolet (UV) and visible irradiation, respectively. Because of synergetic effect of S and Cu, the Cu,S-codoped TiO₂ catalyst has higher activity than undoped and Cu or S doped TiO₂ catalysts.     

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.     

Sonodegradation and photodegradation of methyl orange by InVO4/TiO2 nanojunction composites under ultrasonic and visible light irradiation

The InVO₄/TiO₂ nanojunction composites with different weight ratio of 1:10, 1:25, 1:50 and 1:100 were successfully constructed using an ion impregnate method, followed by calcining temperature 400 °C for 2 h in Ar. The sono- and photo-catalytic activities of the InVO₄/TiO₂ nanojunction composites were evaluated through the degradation of methyl orange (MO) in aqueous solution under ultrasonic and visible light irradiation, respectively. The experimental results determined that the (1:50) InVO₄/TiO₂ nanojunction composite has exhibited the highest sonocatalytic activity. It can be ascribed to vectorial charge transfer at the co-excited InVO₄/TiO₂ interface under ultrasonic irradiation, results in the complete separation of electrons and holes. Interestingly, the (1:25) InVO₄/TiO₂ nanojunction composite displayed superior photocatalytic activity for MO degradation under visible light, indicating that InVO₄ as a narrow band gap sensitizer can expand photocatalytic activity of TiO₂ to visible region, and the charge transfer can be formed from high energy level of InVO₄ conduction band to the low energy level of TiO₂ conduction band in a present of excited InVO₄ alone under visible light irradiation. The sono- and photo-catalytic activities of the InVO₄/TiO₂ nanojunction composites were found to be dependent significantly on different InVO₄ contents, which can be explained by the influence of charge transfer on the basis of the work functions of different catalysis mechanism.     

Oriented single crystalline TiO2 nano-pillar arrays directly grown on titanium substrate in tetramethylammonium hydroxide solution

Oriented single crystalline titanium dioxide (TiO₂) nano-pillar arrays were directly synthesized on the Ti plate in tetramethylammonium hydroxide (TMAOH) solution by one-pot hydrothermal method. The samples were characterized respectively by means of field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Results showed that the TiO₂ nano-pillar with a tetrahydral bipyramidal tip grew vertically on the titanium substrate. HRTEM and Raman results confirmed that the TiO₂ nano-pillar arrays were single crystalline anatase. The controls of morphology, size, and orientation of the nano-pillar could be achieved by varying the solution concentration and hydrothermal temperature. Furthermore, the special morphology of the TiO₂ nano-pillar arrays was caused by the selectively absorption of the tetramethylammonium (TMA) through hydrogen bonds on the lattice planes parallel to (0 0 1) of anatase TiO₂. Less grain boundaries and direct electrical pathway for electron transferring were crucial for the superior photoelectrochemical properties of the single anatase TiO₂ nano-pillar arrays. This approach provides a facile in situ method to synthesize TiO₂ nano-pillar arrays with a special morphology on titanium substrate.     

Acetone gas phase condensation on alkaline metals doped TiO2 sol-gel catalysts

The vapor-phase condensation of acetone was studied over sol-gel prepared TiO₂ and TiO₂-doped with 1 wt.% of Li⁺ or Rb⁺. The acetone condensation was catalyzed by basic sites and the obtained products were mesityl oxide (MO), isomesityl oxide (ISMO), mesitylene (M) as well as isophorone (IP) and phorone (P). The TiO₂ crystalline structure was characterized by XRD and the basicity of catalysts by means of CO₂ chemisorption. The number of basic sites was 0.057 mmol CO₂/g for TiO₂ and Rb/TiO₂ and 0.676 mmol CO₂/g for Li/TiO₂ catalyst. CO₂-TPD showed that the strongest of the basic sites follows the order Rb/TiO₂ > TiO₂ > Li/TiO₂. Activity was found of the same order in the three catalysts. Selectivity patterns shown that MO (70%) was the main product in Li/TiO₂ while for TiO₂ and Rb/TiO₂ comparable amounts of MO (41 and 44%) and M (39 and 39%) were obtained.     

Facile fabrication, characterization, and enhanced photoelectrocatalytic degradation performance of highly oriented TiO2 nanotube arrays

Highly ordered TiO₂ nanotube arrays were successfully fabricated using ethanol and water mixture electrolytes (40 vol% ethanol and 0.2 wt% hydrofluoric acid) by a facile electrochemical anodization method. The as-prepared nanotube arrays were grown perpendicular to the titanium substrate with about 90 nm in diameter, 20 nm in wall thickness, and around 500 nm in length. The formation mechanism of the samples is briefly discussed. A blue shift in the spectrum of UV–Vis absorption was observed with respect to a piece of the sol–gel derived TiO₂ film. Moreover, photocurrent response and photoelectrocatalytic degradation of methyl orange under ultraviolet light irradiation were adopted to evaluate the photoelectrocatalytic properties of the TiO₂ nanotube arrays. We demonstrate that the highly ordered TiO₂ nanotube arrays possess much better photoelectrocatalytic activity than the sol–gel derived TiO₂ film and good stability.     

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.     

Ni-doped TiO2 nanotube arrays on shape memory alloy

Self-organized Ni-Ti-O nanotube arrays were fabricated through a direct anodization of NiTi shape memory alloy in glycerol-based electrolyte. The growth of Ni-doped TiO₂ nanotube arrays was mainly affected by anodization voltage and temperature. Higher anodization voltage facilitated the growth of uniform nanotube arrays. Large-area open-ended Ni-Ti-O nanotube arrays could form on the surface of the shape memory alloy under a higher anodization temperature. The oxide nanotubes had a gradually changed composition along the growth direction of the nanotube and presented a thermal stability up to 400 °C. The nanotubular oxide demonstrated a much better hydrophilic behavior than that of the traditional oxide layer grown on NiTi substrate through air oxidization. The successful fabrication of Ni-doped TiO₂ nanotube arrays here makes it feasible to further explore excellent physical and chemical as well as biomedical properties of the nanotube-modified surfaces of the NiTi shape memory alloy.