Preparation and photoluminescence properties of Eu3+-doped ZrO2 nanotube arrays

Zr–Eu alloy containing 3 at% Eu was prepared by a powder metallurgical method and Eu³⁺-doped ZrO₂ nanotube arrays were prepared by anodising the Zr–Eu alloy. The properties of Eu³⁺-doped ZrO₂ nanotube arrays were studied in contrast to undoped ZrO₂ nanotube arrays under different annealing temperatures. Results showed that the Eu³⁺ ions could not only stabilise the tetragonal phase of zirconium oxide, but also make the crystallite sizes smaller. Annealing temperature exerted a significant influence on the absorbance value, as well as the intensity and position of the photoluminescence peaks. When the excitation wavelength was either 248 nm or 270 nm, the sample annealed at 600 °C displayed the strongest emission peak; while under excitation at 232 nm, the sample annealed at 400 °C exhibited the strongest emission peak.     

Stable TiO2 nanotube arrays with high UV photoconversion efficiency

This work is intended to define an optimal methodology for preparing highly ordered TiO₂ nanotube arrays by a 60-V anodization in a glycol ethylene solution. In order to obtain a mechanically stable structure with a high UV photoconversion efficiency, it is necessary to carefully control the growth mechanism through the anodization process. For this reason, the nanotube arrays have to be formed upon a compact titanium dioxide layer with well-defined thickness. Besides, both the fluoride concentration and anodization time are strictly correlated, because elevated concentrations and/or a long anodization time produce unstable structure with low photoconversion efficiency. The best result in the terms of reproducibility has been obtained previously for a three-minute galvanostatic oxide growth on the pickled titanium sheet, and anodic growth in ethylene glycol solution containing 1 wt.% H₂O and 0.20 wt.% NH₄F for a period lower than 4.5 h. The UV photoconversion efficiency was measured and a maximum value of 28.3% has been obtained, which is the highest result in the literature.     

Preparation of short, robust and highly ordered TiO2 nanotube arrays and their applications as electrode

The short, robust and highly ordered TiO₂ nanotube arrays (TNAs) electrode was prepared by sonoelectrochemical anodization of titanium in HF–H₂O electrolyte solution (referred as short TNAs, STNAs). The self-organized arrays of titania nanotubes of approximately 12–65 nm in diameter and 75–280 nm in length can be synthesized at anodic voltage of 5–20 V. The electron transport process within the STNAs electrode was much favorable in comparison with that for the long TNAs electrode synthesized by conventional magnetic agitation technique (referred as long TNAs, LTNAs), as confirmed by the obviously enhanced photocurrent response of STNAs electrode either in 0.02 M Na₂SO₄ electrolyte solution or in different concentrations of glucose solution or under different intensities of UV illumination. To investigate their photoelectrochemical applications, degradation of tetracycline, a typical pharmaceutical and personal care products (PPCPs), was carried out using photoelectrocatalytic (PEC) means, comparing with electrochemical (EC) and photocatalytic (PC) processes. The kinetic constant of the PEC process of STNAs electrode was 3.17 times as high as its PC process. The color removal rate of tetracycline by STNAs electrode achieved 81% within 3 h, which was 21% higher than that for LTNAs electrode. In degrading tetracycline, 41% of TOC was mineralized using the STNAs electrode against 23% using LTNAs electrode under similar conditions. Such kind of titania nanotubes will have many potential applications in various areas as an outstanding photoelectrochemical material.     

Synthesis and dielectric properties of polyaniline/titanium dioxide nanocomposites

Two series of polyaniline–TiO₂ nanocomposite materials were prepared in base form by in situ polymerization of aniline with inorganic fillers using TiO₂ nanoparticles (P25) and TiO₂ colloids (Hombikat), respectively. The effect of particle sizes and contents of TiO₂ materials on their dielectric properties was evaluated. The as-synthesized polyaniline–TiO₂ nanocomposite materials were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR), thermal analysis (DTA/TGA), and X-ray diffraction (XRD). Dielectric properties of polyaniline–TiO₂ nanocomposites in the form of films were measured at 1 KHz–1 MHz and a temperature range of 35–150 °C. Higher dielectric constants and dielectric losses of polyaniline–TiO₂ nanocomposites than those of neat PANI were found. PANI–TiO₂ nanocomposites derived from P25 exhibited higher dielectric constants and losses than those from Hombikat TiO₂ colloids. Electrical conductivity measurements indicate that the conductivity of nanocomposites is increased with TiO₂ content. The dielectric properties and conductivities are considered to be enhanced due to the addition of TiO₂, which might induce the formation of a more efficient network for charge transport in the base polyaniline matrix.     

Self-organized vanadium and nitrogen co-doped titania nanotube arrays with enhanced photocatalytic reduction of CO2 into CH4

Self-organized V-N co-doped TiO₂ nanotube arrays (TNAs) with various doping amount were synthesized by anodizing in association with hydrothermal treatment. Impacts of V-N co-doping on the morphologies, phase structures, and photoelectrochemical properties of the TNAs films were thoroughly investigated. The co-doped TiO₂ photocatalysts show remarkably enhanced photocatalytic activity for the CO₂ photoreduction to methane under ultraviolet illumination. The mechanism of the enhanced photocatalytic activity is discussed in detail.     

Photoelectrochemical oxidation of salicylic acid and salicylaldehyde on titanium dioxide nanotube arrays

We report on the kinetics of photoelectrochemical oxidation of salicylic acid (SA) and salicylaldehyde (SH) on titanium dioxide nanotube arrays. The TiO₂ nanotubes were prepared by the electrochemical oxidation of titanium substrates in a nonaqueous electrolyte (DMSO/HF). Scanning electron microscopy (SEM) was employed to examine the morphology of the formed nanotubes. Linear voltammetry was used to study the electrochemical and photoelectrochemical behavior of the synthesized TiO₂ nanotube arrays. The photoelectrochemical oxidation of SA and SH on the TiO₂ nanotubes was monitored by in situ UV-vis spectroscopy, showing that the kinetics of the photoelectrochemical oxidation of SA and SH follows pseudo first-order and that the rate constant of SH oxidation is 1.5 times larger than that of SA degradation. Quantum chemical calculations based on the DFT method were performed on SA and SH to address the large difference in kinetics. The relatively higher E_LUMO − E_HOMO makes SA more stable and thus more difficult to be oxidized photoelectrochemically. The impact of temperature and initial concentrations on the kinetics of SA and SH photoelectrochemical degradation was also investigated in the present work.     

Fabrication of Ti–Al–Zr alloy oxide nanotube arrays in organic electrolytes by anodization

We report the formation of self-organized Ti–Al–Zr alloy oxide nanotube arrays at different fluoride concentrations in formamide and glycerol electrolytes. The nanotube morphology is strongly influenced by the fluoride concentration, applied potential and reaction time. Titanium alloy (Ti–Al–Zr) oxide nanotube arrays with a length of about 6.13 μm, a pore diameter of 116 nm and wall thickness of 55 nm were prepared at 50 V for 24 h in formamide and glycerol mixtures (volume ratio 1:1) with addition of 1.0 wt% NH₄F. The as-prepared nanotubes were amorphous and alloy oxide crystals appeared after annealing in air at 400 and 600 °C for 3 h.     

Synthesis and magnetic properties of Cu-doped ZnO nanowire arrays

Arrays of Cu-doped ZnO nanowires were successfully fabricated by electrodeposition of Zn²⁺ and Cu²⁺ into anodic aluminum oxide template and post-oxidation annealing in air atmosphere. The transmission electron microscopy result shows that the nanowires are uniform, about 100 nm in diameter and with the aspect ratio of up to 40. Selected area electron diffraction and X-ray diffraction results indicate that the nanowires are in hexagonal wurtzite structure. Magnetization measurements show that the Zn_1−xCu_xO (x = 0.07 and 0.11) nanowires exhibit room-temperature ferromagnetism and the enhancement of the ferromagnetism is revealed for the Zn_0.93Cu_0.07O nanowires annealed in vacuum.     

Photoelectrocatalytic properties of Ag nanoparticles loaded TiO2 nanotube arrays prepared by pulse current deposition

A pulse current deposition technique was adopted to construct highly dispersed Ag nanoparticles on TiO₂ nanotube arrays which were prepared by the electrochemical anodization. The morphology, crystallinity, elemental composition, and UV-vis absorption of Ag/TiO₂ nanotube arrays were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and diffuse reflectance spectra (DRS). In particular, the photoelectrochemical properties and photoelectrocatalytic activity under UV light irradiation and the photocatalytic activity under visible light irradiation for newly synthesized Ag/TiO₂ nanotube arrays were investigated. The maximum incident photon to charge carrier efficiency (IPCE) value of Ag/TiO₂ nanotube arrays was 51%, much higher than that of pure TiO₂ nanotube arrays. Ag/TiO₂ nanotube arrays exhibited higher photocatalytic activities than the pure TiO₂ nanotube arrays under both UV and visible light irradiation. The photoelectrocatalytic activity of Ag/TiO₂ nanotube arrays under UV light irradiation was 1.6-fold enhancement compared with pure TiO₂ nanotube arrays. This approach can be used in synthesizing various metal-loaded nanotube arrays materials.     

Electrochemical synthesis of self-organized TiO2 nanotubular structures using an ionic liquid (BMIM-BF4)

We show that an ionic liquid consisting of imidazolium salt with a BF₄ counter ion (BMIM-BF₄) can directly be used to grow well-defined layers of self-organized TiO₂ nanotubes. For this a Ti metal substrate is anodized in this electrolyte for potential range between 3 V_Ag/AgCl and 10 V_Ag/AgCl without addition of free fluoride species (fluorides are used in all previous tube growth procedures). Key factors that influence the morphology and geometry of the resulting nanotubular layer are the anodic potential, the anodization time and particularly the water content in the ionic liquid. The resulting nanotubes layers have thickness in the range of approximately 300-650 nm; with individual tubes that have diameters between 27 nm and 43 nm.     

Ordered CoFe2O4 nanowire arrays with preferred crystal orientation and magnetic anisotropy

CoFe₂O₄ nanowire arrays were fabricated by electrodeposition of Fe²⁺ and Co²⁺ into anodic aluminum oxide (AAO) templates and further oxidization. The phase structure of the nanowires is cubic spinel-type, and the XRD result exhibits perfect preferred crystallite orientation along the nanowire axes. Compared with CoFe₂O₄ nanowire arrays synthesized by other methods, the magnetic hysteresis loops demonstrate that the arrays of nanowires exhibit uniaxial magnetic anisotropy with easy magnetization direction along the nanowire axes owing to the large shape anisotropy. This approach provides a facile technology to fabricate oxide nanowires with uniaxial magnetic anisotropy.     

Multifuctional Fe3O4@C/YVO4:Dy nanopowers: Preparation,luminescence and magnetic properties

As-synthesized Fe₃O₄ nanoparticles were encapsulated with carbon layers through a simple hydrothermal process. Fe₃O₄/C nanoparticles were coated with YVO₄:Dy³⁺ phosphors to form bifunctional Fe₃O₄@C@YVO₄:Dy³⁺ composites. Their structure, luminescence and magnetic properties were characterized by XRD, SEM, TEM, HRTEM, PL spectra and VSM. The experimental results indicated that the as-prepared bifunctional composites displayed well-defined core–shell structures. The ∼12 nm diameter YVO₄:Dy³⁺ shell exhibited tetragonal structure. Additionally, the composites exhibited a high saturation magnetization (13 emu/g) and excellent luminescence properties, indicating their promising potential as multifunctional biosensors for biomedical applications.     

Synthesis and electrochemical properties of CeO2 nanoparticle modified TiO2 nanotube arrays

In this paper, a cerium dioxide (CeO₂) modified titanium dioxide (TiO₂) nanotube array film was fabricated by electrodeposition of CeO₂ nanoparticles onto an anodized TiO₂ nanotube array. The structural investigation by X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated that the CeO₂ nanoparticles grew uniformly on the walls of the TiO₂ nanotubes. The composite was composed of cubic-phase CeO₂ crystallites and anatase-phase TiO₂ after annealing at 450 °C. The cyclic voltammetry and chronoamperometric charge/discharge measurement results indicated that the CeO₂ modification obviously increased the charge storage capacity of the TiO₂ nanotubes. The charge transfer process at the surface, that is, the pseudocapacitance, was the dominate mechanism of the charge storage in CeO₂-modified TiO₂ nanotubes. The greater number of surface active sites resulting from uniform application of the CeO₂ nanoparticles to the well-aligned TiO₂ nanotubes contributed to the enhancement of the charge storage density.     

The growth of interfacial compounds between titanium dioxide and bismuth oxide

The growth of interfacial compounds between TiO₂ and Bi₂O₃ during transient liquid phase bonding at 900, 1000 and 1100 °C for various times was investigated. The microstructures and compositions of compounds in joints were analyzed by means of SEM and EPMA. It was found that the compound Bi₄Ti₃O₁₂ forms initially and replaces the Bi₂O₃ interlayer. Bi₂Ti₄O₁₁ then arises at the interface between Bi₄Ti₃O₁₂ and TiO₂ and the metastable Bi₂Ti₂O₇ phase appears last at the interface between Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁. The modes and activation energies of the growth of Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁ were determined respectively. Holes in the middle of the joint heated at 1100 °C for 24 h were also found.     

Electrochemical tuning of titania nanotube morphology in inhibitor electrolytes

The electrochemical behavior of fluorine containing electrolytes and its influence in controlling the lateral dimensions of TiO₂ nanotubes is thoroughly investigated. Potentiostatic anodization is carried out in three different electrolytes, viz., aqueous hydrofluoric acid (HF), HF containing dimethyl sulphoxide (DMSO) and HF containing ethylene glycol (EG). The experiments were carried out over a broad voltage range from 2 to 200 V in 0.1-48 wt% HF concentrations and different electrolytic compositions for anodization times ranging from 5 s to 70 h. The chemistry that dictates how the nature of electrolytes influences the morphology of nanotubes is discussed. Electrochemical impedance spectra were recorded for varying compositions of all the electrolytes. It was observed that composition of the electrolyte and its fluorine inhibiting nature has significant impact on nanotube formation as well as in controlling the aspect ratio. The inhibiting nature of EG is helpful in holding fluorine at the titanium anode, thereby allowing controlled etching at appropriate voltages. Thus our study demonstrates that HF containing EG is a promising electrolytic system providing wide tunability in lateral dimensions and aspect ratio of TiO₂ nanotubes by systematically varying the anodization voltage and electrolyte composition.     

Photocurrent response and semiconductor characteristics of Ce-Ce2O3-CeO2-modified TiO2 nanotube arrays

We reported Ce and its oxide-modified TiO₂ nanotube arrays (TNTs) and their semiconductor properties. The TNTs were prepared by anodic oxidation on pure Ti and investigated by electrochemical photocurrent response analysis. Then, the TNT electrodes were deposited of Ce by cathodic reduction of Ce(NO₃)₃ 6H₂O. After deposition, the TNT electrodes were fabricated by anodic oxidation at E = 1.0 V_(SCE) for various electricity as Ce-Ce₂O₃-CeO₂ modification. The Ce-deposited TNTs (band gap energy E_g = 2.92 eV) exhibited enhanced photocurrent responses under visible light region and indicated more negative flat band potential (E_fb) compared with the TNTs without deposition. After anodic oxidation, the mixed Ce and its oxide (Ce₂O₃-CeO₂)-modified TNT photoelectrodes exhibited higher photocurrent responses under both visible and UV light regions than the TNTs without deposition. The photocurrent responses and E_fb were found to be strongly dependent on the contents of Ce₂O₃ and CeO₂ deposited on TNTs. A new characteristic of E_g = 2.1 ± 0.1 eV was investigated in the Ce₂O₃- and CeO₂-modified photoelectrodes. X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were also employed to characterize various modified TNTs photoelectrodes.     

Synthesis and characterization of Fe doped titanium dioxide nanopowders

Titanium dioxide nanopowders, doped with different amounts of Fe³⁺ ions (0.3-5 mass%), were synthesized by acid-catalyzed sol-gel method in a non-aqueous medium. The obtained powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and determination of izoelectric points as well as particle diameters. Careful investigation of porous structure was provided by application of nitrogen adsorption-desorption method. Structure analysis showed that the obtained nanopowders exhibited the anatase crystal structure, independent of the amount of iron dopants. The presence of Fe³⁺ ions in anatase decreases the value of isoelectric point of undoped TiO₂. Unlike crystal structure, porosity parameters are strongly affected by the amount of iron ions incorporated in TiO₂ lattice. The mesoporosity of TiO₂ can be successfully controlled by changing the amount of iron dopants.     

Cyclic voltammetry studies of TiO2 nanotube arrays electrode: Conductivity and reactivity in the presence of H and aqueous redox systems

In this paper, we use cyclic voltammetry to investigate the effect of protons on the conductivity and reactivity of TiO₂ nanotube array (NTA) electrodes in an aqueous redox system. The H⁺ ion can change the TiO₂ nanotube (anatase phase) surface states for electrons transfer. It can also act as an intermediate state for electron transfer to the acceptor species in the electrolyte surrounding the TiO₂. The higher the concentration of H⁺ ions in the aqueous electrolyte, the easier it is for the electrons transfer from the TiO₂ to the electrolyte oxidized species. The reduction is facile, with a similar reduction potential for various acceptor species, but re-oxidation is not possible. It is apparently an electrochemical reduction involving a single-proton transfer and single-electron transfer. Based on this conclusion, an electrode (PB/Au/TiO₂ NTAs) was fabricated by means of electrodeposition. The electrode used to detect hydrogen peroxide. The sensitivity of the detector is high, and its the detection limit can be as low as 100 nM.     

Morphology and Microstructure of As-Synthesized Anodic TiO2 Nanotube Arrays

The as-grown structure of electrochemically synthesized titania nanotube arrays is investigated by scanning electron microscope (SEM) in combination with transmission electron microscope (TEM) as well as X-ray diffraction (XRD). The analysis reveals a preferred growth direction of the nanotubes relative to the substrate surface and the well control on the nanotube arrays morphology. The crystal structure of the anatase phase is detected and exists in the tube walls without any thermal treatment, which makes it possible to realize the application of as-formed TiO₂ nanotubes avoiding the degradation of the nanotube structures when sintering. In addition, a new growth, layered model of the anodic TiO₂ nanotubes is presented to obtain further understanding of the growth mechanism.