Formation mechanism and packing options in tubular anodic titania films

Titania nanotube arrays were synthesized via anodic oxidation of titanium foils in glycerol electrolyte containing NH₄F at anodization voltage ranging from 10 V to 30 V. The structural parameters of self-organized periodic arrays of titania nanotubes were determined by small-angle neutron scattering and scanning electron microscopy techniques. Transmission electron microscopy and electron diffraction studies of single-standing nanotubes revealed the presence of nanocrystalline titanium oxide phases with oxidation states lower than +4 (TiO, Ti₂O₃). Several assumptions on growth and self-organization mechanism of nanotube arrays have been made.     

Fabrication of complete titania nanoporous structures via electrochemical anodization of Ti

We present a novel method to fabricate complete and highly oriented anodic titanium oxide (ATO) nano-porous structures with uniform and parallel nanochannels. ATO nano-porous structures are fabricated by anodizing a Ti-foil in two different organic viscous electrolytes at room temperature using a two-step anodizing method. TiO₂ nanotubes covered with a few nanometer thin nano-porous layer is produced when the first and the second anodization are carried out in the same electrolyte. However, a complete titania nano-porous (TNP) structures are obtained when the second anodization is conducted in a viscous electrolyte when compared to the first one. TNP structure was attributed to the suppression of F-rich layer dissolution between the cell boundaries in the viscous electrolyte. The structural morphologies were examined by field emission scanning electron microscope. The average pore diameter is approximately 70 nm, while the average inter-pore distance is approximately 130 nm. These TNP structures are useful to fabricate other nanostructure materials and nanodevices.     

Influence of molybdenum species on growth of anodic titania

TiMo, bcc, solid-solution alloys, containing 11.5-37.0 at.% molybdenum, have been anodised galvanostatically in 0.1 mol dm⁻³ ammonium pentaborate and 1.0 mol dm⁻³ phosphoric acid electrolytes, with resultant anodic films characterised by scanning electron microscopy, transmission electron microscopy, Rutherford backscattering spectroscopy and glow discharge optical emission spectroscopy. Uniform amorphous films are formed at high current efficiency to >100 V, with formation ratios of 2.3 and 2.2 nm V⁻¹ in the respective electrolytes, contrasting with the amorphous-to-crystalline transition of anodic titania on titanium that occurs at ∼20-50 V. Apart from minor incorporation of electrolyte species, the films comprise an outer layer of TiO₂ and an inner oxide layer containing Ti⁴⁺ and Mo⁶⁺ ions. The films grow by migration of both cations and anions, with Ti⁴⁺ ions migrating faster than Mo⁶⁺ ions that is related to the energies of Ti⁴⁺O and Mo⁶⁺O bonds.     

Mesoporous Ti2Nb10O29 microspheres constructed by interconnected nanoparticles as high performance anode material for lithium ion batteries

Mesoporous Ti₂Nb₁₀O₂₉ microspheres are prepared through a solvothermal method in isopropanol-glycerol solvent followed by calcination treatment. The Ti₂Nb₁₀O₂₉ microspheres with diameter in the range between 1.0 and 2.0?µm are assembled from nanoparticles. The special hierarchical mesoporous structure of Ti₂Nb₁₀O₂₉ microspheres can reduce the Li⁺ ions diffusion distance and supply large interfacial area between electrolyte and electrode, endowing them with better electrochemical properties compared to Ti₂Nb₁₀O₂₉ nanoparticles. At different current densities of 1, 5, 10, 20 and 30?C, the first discharge capacity of mesoporous microspheres is respectively 268.9, 218.5, 205.3, 180.5, and 158.1 mA?h g^?1. By contrast, the corresponding values of Ti₂Nb₁₀O₂₉ nanoparticles are quickly reduced from 254.6, 196.3, 159.4, 127.5, to 99.0 mA?h g^?1. It worth noting that Ti₂Nb₁₀O₂₉ microspheres exhibit excellent rate capacities. Furthermore, at a current density of 10?C, the specific capacity of Ti₂Nb₁₀O₂₉ microspheres can be maintained at 173.5 mA?h g^?1 after 500 cycles, with 86.8% retention of the first discharge capacity. These results suggest that the Ti₂Nb₁₀O₂₉ mesoporous microspheres would be developed as potential anodic materials for high performance lithium-ion batteries.     

Electrical instability of composite aluminum oxide films

Composite films of hydrous + anodic Al₂O₃ have the peculiar property of losing field strength after anodization. After some time on open circuit a film that had supported several hundred volts will suddenly sustain no more than 10–20 V. A “reformation” to the original voltage brings the film to a stable state. Properties of this instability are described here. It is shown to result from diffusion of water into internal voids within the barrier oxide layer. The voids become filled with anodic oxide during the reformation.     

Etude des reactions anodiques des ions oxydes dissous dans l'eutectique LiCl-KCl sur electrodes de graphite par des methodes electrochimiques impulsionnelles

The oxide anodic reaction on a graphite electrode in fused LiClKCl eutectic has been investigated by use of chronopotentiometry, chronoamperometry and cyclic voltammetry. The analysis of the anode response shows the existence of two steps; the first is due to adsorbed oxide ions, whereas a second part of the signal corresponds to the diffusion of electroactive species towards the electrode. The amount of oxide ions γ_eq adsorbed on the graphite anode and the diffusion coefficent D of the electroactive species are calculated: γ_eq ? 8.10^?8 moles cm^?2 for a concentration of oxide ions in the liquid phase C_O^2? = 4,96.10^?5 moles cm^?3; D_O^2? = 2.10^?5 cm² s^?1 at 442°C.A rapid determination of the oxide ions content in the fused LiClKCl eutectic is proposed for a range of relatively high concentration values (C_O^2? > 20 ppm).     

Anodic titanium oxide: A new template for the synthesis of larger diameter multi-walled carbon nanotubes

Carbon nanotubes (CNTs) with larger diameter were synthesized over anodic titanium oxide (ATO) template by CVD method using acetylene as carbon source. The porous titanium oxide was obtained by anodization of titanium metal in a mixture of 1 M H₂SO₄ + 0.5% HF electrolyte at a constant applied potential of 40 V. The XRD analysis of anodized titanium revealed that rutile and anatase forms of TiO₂ are formed due to anodization. Further, SEM analysis was used to follow the development of pores on titanium surface. The TEM analysis revealed that the formed CNTs are straight and hollow with uniform wall thickness as well as larger diameter (70–80 nm). HRTEM study showed that the formed CNTs are multi-walled and their wall thickness is around 2–3 nm. Further, the structural features of the formed CNTs were studied by XRD. Raman spectroscopy was used to study the degree of graphitization of CNTs. The Lewis acid sites of TiO₂ present in the internal surface of the pores play an important role in the catalytic decomposition of acetylene and hence the formation of CNTs. When increasing the carbon deposition time, the wall thickness of CNTs is not increased significantly, indicating that the decomposition of acetylene is due to Lewis acid sites of TiO₂ and not due to thermal decomposition. Further, the morphology of CNTs formed over ATO template was compared with that of CNTs formed on Co electrodeposited ATO. There is no significant difference in morphology as well as wall thickness was observed between the CNTs grown over ATO with and without Co catalyst. But, still further investigations are necessary to study the structural differences between the CNTs grown over ATO with and without Co catalyst.     

New effective process to fabricate fast switching and high contrast electrochromic device based on viologen and Prussian blue/antimony tin oxide nano-composites with dark colored state

We developed a new electrochromic device by using compact Prussian blue (PB)/antimony tin oxide (ATO) nano-composites as anodic electrode and viologen anchoring on titanium dioxide (TiO₂) nano-particles as cathodic electrode. The anodic electrode was based on a transparent nanostructured ATO nano-particle film and was electro-deposited by Prussian blue to form compact Prussian blue/ATO nano-composites by means of galvanostatic electrodeposition process. Nanocrystalline TiO₂ thin films on conducting glass were modified with a mono-layer of viologen with two anchoring groups, which were much strongly adsorbed onto the surface of TiO₂ nano-particles. A polymer gel electrolyte sandwiched between the anodic and cathodic layers is used as the ionic transport layer. The 2.5 cm × 2.5 cm electrochromic device shows high contrast (64.8%, at 600 nm) very low transmittance at colored stage (0.1%, at 600 nm), fast switching time (600 and 720 ms for coloration and bleaching, respectively), high coloration efficiency of 912 cm² C⁻¹ at 600 nm and good stability. The enhanced performance of the electrochromic device can be attributed to the ATO nano-particles as inter-conductive materials.     

Eightfold increased membrane flux of NF 270 by O3 oxidation of natural humic acids without deteriorated permeate quality

BACKGROUND: Membrane fouling by humic acids limits the water recovery of nanofiltration in drinking water production. This article investigates if membrane fouling can be reduced by decomposition of humic acids in the concentrate stream by O₃ oxidation. RESULTS: At a specific O₃ dose of 2.11 g O₃ per g COD (17.0 g m^?3 O₃ (g) for 20 min), a COD reduction of 38% and a hydrophobic COD reduction of 69% is achieved. The membrane permeability of the ozonated solution by NF 270 membranes is higher (20.6 × 10^?9 L s^?1 m^?2 Pa^?1) than the permeability if the untreated solution is filtered (2.4 × 10^?9 L s^?1 m^?2 Pa^?1). The COD retention of the ozonated solution was similar to the retention of the untreated samples. The addition of H₂O₂ allows a better mineralization degree, i.e. UVA removal increased from 53% to 66% if H₂O₂ was added as from 10 min oxidation at the same molar flow rate as O₃. CONCLUSION: O₃ oxidation can substantially alleviate membrane fouling by humic acids in nanofiltration systems and the addition of H₂O₂ can slightly improve its decomposition. Copyright © 2010 Society of Chemical Industry     

Effect of the solvent on growth of titania nanotubes prepared by anodization of Ti in HCl

The effect of the solvent on the anodic growth of titania nanotubes in HCl dissolved in water, ethylene glycol and 2-propanol was studied. These nanotubes grow with locally rapid breakdown of the passive TiO₂ film forming a forest of nanotubes-bearing microtowers with the background of passive TiO₂ Film. These bundles of assembled-groups of titania nanotubes look like Pillars corals. The low relative permittivity of the 2-propanol led to lowering of dissociation of HCl and hence lowering the activity of H⁺ and Cl⁻ ions which in turn led to suppress of dissolution of titania and increasing the growth rate of the titania nanotubes. The X-ray diffraction pattern showed that the titania nanotubes after annealing change to the crystalline anatase phase. The anodic films showed characteristic coloration with intensity and color that changes (qualitatively) with time of anodization.     

Effect of calcination temperature on morphology and electrochemical performance of PbLi2Ti6O14

As a promising anode material, PbLi₂Ti₆O₁₄ has attracted the attention of many researchers. In this work, a series of PbLi₂Ti₆O₁₄ are prepared by solid state method at five different calcination temperatures and used as anode materials in lithium ion batteries. Through a series of tests, the results show that the phase purity, morphology and electrochemical performance of PbLi₂Ti₆O₁₄ can be seriously influenced by calcination temperature. When the calcination temperature is 900?°C, the phase-pure PbLi₂Ti₆O₁₄ can be obtained with relatively small particle size, excellent cycle performance and outstanding lithium ion diffusion behavior. It provides an initial charge capacity of 151.3?mA?h?g^?1 at 100?mA?g^?1. After 100 cycles, it shows a reversible capacity of 142.0?mA?h?g^?1 with superior capacity retention of 93.85%. In contrast, PbLi₂Ti₆O₁₄ formed at 800?°C displays an unsatisfactory performance due to the presence of impurity, even though it has the smallest particle size and the largest lithium ion diffusion coefficient among the five samples. The reversible capacity is only 82.6?mA?h?g^?1 after 100 cycles with capacity retention of 53.9%. In order to further study the lithium ion diffusion behavior of PbLi₂Ti₆O₁₄, the in-situ X-ray diffraction technique is also implemented. It is found that during the lithiation/delithiation process, the stable framework can effectively inhibit the volume change and ensures the excellent electrochemical performance of PbLi₂Ti₆O₁₄.     

Enhancement of reaction-sintering of alumina-excess magnesium aluminate spinel in presence of titania

Alumina-excess magnesium aluminate spinel finds use in different high temperature applications including steel ladles. Alumina-excess spinel was prepared by solid oxide reaction using magnesia (MgO=10?wt%) and calcined alumina (Al₂O₃ = 90?wt%), in the sintering temperature range of 1500–1700?°C. The role of titania on the densification, spinelisation, evolution of microstructure and phase assemblage was investigated in this MgO-Al₂O₃ system. Titania addition increased the rate of densification 20x compared to undoped composition at 1500?°C under dynamic heating condition. However, under static firing, the beneficial effect of titania on densification could only be discerned at lower temperatures. The microstructure of titania doped sintered alumina-excess spinel compacts contain magnesium aluminium titanate phase in the grain boundary of corundum and spinel grains. The beneficial effect of titania on densification is attributed to magnesium aluminium titanate phase (Mg_xAl_2(1-x)Ti_(1+x)O₅) development and also by incorporation of Ti⁴⁺ into the spinel structure.     

Sol-gel processed high-k aluminum oxide dielectric films for fully solution-processed low-voltage thin-film transistors

High-k oxide dielectric films have attracted intense interest for thin-film transistors (TFTs). However, high-quality oxide dielectrics were traditionally prepared by vacuum routes. Here, amorphous high-k alumina (Al₂O₃) thin films were prepared by the simple sol-gel spin-coating and post-annealing process. The microstructure and dielectric properties of Al₂O₃ dielectric films were systematically investigated. All the Al₂O₃ thin films annealed at 300–600?°C are in amorphous state with ultrasmooth surface (RMS ~ 0.2?nm) and high transparency (above 95%) in the visible range. The leakage current of Al₂O₃ films gradually decreases with the increase of annealing temperature. Al₂O₃ thin films annealed at 600?°C showed the low leakage current density down to 3.9?×?10^?7 A/cm² at 3?MV/cm. With the increase of annealing temperature, the capacitance first decreases then increases to 101.1?nF/cm² (at 600?°C). The obtained k values of Al₂O₃ films are up to 8.2. The achieved dielectric properties of Al₂O₃ thin films are highly comparable with that by vapor and solution methods. Moreover, the fully solution-processed InZnO TFTs with Al₂O₃ dielectric layer exhibit high mobility of 7.23?cm² V^?1 s^?1 at the low operating voltage of 3?V, which is much superior to that on SiO₂ dielectrics with mobility of 1.22?cm²/V^?1 s^?1 at the operating voltage of 40?V. These results demonstrate that solution-processed Al₂O₃ thin films are promising for low-power and high-performance oxide devices.     

Fabrication of nanoporous TiO2 by electrochemical anodization

The formation of self-organized porous titania is achieved by electrochemical anodization under a potentiostatic regime. Anodic titanium oxide (ATO) was fabricated by a three-step self-organized anodization of the Ti foil in an ethylene glycol electrolyte containing 0.38 wt% of NH₄F and 1.79 wt% of H₂O. Anodizing was carried out at the constant cell potential ranging from 30 to 70 V at the temperature of 20 °C. It was found that nanoporous TiO₂ arrays can be obtain only after a short duration of the third step (10 min). The influence of anodizing potential on the structural parameters of porous anodic titania including pore diameter, interpore distance, wall thickness, porosity and pore density was extensively studied. The linear dependencies between interpore distance, pore diameter and wall thickness upon the anodizing potential were found. The regularity of pore arrangement was monitored qualitatively by fast Fourier transforms (FFTs) of top-view FE-SEM images. It was found that the best arrangement of nanopores is observed at 40 V. This finding was confirmed by the analysis of pore circularity. The highest circularity of pores was observed once again at 40 V.     

Iron modified titanium–hafnium binary oxides as catalysts in total oxidation of ethyl acetate

Multicomponent iron–titanium–hafnium oxide materials with different compositions were prepared by combination of homogeneous precipitation with urea and incipient wetness impregnation techniques and tested as catalysts for ethyl acetate oxidation as representative VOCs. Nitrogen physisorption, XRD, Raman, UV–Vis, XPS, Mössbauer spectroscopy and TPR analyses reveal co-existence of substituted Fe_xTi_1 − xO₂ oxide, finely dispersed iron oxide species with supper paramagnetic behavior and well crystallized α-Fe₂O₃ particles, which relative part depends on hafnium content in titania lattice. The effect of phase composition on the catalytic behavior of these materials in ethyl acetate oxidation was discussed.     

Water Vapor‐Enhanced Diffusion in Alumina

Experimental results indicate effective faster diffusion of oxygen in polycrystalline alumina when exposed to water vapor at high temperatures. α‐Al₂O₃ containing Ni metal particles was exposed to dry air or humid environments at 1300°C for up to 20 h. Oxidation of Ni in α‐Al₂O₃ to form NiAl₂O₄ was used to determine oxygen diffusion depth from the surface. The apparent kinetic rate constant for oxygen diffusion in the presence of water vapor was 79% higher compared with that in a dry atmosphere at 1300°C (1.4 × 10^?13 m²/s for 0.2 atm versus 7.9 × 10^?14 m²/s for dry air). (OH)^? ions are smaller and more polarizable than O^2?, which would be expected to lead to faster diffusion. This effect may impact sintering, creep, corrosion, oxidation, and the performance of thermal barrier coatings (TBC).     

Anodic oxide films at nickel electrodes in alkaline solutions—I. Kinetics of growth of the β-Ni(OH)2 phase

The growth kinetics of thin anodic oxide films at nickel electrodes are examined in 0.1 N KOH solutions at room temperature under potentiodynamic and galvanostatic conditions. At electrodes that have been mechanically polished and cathodically reduced, growth of the lower oxidation state phase, β-Ni(OH)₂, follows, under both experimental conditions, the formalism of the Cabrera—Mott model of high field assisted formation of thin oxide films. At 10^?3 Acm^?2, the field within the oxide phase is 8.5 × 10⁶ V/cm. The field decreases 1.3 × 10⁶ V/cm for each ten-fold decrease of current density. The potential at which β-Ni(OH)₂ begins to grow, V₀ = ?0.39 V vs she, is identified as the reversible potential of the phase. It is, however, 270 mV positive than the reversible potential listed in literature for Ni(OH)₂ formation in solutions of the same pH. It is suggested that the observed reversible potential refers to the β-Ni(OH)₂ phase. The exchange current density for the oxide film growth is 2.3 × 10^?10 Acm^?2. Comparison of the kinetics at Pt and Ni electrodes indicates that the activation energy for the growth of the β-Ni(OH)₂ phase is 21 kcal/mole.     

Oxygen permeability and structural stability of a novel tantalum‐doped perovskite BaCo0.7Fe0.2Ta0.1O3−δ

Dense BaCo_0.7Fe_0.2Ta_0.1O_3?δ (BCFT) perovskite membranes were successfully synthesized by a simple solid state reaction. In situ high‐temperature X‐ray diffraction indicated the good structure stability and phase reversibility of BCFT at high temperatures. The thermal expansion coefficient (TEC) of BCFT was determined to amount 1.02 × 10^?5 K^?1, which is smaller than those of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) (1.15 × 10^?5 K^?1), SrCo_0.8Fe_0.2O_3?δ (SCF) (1.79 × 10^?5 K^?1), and BaCo_0.4Fe_0.4Zr_0.2O_3?δ (BCFZ) (1.03 × 10^?5 K^?1). It can be seen that the introduction of Ta ions into the perovskite framework could effectively lower the TEC. Thickness dependence studies of oxygen permeation through the BCFT membrane indicated that the oxygen permeation process was controlled by bulk diffusion. A membrane reactor made from BCFT was successfully operated for the partial oxidation of methane to syngas at 900°C for 400 h without failure and with the relatively high, stable oxygen permeation flux of about 16.8 ml/min cm². © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Synthesis and structural properties of lithium titanium oxide powder as-synthesized by two step calcination process

A two-step calcination synthesis, considering the potential for mass production, of lithium titanium oxide powder was carried out to fabricate a single Li₄Ti₅O₁₂ phase, which is useful for anode electrode material of Li-based rechargeable battery as well as an electrode for supercapacitor. The final composition is controlled by adding more TiO₂ powder into powder gained at one calcination process during the two calcination process. We investigated the influence of excess TiO₂ on the structural characteristics of lithium titanium oxide synthesized by the two-step calcination method. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) measurements showed that the as-synthesized powder had a spinel crystal structure as well as A composition of 4: 5: 12. In addition, a high resolution transmission electron microscopy (HRTEM) analysis revealed that the fabricated powder exhibited a single crystalline phase formation. These results indicated that the powder synthesized in the one-step calcination process showed coexistence crystalline phases, which are the Li₄Ti₅O₁₂ and Li_2.39Ti_3.4O₈ phase. However, in the two-step calcination process, the powder synthesized showed the single crystalline Li₄Ti₅O₁₂ phase. A very uniform grain size of the as-synthesized powder was shown in a field emission scanning electron microscopy (FESEM). These results suggested that the two-step calcination process can be used for synthesis of single crystalline Li₄Ti₅O₁₂ powder with uniform grain shape and provide motivation to pursue mass production of lithium titanium based oxide powder for bulk type batteries.     

The anodic oxidation of neodymium

The anodic oxidation of neodymium was investigated by the galvanostatic technique in Na₂-HPO₄ - ethylene glycol solution. According to the combination of measurments of ac capacity and of specular reflectivity, we obtained a refractive index for the oxide films as a function of wavelength and the value of apparent dielectric constant of the films for which sesquioxide Nd₂O₃ was assumed to have been produced. It was found that the film thickness could be determined from the wavelength of both maximum and minimum specular reflectivities due to interference, and that a linear relationship existed between the film thickness and the formation voltage. The slope of this linear line was 10.0Å V^?1 or a mean field of 1.00 × 10⁷ V cm^?1 at 1 mA cm^?2. The apparent current efficiency estimated from the thickness of the film vs passed charge plots scarcely depended on latter and its value was about 0.52, assuming that the film composition was Nd₂O₃.