Showing papers on "Rutile published in 2001"

Band-Gap Narrowing of Titanium Dioxide by Nitrogen Doping

Abstract: TiO2-based powder, including 0.1 at% of N doped in the rutile lattice, has been synthesized by oxidation of TiN. As a result, a significant shift of the absorption edge to a lower energy in the visible-light region has been observed. The substitutional doping of N into the TiO2 lattice is found to be effective; its 2p states contribute to the band-gap narrowing by mixing with O 2p as shown in ab initio electronic structure calculations.

Preparation and structure analysis of titanium oxide nanotubes

Abstract: Well crystallized nanoscale tubular materials have been synthesized via the reaction of TiO2 crystals of either anatase or rutile phase and NaOH aqueous solution. The atomic structure of the synthesized tubular material is imaged by high-resolution transmission electron microscopy (HRTEM), and the composition of individual tubular structures is determined using selected area energy dispersive X-ray spectroscopy (EDX). Our results show that the tubular materials are well crystallized tubes with an average diameter of about 9 nm and little dispersion, and are composed of mainly titanium and oxygen. The atomic ratio of O/Ti is found, however, to vary from tube to tube. Detailed electron and x-ray diffraction studies show that the structure of our titanium oxide nanotubes do not agree with those made of TiO2 crystals with either anatase or rutile phase. HRTEM observations revealed that the titanium oxide nanotubes usually have multiple shells, in analogy with multiwalled carbon nanotubes, but the shell spacin...

Hydrothermal synthesis of nanosized anatase and rutile TiO2 using amorphous phase TiO2

Abstract: Phase-pure TiO2 nanocrystallites with narrow particle-size distributions were selectively prepared by hydrothermal processes starting from amorphous TiO2. Autoclaving amorphous TiO2 in the presence of HF and HCl as cooperative catalysts led to the formation of narrow-sized anatase TiO2 with a regular crystalline surface. Use of nitric acid as a cooperative catalyst with HF also gave the anatase TiO2 with a narrow size distribution but with a rather irregular crystalline surface. On the other hand, amorphous TiO2 was converted to phase-pure rutile TiO2 nanocrystallites by autoclaving in the presence of citric and nitric acids. Chelation of TiO6 octahedra with citric acid and acidification with nitric acid are critical for the phase transition from amorphous to rutile. The shape of all rutile nanocrystallites was rod-like. The crystal growth of TiO2 to phase-pure anatase or rutile was proposed to proceed via respective face-sharing and edge-sharing processes through dissolution–precipitation of the dissolved TiO6 octahedra from the amorphous phase. The photocatalytic activity in the redox reaction of 2-propanol with oxygen was quite comparable among the synthesized anatase and rutile nanocrystallites.

Photocatalytic Activities of Pure Rutile Particles Isolated from TiO2 Powder by Dissolving the Anatase Component in HF Solution

Abstract: We have found that anatase TiO2 dissolves into an HF solution more easily than rutile TiO2. Using this property, we succeeded in isolating pure rutile particles from TiO2 powder consisting of rutile and anatase phases by the HF treatment. The isolation of pure rutile particles was confirmed by X-ray diffraction patterns and diffuse reflection spectra. The rutile particle showed much lower photocatalytic activity than the original powder, if compared for the oxidation of 2-propanol in aqueous solution using dissolved oxygen as the electron acceptor. On the other hand, when Fe(III) ions were used as the electron acceptor for oxidation of 2-propanol and water, the activity scarcely changed after the HF treatment. These results indicate that rutile TiO2 has poor activity for the reduction of oxygen, and the presence of a small amount of an anatase component is crucial for an efficient photocatalytic reaction on TiO2 particles using oxygen as the electron acceptor.

Anatase-to-rutile transformation in titania powders

Abstract: Titania (TiO2) is an important electronic ceramic material for use in diverse applications such as gas sensors, catalysts, dielectrics, and ceramic membranes. TiO2 exists as several polymorphic phases, most commonly as rutile or anatase. This paper investigates the microstructural evolution of anatase-based commercial TiO2 powders, with an average size of 100 nm, at high temperatures. These powders transform to the rutile structure at 1000°C. The characteristics of the anatase-to-rutile transformation have been studied using transmission electron microscopy analysis, and new information regarding the nature and mechanisms of this polymorphic reaction has been revealed.

Composite n–p semiconducting titanium oxides as gas sensors

Abstract: Anatase and rutile thick films generated from the same source of titania were found to exhibit different types of conductivity upon exposure to CO and CH 4 at 600°C in a background of 5% O 2 /95% N 2 . Anatase behaved as a n-type semiconductor, with a decrease in resistance with reducing gas, whereas rutile exhibited p-type conductivity. The morphology of the particles were different, with anatase consisting of spherical particles of 100–200 nm dimensions, whereas rutile appeared as elongated rods of ∼1 μm lengths. The n-type behavior of anatase can be explained based on the oxygen vacancies. For explanation of the p-type behavior of rutile, impurities in the sample have to be taken into account. The impurity contents in both samples were similar, and doping of the lower valent impurities into the TiO 2 lattice should lead to creation of interstitial titanium defects. During anatase to rutile conversion at temperatures of 1000°C, the titanium interstitials can help incorporate excess oxygen, leading to formation of holes and p-type conductivity in the rutile phase. Resistance changes upon interaction of reducing gas with composites of anatase–rutile was also studied. It was found that samples with 4 followed a similar pattern. However, since the overall response of CH 4 was smaller than that of CO, the 75% rutile sample showed no change upon exposure to CH 4 , while exhibiting an n-type response to CO, indicative of a selective CO sensor at temperatures of 600°C. A polychromatic percolation model was developed to explain the electrical data. Two independent, parallel pathways involving the n-type anatase and p-type rutile were considered to be important in the conductivity. Using experimental data related to the extent of sintering, and appropriate particle sizes, the model predicted that n–n percolation would occur from 0 up to 94.5% rutile and p–p percolation would begin at 75.1% rutile. In between 75.1 and 94.5% rutile, both n- and p-pathways would percolate, resulting in the observed diminished changes in resistance.

Influence of solution concentration on the hydrothermal preparation of titania crystallites

Abstract: Nano/micro-sized titania powders were prepared by a hydrothermal method using an aqueous solution of TiCl4 as precursor. The influence of the concentration of the aqueous TiCl4 solution on the formation, phase, morphology, and grain size of the products was investigated at various reaction temperatures and times. Increasing the concentration of TiCl4 favored the formation of rutile type TiO2. When [TiCl4] < 0.5 mol dm−3, the products were a mixture of rutile type and anatase type powders, the percentage of rutile type powders in all crystalline products varied with reaction time. When [TiCl4] = 0.5 mol dm−3, the product was a pure rutile type powder. The powder size was related to the concentration based on the nucleation and growth theory of supersaturated solutions. A growth unit model was proposed to explain the phase formation.

Ferromagnetism in Co-Doped TiO2 Rutile Thin Films Grown by Laser Molecular Beam Epitaxy

Abstract: Epitaxial TiO2 rutile films were fabricated on α-Al2O3 (1012) substrate in the layer-by-layer fashion by laser molecular beam epitaxy. Ablation with a Co-doped TiO2 target produced single phase of rutile film with the concentration of Co between 0 and 5%. Some ferromagnetic domain structures were observed in CoxTi1-xO2 rutile films by a scanning superconducting quantum interference device microscope at 3 to 90 K. The magnetic hysteresis could also be observed even at room temperature with a magnetic moment of ~ 1 µB/Co atom.

Effects of thermally induced anatase-to-rutile phase transition in MOCVD-grown TiO2 films on structural and optical properties

Abstract: Titanium dioxide (TiO2) films with a thickness of 550 nm were deposited on quartz glass at 300 °C by metalorganic chemical vapor deposition. The effects of post-annealing between 600 °C and 1000 °C were investigated on the structural and optical properties of the films. X-ray diffraction patterns revealed that the anatase phase of as-grown TiO2 films began to be transformed into rutile at the annealing temperature of 900 °C. The TiO2 films were entirely changed to the rutile phase at 1000 °C. From scanning electron spectroscopy and atomic force microscopy images, it was confirmed that the microstructure of as-deposited films changed from narrow columnar grains into wide columnar ones. The surface composition of the TiO2 films, which was analyzed by X-ray photoelectron spectroscopy data, was nearly constant although the films were annealed at different temperatures. When the annealing temperature increased, the transmittance of the films decreased, whereas the refractive index and the extinction coefficient calculated by the envelope method increased at high temperature. The values of optical band gap decreased from 3.5 eV to 3.25 eV at 900 °C. This abrupt decrease was consistent with the anatase-to-rutile phase transition.

Ion Pair Formation and Primary Charging Behavior of Titanium Oxide (Anatase and Rutile)

Abstract: The primary charging behavior of titanium oxide (anatase, rutile, and P25) and the ion pair formation of the electrolyte ions with the surface groups have been extensively studied. A large number of titration and electrokinetic data sets available in the literature have been successfully described, using the MUSIC (MultiSite Complexation) model with a Basic Stern double-layer option and applying the ion pair formation concept. The systematic analysis of the data, over a large number of different monovalent electrolytes and various ionic strength values, allowed the determination of a number of “best estimated” values for the ion pair formation constants. The values suggest that the interaction of the cations with the titania surface is stronger than that of the anions. This is in accordance with the observed shift of the IEP of titanium oxide to higher pH values, at high electrolyte concentrations. The binding of the cations follows the sequence Cs+ < K+ < Na+ < Li+ and that of the anions follows the sequ...

Oxidation and phase transitions of epitaxial tin oxide thin films on (1̄012) sapphire

Abstract: We studied the structural behavior and electrical transport properties of epitaxial α-SnO thin films grown on the (1012) α-Al2O3 (sapphire) substrate Hall effect measurements revealed that the epitaxial as-deposited SnO film is a p-type semiconductor In situ x-ray diffraction studies show that the α-SnO phase is metastable and will transform into SnO2 with the rutile type structure when annealed at high temperatures in air The onset of this phase transformation was observed to begin approximately at 300 °C during heating Shortly thereafter, rutile SnO2 was observed to coexist with α-SnO and intermediate products such as Sn and Sn3O4 After being annealed at temperatures above 600 °C, the film then fully transformed into the rutile SnO2 phase Our results show that the α-SnO to SnO2 structural transformation proceeds initially by the localized disproportionate redistribution of internal oxygen at low temperature, followed by the transformation of the remaining SnO phase and intermediate phases into Sn

Photonic Crystals from Emulsion Templates

Abstract: Macroporous titania, which undergoes transition to the rutile phase by calcination without collapse of the pore structure, is obtained by polymerizing a titania sol suspended around “colloidal crystals” of oil droplets. The deformable template counteracts cracking of the titania phase. The Figure shows a scanning electron micrograph of a rutile sample with 200 nm pores obtained by the method described.

Surface characterizations of variously treated titanium materials.

Abstract: The attachment of cells to titanium surfaces is an important phenomenon in the area of clinical implant dentistry. A major consideration in designing implants has been to produce surfaces that promote desirable responses in the cells and tissues. To achieve these requirements, the titanium implant surface can be modified in various ways. This research was designed to elucidate the relationship between surface roughness (Ra) and contact angle (theta) of various engineered titanium surfaces of commercially pure titanium, titanium-aluminum-vanadium alloy (Ti-6Al-4V), and titanium-nickel (TiNi) alloy. The contact angle was measured using distilled water, 1% sodium chloride solution, human neutrophils, and osteoblast-like cells. Surface oxide crystallography was identified by transmission electron diffraction. It was found that: (1) there were no significant differences in contact angles among the 4 media; (2) for commercially pure titanium, a combined treatment (hydrofluoric acid/nitric acid/water --> sodium hydroxide --> oxidation) showed the lowest theta (10.51 degrees in water), while the surface treated with sulfuric acid showed the highest value (72.99 degrees in water); (3) for all commercially pure titanium samples, when theta is greater than 45 degrees, the contact angle increases linearly with Ra (hydrophobic nature) and the surface is covered with rutile-type oxide only, while the contact angle decreases linearly with Ra when theta is less than 45 degrees (hydrophilic nature) and the surface is covered with a mixture of rutile and anatase oxides; and (4) a similar trend was found on Ti-6Al-4V and TiNi surfaces.

An ultradense polymorph of rutile with seven-coordinated titanium from the Ries crater.

Abstract: We report the discovery of an ultradense post-rutile polymorph of titanium dioxide in shocked gneisses of the Ries crater in Germany. The microscopic diagnostic feature is intense blue internal reflections in crossed polarizers in reflected light. X-ray diffraction studies revealed a monoclinic lattice, isostructural with the baddeleyite ZrO2 polymorph, and the titanium cation is coordinated with seven oxygen anions. The cell parameters are as follows: a = 4.606(2) angstroms, b = 4.986(3) angstroms, c = 4.933(3) angstroms, beta (angle between c and a axes) = 99.17(6) degrees; space group P2(1)/c; density = 4.72 grams per cubic centimeter, where the numbers in parentheses are standard deviations in the last significant digits. This phase is 11% denser than rutile. The mineral is sensitive to x-ray irradiation and tends to invert to rutile. The presence of baddeleyite-type TiO2 in the shocked rocks indicates that the peak shock pressure was between 16 and 20 gigapascals, and the post-shock temperature was much lower than 500 degrees C.

Low temperature growth of rutile TiO2 films in modified rf magnetron sputtering

Abstract: Low temperature (≤300°C) growth of rutile TiO2 films with high refractive index which is equal to bulk TiO2 crystal was achieved by using a modified sputtering method. Depositions were carried out in rf magnetron sputtering apparatus equipped with an auxiliary permanent magnet just under the grounded electrode. The as-deposited films showed rutile polycrystalline structure and fine surface morphology indicating higher densification. Remarkable changes in composition, total current and energy of incident ions were presented at a Ar–O2 total pressure of 2.7 Pa. The rutile phase grows in a modified sputtering method in contrast with anatase phase growth in conventional sputtering apparatus.

Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride.

Abstract: Many reactive metals are difficult to prepare in pure form without complicated and expensive procedures. Although titanium has many desirable properties (it is light, strong and corrosion-resistant), its use has been restricted because of its high processing cost. In the current pyrometallurgical process--the Kroll process--the titanium minerals rutile and ilmenite are carbochlorinated to remove oxygen, iron and other impurities, producing a TiCl4 vapour. This is then reduced to titanium metal by magnesium metal; the by-product MgCl2 is removed by vacuum distillation. The prediction that this process would be replaced by an electrochemical route has not been fulfilled; attempts involving the electro-deposition of titanium from ionic solutions have been hampered by difficulties in eliminating the redox cycling of multivalent titanium ions and in handling very reactive dendritic products. Here we report an electrochemical method for the direct reduction of solid TiO2, in which the oxygen is ionized, dissolved in a molten salt and discharged at the anode, leaving pure titanium at the cathode. The simplicity and rapidity of this process compared to conventional routes should result in reduced production costs and the approach should be applicable to a wide range of metal oxides.

Al-doped TiO2 pigments: influence of doping on the photocatalytic degradation of alkyd resins

Abstract: Rutile white pigments are generally doped with Al2O3 to suppress their photocatalytic activity, the mechanism of this effect being however obscure. In this investigation, rutile microcrystals were used that had been doped with increasing amounts of Al2O3. Parts of the dopant were homogeneously dissolved in the crystal while the rests had segregated to the particle surface. To study the effects of dissolved Al2O3 alone, the Al2O3 at the surface could be removed by etching. Photocatalytic activities of the pigments with and without surface-Al2O3 were determined by artificial weathering of pigmented alkyd paint films and a kinetic analysis in terms of Weibull statistics of failure. Results were interpreted using basic models of photosemiconductors. It is found that the reduction of photocatalytic activities by Al2O3 in the bulk is more important than at the surface although its amounts in the bulk are smaller. Bulk-Al2O3 dopant states provide efficient recombination sites for the electrical charges photogenerated within the TiO2 crystal. However, among the two modes of incorporation of Al2O3 into rutile only the one providing oxygen vacancies is effective. The slight activity reduction by surface-Al2O3 coincides with the reduction of surface hydroxyl concentration measured with these pigments.

Methods of obtaining photoactive coatings and/or anatase crystalline phase of titanium oxides and articles made thereby

Abstract: Hydrophilic and/or rutile and anatase titanium oxide are obtained by sputter depositing titanium metal oxide on a film of zirconium oxide in the cubic phase. Another technique is to deposit a titanium metal on a film of zinc oxide in the cubic phase and heating the coating in an oxidizing atmosphere to provide an anatase and/or rutile phase(s) of titanium oxide.

Modification of TiO2 Network Structures Using a Polymer Gel Coating Technique

Abstract: A variety of polymer gels with different chemical composition, architecture, porosity, and surface area have been used as templating materials for the fabrication of porous TiO2 networks. Titanium isopropoxide was incorporated in the bicontinuous polymer structure where hydrolysis and condensation reactions were carried out, producing a hybrid of polymer and amorphous titania. Calcination of this hybrid resulted in the formation of a continuous, purely inorganic network of either the anatase or rutile crystal phase, with the individual titanium dioxide particles contacting a number of neighbors, thereby forming a continuous three-dimensional structure. The resulting networks have vastly different structures, with porosities as high as 99% and surface areas ranging from 5 to 100 m2 g-1. These structures may be of importance to various fields of research, including photovoltaics and photocatalysis, because the open “coral-like” network structure of the titanium dioxide allows high access of the titanium dio...

Photocatalytic Effects of Rutile Phase TiO2 Ultrafine Powder with High Specific Surface Area Obtained by a Homogeneous Precipitation Process at Low Temperatures

Abstract: The photocatalytic characteristics of nanostructured TiO2 ultrafine powder with rutile phase produced using the homogeneous precipitation process at low temperatures (HPPLT) were compared with those of commercial P-25 TiO2 powder by flame hydrolysis. The TiO2 powder by the HPPLT showed much higher photoactivity in the removal rate, showing lower pH values in the solution than the P-25 powder when eliminating metal ions such as Pb and Cu from the aqueous metal-EDTA solutions. This can be inferred as the more rapid photo-oxidation or -reduction of metal ions from the aqueous solution, together with relatively higher efficiencies in the use of an electron-hole pair formed on the surface of the TiO2 particles under UV light irradiation. Also, in the view of the TiO2 particle morphology, compared to the well-dispersed spherical P-25 particles, the agglomerated TiO2 secondary particles by the HPPLT consist of acicular typed primary particles with a thickness in the range of 3–7 nm and the primary particles radialize in all directions, which would be more effective to photocatalytic reactions without the large electron-hole recombination on the surface of the TiO2 particle under UV light irradiation. It can be, therefore, thought that the higher photoactivity of the rutile TiO2 powder by the HPPLT in the aqueous solutions results mainly from having a larger surface area by the acicular shaped primary particles with very thin thickness and radialization in all directions.

The crystalline phase stability of titania particles prepared at room temperature by the sol–gel method

Abstract: Titania particles having anatase, brookite and rutile phase were prepared at various H+/TTIP (Titaniumtetraisopropoxide) mole ratios and room temperature by the sol-gel method The crystalline phases according to the variation of the post heat treatment temperature were observed The crystalline phase and the phase transformation, morphology, and crystallite size were identified by using XRD, TG/DTA, Raman spectroscopy and TEM The brookite phase of titania particles prepared at the H+/TTIP mole ratio of 002 and room temperature was not transformed into anatase or rutile even with the heat treatment at 750°C, and also the anatase phase was stable at the temperature as high as 850°C However, the titania particles prepared at the H+/TTIP mole ratio of 067, which contained the mixed phases of anatase, brookite, and rutile at room temperature, showed only rutile phase at temperature of 750°C It was thus shown that the initial crystalline phase of the primary particles prepared at room temperature had an important effect on the phase transformation behavior upon post heating Phase transformation from brookite to anatase and subsequently to rutile occurred with heating