Showing papers on "Rutile published in 1996"

Electrochemical and photoelectrochemical investigation of single-crystal anatase

Abstract: Single crystals of TiO2 anatase containing 0.22% of Al and traces of V, Zr, Nb, and La were grown by chemical transport reactions employing TeCl4 as the transporting agent. Electrodes having the (101) face exposed doped by reduction with hydrogen were employed. The electrochemical and photoelectrochemical behavior of a single crystal of anatase were scrutinized for the first time. Properties were compared to those of single-crystal rutile having the (001) face exposed. Impedance analysis established that the flatband potential of anatase (101) is shifted negatively by 0.2 V with regards to that of rutile (001). Interfacial capacitance measurements under forward bias indicate smaller density of surface states on anatase. Photoelectrochemical oxidation of water occurs on both rutile and anatase with incident photon-to-current conversion efficiencies close to unity at λ = 300 nm. From the comparison of Ufb and Eg, it follows that anatase (101) and rutile (001) electrodes differ mainly in the position of the ...

Comparison of the Photoelectronic and Photocatalytic Activities of Various Anatase and Rutile Forms of Titania in Pure Liquid Organic Phases and in Aqueous Solutions

Abstract: Various titania samples of industrial origin (Degussa and Tioxide) have been characterized by electrical photoconductance measurements and tested as photocatalysts in various liquid media (either pure organic liquids or aqueous solutions) as a function of their structure (anatase versus rutile). Anatase was constantly found more active than rutile, whatever the reaction chosen (mild oxidation of pure cyclohexane and 2-propanol; total degradation of phenol and nitrophenol isomers in water). In identical conditions, Degussa was found more active, but the intrinsic activity, expressed in moles converted per hour and per square meter of active surface, was found slightly higher for anatase Tioxide. The higher activity of anatase with respect to that of rutile could be explained by the interpretation of the photoconductance measurements, which evidence the higher aptitude of anatase (i) to photoadsorb oxygen, in O2- and O- forms; (ii) to photodesorb it; and (iii) to have a lower relative electron−hole recombin...

Theoretical study of lithium intercalation in rutile and anatase

Abstract: Motivated by recent developments concerning coloration and energy storage in lithium intercalated nanostructural TiO2, quantum chem. Hartree-Fock calcns. have been carried out to study lithium atom intercalation in rutile and anatase. Equil. geometries and effective at. charge were obtained for the rutile (110) and anatase (101) clean surfaces. Li-induced local one-electron energy levels were found in the gap between the upper valence band and the conduction band and could be attributed to Ti3+ states. The absorption energies obtained are compared with available exptl. data. The equil. positions of the Li atom and its surrounding host atoms have been calcd. for both structures. The results predict a higher possibility of lithium intercalation in the anatase structure than in rutile.

Grain size dependence of anatase-to-rutile structural transformation in gel-derived nanocrystalline titania powders

Abstract: Nanocrystalline materials, which are composed of ultrafine crystals with dimensions ranging from several to tens of nanometres, are currently receiving much attention by virtue of their special chemical, physical and mechanical properties, which are suited for various applications. For the past two decades, the sol-gel process has become one of the successful techniques for preparing nanocrystalline metallic oxide materials. Among these oxides, titania is a very useful material for its gas sensitivity [1, 2] and excellent dielectric properties [3], as well as having catalysis applications [4-7]. Titania has three naturally occurring polymorphs: brookite (orthorhombic, Pcab), anatase (tetragonal, I41/amd), and rutile (tetragonal, P42/mnm). Rutile is the only stable phase, whereas anatase and brookite are both metastable at all temperatures. It has been widely demonstrated that some properties of titania are very sensitive to its microstructure. For example, the anatase phase is fotmd to be a superior support of V205/TiO2 catalyst for the selective i~artial oxidation reaction relative to the rutile phase [5], while rutile phase has proved to be rauch efficient to catalyse the decomposition of H2S gas than anatase phase in nanocrystalline titania powders with approximately the same specific surface area [6]. Recently [7], it has also been found that a 70%/30% anatase/rutile mix makes the best photocatalyst for the oxidation of organics when titania was applied to the treatment of wastewater. Therefore, it is of importance to investigate, in detail, the structural transformation of the titania system. In our previous papers, we have found that the transformation behaviour of anatase-to-rutile (A ~ R) in gel-derived nanocrystalline titania powders can be affected by many factors, such as impurities [8, 9], preparation conditions [10, 11] and so on. In this letter, the grain size dependence of the A--+ R transformation is reported. Nanocrystalline titania powders were prepared by a sol-gel method [8]. Tetrabutyl titanate was used as the precursor, ethanol as the solvent and hydrochloric acid as the hydrolysis catalyst. The molar ratio of these reactants was: Ti(Osu)4:EtOH:HC1 :H20 = 1:15:0.3:1. The experimental procedure has been described in detail previously. The as-prepared dry gel was confirmed to be of amorphous structure, and then crystallized into an anatase structure after annealing at 523K for 2h. The A + R phase

Preparation of anatase, brookite and rutile at low temperature by non-hydrolytic sol–gel methods

Abstract: Titania samples prepared by different non-hydrolytic sol–gel methods, mainly based on the etherolysis and alcoholysis of titanium tetrachloride, have been found to differ in both structure and texture. Thus, the reaction of diethyl ether with TiCl4 at 110 °C affords anatase, which begins to convert into rutile only around 1000 °C. The reaction of TiCl4 with ethanol leads to rutile as early as 110 °C, whereas the reaction of tert-butyl alcohol at 110 °C leads to the singular formation of brookite.

Open air plasma chemical vapor deposition of highly dielectric amorphous TiO2 films

Abstract: By using the cold plasma torch we developed, TiO2 films were deposited on substrates exposed to air by feeding Ti(OEt)4 into the plasma. XPS and x‐ray analyses revealed that all films were stoichiometric and amorphous TiO2, but Raman spectra indicated the existence of short‐range crystallinity in films deposited above 350 °C. The short range structure changed from anatase to rutile by admixing of hydrogen to the plasma. The rutile containing TiO2 film had a higher breakdown electric field as well as a higher dielectric constant than the anatase containing film.

Molecular Dynamic Simulation in Titanium Dioxide Polymorphs: Rutile, Brookite, and Anatase

Abstract: Molecular dynamic (MD) simulations with a quantum correction were performed on the titanium dioxide polymorphs. Interatomic potential functions of our new model are composed of Coulomb, short-range repulsion, van der Waals, and Morse interactions. The energy parameters were empirically determined to reproduce the fundamental properties of rutile crystal. The optimized crystal structure of TiO2, rutile, was in very good agreement with experimental data in the literature. For brookite and anatase, our MD simulations reproduced well the crystal structures and several physical properties, including volume thermal expansivity and bulk modulus. The present MD simulations with a new interatomic potential function and parameters successfully predicted the crystal structures of the titanium dioxide polymorphs.

Phase development in sputter deposited titanium dioxide

Abstract: We report the formation of highly oriented rutile titanium dioxide films Films of a primarily amorphous nature were grown on fused silica by rf reactive sputter deposition using a Ti target and rare gas (Ne or Ar)‐O2 discharges Post‐deposition annealing was done at 350–1150 °C in air for 60 minute intervals, followed by an air cool The phase mixture of the as‐deposited films, determined by x‐ray diffraction, was of two types: (I) amorphous + rutile + anatase, or (II) amorphous + rutile All phases were highly oriented with (110) rutile planes and (101) anatase planes parallel to the substrate Upon annealing, the amorphous component of films containing no anatase transformed entirely to rutile, even at temperatures where it is possible to form anatase, <800 °C, indicating that anatase requires ‘‘seeds’’ to form The results of this study clearly demonstrate that the crystal structure of the as‐deposited film determines the development of the rutile phase with post‐deposition annealing

Fluid-absent melting of F-rich phlogopite+rutile+quartz

Abstract: Abstract Fluid-absent melting is believed to be an important process in the generation of melts in the lower crust and upper mantle. Breakdown of phlogopite makes H2O available and thus controls the conditions at which fluid-absent melting occurs. Both F and Ti in biotite have been shown to affect strongly the thermal stability of biotite. To model better the fluid-absent melting of assemblages containing phlogopite, the reaction F-phlogopite + quartz + rutile = enstatite + melt has been studied experimentally. Experiments were performed at 7, 10, and 15 kbar using a natural F-rich phlogopite with a starting composition of F/(F + OH) = 0.43 and Mg/(Mg + Fe) (in molar proportions) = 0.94. Results indicate that the thermal stability of F-rich phlogopite + quartz + rutile is extended by as much as 450 °C relative to the KMASH system and by 300 °C relative to the Ti-free F-KMASH system. Approach to equilibrium in the experiments was assessed by convergence of results of melting and crystallization experiments. Phlogopite compositions from experimental products show that, although F-rich phlogopite incorporates relatively little Ti (2-3 wt% TiO2), the combination of F and Ti increases the stability of phlogopite to considerably higher temperatures (~300 °C) than that of either component alone. Melts formed by the fluid-absent melting of F-rich phlogopite + quartz + rutile at temperatures > 1000 °C are granitic and strongly peraluminous. The compositions of these melts suggest that the formation of metaluminous to peralkaline A-type granites by fluid-absent melting of halogen-enriched sources is unlikely.

Dielectric behaviour of nano‐TiO2 bulks

Abstract: Two kinds of nano-TiO 2 samples, including nano-anatase and nano-rutile, were prepared by the hydrothermal method and the gas condensation method, respectively. Results of dielectric measurements are as follows: 1. Nano-TiO 2 bulk samples (anatase and rutile) exhibit much higher dielectric constant than coarse-grain samples in the low measuring frequency range. 2. For both nano-anatase and nano-rutile, one peak appears in the spectrum of dielectric constant versus temperature, and it is located in the measuring temperature range of -150 to 150 °C for nano-anatase and in the measuring temperature range of 25 to 450 °C for nano-rutile. 3. For both nano-anatase and nano-rutile, in the spectrum of dielectric loss versus temperature there exists one peak. The mechanisms of the above-mentioned phenomena were discussed in detail.

Pearlescent glass pigment

Abstract: A pearlescent pigment consists essentially of C glass flakes having a layer comprising rutile titanium dioxide or iron oxide thereon. A hydrous layer of the rutile forming titanium dioxide or iron oxide is formed on the glass flakes and the resulting coated flakes are calcined.

Efficient TiO2 Powder and Film Photocatalysts with Rutile Crystal Structure

Abstract: Efficient TiO2 powder and film photocatalysts with the rutile structure were prepared. Their photocatalytic properties were assessed by measuring the photooxidative decomposition of gaseous acetaldehyde. The rutile powder showed much higher photoactivity than Degussa P-25 powder, both in degradation rate and in quantum efficiency. Moreover, the rutile film prepared by sintering the rutile sol at 450 °C also showed rather high photocatalytic activity.

Integrity and thermal decomposition of apatite in coatings influenced by underlying titanium during plasma spraying and post-heat-treatment

Abstract: The integrity and thermal decomposition of calcium apatite are influenced by the underlying titanium during plasma-spraying deposition, especially at the apatite/titanium interface. The destruction of apatite at the interface is governed by substrate temperature, titanium catalysis, and its reaction with titanium dioxide produced from oxidation of titanium in the plasma gas. The apatite in the outer layer of coatings is affected mainly by the substrate temperature and can keep its integrity with a suitable plasma-spraying procedure to minimize the increase of substrate temperature. The heat treatment of the coatings in vacuum results in the decomposition of apatite to α-tricalcium phosphate (α-TCP) and tetracalcium phosphate monoxide (TCPM) with the increase of intensity approaching the interface, which roughens the surface of the coatings. In the air-heat treatment, oxidation of titanium produces a thickened, dense rutile layer at the interface which prevents titanium atoms from diffusing into the coatings and inhibits the titanium-catalyzed decomposition of apatite. The apatite adjacent to the rutile layer reacts moderately with rutile to produce calcium titanate (CaTiO3), α- and β-TCP, while the apatite in the outer layer, separated from the rutile layer, maintains its integrity without decomposition even in a prolonged air-heat treatment. The retention of apatite integrity leads to a decreased surface roughness of the coating. © 1996 John Wiley & Sons, Inc.

Temperature effects on the size of anatase crystallites in MoTiO2 and WTiO2 powders

Abstract: The substitution effects of Ti by Mo and W ions on the stability of anatase in commercially available TiO 2 powders have been studied by X-ray diffraction, transmission electron microscopy, and surface area measurements in the 300–1000 K temperature range. The powder containing W shows smaller microstrains compared to those of pure anatase phase and those containing Mo. The stabilization effect of vacancies in the anatase phase is discussed and the anatase to rutile transformation caused by annealing is proposed to be related to the disappearance of Ti 3+ ions in the anatase structure. Finally, a possible redox mechanism Ti 3+ ⇔ Ti 4+ + e − is proposed to play a fundamental role in the catalytic and sensor properties of this material.

Method for the production of synthetic rutile

Abstract: Synthetic Rutile is prepared from raw ilmenite ore by a method comprising activating ilmenite and subjecting it to a multi-stage countercurrent leaching process in hot hydrochloric acid.

Atomic-layer growth of TiO2-II thin films

Abstract: Thin films of the high-pressure polymorph of titania, TiO2-II, were grown by atomic-layer deposition (ALD) using TiCl4 and H2O as precursors. Pure TiO2-II was synthesized at substrate temperatures ranging from 375 to 550degreesC while the films grown at 350 and 600degrees C contained mixed anatase-TiO2-II and rutile-TiO2-II phases respectively. Transformation of ALD-grown TiO2-II into rutile occurred on post-growth annealing at temperatures as high as 700degreesC.

Method for preparing hydrous titanium oxide spherules and other gel forms thereof

Abstract: The present invention are methods for preparing hydrous titanium oxide spherules, hydrous titanium oxide gels such as gel slabs, films, capillary and electrophoresis gels, titanium monohydrogen phosphate spherules, hydrous titanium oxide spherules having suspendible particles homogeneously embedded within to form a composite sorbent, titanium monohydrogen phosphate spherules having suspendible particles of at least one different sorbent homogeneously embedded within to form a composite sorbent having a desired crystallinity, titanium oxide spherules in the form of anatase, brookite or rutile, titanium oxide spherules having suspendible particles homogeneously embedded within to form a composite, hydrous titanium oxide fiber materials, titanium oxide fiber materials, hydrous titanium oxide fiber materials having suspendible particles homogeneously embedded within to form a composite, titanium oxide fiber materials having suspendible particles homogeneously embedded within to form a composite and spherules of barium titanate. These variations of hydrous titanium oxide spherules and gel forms prepared by the gel-sphere, internal gelation process offer more useful forms of inorganic ion exchangers, catalysts, getters and ceramics.

Effects of titanium dioxide passive film crystal structure, thickness, and crystallinity on C3 adsorption.

Abstract: The effects of titanium dioxide passive film crystal structure, thickness, and crystallinity on C3 adsorption from diluted human plasma were measured. Titanium dioxide surfaces created include (1) 70-nm anatase and rutile films comprising a mixture of amorphous and microcrystalline titanium dioxide, (2) 140-nm anatase and rutile films with greater crystallinity than the 70-nm films, (3) 70-nm aged anatase films with approximately the same crystallinity as that of the 140-nm anatase films, (4) sintered anatase and sintered rutile with no underlying metal, representing completely crystalline oxide films of infinite thickness, and (5) electropolished titanium. All combinations of experimental variables of surface type, exposure time, and plasma dilution were replicated four times. Anatase and rutile C3 surface concentrations increased with increased oxide thickness and crystallinity. The 70-nm aged anatase film isotherm more closely matched that of the 140-nm than of the 70-nm film, indicating that some property associated with oxide crystallinity is more influential in C3 adsorption than properties associated with oxide thickness alone. Concentrations of C3 were equal for equivalent anatase and rutile surfaces, except at high plasma concentrations and long exposure times, in which more C3 was adsorbed to anatase, suggesting that crystal structure may not be a significant controlling factor.