Showing papers on "Rutile published in 2006"

Crystallization thermometers for zircon and rutile

Abstract: Zircon and rutile are common accessory minerals whose essential structural constituents, Zr, Ti, and Si can replace one another to a limited extent. Here we present the combined results of high pressure–temperature experiments and analyses of natural zircons and rutile crystals that reveal systematic changes with temperature in the uptake of Ti in zircon and Zr in rutile. Detailed calibrations of the temperature dependencies are presented as two geothermometers—Ti content of zircon and Zr content of rutile—that may find wide application in crustal petrology. Synthetic zircons were crystallized in the presence of rutile at 1–2 GPa and 1,025–1,450°C from both silicate melts and hydrothermal solutions, and the resulting crystals were analyzed for Ti by electron microprobe (EMP). To augment and extend the experimental results, zircons hosted by five natural rocks of well-constrained but diverse origin (0.7–3 GPa; 580–1,070°C) were analyzed for Ti, in most cases by ion microprobe (IMP). The combined experimental and natural results define a log-linear dependence of equilibrium Ti content (expressed in ppm by weight) upon reciprocal temperature: $$\log ({\text{Ti}}_{{{\text{zircon}}}}) = (6.01 \pm 0.03) - \frac{{5080 \pm 30}}{{T\;(\hbox{K})}}.$$ In a strategy similar to that used for zircon, rutile crystals were grown in the presence of zircon and quartz (or hydrous silicic melt) at 1–1.4 GPa and 675–1,450°C and analyzed for Zr by EMP. The experimental results were complemented by EMP analyses of rutile grains from six natural rocks of diverse origin spanning 0.35–3 GPa and 470–1,070°C. The concentration of Zr (ppm by weight) in the synthetic and natural rutiles also varies in log-linear fashion with T −1: $$\log ({\text{Zr}}_{{{\text{rutile}}}}) = (7.36 \pm 0.10) - \frac{{4470 \pm 120}}{{T\;(\hbox{K})}}.$$ The zircon and rutile calibrations are consistent with one another across both the synthetic and natural samples, and are relatively insensitive to changes in pressure, particularly in the case of Ti in zircon. Applied to natural zircons and rutiles of unknown provenance and/or growth conditions, the thermometers have the potential to return temperatures with an estimated uncertainty of ±10 ° or better in the case of zircon and ±20° or better in the case of rutile over most of the temperature range of interest (∼400–1,000°C). Estimates of relative temperature or changes in temperature (e.g., from zoning profiles in a single mineral grain) made with these thermometers are subject to analytical uncertainty only, which can be better than ±5° depending on Ti or Zr concentration (i.e., temperature), and also upon the analytical instrument (e.g., IMP or EMP) and operating conditions.

Influence of Nitrogen Doping on the Defect Formation and Surface Properties of TiO 2 Rutile and Anatase

Abstract: Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of TiO2 rutile(110) and anatase(101) single crystals were investigated. No band gap narrowing is observed, but N doping induces localized N 2p states within the band gap just above the valence band. N is present in a N(III) valence state, which facilitates the formation of oxygen vacancies and Ti 3d band gap states at elevated temperatures. The increased O vacancy formation triggers the 1 x 2 reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications.

Photoluminescence of anatase and rutile TiO2 particles.

Abstract: Nonaqueous reactions between titanium(IV) chloride and alcohols (benzyl alcohol or n-butanol) were used for the synthesis of anatase TiO2 particles, while rutile TiO2 particles were synthesized in aqueous media by acidic hydrolysis of titanium(IV) chloride. The X-ray diffraction measurements proved the exclusive presence of either the anatase or the rutile phase in prepared samples. The photoluminescence of both kinds of particles (anatase and rutile) with several well-resolved peaks extending in the visible spectral region was observed, and the quantum yield at room temperature was found to be 0.25%. Photon energy up-conversion from colloidal anatase and rutile TiO2 particles was observed at low excitation intensities. The energy of up-converted photoluminescence spans the range of emission of normal photoluminescence. The explanation of photon energy up-conversion involves mid-gap energy levels originating from oxygen vacancies.

EPR investigation of TiO2 nanoparticles with temperature-dependent properties.

Abstract: An in situ electron paramagnetic resonance (EPR) study has been carried out for anatase (Hombikat UV100) and rutile TiO2 nanoparticles at liquid helium (He) temperature (4.2 K) under UV irradiation. Rutile titania was synthesized by ultrasonic irradiation with titanium tetrachloride (TiCl4) as the precursor. XRD and Raman results evidence the crystallinity of titania phases. The nature of trapped electrons and holes has been investigated by EPR spectroscopy under air and vacuum conditions. Illumination of TiO2 powder (anatase and rutile) at 4.2 K resulted in the detection of electrons being trapped at Ti4+ sites within the bulk and holes trapped at lattice oxide ions at the surface. The stability of electron traps was very sensitive to temperature in both phases of TiO2. The annealing kinetics of the EPR detected radicals has been studied from 4.2 K to ambient temperature and also for calcined titania particles from 523 to 1273 K.

Size-dependent modifications of the Raman spectrum of rutile TiO2

Abstract: Crystallite-size-dependent variations in the Raman spectrum of rutile TiO2 have been characterized and compared with those of the well-investigated rutile SnO2. For an average crystallite size below ∼25nm, the Raman spectrum of rutile TiO2 nanocrystals displays an additional low-frequency, possibly surface vibrational, mode at ∼105cm−1. The disorder-activated, high-frequency surface modes seen in the Raman spectrum of rutile SnO2 nanocrystals are absent. The Eg and A1g vibrational modes of rutile TiO2 show systematic redshifts, broadening, and intensity reductions with decreasing crystallite size, which are consistent with phonon confinement behavior. A phonon confinement model provides reasonable crystallite size quantification, as in the case of rutile SnO2 and RuO2.

Transformation of brookite-type TiO2 nanocrystals to rutile: correlation between microstructure and photoactivity

Abstract: Nanometric particles of pure brookite TiO2 were synthesized by modified thermolysis of reactant solutions containing titania powder, HCl, urea and PEG 10000. Unique flower-like brookite agglomerates with an average diameter of ∼400–450 nm composed of single brookite nanocrystals of ∼4–5 nm were obtained at 105 °C. The brookite → rutile transformation has been studied and TiO2 mixtures with variable amount of anatase, brookite and rutile polymorphs at different temperatures (from 200 to 800 °C) were obtained. High resolution transmission electron microscopy (HRTEM), electron diffraction pattern and BET/BJH analyses were used to characterize the phase assemblages, crystallite size and pore volume of the pure-phase brookite and TiO2 mixtures. In order to understand the metastable–stable TiO2 phase transformation X-ray powder diffraction (XRD) was performed. The photoactivity of pure brookite and TiO2 powders with different compositions of the brookite–anatase–rutile and anatase–rutile polymorphs obtained during the transitions was examined by photocatalyzed degradation of 4-chlorophenols in aqueous solution. The titania sample having the highest catalytic activity was obtained at 500 °C, contained 3.2% brookite, 42.9% anatase and 53.9% rutile and is referred to as TiO[B])/500 .

The critical size mechanism for the anatase to rutile transformation in TiO2 and doped-TiO2

Abstract: A series of titania and doped titania materials have been prepared from sol–gel methods using a titanium isopropoxide precursor. Powder X-ray diffraction (PXRD) and secondary electron microscopy (SEM) have been used to follow the anatase to rutile transformation (ART) phase change in this system. PXRD was used to estimate the relative amounts of each phase and the average particle size at a series of temperatures. Importantly, very careful choice of reaction precursors were made so that a wide range of similar samples could be compared, thus removing effects due to preparation. It was found that doping with Si, Zr, Al and tertiary mixtures of these produced an elevated ART temperature whilst Co, Mn and V had the opposite effect. The most likely explanation for the elevation in the ART temperature is the presence of dopant strain fields, which limit mass transport in the system. Lowering of the ART temperature is probably related to creation defect sites, which provide low energy mass transport routes. It was also found that in the majority of samples, the mechanism for phase change was related to attaining a critical particle size. This was measured at around 450 A independent of the dopant used. Results are discussed in terms of previous work.

Theoretical study of N-doped TiO2 rutile crystals.

Abstract: The N-doping effects on the electronic and optical properties of TiO2 rutile crystal have been studied using density functional theory (DFT). The calculations of several possible N-doped structures show that band gaps have little reduction but some N 2p states lie within the band gap in the substitutional N to O structure and interstitial N-doped rutile supercell, which results in the reduction of the photon-transition energy and absorption of visible light. In contrast, substitutional N to Ti doped model has a significant band-gap narrowing. The results maybe clarify confusions in nitrogen-doped TiO2 rutile crystal.

Large‐Scale Synthesis and Characterization of TiO2‐Based Nanostructures on Ti Substrates

Abstract: Na 2 Ti 6 O 13 nanoplates, nanowires, and continuous nanowire network films are hydrothermally formed on a large scale directly on Ti substrates for the first time. The morphology of the formed Na 2 Ti 6 O 13 nanostructures can be easily tuned by varying the experimental parameters of temperature, reaction duration, and the NaOH concentration. Our study demonstrates that the synthesized Na 2 Ti 6 O 13 nanostructures are easily converted into H 2 Ti 3 O 7 nanostructures-a desirable precursor for the fabrication of various TiO 2 -based nanomaterials-with shape preservation, by an ion-exchange process. Anatase, a mixture of anatase and rutile, and rutile TiO 2 nanowires are formed when the H 2 Ti 3 O 7 nanowires are annealed at 450, 600, and 750 °C, respectively. The optical properties and the photocatalytic activity of H 2 Ti 3 O 7 nanowires and of the TiO 2 -based nanomaterials are also addressed. The approach described in this study provides a simple and novel method for the large-scale synthesis of various TiO 2 -based nanostructured materials that grow directly on Ti substrates and are ready for a wide range of practical applications, such as the photodegradation of wastewater.

Visible-light-responsive nano-TiO2 with mixed crystal lattice and its photocatalytic activity

Abstract: Ultraviolet- and visible-light-responsive titania-based photocatalysts were synthesized and employed in the photocatalytic oxidation of NOx. Sol–gel processes using tetrabutyl orthotitanate and ethanol under acid catalyzed condition and controlled calcination were performed to synthesize titanium dioxide with a mixed crystal lattice of anatase, brookite and rutile phases. The TiO2 prepared under calcination at 200 °C exhibited high photocatalytic activity for degradation of NOx under both ultraviolet (UV) and visible-light illumination. The experimental results showed that up to 70% removal of NOx could be obtained in a continuous flow type reaction system under irradiation with visible light. The calcination temperature has an important influence on the particle size and lattice structure of TiO2. It is also found that the peculiar mixed-phase structure of TiO2, evidenced from Raman, x-ray diffractometry (XRD), and UV–vis spectroscopy, was inferred to be an important factor for visible-light absorption and NOx removal activity under a wide range of visible-light illumination.

Carbon-modified TiO2 photocatalyst by ethanol carbonisation

Abstract: The new photocatalysts based on commercially available titanium dioxide powders: Tytanpol A11 (Police, Poland), pure anatase and P-25 (Degussa, Germany) containing about 20% rutile were modified by carbon via ethanol carbonisation. Titanium dioxides were heated at different temperature from 150 to 400 °C for 1 h in an atmosphere of ethanol vapour. The photocatalytic activity of carbon-modified TiO2 was studied by oxidation of phenol in water under UV and artificial solar light irradiation. With increasing of carbon content in TiO2 photocatalysts the activity for phenol decomposition under UV light was decreasing but that under visible light was stable. Turbidity of the slurry solution decreased with increasing of carbon content for all prepared photocatalysts because of the change of their surface character from hydrophilic to hydrophobic.

Orientation dependence of the isoelectric point of TiO2 (rutile) surfaces.

Abstract: The electroosmotic behavior of the rutile polymorph of titanium dioxide was explored as a function of the crystallographic orientation. Atomic force microscopy (AFM) was employed to make high-resolution force spectroscopy measurements between a silica sphere attached to a traditional, contact-mode AFM cantilever and TiO2(110), TiO2(100), and TiO2(001) surfaces in aqueous solutions. Measurements were taken in multiple solution conditions across a broad range of pH values, and the resultant force−distance curves were used to deduce relative behaviors of each orientation of rutile, with particular interest in changes of the isoelectric point (iep). Differences in the iep as a function of orientation are explained in terms of differences in both the coordination number and density of acidic and basic sites on the surface. The results were supported by angle-resolved X-ray photoelectron spectroscopy (XPS) measurements of a nominal monolayer of palladium metal deposited on each of the three orientations studied...

Antibacterial activity of zinc modified titanium oxide surface.

Abstract: Titanium-based implants are successfully used for various biomedical applications. However, in some cases, e.g. in dental implants, failures due to bacterial colonization are reported. Surface modification is a commonly proposed strategy to prevent infections. In this work, titanium oxide, naturally occurring on the surface of titanium, was modified by promoting the formation of a mixed titanium and zinc oxide, on the basis of the idea that zinc oxide on titanium surface may act as the zinc oxide used in pharmaceutical formulation for its lenitive and antibacterial effects. The present work shows that it is possible to form a mixed titanium and zinc oxide on titanium surfaces, as shown by Scanning Electron Microscopy and XPS analysis. To this end titanium was preactivated by UV on crystalline titanium oxide, both in the anatase form or in the co-presence of anatase and rutile. By performing antibacterial assays, we provide evidence of a significant reduction in the viability of five streptococcal oral strains on titanium oxide surfaces modified with zinc. In conclusion, this type of chemical modification of titanium oxide surfaces with zinc might be considered a new way to reduce the risk of bacterial colonization, increasing the lifetime of dental system applications.

Photocatalytic Activity of Ni 8 wt%‐Doped TiO2 Photocatalyst Synthesized by Mechanical Alloying Under Visible Light

Abstract: Ni 8 wt%-doped titanium dioxide (TiO 2 ) was synthesized by mechanical alloying. The photocatalytic activity of Ni 8 wt%-doped TiO 2 powder was evaluated by measuring the visible light absorption ability by ultraviolet visible diffuse reflectance spectroscopy (UV/Vis-DRS) and photoluminescence (PL) spectroscopy. Ni 8 wt%-doped TiO 2 powders had only a rutile phase and spherical particles with an average grain size of less than 10 nm. The UV/Vis-DRS analysis showed that the UV absorption for the Ni 8 wt%-doped TiO 2 powder moved to a longer wavelength and the photoreactivity was rapidly enhanced. And PL results revealed that the new absorption was believed to be induced by localization of the trapping level near the valance band or conduction band. Moreover, Ni 8 wt%-doped TiO 2 had a high reaction activity for decomposition of 4-chlorophenol in aqueous solution under UV and visible light. To obtain the electronic structure of Ni-doped TiO 2 , we have performed ab initio pseudopotential plane wave methods based on the density functional theory. The band gap of Ni-doped TiO 2 narrowed more than pure TiO 2 . These results agree with the experimentally observed phenomenon.

Visible Light Photocatalysis of Nitrogen-Doped Titanium Oxide Films Prepared by Plasma-Enhanced Chemical Vapor Deposition

Abstract: N-doped TiO 2 films were prepared by plasma-enhanced chemical vapor deposition (PECVD) using titanium tetraisopropoxide and NH 3 mixture. Two nitrogen bonding states, substitutional and interstitial nitrogen atoms, are observed and about 80% of nitrogen atoms exits the substitutional sites irrespective of nitrogen content. The transition temperature from the anatase to rutile becomes lower in the N-doped TiO 2 films compared with that of nondoped TiO 2 films. Visible light photocatalysis, both photocatalytic decomposition of organic compounds and photoinduced hydrophilicity, was observed in the N-doped TiO 2 films. It is considered that both nitrogen atoms substituted for oxygen atoms and crystalline structure contribute to the visible-light photocatalytic activity.

FTIR study on the formation of TiO2 nanostructures in supercritical CO2.

Abstract: TiO2 nanospherical and fibered structures were obtained via a one-step sol−gel method in supercritical carbon dioxide (scCO2) involving polycondensation of the alkoxide monomers titanium isopropoxide (TIP) and titanium butoxide (TBO) with acetic acid (HAc). The resulting materials were characterized by means of electron microscopy (SEM and TEM), X-ray diffraction (XRD), thermal analysis (TGA), and attenuated total reflection Fourier transmission infrared (ATR-FTIR) analysis. Depending on the experimental conditions, TiO2 anatase nanospheres with a diameter of 20 nm or TiO2 anatase/rutile nanofibers with a diameter of 10−100 nm were obtained. Fiber formation was enhanced by a higher HAc/Ti ratio and the use of the titanium isopropoxide (TIP) monomer. The mechanism of the microstructure formation was studied using in situ FTIR analysis in scCO2. The FTIR results indicated that the formation of nanofibers was favored by a titanium hexamer that leads to one-dimensional condensation, while nanospheres were fav...

Room temperature synthesis of rutile nanorods and their applications on cloth

Abstract: In order to achieve better photocatalytic performance, rutile nanorods dispersed in anatase and brookite phases were synthesized from titanium isopropoxide (TIP) in a concentrated HNO3 solution at room temperature (23 °C). X-ray diffraction results indicated that the percentage of rutile increased with increasing peptization time. Scanning electron microscopy and and high-resolution transmission electron microscopy measurements revealed that the nanosized titania particles mainly consisted of granular anatase and brookite, and rod-like rutile. It was interesting that the stability of the colloid increased with increasing nanoparticle concentration, and the tricrystalline titania showed a photocatalytic activity higher than that of pure anatase. These nanocrystals were applied onto cotton fabrics, and achieved a promising bactericidal photocatalytic activity and excellent protection against UV radiation.