Showing papers on "Rutile published in 1998"

Thermodynamic analysis of phase stability of nanocrystalline titania

Abstract: The phase stability of nanocrystalline anatase and rutile was analyzed thermodynamically. According to the present analysis, anatase becomes more stable than rutile when the particle size decreases belowca. 14 nm. The calculated phase boundary between nanocrystalline anatase and rutile coincides with the experimental data for appearance of rutile during coarsening of nanocrystalline anatase. Both surface free energy and surface stress play important roles in the thermodynamic phase stability, which is a function of particle size.

Effect of rutile phase on the photocatalytic properties of nanocrystalline titania during the degradation of p-coumaric acid

Abstract: Nanocrystalline titania catalysts with high surface area (68–100 m 2 /g) with varying amounts of anatase and rutile phases were tested during the photocatalyzed degradation of p -coumaric acid. This is a pollutant found in agricultural waste waters originating from the wine and olive oil industry. The catalysts were prepared by the same route namely, the hydrolysis of alkoxides followed by densification under hydrothermal conditions (1500–4000 kPa). Hydrolysis of Ti-butoxide gives rise to predominantly anatase containing titania whereas the Ti-ethoxide hydrolysis leads to mixtures of anatase and rutile. Compared to pure anatase or rutile, titania containing both phases shows a significantly higher catalytic activity during the degradation of p -coumaric acid. After preliminary optimization experiments, the degradation of 0.1 mM p -coumaric acid was achieved in 45 min under light irradiation in the presence of H 2 O 2 . The most efficient catalyst is TiO 2 containing 30% rutile and 70% anatase and is prepared by the hydrolysis of tetraisopropyl–orthotitanate. Pore size distribution measurements indicate the presence of mesopores of radii in the range 6–25 A which were responsible for the effective adsorption of the pollutant on the catalyst. Electrophoretic mobility measurements show the isoelectric point for the most efficient catalyst at pH 5.5.

Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media

Abstract: The influence of deposited silver upon the photocatalytic activity of titania in the rutile and anatase allotropic forms has been studied in three different reactions. The common feature found for these three reactions was an electron transfer from illuminated TiO 2 to silver particles. In the first reaction (platinum photodeposition). it has been shown that the initial presence of metallic silver orientates the localization of subsequent Pt photodeposits with, in particular, a 100% selectivity to Pt deposition on top of silver particles or agglomerates previously deposited on anatase. In the second reaction (2-propanol oxidation), Ag deposit was found beneficial for the activity of rutile and detrimental for that of anatase. For rutile which is less active, silver helps for the electron-hole pair dissociation. By contrast, for anatase, the negatively charged Ag particles preferentially attract photoholes and become recombination centers, thus decreasing the photocatalytic activity. For the third reaction (dehydrogenation of 2-propanol), Ag/TiO 2 catalysts were found very poorly active, as expected for a group I-B metal. However, in the additional presence of platinum deposits, Pt/Ag/TiO 2 catalysts are as active as their Pt/TiO 2 homologues, thus confirming that Pt is deposited on top of silver with good electronic contacts between both metals. The three photocatalytic reactions have common electronic processes based on the photoelectron generation on titania and the subsequent electron transfer to the metal (s).

Orientation Dependence of Photochemical Reactions on TiO2 Surfaces

Abstract: The photochemistry of oriented rutile surfaces has been examined using the photoreduction of Ag+ to Ag metal from an aqueous solution onto the TiO2 surfaces. The photochemical reaction rates and quantum yields of rutile films were found to be a function of the orientation of the rutile surface. The (100) and (110) rutile orientations have lower photoreduction rates than the (101), (111), and (001). This dependence of the photochemical reaction rates on film surface orientation is observed over a range of light intensities, illumination wavelengths, and film thicknesses. Atomic force microscopy has been used to characterize the morphologies of the film surfaces and the photoreduced Ag at these surfaces. Several properties of rutile, such as the surface morphology, surface chemistry, space charge phenomena, and charge transport, are discussed in relation to their possible contributions to the orientation dependence of the photochemistry on the rutile films.

Electronic absorption by Ti 3+ ions and electron delocalization in synthetic blue rutile

Abstract: Polarized absorption spectra, σ and π, in the spectral range 30000–400 cm−1 (3.71–0.05 eV) were obtained on crystal slabs // [001] of deep blue rutile at various temperatures from 88 to 773 K. The rutile crystals were grown in Pt-capsules from carefully dried 99.999% TiO2 rutile powder at 50 kbar/1500 °C using graphite heating cells in a belt-type apparatus. Impurities were below the detection limits of the electron microprobe (about 0.005 wt% for elements with Z≥13). The spectra are characterized by an unpolarized absorption edge at 24300 cm−1, two weak and relatively narrow (Δν1/2≈3500–4000 cm−1), slightly σ-polarized bands ν1 at 23500 cm−1 and ν2 at 18500 cm−1, and a complex, strong band system in the NIR (near infra red) with sharp weak peaks in the region of the OH stretching fundamentals superimposed on the NIR system in the σ-spectra. The NIR band system and the UV edge produce an absorption minimum in both spectra, σ and π, at 21000 cm−1, i.e. in the blue, which explains the colour of the crystals. Bands ν1 and ν2 are assigned to dd transitions to the Jahn-Teller split upper Eg state of octahedral Ti3+. The NIR band system can be fitted as a sum of three components. Two of them are partly π-polarized, nearly Gaussian bands, both with large half widths 6000–7000 cm−1, ν3 at 12000 cm–1 and the most intense ν4 at 6500 cm−1. The third NIR band ν5 of a mixed Lorentz-Gaussian shape with a maximum at 3000 cm−1 forms a shoulder on the low-energy wing of ν4. Energy positions, half band widths and temperature behaviour of these bands are consistent with a small polaron type of Ti3+Ti4+ charge transfer (CT). Polarization dependence of CT bands can be explained on the basis of the structural model of defect rutile by Bursill and Blanchin (1983) involving interstitial titanium. Two OH bands at 3322 and 3279 cm−1 in σ-spectra show different stability during annealing, indicating two different positions of proton in the rutile structure, one of them probably connected with Ti3+ impurity. Total water concentration in blue rutile determined by IR spectroscopy is 0.10 wt-% OH. The EPR spectra measured in the temperature interval 20–295 K show the presence of an electron centre at temperatures above 100 K and Ti3+ ions in more than one structural position, but predominantly in compressed interstitial octahedral sites, at lower temperatures. These results are in good agreement with the conclusions based on the electronic absorption data.

Anisotropic photochemical reactivity of bulk tio2 crystals

Abstract: Polished and annealed surfaces of randomly textured rutile polycrystals were used to photochemically reduce Ag+ to Ag metal in an aqueous solution. By correlating the surface orientations of more than 100 individual crystallites with the amount of deposited Ag, we conclude that the most reactive orientations lie near the {101} plane. Most annealed rutile surfaces are microscopically faceted so that they locally expose planes that are not parallel to the crystallite's average surface. The observations reported here indicate that the anisotropy of rutile's photochemical properties derives from differences in the properties of specific surface planes rather than the bulk crystallite orientation.

Selective oxidation of propylene over gold deposited on titanium-based oxides

Abstract: Gold supported on titanium-based metal oxides can assist the selective partial oxidation of propylene at temperatures from 313 K to 573 K in a gas containing both H2 and O2 The preparation method was found to be crucial in controlling the selectivities In general, impregnation and chemical vapor deposition methods do not produce selective catalysts Only the deposition-precipitation method makes gold selective to propylene oxide or propanal, suggesting that a strong contact between the gold particles and the titanium ion sites on the support is important The effect of changing the support was also dramatic; the use of the anatase form of TiO2 and Ti-MCM-41 results in propylene oxide production, while the rutile structure of TiO2 caused complete oxidation to CO2 Microporous crystalline titanium silicates such as TS-1, TS-2, and Ti-β zeolite make gold relatively selective to propanal and of the three TS-1 gives the highest selectivity These results indicate that the oxidation of propylene in the copresence of H2 must involve the surface of the supports and that the reaction takes place at the interface perimeter around the gold particles

Anisotropic complex permittivity measurements of mono-crystalline rutile between 10 and 300 K

Abstract: The dielectric properties of a single crystal rutile (TiO2) resonator have been measured using whispering gallery modes. Q factors and resonant frequencies were measured from 300 to 10 K. Q factors as high as 104, 105, and 107 were obtained at 300, 80, and 10 K, respectively. Using the whispering gallery mode technique we have determined accurately the loss tangent and dielectric constant of monocrystalline rutile and obtained much more sensitive measurements than previously reported. We show that rutile exhibits anisotropy in both the loss tangent and permittivity over the range from 10 to 300 K.

Micro-Raman Spectroscopic Characterization of Nanosized TiO2 Powders Prepared by Vapor Hydrolysis

Abstract: Micro-Raman analysis was used to study the structure of TiO2 powders produced at low (260 °C) and high (600–900 °C) temperatures by vapor hydrolysis of titanium tetraisopropoxide (TTIP). Spatial inhomogeneity was discovered after the amorphous TiO2 powders produced at low temperature were calcined at 700, 800, and 900 °C for 3 h. The TiO2 powders produced at high temperatures (from 600 to 900 °C) were found to be spatially homogeneous and predominately anatase in structure. Small amounts of rutile and brookite are found for powders produced at 700, 800, and 900 °C after calcination at 600 °C for 3 h. The rutile and brookite impurities are believed to be concentrated on the surface of anatase based on a comparison of results of Raman and x-ray diffraction studies.

Thermo-Raman studies on anatase and rutile

Abstract: One application of Raman spectroscopy is in the identification of the structure and the monitoring of the transformation of a sample in a thermal process. In this work, Raman spectra were taken continuously from xerogel TiO2 in a thermal process with heating, sintering and cooling. The variations in the Raman spectra indicated the transformation of xerogel first to amorphous TiO2, then to anatase and finally to rutile. The intensities of the characteristic Raman bands of anatase and rutile were studied by thermo-Raman spectroscopy as a function of temperature, and they were found to vary considerably. The intensities for both anatase and rutile became weak at high temperatures and recovered during the cooling period. © 1998 John Wiley & Sons, Ltd.

Porous ’coral-like‘ TiO2 structures produced by templating polymer gels.

Abstract: A new synthetic approach to generate highly porous semiconductor networks that could be used for catalytic and solar cell applications is reported. Titanium dioxide networks were produced using a polymer gel templating procedure, resulting in a coral-like structure with a wall thickness of about 100−150 nm and pore sizes ranging from 100 nm to micrometers in diameter. Calcination results in the production of either the anatase or rutile crystal phases without carbon contamination from the starting polymer materials, with the individual titanium dioxide particles contacting neighbors, thereby forming a continuous network. The results obtained are of importance to various fields of research as the open “coral-like” network structure of the titanium dioxide allows high access of the titanium dioxide surface to the reaction medium.

Amount of hydroxyl radical on calcium-ion-implanted titanium and point of zero charge of constituent oxide of the surface-modified layer.

Abstract: To compare the surface properties of calcium-ion (Ca2+)-implanted titanium with those of titanium and to investigate the mechanism of bone conductivity of Ca2+-implanted titanium, amounts of hydroxyl radical of Ca2+-implanted titanium and titanium were estimated. Also, the point of zero charge (p.z.c.) of oxide constituting surface oxides of Ca2+-implanted titanium and titanium was determined. Results showed that the amount of active hydroxyl radical on Ca2+-implanted titanium was found to be significantly larger than that on titanium, indicating that the number of electric-charging sites of Ca2+-implanted titanium in electrolyte is more than that of titanium. The p.z.c. values of rutile (TiO2), anatase (TiO2), and perovskite (CaTiO3), were estimated to be 4.6, 5.9, and 8.1, respectively. Thus, Ca2+-implanted titanium surface is charged more positively in bioliquid than titanium, accelerating the adsorption of phosphate ions. © 1998 Chapman & Hall

Tungsten-bearing rutile from the Kori Kollo gold mine, Bolivia

Abstract: W-bearing rutile formed during alteration of jarosite by resurgent hydrothermal fluids in the oxide zone of the Kori Kollo gold deposit. The rutile shows sector zoning in basal sections and well developed multiple growth zones, both defined in backscatter electron images by variations in the W content. The maximum WO3 content is 5.3 wt.% and W substitutes for Ti with double substitution of Fe to maintain charge balance. The causes of multiple growth bands are considered to be changes in externally controlled variables occurring in a shallow hydrothermal system. Whereas Ti is probably leached from biotite in dacitic rocks, the W is introduced by hydrothermal fluids.

Niobium-titanium oxide powders obtained by laser-induced synthesis: Microstructure and structure evolution from diffraction data

Abstract: The influence of the niobium content on the anatase-to-rutile phase transition in nanopowders of Nb–Ti oxides was studied and the changes in the particle size and microstrain distribution obtained at different temperatures were analyzed. A correlation is found between the initial microstructure in the Ti1 – xNbxO2 (x = 0.03, 0.2) powder and the niobium content. The presence of Nb was found to inhibit the growth of both the anatase and the rutile phases.

Artificial heart valves: improved hemocompatibility by titanium oxide coatings prepared by ion beam assisted deposition

Abstract: Titanium oxide films were prepared by ion beam assisted deposition on an Eaton Z-200 system, in which the deposition of titanium evaporated from an electron beam evaporator and the bombardment with xenon ions at an energy of 40 keV extracted from an arc discharge-type ion source proceeded simultaneously in an O2 environment. X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD) were used to characterize the deposited films. It was found that the atomic ratio of O/Ti is about 1.85:1. XPS analysis shows that Ti2+, Ti3+ and Ti4+ chemical states exist on the titanium oxide films. The films deposited are in principle crystalline TiO2 with a rutile structure. The films exhibit a highly (100) orientation and increase the degree of (100) orientation with ion beam current density decreasing. The blood compatibility of titanium oxide films was studied by in vitro and in vivo investigation. In vitro investigation includes clotting time measurement and platelet adhesion. Clotting time measurements show that the clotting time of titanium oxide films is longer than that of low temperature isotropic pyrolytic carbon (LTI-carbon), which is widely used as artificial heart valves in clinics. There are also fewer platelets adhered to and deformed on the titanium oxide films compared to the LTI-carbon. In vivo investigation, implanting LTI-carbon and titanium oxide coated LTI-carbon into the ventral aorta of a dog for two weeks, shows that the amount of thrombus on titanium oxide coated LTI-carbon is only about 1/8 compared to that of LTI-carbon. The relationship between chemical states and blood compatibility was discussed. We think that the existence of Ti2+ and Ti3+ is beneficial for blood compatibility.

Ion beam radiation damage effects in rutile (TiO2)

Abstract: Radiation damage accumulation in the rutile phase of titanium dioxide (TiO 2 ) have been studied by Rutherford Backscattering Spectroscopy in channeling geometry (RBS/c) and by transmission electron microscopy (TEM). A large temperature dependence of damage accumulation in rutile was observed. Defect accumulation determined in situ by RBS/c occurs far more rapidly in samples irradiated at 160 K compared to 300 K. A rutile single crystal irradiated at 160 K was rendered fully amorphous at a fluence of 4 × 10 18 Xe 2+ /m 2 , compared to a critical amorphization fluence of 8 × 10 19 Xe 2+ /m 2 for a crystal irradiated at 300 K. In samples irradiated at 300 K a defect denuded zone at the crystal surface was observed to accompany a buried damage layer. In samples irradiated at 160 K, no defect denuded zone was observed. These observations indicate that there is a large temperature dependence associated with thermally activated point defect mobility in rutile. Moreover, in in situ irradiation experiments using 1.5 MeV Xe + ions, a critical amorphization temperature of 200 K was observed. The rutile structure is much more susceptible to radiation damage induced by point defect accumulation, compared to dense collision cascades. In the experiments presented here, only the He + ion irradiation leads to complete amorphization, combined with substantial changes in the micro-hardness and elastic (Young's) modulus.

Rutile-type titanium oxide films synthesized by filtered arc deposition

Abstract: A filtered arc-deposition system was used to synthesize titanium oxide films by evaporating titanium ions in an oxygen environment. Fourier transform infrared spectroscopy and X-ray diffraction analysis show that the films exhibit the rutile-type structure. X-ray photoelectron spectroscopy reveals that a small amount of Ti 2+ and Ti 3+ still exists although Ti 4+ is the main component in the films. The preferred orientation of the films is dependent on the substrate bias and oxygen pressure. Titanium oxide films with (101) and (002) preferred orientation were prepared by changing substrate bias and oxygen pressure. Ultraviolet–visible absorption spectroscopy was used to determine the optical band gap of the prepared films. The results show that the band gap of the films prepared under zero substrate bias is 2.39 eV. When the substrate bias is larger than −100 V, the optical band gap of the films is about 3.33 eV.

In situ growth of evaporated TiO2 thin films using oxygen radicals: Effect of deposition temperature

Abstract: The growth and characterization of TiO2 thin films deposited by electron-beam evaporation of TiO2 have been studied. The growing film was exposed to a flux of atomic oxygen supplied from an oxygen radical beam source at a total deposition pressure of 1×10−5 mbar. The properties of as-deposited 1000 A thick films on silicon substrates have been studied in the growth temperature interval 100–680 °C. X-ray diffractometry demonstrated a phase evolution as a function of growth temperature, from amorphous (100 °C) to anatase (300 °C) and eventually rutile (680 °C). While the amorphous film surface had a smooth film surface as evidenced by atomic force microscopy, the anatase and rutile specimens exhibited a grain-like morphology. No apparent difference in surface roughness was observed between the anatase and rutile phase. Secondary ion mass spectrometry indicated that silicon diffused into the rutile film grown at the highest temperature. Ellipsometry measurements revealed that the crystallized films exhibited...

Photoelectrochemical properties of sol–gel-derived anatase and rutile TiO2 films

Abstract: Anatase and rutile film electrodes with comparable porosities were prepared by the sol–gel dip-coating method, and the photoelectrochemical properties were studied based on photocurrent measurement and impedance analysis in a three-electrode wet cell. The photocurrent was found to increase with the donor density, both in anatase and rutile electrodes. For the same donor density, however, rutile electrodes exhibited higher photocurrents than anatase electrodes, which was ascribed to the more beneficial bandgap structure of the former.

Thickness-dependent crystallinity of sputter-deposited titania

Abstract: In this study, we sputter deposited titania films of thickness ranging from 256 to 705 nm on unheated substrates and studied changes in phase constituency as a function of film thickness. X-ray diffraction, and infrared spectroscopy were used for post-deposition analysis. The results show that the thinnest films consisted of anatase, rutile, and an amorphous structure. As film thickness increased, anatase formed at the expense of the amorphous constituent, whereas the amount of rutile per unit film volume remained constant. We hypothesized that if the thickness-related crystallographic changes were caused by bulk annealing effects due to in situ plasma heating during deposition, then it should be possible to reproduce these changes, ex situ, by furnace annealing. The thinnest films were annealed at three temperatures: 300, 500, and 700 °C. There was no change in phase composition at 300 °C. Metallurgical recovery of both crystalline phases occurred at 500 °C. At 700 °C, rutile grew at the expense of anata...

Solvent-free synthesis of binary inorganic oxides

Abstract: The non-hydrolytic sol–gel route has been used in the solvent-free synthesis of binary inorganic oxides based on silicon, aluminium and titanium. Where necessary, iron(iii) chloride was used as a catalyst. Clear evidence for the formation of true, amorphous binary systems was obtained only in the case of aluminosilicates. A crystalline aluminosilicate was obtained after calcination at 1000 °C. A titanium–silicon binary system gave only crystalline rutile and anatase following calcination, with no evidence for either crystalline silica or a binary oxide.

Computation of light scattering by anisotropic spheres of rutile titania

Abstract: Rutile titania offers an attractive combination of physical properties that has made it the most widely used white pigment in the worldwide marketplace for coatings, plastics, and paper. A fundamental goal of both consumers and manufacturers of rutile titania pigment is to maximize the light scattering efficiency of the pigment in end use. Achieving this goal depends upon identifying, and subsequently controlling, the factors that affect the light scattering efficiency of rutile titania powders. These factors include the particle size distribution, the degree of particle agglomeration, and the degree of optical interaction between neighboring particles in a microstructure. However, the quantitative influence of each of these factors upon the hiding power of rutile titania is not fully understood, due in large part to the difficulty of controlling laboratory experiments to the degree that the effect of each can be studied independently. In the study described here, we applied a finite element method that produces rigorous full-field solutions to Maxwell's equations to the problem of light scattering by anisotropic rutile titania spheres. Owing to the inherent experimental challenges, theoretical modeling has been used to understand light scattering properties of rutile titania. These efforts have typically relied upon Mie theory, which provides an exact treatment of the light scattering properties of a single, optically isotropic sphere. Such studies have included investigation of the theoretical hiding power of rutile titania versus that of air voids, the crowding effect in highly loaded films, and the application of a multiple scattering model to predict the light scattering efficiency of paint films. Mie theory has also been used to study the light scattering properties of core± shell composite particles containing inorganic coatings. In reality, individual rutile titania particles exhibit both optical anisotropy and complex shapes. Mie theory is therefore not capable of rigorously describing the light scattering properties of realistic rutile titania particle shapes and microstructures. Recently, Thiele and French have reported finite element computations on the light scattering properties of anisotropic, morphological rutile titania particles. Attempts to use Mie theory to predict the light scattering properties of a rutile titania particle require approximation to address the optical anisotropy of the crystal. Rutile titania is uniaxial, exhibiting its ordinary refractive index for light polarized parallel to either the aor b-axis of the crystal and its extraordinary refractive index for light polarized parallel to the c-axis. Two methods for approximating rutile using isotropic materials have been proposed in the literature. One approach has been to compute the mean of the refractive indices along the three crystal axes at the light wavelength of interest and subsequently apply Mie theory to compute farfield scattering parameters. We refer to this as the average index approximation. Another method has been to compute separately the far-field scattering parameters for an isotropic sphere having the ordinary and extraordinary refractive indices of rutile titania, and subsequently compute the weighted sum of these separate far-field results. The weighted sum consists of two-thirds the result for the ordinary refractive index plus one-third the result for the extraordinary refractive index. We refer to this as the weighted sum approximation. In the present study, we employ a finite element method to compute the nearand far-field scattering properties of optically anisotropic spheres of rutile titania at the wavelength of 560 nm. These results were then compared to results obtained using Mie theory under both the average index and weighted sum approximations. The finite element method produces rigorous solutions to Maxwell's equations for electromagnetic radiation interacting with arbitrary microstructures, which can contain optically anisotropic materials. The finite element approach can therefore be used to determine the light scattering properties of realistic systems that would otherwise be inaccessible. In the case of light scattering by an optically anisotropic sphere, results obtained using the finite element method contain the effects of the optical activity of the uniaxial crystal, unlike the results of either of the two aforementioned approximations based upon scattering by isotropic spheres.