Photoluminescence of anatase and rutile TiO2 particles

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.     

Features of the anatase transformation to rutile

We showed that the transformation of anatase to rutile is accompanied by a set of various chemical reactions in which the anionic and/or cationic impurities in the anatase partitioned in a separate phase (gaseous or condensed) resulting in the rutile titanium dioxide with a low content of impurities.     

Theoretical study of the structure and optical properties of carbon-doped rutile and anatase titanium oxides

The structure and optical properties of carbon-doped titanium oxides, TiO2, in the rutile and anatase forms have been investigated theoretically from first principles. Two possible doping sites were studied, carbon at an oxygen site (anion doping) and carbon at a titanium site (cation doping). The calculated structures suggest that cation-doped carbon atoms form a carbonate-type structure, whereas anion-doped carbon atoms do not invoke any significant structural change. A density-of-states analysis revealed three in-gap impurity states for anion doping. The optical properties of anion-doped cells qualitatively agree with the experimentally reported visible-light absorbance values. We ascribe part of the absorption to transitions from the valence band to one of the impurity states. These transitions should be able to promote photocatalytic reactions, because electron holes in the valence band are considered to be crucial for this process. Neither in-gap impurity states nor visible-light absorbance were observed in the case of cation doping. The effect of oxygen vacancies was also investigated. Introduction of oxygen vacancies into anion-doped TiO2 populates the impurity states and thus suppresses photocatalysis. The interaction of a doped carbon atom with an oxygen vacancy at a finite spatial separation was also carried out. The possibility of either a carbon-oxygen vacancy pair or higher carbon-oxygen vacancy complex existing is discussed.     

Characterization of rutile and anatase powders by thermal analysis

Anatase and rutile powders prepared by various methods were characterized by thermoanalytical studies up to 1400 °C using a simultaneous TG-DTG-DTA apparatus. Preparation techniques were developed for the manufacture of amorphous titania, pure anatase, stabilized anatase and pure rutile.Enthalpy effects originating from the crystallization of amorphous parts into the anatase modification and from the phase transition of anatase into rutile were investigated.The latter phase transition was detected by means of heat flux DSC measurements. The transition enthalpy was determined.

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Zusammenfassung Nach verschiedenen Methoden dargestellte Anatas- und Rutil-Pulver wurden mittels simultaner TG-DSC bis 1400 °C charakterisiert. Die Präparationsmethoden lieferten amorphes Titan(IV)-oxid, reinen Anatas, stabilisierten Anatas und reinen Rutil. Die Enthalpieeffekte infolge der Kristallisation amorphen Materials zu Anatas und der Phasenumwandlung von Anatas in Rutil wurden untersucht. Mittels Wärmefluss-DSC wurde letztere Umwandlung nachgewiesen und ihre Enthalpie bestimmt.

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- 1400°. , , . . ., .

     

Dispersion behaviors of molybdena on titania (rutile and/or anatase)

Raman and FT-IR spectra were employed to investigate the dispersion of molybdena on mixed TiO2 (rutile and anatase, signed as R and A) with different BET surface ratios of rutile/TiO2(R + A). The results showed that (1) molybdena would preferentially disperse on the rutile surface in mixed TiO2; (2) for MoO3/rutile with low molybdena loading (e.g., 0.20 mmol/100 m2 rutile), a dispersed molybdena species existed on the rutile surface in an isolated tetrahedral coordination environment, while for MoO3/rutile with high molybdena loading (e.g. 0.82 mmol/100 m2 rutile), a polymeric molybdena species could be detected on the rutile surface; (3) for the MoO3/anatase sample, a dispersed molybdena species existed on the anatase surface in a polymeric coordination environment; and (4) the formation of the Bronsted acid site on the surface of rutile and anatase should be related to the polymeric molybdena species. All these results have been discussed via the interaction between OH groups of molybdena and OH groups of rutile and anatase, and it seems reasonable to suggest that, for the lower molybdena loading, the different states of the dispersed molybdena species should result from the different dehydration orders of OH groups of the molybdena and surface OH groups of rutile and anatase.     

The solubility of Co in TiO2 anatase and rutile and its effect on the magnetic properties

Co-doped anatase and rutile bulk-samples prepared by the sol-gel technique are found to be paramagnetic at room-temperature. Only further annealing in Ar/H₂ gas results in a ferromagnetic behavior. X-ray diffraction and electron-microscope studies reveal for low doping levels <4% the formation of Co-doped rutile samples and the formation of CoTiO₃ as a new phase. Co₃O₄ can be detected in anatase samples with Co doping levels ?4%. The observed Co oxides are reduced by Ar/H₂ to Co metal. The room-temperature ferromagnetism can therefore be traced back to a segregation of metallic Co.     

Effect of rutile/anatase ratio on catalytic properties of Co?TiO2 catalysts for fischer-tropsch synthesis

CO hydrogenation on Co–TiO₂ catalysts with similar cobalt dispersity and rutile amount within f=0–1·0 in a rutile-anatase mixture has been studied. The highest activity and selectivity to liquid hydrocarbons is observed for the catalyst with f=0.7.

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CO CO–TiO₂, , , f, - . , f=0,7.

     

Ion exchange behaviour of crystalline niobium antimonate towards alkali metal ions

The potentiometric ion-exchange titrations of niobium antimonate have been performed for the systems Li⁺/H⁺, Na⁺/H⁺, K⁺/H⁺ and NH₄⁺/H⁺. Samples at each step of exchange have been characterised by X-ray diffraction analysis. It is concluded that the alkali metal ion exchange in niobium antimonate occurs without phase transition and the exchanger shows a trifunctional acid behavior. The ion exchange capacity of the material is 3.69 meq g⁻¹. At pH 7 the proton exchange by Li⁺, Na⁺, K⁺and NH₄⁺ is 1.05, 1.70, 1.25 and 1.60 meq g⁻¹ respectively. Thus the exchanger produces a wide range of acidities as is found in noncrystalline or semicrystalline ion exchange materials. The column ion-exchange capacity for the univalent metal ions has been determined and compared with those of a batch experiment. The low ion exchange capacity—observed in column operation has been interpreted in terms of slow kinetics of the ion exchange process.     

Preparation of self-supporting hierarchical nanostructured anatase/rutile composite TiO(2) film

Via the combination of an electrospinning method with a hydrothermal reaction, a large-scale cedar-like hierarchical nanostructured TiO(2) film with an anatase/rutile composite phase was fabricated.     

Ion specificity and viscosity of rutile dispersions

The isoelectric point (IEP) of rutile is shifted to higher pH values in the presence of greater than 10⁻⁴ mol dm⁻³ Ba²⁺, Ca²⁺ and Mg²⁺, and when a critical concentration (5 × 10⁻⁴ mol dm⁻³ for Ba²⁺ and 1 × 10⁻³ mol dm⁻³ for Ca²⁺) is exceeded there is no IEP at all and the ζ potential is always positive. A common intersection point for the ζ-potential curves of the different concentrations of salt is found, but for the various salts the point is shifted from ζ = 0 mV for Mg²⁺ up to ζ = 20 mV for Ba²⁺. Between the IEP and the charge-reversal point a rheologically unstable region is discovered. The shear stress of rutile dispersions (2.5 g rutile + 4 g electrolyte solution) at shear rates of 116 s⁻¹ shows the same pH dependence irrespective of the concentration of alkaline-earth metal cations up to 10⁻² mol dm⁻³. The shear stress is less than 1 Pa below pH 3.8 and in the pH range 5–12 it assumes a value between 50 and 80 Pa at 116 s⁻¹ with some scatter; however, no systematic trend with concentration of alkaline-earth metal cations and a rather insignificant decrease with pH at pristine conditions are observed. The acidic branch of the yield stress (pH) and low shear rate viscosity (pH) curves is insensitive to the presence of alkaline-earth metal cations, and the same behaviour is found for the ζ potential. The alkaline-earth metal cations induce an increase in viscosity in the basic region and a shift in the pH of maximum viscosity to high pH values. It was also discovered that the effect different alkaline-earth metal cations have on the rheological properties at the same concentration is different from the effect induced by indifferent electrolytes. When the ζ potential increases the viscosity at high pH is increased in a series which follows the increase in size of the cation. Received: 9 September 1998 Accepted in revised form: 12 January 1999     

Charge separation at the rutile/anatase interface: a dominant factor of photocatalytic activity

Epitaxial and polycrystalline anatase films were grown by pulse-powered magnetron sputtering. The photoreduction of Ag ions showed the difference in the distribution of the photocatalytic active sites in these films. The polycrystalline anatase film was covered with an Ag layer. In contrast, discrete Ag particles were interspatially deposited on the epitaxial anatase film. Evaluation of the epitaxial film by micro-Raman spectrometry revealed that the rutile coexisted at only the site where the Ag particle was precipitated. These results suggest that the rutile/anatase interface is the active site for photocatalysis and is one of the dominant factors of the photocatalytic activity.     

The synthesis of nanocrystalline anatase and rutile titania in mixed organic media

The synthesis of anatase and rutile titania could be achieved in mixed organic media with the variation of alcohols in the media under mild conditions. Although a nonhydrolytic process cannot be excluded, it is suggested that the formation of titania in these systems is based mainly on the hydrolytic process initiated by the water generated as a result of an esterification reaction between the alcohols and acetic acid. It has been found that the phase of the TiO2 produced depends on the choice of alcohols and temperature. Partial morphology and size are also affected by these factors. It is proposed that the viscosity and pressure of the reaction media influence the particle size. X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and UV-visible adsorption spectroscopy were employed to characterize the final products.     

Effects of material properties on sedimentation and aggregation of titanium dioxide nanoparticles of anatase and rutile in the aqueous phase

This study investigated the sedimentation and aggregation kinetics of titanium dioxide (TiO(2)) nanoparticles with varying material properties (i.e., crystallinity, morphology, and chemical composition). Used in the study were various types of commercially available TiO(2) nanoparticles: three spherical anatase (nominal diameters of 5, 10, and 50 nm) and two rutile nanoparticles (10×40 and 30×40 nm). The 50 nm anatase and 10×40 nm rutile showed higher stability in deionized water and 5 mM NaCl solutions at pH 7 than the 5, and 10 nm anatase nanoparticles in sedimentation experiments. In aggregation experiments, critical coagulation concentration values for the 50 nm anatase were the highest, followed by the 10×40 nm rutile and the 5 nm anatase nanoparticles in NaCl and CaCl(2) solutions. The aggregation kinetics was fitted reasonably well with the Derjaguin-Landau-Verwey-Overbeek (DLVO) equations for the TiO(2) nanoparticles tested. Results showed that crystallinity and morphology are not influential factors in determining the stability of TiO(2) nanoparticle suspensions; however, the differences in their chemical compositions, notably, the varying concentrations of impurities (i.e., silicon and phosphorus) in the pristine materials, determined the surface charge and therefore the sedimentation and aggregation of TiO(2) nanoparticles in the aqueous phase.     

Hydrothermal synthesis and enhanced photocatalytic activity of mixed-phase TiO2 powders with controllable anatase/rutile ratio

In this study, mixed-phase TiO₂ powders were novelly synthesized via a facile and mild hydrothermal method without any post-heat treatment. TiOSO₄ and peroxide titanic acid (PTA) were used as inorganic titanium sources, while no special solvent or additive were introduced. The XRD and TEM results showed the mixed-phase TiO₂ powders were composed of anatase and rutile phases, and the PTA sol played an important role on forming the rutile nucleus. The proportion of rutile in the mixed-phase TiO₂ could be easily controlled in the range of 0%–70.5% by changing the amount of PTA sol used in the synthesis process. The UV-Visible absorption spectra indicated the prepared mixed-phase TiO₂ showed enhanced visible light absorption with the increase of rutile ratio. The photodegradation experiments revealed the mixed-phase TiO₂ exhibited the best photocatalytic activity at the rutile ratio of 41.5%, while a higher or lower rutile ratio both resulted in the decrease of photocatalytic activity.     

Ion adsorption properties of molybdenum(II) bromide

The adsorption of about 50 ions on molybdenum dibromide, (Mo₆Br₈)Br₄·2H₂O, in nitric acid was investigated. The preparation of the adsorber and its characterization is presented and discussed. Adsorption mechanism studies were carried out for some noble metals and chromium. Sorption cannot be ascribed to ion exchange mechanism but to formation of insoluble species, and to settlement of few ions into surface sorption sites or into a limited number of cavities in the cluster crystal structure of the adsorber.     

Ion exchange properties of heat-treated NH4Cl-containing cellulose-regenerated

Ion exchange properties of carbon fibers obtained by heat treatment in a temperature range of 200–600°C in an inert atmosphere of cellulose-regenerated impregnated by 0.3–3.0 M ammonium chloride were examined.     

Sorption of Eu(III) by amorphous titania, anatase and rutile

Sorption of Eu(III), an analogue of trivalent actinides (Am, Cm), by amorphous titania as well as different crystalline phases of titania, namely anatase and rutile, have been studied as a function of pH, using ¹⁵⁴Eu (half life?=?8.8 yrs, E_???=?123,247?keV) as a radiotracer. The objective of this study was to investigate the effect of the crystalline phase of the titania on their sorption behaviour towards the metal ion. Amorphous titania was prepared by organic route and was converted into anatase and rutile by heating at elevated temperatures based on the differential thermal analysis studies. Eu(III) sorption by all forms of titania rises sharply with the pH of the suspension, with the sorption edge shifting to higher value in the order; amorphous?<?anatase?<?rutile. However, the normalization of the sorption data to the surface area of the sorbents resulted in the overlapping of the sorption curves for amorphous and anatase phases, with the data being higher for rutle in the lower pH region, indicating the effect of the crystal phase on sorption behaviour of Eu(III).