Anatase-rutile Phase Research Articles

Thermal analysis on nanosized TiO 2 prepared by hydrolysis

Nanosized titanium oxide (TiO 2) powders of various amorphous–anatase–rutile phases were prepared by hydrolysis using an economic inorganic compound TiCl 4 as precursor. The powders were characterized by transmission electron mocroscopy (TEM), X-ray diffraction (XRD) and Brunnauer-Emmett-Teller (BET) techniques. Thermogravimetry analysis (TGA), differential thermal analysis (DTA) and DSC were performed on the prepared powders. The effects of small amount of sulphate ions and hydrolysis temperature on the morphology and crystallization were analyzed. The mass fraction of nanosized amorphous TiO 2 was determined by DSC first time.

Influence of manganese ions on the anatase–rutile phase transition of TiO 2 prepared by the sol–gel process

The anatase–rutile phase transformation of TiO 2 containing various amounts of Mn 2+ ions was studied by X-ray diffraction, electron spin resonance spectroscopy, and surface area measurements. The samples were prepared by the sol–gel process from titanium(IV) isopropoxide and manganese(II) acetate tetrahydrate. The amorphous powders were calcined for 2 h at different temperatures. Significant structural changes were observed during the various stages of the phase transformation. It was concluded that at low dopant concentrations, manganese ions are incorporated in the TiO 2 structure, and the anatase phase is stabilized, but at larger amounts, part of the dopant is segregated on the surface of TiO 2 and the rutile formation is accelerated. This was clearly confirmed by electron spin resonance. This experiment allowed the authors to follow the manganese oxidation and the phase transformation in detail.

Preparation and characterization of nanosized titania sensing film

Thick films based on nanosized titania powders have been prepared and characterized. Addition of suitable dopant affected film evolution upon annealing as both anatase–rutile phase transition and grain growth are inhibited. Ta and Nb addition to the TiO 2 powder proved effective for stabilization of the nanophase, as the average grain size was reduced from 110 to 35 and 45 nm, respectively. Electrical measurements indicated selectivity enhancement towards CO and required film preparation to stabilize the film’s grain size below 50 nm.

Study of anatase–rutile transition phase in monolithic catalyst V 2O 5/TiO 2 and V 2O 5–WO 3/TiO 2

Using different commercial TiO 2 (anatase) powders, monolithic honeycomb structures were obtained by cold extrusion. In order to compare the anatase–rutile transformation for the V–Ti–O system with and without the presence of WO 3, two systems were considered: V 2O 5–WO 3/TiO 2 and V 2O 5/TiO 2. The FT-IR, XRD analyses and SEM observations show that the presence of WO 3 influences both the morphology and the rate of formation of the rutile particles. the presence of WO 3 increases the rate of particle propagation, which becomes higher than that of the nucleation process; the rutile particles are already evidently plate-shaped at the beginning of the phase transition. On the contrary, in titania powders which do not contain WO 3, the propagation rate is lower than the nucleation process, the rutile particles have a rod-like shape and their presence can be detected by XRD analysis only when the anatase–rutile phase transformation is in progress.

Photocatalytic Activity in the 2,4-Dinitroaniline Decomposition Over TiO2 Sol-Gel Derived Catalysts

Titania was prepared by the sol-gel method from titanium alkoxide. Depending on the pH of the gelling solution, specific surface areas between 88 and 10 m2/g were obtained. The band gap (Eg) of the samples was found between 3.05 and 3.32 eV. In samples gelled at pH5 and 9 and calcined at 400°C only anatase phase is observed, while for pH3 and pH7 brookite, anatase and rutile or anatase-rutile phases coexist. It was found that the photoactivity in the 2,4-dinitroanailine decomposition depends on the Eg and on the crystalline phases. The highest activity corresponds to the catalysts having the lowest Eg and more than one crystalline phases co-existing.

Titanoxane–siloxane eight-membered rings as precursors of anatase/silica and rutile/silica materials

The eight-membered titanium containing siloxane rings (TiX2)2O4(SiBut2)2 (X = Cl 1, Br 2 or I 3) are efficient single-source precursors to homogeneous titania–silica material. The ceramic conversions which occur pyrolytically at about 550 °C give a mixture of TiO2–SiO2 materials. Differential thermal and thermogravimetric analysis, X-ray diffraction, vibrational spectroscopy as well as scanning electron microscopy were used to characterise the materials. It was found that the nature of the halogen atoms play a crucial role in the thermal transformation. Thus, after thermal treatment, silica mixed with anatase is obtained from 1 and 3, whereas silica mixed with rutile is obtained from the brominated precursor 2. The thermal evolution of 1 studied by using Raman and infrared spectroscopies confirms the inhibiting effect of SiO2 on the anatase–rutile phase transformation. The molecular route to oxide materials described could represent an alternative to the sol–gel process and may offer certain advantages.

Effects of cationic dopants on the phase transition temperature of titania prepared by the sol-gel method

The effect of different cations in the anatase–rutile phase transition temperatures for titania prepared by the sol-gel method was studied. The metal dopants were chosen from different periods and groups of the periodic table to see the role played by the electronic configuration, the oxidizing state, the atomic size, etc. on these temperature modifications. Linear relationships between the anatase–rutile phase transition temperatures and the ionic radii for alkali metal, alkaline earth metal, and group 3 and 13 elements were obtained. For elements of the period 4, there was not such a defined tendency; for most of them the modification of the phase transition temperature was too small. The cations used were Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Al3+, Y3+, La3+, Er3+, Ti4+, Co2+, Ni2+, Cu2+, and Zn2+. In all cases the dopant's concentration was 2 mol% with respect to titanium, and the same anion (nitrate) was used for all salts. A variation of more than 330 °C in the anatase–rutile phase transition temperatures was obtained by using these dopants. The transition temperatures from amorphous to anatase and from anatase to rutile phases were obtained from the x-ray diffractograms.

Enhancing Effect of Iron Chlorides on the Anatase‐Rutile Transition in Titanium Dioxide

The effect of solid Fe2O3 and gaseous iron chlorides on the anatase‐rutile phase transition was investigated in the temperature range of 750°‐950°C via X‐ray diffractometry and scanning electron microscopy. In both cases, iron diffusion in the anatase lattice decreased the transition temperature and increased the anatase‐rutile transformation rate, in comparison with that in TiO2 fired in air. The enhancement effect of iron on the anatase‐rutile transition is understood on the basis of oxygen‐vacancy formation, which favors rutile nucleation. Solid‐state iron diffusion between the contact points of TiO2 and Fe2O3 particles and vapor mass transport through gaseous chlorides were the primary mechanisms of iron mass transport to the TiO2 surface in the presence of both Fe2O3 and gaseous iron chlorides, respectively. The transformation rate at a given reaction temperature increased in the following order of reaction conditions: pure TiO2 in air, TiO2 in the presence of Fe2O3 in air, TiO2 in the presence of Fe2O3 in chlorine, and TiO2 in the presence of gaseous iron chlorides.

Inhibitory effect of the Al 2O 3–SiO 2 mixed additives on the anatase–rutile phase transformation

The inhibitory effect of the mixed alumina and silica additives with molar ratios of [Al]/[Ti]=0.05 and [Si]/[Ti]=0.05 on the phase transformation of anatase to rutile was investigated. The suppressing effect on the anatase–rutile transformation by the mixed additives was observed to be much stronger than that of the single additive. The formation of the anatase solid solution with both alumina and silica was probably responsible for the stronger inhibitory effect.

Kinetics of the anatase–rutile transformation in TiO2 in the presence of Fe2O3

The anatase–rutile phase transition in TiO2 in the presence of Fe2O3 was investigated in air and argon atmospheres by means of X-ray diffraction and scanning electron microscopy. Isothermal curves of rutile transformed from anatase as a function of time were obtained between 825 and 950 °C. The data were well fitted by various rate laws. In the presence of Fe3+, the anatase–rutile transition temperature is lower and the transformation rate in air is higher than the corresponding one in pure TiO2. The transformation in the presence of Fe3+ in an argon atmosphere is more rapid than in air. The enhancement effect of Fe3+ on the anatase–rutile transformation in both atmospheres is understood on the basis of the formation of oxygen vacancies.

Modification of the phase transition temperatures in titania doped with various cations

Titania matrices prepared by a sol-gel technique were doped with several cations (La, Zn, Al, K, Na, Ca, Ba, and Co). The effect of the dopants on the thermal and structural properties of the materials is analyzed. The dopant concentration was 2% mol with respect to titanium, and in all cases the same anion (nitrate) was used. The transition temperatures from amorphous to anatase and from anatase to rutile were measured using x-ray diffraction. The amorphous-anatase transition is independent, for almost all samples, of the type of dopant used; however, the anatase-to-rutile phase transition depends strongly on the kind of cation. This means that the temperature range where the anatase phase exists can be controlled by choosing the appropriate dopant. We have found a correlation between the anatase-rutile phase transition temperature and the radius of the cations and their electric charge.

Sintering of nanophase oxide ceramics by using millimeter-wave radiation

Sintering of nanocrystalline powder materials by microwave heating has been studied. Two types of materials have been investigated: compacts of Al 2 O 3  ZrO 2 (Y 2 O 3) (average grain size 15 nm), prepared by the sol gel method, and TiO 2 compacts (∼100 nm) produced by the blasted-wire method. The kinetics of Al 2O 3  ZrO 2 compact densification and grain growth under microwave and conventional heating are intercompared With microwave heating densification proceeds in two stages, the first stage is characterized by a decrease in the apparent activation energy in comparison with conventional heating. It has been found that TiO 2 — ceramics with densities ∼99% of T.D. can be obtained by fast microwave heating. However, because of fast sintering, the samples exhibit incomplete anatase-rutile phase transformation and rather low values of microhardness.

Effect of SiO 2, NaCl, Al 2O 3, and FeCl 3 on Phase Change Behavior of Supported and Unsupported TiO 2

The anatase-rutile phase change temperatures in TiO 2 powders precipitated from aqueous TiCl 4 and the same deposited on muscovite and phlogopite micas were investigated. Doping with Al 2O 3, NaCl, and SiO 2 increased the phase change temperature of supported and unsupported TiO 2, but doping with FeCl 3 · 6H 2O decreased the beginning temperature of the phase change on unsupported TiO 2. The difference in phase change dependence on temperature between undoped TiO 2 supported by micas and unsupported TiO 2 was caused by diffusion of aluminum, silicon, and sodium ions from the mica substrate during the calcination step.

Structural and morphological modifications of sintering microcrystalline TiO 2: an XRD, HRTEM and FTIR study

Degussa P25 (mostly anatase) activated at T ≤ 873 K exhibits fairly regular particles and mainly one family of strong coordinatively unsaturated Ti 4+ sites, active towards CO at 77 K. After firing at T ≥ 1000 K, the phase becomes rutile, the crystallites become larger, and their morphology becomes more irregular, while less and less uncoordinated Ti 4+ sites are produced on activation, yielding CO bands absorbing at lower ṽ. The modifications of crystal morphology and of surface activity towards CO are not due to the anatase-rutile phase transition, as a similar behaviour with firing temperature is exhibited by pure rutile.

Densification of nanostructured titania assisted by a phase transformation

NANOPHASE materials, characterized by an ultrafine grain size, have stimulated much interest in recent years1–11 by virtue of their unusual mechanical, electrical, optical and magnetic properties. Nanophase ceramics are of particular interest because they are more ductile at elevated temperatures than are coarse-grained ceramics11—an important property for the fabrication of ceramic components. Preparing materials that are both dense and fine-grained, however, has proved difficult: the high sintering temperatures generally required to obtain high densities can also lead to exaggerated grain growth, resulting in coarse-grained, nonuni-form materials. Sintering at lower temperatures gives a much finer grain size, but does not in general result in high-density materials. We show here that dense nanostructured titania, with density >99% of the theoretical maximum and an average grain size of less than 60 nm, can be prepared by sintering a titanium oxide sol gel near the anatase–rutile phase transformation temperature (about 600 °C). The increased mobility of the atoms during the phase transformation enhances the sintering rate at lower temperatures, suggesting that this method could be used more generally to produce nanophase materials with near theoretical densities.

Raman spectroscopy of thermally annealed TiO 2 thin films obtained by chemical vapour deposition and the spray method

Thin polycrystalline films of TiO 2 deposited by chemical vapour deposition (CVD) and the spray method have been examined by Raman spectroscopy before and after thermal annealing at 100, 200, 300, 500, 800 and 900°C in a non-oxidizing atmosphere of H 2. According to Parker and Siegel [ J Mater Res, 5, 1246 (1990)] it is possible to calibrate a Raman spectrum to the oxygen non-stoichiometry of TiO 2. Curves are used as calibrational ones for the peaks of ‘144 cm −1’ anatase vibrational mode and ‘443 cm −1’ vibrational rutile mode. It can be seen that for CVD samples obtained non-stoichiometry reaches a maximum at 500°C and goes back to stoishiometry at the phase transition temperatures in the range of 800–900°C. For the samples obtained by the spray method nonstoichiometry reaches a maximum at the lower temperature of 200°C. In the whole range from room temperature to 900°C the spray method obtained samples are much more non-stoichiometric than CVD obtained samples. Furthermore, Raman spectra of CVD obtained TiO 2 films annealed at 900°C in hydrogen reveal an amorphous phase immediately before the anatase-rutile phase transition. For samples obtained by the spray method we did not observe the same phenomenon.

Raman spectroscopy of thermally annealed TiO 2 thin films

Thin polycrystalline films of TiO 2 obtained by the chemical vapour deposition method have been examined by the Raman spectroscopy method before and after thermal annealing at 500°C, 800°C and 900°C, in atmospheres of H 2, O 2 and N 2. Raman spectra of TiO 2 films annealed at 800°C in hydrogen reveal an amorphous phase immediately before the anatase-rutile phase transition.

Untersuchungen zur Kinetik der Anatas-Rutil-Umwandlung an DeNOx-Katalysatoren für V2O5-WO3-TiO2 (Anatas) und V2O5-MoO3-TiO2 (Anatas)

The effect of preparation — specific parameters such as energy input, thermal treatment, V2O5-content — on the kinetics of the anatase-rutile transformation, were investigated and their relative contributions analysed. The various degrees of transformation were determined by quantitative X-ray diffraction measurements and checked for various rate laws. The rate constants evaluated according to Avrami for the V2O5—WO3—TiO2 (anatase)-system were extrapolated from Arrhenius plots for temperature values corresponding to conditions in power plants. The experimental results yield information on the stabilities (exposition periods) of the anatase-modification which react sensitively towards changes in phase composition at the anatase phase boundary, thus reflecting activation and deactivation processes which have an effect on the kinetics of the anatase-rutile phase transformation. The kinetic behaviour determined according to Avrami for the V2O5—WO3—TiO2 (anatase)-system has not been found for the V2O5—MoO3—TiO2 (anatase)-system. A unique feature of MoO3-systems consists in the following: after passing through a kinetically controlled region they form stable phases which can no longer be influenced kinetically. The anatase content of such phases can be adjusted by maintaining specific preparation parameters.

Importance of the anatase-rutile phase transition and titania grain enlargement in the strong metal-support interaction phenomenon in [formula omitted] catalysts

The strong metal-support interaction (SMSI) in the system consisting of iron supported on high-surface-area titania powder was investigated. By observation of the anatase-rutile phase transition, titania grain enlargement, and the SMSI effect at several reduction temperatures in the range 643–873 K, it was found that these three phenomena occur simultaneously at reduction temperatures above 723 K. This suggests that the three phenomena are interrelated, and that the phase transition and grain enlargement are important factors concerning SMSI in high-surface-area powder samples. A mechanism has been proposed whereby Ti 3+ defect centers in the titania lattice formed from support reduction are responsible for acceleration of phase transition and grain enlargement, and the TiO x , species that migrates onto supported metal particles is formed during the chaotic state of simultaneous phase transition and grain enlargement. At the temperature of the onset of the above three phenomena (~723 K), a stabilization against high-temperature sintering of iron particles became evident, suggesting that this effect becomes operative at the onset of SMSI. Iron particles supported on rutile were significantly smaller than anatase-supported particles which were reduced at the same temperature. The rutile was formed from anatase before impregnation with iron by reduction in hydrogen at 873 K followed by oxidation with oxygen at 473 K. The smaller iron particles and incomplete iron reduction were attributed to O − 2 ions adsorbed to the titania surface which acted to anchor Fe 3+ and/or Fe 2+ ions to the support, preventing their migration and combination to form large fully reduced iron particles.

Inhibition mechanism of the anatase-rutile phase transformation by rare earth oxides

Inhibition mechanism of the anatase-rutile phase transformation by rare earth oxides