Physical stabilization of anatase (TiO2) by freeze-drying

Journal of Materials Science, 1988

Spherical, submicrometre, amorphous hydrous titania powder synthesized by controlled hydrolysis and polymerization from titanium tetraethoxide solutions was hydrothermally converted to spherical polycrystalline anatase particles by autoclaving or refluxing. Green compacts produced with either autoclaved or refluxed powder via a colloid filtration route had a high density and were crack-free; processing with untreated hydrous titania resulted in cracked green compacts. Compacts of the hydrothermally treated powders could be sintered to 98% theoretical density at temperatures as low as 900 ~ C. A compact of commercial powder produced in the same fashion was not observed to densify at such temperatures. Using various firing techniques, compacts of the hydrothermally treated powder could be sintered to 98% theoretical density or greater while controlling the titania phase assemblage as (1) anatase, (2) rutile, or (3) a mixture of anatase and rutile. By scaling the phase transformation and sintering kinetics, the grain size of the sintered microstructure can be controlled from a submicrometre to a micrometre scale.

Journal of Colloid and Interface Science, 2001

Titania powders were synthesized by a sol-gel process using titanium tetrabutoxide as a precursor. The syntheses were performed in water or in saturated aqueous solutions of KCl, CaCl 2 , NiCl 2 , CoCl 2 , and MnCl 2. It is demonstrated, by X-ray diffraction patterns of the synthesized powders that the samples obtained in saturated aqueous solutions of metal chlorides are crystalline (anatase phase) with some minor amount of brookite phase, whereas the sample synthesized in water is amorphous in nature. Thus, it is shown that the anatase phase can be obtained independently on any previous or further treatment of the synthesized powder, such as hydrothermal or heat treatment, providing a new, simple, quick, and inexpensive route to synthesize anatase powders.

2021

TiO 2 photocatalyst is one of the advanced oxidation process strategy which may be very potential to be implemented to deal with wastewater. The properties of the organic solvents and alkoxide precursors can impose significant effects on the final products' microstructures and properties of TiO 2. In this case, two organic compounds, ethanol, and methanol were selected for sol-gel synthesis. Besides these two solvents, two precursors, tetra isopropyl orthotitanate and tetra-n-butyl orthotitan-ate were used. Attention was paid to the structure, bad-gap, morphology, and photocatalytic activity of the samples. XRD confirmed the anatase crystal structure for all samples. The formation of brookite phase was facilitated by the use of tetra isopropyl orthotitanate with ethanol as solvent. The photoluminescence spectra displayed a broad, intense emission in the upper UV region. A band-gap near the theoretical band-gap at 3.2 eV was observed; hence, the photoactivity will occur in the ultraviolet area. TNBm was observed to have a higher percentage (66.5%) and a higher rate (0.5/min) of methylene blue degradation to test the photoactivity of synthesized TiO 2 NPs.

2004

During the sulfate process of TiO2 pigment production, hydrated titanium dioxide is calcined. As a result of the polymorphous conversion, rutile is obtained. Roasting additives are introduced into the calciner, in order to achieve required process temperature, crystallite size of individual phases, optical properties, and the pigment photostability. Calcination process was investigated on the laboratory scale with the use of hydrated titanium dioxide containing rutile nuclei from the industrial installation. The influence of both temperature and calcination time on the anatase-rutile transformation ratio and the crystallites growth was determined. The obtained dependences were described using exponential equations. The ratio of anatase-rutile transformation was higher at elevated temperatures. It was noticed that the process temperature affects the conversion ratio considerably more than the calcination time. During the experiments alkaline metals (potassium, lithium) and phosphate were introduced into the calcination suspension. It was noted that the increase of phosphates content in the calcinated TiO2 • nH2O (0.1-0.5 mass % P2O5 in relation to TiO2) caused the anatase-rutile transformation ratio to drop 2 to 10 times. Moreover, the phosphates presence restricted an unfavourable anatase and rutile crystallites growth. In fact, their crystallites size was 2-3 times lower, compared to the crystallites size measured when the phosphates were not added. If the potassium content in hydrated TiO2 was increased (0.05-1.0 mass % K2O in relation to TiO2), during calcination the rutile formation passed through a maximum. Similar behaviour was observed when lithium (0.025-0.5 mass % in relation to TiO2) was introduced instead of potassium. Neither anatase nor rutile average crystallites size was influenced by the potassium or lithium addition.

Industrial & Engineering Chemistry Research, 2007

jusami.batan.go.id

IMPROVING ANATASE TO RUTILE TRANSITION TEMPERATURE BY MILLING. An intensive investigation using X-ray diffractometry has been conducted to study the optimum mechanical and thermal conditions for the transition of anatase-to-rutile from a titanium dioxide (TiO 2) powder. Milling was applied by employing a conventional ball milling instrument and a quasi-high-energy pulveriser and varying the milling period. Heat treatment was performed by calcination at 850, 900, 950, 975, 1000, 1050 and 1100°C for 1 hour, being the predicted optimum temperature to obtain fully rutile powder was 975°C. Each powder was milled and then calcined prior to the X-ray diffraction investigation. X-ray diffraction data were analysed using (1) standard identification and peak characterisation, (2) the Rietveld method to give the weight fractions, lattice constants and crystallite size and strain estimates, and (3) Mozaix, an own-developed software to provide strain, crystallite size and size distribution of phases. Results showed that milling speed and milling up to 24 hours does not significantly change the phases' composition, but enhances the transition temperature. Conventional milling gives better results than pulverising. Conventional milling at 100 rpm for 3 hours reduces the transition temperature from 1100°C to 1000°C. Longer milling, however, does not improve the transition temperature. The optimum process is discussed and SEM micrographs are used to support the argument.

Langmuir, 2004

Thermodynamic stability of anatase nanoparticles and their transformation behaviors to rutile phase in an acidic solution was investigated in situ at two different peptization temperatures using a freezedrying method. When peptized at 30°C, the initial product was anatase with a significantly distorted atomic structure, a significant amount of hydroxyl group and Ti 3+ ions, and, thus, a thermodynamically unstable state. The instability of 30°C-peptized anatase was responsible for a suitable transformation to rutile later via dissolution of the anatase to form a titanium hydroxylate, followed by reprecipitation into rutile. On the other hand, 80°C-peptized anatase had a relatively more ordered atomic structure, a much reduced amount of hydroxyl group, negligible Ti 3+ ions, and, thus, a thermodynamically more stable state. Plausible reasons why the 80°C-peptized anatase does not transform to rutile were deduced.

The work deals with the evaluation of the application of a novel treatment of the titanium surface in order to increase anatase growth. Two different anodizing methodologies, previously developed [6] were considered, followed by a thermal post treatment able to convert the amorphous oxide film, obtained by anodizing, in a crystalline anatase structure. X ray thin film diffractometry and Raman spectroscopy have been used to identify anatase on the treated titanium surface. Combining the information obtained by the two analysis techniques allowed to verify that the best anatase crystallinity could be obtained by the treatment 1A followed by heat treatment.

Journal of the American Ceramic Society, 2010

The influence of sodium chloride (NaCl) and dibasic sodium phosphate (Na 2 HPO 4. 2H 2 O, DSP) on the phase transformation and particle size of titanium dioxide powder had been investigated. The salt matrix was shown to suppress both the amorphous to anatase transition and the anatase to rutile transition. DSP was particularly effective. Irrespective of the additives, transformation versus time curves of the anatase to rutile conversion were observed to be sigmoidal, and were interpreted in terms of a first order, nucleation-growth controlled phenomenon. Analysis of these curves using appropriate rate laws yielded activation energies for nucleation of B4.3 eV for NaCl salt matrix and B8.6 eV for DSP salt matrix, compared with B3.2 eV for a sample with no salt. Activation energies for the growth or propagation stage were B5 and B8.9 eV for NaCl and DSP powders, respectively, compared with B3.8 eV for no salt. In the solid state, salt matrices suppressed the particle growth as well. These behaviors, in general are thought to be governed by the presence of salt impurities, especially anions chemisorbed at the surface.

Polish Journal of Chemical Technology, 2000

TiO 2 attracts much interest because of its many potential applications. The use of titanium dioxide strongly depends on its polymorphic form: brookite, anatase, or rutile. Only rutile and anatase play an important role in industry. Anatase as a metastable form undergoes a non-reversible transformation into rutile. Understanding the kinetics of phase transformation and the processes of crystal growth of a material is essential for controlling its structure and, thus, its specifi c properties. The main purpose of this paper is to explain the anatase to rutile recrystallization kinetics in the modifi ed TiO 2 calcined from industrial hydrated titanium dioxide. The apparent activation energy of anatase to rutile transformation and the average size of titanium dioxide crystallites were determined for the unmodifi ed TiO 2 and TiO 2 modifi ed with P, K, Al, B, Zn, Zr, Ce, Sn, or Sb introduced in the amount of 0.5 mol% and 1.0 mol% when recalculated for their oxides. The growth of TiO 2 crystallites during calcination was strongly inhibited by P, Ce and Zr, and inhibited to a lesser degree by Al, Sn and Sb. B and Zn did not affect the investigated process and K accelerated crystallites growth. The values of apparent activation energy depending on a modifi er formed a relationship: Al<Sb<Sn<P<B<Ce<0=Zn=K<Zr. The observed dependencies can be explained by reactions occurring between the modifi ers and titanium dioxide.

Journal of Alloys and Compounds, 2015

The synthesis of anatase TiO 2 nanoparticles by an acid-assisted solegel method at 25 and 80 C is described. Specifically, acetic acid (AA) was used and the evolution of the anatase phase with the amount of AA was observed. The results of X-ray diffraction (XRD) and transmission electron microscopy (TEM) both showed that a pure anatase phase was obtained with particle size smaller than 5 nm. Structural refinements and quantitative determination of phase composition was achieved by using the Rietveld method. The particle size distribution became slightly narrower as the amount of AA was increased. Raman spectroscopy showed that when the amount of AA was increased a small amount of brookite was present at the contamination level. The anatase phase was studied by differential thermal analysis (DTA), providing phase stability up to 600 C. These and other results were discussed in terms of particle size and structure. Likewise, the formation of the anatase phase under these synthesis conditions was explained.

International Journal of Nanotechnology, 2009

TiO 2 nanocrystals were prepared by hydrolysis and peptisation of titanium isopropoxide under different pH values. The as-prepared powder of very fine anatase crystallites ranges from 8 nm in acidic solution of pH 3 to 10 nm in basic solution of pH 8. Heat treatment of the powders leads to grain growth and anatase-rutile transformation. Experimental results have shown that the anatase to rutile phase transformation in the heat-treated powders depend not only on primary crystallite size but also the presence of brookite phase. It is observed that in the powders with the same size the presence brookite phase would accelerate the anatase to rutile transformation. Furthermore it has been found that the transition route is different among samples prepared in acidic or basic solutions. It is also proved that in case of acidic solutions, the transition route follows the brookite-anatase-rutile path, but in case of basic solutions, it would be done via anatase-brookite-rutile. Apart from these observed paths, the main route of A-R remains unchanged for both acidic and basic conditions. So it could be justified that brookite phase together with anatase grain size in titania powder plays very important and different roles and accelerates phase transformation.

Journal of the Serbian Chemical Society, 2005

Titanium dioxide powders were synthesized by the sol-gel process using titanium tetrabutoxide as the precursor. The syntheses were performed in saturated aqueous solutions of KCl, CaCl 2 , NiCl 2 , CoCl 2 and MnCl 2 , and in dimethylformamaide (DMF) and dimethylsulfoxide (DMSO) solutions. The obtained X-ray diffraction patterns show that all samples were crystalline (anatase phase) with some minor amounts of a brookite phase. It is worth noting that the anatase phase was obtained independent of any previous or further treatment of the synthesized powder, such as hydrothermal or heat treatment. For the titanium dioxide powders synthesized in saturated aqueous solution of metal chlorides (mean crystallite size = 11 nm), the anatase-rutile transition occurred in the range 455-570°C, depending on the considered sample, as verified by DTA analysis. In the powders synthesized in DMF or DMSO solutions (means crystallite size = 6 nm), the same structural transition occurred at 485°C.

Journal of Applied Sciences, 2011

Pure anatase TiO 2 nanoparticles were synthesized by microwave assisted sol-gel method and further characterized by powder X-ray diffraction (XRD), energy dispersive x-ray analysis (EDAX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV-Visible spectrophotometer, SEM images showed that TiO 2 nanoparticles were porous structure. The XRD patterns indicated that TiO 2 after annealed at 300°C for 3 h was mainly pure anatase phase. The crystallite size was in the range of 20-25 nm, which is consistent with the results obtained from TEM images. Microwave heating offers several potential advantages over conventional heating for inducing or enhancing chemical reactions.