Anatase Phase Stability Research Articles

Surfactant mediated synthesis of spherical binary oxides photocatalytic with enhanced activity in visible light

Spherical silica and zirconia mixed titania and pure titania samples were prepared in presence of cetyltrimethylammonium bromide (CTAB) through controlled hydrolysis of corresponding metal alcoxides. Effect of surfactant amount and calcinations temperature on morphology, surface area and photocatalytic activity is studied using PXRD, SEM, FTIR, Solid state UV–vis spectroscopy and BET surface area. It is well observed that in presence of 2 mol% CTAB, uniform sized spherical oxide particles can be synthesized. However, increasing or decreasing the surfactant amount does not favor the spherical particle formation. Material synthesis in presence of CTAB not only helps in the spherical particle formation but also increases the surface area and visible light absorption. Studies on photocatalytic lead removal with respect to calcination temperature indicate that the calcination at 500 °C is most suitable for the best photocatalytic activity. Mixing of zirconia and silica helps in anatase phase stabilization even at 900 °C calcination. Accordingly low decrease in surface area even at 900 °C calcination is observed. Due to the phase stabilization and higher surface area binary oxide materials showed comparatively better photocatalytic activity even after calcination at 900 °C. So it can be concluded that present synthesis approach can produce uniform sized spherical binary oxide materials with better photocatalytic activity in visible light.

Formation and stability of anatase phase of phosphate incorporated and carbon doped titania nanotubes

Phase transformation analysis of the phosphate containing and carbon doped titania nanotubes, prepared by a simple anodization method, reveals complete transformation from amorphous to anatase phase in air between 360 and 400 °C. Activation energies for formation of anatase phase are evaluated and compared for the two types of titania nanotubes. A detailed analysis of the phase transformation characteristics and stability of the anatase phase is reported.

Effect of Nb and Sc Doping on the Phase Transformation of Sol–Gel Processed TiO2 Nanoparticles

Nb and Sc doped TiO2 nanoparticles were synthesized via sol-gel technique. Dopant concentration of each element was varied from 0.5 to 1.5 atomic%. The effect of metal ion doping and calcination temperatures on anatase to rutile phase transformation has been investigated. Samples were analyzed by various analytical methods such as X-ray diffraction (XRD), Transmission Electron Microscope (TEM), X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray Spectroscopy (EDS). XRD analyses showed that Nb and Sc doped samples calcined at 300 degrees C and 350 degrees C, respectively, were crystalline and had an anatase structure. Results showed that anatase was stable up to 700 degrees C annealing temperature for samples doped with 0.5 atomic% Nb. There was a sharp transition from anatase to rutile phase above 700 degrees C and complete rutile structure was obtained at 750 degrees C. However, the transformation from anatase to rutile was not so sharp in samples doped with 1.0 atomic% and 1.5 atomic% Nb. Results indicated that higher concentration of Nb helps to stabilize the anatase phase. For samples doped with 0.5 atomic% Sc, anatase phase is stable up to 650 degrees C. Transformation from anatase to rutile starts at temperature above 650 degrees C and 100% rutile phase was obtained at 800 degrees C while for samples doped with 1.0 atomic% and 1.5 atomic% Sc, the complete transformation from anatase to rutile takes place at an even higher temperature. Results indicate that increasing the calcination time from 0.5 to 2.0 hours at 500 degrees C does not affect the stability of anatase phase. However, TEM and XRD data showed that the increase in the annealing time leads to an increase in particles size. The rutile to anatase concentration ratio increased with temperature above the phase transformation temperature. The activation energy for the phase transformation from anatase to rutile for doped and undoped samples was also measured. There was a general rise in the activation energy with increasing dopant concentration.

Improved High-Temperature Stability and Sun-Light-Driven Photocatalytic Activity of Sulfur-Doped Anatase TiO2

Of the various forms of titania (anatase, rutile, and brookite), anatase is reported to be the best photocatalyst. The anatase-to-rutile transformation in pure synthetic titania usually occurs at a temperature range of 600−700 °C. High-temperature (≥800 °C) stable anatase titania photocatalyst is required for antibacterial applications in building materials. A simple methodology to extend the anatase phase stability by modifying the titanium isopropoxide precursor with sulfur modification using sulfuric acid is presented. Various TTIP/H2SO4 molar ratios such as 1:1, 1:2, 1:4, 1:8, and 1:16 were prepared, and these samples were characterized by XRD, DSC, Raman spectroscopy, XPS, and BET surface area analysis. Sulfur-modified samples showed extended anatase phase stability up to 900 °C, while the control sample prepared under similar conditions completely converted to rutile at 800 °C. Stoichiometric modification up to 1:4 TTIP/H2SO4 composition (TS4) was found to be most effective in extending the anatase-...

Smooth titania nanotubes: Self-organization and stabilization of anatase phase

We present transmission electron microscopy and Raman scattering measurements showing that niobium inhibits the processes of nucleation and growth of anatase crystallites in the initial amorphous titania nanotubes and thus shifts the temperature of the complete amorphous-to-anatase phase transition to higher values up to 550°C. Niobium dopants stabilize the anatase phase in titania nanotubes up to 650°C. The size of anatase crystallites can reach 30–50nm. Excess niobium atoms which are pulled off from the volume of anatase crystallite form polymeric or monomeric Ti–O–NbO groups at the interface area. Slight shift and broadening of Eg (144cm−1), A1g (515cm−1) and Eg (630cm−1) modes in Raman spectra can be explained by niobium insertion into the anatase structure.

Titania−Sepiolite Nanocomposites Prepared by a Surfactant Templating Colloidal Route

Microfibrous TiO2−sepiolite and TiO2/SiO2−sepiolite nanocomposites were prepared following a colloidal route based on the controlled hydrolysis of alkoxides in the presence of cetyltrimethylammonium−sepiolite gel. In this way, titanium(IV) isopropoxide and tetramethoxysilane were used as titania and silica precursors, respectively, being incorporated within the organophilic layer developed on the surface of the silicate microfibers and further hydrolyzed in this region. The hydrolysis of the precursor gives highly viscous colloidal systems leading to a spontaneous heterocoagulation process of the colloidal system. Drying and thermal treatments of the resulting gels ensure the elimination of water and organic matter, driving to the formation of TiO2 nanoparticles homogeneously distributed on the surface of the sepiolite microfibers. The nanocomposites were characterized by chemical analyses, transmission electron microscopy, field emission scanning electron microscopy, X-ray diffraction, thermogravimetric/differential thermal analysis, and N2 adsorption. They were tested as photocatalysts in the photo-oxidation of phenol in an aqueous medium, showing the efficiency of the anatase containing nanocomposites in the removal of this selected pollutant model molecule. Thiourea was incorporated to increase the TiO2 anatase phase stability, improving in this manner the photocatalytic activity of the TiO2−sepiolite nanocomposites. It has been shown that sepiolite has a positive synergistic effect on the TiO2 photocatalysis.

Anatase phase stability and doping concentration dependent refractivity in codoped transparent conducting TiO2 films

Nb0.06SnxTi0.94−xO2 (x ≤ 0.3) thin films were grown by a pulsed-laser deposition method with varying Sn concentration. Through a combinatorial technique, we find that Sn concentration can reach a maximum of about x = 0.3 while maintaining the stable anatase phase and epitaxy. A doping concentration dependence of the refractivity is revealed, in which refractivity reduction at a wavelength of λ = 500 nm is estimated to be 12.4% for Nb0.06Sn0.3 Ti0.64O2 thin film. Sn doping induced band-gap blue shift can be contributed to the mixing of extended Sn 5s orbitals with the conduction band of TiO2. Low resistivity on the order of 10−4 Ω cm at room temperature and high internal transmittance of more than 95% in the visible light region are exhibited for Nb0.06Snx Ti0.94−xO2 thin films (x ≤ 0.2). Optical and transport analyses demonstrate that doping Sn into Nb0.06 Ti0.94O2 can reduce the refractivity while maintaining low resistivity and high transparency.

A Simple Sol−Gel Processing for the Development of High-Temperature Stable Photoactive Anatase Titania

A method for the preparation of anatase TiO2, which is stable to a temperature as high as 900 °C, without using any complex cationic dopants is presented. The synthetic procedure involves the reaction of titanium tetraisopropoxide (TTIP) with trifluoroacetic acid (TFA) followed by hydrolysis and sol−gel conversion to the xerogel and further calcination. The retention of the anatase phase to high temperatures can be attributed to the presence of small amounts of fluorine in the lattice, which is gradually removed between 500 and 900 °C as confirmed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopic analysis. Samples prepared with a 1:16 TTIP/TFA composition calcined at 900 °C showed significantly higher photocatalytic activity compared to the control sample, standard commercial photocatalyst Degussa P25, and samples prepared using acetic acid and oxalic acid. The high-temperature anatase phase stability, determined by X-ray diffraction and Raman spectroscopy, coupled with its high crystallinity, microporosity, and minimal oxygen vacancy contributes to improved photocatalytic activity.

Enhanced photoactivity of neodymium doped mesoporous titania synthesized through aqueous sol–gel method

Nanosize neodymium doped titania has been prepared by hydrolysis of titanium oxychloride followed by peptisation under acidic condition. The anatase to rutile phase transformation temperature was found to increase by 150 °C as a result of neodymium doping. The doped sample shows 10 times higher surface area than the undoped one after calcining at 700 °C. All the samples calcined at 500, 600 and 700 °C show type IV isotherm, which is characteristic of mesoporous material. The pore size distribution curves also show that the pores are in mesoporous region. Further, the neodymium doped titania shows increased photoactivity than the undoped titania with respect to decomposition of methylene blue when subjected to UV light. The transmission electron micrograph indicates that a nanocrystalline doped titania is obtained through the present method. The effect of neodymium doping on the anatase phase stability, specific surface area and photoactivity are reported.

Effect of tantalum addition on anatase phase stability and photoactivity of aqueous sol–gel derived mesoporous titania

The effect of the addition of tantalum to nanocrystalline titania matrix on the photocatalytic activity is presented. The tantalum addition increases the anatase phase stability to above 1000 °C by inhibiting the increase in crystallite size of titania. All the tantalum added titania compositions have higher surface area than pure titania. Transmission electron micrograph images support the reduction in crystallite size of titanium oxide in presence of tantalum. Addition of tantalum above 1 mole percentage increases photocatalytic activity considerably and the activity is much higher than that of commercially available Hombikat UV 100 titania. All the compositions containing tantalum have higher photoactivity than pure titania. The synergistic effect of high surface area and small crystallite size is found to be responsible for the observed photocatalytic activity rates. The aqueous sol–gel method adopted for the preparation of tantalum added titania from cheaper salt precursors and the resultant highly photoactive titania make this investigation promising for various technological applications.

Aqueous colloidal sol–gel route to synthesize nanosized ceria-doped titania having high surface area and increased anatase phase stability

Nanocrystalline sol–gel derived titania doped with ceria (1, 2, 5 and 10-mole%) has been prepared from titanyl oxysulphate. The titania doped with 5-mole% CeO2 after calcining to 500 °C, possesses specific surface area of 97 m2 g−1 and has anatase phase stability up to 900 °C. Moreover it retains a surface area of 37 m2 g−1 at 700 °C. In comparison, the undoped calcined material has anatase stability only up to 700 °C and specific surface area only 48 m2 g-1 and 6 m2 g-1 at 500 °C and 700 °C, respectively. The diffuse reflectance spectra show that, as the cerium content increases, the absorption undergoes a red shift and reaches the visible range. The exceptionally high phase stability, crystallinity and high surface area are due to the extremely fine particle size and effective doping achieved by the specific synthesis method. The results based on X-ray diffraction, specific surface area and diffuse reflectance spectra indicated that the maximum threshold limit of doping is up to a value of 5-mole%.

Stabilization of anatase phase in the rare earth; Eu and Sm ion doped nanoparticle TiO2

The extension of temperature range of anatase phase for nanoparticle TiO 2 has been achieved by doping the rare earth; Sm and Eu ions, respectively. The TiO 2 films and nanoparticles were prepared from Ti-(IV)-isopropoxide (TTIP) based on the sol–gel method at the varied sintering temperature from 400 to 950 °C for 1 h. The powder X-ray diffraction (XRD) spectra revealed significantly delaying of the phase transition from anatase to rutile by RE ions doping. The film morphology by atomic force microscopy (AFM) for doped films showed reduction of particle sizes as well as formation of uniform films. According to the structural studies, it is concluded that the RE ions has influenced greatly on the stabilization of anatase phase.

Enhanced photoactivity and anatase thermal stability of silica–alumina mixed oxide additives on sol–gel nanocrystalline titania

Nanocrystalline titania with silica and silica–alumina mixed oxide as additives has been prepared through a sequential approach sol–gel method starting from titanyl sulphate in aqueous medium. The mixed oxide added titania shows increased anatase phase stability and high surface area. The complete transformation of anatase to rutile in the mixed oxide added titania occurs only above 1100 °C. The silica and silica–alumina added titania precursor even after calcination at 800 °C show specific surface area of 53 m 2 g − 1 and 63 m 2 g − 1 respectively. Further, the mixed oxide added titania sample shows excellent photoactivity compared to the commercially available Hombikat UV 100 titania, with respect to degradation of methylene blue. The addition of mixed oxide has resulted in better properties with respect to specific surface area, increased anatase to rutile phase transformation and photocatalytic activity of titania.

Characterization of CaO–TiO2 and V2O5/CaO–TiO2 catalysts and their activity for cyclohexanol conversion

Influence of CaO on TiO2 (anatase) phase stabilization and dispersion behaviour of V2O5 over CaO–TiO2 mixed oxide support have been investigated using XRD, FTIR, Raman, XPS, and BET surface area techniques. The CaO–TiO2 binary oxide (1:1 molar ratio) was synthesized by a homogeneous co-precipitation method from ultrahigh dilute solutions of the corresponding chlorides and calcined at various temperatures from 723 to 1273K. On the calcined CaO–TiO2 support (723K) various amounts of V2O5 (2.5–10wt.%) were deposited from ammonium metavanadate by a wet impregnation method and calcined at different temperatures. Conversion of cyclohexanol to cyclohexanone/cyclohexene was performed as a model reaction to assess the acid–base properties of the prepared catalysts. The CaO–TiO2 composite oxide exhibits reasonably high specific surface area and high thermal stability up to 1273K retaining titania-anatase phase. The deposited V2O5 over CaO–TiO2 support is present mostly in a dispersed form. Further, a strong and preferential interaction between the basic CaO and the dispersed V2O5 leads to the formation of CaVO3 and the physicochemical properties of the prepared catalysts are clearly reflected in their catalytic activity.

An aqueous sol–gel route to synthesize nanosized lanthana-doped titania having an increased anatase phase stability for photocatalytic application

A novel synthesis method is reported, involving peptisation and doping of lanthana on colloidal sol–gel titania derived from titanyl sulphate, where the anatase phase has a high stability above 800 °C and a specific surface area of 101 m 2 g −1 for the gel calcined at 500 °C. The undoped gel calcined at 600 °C has an average crystallite size of 12 nm, but under identical experimental conditions, the doped one shows an average crystallite size of 7 nm. The doped titanium oxide has a specific surface area of 25 m 2 g −1 even after calcination at 800 °C. The exceptionally high phase stability and surface area are due to the specific synthesis method. Doped titania also shows an excellent photoactivity as evaluated by the extent of decomposition of methylene blue when exposed to ultraviolet light.

A study about the stabilization of anatase phase at high temperatures on sol–gel cerium and copper doped titania and titania–silica powders

Copper and cerium oxides doped titania samples, as well as silica–titania samples were prepared through sol–gel process by using titanium butoxyde as precursor. The obtained samples were characterized by X-ray diffractometry and scanning electron microscopy. The main objective of the proposed study is to promote the stabilization of anatase phase at high temperatures. The obtained experimental results show that cerium doped titania samples stabilizes the anatase phase until 800°C, whereas copper doped samples calcinated at the same temperature are a mixture of anatase and rutile phases. So, CeO2 is a better stabilizer for anatase phase than CuO. On the other hand, based on previously reported data it can be concluded that both, CeO2 and CuO are better stabilizers for anatase phase than SnO2. It is also verified that the total amount of silica in the titania–silica double oxide increases the temperature required for crystallization, and as a consequence, the anatase phase is stabilized at higher temperatures i.e. up to 1200°C.

First-principles calculations of the phase stability ofTiO2

First-principles calculations of the crystal structures, bulk moduli, and relative stabilities of seven known and hypothetical ${\mathrm{TiO}}_{2}$ polymorphs (anatase, rutile, columbite, baddeleyite, cotunnite, pyrite, and fluorite structures) have been carried out with the all-electron linear combination of atomic orbitals (LCAO) and pseudopotential planewave (PW) methods. The anatase versus rutile relative phase stability at 0 K and zero pressure has been investigated using high-quality basis sets and carefully controlled computational parameters. From the optimal crystal structures obtained with the Hartree-Fock theory at various pressures, the bulk modulus and phase transition pressures of various high-pressure polymorphs have been derived at the athermal limit. In most cases, the calculated unit cell data agree to within 2% of the corresponding experimental determination. Complete predicted structural data (unit cell constants and fractional atomic coordinates) are presented for the baddeleyite and pyrite forms. The calculated bulk moduli are within 10% of the most reliable experimental results. Both the all-electron LCAO and pseudopotential PW methods predict anatase to be more stable than rutile at 0 K and ambient pressure. The computed anatase-columbite, rutile-columbite, columbite-baddeleyite, and baddeleyite-cotunnite phase transitions appear in the same order as observed in experiments, and the transition pressures agree semiquantitatively with those measured. The pyrite and fluorite structures are predicted to be less stable than other polymorphs at pressures below 70 GPa in agreement with experiments. Finally, the elastic properties, compressibilities and phase transformations of the various polymorphs are discussed in terms of simple models based on the behavior of the constituent Ti-O polyhedra under compression.

Effect of calcination temperature and addition of silica, zirconia, alumina on the photocatalytic activity of titania

Nanophase titania was prepared by sol-gel method and spray pyrolysis. We tried to elucidate the relationship between the photoactivity and the crystallite size of anatase phase. To better understand the changes in the bulk and the surface of titania as the calcination temperature is changed, EPR and photoluminescence analysis were carried out. The effect of the secondary metal oxide embedded into titania matrix on the photoactivity was also investigated. It was found that the photoactivity of titania has a linear relationship to the crystallite size. For the analysis of EPR and photoluminescence for pure titania, the increase of photoactivity with increasing the calcination temperature is due to the formation of surface active sites such as O- as well as the increase of crystallinity resulting from the removal of bulk defects. For silica/titania mixed oxide, it was found that the improvement of the thermal stability of anatase phase is important to enhance the photoactivity of titania because the prepared catalyst was calcined at a higher temperature than 700 °C without forming rutile phase. It was also concluded that the simultaneous increase of the surface area and the crystallinity promises to improve the photoactivity achieved by increasing the content of silica up to 60%. By the analysis of EPR and photoluminescence, it was found that the embedding of silica into titania matrix suppresses the formation of Ti3+ and produces a new active site of Ti-O-Si, which easily interacts with the oxygen. In the investigation of zirconia/titania and alumina/titania mixed oxide, it was found that the increase of the surface OH is essential to positively affect of the improved thermal stability on the photoactivity.

Synthesis of thermally stable, high surface area anatase–alumina mixed oxides

Mixed oxides containing anatase and 5–20 mol% alumina have been synthesized from a precursor sol containing acetic acid modified titanium isopropoxide and boehmite (AlOOH). The resultant oxides retain considerable anatase phase at 1000°C and have specific surface area many times higher than that of pure titania. A composite precursor containing titania–20 mol% alumina after calcination at 450°C, has a specific surface area as high as 318 m 2/g compared to a value of 100 m 2/g for pure titania prepared under identical conditions. The uniform dispersion of the nanoparticles of alumina in the titania gel matrix causes reduction in the anatase/anatase contact points and thus results in a decrease of nucleation sites leading to increase in thermal stability of anatase phase. The high surface area and porosity could arise from the highly branched acetate polymeric gel. The sintered density of compacts derived from mixed oxides is also brought down considerably compared to pure titania counter parts.

Titania based platinum monolithic catalysts for lean-burn DeNO x process

Monolithic catalyst supports were prepared with mixtures of titanium dioxide and a natural magnesium silicate. Some properties of relevance to their scale-up for industrial applications using monoliths manufactured with various commercial titanias and treated at different temperatures were compared. Thus, their textural properties, anatase phase stability and axial strength were evaluated. Taking into account these results, a support was selected for the preparation of platinum catalysts, which were prepared by varying the impregnation time and the platinum solution concentration. Equations were obtained describing the dependence of the catalyst platinum content on each of these parameters. Catalyst activities were tested in the catalytic reduction of nitrogen oxide with propylene in lean-burn conditions.