Effect of heat treatment on structure evolution and properties of nano zinc titanate ceramics

Nano zinc titanate ceramics are prepared using a conventional solid state method. The obtained compacts are sintered at 800, 900, 1000 and 1100 °C for 3 h. The prepared compacts are analyzed using X‐ray diffraction (XRD) and scanning electron microscopy (SEM) for structural and microstructural studies. Based on the X‐ray diffraction (XRD) data, it is observed the coexistence of ZnTiO₃ and α‐Zn₂TiO₄ phases together at low temperature (800 °C) without the presence of TiO₂ (rutile) contradicting the general mechanism stating the transformation of ZnTiO₃ to α‐Zn₂TiO₄ and TiO₂ at higher temperatures. A new mechanism is proposed to explain the formation of nano ZnTiO₃ and α‐Zn₂TiO₄ structures depending on the role of TiO₂ in achieving this mission. According to this mechanism, we propose a partial diffusion of TiO₂ in the ZnO lattice forming the ZnTiO₃ phase. The second part of TiO₂ acts as a catalyst that facilitates the transformation of nano ZnTiO₃ to nano α‐Zn₂TiO₄. The catalytic power of rutile is achieved from its enhanced tensile stress that induces the phase transition from nano ZnTiO₃ to nano α‐Zn₂TiO₄.     

Mechanochemical effects on the kinetics of zinc titanate formation

In this work, mixtures Zn-TiO₂ (anatase) in molar ratio 1:1 were mechanochemically activated in air atmosphere, and submitted to thermal treatments in order to study its thermal transformations. The behavior of the system during the milling was followed by X-ray diffraction (XRD), differential thermal analyses (DTA) and thermogravimetric analyses (TGA). Mechanochemical activation produces a progressive loss in crystallinity of the starting powders, with simultaneous oxidation of metallic Zn. However, the formation of neither ZnTiO₃ nor Zn₂TiO₄ could be detected. At temperatures above 600°C, the thermal treatments resulted in the formation of ZnTiO₃ and Zn₂TiO₄, at lower temperatures and shorter holding times for samples activated during longer times. The non-activated mixture exhibited a very different behavior, yielding Zn₂Ti₃O₈ and Zn₂TiO₄ without evidence of ZnTiO₃ formation. The obtained results are explained on the basis of reaction mechanisms taking place in the activated and non-activated samples.     

High-energy ball milling of intermetallic Ti-Cu alloys for the preparation of oxide nanoparticles

Copper and titanium oxides in the nano-size range show unique chemical and physical properties and thus have been intensively considered for novel and smart applications. In this work, oxide nanoparticles were prepared by high-energy ball milling of Ti-Cu alloys followed by a controlled oxidation process. Alloys of the Ti-Cu system Ti-50Cu, Ti-57Cu, and Ti-65Cu (in wt.%) prepared by arc melting were selected considering they provide different starting brittle intermetallic phases before milling. Microstructural investigation indicated that Ti-50Cu was composed of Ti₂Cu and TiCu, while Ti-57Cu was single-phase TiCu. Ti-65Cu was dual-phase and consisted of Ti₃Cu₄ and Ti₂Cu_3. A mean particle size below 10 nm was achieved after high-energy ball milling for all compositions. The oxidation process was then investigated in two temperature ranges. At high oxidation temperatures of 700–800 °C, a complete oxidation took place leading to oxides TiO₂-rutile and CuO in all alloys. However, at a low oxidation temperature (350 °C), partial oxidation occurred and different oxides were obtained. Ti-50Cu was the most promising alloy and led to a mix of TiO₂ (rutile and anatase), CuO, Cu₂O, and Ti₃Cu₃O. After long exposure to thermal oxidation, the resulting oxides remained in the nanometric range with a particle size distribution showing a D50 of approximately 6 nm.     

Modeling of the Phase Transformation Induced by Ball Milling in Anatase TiO2

A high-pressure and high-temperature phase of TiO₂ : TiO₂ II is formed transiently during room-temperature high-energy ball milling of anatase TiO₂ : TiO₂ anatase TiO₂ II TiO₂ rutile. Rutile is the only phase present after prolonged ball milling. The present paper focuses on the influences of physical and chemical processing conditions on the transformation kinetics. The effects of two milling parameters on the kinetics of phase transformation of anatase TiO₂ were investigated: the nature of milling tools and the powder-to-ball weight ratio R. Granulometric characterizations and TEM observations have demonstrated that the transformation of TiO₂ anatase into TiO₂ II occurs without fracturing of particles and that TiO₂ II nanograins form at the surface of anatase particles. The parameter R affects only the transformation rate. For a given R, the transformation rate is the largest with alumina grinding tools, intermediate with zirconia tools, and the smallest with steel tools. The parameters involved in current models of the mechanical alloying process do not suffice to explain the differences in transformation rates observed here. A parameter, which takes into account the influence of the mechanical properties of grinding materials, is considered.     

Synthesis and photocatalytic properties of nanomaterials based on titanium(IV) and zinc(II) oxides

We have synthesized materials based on titanium(IV) and zinc(II) oxides, containing 1 to 60 wt % Zn, at heat-treatment temperatures from 80 to 1150°C, with the formation of multiphase compositions (X-ray amorphous phase, anatase, rutile, ZnTiO₃, and/or Zn₂TiO₄) and studied their phase transitions, morphology, and photocatalytic activity. Increasing the Zn content of the materials is favorable for their spectral sensitization, including the range λ ≥ 670 nm.     

Sintering and microwave dielectric properties of LTCC-zinc titanate multilayers

The effects of ZnO–B₂O₃–SiO₂ (ZBS) on the sintering behavior and microwave dielectric properties of ZnO–TiO₂ system were investigated as a function of ZBS content and sintering temperature. Densities of the specimens were enhanced with an increase of ZBS up to 2 wt.% and then decreased. X-ray diffractometry analyses results indicated that the phase stability region of the hexagonal ZnTiO₃ extended to lower temperatures as the amount of ZBS increased. The dielectric properties of ZnO–TiO₂ system with ZBS are strongly dependent on the sintering conditions, especially near the phase decomposition temperature. The sintering temperature of the specimens could be reduced to 900 °C without the degradation of the microwave dielectric properties. From 900 °C, the temperature compensation characteristics occurred as the phase composition changed from ZnTiO₃ to two phases: Zn₂TiO₄ and rutile. The dielectric constant (ɛ_r) increased and Q×f value decreased due to the phase decomposition. The ɛ_r value of 27, Q×f value of 19,396 (at 6 GHz) and τ_f value of 2 ppm/°C were obtained for ZnO–TiO₂ ceramics with 2.0 wt.% ZBS sintered at 900 °C for 3 h. The low-temperature sintering ceramics powders were suitable for the tape casting process. Also, the material is compatible with Ag electrodes and, therefore, is suitable for LTCC application.     

EXAFS investigation of iron local environment in metal-doped titania photocatalysts prepared by hydrothermal and high-energy ball milling routes

Iron local environment was investigated by EXAFS in Fe- and (Fe, Eu)-doped TiO₂ photocatalysts, prepared by hydrothermal and high-energy ball milling (HEBM) routes. In the case of the hydrothermal samples, the substitution of Ti⁴⁺ by Fe³⁺ ions was evidenced. For the samples prepared by HEBM, the iron environment corresponds to mixed metallic and oxidized (FeO, α-Fe₂O₃) configurations, without a clear evidence of iron incorporation into the TiO₂ lattice. This could be related to the catalyst contamination by iron microparticles detached from the balls during milling process.     

Direct formation of new,phase-stable,and photoactive anatase-type Ti1−2XNbXScXO2 solid solution nanoparticles by hydrothermal method

A new anatase phase of photoactive Ti_1?2XNb_XSc_XO₂ (X = 0–0.2) solid solutions was directly formed as nanoparticles from precursor solutions of TiOSO₄, NbCl₅, and Sc(NO₃)₃ under mild hydrothermal conditions at 180 °C for 5 h using the hydrolysis of urea. With the increase of the content of niobium and scandium from X = 0 to 0.2, the lattice parameters a₀ and c₀, the crystallite size, and the optical band gap of anatase gradually increased. Their photocatalytic activity and adsorptivity were evaluated separately by the measurement of the concentration of methylene blue (MB) remained in the solution in the dark or under UV-light irradiation. The anatase-type Ti_1?2XNb_XSc_XO₂ (X = 0.05) showed approximately two times and three times as high photocatalytic activity as those of the hydrothermal anatase-type pure TiO₂ and commercially available reference pure TiO₂ (ST-01), respectively. The anatase phase of Ti_1?2XNb_XSc_XO₂ (X = 0–0.2) existed stably up to 900 °C during heat treatment in air. New rutile-type Ti_1?2XNb_XSc_XO₂ solid solutions are formed through the phase transformation. The starting temperature of anatase-to-rutile phase transformation for Ti_1?2XNb_XSc_XO₂ (X = 0–0.2) solid solutions was delayed but its completing temperature was accelerated.     

The preparation of the anatase and rutile forms of Ag-TiO2 and hydrogen production from methanol/water decomposition

This study examined hydrogen production over Ag-TiO₂ photocatalysts containing Ag_xO, a conducting component. X-ray photon spectroscopy (XPS) confirmed that the Ag and Ag₂O components were dominant in the Ag-TiO₂ photocatalysts treated at 500 and 800 °C, respectively. The Ti2p bands in Ag-TiO₂ were shifted to lower binding energies, which were assigned to Ti³⁺, compared to pure TiO₂, and the shift was greater in the rutile structure than in the anatase. The measured full widths at half maximum (FWHM) of the Ag3d and Ti2p peaks were larger in the anatase structure than in the rutile structure in both TiO₂ and Ag-TiO₂. The H₂ production from methanol photodecomposition was greater over the rutile structure than over the anatase structure of TiO₂. Moreover, the amount of hydrogen was enhanced over Ag-TiO₂ compared to pure TiO₂; the production reached 17,124 μmol after 24 h over rutile Ag-TiO₂. After methanol photodecomposition, the amount of Ag component in the Ag-TiO₂ photocatalysts increased, while the Ag₂O component decreased.     

Formation and transformation of ZnTiO3 prepared by sol–gel process

ZnTiO₃ powders with pure hexagonal phase were prepared by the sol–gel process with Zn(NO₃)₂·6H₂O and Ti(OC₄H₉)₄ materials. The thermal behavior and phase transformation of the gels were investigated by the differential scanning calorimetry–thermogravimetry (DSC–TG) analysis, X-ray diffraction (XRD) patterns, Fourier-transforming infrared (FT-IR) spectroscopy, and Raman scattering spectroscopy. The results revealed that pure hexagonal phase of ZnTiO₃ could be obtained at low temperature of 800 °C. However, in further increased temperature above 900 °C, hexagonal ZnTiO₃ would decompose into cubic Zn₂TiO₄ and rutile TiO₂.     

Effect of Cu dopant on microstructure and phase transformation of ZnTiO3 thin films prepared by radio frequency magnetron sputtering

Cu doped zinc titanate (ZnTiO₃) films were prepared using radio frequency magnetron sputtering. Subsequent annealing of the as-deposited films was performed at temperatures ranging from 600 to 900 °C. It was found that the as-deposited films were amorphous and contained 0.84 at.% Cu. This was further confirmed by the onset of crystallization that took place at annealing temperatures 600 °C. The phase transformation for the as-deposited films and annealed films was investigated in this study. The results showed that Zn₂Ti₃O₈, ZnTiO₃, and TiO₂ can coexist at 600 °C. When annealed at 700 °C, the results revealed that mainly the hexagonal ZnTiO₃ phase formed, accompanied by minority amounts of TiO₂ and Zn₂Ti₃O₈. Unlike pure zinc titanate films, this result showed that the Zn₂Ti₃O₈ phase can be stable at temperatures above 700 °C. Moreover, Cu addition in zinc titanate thin film could result in the decomposition of hexagonal (Zn,Cu) TiO₃ phase at 800 °C. When the Cu content was increased in zinc titanate thin films from 0.84 at.% to 2.12 at.%, there were only two phases; Zn₂Ti₃O₈ and ZnTiO₃, coexisting at temperatures between 700 and 800 °C. This result indicated that a greater presence of Cu dopants in zinc titanate thin films leads to the existence of the Zn₂Ti₃O₈ phase at higher temperatures.     

Study on the formation mechanism of Y-Ti-O oxides during mechanical milling and annealing treatment

Y-Ti-O nano-scale oxides play important roles in ensuring the excellent performance of oxide dispersion strengthened (ODS) steels. In this study, a model powder system of Y₂O₃ and Ti was designed to investigate the formation and evolution mechanism of Y-Ti-O oxides. The morphology of powders tended to be stable after high energy ball milling for 240 min in Ar. X-ray diffraction (XRD) results suggested that there was no formation of new phase after mechanical milling. Thermo-gravimetric and differential thermal analysis (TG-DTA) was applied to analyze physical and chemical reactions of milled powders respectively in Ar and air. The corresponding annealing and XRD were performed to study the types and structures of oxides at different temperatures. It shows that oxygen concentration and temperature are the critical factors affecting the formation of oxides. Ti was evolved into Ti₆O, Ti₃O and TiO₂ in turn with temperature increasing. Then only TiO₂ was reacted with Y₂O₃ to form Y₂Ti₂O₇. The formation of Y₂Ti₂O₇ began at around 500 ℃ and was completed around 1004 ℃. A maximum formation rate occurred at about 779 ℃. High resolution transmission electron microscopy (HRTEM) suggested that the main phase in powders sintered at 1100 ℃ was identified as pyrochlore structure Y₂Ti₂O₇.     

Thermodilatometric behaviour of pure and doped ZrTiO4-SnO2

Thermodilatometric tests have been performed to investigate the shrinkage of green compacts of pure and ZnO-doped 4ZrO₂-5TiO₂-SnO₂ prepared by dry ball milling, flo-deflocculation and coprecipitation. Experiments have shown that the powder preparation procedure has a significant influence on the sintering process. Optimizing the homogeneity composition, which is enhanced from dry ball milling to coprecipitation, raises the starting sintering temperature. The reduction of the dimension of the starting particles increases the sintering rate and the addition of ZnO favours the shrinkage of the green bodies. Coprecipitated products lead to the highest final density because the evaporation of tin oxide on firing is reduced.     

Structure and luminescence properties of TiO2:Er3+ nanocrystals annealed at different temperatures

Erbium doped TiO₂ nanocrystals with the structures of anatase, pyrochlore Er₂Ti₂O₇, and rutile, characterized by X-ray diffraction, have been obtained at different annealing temperatures from 300 °C to 900 °C. The nanocrystalline size for anatase TiO₂ is reduced with increasing doped erbium concentration. Following ultraviolet 325 nm irradiation, the intensity of the green emission is the most intense for the TiO₂:Er³⁺ nanocrystals with a structure of pyrochlore Er₂Ti₂O₇, which evolves from the structure of anatase annealed at 800 °C. Moreover, following ultraviolet 325 nm and infrared 980 nm irradiation, the visible emission spectra for the nanocrystals annealed at 900 °C change drastically. Correspondingly, the structure of anatase disappears, while that of rutile becomes dominant, which indicates that phase transformation occurs.     

Study of Fe-doped ZnO by Mechanical Alloying

Zn_100?x OFe _x samples were obtained by mechanical alloying (MA) with different milling times. The purpose of this work was to analyze the variation of hyperfine parameters of the system with milling time. X-ray patterns show that pure ZnO under mechanical milling is not contaminated; additionally the system obtained by mechanical alloying has a wurtzite structure similar to pure ZnO. M?ssbauer spectra show that all samples present two phases: paramagnetic and ferromagnetic. The milling time contributes to increase the paramagnetic phase; however, the M?ssbauer spectra taken for samples milled during 36 and 99 hours indicates that there is a solubility limit for Fe atoms in ZnO lattice due that the spectra area remains nearly constant after 36?h. M?ssbauer spectrometry shows that the Fe atoms which penetrate inside the ZnO matrix behave like Fe³⁺ and Fe²⁺.     

Synthesis of visible-light responsive nitrogen/carbon doped titania photocatalyst by mechanochemical doping

Nitrogen and/or carbon doped titania photocatalysts TiO_2−x A _y (A = N, C) were prepared by a novel mechanochemical method. The samples were prepared by a high-energy ball milling of P25 titania with different nitrogen/carbon sources such as hexamethylenetetramine, admantane or ammonium carbonate, followed by calcination in air at 400 °C. The high mechanical energy accelerated the phase transformation of anatase to rutile, while the existence of the chemical reagents tended to block the transformation. The prepared powders possessed two absorption edges around 400 and 540 nm and showed excellent photocatalytic ability for nitrogen monoxide oxidation under visible light irradiation. Under the irradiation of visible light of wavelength >510 nm, 37% of nitrogen monoxide could be continuously removed by the carbon and nitrogen co-doped titania prepared by planetary ball milling of P25 titania–10% hexamethylenetetramine mixture followed by calcination in air at 400 °C. This mechanochemical technique might be widely useful for doping oxides with nonmetallic elements.     

Electron trapping at the lattice Ti atoms adjacent to the Nb dopant in Nb-doped rutile TiO2

The Nb-doped anatase TiO₂ is considered one of the most promising alternative transparent conducting oxides to substitute for indium tin oxide. However, studies have found that the conductivity emerges only in the anatase form, not in the rutile form. We applied the first-principle band structure method for the Nb-doped TiO₂ in both polymorphs. The calculation was carried out using the spin-restricted and spin-polarized GGA+U level of the theory. Special care was taken in the calibration of +U parameters to satisfy the generalized Koopman’s theorem. A significant difference was found between the spin-polarized and spin-restricted calculations. We noticed that spin polarization was necessary to reproduce the electron trapping in rutile. In addition, electrons are trapped at two lattice Ti atoms adjacent to the Nb_Ti dopant along the [001] direction, as described with the formal charge state of Ti^3.5+–Nb⁵⁺–Ti^3.5+. A careful convergence of the electron trapping character was conducted against the unit cell size based on the Bader population analysis.     

Estimation of reaction conditions for synthesis of nanosized brookite-type titanium dioxide from aqueous TiOCl<Subscript>2</Subscript> solution

Ti O₂ nanoparticles with a mixture of brookite and rutile phases were prepared from aqueous TiOCl₂ solution at 80–150°C and pure rutile phase at 200°C. The volume fraction of brookite was gradually increased with increase of HCl concentration in the range of about 4.43 M to 6.28 M. The maximum volume fraction of brookite in the as-prepared TiO₂ particles was obtained when oxidation of Ti⁴⁺ to TiO₂ was completed but it was gradually decreased with increase of reaction time. The reaction time for complete oxidation of Ti^{4 +} to TiO₂ was about 15 h at 80°C, about 5 h at 100°C, about 2 h at 120°C, and about 1 h at 150°C, respectively, showing that the kinetics of oxidation is very dependent on the reaction temperature. Brookite phase was not transformed directly to rutile phase but to anatase phase by heat-treatment at about 750°C, which finally converted to rutile phase at 1100°C.     

Polymorphic transformation and powder characteristics of TiO2 during high energy milling

Many studies have indicated that the reactivity of reactants can be enhanced greatly by mechanical activation through high energy ball milling. To understand this enhanced reactivity, the polymorphic transformation and the evolution of the powder characteristics of TiO₂ and graphite mixtures during high energy ball milling was investigated using various analytical instruments. It was found that polymorphic transformation of anatase to srilankite and rutile took place during milling. Furthermore, amorphization of crystalline phases and crystallization of the amorphous phase occurred at the same time during milling. High energy milling also led to ultrafine crystallites, large specific surface areas, and substantial amounts of defects in the powder particles. Effects of the graphite addition and the milling temperature on the polymorphic transformation and the evolution of the powder characteristics were also investigated. It was proposed that the polymorphic transformation of TiO₂ during milling could be explained in terms of the temperature-pressure phase diagram if the temperature rise and high pressure at the collision site were taken into consideration.     

Formation and transformation of ZnTiO3 prepared by sputtering process

Zinc titanate (ZnTiO₃) films were prepared using RF magnetron sputtering at substrate temperatures ranging from 30 to 400 °C. Subsequent annealing of the as-deposited films was performed at temperatures ranging from 600 to 900 °C. It was found that all as-deposited films were amorphous, as confirmed by XRD. This was further confirmed by the onset of crystallization that took place at annealing temperatures 600 °C. The phase transformation for the as-deposited films and annealed films were investigated in this study. The results revealed that pure ZnTiO₃ (hexagonal phase) can exist, and was obtained at temperatures between 700 and 800 °C. However, it was found that decomposition from hexagonal ZnTiO₃ to cubic Zn₂TiO₄ and rutile TiO₂ took place with a further increase in temperature to 900 °C.