Preparation of TiO<Subscript>2</Subscript> nanoparticles by sonochemical method,isotherm, thermodynamic and kinetic studies on the sorption of strontium

The objective of this study is to evaluate the use of titanium dioxide nanoparticles which were prepared by novel sonochemical method as an ion exchange material for the removal of Sr from aqueous solution. The pH effect on the Sr²⁺ sorption was investigated. The data obtained have been correlated with Freundlich, Temkin and Dubinin–Radushkevich (D–R) isotherm models. Thermodynamic parameters fort he sorption system have been determined at four temperatures. Simple kinetic models have been applied to the rate and isotherm sorption data and the relevant kinetic parameters were determined from the graphical presentation of these models at 298°K. Results explained that the pseudo second-order sorption mechanism is predominant and the overall rate constant of sorption process appears to be controlled by chemical sorption process. The value of sorption energy E = 13 kJ/mol at 298°K and the value of Gibbs free energy ∆G° = 3,222 kJ/mol at 298°K prove that the sorption of strontium on titanium dioxide nanoparticles is an endothermic and non-spontaneous process.     

Phase transition of RbH<Subscript>2</Subscript>PO<Subscript>4</Subscript> and its composite with SiO<Subscript>2</Subscript> studied by thermal analysis

The phase transition at T _p (~109 °C) of RbH₂PO₄ and its composite with SiO₂ has been investigated by thermal analysis here. In the case of neat RbH₂PO₄, there is a linear relationship between endothermic peak temperature (T _m) and square root of heating rate (Φ ^1/2), from which the onset temperature of phase transition can be determined. Besides, Kissinger method and another calculation method were employed to obtain the activation energy of phase transition. The detailed deduction process was presented in this paper, and the estimated activation energies are E ₁ ≈ 126.3 kJ/mol and E ₂ ≈ 129.2 kJ/mol, respectively. On the other hand, the heterogeneous doping of RbH₂PO₄ with SiO₂ as dopant facilitates its proton conduction and leads to the disappearance of jump in conductivity at T _p. The heats of transition in the composites decrease gradually with increasing the molar fraction of SiO₂ additives. In the cooling process, a new and broad exothermic peak appeared between ~95 and ~110 °C, and its intensity also changes with the SiO₂ amount. These phenomena might be related to the formation of amorphous phase of RbH₂PO₄ on the surface of SiO₂ particles due to the strong interface interaction.     

Effect of protonation and deprotonation on surface charge density of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>

The protonation and deprotonation of the Nb₂O₅ surface has been followed in order to understand the reactions of surface of this catalyst. The simultaneous potentiometric and conductometric titrations had been carried by using 50 mL of water suspension of Nb₂O₅ 40 g L⁻¹. The oxide was entirely deprotonated when adding 0.4 mL NaOH 1 mol L⁻¹, and later titrated with 0.1 mol L⁻¹. The titration had supplied K ₁ and K ₂ and the obtained values were 3.24 × 10⁻³ and 4.17 × 10⁻⁸, respectively. The zero point charge was pH_pcz = 4.94. The thermodynamic studies were carried out by using 50 mL of a 40 g/L Nb₂O₅ aqueous suspension with the pH adjusted to pH_PZC value. The suspension was titrated with 0.5 mol/L of HNO₃ or NaOH for protonation or deprotonation studies, respectively, in an isoperibol calorimeter CSC ISC-4300. Thus, the obtained thermodynamic values of the protonation and deprotonation of Nb₂O₅ were Δ_dp G = −37.60 kJ/mol, Δ_dp H = −23.72 kJ/mol and Δ_dpS = 47 J/(mol K).     

Structural evolution and magnetic properties of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers by electrospinning

The SrFe₁₂O₁₉/poly (vinyl pyrrolidone) (PVP) composite fiber precursors were prepared by the sol-gel assisted electrospinning with ferric nitrate, strontium nitrate and PVP as starting reagents. Subsequently, the M-type strontium ferrite (SrFe₁₂O₁₉) nanofibers were derived from calcination of these precursors at 750–1,000 °C.The composite precursors and strontium ferrite nanofibers were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. The structural evolution process of strontium ferrite consists of the thermal decomposition and M-type strontium ferrite formation. After calcined at 750 °C for 2 h the single M-type strontium ferrite phase is formed by reactions of iron oxide and strontium oxide produced during the precursor decomposition process. The nanofiber morphology, diameter, crystallite size and grain morphology are mainly influenced by the calcination temperature and holding time. The SrFe₁₂O₁₉ nanofibers characterized with diameters of around 100 nm and a necklace-like structure obtained at 900 °C for 2 h, which is fabricated by nanosized particles about 60 nm with the plate-like morphology elongated in the preferred direction perpendicular to the c-axis, show the optimized magnetic property with saturation magnetization 59 A m² kg⁻¹ and coercivity 521 kA m⁻¹. It is found that the single domain critical size for these M-type strontium ferrite nanofibers is around 60 nm.     

Sol–gel synthesis,characterization and photocatalytic activity of mesoporous TiO<Subscript>2</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3 </Subscript>granules

Mesoporous TiO₂/γ-Al₂O₃ composite granules were prepared by combining sol–gel/oil-drop method, using various titania solution. The product granules can be used as a photocatalyst or adsorbent in moving, fluidized bed reactors. The phase composition and pore structure of the granules can be controlled by calcination temperature and using different titania solution. In the photocatalysis of NH₃ decomposition, TiO₂/γ-Al₂O₃ granules using Degussa P25 powder treated thermally at 450 °C showed the highest catalytic ability. However, TiO₂/γ-Al₂O₃ granules using titania made by hydrothermal method had comparable performance in NH₃ decomposition.     

Kinetics of formation of barium tungstate in equimolar powder mixture of BaCO<Subscript>3</Subscript> and WO<Subscript>3</Subscript>

The formation of Barium monotungstate (BaWO₄) particles in equimolar powder mixtures of BaCO₃ and WO₃ was examined under isothermal and non-isothermal conditions upon heating in air at 25–1200 °C, using thermogravimetry. Concurrence of the observed mass loss (due to the release of CO₂) to the occurrence of the formation reaction was evidenced. Accordingly, the extent of reaction (x) was determined as a function of time (t) or temperature (T). The x–t and x–T data thus obtained were processed using well established mathematical apparatus and methods, in order to characterize nature of reaction rate-determining step, and derive isothermal and non-isothermal kinetic parameters. Moreover, the reaction mixture quenched at various temperatures (600–1,000 °C) in the reaction course was analyzed by various spectroscopic and microscopic techniques, for material characterization. The results obtained indicated that the reaction rate may be controlled by unidirectional diffusion of WO₃ species across the product layer (BaWO₄), which was implied to form on the barium carbonate particles. The isothermally determined activation energy (118–125 kJ/mol) was found to be more credible than that (245 kJ/mol) determined non-isothermally.     

Electrical conductivity studies in the system Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript>-Li<Subscript>1.33</Subscript>Ti<Subscript>1.67</Subscript>O<Subscript>4</Subscript>

Electrical conductivity in the monoclinic Li₂TiO₃, cubic Li_1.33Ti_1.67O₄, and in their mixture has been studied by impedance spectroscopy in the temperature range 20–730 °C. Li₂TiO₃ shows low lithium ion conductivity, σ₃₀₀≈10^–6 S/cm at 300 °C, whereas Li_1.33Ti_1.67O₄ has 3×10^–8 at 20 °C and 3×10^–4 S/cm at 300 °C. Structural properties are used to discuss the observed conductivity features. The conductivity dependences on temperature in the coordinates of 1000/T versus log_e(σT) are not linear, as the conductivity mechanism changes. Extrinsic and intrinsic conductivity regions are observed. The change in the conductivity mechanism in Li₂TiO₃ at around 500–600 °C is observed and considered as an effect of the first-order phase transition, not reported before. Formation of solid solutions of Li_{2– x} Ti_{1+ x} O₃ above 900 °C significantly increases the conductivity. Irradiation by high-energy (5 MeV) electrons causes defects and the conductivity in Li₂TiO₃ increases exponentially. A dose of 144 MGy yields an increase in conductivity of about 100 times at room temperature. Electronic Publication     

Effect of heat treatment on electrochromic properties of TiO<Subscript>2</Subscript> thin films

Electrochromic titanium oxide (TiO₂) films were deposited on ITO/glass substrates by chemical solution deposition (CSD). The stock solutions were spin-coated onto substrates and then heated at various temperatures (200–500 °C) in various oxygen concentrations (0–80%) for 10 min. The effects of the processing parameters on the electrochromic properties of TiO₂ films were investigated. X-ray diffraction measurements demonstrated that the amorphous TiO₂ films were crystallized to form anatase films above 400 °C. The electrochromic properties and transmittance of TiO₂ films were measured in 1 M LiClO₄–propylene carbonate (PC) non-aqueous electrolyte. An amorphous 350 nm-thick TiO₂ film that was heated at 300°C in 60% ambient oxygen exhibited the maximum transmittance variation (ΔT%), 14.2%, between the bleached state and the colored state, with a ΔOD of 0.087, Q of 10.9 mC/cm², η of 7.98 cm²/C and x in Li _x ClO₄ of 0.076 at a wavelength (λ) of 550 nm.     

Some studies on the phase formation and kinetics in TiO<Subscript>2</Subscript> containing lithium aluminum silicate glasses nucleated by P<Subscript>2</Subscript>O<Subscript>5</Subscript>

Lithium aluminum silicate (LAS) glasses of compositions (wt%) 10.6Li₂O–71.7SiO₂–7.1Al₂O₃–4.9K₂O–3.2B₂O₃–1.25P₂O₅–1.25TiO₂ were prepared by the melt quench technique. Crystallization kinetics was investigated by the method of Kissinger and Augis–Bennett using differential thermal analysis (DTA). Based on the DTA data, glass ceramics were prepared by single-, two-, and three-step heat treatment schedules. The interdependence of different phases formed, microstructure, thermal expansion coefficient (TEC) and microhardness (MH) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-mechanical analysis (TMA), and microhardness (MH) measurements. Crystallization kinetics revealed that Li₂SiO₃ is the kinetically favored phase with activation energy of 91.10 kJ/mol. An Avrami exponent of n = 3.33 indicated the dominance of bulk crystallization. Based upon the formation of phases, it was observed that the two-stage heat treatment results in highest TEC glass ceramics. The single-step heat treatment yielded glass ceramics with the highest MH.     

A kinetic analysis of thermal decomposition of polyaniline/ZrO<Subscript>2</Subscript> composite

Synthesis, characterization and thermal analysis of polyaniline (PANI)/ZrO₂ composite and PANI was reported in our early work. In this present, the kinetic analysis of decomposition process for these two materials was performed under non-isothermal conditions. The activation energies were calculated through Friedman and Ozawa-Flynn-Wall methods, and the possible kinetic model functions have been estimated through the multiple linear regression method. The results show that the kinetic models for the decomposition process of PANI/ZrO₂ composite and PANI are all D₃, and the corresponding function is ƒ(α)=1.5(1−α)^2/3[1−(1-α)^1/3]−1. The correlated kinetic parameters are E _a=112.7±9.2 kJ mol⁻¹, lnA=13.9 and E _a=81.8±5.6 kJ mol⁻¹, lnA=8.8 for PANI/ZrO₂ composite and PANI, respectively.     

Synthesis of electrospun BaSrTiO<Subscript>3</Subscript>/PVP nanofibers

Ba_1−x Sr _x TiO₃(x = 0–0.5, BST) nanofibers with diameters of 150–210 nm were prepared by using electrospun BST/polyvinylpyrrolidone (PVP) composite fibers by calcination for 2 h at temperatures in the range of 650–800 °C in air. The morphology and crystal structure of calcined BST/PVP nanofibers were characterized as functions of calcination temperature and Sr content with an aid of XRD, FT-IR, and TEM. Although several unknown XRD peaks were detected when the fibers were calcined at temperatures less than 750 °C, they disappeared with increasing the temperature (above 750 °C) due to its thermal decomposition and complete reaction in the formation of BST. In addition, the FT-IR studies of BST/PVP fibers revealed that the intensities of the O–H stretching vibration bands (at 3430 and 1425 cm⁻¹) became weaker with increasing the calcination temperature and a broad band at 540 cm⁻¹, Ti–O vibration, appeared sharper and narrower after calcination above 750 °C due to the formation of metal oxide bonds. However, no effect of Sr content on the crystal structure of the composites was detected.     

Fabrication and characterization of TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> composite nanoparticles and polyurethane/(TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript>) nanocomposite films

TiO₂–SiO₂ composite nanoparticles were prepared by a sol–gel process. To obtain the assembly of TiO₂–SiO₂ composite nanoparticles, different molar ratios of Ti/Si were investigated. Polyurethane (PU)/(TiO₂–SiO₂) hybrid films were synthesized using the “grafting from” technique by incorporation of modified TiO₂–SiO₂ composite nanoparticles building blocks into PU matrix. Firstly, 3-aminopropyltriethysilane was employed to encapsulate TiO₂–SiO₂ composite nanoparticles’ surface. Secondly, the PU shell was tethered to the TiO₂–SiO₂ core surface via surface functionalized reaction. The particle size of TiO₂–SiO₂ composite sol was performed on dynamic light scattering, and the microstructure was characterized by X-ray diffraction and Fourier transform infrared. Thermogravimetric analysis and transmission electron microscopy (TEM) employed to study the hybrid films. The average particle size of the TiO₂–SiO₂ composite particles is about 38 nm when the molar ratio of Ti/Si reaches to1:1. The TEM image indicates that TiO₂–SiO₂ composite nanoparticles are well dispersed in the PU matrix.     

Kinetics of Cu<Superscript>2+</Superscript> binding to the poly(acrylic acid) hydrogel

Isothermal kinetics of copper (ion) binding to poly(acrylic acid) (PAA) hydrogel at 20, 25, 35 and 45°C was investigated. Isothermal conversions and kinetic curves of Cu²⁺ binding to the PAA hydrogel were determined. It was found that the well-known kinetic models of Peppas cannot be applied to describing the entire process of Cu²⁺ binding. The new method for the determination of the kinetic model of the Cu²⁺ binding process, as well as the activation energy density distribution functions of PAA hydrogel interaction with Cu²⁺, were established. It was found that Cu²⁺ diffusion to the active centers (with E _a = 9 kJ/mol) has a dominant influence on the kinetics of the process at temperatures T ≥ 30°C, but at T ≥ 30°C and for the degree of bound Cu²⁺ α ≥ 0.2, the interaction of Cu²⁺ from the adsorption center with E _a = 26 kJ/mol is dominant. The text was submitted by the authors in English.     

Thermochemical properties of MnNb<Subscript>2</Subscript>O<Subscript>6</Subscript>

Homogeneous manganocolumbite (MnNb₂O₆) was synthesized from Nb₂O₅ and MnO oxides. Powder sample was orthorhombic with unit cell parameters: α = 0.5766 nm, b = 1.4439 nm, c = 0.5085 nm and V = 0.4234 nm³. Heat capacity over the temperature range of 313–1253 K was measured in an inert atmosphere with combined thermogravimetry and calorimetry using NETZSCH STA 449C Jupiter thermoanalyzer. Melting point was 1767 ± 3 K, enthalpy of melting was 144 ± 4 kJ mol⁻¹. Experimental heat capacity of MnNb₂O₆ is fitted to polynomial C _pm = 221.46 + 3.03 · 10⁻³ T + −39.79 · 10⁵ T ⁻² + 40.59 · 10⁻⁶ T ².     

Structure and lithium mobility of Li<Subscript>4</Subscript>Pt<Subscript>3</Subscript>Si

The lithium-rich silicide Li₄Pt₃Si was synthesised from the elements by high-temperature synthesis in a sealed niobium ampoule. Its structure was refined on the basis of single-crystal X-ray diffraction data: R32, a = 693.7(2), c = 1627.1(4) pm, wR2 = 0.0762, 525 F² values and 21 variables. The striking structural motifs of the Li₄Pt₃Si structure are silicon atoms with a slightly distorted trigonal prismatic platinum coordination with short Si–Pt distances (238–246 pm). Always two trigonal prisms are condensed via a common Pt₃ triangle, and these double units built up a three-dimensional network by condensation via common corners. The channels left by this prismatic network are filled by two crystallographically independent lithium sites in a 3:1 ratio. The single crystal X-ray data were fully confirmed by neutron powder diffraction and ⁷Li magic-angle spinning (MAS)–nuclear magnetic resonance (NMR) results. The two distinct lithium sites are well differentiated by their ⁷Li isotropic chemical shift and nuclear electric quadrupolar interaction parameters. MAS-NMR spectra reveal signal coalescence effects above 300 K, indicating chemical exchange between the lithium sites on the millisecond timescale. The spectra can be simulated with a simple two-site exchange model. From the resulting temperature-dependent correlation times, an activation energy of 50 kJ/mol is extracted.     

XAFS investigation of [Pd(NH<Subscript>3</Subscript>)<Subscript>4</Subscript>][AuCl<Subscript>4</Subscript>]<Subscript>2</Subscript> and its thermolysis products

Thermolysis of double complex salt [Pd(NH₃)₄][AuCl₄]₂ has been studied in helium atmosphere from ambient to 350 °C. The XAFS of Pd K and Au L₃ edges and thermogravimetry measurements have been carried out to characterize the intermediates and the final product. In the temperature range 115–160 °C the complex is decomposed to form Pd(NH₃)₂Cl₂ and AuCl_4−x N _x species with x ranging from 2 to 3. Subsequent heating of the intermediate up to 300 °C leads to the total loss of NH₃. The Au–Cl and Au–Au bonds form the local environment of Au at the stage of decomposition while only four chlorine atoms are around Pd. At the temperature of 330 °C the Au and Pd nanoparticles as well as residues of palladium chloride are detected. The final product consists of separated Au and Pd nanoparticles.     

Preparation and photocatalytic activity of TiO<Subscript>2</Subscript> films with Ni nanoparticles

Titania thin films were synthesized by sol–gel dip-coating method with metallic Ni nanoparticles synthesized separately from an organometallic precursor Ni(COD)₂ (COD = cycloocta-1,5-diene) in presence of 1,3-diaminopropane as a stabilizer. Titania was obtained from a titanium isopropoxide precursor solution in presence of acetic acid. A Ni/TiO₂ sol system was used to coat glass substrate spheres (6, 4 and 3 mm diameter sizes), and further heat treatment at 400 °C was carried out to promote the crystallization of titania. XRD analysis of the TiO₂ films revealed the crystallization of the anatase phase. Transmission Electron Microscopy (TEM) and High Resolution TEM studies of Ni nanoparticles before mixing with the TiO₂ solution revealed the formation of Ni nanostructures with an average size of 5–10 nm. High-angle annular dark-field images of the Ni/TiO₂ system revealed well-dispersed Ni nanoparticles supported on TiO₂ and confirmed by AFM analysis. The photocatalytic activity of the Ni/TiO₂ films was evaluated in hydrogen evolution from the decomposition of ethanol using a mercury lamp for UV light irradiation. Titania films in presence of Ni nanoparticles show higher efficiency in their photocatalytic properties in comparison with TiO₂.     

Phase diagrams of the KF-K<Subscript>2</Subscript>TaF<Subscript>7</Subscript> and KF-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> systems

The phase diagrams of the systems KF-K₂TaF₇ and KF-Ta₂O₅ were determined using the thermal analysis method. The phase diagrams were described by suitable thermodynamic model. In the system KF-K₂TaF₇ eutectic points at x _KF=0.716 and t=725.4°C and at x _KF=0.214 and t=712.2°C has been calculated. It was suggested that K₂TaF₇ melts incongruently at around 743°C forming two immiscible liquids. The system KF-Ta₂O₅ have been measured up to 8 mol% of Ta₂O₅. The eutectic point was estimated to be at x _KF∼0.9 and t∼816°C. The formation of KTaO₃ and K₃TaO₂F₄ compounds has been observed in the solidified samples.     

Synthesis and colour properties of pigments based on terbium-dopped Mg<Subscript>2</Subscript>SnO<Subscript>4</Subscript>

The main aim of this work was to synthesize the magnesium orthostannate doped by terbium cations and tested whether these materials can be used for colouring of the different materials, e.g. organic binder and ceramic glazes. Initial composition of pigments was counted according the general formula 2MgO(1 − x)SnO₂–xTbO₂, where values of x varied from 0.1 to 0.5 in 0.1 steps. The simultaneous TG/DTA measurements of mixture containing tin oxide, magnesium carbonate hydroxide and terbium oxide showed that the formation of a new compound started at temperature 1,029 °C, but single-phase system was not prepared. Granulometric compositions of samples that were prepared by calcining at temperatures 1,300–1,400 °C are characterized by values of median (d ₅₀) in range 4–8 μm. The calcining temperature 1,500 °C caused the increase of the particle sizes at around 12 μm. The composition of sample 2MgO–1.5SnO₂–0.5TbO₂ and heating temperature 1,500 °C are the most suitable conditions for preparation of colourfully interesting pigment that can be recommended also for colouring of ceramic glazes. Especially, for colouring of decorative lead containing glaze G 07091 containing 5 wt% of PbO and 8 wt% of Al₂O₃.     

Opposite effect of Li<Superscript>+</Superscript> codoping on the upconversion emissions of Er<Superscript>3+</Superscript>-doped TiO<Subscript>2</Subscript> powders

The Er³⁺-Li⁺ codoped TiO₂ powders have been prepared by the non-aqueous sol–gel method. The green and red upconversion emissions centered at about 526, 550 and 663 nm were observed by the ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively. Li⁺ codoping has opposite effect on the upconversion emissions intensities for Er³⁺-doped TiO₂ at sintering temperatures of 1,073 and 1,273 K. At 1,073 K, the Er³⁺-doped TiO₂ phase transition from anatase to rutile was accelerated with increasing Li⁺ codoping concentration, leading to the increase of crystal field symmetry of Er³⁺, thus the upconversion emissions intensities decreased. At 1,273 K, Li⁺ codoping had no effect on the phase structure of Er³⁺-doped TiO₂ and only increased the Er–O bond length, it indicated that the upconversion emissions intensities greatly enhanced because of the decrease of crystal field symmetry of Er³⁺.