Influence of temperature schedules on particle size and crystallinity of titania synthesized by vapor-phase oxidation route

Crystalline TiO₂ particles were produced in a tubular flow reactor by chemical vapor synthesis using titanium tetrachloride as a starting material in oxygen containing atmospheres. The dependence of particle size, morphology and crystalline phase of titania on temperature schedules including the reactor temperature, the oxygen preheated temperature and the product cooling measure were explored. It is found that there are two opposite effects of temperature on particle size and crystalline phase content. The particle size distribution, SEM and TEM of resulting powders show that the grain size is controlled by the relative magnitudes of the nucleation rate and growth rate, both of them being subject to the temperature schedules. XRD indicates that particles crystalline phase is predominately anatase and the rutile content increment is not consistent with temperature increase. Anatase titania can be converted to rutile by addition of crystal modifier AlCl₃. The element analysis by EDS shows that Al enriches on the particle outer surface.     

Thermally stable nanophase anatase titania with mesoporous texture by pseudo-inorganic templating

Thermally stable mesoporous nanocrystalline anatase titania has been prepared by a pseudo-inorganic templating method by codoping with lanthanum and silica followed by chemical leaching. The methodology uses lower temperatures that than reported previously and is applicable to the formation of both powders and coated glass surfaces.     

Influence of molybdenum species on growth of anodic titania

TiMo, bcc, solid-solution alloys, containing 11.5-37.0 at.% molybdenum, have been anodised galvanostatically in 0.1 mol dm⁻³ ammonium pentaborate and 1.0 mol dm⁻³ phosphoric acid electrolytes, with resultant anodic films characterised by scanning electron microscopy, transmission electron microscopy, Rutherford backscattering spectroscopy and glow discharge optical emission spectroscopy. Uniform amorphous films are formed at high current efficiency to >100 V, with formation ratios of 2.3 and 2.2 nm V⁻¹ in the respective electrolytes, contrasting with the amorphous-to-crystalline transition of anodic titania on titanium that occurs at ∼20-50 V. Apart from minor incorporation of electrolyte species, the films comprise an outer layer of TiO₂ and an inner oxide layer containing Ti⁴⁺ and Mo⁶⁺ ions. The films grow by migration of both cations and anions, with Ti⁴⁺ ions migrating faster than Mo⁶⁺ ions that is related to the energies of Ti⁴⁺O and Mo⁶⁺O bonds.     

Fast preparation of ordered crystalline mesoporous titania with high thermal stability and photo oxidation performance

Ordered mesoporous titania with crystalline anatase walls has been synthesized through fast evaporation-induced self-assembly method in a non-aqueous solution that only needs a 30 h synthetic period. The ordered mesostructure and crystalline anatase frameworks are characterized by the low-angle and wide-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM). The ordered titania mesostructure is thermally stable to 733 K, and the corresponding N₂ adsorption–desorption analysis exhibits that it has a surface area of 246 m²/g and a narrow pore distribution centered at 3.7 nm. Crystalline mesoporous titania exhibits the higher catalytic performance in photooxiding α-methylstyrene to acetophenone.     

Use of mesoporous SBA-15 for nanostructuring titania for photocatalytic applications

SBA15–TiO₂ samples prepared by introducing titanium with a grafting method and having TiO₂ loadings below 15 wt.% have been characterized by XRF, XRD, IR, porosimetry, SEM, HRTEM, and UV–Visible diffuse reflectance. Differently from the samples reported in the literature characterized by a high TiO₂ loading, no evidences have been found for the presence of titania particles inside or outside the mesopores of SBA-15. Three different titanium species were instead evidenced to be present. The first two derive from the reaction of titanium with silanol groups in the corona area of inner SBA-15 walls leading to the formation of either TiO₄ tetrahedral sites (by reaction by hydroxyl nests of surface defect sites) and/or pseudo-octahedral surface sites anchored by two (or more) Si or Ti ions through bridging oxygens. The third species derives from the reaction of titanium in the regions with high sylanol density, e.g. in the micropores located in the corona of SBA-15 channels, leading to the formation of TiO₂-like nanoareas (probably Si-doped) with dimensions of around 1–2 nm maximum. The potential interest of these materials as photocatalysts, for the presence of a TiO₂-like nanoareas highly accessible by reactants, is discussed.     

Synthesis and characterization of mesoporous materials: Silica–zirconia and silica–titania

An experimental strategy was developed to obtain mesoporous SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides by a sol–gel method, treating the gels hydrothermally. The solids were characterized by nitrogen physisorption, pyridine thermodesorption, ²⁹Si nuclear magnetic resonance, SEM and X-ray diffraction. The effects of ZrO₂ content, the generated pressure in the synthesis vessel and further modification of this type of procedure on the solids properties were studied. It was found that SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides dried at atmospheric pressure developed type I isotherms. On the other hand, for the SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides that were treated under pressure in the autoclave (at high SiO₂ content) the porosity was improved and mesoporous materials exhibiting type IV adsorption isotherms. Specific surface area and pore size distribution were a function of ZrO₂ and TiO₂ content. The materials exhibited narrow pore size distributions with pore diameters in the region of mesopores at about 4 nm and high surface areas, the highest being 481 m²/g for the 10 wt% ZrO₂ Si–Zr material. Differences in acidity as determined by pyridine thermodesorption were observed to depend on the synthesis parameters and ZrO₂ and TiO₂ concentration.     

Optimal synthesis of mesostructured hollow titania nanotubes templated on CaCO3 nanoparticles

Approximate 10 µm length of mesostructured hollow titania nanotubes with intact configuration was successfully prepared by using needle-like calcium carbonate and octadecylamine as double templates at room temperature in nonaqueous system. During the whole preparation, two parameters i.e. tetrabutoxytitanium/calcium carbonate molar ratio and annealing temperature, were optimized to obtain titania nanotubes with well-defined tubular morphology and mesoporous structure in tube walls. The as-prepared samples were characterized by scanning electron microscopy, transmission electron microscopy, broad-angle X-ray diffraction, pore size distribution and Brunauer-Emmett-Teller. The results showed that under optimal experimental conditions i.e. tetrabutoxytitanium/calcium carbonate molar ratio (50 wt.%) and annealing temperature (773 K), the tube materials exhibited uniform tubular structure with a length of 8-15 µm and an inner diameter of ∼ 400 nm, a wall thickness of ∼ 40 nm, a surface area of 112.2 m²/g and a pore volume of 0.18 cm³/g. The optimized titania nanotubes were utilized as a carrier for the immobilized of ibuprofen via a simple adsorption method. It was found that the loaded drug presented good sustained release property in three release media, i.e. simulated body fluid, normal saline and pure water.     

Low temperature synthesis of monolithic mesoporous magnetite nanoparticles

Magnetite nanoparticles are commonly used for drug delivery, as MRI contrast agents, and as adsorbents for the removal of heavy metal cations from waste water. The smaller the particle sizes the higher the efficiency of these particles in these applications. Different methods have been explored for the preparation of magnetic nanoparticles with this size limitation. Co-precipitation is one of the most versatile methods in this regard, and is characterized by the ability of preparation of a high yield of nanoparticles. Control of the particle size distribution, phase purity and type of porosity of the formed magnetite nanoparticles has been always considered a challenge. In the current study, magnetite mesoporous nanoparticles with an average particle size of 55 nm were prepared in pre-adjusted highly alkaline aqueous media at relatively low temperatures. Phase purity of the deposited magnetite was confirmed by X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Scanning (SEM) and transmission electron microscopy (TEM) graphs showed homogenously dispersion of spherical magnetite nanoparticles. Agglomeration of the mesoporous nanoparticles took place forming clusters with unified pore size distribution due to the homogenous particle size distribution. Magnetic susceptibility measurements at room temperature confirmed the magnetization characteristics of the nanoparticles.     

Limiting withdrawal rate and maximum film thickness during dip-coating of titania sols onto a Si substrate

The article highlights new insights into production of thin titania films widely used as catalyst support in many modern reactors including capillary microreactors, microstructured fixed-bed reactors and falling film microreactors. Dip-coating of a titania sol onto a Si substrate has been studied in the range of the sol viscosities of 1.5-2.5 mPa s and the sol withdrawal rates of 0.2-18 mm/s. Different viscosities of sols were created by addition of desired amounts of nitric acid to the synthesis mixture of titanium isopropoxide and Pluronic F127 in ethanol which allowed to control the rate of the condensation reactions. Uniform mesoporous titania coatings were obtained at the solvent withdrawal rates below 10 mm/s at sol viscosities in the range from 1.6 mPa s to 2.5 mPa s. There exists a limiting withdrawal rate corresponding to a capillary number of ca. 0.01 beyond which uniform titania films cannot be obtained. Below the limiting withdrawal rate, the coating thickness is a power function of the sol viscosity and withdrawal rate, both with an exponent of 2/3. The limiting withdrawal rate increases as the solvent evaporation rate increases and it decreases as the sol viscosity increases.     

Synthesis of Ti-containing mesoporous silicates from inorganic titanium sources

Titanium-containing mesoporous molecular sieves including periodic mesoporous silicas (SBA-15-type) and organosilicas (PMO-type) can be assembled by using mixed inorganic acid–base pairs (TiCl₄ and tetrabutyl titanate) or a single inorganic TiCl₃ as the titanium sources and tetraethoxysilane and/or 1,2-bis(triethoxysilyl)ethane as the silica sources and triblock copolymer as the structure-directing agent in acidic media through the hydrothermal method. Characterization using XRD, nitrogen sorption isotherms, UV–vis, FT-IR and NMR techniques reveals that the Ti-containing mesoporous materials possess ordered 2D hexagonal mesostructures, high surface areas (421–1070 m²/g), uniform pore sizes (5.1–8.0 nm), large pore volumes (0.5–1.3 cm³/g), and tetrahedrally incorporated titanium (IV) species in the silica network. The maximum incorporated Ti content is about 0.34 wt% for the ordered mesostructure regardless of the titania and silica sources and the initial Si/Ti ratio.     

Achievement of thick mesoporous TiO2 crystalline films by one-step dip-coating approach

A serial of surfactant-templated mesoporous TiO₂ films with the thickness over several micrometers have been successfully synthesized by one-step dip-coating and subsequent evaporation induced self-assembly method. Three different pre-condensed TiO₂ sols in the presence of surfactant (Pluronic F127) micelles with high viscosities were employed as the precursors for dip-coating. By treating the films in liquid paraffin as “shape protector” at certain high temperature for sufficient time, thick mesoporous films can be kept crack-free after calcinations. By employing the size-controlled titanium-oxo clusters in the sols as building blocks for self-assembly, the final obtained films represent tunable mesostructures. The mesoscopic characteristics of the films, such as Brunauer–Emmett–Teller surface areas, pore size distributions and pore wall crystallizations, have been comparatively studied. The results demonstrate that such tunable mesoscopic characteristics are greatly dependent on the structural and shape parameters of the initial formed inorganic clusters.