Direct Formation and Photocatalytic Performance of Anatase (TiO2)/Silica (SiO2) Composite Nanoparticles

Anatase (TiO₂)/silica (SiO₂: 23.9–27.7 mol%) composite nanoparticles were directly synthesized from (i) the reaction of titanyl sulfate (TiOSO₄) and sodium metasilicate (Na₂SiO₃) under mild hydrothermal conditions, (ii) the acidic precursor solutions of TiOSO₄ and tetraethylorthosilicate (TEOS) by thermal hydrolysis, and (iii) the metal alkoxides, i.e., tetraisopropoxide (TTIP) and TEOS, by the sol–gel method. Their photocatalytic activities were evaluated by measurements of the relative concentration of methylene blue after UV irradiation. The as-prepared TiO₂/SiO₂ composite nanoparticles showed far more improved photocatalytic activity than the pure anatase-type TiO₂. The composite nanoparticles formed from (i) TiOSO₄ and Na₂SiO₃ as well as those from (ii) TiOSO₄ and TEOS showed fairly good photocatalytic activity, and it was better than that of those synthesized from (iii) the metal alkoxides, which was suggested to be due to the difference in crystallinity of the anatase.     

Formation and Sintering of TiO2 (Anatase) Solid Solution in the System TiO2-SiO2

In the TiO₂-SiO₂ system, anatase solid solutions (ss) containing up to similar/congruent ∼15 mol% SiO₂ are formed in the as-prepared state by the hydrazine method. The lattice parameters a and c decrease linearly from 0.3785 to 0.3776 nm and from 0.9514 to 0.9494 nm, respectively, with increased SiO₂ content. At high temperatures, the solid solutions by transformation decompose into rutile and amorphous SiO₂. The anatase(ss) powders have been characterized for particle size and surface area. They consist of very fine particles (7-25 nm). Surface areas at low temperatures are very high and do not drop below 60 m²/g at 1000°C. Nanostructured anatase(ss) ceramics, with greaterthan/equal to 99.5% of theoretical density and an average grain size of 72 nm, have been fabricated by hot isostatic pressing for 1 h at 850°C and 196 MPa. Their mechanical and electrical properties have been examined.     

Formation and Transformation of TiO2 (Anatase) Solid Solution in the System TiO2—Al2O3

In the system TiO₂—Al₂O₃, TiO₂ (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al₂O₃) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al₂O₃. At higher temperatures, anatase solid solutions transform into TiO₂ (rutile) with the formation of α-Al₂O₃. Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C.     

Photoactive and Adsorptive Niobium-Doped Anatase (TiO2) Nanoparticles: Influence of Hydrothermal Conditions on their Morphology, Structure, and Properties

Anatase-type TiO₂ nanoparticles with adsorptivity and improved photocatalytic activity for the decomposition of methylene blue (MB) in its aqueous solution, which contained up to 10 mol% niobium by forming solid solutions with niobium oxide, were directly synthesized from precursor solutions of TiOSO₄ and NbCl₅ under three hydrothermal conditions in the absence and presence of urea and aqueous ammonia at 180°C for 5 h. The influence of the hydrothermal conditions on the crystallite growth, morphology, specific surface area, adsorptivity, and photocatalytic activity of niobium-doped TiO₂ was investigated. The crystallite growth of anatase was enhanced by the presence of the niobium component. The 10 mol% niobium-doped TiO₂ that was prepared under the hydrothermal condition in the presence of urea had fine crystallites (11 nm) and high specific surface areas (135 m²/g), which showed the most enhanced photocatalytic activity and the highest adsorptivity. The hydrothermal treatment under weak basic conditions and formation of solid solutions with niobium oxide brought about a considerable increase in the adsorption of MB for the anatase-type TiO₂.     

Anatase-Type TiO2 and ZrO2-Doped TiO2 Directly Formed from Titanium(III) Sulfate Solution by Thermal Hydrolysis: Effect of the Presence of Ammonium Peroxodisulfate on their Formation and Properties

Anatase-type TiO₂ powder containing sulfur with absorption in the visible region was directly formed as particles with crystallite in the range 15–88 nm by thermal hydrolysis of titanium(III) sulfate (Ti₂(SO₄)₃) solution at 100°–240°C. Because of the presence of ammonium peroxodisulfate ((NH₄)₂S₂O₈), the yield of anatase-type TiO₂ from Ti₂(SO₄)₃ solution was accelerated, and anatase with fine crystallite was formed. Anatase-type TiO₂ doped with ZrO₂ up to 9.8 mol% was directly precipitated as nanometer-sized particles from the acidic precursor solutions of Ti₂(SO₄)₃ and zirconium sulfate in the presence and the absence of (NH₄)₂S₂O₈ by simultaneous hydrolysis under hydrothermal conditions at 200°C. By doping ZrO₂ into TiO₂ and with increasing ZrO₂ content, the crystallite size of anatase was decreased, and the anatase-to-rutile phase transformation was retarded as much as 200°C. The anatase-type structure of ZrO₂-doped TiO₂ was maintained after heating at 1000°C for 1 h. The favorable effect of doping ZrO₂ to anatase-type TiO₂ on the photocatalytic activity was observed.     

Kinetic Model for TiO2 Polymorphic Transformation from Anatase to Rutile

We propose a distribution kinetics model for the polymorphic transformation (anatase to rutile) and coarsening of TiO₂. Based on population balance equations for the size distributions of the dimorphs, the simplified model applies a first-order rate expression for transformation combined with Smoluchowski coalescence for coarsening of anatase and rutile particles. Two moments of the size distributions (number and mass of particles) lead to dynamic expressions for extent of reaction and average particle diameter. The model describes the time-dependent data of Banfield and colleagues fairly accurately, and provides activation energies for anatase coalescence and transformation. The equilibrium constant for the microscopically reversible transformation, occurring without coarsening, yields a small endothermic enthalpy change per mole of TiO₂. This probably reflects contributions of the transformation enthalpy of the anhydrous phases at the given particle size, which is very close to 0, and the enthalpy associated with a small amount of dehydration (endothermic water evaporation) during transformation.     

Low-Temperature Preparation and Magnetic Properties of Nanoparticle Iron-Doped Anatase TiO2

Nanoparticle iron (Fe)-doped anatase TiO₂ was prepared at a low temperature (100°C) and at room pressure. The product was obtained from a boiling solution of an amorphous TiO₂ gel mixed with an iron nitrate solution and stirred for 5 h. An amorphous TiO₂ gel was obtained from TiCl₃ solution and NH₄OH as a precipitating agent stirred at room temperature for 1 day. EDAX results on different selected areas of as-prepared Fe-doped anatase TiO₂ revealed a homogeneous composition of 17 at.% Fe. Fe–TiO₂ has a superparamagnetic state with a possibility of antiferromagnetism at low temperatures. Fe seems to substitute titanium ions without any evidence of other impurities such as Fe nanoclusters or Fe-based oxides.     

Preparation of TiO2 Nanoparticles on Different SiO2 Supports by the Adsorption Phase Technique

The influence of supports on the preparation of TiO₂ nanoparticles by the adsorption phase technique is studied in detailed. Series temperature experiments of two types of supports (named as SiO₂ A and B) were used. Energy-dispersive analysis by X-ray indicates that the concentration of TiO₂ on both supports decreases with temperature increasing. TiO₂ quantity on SiO₂ A decreases sharply between 40° and 60°C, whereas the temperature range for SiO₂ B is between 30° and 50°C. X-ray diffraction (XRD) shows that grain size of TiO₂ particles on two SiO₂ surfaces is all below 7 nm. It is also shown by XRD that particles on SiO₂ A decrease sharply as in the quantity curve of TiO₂, but particles on SiO₂ B all change gradually and TiO₂ particles on SiO₂ B are more uniform in transmission electron spectroscopy. The similarly of both supports is considered to be the reason for the similar changes in Ti concentration, and the different characteristics of the internal/external surface lead to variant quantity and grain size, as well as characteristics of TiO₂.     

Crystallization of Monoclinic 2TiO2·5Nb2O5

Monoclinic 2TiO₂·5Nb₂O₅ crystallizes at 810° to 835°C from an amorphous material prepared by the simultaneous hydrolysis of titanium and niobium alkoxides. Crystallization isotherms are described by the contracting cube equation 1 − (1 − f)¹¹³= k(t − t₀); the activation energy is 315 kJ·mol⁻¹. Monoclinic 2TiO₂·5Nb₂O₅ transforms to the orthorhombic modification at ∼1200° to 1300°C.     

Ultrasonic Dispersion of TiO2 Nanoparticles in Aqueous Suspension

Aggregation and dispersion behavior of nanometer and submicrometer scale TiO₂ particles in aqueous suspension were investigated using three kinds of mechanical dispersion methods: ultrasonic irradiation, milling with 5-mm-diameter balls, and milling with 50 μm beads. Polyacrylic acids with molecular weights ranging from 1200 to 30 000 g/mol were used as a dispersant, and the molecular weight for each dispersion condition was optimized. Viscosities and aggregate sizes of the submicrometer powder suspensions were not appreciably changed in the ultrasonic irradiation and 5-mm-ball milling trials. In contrast, in the trials in which nanoparticle suspension was used, ultrasonic irradiation produced better results than 5-mm-ball milling. Use of ultrasonication enabled dispersion of aggregates to primary particle sizes, which was determined based on the specific surface area of the starting TiO₂ powders, even for relatively high solid content suspensions of up to 15 vol%. Fifty-micrometer-bead milling was also able to disperse aggregates to the same sizes as the ultrasonic irradiation method, but 50-μm-bead milling can be used only in relatively low solid content suspensions. It was concluded that the ultrasonic dispersion method was a useful way to prepare concentrated and highly dispersed nanoparticle suspensions.     

DADD-Assisted Hydrothermal Synthesis of t-ZrO2 Nanoparticles

Tetragonal ZrO₂ (t-ZrO₂) nanoparticles (diameter: ca. 10 nm; BET-specific surface area: 100 m²/g) are hydrothermally prepared via crystallization of 1,12-diaminododecane (DADD)-hydrous zirconia gels at 230°C for 3 days. The volume content of monoclinic phase is ca. 0.1. The surface-adsorbed hydrocarbon layer (ca. 3 wt%) is observed by FTIR and TGA. They account for the formation of our t-ZrO₂ nanoparticles with high-phase purity.     

Solubility of TiO2 in ZrO2

The solubility of TiO₂ in tetragonal ZrO₂ is 13.8±0.3 mol% ui 1300°C, 14.9±0.2 mol% at 1400°C, and 16.1±0.2 mol% at 1500°C. These solid solutions transform to metastable monoclinic solid solutions without compositional change on cooling to room temperature.     

Direct Observation of the Crystallization of Li2O-Al2O3-SiO2 Glasses Containing TiO2

The nucleation and crystallization of Li₂O-Al₂O₃-SiO₂ glasses containing TiO₂ were investigated using transmission electron microscopy of thin sections produced from bulk samples. Phase separation occurs during cooling from the melt, and on heating, a large number of titanium-aluminum crystals approximately 50 A in diameter are formed. These crystals are the heterogeneous nuclei for the crystallization of the remaining glass. Photomicrographs of various stages of crystallization show the development of the fine-grained glass-ceramic.     

Solubility of Iron in TiO2 (Rutile)

An upper limit to the solubility of iron in rutile was determined in the range 800° to 1350°C by X-ray examination of mixtures of TiO₂ and α-Fe₂O₃ powders reacted in 1 atm oxygen pressure and quenched to room temperature. The results indicate somewhat lower solubilities than would be inferred from previous measurements; the solubility ranges from 3 cation % iron at 1350°C to 1% at 800°C. There is no detectable change in rutile lattice parameters with iron content up to the limit of solubility.     

TiO2 Produced by Vapor-Phase Oxygenolysis of TiCI4

The formation of TiO₂ powders by oxygenolysis of TiCI₄ was studied with emphasis on the effects of reaction conditions on the particle size of the products. The particle size of TiO₂(anatase) decreased with increasing reaction temperature or O₂concentration and with decreasing TiCI₄ concentration. The results are compared with those for the oxygenolysis of AlBr₃and SiCI₄. It was found that the reactivity of metal halides with O₂ is closely related to the ease of dissociation of the first halogen atom.     

Formation of Monodisperse Spherical TiO2 Powders by Thermal Hydrolysis of Ti(SO4)2

Titania powders have been prepared by the thermal hydrolysis of titanium sulfate at 80°C in a mixed solvent of n -propyl alcohol ( n -PrOH) and water. The morphology of the powders was greatly influenced by the volume ratio in n -PrOH to water (RH ratio) of the solvent. An RH ratio of 1.0 was essential for the formation of spherical TiO₂ powders. When hydroxypropyl cellulose (HPC) was used as a steric dispersant, monodisperse spherical powders 0.7 (μm in diameter were produced. As the RH ratio was increased, the zeta potential of the powders was negatively increased and the dielectric constant of the solvent was decreased. However, powders obtained at an RH ratio of 1.0 were observed to have the highest energy barrier. The colloidal stability of the powders in a mixed solvent of n -PrOH and water is discussed. As-precipitated powders were amorphous hydrates of titania and were crystallized by calcination into anatase (>600°C) and rutile (>800°C).     

Nanostructure of TiO2 Nano-Coated SiO2 Particles

We characterized SiO₂–TiO₂ nano-hybrid particles, prepared using the sol–gel method, using high-resolution transmission microscopy. A few nanometer-ordered TiO₂ anatase crystallites could be observed on the monodispersed SiO₂ nanoparticle surface. The quantum size effect of the TiO₂ anatase crystallites is attributed to the blue shift of the absorption band. The rough surface of the SiO₂–TiO₂ nano-hybrid particles was derived from the developed growth planes of the TiO₂ anatase crystallites, grown from fully hydrolyzed Ti alkoxide that did not react with acetic acid during the crystallization process at 600°C thermal annealing.     

Formation of Nanometric TiB2 from TiO2

Titanium diboride can be produced by ball-milling a mixture of TiO₂, B₂O₃, and Mg metal for between 10 and 15 h. The reaction was found to be completed during the milling with no evidence of residual Mg. The unwanted phase, MgO, was readily removed by leaching in acid. The leached powder obtained after 15 h milling had a particle size of <200 nm and was highly faceted. The particle size decreased to ∼50 nm after 100 h milling and seemed to be relatively monodisperse. Scherrer calculation of the crystallite size showed that the product particles were probably single crystal.     

Low-Temperature Nucleation of Rutile Observed by Raman Spectroscopy during Crystallization of TiO2

The thermal evolution of amorphous TiO₂ powders, consisting of spherical particles and prepared by hydrolysis of a titanium ethoxide aerosol, was studied by using Raman spectroscopy. On calcination at 350°C, the solid crystallized, giving anatase as a major phase. A small amount of rutile was also detected and attributed to small seeds localized at the particle outlayer. The nucleation of rutile at so low a temperature was ascribed to the presence of organic impurities in the powders. The transformation of anatase into rutile was clearly observed after heating at 660°C.