Preparation and Properties of Nanocrystalline α-Fe2O3-Sensitized TiO2 Nanosheets as a Visible Light Photocatalyst

We present a novel approach for the fabrication of α-Fe₂O₃-sensitized TiO₂ nanosheets. The TiO₂ nanosheets with partly curly surfaces and large surface areas can rapidly absorb the methylene blue (MB). The absorption and degradation of MB are investigated. Results show that the photocatalytic activities of α-Fe₂O₃ make the MB degradation efficient under visible light irradiation. By coupling two semiconductor systems, electrons can be injected from an excited small band gap semiconductor into TiO₂ under visible light. Such a modification is expected to lead to new or enhanced properties.     

Low-Temperature Growth of Monolayer Rutile TiO2 Nanorod Films

Low-temperature growth of well-crystallized titania thin films with controlled nanofeatures are of great interest because of their potential uses in catalysts, gas sensors, photovoltaic cells, photonic crystals, etc. This paper reports the synthesis of a well-crystallized, pure rutile monolayer consisting of well-aligned nanorods with average diameters of ca. 25 nm and an aspect ratio of ca. 6 through a simple solution approach at a low temperature of 80°C. The monolayer nanorods precipitate from the precursors that were obtained through the reaction between metallic titanium and hydrogen peroxide solutions at 80°C for 24–60 h. The nanoporous titania thin layer derived by oxidizing the titanium substrate with hydrogen peroxide at 80°C for 10 min facilitates the growth of the monolayer rutile TiO₂ nanorod films.     

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.     

Ab Initio Calculations of Intrinsic Defects in Rutile TiO2

This paper presents the results of density functional theory calculations of the defect formation enthalpies of Frenkel and Schottky defects in rutile TiO₂. The calculations are performed within the generalized gradient, supercell, and pseudopotential approximations and predict that Frenkel defects are more energetically favorable in TiO₂ than Schottky defects. The calculations also indicate that both Frenkel and Schottky defects prefer to cluster together rather than spread out across the unit cell. Possible diffusion routes for interstitial Ti atoms are investigated and strong anisotropy in interstitial diffusion in TiO₂ is supported by the results.     

Photocatalytic Activity of Ni 8 wt%-Doped TiO2 Photocatalyst Synthesized by Mechanical Alloying Under Visible Light

Ni 8 wt%-doped titanium dioxide (TiO₂) was synthesized by mechanical alloying. The photocatalytic activity of Ni 8 wt%-doped TiO₂ powder was evaluated by measuring the visible light absorption ability by ultraviolet visible diffuse reflectance spectroscopy (UV/Vis-DRS) and photoluminescence (PL) spectroscopy. Ni 8 wt%-doped TiO₂ powders had only a rutile phase and spherical particles with an average grain size of less than 10 nm. The UV/Vis-DRS analysis showed that the UV absorption for the Ni 8 wt%-doped TiO₂ powder moved to a longer wavelength and the photoreactivity was rapidly enhanced. And PL results revealed that the new absorption was believed to be induced by localization of the trapping level near the valance band or conduction band. Moreover, Ni 8 wt%-doped TiO₂ had a high reaction activity for decomposition of 4-chlorophenol in aqueous solution under UV and visible light. To obtain the electronic structure of Ni-doped TiO₂, we have performed ab initio pseudopotential plane wave methods based on the density functional theory. The band gap of Ni-doped TiO₂ narrowed more than pure TiO₂. These results agree with the experimentally observed phenomenon.     

Preparation of Nanosized Rutile TiO2 from an Aqueous Peroxotitanate Solution

A new facile method for direct preparation of well-crystallized rutile TiO₂ nanoparticles without any ionic impurities was reported. The nanosized TiO₂ was prepared by aging a peroxotitanate solution at 100°C for 0–12 h, formed by reaction of H₂O₂ and titanium tetraisopropoxide (TTIP). The method involves hydrolysis of TTIP and simultaneous digestion of hydrolyzed precipitates, and hydrothermal treatment into crystalline phases. It was found that the TTIP/H₂O₂ molar ratio in the preparation of peroxotitanate as a precursor for TiO₂ was crucial in the formation of a rutile phase. Transmission electron microscope observation for sols showed rod-like shapes with average particle sizes of around 100 nm in the elongated direction.     

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.     

Conventional and Microwave-Hydrothermal Synthesis of TiO2 Nanopowders

The objective of this study was to compare the conventional and microwave hydrothermal treatment of TiOCl₂ solutions. Colloidal titania suspensions were prepared by forced hydrolysis at 195°C for different times ranging from 1 to 32 h for the conventional synthesis and from 5 min to 1 h for the microwave ones. The effect of the microwave technology on both the synthesis conditions and titania nanoparticles properties has been evaluated. Particles morphology and crystallinity were studied by using transmission electron microscopy and X-ray diffraction. The thermal and chemical stability of the obtained powders were determined by TG/DTA analysis.     

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.     

Mechanism of ZrTiO4 Synthesis by Mechanochemical Processing of TiO2 and ZrO2

High-energy ball milling initiates a solid-state reaction in an equimolar mixture of TiO₂ and ZrO₂. The first stage of ball milling induced the transformation of anatase TiO₂ to high-pressure phase TiO₂ (II), isostructural with ZrTiO₄. The formation of solid solutions monoclinic ZrO₂/TiO₂ and TiO₂ (II)/ZrO₂ was observed in the intermediate stage. Afterward, a nanosized ZrTiO₄ phase was formed in the milled product from the TiO₂ (II)/ZrO₂ solid solution. The sintering of the milled product at a temperature <1100°C was examined in situ by Raman spectroscopy. The full solid-state reaction toward ZrTiO₄ ceramic is completed at a temperature considerably lower than reported in the literature.     

Synthesis of Anatase-Type TiO2 Nanocrystallites Via a Redox Route

Anatase nanocrystallites showing high surface area (∼62 m²/g) and good photocatalytic property have been obtained by pyrolyzing at 600°C for 4 h an ammonium titanyl double sulfate precursor (α-(NH₄)₂TiO(SO₄)₂) synthesized via a redox approach, that is, by oxidizing an aqueous solution of titanium trichloride (TiCl₃) with ammonium peroxodisulfate ((NH₄)₂S₂O₈), followed by reacting with ammonium sulfate ((NH₄)₂SO₄).     

Synthesis and Characterization of Monosized Doped TiO2 Powders

Monosized spheroidal doped TiO² particles with an average diameter of 0.3 to 0.7 μm were prepared by the controlled cohydrolysis of mixtures of titanium alkoxide and either niobium or tantalum alkoxide. An inorganic salt precipitation technique was used to put Ba, Cu, or Sr onto the surface of singly doped TiO₂ particles. Chemical analyses by several methods demonstrated the reproducibility and reliability of the doping procedures     

Sol–Gel Synthesis and Photocatalytic Activity of CeO2/TiO2 Nanocomposites

Uniform CeO₂ / TiO₂ composite nanoparticles with different Ce/Ti molar ratios have been successfully synthesized via the sol–gel method. The samples were characterized using differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The surface state analysis by means of X-ray photoelectron spectroscopy (XPS) shows that the Ti element mainly exists as a chemical state of Ti⁴⁺, while the Ce element exists as a mixture of Ce³⁺ and Ce⁴⁺ oxidation states. The photocatalytic degradation of methyl orange (MeO) in CeO₂ / TiO₂ suspension was investigated. The results indicate that the CeO₂/TiO₂ nanocomposites show higher photocatalytic activity than pure TiO₂. Photodegradation of MeO can be improved by increasing the Ce/Ti molar ratio in the initial 15 min.     

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.     

Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.