Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders

Bimodal nanocrystalline mesoporous TiO₂ powders with high photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate (TiO(C₄H₉)₄, TBOT) as precursor. The as-prepared TiO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption–desorption measurements. The photocatalytic activity of the as-prepared TiO₂ powders was evaluated by the photocatalytic degradation of acetone (CH₃COCH₃) under UV-light irradiation at room temperature in air. The effects of hydrothermal temperature and time on the microstructures and photocatalytic activity of the TiO₂ powders were investigated and discussed. It was found that hydrothermal treatment enhanced the phase transformation of the TiO₂ powders from amorphous to anatase and crystallization of anatase. All TiO₂ powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous region: one was intra-aggregated pores with maximum pore diameters of ca. 4–8 nm and the other with inter-aggregated pores with maximum pore diameters of ca. 45–50 nm. With increasing hydrothermal temperature and time, the average crystallite size and average pore size increased, in contrast, the Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes and porosity steadily decreased. An optimal hydrothermal condition (180 °C for 10 h) was determined. The photocatalytic activity of the prepared TiO₂ powders under optimal hydrothermal conditions was more than three times higher than that of Degussa P25.     

Formation and Characterization of Monodisperse, Spherical Organo-Silica Powders from Organo-Alkoxysilane-Water System

Monodisperse, spherical organo-silica powders were synthesized from an immiscible mixture of an aqueous NH₄OH solution and organo-alkoxysilanes, such as phenyl-trimethoxysilane (PTMS), methyltrimethoxysilane (MTMS), and methyltriethoxysilane (MTES). When the organo-alkoxysilanes were added to the aqueous NH₄OH solution with stirring, organo-alkoxysilane/water macroemulsions were formed because of the immiscibility of the aqueous NH₄OH solution and the organo-alkoxysilane. In the emulsion, organo-alkoxysilane droplets dissolved into the water phase through the droplet-water interface, and the organo-silica powder was formed by the hydrolysis and condensation of the organo-alkoxysilane. The powders were obtained reproducibly under broad concentrations of organo-alkoxysilanes; 0.3-1.5 mol/L for PTMS, 0.3-0.6 mol/L for MTMS, and MTES, with concentrations of ammonia between 0.0625 and 2.0 mol/L. According to the analysis of the solid-state MAS ²⁹Si NMR and the ¹³C NMR analysis, the organo-silica powder did not contain any residual alkoxy groups, only carbon sources from nonhydrolyzable R groups: phenyl groups in PTMS, and methyl groups in MTMS and MTES.     

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.     

Preparation and Upconversion Properties of Yb3+, Ho3+: Lu2O3 Nanocrystalline Powders

Yb³⁺, Ho³⁺: Lu₂O₃ nanocrystalline powders were synthesized by a reverse-strike co-precipitation method using NH₄HCO₃ and NH₃·H₂O as precipitators. X-ray diffraction analysis and field emission scanning electron microscopy observation showed that the phase composition of the powders was cubic and the particle size was 30–50 nm. Under the excitation of a 980 nm continuous wave diode laser, green and red emissions centered around 548 and 667 nm, respectively, were observed and the green emission dominated the upconversion spectrum. Power studies revealed that a two-photon process was involved in the upconversion emissions and the possible upconversion mechanisms were discussed.     

pH Effect on Titania-Phase Transformation of Precipitates from Titanium Tetrachloride Solutions

Precipitates in different structures were prepared by the hydrolysis of a TiCl₄ solution at room temperature and 70°C. The pH effect on titania-phase transformation was investigated. The original precipitate formed by the hydrolysis of titanium tetrachloride formed a gel-like structure at pH 4 with the addition of NH₄OH, which could be changed to essentially 100% rutile phase at 600°C. The rutile precipitates prepared at 70°C could be changed to mixed-phase product at pH 0.50 and total anatase phase at pH 3.13 and 5.15. Complete phase transformation from anatase to rutile occurred in the three samples at 700°C.     

Improvement of Hydroxyapatite Sol–Gel Coating on Titanium with Ammonium Hydroxide Addition

Small amounts (1–5 vol%) of ammonium hydroxide (NH₄OH) were added to the ethanol–water-based hydroxyapatite (HA) sol–gel solution. After aging the sol, a Ti substrate was dip-coated and heat-treated at 500°C for 1 h in air. The sol properties were monitored in terms of pH, viscosity, and structure changes with aging time; also, the coating phase and structure on Ti were investigated. During aging, the viscosity of the sol increased while the pH decreased, confirming the polymerization and gelation of the sol. The addition of NH₄OH altered the sol properties significantly. Increase in the NH₄OH concentration increased the pH and viscosity of the sol considerably, especially at short aging period, followed by a saturation with further aging. The Fourier transformed-infrared analysis also confirmed a gradual structure change of the sol with NH₄OH addition. Such changes in pH, viscosity, and structure bands of the sol driven by the NH₄OH addition were attributed to the improved polymerization and gelation of the sol. The improved gelation shortened the aging time needed for crystallization of the apatite coating, i.e., increase in the NH₄OH addition improved the crystallization of the coatings. Within 24 h aging time, the coatings on Ti containing over 3.5 vol% NH₄OH showed characteristic HA phase and structure bands, at which period no apatite peaks were observed on the HA coating obtained without NH₄OH addition.     

Photoluminescence Probing of the Formation of Titanium Dioxide Sols from a Titanium Peroxide Solution

Photoluminescence spectroscopy was used to probe the formation of colloidal TiO₂ sols from a titanium peroxide solution, which were prepared by reaction of H₂O₂ and titanium hydrate gel that was precipitated from acidic TiOCl₂ solution neutralized by NH₄OH, during aging at 100°C for 0–64 h. Emission spectra revealed broad band luminescence which is interpreted as the superposition of a multiple photoluminescence process involving TiO₂ precursors. The maximum peak position of the luminescence band was shifted to a shorter wavelength by the generation of the dominant emission peak as aging time increased, and a peak at ∼390 nm, indicating the anatase phase, was observed after aging for >48 h. However, sols aged for 2 h already were of the anatase phase, according to X-ray diffraction analysis. It was deduced that the interfacial layers between the particles and the liquid in an aqueous solution caused luminescence quenching or accelerated sink.     

Hydrothermal Synthesis for Large Barium Titanate Powders at a Low Temperature: Effect of Titania Aging in an Alkaline Solution

Monodisperse and spherical barium titanate (BaTiO₃) powders with diameters of 200–470 nm were directly prepared by a low-temperature hydrothermal method at 90°C. Spherical titania (TiO₂) powders, ranging in size from 150 to 420 nm, were initially prepared by a controlled hydrolysis and condensation reaction, aged in a highly alkaline solution for 12 h, and then hydrothermally reacted with barium hydroxide to be converted to BaTiO₃ without a morphological change. The aging step of the TiO₂, where the surface of TiO₂ was highly densified through elimination of the pores, was indispensable to retain the sizes and shapes of TiO₂ in the resulting BaTiO₃. This was due to the fact that the formation of BaTiO₃ proceeded by an in situ reaction mechanism. The resulting BaTiO₃ powders exhibited dense and nonporous structures even after calcination at 1000°C.     

Formation Mechanism of Hydrous-Zirconia Particles Produced by Hydrolysis of ZrOCl2 Solutions

The secondary particle size of hydrous-zirconia fine particles that have been produced by the hydrolysis of various ZrOCl₂ solutions, with and without the addition of NH₄OH or HCl, was measured using transmission electron microscopy to investigate the effects of the H+ and Cl−ion concentrations on the formation of secondary particles. The average secondary particle size of hydrous zirconia increased as the H+ion concentration increased, attaining a maximum of 200 nm at an H−ion concentration of 0.44 moldm-3. Further increases in the H+ion concentration then caused a decrease in the average particle size. The present experimental results revealed that the secondary particle size of hydrous zirconia is controlled primarily by the concentration of H+ ions that are produced during hydrolysis. The formation mechanism for the secondary particles in hydrous zirconia was determined on the basis of the present experimental results.     

Synthesis of Titania Composite Membranes by the Pressurized Sol-Gel Technique

Crack-free titania composite membranes have been synthesized from colloidal titania sols (average particle size of 7–16 nm) by the pressurized sol-gel coating technique. The Knudsen permeability of N₂ through the titania membrane layer heat-treated at 300°C is about 21 × 10^-7–35 × 10^-7 mol/m²·s·Pa at room temperature. The permselectivity and the separation factor of He to O₂ in the temperature range of 25°-245°C are 2.49–2.74 and 1.66–1.84, respectively. The average pore diameters of these membranes estimated from the molecular weight cutoff data (6000–9000) using poly(ethylene glycol) are 3.0–4.0 nm. By doping zirconia into titania membranes, their thermal stability could be improved up to 800°C.     

Electroceramics from Source Materials via Molecular Intermediates: BaTiO3 from TiO2 via (Ti(catecholate)3)2-

Rutile or anatase may be depolymerized and complexed by sequential treatment with (i) H₂SO₄/(NH₄)₂SO₄, (ii) H₂O, and (iii) catechol/NH₄OH to produce the intermediate (NH₄)₂(Ti(catecholate)₃) · 2H₂O. Treatment with Ba(OH)₂· 8H₂O leads to an acid-base reaction generating Ba(Ti(catecholate)₃) · 3H₂O, in which the Ba:Ti ratio is held at 1:1 at the molecular level. Calcination produces BaTiO₃ powder.     

Dopants for synthesis of stable bimodally porous titania

Bimodally porous titania powders doped with alumina, zirconia, and silica were made by wet precipitation from organometallic precursors (for Al/Ti=0.05-0.4, and Zr/Ti=Si/Ti=0.1). Doping retards not only the anatase-to-rutile phase transformation, but also the crystallite growth of titania. So it was used to control the powder phase composition and pore structure at high temperatures. The extent of the retarding effect on pore structure and phase transformation increased with increasing alumina concentration. The effectiveness of these dopants follows the order of: zirconia>silica>alumina. The dopants also reduce the loss of surface area of the calcined powders by decreasing the sintering and phase transformation rates. All powders exhibited bimodal pore size distributions (PSD) with fine intra-particle pores (1–4 nm) and larger inter-particle pores (10–120 nm). However, the intra-particle pores of the pure titania disappeared at 600°C, while the bimodal PSD of doped titania was maintained up to 750°C.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

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.     

Fabrication and Photoluminescence Characteristics of Eu3+-Doped Lu2O3 Transparent Ceramics

A novel co-precipitation process was adopted for the preparation of highly sinterable europium-doped lutetia powders using ammonium hydroxide (NH₃·H₂O) and ammonium hydrogen carbonate (NH₄HCO₃) as the mixed precipitant. The resultant powders calcined at 1000°C for 2 h showed good dispersity and excellent sinterability. Highly transparent polycrystalline lutetia ceramics with a relative density of ∼99.9% were fabricated by pressureless sintering in flowing H₂ atmosphere at 1850°C for 6 h without any additives. The average grain sizes of the transparent material were estimated to be 50–60 μm. Optical in-line transmittance in the visible wavelength region for Lu₂O₃ ceramics (1 mm in thickness) reached 80%. The luminescence and decay behavior of the obtained transparent plate and the corresponding nanophosphors were also investigated.     

Laser-Chemical Vapor Precipitation of Submierometer Silicon and Silicon Nitride Powders from Chlorinated Silanes

The laser-chemical vapor precipitation (L-CVP) of Si₃N₄ powders from miktures of SiH₂Cl₂ and NH₃ or SiC1,₄ and NH₃, was studied. The reactant gases were mixed in the laser beam, thus preventing low-temperature reactions. In a high-temperature electrostatic precipitator (ESP), Si₃N, was collected and separated from the waste product NH₄Cl. In the ESP, Si was collected at a lower temperature. A major problem in utilizing chlorinated silanes was their poor absorption of the 10.6-μm radiation. SF₆ was explored for use as an inert sensitizer. Si was prepared from SiH₂Cl₂. Particle diameters were typically 20 to 40 nm for Si3N₄ and 50 nm for Si.     

Direct Formation of Iron(III)-Doped Titanium Oxide (Anatase) by Thermal Hydrolysis and Its Structural Property

Anatase-type TiO₂ (titania) doped with iron up to 19.8 mol% was directly formed as nanometer-sized particles from acidic precursor solutions of TiOSO₄ and Fe(NO₃)₃ by simultaneous hydrolysis, under mild hydrothermal conditions at 180°3C. Iron content in the anatase-type TiO₂ was much less than that of the starting composition of the precursor solutions because of slower hydrolysis rate of Fe(NO₃)₃ than that of TiOSO₄ at 180°3C. The XRD data, TEM selected-area diffraction patterns, and Mössbauer effect measurement showed that iron(III) formed a solid solution in the anatase-type TiO₂ precipitates and that there was no iron oxide precipitated as secondary phase without making a solid solution with TiO₂ present in the precipitates. Doping of Fe₂O₃ into TiO₂ shifted the phase transformation from anatase-type to rutile-type structure to a low temperature. On the phase transformation from anatase to rutile, iron oxide was precipitated as Fe₂TiO₅ (pseudobrookite) phase. When the iron content was increased in the anatase phase, onset of optical absorption shifted to longer wavelengths, and absorption in the UV-light region and in the visible-light region over 400–600 nm clearly appeared in the diffuse reflectance spectra of the as-prepared Fe(III)-doped TiO₂.     

Synthesis of Monodispersed Spherical β-Silicon Carbide Powder by a Sol-Gel Process

Monodispersed spherical β-SiC powder was synthesized by heating spherical gel powder derived from the hydrolysis of a mixture of phenyltriethoxysilane and tetraethyl orthosilicate. The solution was prepared in a beaker and hydrolyzed by NH₄OH without stirring. The monodispersed spherical gel powder of submicrometer size was obtained when the added amount of NH₄OH was greater than 16 moles per mole of silane plus alkoxide, and it became monodispersed spherical β-SiC powder by heat-treating at 1500°C for 4 h in an Ar atmosphere. The SiC content of the powder was 92.6 wt%.     

Rapid Formation of Active Mesoporous TiO2 Photocatalysts via Micelle in a Microwave Hydrothermal Process

Rapid formation of active, mesoporous, and crystalline TiO₂ photocatalysts via a novel microwave hydrothermal process is presented. Crystalline anatase mesoporous nanopowders 100–300 nm in size with worm hole-like pore sizes of 3–5 nm were prepared by a modified sol–gel of titanium tetra-isopropoxide, accelerated by a microwave hydrothermal process. The organic surfactant, tetradecylamine, which is used as a self-assembly micelle in the sol–gel and microwave hydrothermal process, enables to harvest crystallized mesoporous anatase nanoparticles with a high-surface area. Mesoporous worm hole-like and crystalline powders with surface areas of 243–622 m²/g are obtained. X-ray diffraction, N₂-adsorption isotherms (Barrett–Joyner–Halenda and Brunauer–Emmet–Teller method), scanning electron microscope, and transmission electron microscope are used to identify the characteristics and morphologies of the powders. It is shown that crystallization by calcination at 400°C/3 h inevitably reduced the surface area, while the microwave hydrothermal process demonstrated a rapid formation of crystalline mesoporous TiO₂ nanopowders with a high-surface area and excellent photocatalytic effects.     

Synthesis of Ultrafine and Spherical Barium Titanate Powders Using a Titania Nano-Sol

A novel synthetic method for the preparation of spherical, homogeneous, and ultrafine barium titanate (BaTiO₃) powders is described. An aqueous titania nano-sol was prepared by peptizing coarse aggregate of hydrous titania with nitric acid. BaTiO₃ powders could be synthesized through a simple reflux method using the titania nano-sol and barium hydroxide. As decreasing the titanium concentration, the particle size of the resulting spherical BaTiO₃ powder was increased from 40 to 130 nm and the porosity also increased. It was revealed that the smaller as-prepared BaTiO₃ powder was less porous and became more tetragonal with less intragranular pores after annealing. With this method, a highly tetragonal BaTiO₃ powder ( c / a ∼1.008) with a particle size of 120.0 nm was successfully prepared and would be very suitable for the thinner dielectrics in higher capacitance multilayer ceramic capacitors.