Preparation of TiO2 hollow microparticles by spraying water droplets into an organic solution of titanium tetraisopropoxide

Hollow, spherical TiO₂ microparticles several tens of micrometers in diameter were prepared by spraying water into an organic phase containing titanium tetraisopropoxide (TTIP) as a titanium source. The rapid hydrolysis of TTIP at the water-oil interface resulted in the formation of a TiO₂ shell covering the water droplet. Hexane and cyclohexane were better solvents than isopropanol for fabricating hollow spherical microparticles, suggesting the importance of immiscibility of the solvent with water in this synthesis method. The average particle size increased as the distance from the nozzle to the surface of the TTIP solution was increased. The shell thickness was reduced by the addition of ethanol to the sprayed water droplet. These results demonstrate the controllability of the structure of TiO₂ hollow microparticles, including the diameter and the shell thickness.     

Preparation of titanium diboride powders from titanium alkoxide and boron carbide powder

Titanium diboride powders were prepared through a sol-gel and boron carbide reduction route by using TTIP and B₄C as titanium and boron sources. The influence of TTIP concentration, reaction temperature and molar ratio of precursors on the synthesis of titanium diboride was investigated. Three different concentrations of TTIP solution, 0.033/0.05/0.1, were prepared and the molar ratio of B₄C to TTIP varied from 1.3 to 2.5. The results indicated that as the TTIP concentration had an important role in gel formation, the reaction temperature and B₄C to TTIP molar ratio showed obvious effects on the formation of TiB₂. Pure TiB₂ was prepared using molar composition of Ti: B₄C = 1: 2.3 and the optimum synthesis temperature was 1200°C.     

Effect of TiCl4 concentration on the structure of its aqueous solutions and on the properties and photoactivity of derived nanocrystalline titania

The influence of the concentration of aqueous TiCl₄ solution on the phase formation, morphology and particle size of the titanium dioxide hydrolysis product was investigated by XRD and TEM. Significant features, observed in the Raman spectra of the TiCl₄ solutions with a concentration >3 M, demonstrated that the TiCl₄ had hydrolysed. As the formal concentration of TiCl₄ decreased from 4.98 to 1 M, the Raman spectra changed qualitatively. Despite the changes in the Raman spectra of these precursor solutions, the TiO₂ product was mainly rutile in all cases. However, at low TiCl₄ concentrations small amounts of anatase were also observed. Electron microscopy suggested that the anatase particles were significantly smaller than the rutile and also indicated increasing aggregation of the product from the more dilute TiCl₄ solutions. The optical properties and photoactivities of the TiO₂ powders prepared at different concentrations were also investigated. The powder synthesized from 5 M TiCl₄ showed the highest UV extinction. The photoactivity of the product, determined by the photocatalytic oxidation of propan-2-ol (isopropanol) to propanone (acetone), was not significantly modified by changes in the concentration of the starting TiCl₄. The possibility that the relatively low area of most rutiles contributes to the reported photocatalytic activity of rutile being lower than that of anatase is discussed.     

Structure and properties of nitrogen-doped titanium dioxide thin films grown by atmospheric pressure chemical vapor deposition

Using TiCl₄, O₂, and N₂O as precursors, N-doped titanium dioxide thin films with large area and continuous surface were obtained by atmospheric pressure chemical vapor deposition. Measurements of X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscope, transmission electron microscope and ultravoilet-Visible transmission spectra were performed. Using N₂O as N-doped source, anatase-rutile transformation is accelerated through oxygen vacancies formation, and the mean grain size of rutile crystallites decreases with the increase of N₂O flow rate. Compared to the pure TiO₂, N-doped TiO₂ films give a relative narrow optical band-gap, and their visible-light induced photocatalysis is much enhanced. Visible-light-induced hydrophilicity of the TiO₂ thin films enhances with the increase of N₂O flow rate, which might be due to the dentritic islands structure on the surface of the N-doped TiO₂ thin films.     

Improving the Electron Transport Performance of TiO2 Film by Regulating TiCl4 Post-Treatment for High-Efficiency Carbon-Based Perovskite Solar Cells

Titanium oxide (TiO₂) has been widely used as an electron transport layer (ETL) in perovskite solar cells (PSCs). Typically, TiCl₄ post-treatment is indispensable for modifying the surfaces of TiO₂ ETL to improve the electron transport performance. However, it is challenging to produce the preferred anatase phase-dominated TiO₂ by the TiCl₄ post-treatment due to the higher thermodynamic stability of the rutile phase. In this work, a mild continuous pH control strategy for effectively regulating the hydrolysis process of TiCl₄ post-treatment is proposed. As the weak organic base, urea has been demonstrated can maintain a moderate pH decrease during the hydrolysis process of TiCl₄ while keeping the hydrolysis process relatively mild due to the ultra-weak alkalinity. The improved pH environment is beneficial for the formation of anatase TiO₂. Consequently, a uniform anatase-dominated TiO₂ surface layer is formed on the mesoporous TiO₂, resulting in reduced defect density and superior band energy level. The interfacial charge recombination is effectively suppressed, and the charge extraction efficiency is improved simultaneously in the fabricated solar cells. The efficiency of the fabricated carbon electrode-based PSCs (C-PSCs) is improved from 16.63% to 18.08%, which is the highest for C-PSCs based on wide-bandgap perovskites.     

Influence of nitric acid-assisted hydrothermal conditions on the characteristics of TiO2 catalysts and their activity in the oxidative steam reforming of methanol

Titania (TiO₂) nanoparticles (NPs) with different morphologies (spherical, rod-shaped, and mixed) were prepared by hydrothermal treatment of different nitric acid (HNO₃)/titanium (IV) isopropoxide (TTIP) molar ratios (0.25, 0.5, 1.0, and 1.7) at different hydrothermal temperatures (90, 150, 200, and 250 °C), hydrothermal times (6, 12, and 24 h), and calcination temperatures (500, 625, and 750 °C). The crystalline structure, morphology, and surface texture of the obtained TiO₂ NPs were characterized by X-ray diffraction, nitrogen adsorption–desorption isotherm, field emission-scanning electron microscopy, and high resolution-transmission electron microscopy analyses. Under a larger HNO₃: TTIP molar ratio, higher hydrothermal temperature, and higher hydrothermal time, the spherical mixed anatase–rutile phase TiO₂ NPs were converted to a nanorod (NR)-shaped rutile phase (TiO₂-R). The TiO₂-R NRs gave the highest methanol conversion level (65%) and hydrogen yield (45%) in the oxidative steam reforming of methanol at 400 °C.     

Morphological control and photodegradation behavior of rutile TiO2 prepared by a low-temperature process

Flower-like rutile titania nanocrystals were prepared via a simple aqueous-phase stirring for 24 h at a low temperature of 75 °C, employing only TiCl₄, HCl as the starting materials. XRD result proved the formation of rutile TiO₂. The observations from TEM and SEM showed that the products were large-scale flower-shaped structures composed of radial nanorods. Comparative experiments demonstrated that pinecone-like, needlelike rutile TiO₂ could be easily achieved by varying the volume ratio of TiCl₄ / H₂O. The growth mechanisms of TiO₂ nanostructures prepared under different conditions and their photodegradation behavior were also discussed. It was found that the flower-like structures exhibited the highest photocatalytic activity in the photodegradation of aqueous brilliant red X-3B solution.     

Synthesis of carbon-doped TiO2 nanoparticles using CO2 decomposition by thermal plasma

This study examined the synthesis of carbon-doped titanium dioxide using TiCl₄ and CO₂ as titanium and carbon sources, respectively, by thermal plasma at atmospheric pressure. The effect of the CO₂ gas flow rate on the preparation of TiO₂ was investigated. The results showed that the decomposition rate of CO₂ was 90% at a CO₂ gas flow rate of 1 L/min. When TiCl₄ was added to produce TiO₂, the decomposition rate of CO₂ reached 95% at a CO₂ gas flow rate of 1 L/min. The resulting powders contained mixed anatase and rutile phases with particle sizes ranging from 20 to 50 nm. The carbon in the CO₂ acted as a dopant to produce the carbon-doped TiO₂. The prepared samples were mainly characterized by X-ray diffraction, X-ray photoelectron spectroscopy, specific surface area measurements and ultraviolet-visible spectroscopy.     

Deformation-induced reactions of ZnO and TiO2

Reactions of ZnO and TiO₂ (anatase and rutile) in different mole ratios induced by high-energy ball milling were studied by X-ray diffraction. It was found that three main reactions could involve during high-energy ball milling: (1) (4 ? X)ZnO + (2 + Y) TiO₂ (anatase or rutile) → Zn_4?X Ti_2+Y O₈; (2) ZnO + TiO₂ (rutile) → ZnTiO₃, and (3) TiO₂ (anatase) → TiO₂ (II) → TiO₂ (rutile). Cubic Zn_4?X Ti_2+Y O₈ nanocrystals with an average crystal size of about 15 nm can be prepared by high-energy ball milling, which could be an attractive process to fabricate material in industrial scale. No decomposition of ZnTiO₃ into Zn₂TiO₄ and rutile was detected during milling. Anatase shows higher reaction activity than rutile and favours the formation of Zn_4?X Ti_2+Y O₈ while rutile favours the formation of ZnTiO₃. During the anatase-to-rutile transformation a transient metastable phase, TiO₂ (II) which is a high-pressure phase of TiO₂, is detected.     

Preparation and photocatalytic activity of TiO2 powders from titanium citrate complex using two-step hydrothermal treatments

White, almost carbon-free TiO₂ powders were prepared from a titanium citrate complex ((NH₄)₄[Ti₂(C₆H₄O₇)₂(O₂)₂]·4H₂O) using a two-step hydrothermal treatment. The product yield, carbon contamination, and crystalline phase of TiO₂ depended on both the temperature and pH value for each treatment. Titanium was precipitated as a solid phase (H₂Ti₂O₅·H₂O) using the first hydrothermal treatment in the basic condition (pH = 12) at temperatures less than 150 °C. Then white rutile or anatase powder was crystallized using the second hydrothermal treatment at 200 °C. By changing the pH condition of the second hydrothermal treatment, rutile and anatase were synthesized selectively. The photocatalytic decomposition activity of obtained rutile powder for gaseous 2-propanol under visible light was increased by Cu-grafting.     

Reaction mechanism and kinetics analysis of the phase transformation of TiO2 from the anatase phase to the rutile phase

The kinetic mechanism of the phase transformation of TiO₂ from the anatase phase to the rutile phase was investigated. TiO₂ powders were prepared with different pH value of the starting solution via the thermal hydrolysis method in this study. The pH values of the starting solutions have significant effects on the phase transformation temperatures of the thermal hydrolysis reaction. As the reaction temperatures were raised, the conversion from the anatase phase to the rutile phase was increased. A core–shell morphology of the prepared TiO₂ samples was suggested via the signals of the anatase phase and the rutile phase in UV–vis spectrum analysis. Through the isothermal heating process of the reaction kinetics, the controlling reaction in the phase transformation process from the anatase phase to the rutile phase was determined to be the three-dimensional phase boundary controlled process. The activation energy of the phase transformation was increased with an increase in the pH value of the starting solution.     

Carbonization of Titanium and Molybdenum by Hexachloroethane

Polycrystalline transition metal carbides (M=Ti, Mo) were deposited by a low-pressure chemical vapor deposition system on quartz and a metal plate. Using C₂C1₆ and the metal plate as the precursors, the carbonization reactions occurred at a temperature exceeding 800°C. The block composition of titanium carbide is Ti/C = 0.66 and that of molybdenum carbide is Mo/C = 1.65. According to our results, the composition of thin films is a mixture of carbon and metal carbide. With respect to the volatile products, in addition to CC1₄, C₂C1₄, and Cl₂, we obtained TiCl₄ while reacting the titanium metal with C₂C1₆. The formation of TiCl₄ implies that titanium metal acts as a reduction reagent. In addition, this compound transfers chlorine from carbon to itself to obtain TiCl, TiC1₂, TiCl₃, and TiC1₄. In this reaction, TiCl₂ and TiCl₃, were freshly prepared and acted as the titanium source of further reaction. The total reaction pathway was studied in detail.     

Nanoporous titanium oxide synthesized from anodized Filtered Cathodic Vacuum Arc Ti thin films

This paper describes the formation of self-organized nanopores in thin films of titanium prepared using a Filtered Cathodic Vacuum Arc (FCVA) deposition system. The post-deposition anodization was performed using 0.5% (wt) NH₄F in ethylene glycol and an aqueous based solution containing 0.5% (wt) NH₄F and 1 M (NH₄)₂SO₄ electrolytes. Homogenously distributed nanopores with dimensions in the range of 10 to 20 nm were obtained. Nanoporous TiO₂ thin films were obtained after annealing the anodized samples at 600 °C for 4 h. Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize these nanoporous films. Raman measurements revealed that the rutile TiO₂ polymorph dominates these structures along with imperfect titanium oxidation resulting in the formation of defect structures, particularly when aqueous electrolyte was used for the anodization.     

Preparation and characterization of Pt–TiO2–SiO2 mesoporous materials and visible-light photocatalytic performance

TiO₂–SiO₂ metal oxide materials with a mesostructure have been prepared by a novel method in which hydrolysis and condensation of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) were controlled by the pH change of acidic solution. The Pt-modified TiO₂–SiO₂ catalysts were synthesized by the photo-reduction method. The resulting materials showed a high photocatalytic activity for the photodegradation of methyl orange in the visible-light range. A reaction mechanism was proposed and discussed.     

Development of the fumed TiO2 having high content of rutile structure and dispesibility using the dry-type of surface modification and the thermal treatment

The effect of dry-type surface modification of a fumed TiO₂ and the thermal treatment were studied to synthesize the new fumed TiO₂ having a high content of rutile structure and high dispersibility. The fumed TiO₂ was modified with various metallic alkoxides to prepare precursors and next the resulted precursors were thermally-treated by the novel natural dropping method with a very short heating time, less than 1 s. The focus of this investigation was on the metallic alkoxide species as a surface modification agent and morphologies of both the precursor and thermally-treated fumed TiO₂. The morphologies and nanostructures of the obtained fumed TiO₂ were characterized. The carbon content and agglomerate of the precursor influenced on the transformation ratio from anatase to rutile structure. It was confirmed that the dry-type surface modification with tetraethoxysilane (TEOS) and titanium tetra-isopropoxide (TTIP) at room temperature is very effective method to prepare precursors for the next thermal treatment. The thermally-treated fumed TiO₂ modified with TTIP showed 100% rutile structure with pure TiO₂ composition at 1400 °C. The thermally-treated fumed TiO₂ modified with small quantities of TEOS exhibited both 100% rutile structure and excellent dispersibility. This high dispersibility caused from a sponge-like structural characteristic of the agglomerate and static electricity repellence by coated SiO₂ layer. The thermally-treated fumed TiO₂ modified with TEOS and TTIP showed the color tone shift can be attributed to rutile structure. It was revealed that the combination of dry-type surface modification and natural dropping thermal treatment is an attractive method to prepare the new fumed TiO₂ with 100% rutile structure maintaining the high dispersiblity.     

New types of hydrous titanium oxides obtained by homogeneous precipitation from (titanium (III) chloride + urea) solutions

Four kinds of hydrous titanium oxide (HTiO) were prepared by refluxing 1 mol dm⁻³ titanium (III) chloride (TiCl₃) solutions containing urea (urea to titanium mole ratio=2.0, 3.0, 4.5, and 6.0; the (TiCl₃ + urea) system) at 371 K. Their physicochemical properties were investigated by means of X-ray diffraction (XRD), differential thermal analysis (DTA-TG), electronmicroscopy, colour measurement, and nitrogen adsorption-desorption at 77K. A rod-like sample 0.2 μ wide and 1.0 μm long was obtained in the case of urea to titanium mole ratio of 2.0. The transmission electronmicrograph showed that the rods were aggregates of acicular crystals in a parallel arrangement. Blue samples of microcrystalline rutile were obtained in the case of an urea to titanium mole ratio above 3.0. The electron spin resonance (ESR) measurement showed that the blue colouration was ascribed to the presence of stable paramagnetic titanium (III) ions. A chemical mechanism for the formation of rod-like and blue-coloured HTiOs is proposed.     

Heat treatment on TiO2 nanoparticles prepared by vapor-phase hydrolysis

TiO₂ nanoparticles are prepared by vapor-phase hydrolysis of TiCl₄ below 550°C and are characterized by TEM, XRD and XPS before and after heat treatment at various temperatures. It shows that anatase/amorphous particles start to transform to rutile at certain temperature. Such transformation temperature tends to decrease with the decrease of particle size. When the rutile phase starts to be formed, the particles grow rapidly, while they grow slowly before the appearance of rutile titania. The Ti2p_3/2 binding energies tend to increase as particle sizes decrease, especially for particles below 50 nm in size.     

In situ investigation of chemical reactions between BaCO3 and anatase or rutile TiO2

The most popular method of preparation of BaTiO₃, which is one of the most widely used ferroelectric materials for multi-layered ceramic capacitors, is the solid-state reaction process between powdered barium carbonate (BaCO₃) and titanium dioxide (TiO₂) at high temperature. The influence of the different structural forms of TiO₂ used (i.e. rutile or anatase) on the reaction process and on the crystallinity of synthesized BaTiO₃ is only known on empirical grounds. The solid-state reaction between BaCO₃ and either TiO₂ anatase or TiO₂ rutile was investigated by in situ by X-ray diffraction and micro Raman scattering measurements. The formation of both barium oxycarbonate BaO_x(CO₃)_1−x and of a very small amount of Ba₂TiO₄ were detected in the samples as intermediate phase before the formation of BaTiO₃. The Raman spectra of the final product obtained in each case is essentially the tetragonal BaTiO₃ containing small amounts of non-reacting BaCO₃ and of hexagonal barium titanate. The tetragonality of the final product was found to be slightly better for BaTiO₃ synthesized from rutile and BaCO₃ than from anatase and BaCO₃, whereas the average particle sizes are essentially the same for both products.     

A simple H2O2-assisted route to hollow TiO2 structures with different crystal structures and morphologies

Hollow TiO₂ structures have been synthesized on a large scale in H₂O₂ aqueous solution by a simple hydrothermal method. The morphologies and crystal structures of hollow TiO₂ structures can be controlled by titanium sources. Hollow anatase TiO₂ microspheres composed of nanoparticles are prepared using titanium powder as titanium source. With the pH value increased, TiO₂ nanoparticles and cuboids with cylindrical hollow interiors are formed, respectively. However, hollow rutile TiO₂ microspheres comprising radially aligned nanorods are fabricated using TiCl₃ as titanium source.     

Synthesis and photocatalytic properties of nanomaterials based on titanium(IV) and zinc(II) oxides

We have synthesized materials based on titanium(IV) and zinc(II) oxides, containing 1 to 60 wt % Zn, at heat-treatment temperatures from 80 to 1150°C, with the formation of multiphase compositions (X-ray amorphous phase, anatase, rutile, ZnTiO₃, and/or Zn₂TiO₄) and studied their phase transitions, morphology, and photocatalytic activity. Increasing the Zn content of the materials is favorable for their spectral sensitization, including the range λ ≥ 670 nm.