The study of the effect of external additives on the synthesis of fumed TiO2 with a high content of rutile structure by the novel natural dropping thermal treatment

The effect of external additives on the synthesis of fumed TiO₂ with a high rutile structure content was studied. The focus of this investigation was on the external additive species, agglomerates of the fumed TiO₂ before and after the thermal treatment. The transformation ratio from the anatase to rutile structure of the thermally-treated fumed TiO₂ was investigated as a function of the fumed TiO₂ agglomerate before the thermal treatment. Small agglomerated powders resulted in a decrease of transformation temperature. Two novel results were obtained in this investigation. One was a new fumed TiO₂ with a 100% rutile structure having an excellent dispersibility being successfully synthesized by the thermal treatment of AEROXIDE® TiO₂ P 25 with a small portion of AEROSIL® R 972. The other was the remarkable acceleration of the transformation from the anatase to rutile structure and grain growth/sintering of the thermally-treated fumed TiO₂ observed at a relatively low thermal treatment temperature by the oxidation reaction of calcium stearate as an external additive.     

The synthesis of a porous-type of TiO2 with rutile structure

The synthesis of a porous-type of TiO₂ with rutile structure was studied. The focuses were on the thermal treatment temperature and time. AEROXIDE® TiO₂ P 25, as a fumed TiO₂, was thermally treated in a vertical-type tubular furnace by the natural dropping method. Even though the thermal treatment time was less than 1?s, a drastic increase of polymorphism from anatase structure to rutile structure was observed. The relationships between the rutile structure transformation ratio and surface area of obtained porous type of TiO₂ were investigated depending on the thermal treatment temperature. The porous-type of fumed TiO₂ showed high dispersibility in the sedimentation test although is showed large particle size.     

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.     

Comparison of nano-particulate TiO2 prepared from titanium tetrachloride and titanium tetraisopropoxide

Nanocrystalline titanium dioxides (TiO₂) prepared from (a) titanium tetrachloride (TiCl₄) and (b) titanium tetraisopropoxide (TTIP) have been compared. The effect of the reaction parameters on the phase formation of TiO₂ was investigated systematically. For TiO₂ prepared by hydrolysis of TiCl₄, the crystal phase changed with the speed of TiCl₄ addition and slow addition favoured the formation of rutile. For TiO₂ prepared by hydrolysis of Ti(OC₃H₇)₄, the reduction in hydrolysis rate associated with lower reaction temperatures promoted rutile formation. For both TiCl₄ and TTIP derived samples, increasing the anatase content decreased the measured rate of photocatalytic oxidation of isopropanol to acetone.     

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.     

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.     

无模板法可控合成多层次结构二氧化钛微球

以1,4-二氧六环为溶剂,采用溶剂热法成功实现无模板法可控合成二氧化钛多层次结构微球。通过系统改变反应体系中浓盐酸与四异丙醇钛(TTIP)相对物质的量比能够有效调控二氧化钛形貌。当浓盐酸与TTIP物质的量比控制在0(或0.7或0.9)、1.8、3.6与5.7时,所得产物分别为纳米颗粒构建二氧化钛微球、纳米棒修饰二氧化钛微球、纳米棒花菜结构以及纳米棒海胆结构。在成功进行形貌调控的基础上,进一步探讨了二氧化钛多种结构的形成机理,并对其光催化产氢性能进行了表征。研究发现,在这4种结构中,纳米棒修饰二氧化钛微球具有最佳的光催化性能,这可能是由于同时存在金红石和锐钛矿两种晶型而形成异质结结构所导致。     

Properties of binary TiO2-SiO2 composite particles with various structures prepared by vapor phase hydrolysis

Various TiO₂/SiO₂ composite nanoparticles were prepared by vapor phase hydrolysis of Titanium Tetraisopropoxide (TTIP) and Tetraethylorthosilicate (TEOS) in a tubular reactor heated by electrical furnace by changing the methods of reactant mixing, the molar ratio of the precursors and the set temperature of the furnace. These variables affected the average size and the bulk composition of the particles as well as their morphological and crystalline structures. The morphological structures of the particles were reconfirmed as T-S, S-T and T/S as in the previous study. Various correlations were obtained between the molar ratio of the precursors, and the size, the composition and the TiO₂ main peak intensity of the particles as a parameter of the mode of reactant mixing. In general, it was found that high content of silicon constituent showed translucent envelopes surrounding primary particles, which increased the dispersion diameter of the particles, reduced the thermal stability, retarded the anatase-to-rutile transformation and the initial photocatalytic degradation of phenol in an aqueous solution.     

Morphology of and surface modification by TiO2 deposits on a porous ceramic substrate

We investigate the morphology of and surface modification by titanium dioxide (TiO₂) deposits on porous ceramic substrates placed in a horizontal, tubular, hot-wall, atmospheric pressure chemical vapor deposition (CVD) reactor with titanium tetraisoperoxide (TTIP) as the precursor. The TiO₂ particles are produced from TTIP through two routes: first by thermal decomposition which then kicks off hydrolysis. The deposit characteristics is found to be location dependent. Those at the reactor entrance and exit are different from that within the reactor. Within the reactor, the deposit characteristics is further found to depend strongly on the deposition temperature (T_d) and is almost independent of the flow-rate and concentration of the reactant. With a T_d of about 380 °C, the deposited TiO₂ forms a forest-like structure with poor adhesivity to the substrate and results in an insignificant pore size reduction for the substrate. If T_d drops down to about 300 °C, the deposited TiO₂ forms a rock-packing structure with good adhesivity to the substrate and can reduce the pore size of the substrate from about 1–2 m down to about 64 nm. At both the reactor entrance and exit, the deposited TiO₂ are loosely-packed spherical particles of average diameter of 140–400 nm. A theory, based on whether or not the reaction is gas-phase or surface dominated, is proposed to explain the dramatic effect of T_d.     

TiO2 coating on silica particles by deposition of sol-gel-derived nanoparticles

TiO₂ was coated on nonporous transparent silica particles of 3.2 μm diameter by deposition of sol-gel-derived TiO₂ nanoparticles. Effects of water concentration, feed rate of titanium tetraisopropoxide (TTIP) solution and amount of supplied TTIP solution on the amount of TiO₂ coated on the silica particles were examined. Scanning electron microscopy observation confirmed that TiO₂ was coated on the silica particles in the form of ‘nanoparticles’ by using this method. Because of that, even though the TiO₂ surface area decreased due to sintering after calcination at high temperature to change the crystalline phase of TiO₂ to the anatase phase, the final surface area was still much larger than that of the original silica particles. The results also showed that as the water concentration increased, the amount of coated TiO₂ decreased. On the other hand, when the amount of supplied TTIP solution increased, the amount of coated TiO₂ increased. It was also confirmed that the feed rate of TTIP solution had little effect on the amount of coated TiO₂. The photocatalytic activities of the resulting TiO₂-coated silica particles were also evaluated by the photocatalytic decomposition of 2,4-dinitrophenol as a model substance. The results showed that the photocatalytic activity of the particles is not a function of the total surface area, but of the surface area in which anatase phase TiO₂ is exposed to the reaction space. The sedimentation velocity of the TiO₂-coated silica particles becomes about 5 orders of magnitude faster than that of the primary particles of the TiO₂. This indicates that the handling of the TiO₂ was also improved considerably by coating on the silica particles.     

One‐Dimensional Densely Aligned Perovskite‐Decorated Semiconductor Heterojunctions with Enhanced Photocatalytic Activity

By using one‐dimensional rutile TiO₂ nanorod arrays as the structure‐directing scaffold as well as the TiO₂ source to two consecutive hydrothermal reactions, densely aligned SrTiO₃‐modified rutile TiO₂ heterojunction photocatalysts are crafted for the first time. The first hydrothermal processing yielded nanostructured rutile TiO₂ with the hollow openings on the top of nanorods (i.e., partially etched rutile TiO₂ nanorod arrays; denoted PE‐TNRAs). The subsequent second hydrothermal treatment in the presence of Sr²⁺ transforms the surface of partially etched rutile TiO₂ nanorods into SrTiO₃ nanoparticles via the concurrent dissolution of TiO₂ and precipitation of SrTiO₃ while retaining the cylindrical shape (i.e., forming SrTiO₃‐decorated rutile TiO₂ composite nanorods; denoted STO‐TNRAs). The structural and composition characterizations substantiate the success in achieving STO‐TNRA nanostructures. In comparison to PE‐TNRAs, STO‐TNRA photocatalysts exhibit higher photocurrents and larger photocatalytic degradation rates of methylene blue (3.21 times over PE‐TNRAs) under UV light illumination as a direct consequence of improved charge carrier mobility and enhanced electron/hole separation. Such 1D perovskite‐decorated semiconductor nanoarrays are very attractive for optoelectronic applications in photovoltaics, photocatalytic hydrogen production, among other areas.     

A low-temperature process to synthesize rutile phase TiO2 and mixed phase TiO2 composites

This works employed K₂Ti₄O₉, a novel Ti source, to prepare TiO₂ powders. By a “low-temperature dissolution-reprecipitation process” (LTDRP), rutile phase TiO₂ was successfully synthesized after reacting at 50 °C for 48 h. The obtained sample showed a specific surface area about 45 m²/g, and excellent activity in photo-destruction of NO_x gas. The coupling of rutile phase TiO₂ with commercial anatase TiO₂ showed significant effect in further enhancing the photocatalytic activity.     

The effect of the H2 flow rate on the structure and optical properties of TiO2 films deposited by inductively coupled plasma assisted chemical vapor deposition

The optical and photocatalytic properties of TiO₂ are closely related to crystalline structures, such as rutile and anatase. In this paper, TiO₂ films were produced by inductively coupled plasma (ICP) assisted chemical vapor deposition (CVD) without extra heating of the substrate, and the effect of H₂ addition on the structure and optical properties of the films was investigated. After increasing the partial pressure of H₂, the structure of the TiO₂ films changed from anatase to rutile, which usually appears at high temperatures (> 600 °C). The light transmittance decreased with increasing the H₂ flow rate due to the increased surface roughness. The photocatalytic activity of the anatase TiO₂ film was better than that of the rutile TiO₂ film.     

Coating of TiO2 thin films on particles by a plasma chemical vapor deposition process

TiO₂ thin films were experimentally coated on glass beads by means of a rotating cylindrical plasma chemical vapor deposition (PCVD) reactor. The morphologies and growth rates of the TiO₂ thin films before and after heat treatment were measured for various process conditions. The precursors for the TiO₂ films were generated from TTIP by plasma reactions, and they were deposited on the glass beads to become TiO₂ thin films. The TiO₂ thin films coated on the glass beads became more uniform by heat treatment. The TiO₂ thin films grew more quickly on the glass beads with increasing mass flow rate of TTIP, reactor pressure, or rotation speed of the reactor. As the applied electric power decreases, the thickness of the thin films on the glass beads increases. This experimental study shows that the use of a rotating cylindrical PCVD reactor can be a good method to coat high-quality TiO₂ thin films uniformly on particles.     

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.     

Real structure of TiO2 in samples of the TiO2-V2O5 system

Measurements of electron spin resonance (ESR) spectra and X-ray structure analysis have been performed on samples of the TiO₂-V₂O₅ system, formed by anatase crystallites on the surface of which a thin layer of V₂O₅ was deposited by thermal decomposition of ammonium vanadate at 800°C. It was found that the deposited V₂O₅ gives rise to a change in the modification of TiO₂ (anatase goes over to rutile) and to a decrease in the volume of the TiO₂ elementary cell. Bands corresponding to Ti atoms of oxidation state +III were identified in the ESR spectrum of the initial TiO₂. The intensity of these bands decreases with increasing content of V₂O₅ in the TiO₂-V₂O₅ samples; the ESR spectra of the samples with high V₂O₅ content exhibit only one band corresponding to vanadium atoms in oxidation state +IV in the rutile lattice. The experimental results allow one to form an idea on the changes of the real structure of TiO₂ in connection with the deposition of V₂O₅ thin films on TiO₂ crystallites and with thermal treatment of these materials.     

Effects of light scattering TiO2 surface-modified by dual oxide coating in dye-sensitized solar cells

The surface of light scattering TiO₂ particles in the dye-sensitized solar cell (DSSC) was dual-coated with Al₂O₃ and SiO₂ nanoparticles. The surface modification of the light scattering TiO₂ particles was performed by a modified sol–gel method using the colloidal alumina and the colloidal silica as surface coating precursors. It was revealed that the dual-coated light scattering TiO₂ particles leads to an increase in short-circuit photocurrent of DSSC device, resulting in an increase in energy conversion efficiency. This seems to be due to the increase of the light scattering by a combination of the light scattering TiO₂ particles and the oxide nanoparticles such as Al₂O₃ and SiO₂.     

Microstructures and Photocatalytic Properties of S Doped Nanocrystalline TiO2 Films

The nanocrystalline S doped titanium dioxide films were successfully prepared by the improved sol-gel process. Here TiO(C₄H₉O)₄ and CS(NH₂)₂ were used as precursors of titania and sulfur, respectively. The as-prepared specimens were characterized using x-ray diffraction (XRD), x-ray energy dispersive spectroscopy (EDS), high-resolution field emission scanning electron microscopy (FE-SEM), Brunauer-Emmett-Teller (BET) surface area, and ultraviolet-visible diffuse reflectance spectroscopy. The photocatalytic activities of the films were evaluated by degradation of organic dyes in aqueous solution. The results of XRD, FE-SEM, and BET analyses indicated that the TiO₂ films were composed of nanoparticles. S doping could obviously not only suppress the formation of brookite phase but also inhibit the transformation of anatase to rutile at high temperature. Compared with pure TiO₂ film, S doped TiO₂ film exhibited excellent photocatalytic activity. It is believed that the surface microstructure of the modified films is responsible for improving the photocatalytic activity.     

Intraparticle structures of composite TiO2/SiO2 nanoparticles prepared by varying precursor mixing modes in vapor phase

TiO₂-SiO₂ composite nanoparticles with different intraparticle structures were prepared by varying modes of reactant mixing in equimolar vapor-phase hydrolysis of TTIP and TEOS. They included the structures of core-shell, either titania core surrounded by silica shell (abbreviated by T-S) or vice versa (S-T), and of mixed type, one oxide dispersed in the other (T/S). Design of mixing junction between TTIP and H₂O was very critical due to high hydrolysis rate of TTIP. That of TEOS was, however, so low that atomic silicon percentage of the particles was always less than 50%. The percentage increased with the temperature of reactor and the free surface of titania. Thus, due to the availability of the surface, the highest and the smallest silicon contents were obtained in the T/S and the S-T particles, respectively. In case of the former, as the temperature increased to 950°C, the silicon content reached 50% and, moreover, silica-rich particles appeared originating from homogeneous nucleation of the component. Average size of primary particles, irrespective of the structures, decreased with the content of silicon. In addition to the silicon effect, the gap in the sizes of the T-S and the S-T particles was further promoted, ultimately by the difference in the hydrolysis rates of the two alkoxides. The difference also caused radial gradient of composition even inside each primary T/S particle. Structural identity of shell component was affected by the existence of core component for both the T-S and the S-T particles.     

Preparation and characterization of phosphate-modified mesoporous TiO<Subscript>2</Subscript> incorporated in a silica matrix and their photocatalytic properties in the photodegradation of Congo red

This study describes the development of mesostructured TiO₂ photocatalysts modified with PO₄^3- to improve its specific surface area and reduce the recombination rate of the electron—hole pairs. The mesoporous photocatalyst was successfully incorporated into a high specific surface area silica matrix by the hydrolysis reaction of tetraethyl orthosilicate (TEOS). Pluronic 123 and phosphoric acid were used as the directing agent for the structure of the mesoporous TiO₂ and as a source of phosphorus, respectively. TiO₂, P/TiO₂, TiO₂-SiO₂ and P/TiO₂-SiO₂ materials were characterized by BET, XRD, TEM-EDS, FTIR and UV-vis DRS measurements. The photoactivity of TiO₂-SiO₂ nanocomposites containing 15 wt.% photocatalyst/silica was evaluated in the degradation reaction of anionic dyes with UV radiation. The proposed nanomaterials showed high potential for applications in the remediation of wastewater, being able to reuse in several cycles of reaction, maintaining its photoactivity and stability. The separation and recovery time of the material is reduced between cycles since no centrifugation or filtration processes are required after the photooxidation reaction.