Structure of TiO2 in TiO2-SiO2 prepared by a complexing agent-assisted sol-gel procedure

Many types of TiO₂-SiO₂ (Ti:Si=50:50 mol%) were prepared by the sol-gel procedure with and without 2-methyl-2, 4-pentanediol (MPD) as an organic ligand. The effect of MPD on the gel structure and the properties of the TiO₂ crystals were studied by XRD and raman spectroscopy, and the effect of the sol standing time on the properties of the TiO₂ crystals were also studied by XRD spectroscopy. In the gels with MPD, anatase of TiO₂ appeared at approximately 580°C, and the crystal structures were similar despite the difference in the gel preparation procedure. The titania gels with MPD were presumed to be dispersed in the silica gel matrix without any Ti-O-Si bond. In the presence of MPD, the formation of titania gels is controlled and the specified TiO₂ crystal is produced.     

Mechanism of formation of YBa2Cu4O8 superconductor in the sol-gel synthesis

It was shown that a single phase YBa₂Cu₄O₈ (124-phase) could be formed from gels at 1 atm oxygen pressure, and the mechanism of its formation was elucidated. It was found that there are two key routes for the sol-gel formation of the 124-phase, one involving the tetragonal YBa₂Cu₃O _y with a low concentration of oxygen defects (tetra-I phase) and the other involving the Ba₂Cu₃O_5.9 as important intermediates of the 124-phase. The rapid formation of these intermediate compounds from the gel was attributed to the small particle size of the oxides and carbonates precipitating at the initial stage of heating. It was thought that the small particles characteristic of sol-gel processing lead to the rapid formation of the intermediate compounds and subsequent precipitation of the 124-phase.     

Preliminary Study on the Effect of pH on Thermal Transformation of Some Diphasic Al2O3-SiO2 Gels

Diphasic Al₂O₃-SiO₂ gels have been synthesized by hydrolyzing ethyl orthosilicate at three different pH ranges in presence of colloidal boehmite. Result shows that dominant mullitization process are sensitive to pH of the gelation process. At highly acidic pH range, solid state reaction between corundum and cristobalite occurs, and develops liquid phase for mullite nucleation. At moderately acidic pH, nucleation and crystallization are most operative for nucleation in aluminosilicate matrix. In highly basic pH region, Al-Si spinel phase develops by incorporation of Si in aluminous phase as intermediary phase. Polymorphic transformation of it may be the cause of sudden mullitization. The changes in crystallization sequence in three distinct processes may be due to variation in the nature and size of silicic acid particle formed by hydrolysis and condensation of TEOS at different pH.     

Formation of Anatase Nanocrystals in Sol-Gel Derived TiO2-SiO2 Thin Films with Hot Water Treatment

We have successfully prepared transparent and porous anatase nanocrystals-dispersed films by treating the sol-gel derived TiO₂-SiO₂ films containing poly(ethylene glycol), PEG, with hot water. This process was done at temperatures lower than 100°C under atmospheric pressure, and thus anatase nanocrystals-dispersed films can be formed on various kinds of substrates including organic polymers with poor heat resistance. The changes in structure and composition of the TiO₂-SiO₂ gel films with hot water treatment were related to the formation process of anatase nanocrystals in the TiO₂-SiO₂ gel films with hot water treatment. The formation of anatase nanocrystals was found to proceed to hydrolysis of Si–O–Ti bonds and dissolution of SiO₂ component. In addition, porous film structure formed by leaching of PEG with hot water treatment led to homogenous dispersion of anatase nonocrystals in the films.     

Nonhydrolytic Sol-Gel Process: Aluminium and Zirconium Titanate Gels

The effectiveness of nonhydrolytic Sol-Gel process to the synthesis of homogeneous bicomponent gels has been studied in the case of aluminium titanate and zirconium titanate. The direct crystallization of Al₂TiO₅ and ZrTiO₄ at low temperature without the intermediate formation of TiO₂ or Al₂O₃ in the former case and TiO₂ or ZrO₂ in the latter indicates that homogeneous bicomponent gels have been formed by this route.     

Fe3O4–SiO2 nanocomposites obtained via alkoxide and colloidal route

The magnetic nanocomposite materials represent an important class of nanomaterials extensively studied nowadays due to their varied applications from medical diagnostic to storage information. The iron oxides in silica matrix systems are highly investigated. The sol-gel method is a suitable way of preparation of Fe₃O₄-SiO₂ nanocomposite materials, since this method allowed the preparation of nanocomposite materials with narrow size distribution of magnetite in silica matrix. In the present work, nanocomposite materials in the Fe₃O₄-SiO₂ system were prepared by sol-gel method via alkoxide and aqueous route. As SiO₂ sources, tetraethoxysilan (TEOS) for the alkoxide route, as well as silica sol Ludox (30%) for the aqueous route, were used. This study shows the influence of the type of silica matrix on the structure, size, and distribution of the Fe₃O₄ nanoparticles in the Fe₃O₄-SiO₂ systems. The gels were annealed at 550°C in order to consolidate the matrices. The structural characterization of the obtained materials via the two preparation routes was performed by DTA/TGA analysis, X-ray diffraction, IR and Mössbauer spectroscopy, Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED).     

Topotaxial formation of titanium-rich barium titanates during solid state reactions on (110) TiO2 (rutile) and (001) BaTiO3 single crystals

The orientation relationships of Ti-rich barium titanate phases formed by solid state reactions at high temperatures were studied using (110) TiO₂ (rutile) and (001) BaTiO₃ single-crystal substrates. Well-oriented Ba₆Ti₁₇O₄₀ islands were observed after a vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 1 nm with the TiO₂ substrate, whereas a thin film consisting of well-oriented BaTiO₃ and Ba₆Ti₁₇O₄₀ grains was formed after vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 50 nm with the rutile substrate. A topotaxial orientation relationship between Ba₆Ti₁₇O₄₀ and TiO₂ was found. Topotaxy is facilitated by a certain similarity in the oxygen sublattices of TiO₂ and Ba₆Ti₁₇O₄₀. The mechanism of the reaction occurring between BaO vapor and the TiO₂ surface at high temperature is discussed. On the other hand, several well-oriented Ba₄Ti₁₃O₃₀, Ba₆Ti₁₇O₄₀ and Ba₂Ti₅O₁₂ phases were observed to be embedded in the mainly forming Ba₂TiSi₂O₈ phase after a solid–solid reaction of amorphous SiO₂ thin films with (001) BaTiO₃ substrates at temperatures above 1000 °C. They were formed by a topotaxial reaction involving the transformation of (111) planes of BaTiO₃ into (001) planes of the Ti-rich phases by removal of BaO and insertion of TiO₂. Cross-sections of the interfaces between the substrates and the various reaction products are studied by (high-resolution) transmission electron microscopy.     

Hybrid RSiO1.5/B2O3 Gels from Modified Silicon Alkoxides and Boric Acid

A new sol-gel route, without any external water addition, for the preparation of hybrid RSiO_1.5/B₂O₃ gels (R = Me, Et, Vi) is reported. The gels are easily synthesized by reacting B(OH)₃ with the liquid silicon alkoxide. The gels have been characterized mainly by FT-IR and ¹¹B MAS NMR spectroscopy. The results show that this new sol-gel process allows the formation of a homogeneous borosilicate gel in which trigonal BO₃ units are incorporated into the siloxane network via B—O—Si bonds.     

The role of precursors in the structure of SiO2-Al2O3 sols and gels by the sol-gel process

Binary sols and gels of SiO₂-Al₂O₃ were prepared using tetraethyl orthosilicate and each of four aluminum compounds; aluminum di (sec-butoxide) ethylacetoacetic ester chelate (AC), aluminum nitrate nonahydrate (AN), aluminum formoacetate (AF), and boehmite sol (BS) made from aluminum i-propoxide. The structure and the evolution of the Si-O-Al bonds in SiO₂-Al₂O₃ sols and gels were investigated by ²⁷Al nuclear magnetic resonance (NMR), Infrared absorption spectra, DTA, and X-ray diffraction. The formation of Si-O-Al bonds differs depending on the aluminum compounds used as raw materials. The ratio of Al(IV) to {Al(IV) + Al(VI)} is related to the microstructural homogeneity of the gels. When AC is used as a raw material, the Si-O-Al bonds are formed in the sol state and resultant gel shows good microstructural homogeneity. In case of AN, the Si-O-Al bonds are not formed either in the sol or the wet gel state. The bonds are formed by drying the gel before heat-treatment temperature reaches 300 400°C, resulting in good microstructural homogeneous gel. When AF is used, the Si-O-Al bonds are formed in the sol state but the ratio of Al(IV) to {Al(IV) + Al(VI)} is lower than when using AC. Microstructural homogeneity of the gel is ranked between AC or AN and BS. Using BS, the Si-O-Al bonds are not formed in the sol solution, and the change in the coordination number of the gel is similar to that of boehmite gel. The Microstructural homogeneity of the gel is the worst among the BS gels, which were prepared by using the four aluminum raw materials.     

Preparation and properties of SiO2-TiO2 thin films from silicic acid and titanium tetrachloride

The preparation of SiO₂-TiO₂ thin films by the sol-gel method using silicic acid and titanium tetrachloride as starting materials was studied. The homogeneous sols were obtained by the condensation reaction of silicic acid with titanium tetrachloride in methanol-tetrahydrofuran. The dipcoating of slide glasses and silicon wafers followed by heat treatment gave oxide thin films of 88–93% transmittance, 3000–4500 Å thickness, and 1.45–1.80 refractive index, depending on heat-treatment temperature and TiO₂ content. FT-IR measurement showed that the Si-O-Ti bond is formed even in the sol and films. The variations of film thickness and refractive index on transformation from the gels into the oxides were found to be quite low.     

Formation and Crystallization Behavior of Gels in the Na2O-SiO2-TiO2-ZrO2 System

SiO₂-TiO₂-ZrO₂ and 5Na₂O·95(SiO₂ + TiO₂ + ZrO₂) gels were synthesized and role of Na₂O in gel formation and crystallization behavior of gels were studied. From Si(OC₂H₅)₄, Ti(iso-OC₃H₇)₄, Zr(n-OC₃H₇)₄ and NaOCH₃ solutions in EtOH without H₂O, transparent and opaque gels were obtained. Opaque bulk gels, rich in TiO₂ or ZrO₂ composition in Na₂O containing SiO₂-TiO₂-ZrO₂ system, contain agglomerated spherical particles of diameter small <10 m, in contrast with opaque gels having large particles <30 m in alkali-free SiO₂-TiO₂-ZrO₂ system. Crystallization temperature (Tc) was measured by DTA on dried gels. Compared with the alkali-free SiO₂-TiO₂-ZrO₂ gels, 5 mol% Na₂O containing gels gave lower Tc in SiO₂ rich compositions and higher in TiO₂ rich or ZrO₂ rich compositions.     

Sol-gel coatings of BaO-TiO2-SiO2 compositions

New alkali resistant BaO-TiO₂-SiO₂ coatings have been developed via the sol-gel process. In the solutions and in the gels (T<300°C) an infrared absorption band at 930 cm^–1 gives the possible evidence of mixed Si-O-Ti bonds, which have not been found in the system SiO₂-TiO₂-ZrO₂. Baking the films at about 500°C for less than 1 h leads to stable layers with negligible residual carbon contents. Compositions near 20 BaO-40 TiO₂-40 SiO₂ showed the best performance.     

三元杂化TiO2-SiO2-POMs催化剂的合成及其在可见光降解罗丹明B中的应用

采用分步溶胶-凝胶法制备的三元杂化催化剂TiO₂-SiO₂-杂多酸(POMs)在可见光降解工业染料罗丹明B中表现出了高效反应活性. 利用时间分辨微波传导(TRMC)和漫反射光谱(DRS)研究了催化剂在可见光区的光催化性能, 实验结果表明: 在三元杂化催化剂内, 二氧化钛和二氧化硅的键合加强了催化剂在可见光区的响应和吸收, 二氧化钛和杂多酸的结合提高了反应活性位(空穴-电子对)的稳定性. 三元杂化催化剂TiO₂-SiO₂-POMs中组分之间的协同效应促进了可见光光催化性能的提高.     

Dielectric and energy storage properties of barium strontium titanate based glass–ceramics prepared by sol–gel method

Ba_0.6Sr_0.4TiO₃ based glass–ceramics were prepared by sol–gel process. Influences of B–Si–O glass content on the microstructure, dielectric, and energy storage properties of the BST based glass–ceramics have been investigated. Perovskite barium strontium titanate phase was found at annealing temperature 800 °C. A secondary phase Ba₂TiSi₂O₈ was detected and lowered by declining the mole ratio of element Si (from 50 to 25 mol%) in glass additive. Microstructural observation indicated that the microstructure homogeneity can be improved by glass addition till 2 mol%, while worsened by excessive glass concentrations. Due to relatively homogeneous microstructure, the maximum discharged energy density and breakdown strength were also obtained in samples with 2 mol% glass additive, which were found to be 0.553 J/cm³ and 43.2 kv/mm, respectively. Microscopic observation of the breakdown area was performed and the mechanical failure, including the formation and accumulation of micro-cracks during the dielectric breakdown process, was considered to be the main cause of dielectric breakdown. Results of the charging and discharging energy densities show that the BST based glass–ceramics prepared by sol–gel method has a potential for pulse power applications.     

Kinetic study of surface nucleated MgO-CaO-Al2O3-SiO2 glasses

The paper investigated, by means of thermal analysis measurements, the validity of three different methods for the evaluation of the activation energy of crystallization for surface nucleated glasses belonging to the MgO-CaO-Al₂O₃-SiO₂ system, separating anorthite and diopside crystals. The values obtained from Kissinger equation of the activation energy for crystallization are comparable with those obtained by other authors for similar glassy systems, 90 to 150 kcal/mol. The results of the kinetic analysis using single-crystallization-peak method should be corrected taking into account the dimensionality of crystal growth.The authors thank the M.U.R.S.T. for financial support.     

掺杂Fe2O3对吸附相反应技术制备TiO2的形貌和光催化性能的影响

利用吸附相反应技术制备得到了掺杂不同浓度的Fe₂O₃的TiO₂复合光催化剂。通过透射电子显微镜(TEM)、紫外可见光谱和X射线衍射(XRD)研究不同掺杂浓度对TiO₂形貌和结晶过程的影响,并利用3种波长光源下的甲基橙光降解实验考评了各个复合光催化剂的催化活性。结果表明,掺杂后复合光催化剂中Fe₂O₃分散性较好较均匀。在TiO₂紫外可见吸收光谱中由于Fe₂O₃的掺杂而出现了红移,而且随着掺杂浓度增加红移越来越明显,复合光催化剂的禁带宽度也越来越小。在焙烧过程中无定形Fe₂O₃或Fe³⁺进入了TiO₂的晶格结构,从而抑制了TiO₂的结晶过程。半导体禁带宽度的减少以及TiO₂结晶过程的抑制作用,都导致紫外光下复合光催化剂催化活性的降低。但Fe₂O₃的掺杂也使得复合光催化剂在可见光区出现了一定的光催化活性。     

V2O5-TiO2复合半导体光催化材料结构及光响应性能研究

采用溶胶凝胶法制备了V₂O₅-TiO₂复合半导体材料，通过Raman、XRD及UV-Vis DRS等实验方法研究了V₂O₅与TiO₂复合对材料表面组成、晶体结构以及光响应性能的影响。结果表明：钒加入后优先与TiO₂作用形成较为稳定的金红石型TiVO₄晶相，其中V⁴⁺是促进TiO₂发生相变的关键；随着钒加入量的增加，V₂O₅由表面高分散状态逐渐聚集形成晶相，并释放部分Ti⁴⁺使之形成锐钛矿型TiO₂晶相，使得体相中金红石型TiO₂的含量有所下降；复合后形成的TiVO₄晶相显著提高了材料对可见光的吸收率，并使其吸光域红移至460 nm左右。     

Effects of the preparation methods on the properties of Ni-La2O3-SiO2 catalysts for m -dinitrobenzene hydrogenation

Ni-La₂O₃-SiO₂ catalysts were prepared by wetness impregnation and sol-gel method followed by conventional drying and supercritical drying, respectively. Their physico-chemical properties and activity for the hydrogenation of m-dinitrobenzene to m-phenylenediamine were investigated by BET, XRD, TPR, H₂-TPD and activity tests. The results showed that the structural and catalytic properties of the Ni-La₂O₃-SiO₂ catalysts obviously depended on the preparation method and the drying mode. The catalyst prepared by the sol-gel method in combination with conventional drying exhibited the highest catalytic activity among the catalysts tested, attributable to its well-dispersed nickel particles and larger active nickel surface area.     

Kinetic study of zirconia crystallization from amorphous ZrO2-SiO2 composite precursors processed by sol-gel chemistry

Powder precursor gels with composition xZrO₂·(100–x)SiO₂, with selected values of x=8, 24, 43 and 75 mol%, were processed by sol-gel chemistry. Differential thermal analysis (DTA) was used to study crystallization in (cubic/tetragonal)-ZrO₂ during the heating of the reactive amorphous precursors. Kinetic parameters such as activation energy, Avrami's exponent and frequency factor have been simultaneously calculated from the computed DTA data using a previously reported kinetic model. The crystallization temperature decreases relative to the increase in the amount of ZrO₂, the value of the kinetic parameter of the crystallization being related to the value of x.     

Thermal behavior of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> mesoporous gels

Summary Titania-based photocatalytic materials were prepared by sol-gel method using Fe³⁺ and polyethyleneglycol (PEG₆₀₀) as additives. Thermogravimetry (TG), differential thermal analysis (DTA) and evolved gas analysis (EGA) with MS detection were used to elucidate processes that take place during heating of Fe³⁺ containing titania gels. The microstructure development of the Fe₂O₃/TiO₂ gel samples with and without PEG₆₀₀ admixtures was characterized by emanation thermal analysis (ETA) under in situ heating in air. A mathematical model was used for the evaluation of ETA results. Surface area and porosity measurements of the samples dried at 120&deg;C and the samples preheated for 1 h to 300 and 500&deg;C were compared. From the XRD measurements it was confirmed that the crystallization of anatase took place after thermal heating up to 600&deg;C.