Effect of calcination temperature and addition of silica, zirconia, alumina on the photocatalytic activity of titania

Nanophase titania was prepared by sol-gel method and spray pyrolysis. We tried to elucidate the relationship between the photoactivity and the crystallite size of anatase phase. To better understand the changes in the bulk and the surface of titania as the calcination temperature is changed, EPR and photoluminescence analysis were carried out. The effect of the secondary metal oxide embedded into titania matrix on the photoactivity was also investigated. It was found that the photoactivity of titania has a linear relationship to the crystallite size. For the analysis of EPR and photoluminescence for pure titania, the increase of photoactivity with increasing the calcination temperature is due to the formation of surface active sites such as O^- as well as the increase of crystallinity resulting from the removal of bulk defects. For silica/titania mixed oxide, it was found that the improvement of the thermal stability of anatase phase is important to enhance the photoactivity of titania because the prepared catalyst was calcined at a higher temperature than 700 °C without forming rutile phase. It was also concluded that the simultaneous increase of the surface area and the crystallinity promises to improve the photoactivity achieved by increasing the content of silica up to 60%. By the analysis of EPR and photoluminescence, it was found that the embedding of silica into titania matrix suppresses the formation of Ti³⁺ and produces a new active site of Ti-O-Si, which easily interacts with the oxygen. In the investigation of zirconia/titania and alumina/titania mixed oxide, it was found that the increase of the surface OH is essential to positively affect of the improved thermal stability on the photoactivity.     

Local structure and photocatalytic activity of B2O3–SiO2/TiO2 ternary mixed oxides prepared by sol–gel method

The local structure and the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides (SiO₂ content was fixed as 30 at.% with respect to TiO₂) was investigated by using XRD, FT-IR, BET, UV-vis spectra, and electron paramagnetic resonance (EPR) measurement. In FT-IR analysis, boron was incorporated into the framework of titania matrix with replacing Ti---O---Si with Si---O---B or Ti---O---B bonds. Also, paramagnetic species such as O⁻ and Ti³⁺ defects were formed by the boron incorporation. In SiO₂/TiO₂ mixed oxides, a blue shift in the light absorption band was observed due to the quantization of band structure. All B₂O₃–SiO₂/TiO₂ samples had pure anatase phase and no rutile phase was formed even though the calcination temperature was over 900 °C. Incorporating boron oxides of more than 10% enlarges the grain size of anatase phase and causes a red shift of the light absorption spectrum. The surface area was monotonically decreased with increasing the content of boron content. As a result, the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides was greatly influenced by the content of boron oxide. The highest photoactivity (g moles/min l) was obtained when the boron content was 5% and seven times higher than that of silica/titania binary mixed oxide. In addition, the specific photoactivity (g moles/m² l) was maximum still at 5%. It was concluded that the large reduction of surface area, the change of band structure, and more formation of bulk Ti³⁺ sites are responsible for the deterioration in the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides when the content of boron is over 10%, although their crystallinity was enhanced by increasing the calcination temperature with keeping anatase phase.     

Control of Phase and Pore Structure of Titania Powders Using HCl and NH4OH Catalysts

Porous titania powders were prepared by hydrolysis of titanium tetraisopropoxide (TTIP) and were characterized at various calcination temperatures by nitrogen adsorption, X-ray diffraction, and microscopy. The effect of HCl or NH₄OH catalysts added during hydrolysis on the crystallinity and porosity of the titania powders was investigated. The HCl enhanced the phase transformations of the titania powders from amorphous to anatase as well as anatase to rutile, while NH₄OH retarded both phase transformations. Titania powders calcined at 500°C showed bimodal pore size distributions: one was intra-aggregated pores with average pore diameters of 3–6 nm and the other was interaggregated pores with average pore diameters of 35–50 nm. The average intra-aggregated pore diameter was decreased with increasing HCl concentration, while it was increased with increasing NH₄OH concentration.     

The influence of preparation method on the physicochemical properties of titania–silica aerogels: Part two

Titania–silica aerogels with different titania content were prepared. Four preparation methods differing mainly in approach to precursors hydrolysis were applied, while only three of them allowed total hydrolysis of silica precursor before titania precursor was added. The preparation of mixed products of titania and silica hydrolysis precursors containing gels was followed by high temperature supercritical drying (HTSCD) and thermal treatment at 500 °C. Obtained mixed oxides in form of aerogels were characterized by BET surface areas up to 1000 m²/g, mesopore volumes up to 1.6 cm³/g and bulk densities as low as 0.04 g/cm³. Even 18 h lasting aging did not allow to produce narrow diameter range mesoporous materials, their broad pore diameter distributions resulted in average pore sizes varying from 10 to nearly 30 nm. XRD measurements proved the presence of anatase crystalline form of titania, while silica was present in amorphous form. SEM studies indicated presence of isolated titania particles on titania–silica surface while joint hydrolysis method was applied. Titania–silica aerogels obtained by the simultaneous hydrolysis of precursors and the impregnation method showed high photocatalytic activity in degradation of salicylic acid in water. Activities of these mesoporous photocatalysts were higher than commercial P25 Degussa TiO₂. Comparison of activity of pure TiO₂ (P25 Degussa) and aerogels indicates higher utilization of titania present in mesoporous mixed oxides.     

Characterization of spherical silica–titania mixed oxide particles synthesized by using a dry process

Silica–titania mixed oxides have excellent properties, such as a low thermal expansion coefficient and a refractive index that can be adjusted by changing the Ti content. However, when the Ti content increases, silica and titania phases in silica–titania mixed oxides can separate. This phase separation leads to the precipitation of the titania component as rutile or anatase crystals. When silica–titania mixed oxides undergo phase separation, their properties become unstable; for example, the refractive index of the particles becomes non-uniform. Therefore, it is preferable to synthesize silica–titania mixed oxides in an amorphous state without causing phase separation. Based on our previous studies on particle size control in silica synthesis, we employed a dry process using organosilicon compounds to synthesize silica–titania mixed oxides. In this study, spherical amorphous silica–titania mixed oxide particles were obtained via flame synthesis using organosilicon and organotitanium compounds. The purpose of this study was to characterize the obtained powder and explore the possibility of controlling particle size during synthesis. By studying the dry process synthesis of spherical silica–titania mixed oxide particles, we confirmed the relationships: between the Si/Ti molar ratio and the obtained crystal structure and between the adiabatic flame temperature and the particle size.     

INFLUENCE OF CALCINATION ON MICROSTRUCTURES AND PHOTOACTIVITIES OF ALKOXIDE-DERIVED TiO2 NANOPARTICLES PREPARED IN W/O MICROEMULSIONS

Ultrafine titania particles were synthesized by hydrolysis of titanium tetraisoproxide (TTIP) in the nanodroplets of water/NP-5/cyclohexane microemulsions. The as-prepared particles were amorphous, transformed into the anatase phase at 450°C, and completely into the rutile phase at 700°C. The amorphous-to-anatase phase transition temperature decreased with increasing water/surfactant molar ratio. With increasing temperature from 500 to 900°C, the crystallite size increased about twice from 11.7 to 24.4 nm, while the size of the secondary particles, agglomerates of the primary panicles, increased by a factor of about 10. The particles grew largely by intra-agglomerate densification below 700°C, whereas they grew by interagglomerate densification above 700°C. The anatase phase formed at 500-600°C showed considerable photoactivity for the degradation of phenol, whereas both the amorphous phase at 300°C and the rutile phase at 700°C were almost inactive for this reaction.     

环氧树脂/稻壳SiO2纳米复合材料的热性能及拉伸性能研究

将稻壳用酸处理后在600 ℃焚烧得到纯度为99.3%、比表面积为212 m2/g的SiO2。经硅烷偶联剂γ-氨丙基三乙氧基硅烷（KH550）改性后的SiO2为无定形态,尺寸在30~50 nm之间。将改性后的稻壳SiO2与环氧树脂复合,利用热分析方法考察了纳米复合材料在N2气氛中的热性能,并采用万能材料试验机测试其拉伸性能。结果表明：稻壳SiO2的加入能有效增加环氧树脂/稻壳SiO2纳米复合材料的热稳定性,复合材料的起始分解温度（Ti）、分解速率最大时的温度（Tmax）以及失重50 %的分解温度（T50 %）均高于纯环氧树脂,并随稻壳SiO2含量的增加而增加。当环氧树脂/稻壳SiO2纳米复合材料的组成相同时,KH550改性的复合材料的Ti、Tmax和T50 %均比未经过KH550改性的高。随KH550用量增加,复合材料T50 %向高温方向移动。此外,复合材料的拉伸强度、断裂伸长率和模量也高于纯环氧树脂。     

Composite powders with titania grafted onto modified fumed silica

A series of powder samples with partially silylated fumed silica A-380 (content of trimethylsilyl (TMS) groups C_TMS = 0.09-0.79 mmol/g corresponding to the substitution degree of 9.6-84.0%) was modified by grafting of CVD-titania (titanium content C_Ti = 1.1-1.8 wt.%) (TiCl₄ as a precursor). Pure fumed silicas A-380 and A-50 were also modified by grafted titania using Ti[OCH(CH₃)₂]₄ (TTIP) as a precursor deposited from isopropyl alcohol, dried and then heated at 1073 K. Interaction of TTIP with A-380 was also studied in the presence of poly(vinyl pyrrolidone), PVP. Unmodified and modified silicas and titania/silicas were studied using adsorption, FTIR, NMR, AFM, TPD-MS, TG, and XRD methods. The size and the crystallinity of grafted titania particles depend on the type of matrix (A-380 and A-50), reaction conditions, presence of the TMS groups or adsorbed PVP, and subsequent treatment. In the case of A-380 with more strongly aggregated primary particles the size of titania particles is smaller and their crystallinity is much lower than that of titania deposits on A-50. Adsorption of water increases on CVD-TiO₂/silylated silica in comparison with silylated silica at high C_TMS values.     

Preparation and characterization of surface bond-conjugated TiO2/SiO2 and photocatalysis for azo dyes

Surface bond-conjugated TiO₂/SiO₂ was prepared by means of the impregnation method. Based on the results of XRD, FTIR, XPS and BET measurements, the growth of titania (predominantly anatase) on the silica substrate seems to occur by anchoring of the TiO₂ phase through Ti–O–Si cross-linking bonds. The structure model of TiO₂/SiO₂ was proposed. Compared to B–TiO₂, the most efficient catalyst is 30 wt.% TiO₂/SiO₂ (Ims30), which showed three times higher photoactivity for the degradation of reactive 15 (R15). In addition, the catalyst had a higher photoactivity on a silica of smaller particle size than on the silica of larger particles. Silica gel plays the basic role of dispersion and support for power TiO₂. The isoelectric point of the catalyst was 3.0 pH units by the measurement of zeta-potential, indicating the presence of the surface acidity of the catalyst. The photodegradation and the adsorption of R15 and cationic blue X-GRL (CBX) were investigated with the change of initial aqueous pH.     

Densification of Alkoxide-Derived Fine Silica Powder Compact by Ultra-High-Pressure Cold Isostatic Pressing

Powder compacts of alkoxide-derived fine silica powders were consolidated into a highly dense and uniform structure by ultra-high-pressure cold isostatic pressing of granules with controlled structure. The diameters of spherical and nearly monosized amorphous silica particles, prepared from metal alkoxide, were successfully controlled in the range of 9 to 760 nm by varying the concentration of ammonia. Close-packed granules of these powders were produced by spray drying. These powders were isostatically pressed up to 1 GPa at room temperature. Although the average particle diameter was less than 100 nm, the maximum relative density of the compacts was more than 78% of theoretical density. The optimum particle size to obtain highly dense compacts was in the range of 30 to 300 nm at 1 GPa. Furthermore, the ratio of mode pore diameter in these compacts to particle diameter was less than 0.155, which corresponded to the minimum ratio of calculated three-particle pore channel radii for hexagonal close packing. Viscous deformation of particles under ultra-high isostatic pressure played an important role in the densification of the compacts.     

Crystallization and Photoactivity of Tio2 Films Formed on Soda Lime Glass by a Sol-Gel Dip-Coating Process

Transparent nanophase TiO₂ thin films on soda lime glass were prepared from titanium tetraisopropoxide (TTIP) by a sol-gel dip-coating method. The TiO₂ films had amorphous phase up to 400°C and anatase phase at 500°C. The amorphous TiO₂ films obtained at 300-400°C showed considerable photoactivity for the degradation of formic acid. The photoactivity of the TiO₂ films was enhanced with increasing calcination temperature from 300° to 500°C. The crystallinity of the anatase films at 500°C was improved with increasing calcination time up to 2 h and reduced with a further increase in calcination time to 4 h due to the significant formation of sodium titanate phase as a result of sodium diffusion. The four-time-dipping anatase films at 500°C exhibited the greatest photoactivity at the calcination time of 2 h. Sodium diffusion into TiO₂ films was retarded by a SiO₂ underlayer of 50 nm in thickness.     

Influences of heat-treatment on the microstructure and properties of silica–titania composite aerogels

Silica–titania composite aerogels were synthesized via ambient pressure drying by using water glass and titanium tetrachloride as raw materials. The influences of heat-treatment at different temperature with different heating rate on the microstructure and properties of the composite aerogels were investigated by differential thermal analyzer, Fourier transform infrared spectrometer, X-ray diffraction, nitrogen adsorption–desorption, scanning electron microscope and transmission electron microscope analysis. The results indicate that the silica–titania composite aerogels heat-treated at 250 °C exhibited highest specific surface area, pore volume and average pore diameter. When the heat-treatment temperature was higher than 450 °C, the –CH₃ groups on the surface of silica–titania composite aerogels would transform into –OH groups gradually, and in the meantime, the composite aerogels network structure would be destroyed gradually and the crystallinity of TiO₂ would be improved with the increase of heat-treatment temperature. Particularly, heat-treatment at temperatures above 750 °C would cause serious damage to the network structure of the composite aerogels. The adsorption/photocatalytic activity experiments showed that the composite aerogels heat-treated at 550 °C exhibit highest darkroom adsorption efficiency, and the 650 °C-heat-treated samples exhibited highest efficiency for removing the Rhodamine B from water.     

Synthesis of mixed silica–titania by the sol–gel method using polyethylenimine: porosity and catalytic properties

By means of the sol–gel method, hybrid xerogels of silica, titania and silica–titania mixed networks of different composition with polyethylenimine (PEI) were formed. After removal of PEI followed by calcination, mesoporous oxides of high surface area with monomodal and narrow porosity distribution were obtained. The surface area of the mixed oxides decreases with increasing titania content, but pore size remains almost constant when a PEI:(SiO₂ + TiO₂) ratio of 1 is kept constant in the xerogel precursor. By doubling and tripling the proportion of PEI, the surface area increases by 15% and 30%, respectively, but pore size remains constant, indicating the formation of a larger number of PEI domains of similar size in the hybrid. Isopropanol decomposition reactions were carried out using the mixed oxides as catalysts, and it was found that the networks contain Lewis acid sites but lack Brønsted acid sites.     

Effect of filler content on the structure‐property behavior of poly(ethylene oxide) based polyurethaneurea‐silica nanocomposites

Poly(ethylene oxide) (PEO) based polyurethaneurea‐silica nanocomposites were prepared by solution blending and characterized by Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Differential Scanning Calorimetry and tensile testing. The colloidal silica nanoparticles with an average size of 50 nm were synthesized by modified Stöber method in isopropanol. Silica particles were incorporated into three cycloaliphatic polyurethaneurea (PUs) copolymers based on PEO oligomers with molecular weights of 2,000, 4,600, and 8,000 g/mol. Hard segment content of PUs was constant at 30% by weight. Silica content of the PU nanocomposites varied between 1 and 20% by weight. Soft segment (SS) glass transition and melting temperatures slightly increased with increasing filler content for all the copolymers. Degree of SS crystallinity first increased with 1% silica incorporation and subsequently decreased by further silica addition. Elastic modulus and tensile strengths of PU copolymers gradually increased with increasing amount of the silica filler. Elongation at break values gradually decreased in PEO‐2000 based PU copolymer with increasing silica content, whereas no significant change was observed in PUs based on PEO‐4600 and PEO‐8000. Enhancement in tensile properties of the materials was mainly attributed to the homogeneous distribution of silica filler in polymer matrices and strong polymer‐filler interactions. POLYM. ENG. SCI., 58:1097–1107, 2018. © 2017 Society of Plastics Engineers     

Thermally stable and photocatalytically active titania for ceramic surfaces

The most photocatalytically active titania modification anatase must be stabilised to achieve high photocatalytic activity in ceramic processes at temperatures above 1000 °C. Thermally stable TiO₂ powders were prepared by the addition of silica and boehmite nanoparticles and deposited on corundum substrates and lead-free glazes. The powders and coatings were fired at increasing temperatures, and stabilisation of the anatase phase was achieved up to 1200 °C. In general, thermal stability was found to be lower when coated on substrates compared to the powder alone, and the extent of reduction depended on the chemical composition of the substrate. Only a slight modification of the titania electronic structure was found, indicating only weak interactions between silica and titania. Based on these results it is possible to assume an amorphous silica and alumina shell encases the titania particles which prevents grain growth and the anatase to rutile phase transformation.     

Effect of excess silicate on the structure formation and textural properties of MTS materials

The synthesis of micellar templated silica (MTS) materials has been conducted with different SiO₂/CTAB molar ratios for two different cetyltrimethylammoniumbromide (CTAB) concentrations of 0.02 and 0.07 M to examine the structure formation for cubic (SBA-1) as well as for hexagonal (SBA-3) surfactant arrangement in acidic reaction medium with increasing silicate excess. Reaction products were analyzed with X-ray diffraction, N₂ physisorption and TG-MS. It was found that an increase of the silicate concentration leads to the formation of additional silica gel in the resulting MTS material, which causes not only a decrease of specific surface area and pore volume, but also of the mesopore diameter. The latter effect is explained by a proposed embedding theory. Furthermore, with increasing silicate content in the synthesis mixture a phase transformation from hexagonal to cubic pore arrangement was found for the CTAB concentration of 0.07 M and the maximum SiO₂/CTAB molar ratio for ordered pore arrangement has been determined.     

Thermal and Morphological Properties of Polypropylene/Styrene-Butadiene-Styrene Nanocomposites

Polypropylene/styrene-butadiene-styrene fumed silica nanocomposites were mixed in a twin screw Brabender Plasticorder, the weight percent of the SBS was varied at (0, 5, 10, 20, and 40), and the fumed silica content was varied at (0, 0.05, 0.1, 0.5, 1, and 2) wt%. The thermal and morphological properties of these nanocomposites were studied using a differential scanning calorimetry and a polarized light microscopy. It has been found that the addition of styrene-butadiene-styrene or fumed silica nanoparticles alone to polypropylene did not change the melting temperature while the existence of both elements changed the melting temperature of polypropylene slightly. Also, increasing the percentage of fumed silica nanoparticles leads to an increase in the crystallization temperature while styrene-butadiene-styrene increment has no effect. The melting peaks obtained by the differential scanning calorimetry revealed that increasing the styrene-butadiene-styrene content reduces the percentage crystallinity as a result of increasing the amorphous portion. The addition of styrene-butadiene-styrene to polypropylene has a very slight effect on the relative percentage crystallinity when calculations are performed based on the quantity of polypropylene within the composite. Although fumed silica nanoparticles aided and accelerating the nucleation step but it could not increase the percentage crystallinity of polypropylene beyond the percentage crystallinity of neat polypropylene.     

Fabrication of porous titania sheet via tape casting: Microstructure and water permeability study

In this article, we report the effects of slurry formulation and sintering conditions on the microstructure and permeability of porous titania sheets prepared by tape casting. It was found that solid concentration and binder content in the titania slurry play a vital role in the porosity and microstructure of the sintered titania sheets. Solid concentration and binder content were optimized based on the green tape quality and open porosity of the sintered titania sheets. The optimum solid concentration with the lowest surface roughness was obtained at 0.61 g/cm³. The effects of temperature and sintering time on the open porosity and crystal structure of the final product were also investigated. Increasing the sintering temperature from 1000 to 1100 °C resulted in increasing the pore size from 170 to 264 nm and decreasing the open porosity. Finally, water permeability of the porous titania sheets was studied to evaluate the permeation flux and maximum operating pressure. The results revealed that the permeability of the porous titania sheet is increased not only by increasing the open porosity but also by increasing the pore size.     

Preparation and characterisation of bioactive hydroxyapatite–silica composite nanopowders via sol–gel method for medical applications

Abstract

Abstract

This study focuses on preparation and characterisation of hydroxyapatite–silica composite nanopowders with different contents of silica. Hydroxyapatite–silica composite nanopowders with 10, 20, 30 and 40?wt‐% silica were prepared using a sol–gel method at 600°C with phosphoric pentoxide and calcium nitrate tetrahydrate as the source of hydroxyapatite, also tetraethylorthosilicate and methyltriethoxisilane as the source of silica. XRD, FTIR, SEM, EDAX and TEM techniques were used for characterisation and evaluation of the phase composition, crystallinity, crystallite size, functional groups, morphology and composition of the products. Dissolution behaviour of the products was evaluated at predetermined time periods by an atomic absorption spectrometer and a pH meter. Results indicated the presence of nanocrystalline hydroxyapatite phase and amorphous silica nanoparticles in composite nanopowders. Also, by increasing the content of silica in composite nanopowders, the crystallite size and crystallinity of hydroxyapatite phase decreased and the Ca ion release rate changed.     

用于选择性激光烧结成型的聚酰胺6/硫酸钙复合粉体的制备及性能研究

采用溶液沉淀法制备了聚酰胺6/硫酸钙（PA6/CaSO₄）复合粉体,通过扫描电子显微镜（SEM）、差示扫描量热仪（DSC）、激光粒度分布仪和Ｘ射线衍射仪（XRD）对复合粉体的微观形貌、热性能、粒度分布、晶体结构和流动性进行了表征。结果表明,随着CaSO₄的加入,PA6/CaSO₄复合粉体由松散的球形结构转变为密实的球形结构;CaSO₄的加入能够提高PA6/CaSO₄复合粉体的结晶度并拓宽其烧结温度窗口,在CaSO₄含量为5 %（质量分数,下同）时,结晶度和烧结温度窗口均达到最大值,分别为61.84 %和18.95 ℃;PA6/CaSO₄复合粉体的粒径随CaSO₄含量的增加呈现先减后增的趋势,休止角先减后增,堆积密度先增后减;当CaSO₄含量为3 %~5 %时,PA6/CaSO₄复合粉体的流动性能最佳,最适用于选择性激光烧结。