Macroporous Morphology of the Titania Films Prepared by a Sol-Gel Dip-Coating Method from the System Containing Poly(Ethylene Glycol). III. Effect of Chemical Additives

The variation of macroporous morphology has been studied for the titania (TIO₂) films prepared by a sol-gel dip-coating method from the system containing poly(ethylene glycol) (PEG) under the addition of various types of organic solvents. The influence of externally-added solvent is interpreted by considering the compatibility between PEG and the solvent mixture, the volatility of the solvent mixture, and the polycondensation rate of titania oligomers. The macroscopic domain formation is remarkable when the compatibility between the solvent mixture and PEG is relatively poor and the boiling point of external solvent is relatively low.     

Macroporous Morphology of the Titania Films Prepared by a Sol-Gel Dip-Coating Method from the System Containing Poly(Ethylene Glycol). I. Effect of Humidity

The influence of humidity upon the macroporous morphology of the titania films has been studied for a sol-gel dip-coating system containing poly(ethylene glycol) (PEG). The water adsorption from the ambient atmosphere modifies the polycondensation rate of TiO2 oligomer and the phase separation rate between the TiO2-PEG complex and solvent mixture, and greatly varies the macroscopic morphology of the resultant TiO2 film.     

Macroporous Morphology of the Titania Films Prepared by a Sol-Gel Dip-Coating Method from the System Containing Poly(ethylene glycol): Effects of Molecular Weight and Dipping Temperature

Macroporous morphology has been examined for the titania (TiO₂) films prepared by a sol-gel dip-coating method from the system containing poly(ethylene glycol) (PEG) under the variations of molecular weight of PEG and dipping temperature. The macroporous morphology is observed only within the limited range of molecular weight because the compatibility between PEG and the solvent mixture is quite sensitive to the molecular weight. The macroscopic domain formation is enhanced at higher withdrawal speed when the dipping temperature is raised, suggesting that the fluidity reduction and the phase separation are more accelerated than the polycondensation due to the enhanced solvent evaporation.     

Sol-Gel Derived Macroporous Titania Films Prepared from the System Containing Poly(ethylene glycol) Together with Long-Chain Alcohols

The effect of alcohol-type on macroscopic morphology has been examined for the macroporous titania (TIO₂) films prepared by a sol-gel dip-coating method from the system containing poly(ethylene glycol) (PEG) and alcohols having larger alkyl groups than ethanol. The phase separation is enhanced by decreasing the compatibility between the solvent mixture and PEG as long as the polycondensation rate does not increase greatly. In the 1-butanol system, macroporous morphology appears clearly even at higher relative humidity as 70% RH due to the slower polycondensation. In contrast, the macroscopic domain formation is completely suppressed by the rapid polycondensation in the 2-propanol system.     

Macroporous Morphology of the Titania Films Prepared by a Sol-Gel Dip-Coating Method from the System Containing Poly(Ethylene Glycol). II. Effect of Solution Composition

The influences of water and alkoxide concentrations, as well as the relative humidity upon the macroporous morphology of the titania (TiO2) films have been studied for a sol-gel dip-coating system containing poly(ethylene glycol) (PEG). The distribution of resultant morphology against the withdrawal speed and relative humidity is varied significantly by mainly modifying the polycondensation and phase separation processes during the dipping operation, even by the small change in starting composition of the dipping solution.     

Morphology and photoluminescence study of titania nanoparticles

Titania nanoparticles are prepared by sol–gel chemistry with a poly(ethylene oxide) methyl ether methacrylate-block-poly(dimethylsiloxane)-block-poly(ethylene oxide) methyl ether methacrylate triblock copolymer acting as the templating agent. The sol–gel components—hydrochloric acid, titanium tetraisopropoxide, and triblock copolymer—are varied to investigate their effect on the resulting titania morphology. An increased titania precursor or polymer content yields smaller primary titania structures. Microbeam grazing incidence small-angle X-ray scattering measurements, which are analyzed with a unified fit model, reveal information about the titania structure sizes. These small structures could not be observed via the used microscopy techniques. The interplay among the sol–gel components via our triblock copolymer results in different sized titania nanoparticles with higher packing densities. Smaller sized titania particles, (∼13–20 nm in diameter) in the range of exciton diffusion length, are formed by 2% by weight polymer and show good crystallinity with less surface defects and high oxygen vacancies.     

Low‐Temperature Route to Crystalline Titania Network Structures in Thin Films

A low temperature route to crystalline titania nanostructures in thin films is presented. The synthesis is performed by the combination of sol‐gel processes, using a novel precursor for this kind of application, an ethylene glycol‐modified titanate (EGMT), and the structure templating by micro‐phase separation of a di‐block copolymer. Different temperatures around 100 °C are investigated. The nanostructure morphology is examined with scanning electron microscopy, whereas the crystal structure and thin film compositions are examined by scattering methods. Optoelectronic measurements reveal the band‐gap energies and sub‐band states of the titania films. An optimum titania thin film is created at temperatures not higher than 90 °C, regarding sponge‐like morphology with pore sizes of 25–30 nm, porosity of up to 71 % near the sample surface, and crystallinity of titania in the rutile phase. The low temperature during synthesis is of high importance for photovoltaic applications and renders the resulting titania films interesting for future energy solutions.     

Effects of Ti Addition on Alumina/Titania Composite Aerogels Synthesized by Sol–Gel Process and Supercritical Ethanol Drying

Alumina/titania composite aerogels with different titania contents were synthesized by the sol–gel process and supercritical ethanol drying. The structures and morphologies of synthesized aerogels were analyzed by X‐ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetry, and N₂ adsorption–desorption tests. Supercritical ethanol drying induced the crystallization of titania, which prompted the transformation of the structure from pseudoboehmite to γ‐Al₂O₃ . Reversely, alumina retarded the anatase‐to‐rutile transformation of titania. The content of titania significantly affected the structure and morphology of alumina/titania composite aerogels. A high content of titania (≥40%) resulted in the phase separation of titania particles, which grew to form the anatase phase octahedral particles with well‐developed facets. When the titania content was low, titania particles could be homogeneously dispersed in alumina particles to form spherical clusters with the poor crystallinity. Titania particles were in the anatase phase, and no rutile phase was formed until the temperature rose to 1000°C. In addition, titania addition resulted in a decrease in the specific surface area (SSA) of alumina aerogels because the SSA of titania was lower than that of alumina aerogels.     

Morphology and dispersion control of titania–silica monolith with macro–meso pore system

Macroporous gels with bicontinuous morphology in micrometer range were prepared in a titania?Csilica system containing 5 and 7.6 mass?% titania using tetraethoxysilane and four kinds of Ti precursors, two titanium alkoxides, titanium chloride and titanium sulfate, under coexistence of poly(ethylene glycol) (PEG) with an average molecular weight of 20,000. In all the systems with different Ti precursors, the addition of PEG induced phase separation, and the macroporous morphology was formed when the transitional structure of phase separation was frozen-in by sol?Cgel transition of inorganic components. However, we can see large differences in phase separation tendency and Ti dispersion in silica network depending on the Ti precursors used. When titanium alkoxides were added into pure silica sol?Cgel system, phase separation tendency largely decreased, so that low temperature reaction was necessary for macropore formation. When we used titanium salts, on the other hand, phase separation tendency does not change much from pure silica system. The difference has been tentatively attributed to the difference in the mixing level of Ti in silica network. Although titania tended to aggregate when titanium alkoxides were used as precursors, Ti could be well dispersed in silica gel matrix when acetylacetone was added in the alkoxide system or when titanium salts were used as Ti precursors.     

Low Temperature Preparation of Titania Coated PAN Fiber and Its Photocatalytical Property

Titania thin film was deposited successfully on polyacrylonitrile (PAN) fiber by the sol‐gel process with the assistance of tetraethyl silicate (TEOS) at low temperature. It was found that the densification and crystallization of the film was a result of the post‐treatment in boiling water because of the hydrolysis of the Si‐O‐Ti bonds and dissolution of the silica component formed in the film. XRD patterns revealed the existence of anatase phase in the continuous titania layer. The product, titania coated polyacrylonitrile fiber (TiO₂/PAN), showed a high photocatalytic property and good repetition on the photodegradation of methylene blue (MB). The proposed method is expected to be used for the preparation of novel photo‐catalysts based on thermally sensitive substrates.     

Sol–gel titania coating on unmodified and surface-modified polyimide films

Sol–gel coating of metal oxides on polymer substrates is a useful process to fabricate various organic–inorganic hybrid materials under mild conditions. However, this process is hardly applicable to pristine polyimide (PI) films because their surfaces do not display effective functional groups for metal oxide coatings. In this study, we firstly examined direct sol–gel coating of titania thin layers on unmodified PI film surfaces. The results confirmed homogeneous, ultrathin titania layer coating and showed that the thickness and microscopic morphology of the titania layers were affected by titanium alkoxide concentrations in the spin coating solutions. We next investigated titania layer coating on surface-modified PI films that prepared using alkaline hydrolysis, which generated carboxylic acid groups on the film surfaces. Optimal hydrolysis time was determined using FT-IR spectroscopy and contact angle measurements. After sol–gel titania coating on the hydrolyzed PI film surfaces, the Scotch tape test was conducted to evaluate adhesion strength between the titania layers and PI film surfaces. Morphological observations of the sample surfaces after the tests clearly showed that surface modification of PI films increased titania layer adhesions. Effect of hydrothermal treatments on film formability and adhesion strength between titania-PI film interfaces was also evaluated.     

Morphology of polyacetylene produced in the presence of the soluble catalyst Ti(OnBu)4-n-BuLi

When the reaction parameters for the polymerization of acetylene in the presence of Ti(OnBu)₄-n-BuLi as catalyst are altered, the macroscopic morphology of the polymer (CH), is also changed, to an extent that is mainly dependent on the concentration of the catalyst. At higher catalyst concentrations, polymerization occurs on the free surface of the catalyst to form a polyacetylene film which retards further polymerization because diffusion of acetylene monomer through the film is thus restricted. At lower catalyst concentrations, however, successive diffusion and polymerization can occur, and yield a gelatinous mass. At extremely low concentrations, the product is instead a powder. Despite these differences in macroscopic morphology, scanning electron microscopy reveals that all forms of polyacetylene are fibrillar in structure. However, the diameter of the fibrils, which is 60 Å on average, may depend on the polymerization conditions.     

Sol–gel titania coating on NiTi alloy with a porous titania film as interlayer

Porous titania film is prepared by alkali treatment of NiTi alloy followed by soaking treatment in HCl solution. The benefit of this porous titania film as an interlayer to improve adhesion and integrity of the sol–gel titania coating on NiTi alloy substrate is evidenced by surface morphological observations. X-ray diffraction analyses indicate the formation of Ni₄Ti₃ phase in the matrix during heat treatment of the NiTi samples. X-ray photoelectron spectroscopy results indicate that the titania coating with two dip-coating layers has completely covered the NaOH–HCl treated NiTi substrate, and potentiodynamic polarization tests show that this titania coating provides good protection for the treated NiTi substrate in 0.9% NaCl solution. Ultraviolet illumination can increase surface hydrophilicity of the NiTi samples by reducing contact angles from 60–80° to 20–10°.     

A synthetic hydrogel with thermoreversible gelation,III: an NMR study of the sol–gel transition

The sol–gel transition mechanism of a thermoreversible hydrogel composed of a copolymer comprising poly(N-isopropylacrylamide) and poly(ethylene glycol) (PNIPAAm–PEG) was studied by NMR. The ¹H– and ¹³C–NMR spectra measured on a PNIPAAm–PEG solution in 99.9% D₂O showed a remarkable line width broadening of the PNIPAAm block of more than that of the PEG block, during thermally induced hydrogel formation. This result suggested that the mobility of the PNIPAAm block is more restricted than that of the PEG block during gelation. A crosslinked polymer network formation was ascertained by a sudden reduction in the spin-lattice relaxation time (T₁) of the residual HDO proton during gelation. The temperature dependency of the T₁ values for the PNIPAAm and PEG blocks revealed that the microscopic condition of the PNIPAAm block in water was drastically changed during gelation, while that of the PEG block was unchanged. The experimental results from NMR supported the following gelation mechanism; that an aggregation of PNIPAAm blocks in the separate copolymers caused by hydrophobic interaction forms crosslinking points to give an infinite three-dimensional network structure. The hydrated PEG chains in the copolymers provide the network with a swelling property in water, and prevent the aggregation from causing a macroscopic phase separation.     

Fabrication and characterization of TiO2 photocatalytic thin film prepared from peroxo titanic acid sol

A novel sol–gel technique using the PTA (peroxo titanic acid) sol as precursor for the fabrication of TiO₂ photocatalytic thin film is introduced in this paper. The peroxo titanic acid sol was synthesized from titanyl sulfate (TiOSO₄), ammonia and peroxide solution (H₂O₂). The transparent and porous TiO₂ thin film was prepared via a sol–gel technique using PTA sol and polyethylene glycol (PEG) as precursor and template, respectively. The TiO₂ thin film samples were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible spectrophotometry (UV–vis), X-ray photoelectron spectrum (XPS) and thermogravimetry and differential thermal analysis (TG-DTA) technique. The PTA sol displayed amorphous TiO₂ below 100 °C. The anatase phase formed at 200 °C to 700 °C. The crystallinity of anatase phase was improved with increasing temperature. The anatase crystals on the surface of TiO₂ film were strip-like, the size being about 100 nm in length and 40 nm in diameter. Addition of PEG to the PTA sol developed porous structures in the film and changed the size and shape of the particles. The surface of the film contained Ti, O and C elements and Na element that diffused into the film from the glass substrate. The photocatalytic performance of TiO₂ film was tested for the degradation of 10 mg/L methyl orange. The degradation of methyl orange solution reached 98.9% after irradiated for 180 min under UV light. The porous TiO₂ thin film exhibited high photocatalytic activity towards degrading methyl orange.     

Preparation and phase transition characterization of VO2 thin film on single crystal Si (100) substrate by sol–gel process

Vanadium dioxide (VO₂) thin films were fabricated on single crystal Si (100) substrates by sol–gel method, including a process of annealing a vanadium pentoxide (V₂O₅) gel precursor at different temperatures. The crystalline structure and morphology of the films were investigated by XRD, FE-SEM and AFM, indicating that the films underwent the grain growth, agglomeration and grain refinement process with increased annealing temperatures. The film annealed at 500 °C exhibits the formation of VO₂ phase with a strong (011) preferred orientation and high crystallinity, the surface of the film is uniform and compact with a grain size of about 120 nm. Meanwhile, the film exhibits excellent phase transition properties, with a decrease of transmittance from 35.5 to 2.5% at λ = 25 μm and more than 3 orders of resistivity magnitude variation bellow and above the phase transition temperature. The phase transition temperature is evaluated at 60.4 °C in the heating transition and 55.8 °C in the cooling transition. Furthermore, the phase transition property of the VO₂ film appears to be able to remain stable over repetitive cycles 100 times.     

Morphology and properties of semi-IPNs of polyetherimide and bisphenol A dicyanate

Semi-interpenetrating polymer networks (semi-IPNs) were synthesized from mixtures of polyetherimide (PEI) and bisphenol A dicyanate (BPACY) at different compositions and different cure temperatures. The phase separation behavior during cure was analyzed in terms of glass transtion temperature (T_g) behavior of fully cured semi-IPNs and the morphology–property relationship was also studied. The mixtures of PEI and BPACY monomer showed upper critical solution temperature behavior and their semi-IPNs showed sea-island morphology in 1–14 wt% PEI composition, dual-phase morphology in 15–19 wt% PEI composition and nodular morphology in 20–60 wt% PEI composition, respectively. The sea-island morphology was formed via nucleation and growth, while the other morphologies were predominantly formed via spinodal decomposition. Cure temperature did not influence the macroscopic morphology, but the domain size changed with temperature. As cure temperature was increased, the PEI domain size in the sea-island morphology decreased, while the BPACY nodule size increased in the nodular morphology. Mechanical and thermal properties were so strongly dependent upon the morphology that they changed dramatically near the phase inversion point.     

Characterization of the pore-surface gel phase in functionalized macroporous polymeric materials

Covalently immobilized pore-surface gel phases were prepared in a functionalized macroporous ultra-high-molecular-weight polyethylene by covalent coupling of lightly cross-linked polymer colloid particles [50% styrene, 49.8% (chloromethyl)stryrene, 0.2% divinylbenzene] to the interstitial pore surfaces. Swelling the covalently coupled colloid particles in a good solvent followed by chemical derivitization resulted in an immobilized pore-surface gel phase rich in primary amine groups. The macromolecular reactivity and molecular size-exclusion characteristics of the aminated pore-surface gel phase were then determined using monofunctional, amine-reactive, poly (ethylene glycol)s (PEG). Pegylated pore-surface gel phases that ranged from 71% (10,000 molecular weight PEG) to 56% (40,000 molecular weight PEG) PEG by weight resulted from reaction of the aminated gel phase with the PEG probe molecules. The number of PEG molecules reacting with the aminated pore-surface gel phase depends only on the Flory radius (or radius of gyration) of the PEG molecule to the negative 2.49th power i.e., 1/R _f ^2.49, corresponding to a M^−1.48 dependence. The immobilized and pegylated polymer colloid particles swell by a factor of 16–25 times the diameter of the original polymer colloid particles in water, thereby demonstrating that pegylation occurred though a substantial fraction of the volume of the immobilized colloid particles. Received: 18 January 1999 Accepted in revised form: 8 June 1999     

Electrochromic and photoelectrochromic properties of sol–gel derived tungsten trioxide/titania composite thin films

Tungsten trixoide/titania (WO₃-titania) composite thin films with W/Ti molar ratios of 100/0, 98/2, 96/4, 94/6 92/8 and 90/10 were prepared on fluorine-doped tin oxide conducting glass, and their electrochromic (EC) and photoelectrochromic (PEC) performances were investigated in this study. The composite thin films were synthesized by sol–gel process using peroxotungstic acid and titanium (IV) n-butoxide as the precursors. The surface morphology and composition of the composite thin films were characterized using scanning electron microscope with energy dispersive spectrometer. Electrochemical experiments with in situ spectroscopic measurement were employed to study the EC properties of the composite thin films. It was found that the presence of titania in the WO₃ matrix might slightly decreases its EC performance. PEC cells using the composite thin films as the working electrode and a sputtered semitransparent platinum thin film on ITO as the counter electrode were fabricated and their PEC performances were investigated. The device using composite thin film prepared from sol solution with a W/Ti molar ratio of 96/4 exhibited the best PEC performance.     

Preparation of MEH‐PPV/nanosized titania hybrids via in situ sol–gel reaction of titanium alkoxide: Optical property

Homogenously dispersed organic (MEH‐PPV)/inorganic (nanosized titania) hybrids were successfully synthesized. The method of preparation was based on a simple one‐step in situ sol–gel technique using titanium isopropoxide (TIP) as the precursor. The key benefit of this preparation was that TIP interacted with both 2‐chlorophenol and MEH‐PPV, so that the degree of aggregation and phase separation could be kept to a minimum with a suitable recipe. MEH‐PPV/TIP/H₂O/2‐chlorophenol of various weight ratios were synthesized to examine the morphology as well as optical properties of the MEH‐PPV/TIP(titania) hybrid. The observation of MEH‐PPV gelation and Fourier transform infrared results verified the interaction existing between MEH‐PPV and TIP. SEM photographs showed that TIP(titania) were homogenously dispersed in the MEH‐PPV film if the hybrid solution was clear from the use of a suitable recipe. UV–vis absorption measurements showed that the addition of TIP decreased the conjugation length of MEH‐PPV. A redshift in the photoluminescence (PL) emission was observed in almost all the hybrids in the solution state, because of the aggregation of MEH‐PPV. However, it was found that spinning destroyed the aggregation of MEH‐PPV, resulting in a blueshift in the PL emission of the hybrids. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 515–529, 2008