Synthesis and electrorheological characteristics of sea urchin-like TiO<Subscript>2</Subscript> hollow spheres

TiO₂ hollow microspheres with sea urchin-like hierarchical architectures were synthesized by a simple hydrothermal method. The as-synthesized hollow microspheres with hierarchical architectures consisting of many rhombic building units exhibit high specific surface area. Electrorheological (ER) properties of hierarchical hollow TiO₂-based suspension were investigated under steady and oscillatory shear. The hollow TiO₂-based suspensions show much higher yield stress and elasticity than pure TiO₂ suspension at the same electric field strength. This phenomenon was elucidated well in view of their dielectric spectra analysis. The sea urchin-like architectures result in stronger interfacial polarization of hollow TiO₂ suspension upon an electric field, showing higher ER activity. Also, hollow interiors of TiO₂ particles increase the long-term stability of suspensions and further merit the ER effect.     

Electrorheological characteristics of polyaniline/titanate composite nanotube suspensions

In this paper, one-dimensional polyaniline/titanate (PANI/TN) composite nanotubes were synthesized by in situ chemical oxidative polymerization directed by block copolymer. These novel nanocomposite particles were used as a dispersed phase in electrorheological (ER) fluids, and the ER properties were investigated under both steady and dynamic shear. It was found that the ER activity of PANI/TN fluids varied with the ratio of aniline to titanate, and the PANI/TN suspensions showed a higher ER effect than that made by sphere-like PANI/TiO₂ nanoparticles. These observations were well interpreted by their dielectric spectra analysis; a larger dielectric loss enhancement and a faster rate of interfacial polarization were responsible for a higher ER activity of nanotubular PANI/TN-based fluids.     

Morphology and film formation of poly(butyl methacrylate)–polypyrrole core–shell latex particles

 Core–shell latex particles made of a poly(butyl methacrylate) (PBMA) core and a thin polypyrrole (PPy) shell were synthesized by two-stage polymerization. In the first stage, PBMA latex particles were synthesized in a semicontinuous process by free-radical polymerization. PBMA latex particles were labeled either with an energy donor or with an energy acceptor, in two different syntheses. These particles were used in a second stage as seeds for the synthesis of the core–shell particles. The PPy shell was polymerized around the PBMA core latex in an oxidative chemical in situ polymerization. Proofs for the success of the core–shell synthesis were obtained using nonradiative energy transfer (NRET) and atomic force microscopy (AFM). NRET gives access to the rate of polymer chain migration between adjacent particles in a film annealed at a temperature above the glass-transition temperature T _g of the particles. Slower chain migration of the PBMA polymer chains was obtained with the PBMA–PPy core–shell particles compared to rate of the PBMA polymer chain migration found with the pure, uncoated PBMA particles. This result is due to the coating of PBMA by PPy, which hinders the migration of the PBMA polymer chains between adjacent particles in the film. This observation has been confirmed by AFM measurements showing that the flattening of the latex film surface is much slower for the core–shell particles than for the pure PBMA particles. This result can again be explained by the presence of a rigid PPy shell around the PBMA core. Thus, these two complementary methods have given evidence that real core–shell particles were synthesized and that the shell seriously hinders film formation of the particles in spite of the fact that it is very thin (thickness close to 1 nm) compared to the size (750 and 780 nm in diameter) of the PBMA core. Transparency measurements confirm the results obtained by NRET and AFM. When the films are placed at a temperature higher than the T _g of PBMA, the increase in transparency is faster for films made with the uncoated PBMA particles than for films made with the coated PBMA particles. This result indicates again that the presence of the rigid PPy layer around the PBMA core reduces considerably the speed at which the structure of the film is modified when heated above the T _g of PBMA. Received: 02 September 1999 Accepted: 21 December 1999     

Characterization of silicone oil emulsions stabilized by TiO2 suspensions pre-adsorbed SDS

To study the effects of pre-adsorbed emulsifier on Pickering emulsion stability, the preparation of silicone oil emulsions by TiO₂ suspensions pre-adsorbed sodium dodecyl sulfate (SDS) at the fixed TiO₂ concentration of 0.15 g was carried out below a fiftieth of critical micelle concentration (cmc) of SDS, where all added amounts of SDS are adsorbed on the TiO₂ particles. The stability of the Pickering emulsions incorporating TiO₂ suspensions pre-adsorbed SDS was investigated by measuring the volume fraction of emulsified silicone oil, adsorbed amounts of TiO₂ suspensions pre-adsorbed SDS, oil droplet size, and some rheological responses such as the stress-strain sweep curve and strain and frequency dependences of dynamic viscoelastic moduli. The silicone oil was almost emulsified by TiO₂ suspensions pre-adsorbed SDS above cmc/10³. Increasing in the adsorbed amount of SDS on the TiO₂ particles leads to an increase in the adsorbed amounts of TiO₂ suspensions pre-adsorbed SDS. Such silicone oil emulsions for the first time showed two yield stresses in the stress-strain sweep curve as well as the oscillatory stress-strain curve. The respective yield stresses also increase with an increase in the adsorbed amounts of TiO₂ suspensions pre-adsorbed SDS. From such characteristic rheological properties and a partial sedimentation of some TiO₂ particles remained in the dispersion medium, we proposed the formation of a three dimensional network of the flocculated TiO₂ particles pre-adsorbed SDS on the silicone oil droplets.     

TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> hybrid nanomaterials: synthesis and variable UV-blocking properties

Silica and core–shell structured titania/silica (TiO₂/SiO₂) nanoparticles with particles size ranging from tens to hundreds of nanometers were prepared and deposited onto cotton fabric substrates by sol–gel process. The morphologies of the nanoparticles were characterized by field-emission scanning electron microscope (FE-SEM). The photocatalytic decomposition properties as well as UV-blocking properties of the fabrics treated with SiO₂ and TiO₂/SiO₂ nanoparticles were investigated.     

Synthesis and electrorheology of rod-like titanium oxide particles prepared via microwave-assisted molten-salt method

The rod-like titanium dioxide (TiO₂) particles were synthesized by a simple and rapid microwave-assisted molten-salt method. The X-ray diffraction analysis revealed the phase composition transformation from the anatase phase of original TiO₂ nanomaterial to the rutile phase of high crystallinity. Scanning electron microscopy proved the conversion of originally globular particles of original anatase TiO₂ sized from 200 to 500 nm into rods with a length of 5–10 μm and a diameter between 0.5 and 2 μm. The electrorheological (ER) measurements performed under steady-state flow as a function of the applied electric field strength and particle concentration showed that suspended rutile rod-like TiO₂ particle-based fluid exhibits much higher ER activity than that of original anatase TiO₂ material powder. These observations were clearly demonstrated by viewing their dielectric spectra analyses.     

Design of anhydrous electrorheological (ER) suspensions based on I2-doped poly(pyridinium salt)

A rigid-rod aromatic poly(pyridinium salt) was synthesized and doped with iodine (I₂) for making anhydrous electrorheological (ER) fluids. The I₂-doped particles were further processed into ones having insulating skins. Dielectric properties and current densities of the suspensions containing these particles were studied to elucidate the roles of conductivity of the dispersed phase in the ER suspension. © 1994 John Wiley & Sons, Inc.     

Synthesis and electrorheological behavior of sterically stabilized polypyrrole-silica-methylcellulose nanocomposite suspension

Various polypyrrole (PPy)-silica-methylcellulose nanocomposite particles were synthesized by suspension polymerization in the presence of silica nanoparticles controlling the ratio of pyrrole, silica, and methylcellulose during the polymerization. The electrorheological (ER) and dielectric properties of the sterically stabilized PPy-silica-methylcellulose nanocomposite suspensions were investigated. The ER response increases with the increase in the silica/pyrrole ratio. The ER behavior also depends on the methylcellulose amount during the polymerization. The yield stress initially increases with the methylcellulose amount, passes through a maximum, and then decreases with the methylcellulose amount. The dielectric constants and dc conductivities of the PPy-silica-methylcellulose nanocomposite particles and the dielectric properties of their suspensions indicate that the increased ER response arises from the enhanced interfacial and particle polarization which depends on the silica/pyrrole ratio and the methylcellulose amount during the polymerization.     

Electrorheology of aniline oligomers

Aniline oligomers were prepared by the oxidation of aniline with p-benzoquinone in aqueous solutions of methanesulfonic acid (MSA) of various concentrations. Their molecular structures were assessed by Fourier transform infrared spectroscopy. The electrorheological (ER) behavior of their silicone oil suspensions under applied electric field has been investigated. Shear stress at a low shear rate, τ _0.9, was used as a criterion of the rigidity of internal structures created by the application of an electric field. It was established from the fitting of the dielectric spectra of the suspensions with the Havriliak–Negami model that dielectric relaxation strength, as a degree of polarization induced by an external field contributing to the enhanced ER effect, increases and relaxation time, i.e., the response of the particle to the application of the field, decreases when a higher molar concentration of MSA is used. The best values were observed for suspensions of the sample prepared in the presence of 0.5 M of MSA. This suspension creates stiff internal structures under an applied electric field strength of 2 kV mm^?1 with τ _0.9 of nearly 50 Pa, which is even slightly of higher value than that obtained for standard polyaniline base ER suspension measured at the same conditions. The concentration of the MSA used in the preparation of oligomers seems to be a crucial factor influencing the conductivity, dielectric properties and, consequently, rheological behavior, and finally ER activity of their suspensions.     

A Simple Method for the Preparation of TiO2/Ag‐AgCl@Polypyrrole Composite and Its Enhanced Visible‐Light Photocatalytic Activity

A novel and facile method was developed to prepare a visible‐light driven TiO₂/Ag‐AgCl@polypyrrole (PPy) photocatalyst with Ag‐AgCl nanoparticles supported on TiO₂ nanofibers and covered by a thin PPy shell. During the synthesis, the PPy shell and Ag‐AgCl nanoparticles were prepared simultaneously onto TiO₂ nanofibers, which simplified the preparation procedure. In addition, because Ag‐AgCl aggregates were fabricated via partly etching the Ag nanoparticles, their size was well controlled at the nanoscale, which was beneficial for improvement of the contact surface area. Compared with reference photocatalysts, the TiO₂/Ag‐AgCl@PPy composite exhibited an enhanced photodegradation activity towards rhodamine B under visible‐light irradiation. The superior photocatalytic property originated from synergistic effects between TiO₂ nanofibers, Ag‐AgCl nanoparticles and the PPy shell. Furthermore, the TiO₂/Ag‐AgCl@PPy composite could be easily separated and recycled without obvious reduction in activity.     

Electrorheological characterization of polyaniline-coated poly(methyl methacrylate) suspensions

Surface-conductive particles consisting of a poly(methyl methacrylate) (PMMA) core and a polyaniline (PA)-coated shell were synthesized and adopted as suspended particles for electrorheological (ER) fluids. The PA-PMMA composite particles synthesized were monodisperse and spherical in shape. The PA-PMMA suspensions in silicone oil showed typical ER characteristics under an applied electric field. The PA-PMMA composite particles possess a higher dielectric constant and conductivity than the pure PA particle, within an acceptable conductivity range for ER fluids, but the PA-based ER fluid showed larger shear-stress enhancement than the PA-PMMA-based systems. This phenomena can be explained by the interfacial polarizability of PA-based ER fluids, which is the difference between the ER fluid's dielectric constant and loss factor - this polarizability was higher than that of PA-PMMA-based ER fluids, as shown by the dielectric spectrum of each fluid. The insulating PMMA core suppressed the interfacial polarization in ER fluids, resulting in reduced interaction among particles under an imposed electric field.     

Raman microscopic investigations of BaTiO<Subscript>3</Subscript> precursors with core–shell structure

Due to their outstanding dielectric and ferroelectric properties, barium titanate (BaTiO₃)-based ceramics have found many applications in electronic devices. To optimise the final quality of such ceramics, a detailed knowledge of the complex processes involved in the formation of BaTiO₃ is required. The phase formation process in ordered structures of the BaCO₃/TiO₂ system was analysed by X-ray diffraction and by Raman spectral imaging (RSI) as a function of the annealing temperature. RSI was used for the first time as a locally resolving method for phase analysis, and proved to be a useful tool in examining the formation process of BaTiO₃ starting from spherical, core–shell structured precursors of the type TiO₂ core/BaCO₃ shell. The Raman spectra of different BaO–TiO₂ phases appearing as intermediate phases during the formation of BaTiO₃ were recorded for separately-prepared pure substances. Using these spectra as fingerprints, and choosing phase filters by setting wave number windows, phase landscape pictures of the samples at different temperatures during the genesis of BaTiO₃ could be created with a lateral resolution of up to 200 nm. These pictures confirm shell-like formation of the different barium titanate phases according to the diffusion of barium and oxygen ions from the Ba-rich shell into the TiO₂ core. At an intermediate state of the phase formation process, the phase sequence Ba₂TiO₄, BaTiO₃, BaTi₂O₅, BaTi₄O₉ and BaTi₅O₁₁ to TiO₂ was detected from the outer to the inner parts of the core–shell structures.     

Electrorheological property and microstructure of acetamide-modified TiO<Subscript>2</Subscript> nanoparticles

A sol–gel method has been proposed to prepare uniform TiO₂ nanoparticles whose average size is about 30 nm. The prepared nanometer TiO₂ particles are modified by acetamide via different self-assembled processes. X-ray diffraction analyses, scanning electron microscopy, and Fourier transform infrared spectrometry are used to determine the structure of the nanoparticles. The results indicate that the different synthesis processes do not change the morphology of the TiO₂/acetamide nanoparticles; nevertheless, they affect the interaction between amide and acetamide. The electrorheological (ER) activity is studied by shear stress under DC electric field. The acetamide-modified TiO₂ ER fluid shows notable ER activity with shear stress about 45 kPa (at 5 kV/mm), which outclasses the shear stress (2 kPa) of unmodified TiO₂ ER fluid. It is also found that the ER effect is very sensitive to the interaction of molecules on TiO₂ particles. The chemical bond between core and shell can enhance the ER activity of the sample.     

Correlation of the Dielectric Properties of Dispersed Particles with the Electrorheological Effect

The dc field rheological properties and frequency dependent dielectric properties of a set of electrorheological (ER) fluids composed of oxidized polyacrylonitrile or aluminosilicate materials dispersed in silicone oil were examined in this paper. Our experimental investigations show that there is a complicated relationship between the dielectric properties of dispersed particles and the ER effect. The dielectric loss of dispersed particles, which has not attracted much attention in previous work, was found to play a considerable role in ER response. The large dielectric loss tangent, experimentally around 0.10 at 1000 Hz, is found to be needed for a strong ER effect. A good ER solid material should first have large dielectric loss, and then the higher the dielectric constant, the stronger the ER effect. The large dielectric loss would facilitate the turning of dispersed particles, and the high dielectric constant would maintain the fibrillation structure stable and strong. Two processes, the particle turning process and the particle polarization process, are thought to be involved in ER activity. Our findings, in connection with the Wagner model, can better explain why the strongest ER effect occurs at particle conductivity of 10⁻⁷S/m; why the shear stress of some ER fluids decreases with frequency while with others the shear stress increases with frequency; and why trace water can enhance the ER effect considerably, which would help in understanding the mechanism of the ER effect. Too large a dielectric loss is thought to be unfavorable for the ER effect, and its suitable range is worth further study. The results also present a method of designing high performance ER fluids, which would significantly promote development of electrorheology and its application in industrial areas.     

Physicochemical and photocatalytic properties of nanosized titanium dioxide deposited on silicon dioxide microspheres

The synthesis of SiO₂ core-TiO₂ shell composites from a titanium dioxide sol and a suspension of microspherical silicon dioxide is described. The main factors ensuring the formation of a composite with a preset morphology are the size and charge of the TiO₂ sol particles (10–45 nm) and silicon dioxide core particles (300–700 nm), the pH values of the suspensions of the starting components and the resulting composite, and the proportions and way of mixing of the siliconand titanium-containing components. The SiO₂ core-TiO₂ shell composites show high photocatalytic activity in the degradation of Rhodamine FL-BM dye (rate constant of k = 0.0813 min⁻¹) and are much more active than precipitated TiO₂ powder (k = 0.0022 min⁻¹). The activity of the composite is determined by the calcination temperature (700–800°C), by the proportion and accessibility of the active component (TiO₂), and by the presence of a dopant (P₂O₅).     

Novel sol–gel synthesis of N-doped TiO<Subscript>2</Subscript> hollow spheres with high photocatalytic activity under visible light

Novel ammonia and triethanolamine assisted sol–gel synthesis method was developed to fabricate the N-doped TiO₂ hollow spheres. The prepared hollow spheres were in submicron size and had good morphology and high specific surface area. Polystyrene (PS) latexes in size of 470 nm were used as the templates to fabricate PS/TiO₂ core–shell spheres. Here ammonia and triethanolamine was first employed together to control the sol–gel process. The N-doped TiO₂ hollow spheres were got after calcinations of the core–shell spheres by using triethanolamine as N source, and the amount of doped N could be easily adjusted by changing the amount of triethanolamine. The hollow spheres had distinct visible light response, and the optical response shifted more to the visible region as the amount of doped N increases. The photodegradation of methylene blue expressed the high photocatalytic activity of the N-doped TiO₂ hollow spheres under visible light.     

Wettability of urea-doped TiO<Subscript>2</Subscript> nanoparticles and their high electrorheological effects

Aimed at the increasement of ER effects, a novel composite, urea doped-TiO₂ particles (TU) were prepared by using a modified sol–gel method. The structure and morphology of the TU particles were observed and analyzed by scanning electron micrpscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometry(FT-IR) and X-ray photoelectron spectrum (XPS). The dielectric properties of the TU particles and the ER effects based on the TU particles were investigated. The influence of wettability on the ER performance between the particles and silicone oil was examined.     

Preparation and characterization of core-shell structured TiO2–BaCO3 particles

Preparation of core-shell structured TiO₂–BaCO₃ particles as precursor of BaTiO₃ genesis, proceeds using a two step procedure, by first coating the TiO₂ core by Ba(OH)₂ shell followed by conversion of the shell region with CO₂ gas by the formation of BaCO₃. Straightforward experimental results reveal environmental scanning electron microscopy (ESEM) and scanning transmission electron microscopy (STEM) as suitable methods for analytical characterization of the core and shell regions from individual TiO₂–BaCO₃ grains. Evidence of coating the whole ensemble of TiO₂ particles is possible using Photo Electro Motive Force (Photo EMF, PEMF) measurements. This method is able to indicate very sensitively changes of surface properties of TiO₂ after coating with Ba(OH)₂ and BaCO₃, respectively. PEMF measurements were used for the first time with concern to this topic.     

Ammonia Gas Sensor Using Polypyrrole‐Coated TiO2/ZnO Nanofibers

Highly porous polypyrrole (PPy)‐coated TiO₂/ZnO nanofibrous mat has been successfully synthesized. The core TiO₂/ZnO nanofibers have an average diameter of ca. 100 nm and the shell of ultrathin PPy layer has a thickness of ca. 7 nm. The NH₃ gas sensor using the as‐prepared material exhibited a fast response over a wide dynamic range and high sensitivity with a detection limit of 60 ppb (S/N=3). Compared to conventional pristine PPy film, the improved performance in NH₃ detection can be attributed to the free access of NH₃ to PPy and a minimized gas diffusion resistance through the ultrathin PPy layer.     

Titanium dioxide/poly(methyl methacrylate) composite microspheres prepared by in situ suspension polymerization and their ability to protect against UV rays

Homogenously dispersed TiO₂/poly(methyl methacrylate) (PMMA) composite microspheres were produced in the size range of 1-10µm and the interfacial characteristics of TiO₂ and PMMA in suspension polymerization were considered. In electron microscopy observation, it was found that TiO₂ nanoparticles were embedded homogeneously in the PMMA phase. This study elucidates that the interfacial compatibility between TiO₂ and PMMA played a decisive role in producing the composite microspheres structured with inner TiO₂ and continuous PMMA, which was achieved by treating the surface of the TiO₂ particles hydrophobically. The TiO₂/PMMA composite microspheres produced showed good ability to protect against UV rays and are therefore of great usefulness in cosmetic formulations.