Characterization of SiO2/Ag composite particles synthesized by in situ reduction and its application in electrically conductive adhesives

In the study, SiO₂/Ag composite particles with silver coating onto the surface of silica have been successfully prepared via a novel and facile approach (Oxidation–Reduction Method). In this approach, the SiO₂ particles were first modified with 3-ammoniatriethoxysilane (APTES) and glyoxalic acid (GA) through two-step reaction, the aldehyde group (CHO) were anchored onto the surfaces of silica spheres via electrostatic attraction, these [Ag(TEA)₂]⁺ ions in the solution were then reduced by the CHO and coated onto the surface of silica to obtain SiO₂/Ag composite particles. The effects of the reaction conditions on silver content and synthetic mechanism had also been discussed. The structure, morphology and optical properties of the SiO₂/Ag composite particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–vis spectroscopy. The results showed the surface of SiO₂ was surrounded by pure silver nanoparticles, and the silver nanoparticles had face-centered-cubic structure, the SiO₂/Ag composite particles with core–shell morphology and special optical properties. And the small content SiO₂/Ag composite particles applied in electrically conductive adhesives (ECAs) improved the electrical bulk resistivity and tensile shear strength.     

Synthesis of Pt-immobilized on silica and polystyrene-encapsulated silica and their applications as electrocatalysts in the proton exchange membrane fuel cell

Nano sized Pt particles were successfully immobilized onto SiO₂ and polystyrene-encapsulated silica core shell (SiO₂@PS). To make the immobilization of Pt onto both silica and polystyrene-encapsulated silica core shell, SiO₂ was first functionalized with -NH₂ using 3-amino propyl trimethoxysilane (APTMS) while for core shell, the negatively charged surface of polystyrene (PS) was changed with positive charge by cationic surfactant such as cetyltrimethylammonium chloride (CTACl) to make the formation of SiO₂ shell on preformed PS sphere. Transmission electron micrograph (TEM) images shows that Pt nanoparticles immobilized onto SiO₂ and SiO₂@PS were to be 3-4 nm without agglomeraiton. The energy dispersive spectroscope (EDS) shows that Pt contents on both SiO₂ and SiO₂@PS were to be 21.45% and 20.28%, respectively. In case of Pt-SiO₂@PS, it is believed that Pt should have been immobilized onto PS surface and pore within SiO₂ shell as well as SiO₂ surface. The MEA fabricated with Pt-SiO₂@PS shows better cell performance than of Pt-SiO₂.     

Core/shell structured sSiO2/mSiO2 composite particles: The effect of the core size on oxide chemical mechanical polishing

In a typical chemical mechanical polishing (CMP) process, the type, morphology, structure, mechanical, and surface characteristics of abrasive particles play an important role in influencing the material removal process. The novel abrasive particles with special mechanical and/or tribochemical properties have been introduced into CMP processes for the improvement of surface quality and finishing efficiency. In this work, the composite particles containing solid silica (sSiO₂) cores and mesoporous silica (mSiO₂) shells were prepared via a developed Stöber method using cetyltrimethylammonium bromide as a structure-templating surfactant. The as-synthesized core/shell structured sSiO₂/mSiO₂ composite particles were characterized by powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and nitrogen sorption–desorption measurements. The effect of the sSiO₂ core size of the composite particles on oxide CMP performance was evaluated in terms of surface roughness and material removal rate (MRR). The root-mean-square surface roughness (0.15–0.31 nm) of the polished substrates slightly increased with increasing of the sSiO₂ core size (168–353 nm) of the composites with a comparable mSiO₂ shell thickness (16–18 nm). The sSiO₂/mSiO₂ composite particles with a relatively smaller or larger core presented a relatively high MRR for silicon oxide films. These oxide CMP results could be rationalized according to the contact area mechanism and indentation-based mechanism, incorporating the total contact area and chemical reactivity between particles and wafers, and the indentation depth of an abrasive particle onto the substrate surface.     

Modified titania and titanium-containing composites as fillers exhibiting an electrorheological effect

SiO₂/TiO₂ nanocomposites and individual titania (S = 30–500 m²/g) have been prepared through cohydrolysis of organomineral silicon and titanium derivatives in the presence of templates. We demonstrate that heterovalent substitution of chromium or cerium for titanium increases the thermal stability of TiO₂ in terms of its crystallite size and specific surface area. After drying at 120°C, fillers based on SiO₂/TiO₂ composites (coprecipitated and core-shell SiO₂/TiO₂ particles) exhibit a significant electrorheological response due to appreciable hydrate and hydroxyl layers on the surface of the particles. Structural modification of titania also allows one to increase the electrorheological effect through an increase in specific surface and defect formation.     

Growth of ultrathin barrier layers with sub-5-nm metallic seeds fabricated by sequential treatments of vacuum plasma and electrochemical solution

This study describes the process of growing densely distributed metallic seeds of sizes ∼4 nm on SiO₂ dielectric layers by sequential treatment in N₂-H₂ vacuum plasma (to activate the surfaces) and an aqueous solution of metallic salt (to adsorb metallic seeds). After the plasma treatment, the metallic seeds can be densely adsorbed onto the SiO₂ dielectric layers, thus facilitating Co-P barrier layers of thickness only ∼10 nm to be grown using electroless plating. Conversely, for the SiO₂ dielectric layers without the plasma pre-treatment, the population density of the metallic seeds is significantly reduced by one order and thus can only initiate the growth of a relatively thick (∼40 nm) barrier layer by the identical seeding and electroless plating conditions. Improvement in seeding by the plasma pre-treatment is discussed based on surface bonding modification.     

Patterned immobilisation of silicon dioxide nanoparticles on the surface of a photosensitive polymer

A photosensitive co-polymer of styrene and 4-vinylbenzyl thiocyanate was synthesised and employed for the immobilisation of aminofunctionalised silica nanoparticles (SiO₂-NP) at the polymer surface. Upon UV irradiation of the co-polymer, isothiocyanate groups are generated by a photo-isomerisation reaction of the thiocyanate groups. The silica nanoparticles were selectively immobilised in irradiated areas by immersing the illuminated polymer surface in a solution of SiO₂-NP. Depending on the time of immersion and the nanoparticle concentration, different amounts of silica can be deposited in the irradiated areas, whilst no immobilisation of SiO₂-NP is observed in the non-irradiated areas. By using photolithographic methods, patterned silica structures (μm scale) were produced on the polymer surface. The SiO₂-NP covered surfaces are of potential interest to generate protective surface layers and to carry out further functionalisation reactions of the immobilised SiO₂-NP particles.     

Gold catalysts for the abatement of environmentally harmful materials: Modeling the structure dependency

FeO_x, TiO₂ and CeO_x layers were deposited by pulsed laser deposition (PLD) technique onto Au films or Au nanoparticles supported on SiO₂/Si(100). The samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and their reactivity was studied in catalytic CO oxidation. Comparison was made with reference samples of FeO_x/SiO₂/Si(100), TiO₂/SiO₂/Si(100), CeO_x/SiO₂/Si(100) and Au/SiO₂/Si(100) layers. The catalytic activity of the metal-oxide/Au/SiO₂/Si(100) samples must be attributed to active sites located on the metal-oxides overlayer modified by gold underneath, since no Au was exposed to the surface according to the XPS and SIMS. We found a promoting effect of gold on the catalytic activity of the FeO_x overlayer and an inhibiting effect of gold on the TiO₂ and CeO_x overlayers. These findings are discussed in terms of electronic interactions at the Au/metal oxide interface.     

Flower-like silicon dioxide/polymer composite particles synthesized by dispersion polymerization route

A vital issue for the manufacture of multifunctional thin films is to synthesize polymer/ceramic hybrid particles. Silicon dioxide (SiO₂)/polymer composite particles were synthesized through dispersion copolymerization of methyl methacrylate (MMA) in the presence of SiO₂ bullet-like particles, using a “grafting-through” approach. The SiO₂ particles were previously modified with the silane-coupling agent 3-(trimethoxysilyl)propyl methacrylate (MPTS). Scanning electron microscopy and transmission electron microscopy analyses confirmed the formation of particles with a rough surface and flower-like morphology. Fourier transform infrared spectroscopy, thermogravimetric analysis, and energy-dispersive X-ray investigations indicated that a nucleation and aggregation process of the growing copolymer MPTS/poly(methyl methacrylate) (PMMA) occurred on the surface of the modified SiO₂ particles. As a result, the SiO₂ core became embedded in a PMMA shell. The influence of MPTS and the concentration of polyvinylpyrrolidone as a steric stabilizer on the flower-like morphology was demonstrated. Dispersion polymerizations have been proven to be simple and effective ways to synthesize composite particles with a high surface area. By using homogeneous systems (i.e., the monomer was soluble in the reaction solvent), no emulsification process was required, and copious amounts of well-dispersed particles were produced. These characteristics open many application possibilities for the use of the synthesized particles in functional coatings and optical devices, for mechanical reinforcement in polymeric materials, and as biomaterials.     

Fabrication of silica/platinum core-shell particles by electroless metal plating

An electroless metal plating method was used to form Pt shells on sub-micrometer-sized silica (SiO₂) particles fabricated by a sol-gel method. The electroless metal plating method was comprised of three steps: (1) surface-modification of SiO₂ particles with polyvinylpyrrolidone (PVP) (SiO₂/PVP) or poly-diallyldimethylammonium chloride (PDADMAC) (SiO₂/PDADMAC), (2) pre-deposition of Pt nuclei or Pt fine particles on the SiO₂ particles by reducing Pt ions in the presence of SiO₂/PVP particles (SiO₂/PVP-Pt) or SiO₂/PDADMAC particles (SiO₂/PDADMAC-Pt), and (3) growth of the pre-deposited Pt by immersing the SiO₂/PVP-Pt or SiO₂/PDADMAC-Pt particles in a Pt-plating solution. The pre-deposition of Pt nanoparticles was successfully performed for the surface-modified SiO₂ particles since the surface modification possibly strengthened the affinity between the SiO₂ particle surfaces and Pt ions. The Pt nanoparticles were pre-deposited more uniformly in the case of PVP because the pre-deposition took place more slowly for the PVP, which provided uniform surface-modification followed by the uniform pre-deposition of Pt nanoparticles. The formation of Pt shells was successfully performed on the SiO₂/PVP-Pt particles in the electroless metal plating process because Pt nuclei were generated by the reduction of H₂PtCl₆ and then further deposited on the Pt particle surfaces on the SiO₂/PVP-Pt particles.     

Formation of silicon dioxide layers at low temperatures (150-400 °C) by atmospheric pressure plasma oxidation of silicon

The formation of silicon dioxide (SiO₂) layers at low temperatures (150-400 °C) by atmospheric pressure plasma oxidation of Si(0 0 1) wafers have been studied using a gas mixture containing He and O₂. A 150 MHz very high frequency (VHF) power supply was used to generate high-density atomic oxygen in the atmospheric pressure plasma. Oxidation rate, structure, and thickness and refractive index profiles of the oxidized layers were investigated by ellipsometry and infrared absorption spectroscopy. Atomic force microscopy was also employed to observe atomic-scale morphologies of the layer surface and wafer Si surface, after chemical removal of the oxidized layers. It was found that stoichiometric SiO₂ layers were obtained at higher oxidation rates than conventional dry O₂ thermal oxidation and radical oxidation processes, even at a very low substrate temperature of 150 °C. Although thickness variations were observed in the plasma region, the refractive index was independent of both substrate temperature and VHF power. In addition, the SiO₂ surface and SiO₂/Si interface roughnesses were comparable to those obtained in conventional dry oxidation at high temperatures.     

SiO2/TiO2 multilayer films grown on cotton fibers surface at low temperature by a novel two-step process

A novel two-step process was developed to synthesize and deposit SiO₂/TiO₂ multilayer films onto the cotton fibers. In the first step, SiO₂ particles on cotton fiber surface were synthesized via tetraethoxysilane hydrolysis in the presence of cotton fibers, in order to protect the fibers against photo-catalytic decomposition by TiO₂ nanoparticles. In the second step, the growth of TiO₂ nanoparticles into the modified cotton fiber surface was carried out via a sol-gel method at the temperature as low as 100 °C. The as-obtained SiO₂/TiO₂ multilayer films coated on cotton fibers were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffraction, respectively.     

Fabrication and mechanical evaluation of hydroxyapatite/oxide nano-composite materials

In the current study, the semiconducting metal oxides such as nano-ZnO and SiO₂ powders were prepared via sol–gel technique and conducted on nano-hydroxyapatite (nHA) which was synthesized by chemical precipitation. The properties of fabricated nano-structured composites containing different ratios of HA, ZnO and SiO₂ were examined using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques. The effect of the variation of ratios between the three components on mechanical, microstructure and in-vitro properties was assessed to explore the possibility of enhancing these properties. The results proved that the mechanical properties exhibited an increment with increasing the ZnO content at the extent of HA. In-vitro study proved the formation and nucleation of apatite onto the surface of the fabricated composites after one week of immersion. It is concluded that HA composites containing SiO₂ or SiO₂/ZnO content had a suitable mechanical properties and ability to form apatite particles onto the composite surface. Based on bioactivity behavior, Si-HA is more bioactive than pure hydroxyapatite and nano-arrangements will provide an interface for better bone formation. Therefore, these nano-composites will be promising as bone substitutes especially in load bearing sites.     

Effect of oxygen on the chemical reactions and electron work function in Ba-Si and BaO-Si structures

Effects of the annealing in oxygen on the chemical composition and electron work function of Ba-Si and BaO-Si structures were studied. Thin Ba or BaO layers deposited onto silicon stimulate oxidation processes on the semiconductor surface. Treatment of the Ba-Si and BaO-Si samples at temperatures T _s ≥500°C leads to the formation of a barium orthosilicate (Ba₂SiO₄) layer. A decrease in the electron work function observed for the silicon surface coated with both barium oxide and barium silicate layers provides for a more than tenfold increase in the electron emission current.     

SiO2 layer thickness effects on the (001) texture of FePtCu:SiO2 nanocomposite films

The influence of SiO₂ layer thickness of (Fe₅₂Pt₄₈)₈₈Cu₁₂:SiO₂ multilayer nanocomposite films on their structural and magnetic properties were investigated. The films were deposited on (001) textured FePt films, and then annealed at 873 K. The crystalline texture of (Fe₅₂Pt₄₈)₈₈Cu₁₂:SiO₂ films changes drastically with respect to the thickness of the SiO₂ layers. In the film with 50-Å thick SiO₂ layers, the (111) peak was strong although the (001) orientation is dominant, and self-organized spherical FePtCu particles were formed in the SiO₂ matrix. However, in the film with 19-Å thick SiO₂ layers, flat FePt grains with perfect (001) orientation were obtained. In addition, twins with different crystalline orientations were seen in the above films with different thicknesses of the SiO₂ layers. Accordingly, different perpendicular hysteresis loops were obtained.     

Fabrication of thermochromic composite using monodispersed VO2 coated SiO2 nanoparticles prepared by modified chemical solution deposition

Monodispersed thermochromic VO₂ particles were fabricated by VO₂ coating onto monodispersed SiO₂ nanoparticles with the modified chemical solution deposition technique using vanadium isopropoxide solution and monodispersed SiO₂ particle suspension solution. The average size of the resultant VO₂–SiO₂ particle was 57 nm and the coating thickness of the VO₂ layer was 6 nm. A thermochromic composite was fabricated using the VO₂–SiO₂ particles and a poly lactose acid polymer as a transparent matrix, and the transmittance of the composite at a high temperature was 10% less than that at a low temperature.     

Nucleation and growth mechanism of apatite on a bioactive and degradable ceramic/polymer composite with a thick polymer layer

Nucleation and growth mechanism of apatite on a bioactive and degradable PLLA/SiO₂–CaO composite with a thick PLLA surface layer were investigated compared to that on a bioactive but non-degradable polyurethane (PU)/SiO₂–CaO composite with a thick PU surface layer. The bioactive SiO₂–CaO particles were made by a sol–gel method from tetraethyl orthosilicate and calcium nitrate tetrahydrate under acidic condition followed by heat treatment at 600 °C for 2 h. The PLLA/SiO₂–CaO and PU/SiO₂–CaO composites were then prepared by a solvent casting method which resulted in thick PLLA and PU surface layers, respectively, due to precipitation of SiO₂–CaO particles during the casting process. Two composites were exposed to SBF for 1 week and this exposure led to form uniform and complete apatite coating layer on the PLLA/SiO₂–CaO composite but not on the PU/SiO₂-CaO composite. These results were interpreted in terms of the degradability of the polymers. A practical implication of the results is that a post-surface grinding or cutting processes to expose bioactive ceramics to the surface of a composite with a thick biodegradable polymer layer is not required for providing apatite forming ability, which has been considered as one of the pragmatic obstacles for the application as a bone grafting material.     

Hollow polymer microspheres containing a gold nanocolloid core adsorbed on the inner surface as a catalytic microreactor

Hollow polymer microspheres with different polarity and functional group for the shell layer containing gold nanocolloid cores adsorbed on the inner surface were prepared by selective removal of sandwiched silica layer from the corresponding gold/silica/polydivinylbenzene (Au/SiO₂/PDVB), Au/SiO₂/poly(ethyleneglycol dimethacrylate) (Au/SiO₂/PEGDMA), and Au/SiO₂/poly(ethyleneglycol dimethacrylate-co-methacrylic acid) (Au/SiO₂/P(EGDMA-co-MAA) tri-layer microspheres, respectively. The tri-layer microspheres were synthesized by distillation precipitation polymerizations of divinylbenzene (DVB), ethyleneglycol dimethacrylate (EGDMA), EGDMA together with methacrylic acid (MAA) in presence of 3-(methacryloxy)propyltrimethoxysilane (MPS)-modified gold/silica (Au/SiO₂) core–shell particles as seeds, which were prepared by coating of a layer of silica onto the surface of Au nanocolloids with the aid of polyvinylpyrrolidone (PVP) via a modified Stöber method. The catalytic property and stability as a microreactor of the hollow polymer microspheres with Au nanocolloid cores adsorbed on the inner surface were studied by the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AnP) with sodium borohydride (NaBH₄) as reductant. Transmission electron microscopy (TEM) and Fourier transform infrared spectra (FT-IR) were used for characterizing the morphology and structure of the resultant microspheres.     

SiO2 nanoparticles surface modified with polyethyleneimine-oleic acid complex as stabilizers of Ni fine particles in dense nonaqueous suspensions

SiO₂ nanoparticles (NPs) surface modified with polyethyleneimine-oleic acid complex (PEI-OA) has successfully prepared in a simple manner as a stabilizer of metal (Ni) fine particles (FPs) as well as a component of Ni/SiO₂ composite particles. Starting from SiO₂ NPs which were collected through centrifugation of commercial SiO₂ colloids, it was found that PEI-OA can effectively adsorbed on collected SiO₂ NPs surface during their redispersion process in toluene with the assistance of ultrasonication. The aggregated particle size (Z-average size) in toluene could be successfully reduced to c.a. 100 nm under saturated adsorption of PEI-OA. It was also found that PEI-OA-modified SiO₂ NPs can effectively attach to the Ni FPs by a simple mixing process in toluene. The FE-SEM observation confirmed the adsorption of the PEI-OA-modified SiO₂ NPs on the Ni FPs without forming severe NP aggregates. Owing to the attachment of the PEI-OA-modified SiO₂ NPs with surfaces that are compatible to toluene and α-terpineol, the suspension stability of the Ni/SiO₂ composite particles in these solvents drastically improved. The result was confirmed by the effective reduction of the sedimentation velocity of diluted suspensions as well as by the reduction of the viscosity of dense suspensions.     

Growth of Ge nanoparticles on SiO2/Si interfaces during annealing of plasma enhanced chemical vapor deposited thin films

Multilayer germanosilicate (Ge:SiO₂) films have been grown by plasma enhanced chemical vapor deposition. Each Ge:SiO₂ layer is separated by a pure SiO₂ layer. The samples were heat treated at 900 °C for 15 and 45 min. Transmission electron microscopy investigations show precipitation of particles in the layers of highest Ge concentration. Furthermore there is evidence of diffusion between the layers. This paper focuses mainly on observed growth of Ge particles close to the interface, caused by Ge diffusion from the Ge:SiO₂ layer closest to the interface through a pure SiO₂ layer and to the interface. The particles grow as spheres in a direction away from the interface. Particles observed after 15 min anneal time are 4 nm in size and are amorphous, while after 45 min anneal time they are 7 nm in size and have a crystalline diamond type Ge structure.     

Formation of gold nanoparticles in silicon suboxide films prepared by plasma enhanced chemical vapour deposition

Nanostructured materials fabricated by dispersing metal particles on the dielectric surface have potential application in the field of nanotechnology. Interfacial metal particles/dielectric matrix interaction is important in manipulating the structural and optical properties of metal/dielectric films. In this work, a thin layer of gold (Au) was sputtered onto the surface of silicon oxide, SiO_x (0.38 < x < 0.68) films which was deposited at different N₂O/SiH₄ flow rate ratios of 5 to 40 using plasma enhanced chemical vapor deposition (PECVD) technique prior to the annealing process at 800 °C. FTIR spectra demonstrate the intensity and full-width at half-maximum (FWHM) of Si-O-Si stretching peaks are significantly dependent on the N₂O/SiH₄ flow-rate ratio, η. The films deposited at low and high N₂O/SiH₄ flow rate ratios are dominated by the oxygen and silicon contents respectively. The size and concentration of Au particles distributed on the surface of SiO_x films are dependent on the N₂O/SiH₄ flow-rate ratio. High concentrations of Au nanoparticles are distributed evenly on the surface of the film deposited at N₂O/SiH₄ flow-rate ratio of 30. Crystallinity and crystallite sizes of Au are enhanced after the thermal annealing process. Appearance of surface plasma resonance (SPR) absorption peaks at 524 nm for all samples are observed as a result of the formation of Au particles. The annealing process has improved SPR peaks for all the as-deposited films. The energy gap of the as-deposited Au/SiO_x films are in the range of 3.58 to 4.38 eV. This energy gap increases after the thermal annealing process except for the film deposited at η = 5.