Low temperature in situ formation of cobalt in silicon nitride toward functional nitride nanocomposites.

This work highlights the first demonstration of a low-temperature in situ formation of Co nanocrystallites embedded within an amorphous silicon nitride matrix through careful control of the chemistry behind material design using perhydropolysilazane (PHPS) as a Si3N4 precursor further coordinated with CoCl2 and ammonia as a pyrolysis atmosphere. The Co nucleation was allowed to proceed at temperatures as low as 400 °C via thermal decomposition of Co2N pre-formed in situ by the reaction of CoCl2 with the Si centers of PHPS at the early stage of pyrolysis (220-350 °C).

Reversible Redox Property of Co(III) in Amorphous Co-doped SiO2/γ-Al2O3 Layered Composites.

This paper reports on a unique reversible reducing and oxidizing (redox) property of Co(III) in Co-doped amorphous SiO2/γ-Al2O3 composites. The Fenton reaction during the H2O2-catalyzed sol-gel synthesis utilized in this study lead to the partial formation of Co(III) in addition to Co(II) within the composites. High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analyses for the composite powder sample with a composition of Al:Si:Co = 85:10:5 showed the amorphous state of the Co-doped SiO2 that modified γ-Al2O3 nanocrystalline surfaces. In situ X-ray absorption fine structure (XAFS) spectroscopic analysis suggested reversible redox reactions of Co species in the composite powder sample during heat-treatment under H2 at 500 °C followed by subsequent cooling to RT under Ar. Further analyses by in situ IR spectroscopy combined with cyclic temperature programmed reduction/desorption (TPR/TPD) measurements and X-ray photoelectron spectroscopic (XPS) analysis revealed that the alternating Co(III)/(II) redox reactions were associated with OH formation (hydrogenation)-deformation (dehydrogenation) of the amorphous aluminosilicate matrix formed in situ at the SiO2/γ-Al2O3 hetero interface, and the redox reactions were governed by the H2 partial pressure at 250-500 °C. As a result, a supported mesoporous γ-Al2O3/Co-doped amorphous SiO2/mesoporous γ-Al2O3 three-layered composite membrane exhibited an H2-triggered chemical valve property: mesopores under H2 flow (open) and micropores under He flow (closure) at 300-500 °C.

Hydrogen Selective SiCH Inorganic-Organic Hybrid/γ-Al2O3 Composite Membranes.

Solar hydrogen production via the photoelectrochemical water-splitting reaction is attractive as one of the environmental-friendly approaches for producing H2. Since the reaction simultaneously generates H2 and O2, this method requires immediate H2 recovery from the syngas including O2 under high-humidity conditions around 50 °C. In this study, a supported mesoporous γ-Al2O3 membrane was modified with allyl-hydrido-polycarbosilane as a preceramic polymer and subsequently heat-treated in Ar to deliver a ternary SiCH organic-inorganic hybrid/γ-Al2O3 composite membrane. Relations between the polymer/hybrid conversion temperature, hydrophobicity, and H2 affinity of the polymer-derived SiCH hybrids were studied to functionalize the composite membranes as H2-selective under saturated water vapor partial pressure at 50 °C. As a result, the composite membranes synthesized at temperatures as low as 300-500 °C showed a H2 permeance of 1.0-4.3 × 10-7 mol m-2 s-1 Pa-1 with a H2/N2 selectivity of 6.0-11.3 under a mixed H2-N2 (2:1) feed gas flow. Further modification by the 120 °C-melt impregnation of low molecular weight polycarbosilane successfully improved the H2-permselectivity of the 500 °C-synthesized composite membrane by maintaining the H2 permeance combined with improved H2/N2 selectivity as 3.5 × 10-7 mol m-2 s-1 Pa-1 with 36. These results revealed a great potential of the polymer-derived SiCH hybrids as novel hydrophobic membranes for purification of solar hydrogen.

Formation and Thermal Behaviors of Ternary Silicon Oxycarbides derived from Silsesquioxane Derivatives.

Silsesquioxane (SQ) derivatives possessing intramolecular H2C = CH- groups and Si-H groups were designed as precursors for ternary silicon oxycarbide (SiOC). By using R-Si(OMe)3, H-Si(OEt)3 and (H-Si(Me)2)2O as starting compounds, SQ derivatives of VH-SQ (R = vinyl) and St-H-SQ (R = stylyl) were successfully synthesized through the conventional sol-gel route. Simultaneous thermogravimetric and mass spectroscopic analyses up to 1000 °C revealed that in situ cross-linking via hydrosilylation and demethanation of VH-SQ suppressed the evolution of gaseous hydrocarbon species to afford amorphous SiOC having a composition close to the desired stoichiometric SiO2(1-x)Cx (x = ca. 0.3) with a high yield. The effect of carbon content on the phase separation and crystallization of the SQ-derived amorphous SiOC was studied by several spectroscopic analyses and TEM observation. The results were discussed aiming to develop a novel polymer-derived ceramics (PDCs) route for in situ formation of binary ß-SiC-amorphous SiO2 nanocomposites with enhanced thermal and mechanical stability.

Void Formation/Elimination and Viscoelastic Response of Polyphenylsilsesquioxane Monolith.

Polyphenylsilsesquioxane (PhSiO3/2) particles as an organic-inorganic hybrid were prepared using sol-gel method, and monolithic samples were obtained via a warm-pressing. The reaction mechanism of particles' polymerization and transformation to the monolith under the warm-press were investigated using solid state 29Si nuclear magnetic resonance (NMR) spectrometer, thermal gravimetric-differential thermal analyzer (TG-DTA), mass spectrometer (MS) and scanning electron microscope (SEM). Transparent and void-free monoliths are successfully obtained by warm-pressing above 180 °C. Both the terminal ⁻OH groups on particles' surface and warm-pressing are necessary for preparation of void-free PhSiO3/2 monolith. From the load-displacement measurement at various temperatures, a viscoelastic deformation is seen for PhSiO3/2 monolith with voids. On the other hand, an elastic deformation is seen for void-free PhSiO3/2 monolith, and the void-free monolith shows much higher breakdown voltage.

Microporosity and CO2 Capture Properties of Amorphous Silicon Oxynitride Derived from Novel Polyalkoxysilsesquiazanes.

Polyalkoxysilsesquiazanes ([ROSi(NH)1.5]n, ROSZ, R = Et, nPr, iPr, nBu, sBu, nHex, sHex, cHex, decahydronaphthyl (DHNp)) were synthesized by ammonolysis at -78 °C of alkoxytrichlorosilane (ROSiCl3), which was isolated by distillation as a reaction product of SiCl4 and ROH. The simultaneous thermogravimetric and mass spectrometry analyses of the ROSZs under helium revealed a common decomposition reaction, the cleavage of the oxygen-carbon bond of the RO group to evolve alkene as a main gaseous species formed in-situ, leading to the formation of microporous amorphous Si-O-N at 550 °C to 800 °C. The microporosity in terms of the peak of the pore size distribution curve located within the micropore size range (<2 nm) and the total micropore volume, as well as the specific surface area (SSA) of the Si-O-N, increased consistently with the molecular size estimated for the alkene formed in-situ during the pyrolysis. The CO2 capture capacity at 0 °C of the Si-O-N material increased consistently with its SSA, and an excellent CO2 capture capacity of 3.9 mmol·g-1 at 0 °C and CO2 1 atm was achieved for the Si-O-N derived from DHNpOSZ having an SSA of 750 m²·g-1. The CO2 capture properties were further discussed based on their temperature dependency, and a surface functional group of the Si-O-N formed in-situ during the polymer/ceramics thermal conversion.

Synthesis of a Novel Polyethoxysilsesquiazane and Thermal Conversion into Ternary Silicon Oxynitride Ceramics with Enhanced Thermal Stability.

A novel polyethoxysilsesquiazane ([EtOSi(NH)1.5]n, EtOSZ) was synthesized by ammonolysis at -78 °C of ethoxytrichlorosilane (EtOSiCl3), which was isolated by distillation as a reaction product of SiCl4 and EtOH. Attenuated total reflection-infra red (ATR-IR), 13C-, and 29Si-nuclear magnetic resonance (NMR) spectroscopic analyses of the ammonolysis product resulted in the detection of Si-NH-Si linkage and EtO group. The simultaneous thermogravimetric and mass spectrometry analyses of the EtOSZ under helium revealed cleavage of oxygen-carbon bond of the EtO group to evolve ethylene as a main gaseous species formed in-situ, which lead to the formation at 800 °C of quaternary amorphous Si-C-N with an extremely low carbon content (1.1 wt %) when compared to the theoretical EtOSZ (25.1 wt %). Subsequent heat treatment up to 1400 °C in N2 lead to the formation of X-ray amorphous ternary Si-O-N. Further heating to 1600 °C in N2 promoted crystallization and phase partitioning to afford Si2N2O nanocrystallites identified by the XRD and TEM analyses. The thermal stability up to 1400 °C of the amorphous state achieved for the ternary Si-O-N was further studied by chemical composition analysis, as well as X-ray photoelectron spectroscopy (XPS) and 29Si-NMR spectroscopic analyses, and the results were discussed aiming to develop a novel polymeric precursor for ternary amorphous Si-O-N ceramics with an enhanced thermal stability.

Photoinduced reduction and heat-induced oxidation of silver in transparent RSiO(3/2) and RSiO(3/2)-TiO(2) films.

Photoinduced reduction, resulting in nanoparticle formation, and heat-induced oxidation of doped silver have been demonstrated in silsesquioxane (RSiO(3/2)) and RSiO(3/2)-TiO(2) films (R = methyl, vinyl, and glycidoxypropyl) synthesized via the sol-gel method. The occurrence of redox behavior in the doped silver, i.e. formation and dissociation of silver nanoparticles (AgNPs) in the films, was confirmed by UV-Vis spectroscopy, IR spectroscopy, X-ray diffraction and transmission electron microscopy. The films containing vinyl and glycidoxypropyl organic functional groups were found to effectively reduce silver ions upon UV irradiation, whereas the film containing the methyl group substituent did not. Incorporation of a TiO(2) component into the RSiO(3/2) films caused thermal dissociation of the AgNPs. The formation and dissociation of AgNPs in the films was reversible by reiterating the cycle of UV irradiation and heat treatment.

Highly hydrophobic flip-flop-type ultrathin coating films prepared via electrostatic self-assembly.

A flip-flop-type hydrophobic coating film consisting of oppositely charged poly(allylamine hydrochloride) (PAH) and a perfluorosulfonate ionomer (Nation) ultrathin layers was prepared via layer-by-layer (LbL) assembly. The thickness of a Nafion layer was estimated to be 6.7 nm from the amount of deposited Nafion. The PAH/Nafion multilayers are optically transparent in UV-Vis region from 300 to 800 nm, and extremely small amount of Nafion was used for the coating (approximately 1.1 microg/cm2). The contact angle for water droplet of the PAH/Nafion multilayers was approximately 105 degrees. On the other hand, the contact angle for water droplet decreased rapidly from 105 degrees to approximately 85 degrees after being kept in closed vessel (approximately 100% of relative humidity) for 90 min. (approximatley 100% of relative humidity) for 15 h. The PAH/Nafion multilayer again showed hydrophobicity with the contact angle of 102 degrees after being dried at 125 degrees C for 30 min. PAH/Nafion multilayers obtained are hydrophobic in air and hydrophilic in water, namely flip-flop-type amphiphilic coating films.