Polymer nano-encapsulation of templated mesoporous silica monoliths with improved mechanical properties

Macroporous (1–5 μm) monolithic silica aerogels consisting of both random but also ordered mesoporous walls have been synthesized via an acid-catalyzed sol–gel process from tetramethoxysilane (TMOS) using a triblock co-polymer (Pluronic P123) as a structure-directing agent and 1,3,5-trimethylbenzene (TMB) as a micelle-swelling reagent. Pluronic P123 was removed by Soxhlet extraction, and materials in monolithic form were obtained by extracting the pore filling solvent with liquid CO₂, which eventually was taken out supercritically. Although these monoliths are more robust than base-catalyzed silica aerogels of similar density, nevertheless, the mechanical properties can be improved dramatically by letting an aliphatic di-isocyanate (Desmodur N3200) react with the silanols on the macro- and mesoporous surfaces. As it turns out, the polymer fills the mesopores and coats conformally the macropores of templated samples, so that BET surface areas decrease dramatically, from 550–620 m² g^?1 to <5 m² g^?1. By comparison, polymer nano-encapsulation of non-templated acid-catalyzed aerogels preserves a large fraction of their mesoporous surface area, and BET values decrease from 714 m² g^?1 to 109 m² g^?1. Finally, since polymer nano-encapsulation preserves the macroscopic physical dimensions of the monoliths before drying, comparative analysis of the physical dimensions against XRD data of native versus polymer nano-encapsulated samples provides evidence that upon drying macropores (micron size regime) shrink less than mesopores (nanometer size regime).     

Preparation of core–shell nanospheres of silica–silver: SiO2@Ag

We prepared SiO₂@Ag core–shell nanospheres: silver nanoparticles (～4 ± 2 nm in diameter) coated silica nanospheres (～50 ± 10 nm in diameter). The preparation route is a modification of the Stöber method, and involves the preparation of homogeneous silica spheres at room temperature, combined with the deposition of silver nanoparticles from Ag⁺ in solution, by using water/ethanol mixtures, tetraethyl-orthosilicate as Si source and silver nitrate as Ag source in a single-pot wet chemical route without an added coupling agent or surface modification, which leads to the formation of core@shell homogeneous nanospheres. We present the preparation and characterization of the SiO₂@Ag core–shell nanospheres and also of bare silica spheres in the absence of silver, and propose a reaction mechanism for the formation of the core–shell structure.     

Synthesis of Pt-containing mesoporous silica using Pt precursor as a pore-forming agent

We synthesized Pt nanoparticle-containing mesoporous silica in a one-pot process using tetraammineplatinum(II) hydroxide (TPH) precursor as a pore-forming agent and silica nanospheres as a silica source. The TPH precursor was added into as-prepared colloidal silica sol with silica nanospheres (SN) of about 8 nm in particle diameter, to obtain the SN–TPH sol. During drying process of the SN–TPH sol, an amorphous SN–TPH nanocomposite was formed via hydrogen-bonding interaction between silanol groups and amine groups of the TPH precursor. The hydrogen-bonding interaction was confirmed by thermal gravimetry and differential thermal analysis (TG–DTA) profiles and Fourier transform infrared (FTIR) spectra. Using the TPH precursor as a pore-forming agent, incorporation of the Pt nanoparticles into the mesoporous silica can be simultaneously achieved with the synthesis of the mesoporous silica in a one-pot process. In addition, Pt nanoparticle size and pore diameter of the mesoporous Pt/silica were simultaneously controlled by simply varying the concentration of the TPH precursor. The pore diameter of the mesoporous silica was easily controlled from 3.2 nm to 6.5 nm with an increase in the TPH concentration.     

A novel process to prepare a hollow silica sphere via chitosan-polyacrylic acid (CS-PAA) template

Hollow silica spheres were synthesized by using a chitosan-polyacrylic acid (CS-PAA) template. Polyacrylic acid (PAA) was titrated into the dissolved chitosan solution to form CS-PAA particles. For forming the core-shell particle, colloidal silica which was prepared from homogeneous nucleation was mixed with a solution of CS-PAA particles. Colloidal silica was adsorbed on to the surface of the CS-PAA particles to form a shell structure. CS-PAA could be removed from the core of the core-shell particles by adjusting with HCl to pH < 1 for forming the hollow silica structure. The outside particle size diameter, shell thickness, specific surface, pore diameter and pore volume of the final hollow silica sphere are about 200 nm, 20 nm, 350.95 m²/g, 5.4078 nm and 0.516768 cm³/g, respectively.     

Facile synthesis of ordered large-pore mesoporous silica thin film with Im3m symmetry using n-butanol as the cosurfactant

A facile synthesis route to ordered large-pore (10.7 nm) mesoporous silica film with the cubic Im3m mesostructure is reported in a TEOS–F127–BuOH–HCl–H₂O system through dip-coating method. Characterization by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen sorption reveals that the obtain mesoporous silica material possessed high surface area and large pore diameter. A relative comparison between the mesoporous silica films synthesized with and without BuOH is also presented. A reasonable formation mechanism of the large-pore mesoporous silica film is depicted in this work.     

The Eγ′ center as a probe of structural properties of nanometer-sized silica particles

A Q-band electron spin resonance (ESR) study is reported of E′ type point defects observed in ～7 nm-sized fumed silica nanoparticles following 10-eV irradiation to photodissociate H from passivated defects. In a comparative study with bulk silica (suprasil), the E′ center is used as an atomic probe to get more in depth information on the network structure of the nm-sized particles. The nanoparticles were brought into contact with ‘bulk’ Si/SiO₂ entities at an elevated temperature in vacuum (T_an = 1105 °C), i.e., the presence of an Si/SiO₂ interface. As a result of this post manufacture treatment, the E′ density increased drastically (>order of magnitude), enabling us to resolve hyperfine (hf) structure of the E′ centers located in the core region of the nanoparticles. Two doublet structures are observed, one each assigned to O₂Si–H entities and the primary ²⁹Si hf structure of the E′ centers. Analysis of these hf spectra reveals interesting information on the network structure of the core region of the nanoparticles: (1) Fumed silica is found to contain relatively less hydrogen than suprasil. (2) An increased ²⁹Si hf splitting (439 ± 2 G) is observed compared to bulk silica (418 ± 2 G), indicating that the E′ centers located in the core of the nanoparticles exhibit on average a slightly more pyramidal defect structure, and moreover, providing evidence that the fumed silica particles are densified compared to standard bulk silica, possibly originating from the presence of more low-membered rings (n < 5) in the nm-sized silica network.     

27Al NMR structural study on aluminum coordination state in bismuth doped silica glass

The aluminum coordination state in bismuth doped silica glass, which has new broad infrared emission at 1.3 μm regions, was investigated by using ²⁷Al NMR, and it is demonstrated that 6-fold coordinated aluminum ions with corundum structure are dominant in bismuth doped silica glass until Bi₂O₃ concentrations of 1.0 mol% with Al₂O₃. The aluminum ion efficiently affects the creation of a Bi luminescent center at an intensity of Bi₂O₃ (1.0 mol%)–Al₂O₃ (2.3 mol%)–SiO₂ (96.7 mol%); the sample is three orders of magnitude larger than the Bi₂O₃ (1.0 mol%)–SiO₂ (99.0 mol%) sample. Aluminum ions with corundum structure in silica glass have a very important role for the configuration of peculiar Bi luminescent centers.     

Microstructural and photocatalytic properties of sol–gel-derived vanadium-doped mesoporous titanium dioxide nanoparticles

The vanadium (V)-doped mesoporous titanium dioxide (TiO₂) nanoparticles at low V/Ti ratios ranging from 0 to 2 wt% were prepared using hydrolytic sol–gel method in the presence of tri-block copolymer Pluronic F127. The microstructures of TiO₂ in terms of morphology, crystallization, chemical states of species, surface area, and band gap were characterized by SEM, TEM, XRPD, XPS, surface area analyzer, and UV–Vis spectrophotometer, respectively. SEM images showed that the V-doped TiO₂ nanoparticles were porous structures, and the surface areas and pore sizes ranged from 86 ± 9 to 96 ± 15 m²/g and from 12 ± 4 to 15 ± 2 nm, respectively. The XRPD patterns indicated that V-doped mesoporous TiO₂ after calcination at 500 °C was mainly anatase phase, and the crystallite sizes were in the range 14–16 nm, which are consistent with the results obtained from SEM images. XPS spectra and HRTEM images showed that vanadia was doped both on the surface and in the lattice of anatase TiO₂. A slight red-shift in wavelength absorption was observed when V/Ti ratio increased from 0 to 2 wt%. Methylene blue (MB) was further used as the target compound to examine the photocatalytic activity of V-doped mesoporous TiO₂ nanocatalysts under illumination of solar simulator or UV light. Addition of vanadium ions slightly decreased the photocatalytic activity of TiO₂ toward the decolorization of MB under the illumination of UV light at 305 nm. However, a 1.6–1.8 times increase in rate constants for MB photodegradation was observed when 0.5–1.0 wt% V-doped TiO₂ was illuminated with sunlight at AM 1.5.     

Improved method for preparation of anhydrous silica by microwave irradiation with spectroscopic characterization and toxicity assay

Amorphous anhydrous silica SiO_{2 (mw)} (99.99%) is successfully synthesized through microwave irradiation technique and time of reaction is reduced up to 1 h. The dehydration phase study of Si–water, Si–OH, Si–O–Si networking, elemental analysis and surface morphology was carried out by FTIR, FTNIR, SEM and EDAX spectroscopic techniques. The broad absorption stretching and bending of Si–OH and H₂O at 3695.38–2832.96 cm^? 1, 1638 cm^? 1 and 1191.20–1017.14 cm^? 1 completely disappeared and appearance of new bands at 946.93 and 797.63 cm^? 1 confirmed the amorphous anhydrous silica with Si–O–Si networking. The SEM images of SiO_{2 (mwc)} described the smooth and fine particle texture and confirmed 99.99% Si–O–Si networking of anhydrous silica. The 99.99% purity was verified by EDAX spectra which exhibited sharp signals only for oxygen and silicon. Toxicity against Monomorium minimum and Tribolium castaneum with 100% mortality and LT₅₀ 91 min and 7.5 h respectively is being reported. It can be used for long storage of grains in the future.     

Fluorine laser-induced silicon hydride Si–H groups in silica

Formation and destruction of silicon hydride (Si–H) groups in silica by F₂ laser irradiation and their vacuum ultraviolet (VUV) optical absorption was examined by infrared (IR) and VUV spectroscopy. Photoinduced creation of Si–H groups in H₂-impregnated oxygen deficient silica is accompanied by a growth of infrared absorption band at 2250 cm⁻¹ and by a strong increase of VUV transmission at 7.9 eV. Photolysis of Si–H groups by 7.9 eV photons in this glass was not detected when the irradiation was performed at temperature 80 K. However, a slight destruction of Si–H groups under 7.9 eV irradiation was observed at the room temperature. This finding gives a tentative estimate of VUV absorption cross section of Si–H groups at 7.9 eV as 4 × 10⁻²¹ cm², showing that Si–H groups do not strongly contribute to the absorption at the VUV fundamental absorption edge of silica glass.     

Novel monolithic mesoporous foamed carbons prepared using micro-colloidal particles as templates

Novel mesoporous foamed carbons were synthesized from carbonization of organic gels templated by polymer micro-colloidal particles. Resorcinol and formaldehyde were allowed to gel in dilute polymethylmethacrylate (PMMA) microemulsion latex, subsequently the water in the gel was solvent exchanged with methanol and the wet gel was dried under ambient pressure. Pyrolysis was carried out at 800 °C to afford carbon xerogels with porous structures similar to those of resorcinol–formaldehyde (RF) carbon aerogels, but of higher density (>1.2 g/cm³), which provide the carbon materials with relatively higher volumetric surface area (up to 918 m²/cm³). Brunauer–Emmett–Teller (BET) adsorption results indicate that PMMA micro-colloid particles with mean diameter 25 nm contributed to the formation of mesopores of mean diameter at 5 nm.     

Roughness of glass surfaces formed by sub-critical crack growth

This paper presents a study on the roughness of glass fracture surfaces formed as a consequence of sub-critical crack growth. Double-cantilever-beam specimens were used in these studies to form fracture surfaces with areas under well-defined crack velocities and stress intensity factors. Roughness depends on crack velocity: the slower the velocity, the rougher the surface. Ranging from approximately 1 × 10⁻¹⁰ m/s to approximately 10 m/s, the velocities were typical of those responsible for the formation of fracture mirrors in glass. Roughness measurements were made using atomic force microscopy on two glass compositions: silica glass and soda lime silica glass. For silica glass, the RMS roughness, R_q, decreased from about 0.5 nm at a velocity of 1 × 10⁻¹⁰ m/s to about 0.35 nm at a velocity of 10 m/s. For soda lime silica glass, the roughness decreased from about 2 nm to about 0.7 nm in a highly non-linear fashion over the same velocity range. We attributed the roughness and the change in roughness to microscopic stresses associated with nanometer scale compositional and structural variations within the glass microstructure. A theory developed to explain these results is in agreement with the data collected in the current paper. The RMS roughness of glass also depends on the area used to measure the roughness. As noted in other studies, fracture surfaces in glass exhibit a self-affine behavior. Over the velocities studied, the roughness exponent, ζ, was approximately 0.3 for silica glass and varied from 0.18 to 0.28 for soda lime silica glass. The area used for these measurements ranged from (0.5 μm)² to (5.0 μm)². These values of the roughness exponent are consistent with values obtained when the scale of the measurement tool exceeds a critical size, as reported earlier in the literature.     

Silica-doped alumina cryogels with high thermal stability

Alumina cryogels with a dopant of silica in the content range of 0–10 wt% were synthesized from aqueous boehmite sol through the sol-gel technique and subsequent freeze drying. The higher thermal stability was achieved by the addition of 10 wt% silica; a γ-Al₂O₃ phase still remained after heating at 1200 °C for 5 h, and the surface area and pore volume were 47 m² g⁻¹ and 105 mm³ g⁻¹, respectively. The marked stability was ascribed to the synergetic effect of the very low bulk density (0.05 g cm⁻³) and the dopant. The thermal stability was lower for the cryogels than for the corresponding aerogels; however, it was also suggested that cryogel was highly durable in water in contrast to aerogel.     

X-ray photoelectron spectroscopy of erbium-activated-silica–hafnia waveguides

X-ray photoelectron spectroscopy (XPS) has been used in the study of sol gel-derived Er³⁺-activated xHfO₂–(100 − x)SiO₂ (x = 10, 20, 30, 40, 50 mol) planar waveguides. The analysis of Si 2p and O 1s core lines were related to the Hf/Si molar ratio to assess the role of hafnia in modifying the silica network. Increasing the HfO₂ content brings about a change of the Si 2p and O 1s binding energy respect to those from pure silica. This trend is explained with a formation of hafnium silicate in the matrix with successive phase separation between HfO₂ and SiO₂ rich phases. XPS results show that hafnia is well dispersed in the silica matrix for molar concentration below 30%. Formation of pure HfO₂ domains was detected at higher hafnia concentrations in agreement with previous spectroscopic analyses.     

An alumina mat with a nano microstructure prepared by centrifugal spinning method

Ceramic fiber products specially alumina mat because of low thermal conductivity and high melting point are used as high temperature insulating materials. Alumina has so high melting point (T_m > 2040 °C) that its mat can be produced through sol–gel method. In this research alumina mat has been manufactured by sol–gel spinning method using our laboratory-designed centrifugal spinneret. The desired viscosity of sol for spinning is 150 P. Phase transformation of the product begins at 600 °C and there is not any amorphous phase at 800 °C and theta alumina (θ-Al₂O₃) is the main phase. In this work, transformation of transitional phase to alpha alumina (α-Al₂O₃) takes place from 1000 °C to 1200 °C. The optimum percent of silica in alumina mat is 4 wt%. Fibers constitute network structure that their average diameter is about 10 μm and contains very fine grains (100 nm). The silica percent concerning the limits of this study (<10 wt%) does not effect on fiber diameter, but grain size decreases from about 200 nm to less than 100 nm while increasing silica percent.     

Thermal decomposition and new luminescence bands in wet,dry, and additional oxygen implanted silica layers

Wet and dry silica oxide layers have been treated thermally up to T_a = 1300 °C and were investigated by cathodoluminescence (CL) spectroscopy. Whereas the dry oxides after high temperature treatment show an increase of the yellow–red spectra region, contrary, in wet oxides the UV–blue region is enhanced. Even a new strong band in the near-UV region (NV) at 330 nm (3.76 eV) is found for wet oxides at liquid nitrogen temperature (LNT), but much broader and with lower intensity for room temperature (RT) in a triple band structure UV: 290 nm, NV: 330 nm, and V: 400 nm. These violet bands should be associated with a thermally decomposed and rapidly cooled-down silica network in presence of OH groups or even dissociated oxygen. Additional oxygen implantation into dry silica with high doses up to 10¹⁷ ions/cm² and high thermal treatment T > 1100 °C leads as well to enhanced UV–NV–V luminescence emission bands supporting the fact that oxygen and structural decomposition play a decisive role in formation of near-UV luminescent defects in silica.     

Thulium environment in a silica doped optical fibre

Thulium-doped optical fibre amplifiers (TDFA) are developed to extend the optical telecommunication wavelength division multiplexing (WDM) bandwidth in the so-called S-band (1460–1530 nm). The radiative transition at 1.47 μm (³H₄ → ³F₄) competes with a non-radiative multi-phonon de-excitation (³H₄ → ³H₅). The quantum efficiency of the transition of interest is then highly affected by the phonon energy (E_p) of the material. For reliability reasons, oxide glasses are preferred but suffer from high phonon energy. In the case of silica glass, E_p is around 1100 cm^?1 and quantum efficiency is as low as 2%. To improve it, phonon energy in the thulium environment must be lowered. For that reason, aluminium is added and we explore three different core compositions: pure silica, and silica slightly modified with germanium or phosphorus. The role of aluminium is studied through fluorescence decay curves, fitted according to the continuous function decay analysis. From this analysis, modification of the thulium local environment due to aluminium is evidenced.     

Photoluminescence of Sm3+, Dy3+, and Tm3+-doped transparent glass ceramics containing CaF2 nanocrystals

Photoluminescence properties of Sm³⁺, Dy³⁺, and Tm³⁺-doped transparent oxyfluoride silicate glass ceramics containing CaF₂ nanocrystals were reported. Emission bands of ⁴G_5/2 → ⁶H_5/2 (562 nm), ⁴G_5/2 → ⁶H_7/2 (598 nm), ⁴G_5/2 → ⁶H_9/2 (645 nm) and ⁴G_5/2 → ⁶H_11/2 (706 nm) for the Sm³⁺: glass and glass ceramic, with an excitation at ⁶H_5/2 → ⁴F_7/2 (402 nm) have been recorded. Of them, ⁴G_5/2 → ⁶H_7/2 (598 nm) has shown a bright orange emission. With regard to the Dy³⁺: glass, a bright fluorescent yellow emission at 575 nm (⁴F_9/2 → ⁶H_13/2) and blue emission at 481 nm (⁴F_9/2 → ⁶H_15/2) have been observed, apart from 662 nm (⁴F_9/2 → ⁶H_11/2) emission transition with an excitation at 386 nm (⁶H_15/2 → ⁴I_13/2 + ⁴F_7/2) wavelength. Emission bands of ¹G₄ → ³F₄ (650 nm) and ¹G₄ → ³H₅ (795 nm) transitions for the Tm³⁺: glass and glass ceramic, with an excitation at ³H₆ → ¹G₄ (467 nm) have been observed. Of them, ¹G₄ → ³F₄ (650 nm) has shown bright red emission. Decay lifetime measurements were also carried out for all the observed Sm³⁺, Dy³⁺, and Tm³⁺-doped glass and glass ceramic emission bands.     

Continuous silica fiber reinforced silica composites densified by polymer-derived silicon nitride: Mechanical properties and microstructures

Continuous silica glass fiber reinforced silica composites were prepared via silica sol suspension infiltration/sintering method followed by densification treatment by preceramic polymer infiltration/pyrolysis process, and the mechanical properties and microstructures of the composites were investigated. Pyrolyzed at 873 K in ammonia atmosphere, the preceramic polymer polyhydridomethylsilazane resulted in a low-carbon near-stoichiometric ceramic with an empirical formula of Si_1.0N_1.38C_0.01O_0.04H_0.78. After the densification treatment, the density of the composites increased a little from 1.64 g/cm³ to 1.79 g/cm³, and the flexural strength increased greatly from 69.4 MPa to 95.9 MPa. However, the densified composites exhibited a tendency to brittle failure due to the relatively strong interfacial bonding between the glass fibers and polymer-derived silicon nitride, and also the degradation of glass fibers during post-treatment.     

Synthesis of hybrid mesoporous spheres using the chitosan as template

Hybrid mesoporous spheres of Al and Si oxides were synthesized for the mixture of organic material (chitosan) with inorganic material (aluminum and silicon hydroxide). It was observed that chitosan with larger polymerization degree, resulted in a larger mechanical resistance of the spheres. The oxides were characterized by the following: Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), differential scanning calorimetry (DSC), as well as, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and adsorption isotherms of N₂ (BET). Highly uniform oxide sphere diameters were obtained (average of 1.0 mm). The results of the adsorption isotherms indicated that the material is mesoporous. The surface area of the materials ranged between 620 and 245 m²/g, and the pore volume varied between 0.82 and 0.28 cm³/g, depending on the molar ratio of the organic and inorganic materials.