Preparation and growth mechanism of clustered one-dimensional SiOx amorphous nanowires by catalytic pyrolysis of a polymer precursor

Large-scale synthesis of clustered one-dimensional amorphous silica nanowires was achieved by simple thermal pyrolysis of an amorphous preceramic powder from perhydropolysilazane on alumina wafers coated with catalyst FeCl₂. Scanning electron microscopy and transmission electron microscopy observations showed that the silica nanowires had smooth surface, and lengths of hundreds of micrometers and diameters in the range of 30–40 nm. Energy dispersive X-ray spectroscopy revealed that these nanowires consisted of Si and O elements in an atomic ratio of approximately 1:2, consistent with the stoichiometric formula SiO₂. The two amorphous bulges in Raman spectrum at the centers of around 260 cm^?1 and 800 cm^?1 were identified to be those of amorphous silica. The growth mechanism of the as-produced silica nanowires could be attributed to vapor–liquid–solid mechanism. These results provide an alternative and simple preparation procedure for nanostructures with controlled morphology, and it will be helpful to understand the growth mechanism of one-dimensional SiO₂ nanostructures.     

Preparation and characterization of nickel based catalysts on silica,alumina and titania obtained by sol–gel method

This work reports the investigation of the characteristics of catalysts produced by the sol gel process. Our analysis focuses on the properties related to heterogeneous catalysis, such as methane steam reforming, dry reforming, hydrogenation of organic compositions. Alumina, silica and titania based materials doped with Ni were synthesized. The characterization techniques used were: temperature programmed reduction, thermogravimetric analysis, specific surface area and X-ray powder diffraction. The specific surface area values obey the following sequence: Ni–SiO₂ > Ni–Al₂O₃ > Ni–TiO₂. The temperature programmed reduction indicated that in the Ni–SiO₂ sample, the nickel oxide is present in two different forms on the surface; the Ni–Al₂O₃ material presented one peak at high temperature, suggesting the presence of a nickel aluminate form. However, Ni–TiO₂ did not present reduction peaks. The thermogravimetric analysis, which was performed under inert atmosphere, showed the decomposition of the organic residues adsorbed on surface. The X-ray powder diffraction patterns of calcined materials showed two crystalline forms for TiO₂ in Ni–TiO₂ rutile and anatase. In Ni–Al₂O₃, crystalline areas composed of α-Al₂O₃, γ-Al₂O₃ and NiO were observed and finally for Ni–SiO₂, amorphous areas and NiO were found.     

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.     

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.     

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.     

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.     

Structural investigations of nitrided Nb2O5 and Nb2O5–SiO2 sol–gel derived films

Sol–gel derived xNb₂O₅–(100 ? x)SiO₂ films (where x = 100, 80, 60, 50, 40, 20, 0 mol%) were nitrided at various temperatures (800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C). The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The XRD results have shown that the temperatures below 1100 °C were too low to obtain a pure NbN phase in the samples. The AFM observations indicate that the formation of the NbN phase and the size of NbN grains are related to the silica content in the layer. NbN grains become more regular and larger as the niobium content increases. The maximum grain size of about 100 nm was observed for x = 100. Preparation of the Nb₂O₅–SiO₂ sol–gel derived layers and the subsequent nitridation is a promising method of inducing crystalline NbN in amorphous matrices. It follows from the XPS results that a small amount of Nb₂O₅ remains in the films after nitridation at 1200 °C and that nitrogen reacted not only with Nb₂O₅ but also with SiO₂.     

Precipitation of zinc ferrite nanoparticles in the Fe2O3–ZnO–SiO2 glass system

Glass materials in the ZnO–Fe₂O₃–SiO₂ system, containing zinc ferrite nanoparticles, were prepared by the sol–gel method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy, AC- and DC-magnetization techniques. The gel samples, dried at 130 °C, were further heat treated in air at 500 and 800 °C. At 500 °C zinc ferrite and hematite nanoparticles, with an average size of approximately 24 nm, were precipitated in the brown and opaque 10ZnO–10Fe₂O₃–80SiO₂ and in the ruby colored transparent 5ZnO–5Fe₂O₃–90SiO₂ and 2.5ZnO–2.5Fe₂O₃–95SiO₂ glass matrices. In the 5ZnO–5Fe₂O₃–90SiO₂ sample the nanoparticles exhibited ferro or ferrimagnetic interactions combined with superparamagnetism with a blocking temperature of approximately 14 K. Heating at 800 °C seems to cause partial dissolution of the zinc ferrite and hematite particles in all the investigated compositions. Accordingly at 800 °C the 5ZnO–5Fe₂O₃–90SiO₂ glass shows a paramagnetic behavior down to 2 K.     

Effect of TiO2 addition on the chemical durability of Bi2O3–SiO2–ZnO–B2O3 glass system

The effect of the substitution of ZnO for TiO₂ on the chemical durability of Bi₂O₃–SiO₂–ZnO–B₂O₃ glass coatings in hot acidic medium (0.1 N H₂SO₄ at 80 °C) for different times was studied. The thick films produced by a screen-printing method and heat treated at 700 °C/5 min were analyzed by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The glass from the Bi₂O₃–SiO₂–ZnO–B₂O₃ system developed Zn₂SiO₄ and a glassy phase that were readily attacked by hot 0.1 N sulfuric acid, whereas the heat treated coating from the Bi₂O₃–SiO₂–TiO₂–ZnO–B₂O₃ system presented a finer microstructure with thin interconnected Bi₄Ti₃O₁₂ crystals and a glassy phase more resistant to hot 0.1 N sulfuric acid attack etching.     

Quantitation of sulfate solubility in borosilicate glasses using Raman spectroscopy

Raman spectroscopy is used here as an innovative technique to investigate sulfate content in borosilicate glasses. Using Raman spectroscopy after having heated the material, the evolution of sulfate amounts can be followed as a function of temperature, time and chemical composition of the starting matrix. The accuracy of this technique was verified using electron probe micro analysis (EPMA), on two systems of glasses (SiO₂–B₂O₃–Na₂O (SBNa) and SiO₂–B₂O₃–BaO (SBBa)) in order to compare the effect of alkaline or alkaline-earth elements on sulfur speciation and incorporation. To quantitate sulfate content with Raman spectroscopy, the integrated intensity of the sulfate band at 990 cm^?1 was scaled to the sum of the integrated bands between 850 and 1250 cm^?1, bands that are assigned to Qⁿ silica units. Calibration curves were then determined for different samples. The determination of sulfate contents with Raman spectroscopy analysis is possible with an accuracy of approximately 0.1 wt% depending on the composition of the glass. It mainly allows us to follow sulfate removal during the elaboration process and to establish kinetic curves of sulfate release as a function of the viscosity of the borosilicate glass.     

Electron beam poling in amorphous Ge-doped H:SiO2 films

Amorphous Ge-doped H:SiO₂ films on silica, deposited by matrix-distributed electron cyclotron resonance – plasma enhanced chemical vapor deposition, were irradiated with an electron beam while varying the dose. Using the Maker fringe method, second-harmonic generation was measured in the irradiated regions of the films. With a current of 5 nA, and an acceleration voltage of 25 kV for 25 s, a Ge-doped H:SiO₂ film (3.8 at.% Ge) showed a maximum second-order nonlinearity of d₃₃ = 0.0005 pm/V. In contrast, a H:SiO₂ film with a smaller Ge content (1.0 at.% Ge), showed a large SHG: d₃₃ = 0.06 pm/V when irradiated for 15 s. The second-harmonic generation in the films is caused by a frozen-in electric field induced by charge implantation from the electron beam. The strength of the electric field is determined by two conditions: the trapping centers (numbers, depth) and the remaining conductivity under large electric field.     

Nitridation of SiO2–B2O3 aerogels

SiO₂–B₂O₃ aerogels have been prepared by drying wet gels at a supercritical condition for ethanol in an autoclave. Aerogels have been nitrided for 6 h in flowing ammonia at the temperature of 1200 °C. It has been found that the amount of nitrogen incorporated in these aerogels always exceeds 20 wt%. This is a much higher value compared with the amount of nitrogen incorporated in a pure silica aerogel nitrided at the same conditions. The specific surface area of SiO₂–B₂O₃ aerogels has been between 312 and 359 m²/g. After nitridation some shrinkage of aerogels has been observed and the surface area decreases about 20%. In FTIR spectra of SiO₂–B₂O₃ aerogels a typical bands for SiO₂ are observed. After nitridation a shift and broadening of 1100 cm^?1 band to lower wavenumbers indicates that Si–N and B–N bonds are formed in nitrided aerogels.     

Characterization of thermo-optical and mechanical properties of calcium aluminosilicate glasses

In this work several different compositions of CaO:Al₂O₃:SiO₂ were prepared under vacuum atmosphere to study the glass forming ability of this system as a function of the SiO₂ content. Samples containing 25–45 wt% of Al₂O₃, 31–44 wt% of CaO, 14–39 wt% of SiO₂ and 4.1 wt% of MgO were prepared in graphite crucibles, for approximately 2 h at ∼ 1600 °C. The influence of silica content is discussed in terms of the mechanical properties, glass transition temperature, crystallization temperature, transmittance spectrum, refractive index, mass density, specific heat, thermal diffusivity, thermal conductivity and the temperature coefficient of optical path length change. The results reinforce the idea that these glasses are strong materials, having useful working-temperature range, good combination of thermal, mechanical and optical properties that could be exploited in many optical applications, in particular, as glass laser materials.     

Silica xerogel films hybridized with carbon nanotubes by single step sol–gel processing

Synthesis of multi-walled carbon nanotubes (MWCNTs) doped silica xerogel films was reported in this work. A crucial step of introducing MWCNTs was achieved by functionalizing them by acid treatment to form stable and homogenous SiO₂/MWCNTs sol. Scanning electron microscopy showed spherical particles in honeycomb network structure for undoped xerogel films whereas dispersion and wrapping of MWCNTs in silica matrix was observed for MWCNTs doped films. Various bond formations during the sol–gel process and surface modification were confirmed using Fourier transform infra-red and detailed study on the chemical bonding state of the films was carried out using X-ray photoelectron spectroscopy. Nanoindentation studies showed that the mechanical properties of MWCNTs doped xerogel film increase dramatically: higher modulus (E = 2.127 ± 0.095 GPa) and hardness (H = 0.035 ± 0.017 GPa) values than those of pristine xerogel film (E = 0.234 ± 0.058 GPa, H = 0.01 ± 0.003 GPa).     

Effect of iron oxide addition on structural properties of calcium silico phosphate glass/glass-ceramics

Glasses with nominal composition 34SiO₂–(45 ? x) CaO–16 P₂O₅–4.5 MgO–0.5 CaF₂–x Fe₂O₃ (where x = 5, 10, 15, 20 wt.%) have been synthesized by melt quench technique. These have been investigated for structural features by using Fourier transform infra-red (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Results have shown an increase in fraction of non‐bridging oxygen in glasses with an increase in iron oxide content up to 15 wt.% and subsequently decreases with further increase in iron oxide content to 20 wt.%. These effects are originated by the incorporation of Fe₂O₃ into the silica network. Iron oxide behaves as a network modifier at low concentration and stabilizes the glass network at higher content. The glass-ceramics exhibit an increase in the formation of magnetite phase with an increase in iron oxide. The glass phase in the glass-ceramics matrix, controls the surface dissolution, which in turn decides the response of the material in-vitro. The glass-ceramics with 15 wt.% iron oxide has shown optimum response in simulated body fluid.     

Molecular dynamics simulation of temperature-induced structural changes in cristobalite,coesite and amorphous silica

Structural changes in cristobalite, coesite, and amorphous silica have been investigated from 100 to 3000 K by molecular dynamics simulations. These have been made with ‘soft potentials’ that yield melting and glass transition temperatures in fair agreement with experimental data. The simulated density of cristobalite increases abruptly at the α–β transition, that of coesite increases gradually, whereas that of amorphous silica goes through a density maximum around 2000 K. In the three phases, the complex temperature-induced structural changes have been analyzed in terms of variations of the abundances of four ‘structons’ which represent different kinds of Si–O bond lengths and torsional angles between two SiO₄ units. These changes in cristobalite, coesite and amorphous silica can then be rationalized in terms of loosening of network and of dynamical rotation of SiO₄ units. The relation between structural changes and ring statistics is also discussed.     

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.     

Improvement in optical transmission of the ambient pressure dried hydrophobic nanostructured silica aerogels with mixed silylating agents

The experimental results on the preparation of transparent and hydrophobic silica aerogels based on ion exchanged sodium silicate (Na₂SiO₃) using mixed silylating agents of trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDZ) are reported. Hydrogels were prepared with ion exchanged Na₂Si0₃ of 1.10 specific gravity with 1 N ammonium hydroxide (NH₄OH). The resulted gels were exchanged with methanol solvent followed by silylation using HMDZ + HMDSO or TMCS + HMDZ or TMCS + HMDSO mixture in methanol and hexane and dried the gels at room temperature for 24 h, at 50 and 200 °C for 1 h each. It has been observed that the percentage of the silylating agent in the mixture, time interval of addition of one agent followed by another and volume of silylating mixture have an effect on density, % of optical transmission, % of porosity, porevolume, thermalconductivity and refractive index of the aerogels. Hydrophobicity of the aerogels was studied by contact angle measurements. The TMCS + HMDSO aerogels have been found more hydrophobic (contact angle > 150°) than the other aerogels. Further, aerogels have been characterized by pore size distribution, Fourier Transform Infra Red Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and thermogravimetric analysis and differential thermal analysis (TGA–DTA) techniques. It has been found that the weight increase is the highest (325%) for HMDSO + HMDZ aerogels and lowest (1.2%) for HMDSO + TMCS aerogels by keeping the aerogels in the atmosphere over a period of 18 months. Low density (0.042 g/cc) transparent (85%), low thermal conductive (0.047 W/m K), low refractive index (1.0088) and hydrophobic (152°) silica aerogels were obtained with equivolume TMCS + HMDSO mixture of 10 ml, time interval of addition of 12 h between the two reagents prior addition of TMCS followed by HMDSO for 24 h of silylation period. It was found from the TGA–DTA of the aerogels that hydrophobicity of the aerogels remained up to the temperature of 425 °C.     

Non-isothermal crystallization kinetic studies on MgO–Al2O3–SiO2–TiO2 glass

Thermal stability and crystallization kinetics of the glass 21% MgO, 21.36% Al₂O₃, 53.32% SiO₂ and 4.11% TiO₂ (mol%) has been studied using differential thermal analysis (DTA), dilatometry and X-ray diffraction (XRD). Glass in both bulk and frit forms were produced by melting in platinum crucible at 1600 °C for 1–2 h. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 340 kJ mol⁻¹ and 498 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Crystallization of bulk glass was carried out at various temperatures and for different time durations in the range of 850–1000 °C. The influence of the addition of TiO₂ on the crystallization sequence of the glass was experimentally determined and discussed.     

Crystallization of amorphous Al2O3–SiO2 precursors doped with nickel

The crystallization of amorphous diphasic Al₂O₃–SiO₂ precursors doped with nickel has been studied by differential scanning calorimetry (DSC), XRD diffraction (XRD) and scanning and transmission electron microscopy (SEM, TEM) equipped with energy dispersive X-ray spectroscopy (EDS). Diphasic gels with constant atomic ratio (Al + Ni)/Si = 3:1, where 0, 1, 2 and 3 at.% of aluminum were replaced by nickel, have been prepared by hydrolyzing of TEOS in aqueous solution of aluminum nitrate. Crystallization of Ni-containing γ-Al₂O₃ preceded the crystallization of Al–Si spinel. Activation energy of 603 ± 16 kJ mol⁻¹ for crystallization of Ni-containing γ-Al₂O₃ was obtained in non-isothermal conditions. Ni-incorporated γ-Al₂O₃ transforms gradually with the temperature increase into Ni aluminate spinel, while Al–Si spinel reacts with amorphous silica forming mullite at about 1200 °C. Rietveld structure refinement of phases present in the samples annealed at 1600 °C and SEM-EDS and TEM-EDS analyses of related phases have shown that nickel predominantly crystallizes as NiAl₂O₄, but small amount of nickel is incorporated in mullite structure, as well as, dissolved in the glassy phase of the system.