Vanadia-silica xerogels: optical and structural properties

Vanadium doped silica gels were prepared from tetraethyl orthosilicate and three different inorganic vanadium precursors with formal oxidation states of V³⁺, V⁴⁺ and V⁵⁺ respectively. Optical and EPR studies were carried out on the dried gels to understand the changes in the oxidation state and coordination of vanadium in the doped silica gel matrix. The observed optical and EPR results provide very strong evidence to establish that irrespective of the starting material, vanadium is stabilized as vanadyl ion in the gel monoliths. EPR studies on the powdered samples corroborate the optical data on the gel samples and confirmed that the stabilized vanadyl ion is situated in a distorted octahedral geometry in these silica gels.     

Raman spectroscopic investigations of Mn2+ doping effects on the densification of acid-catalyzed silica xerogels

Undoped and Mn²⁺-doped silica xerogels were prepared from hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The xerogels were characterised by density measurements and fluorescence and Raman spectroscopies. Raman measurements over the range 4–1200 cm⁻¹ showed that the number of three- and four-membered rings in the xerogel network depends on the thermal treatment and on the concentration of Mn²⁺ ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in the undoped one, showing that doping with Mn²⁺ hampers the destruction of three- and four-membered rings. In the low-wave number region (4–100 cm⁻¹), doping with manganese ions was found to affect the position of the boson peak. The boson peak profiles were used to deduce that the sizes of the cohesive domains in the gel-derived silica network are much larger for doped samples (11 nm for 500 ppm) than for undoped ones (2.1 nm).     

Structure and spectroscopic properties of trapped holes in silica

We present the first consistent comparison of the theoretically predicted structures and spectroscopic characteristics of holes trapped in crystalline and amorphous silica. The structure and spectroscopic properties of Ge and [AlO₄]⁰ hole centers in α-quartz and self-trapped holes (STH₁ and STH₂) in silica glass were calculated using an ab initio embedded cluster method. The calculated principal values of g-tensor and hyperfine constants as well as the positions of maxima of optical absorption bands are in good agreement with the experimental data. In the case of Ge and [AlO₄]⁰ hole centers, as well as STH₁, the hole is almost fully localized on one O atom. In STH₂ the hole is delocalized over two bridging O atoms. The calculated principal values of the g-tensor and hyperfine constants of STH₁ and STH₂ support the assignment based on ESR studies. The calculated optical absorption energies of one-center holes in a amorphous silica agree with the experimentally observed absorption bands assigned to STH₁.     

Multigram scale synthesis and characterization of low-density silica xerogels

The synthesis of silica with preserved porosity and tailored morphology by sol–gel process can be achieved by hybrid organic–inorganic synthesis: a modified alkoxide, viz. 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), is introduced during the base catalysed synthesis with TEOS as main silica precursor. Additives with methoxy groups induce a nucleation mechanism because of their higher reactivity compared to main reagents with ethoxy groups. The nucleation model presented in previous papers was refined by taking into account the porosity of the particles and calculating the number of additive molecules by nucleus for each value of the ratio of additive/main reagent. The extrapolation of the synthesis process to semi-industrial scale goes through the replacement of laboratory grade reagents by industrial grade reagents and the scaling up to the production of higher quantities. At each of these two steps, the morphology and porosity of the samples has been compared to those of laboratory grade samples. It was shown that the texture and particle size has quasi totally been preserved.     

Textural properties of low-density xerogels

The extent of shrinkage during drying is controlled by the balance between the capillary pressure developed in the pore liquid and the modulus of the solid network. One first method to obtain low-density xerogels consists in strengthening TEOS-based alcogels by providing new monomers to the alcogel after gelation. In the second method, low-density xerogels are produced by surface modification (silylation) of the wet gel with trimethylchlorosilane. The capillary pressure is reduced and the presence of non-reactive species on the surface makes the shrinkage reversible. A reduction of the capillary pressure can be achieved by introduction of a substituted alkoxide 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) to a TEOS-based alcogel, synthesised in a single base-catalysed step. This additive acts as a nucleation agent leading to big silica particles (20 nm) with a low EDAS/TEOS ratio (0.03). The pores between those particles are also large and the drying stress is reduced. The textural properties of those three materials are compared: bulk densities of the samples modelled on the first and third method are varying in the same range (0.25–0.35 g/cm³) while xerogels obtained by the surface modification process are less dense (0.1–0.15 g/cm³). The biggest pores are observed in the third method.     

Cobalt porphyrin covalently bonded to organo modified silica xerogels

Some of the fine physicochemical properties displayed by porphyrin solutions can be preserved when these species are trapped within inorganic oxide pore networks, such as silica. A successful outcome is related to inhibiting the interaction between the macrocycle and SiOH surface groups. Here, when porphyrins are chemically bonded to the walls of a silica network through molecular bridges arising from functionalized alkoxides alone or combined with monomer precursors (i.e. lactams, diamines, etc.), their spectroscopic properties can be kept similar to those shown in solution. This latest outcome consists in bonding porphyrinic species to the pore walls of an organo-modified silica networks. In the present work, a strategy has been designed to uphold these useful properties by covalently bonding cobalt porphyrin molecules inside a SiO₂ pore network where SiOH surface groups are exchanged by alkyl groups proceeding from organo-substituted alkoxides. In these hybrid systems, the electronic transitions are well preserved when SiOH surface groups are exchanged by groups such as methyl, ethyl, vinyl, etc. This separation action prevents flocculation among macrocyclic molecules (even if a change in polarity occurs inside the pores) due to the adsorption of bridging alkyl groups. A proper selection of both macrocyclic molecules and bridging molecules, in conjunction with a proper choice of the synthesis conditions, lead to the attainment of hybrid solid systems in which the spectroscopic properties are not only preserved but can even be adjusted by selecting suitable sizes and identities of the bridging alkyl groups.     

Low-temperature specific heats of porous silica xerogels of low densities

Low-temperature specific heat measurements have been performed in porous silica xerogels with densities varying from 670 to 1730 kg m⁻³ to study the low-energy vibrational dynamics. The specific heat, C_p, shows a bump in the temperature range above 4 K, when reported in a plot of C_p/T³ against the temperature, T. The bump is almost independent of the sample density and is close to the boson peak observed in melt-quenched amorphous silica (a-SiO₂). At temperatures <4 K, an additional contribution to that predicted by the Debye theory is observed. It follows an approximately linear temperature dependence (C_exc=aT^1+v, v being equal to about 0.25). In the xerogel with the largest density, specific heat of about a factor 5 larger than that of a-SiO₂ is measured, which increases with decreasing sample density. By comparison with the corresponding properties of a-SiO₂, we conclude that the disorder introduced by the presence of pores does not measurably affect the excess density of vibrational states in a frequency range of the boson peak (BP), but increases the density of the two-level systems (TLS).     

Influences of Yb3+ ion concentration on the spectroscopic properties of silica glass

We investigated optical spectroscopic properties of silica glass doped with ytterbium (Yb³⁺) ions to assess their lasing performance in diode-laser pumped systems. The Yb-doped silica glass preforms were fabricated by solution doping technique using the MCVD process. The stimulated emission cross-section and laser performance parameters were determined from the measured absorption spectra using the method of reciprocity. Fluorescence decay characteristics were observed to be deviated from exponential behavior as the Yb³⁺ ions doping level increases and there exists a lifetime quenching behavior related to the cluster effect of Yb³⁺ ions into silica glass. Nevertheless, lasing parameters indicated that clustering of Yb³⁺ ions does not significantly affect the spectral properties relevant to the predicted lasing performance when concentration is low, but becomes predominant at higher concentration.     

Relationship between structure and optical properties in rare earth-doped hafnium and silicon oxides: Modeling and spectroscopic measurements

The SiO₂–HfO₂ binary system is recognized as a promising candidate for Erbium-doped waveguides amplifiers fabrication. Recently, it was demonstrated that Er³⁺-activated 70SiO₂–30HfO₂ planar waveguides with valuable optical and structural properties can be prepared by sol–gel technique with dip-coating processing. The important role played by hafnium in the silica network was evidenced by the particular spectroscopic properties presented by Er³⁺-ions in the silica–hafnia planar waveguides. In this work we present preliminary results on HfO₂–SiO₂ bulk xerogels doped with Eu³⁺ ions, with the aim to go inside the role of hafnium on the rare earth ions local environment. Spectroscopic measurements of the Eu³⁺ photoluminescence emission are given. Numerical simulations by the molecular dynamics method have been performed showing clearly a phase separation for the HfO₂ richer samples. Moreover it is found than the rare earth-doping ions stay preferentially in hafnium rich domains, thus explaining why the rare earth spectroscopic properties are strongly modified.     

Raman spectroscopic study of hydrogen-containing vitreous silica

It is shown by Raman spectroscopy that high-temperature hydrogen-impregnated vitreous silica contains, in addition to physically dissolved hydrogen, ≡SiOH and ≡SiH groups. A Raman-scattering peak with a wave-number shift of 3685 cm^?1 is assigned to the OH stretching vibration, a peak with a wave-number shift of 2254 cm^?1 to a SiH stretching vibration and a peak with a wave-number shift of 4135 cm^?1 to the presence of physically dissolved hydrogen. When hydrogen was replaced by deuterium, appropriate shifts of the peaks mentioned were found.     

Size‐dependent spectroscopic,optical, and electrical properties of PbSe nanoparticles

Stable crystalline Lead Selenide nanoparticles (PbSe NPs) were prepared with a cubic (sphalerite) phase in polyvinyl alcohol (PVA) matrix. Moreover, XRD was used to characterize the PbSe NPs in PVA. The size were controlled by Pb:Se ions ratio in which the particle size was found to be inversely proportional to this ratio up to 16:1. FT‐Raman and FT‐IR measurements indicated the decrease of crystallization of PVA due to doping with PbSe NPs as indicated by the appearance of a peak at 1124 cm^‐1 that increases by size reduction of PbSe NPs. The Photoluminescence and electrical measurements monitored the interaction of PbSe NPs with PVA which increased by decreasing the particle size. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The role of the main silica precursor and the additive in the preparation of low-density xerogels

The incorporation of an additive during sol-gel synthesis reduces shinkage during ambient drying. The following additives have been studied: 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltriethoxysilane (AES) and 3-(2-aminoethylamino)propyltriethoxysilane (EDAES) and the main silica precursors were tetraethylorthosilicate (TEOS) and tetrapropylorthosilicate (TPOS). When the additive contains methoxy groups (EDAS), it acts as a nucleation agent of the silica particles and exactly the same properties (pore volume, specific surface area, particle and aggregate size) are obtained whether the main reagent is TEOS or TPOS. The nucleation mechanism is based on the difference in reactivity between additive and main reagent. In case of nucleation by the additive, the nucleation agent fixes the properties whatever the main silica precursor is. When both the additive and the main reagent contain ethoxy groups (series AES-TEOS and EDAES-TEOS), there is no nucleation mechanism by the additive, and the silica particle size remains nearly constant. With less reactive main reagent (series AES-TPOS and EDAES-TPOS), pore volumes up to 17 cm³/g have been obtained with pore sizes up to nearly 10 μm and very big particles (∼100 nm). The absence of nucleation by the additive for the couples AES-TPOS and EDAES-TPOS could be due to the fact that the difference in reactivity between ethoxy groups and propoxy groups is not sufficient to initiate the nucleation mechanism by the additive. In the absence of nucleation by the additive, the main reagent plays a role: highly porous materials with very large pores are prepared with TPOS.     

High porosity silica xerogels prepared by a particulate sol–gel route: pore structure and proton conductivity

The lower cost and higher hydrophilicity of silica xerogels could make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane fuel cells (PEMFCs). For that purpose, we need to obtain micro or micro and mesoporous silica xerogels with a high porosity. The preparation of micro (<2 nm) and micro and mesoporous silica xerogels (2<d_poresize10 nm) from particulate as oppossed to polymeric suspensions of silica using tetraethyl orthosilicate (TEOS) as precursor is used. Two techniques of varying packing density have been performed in this work: (1) Control of the aggregation degree in the sol by adjusting its pH before gelation (pH 5, 6 and 8) and (2) Mixture of sols with a different average particle size (particles formed under acid and base catalyzed reactions). Proton conductivity of the obtained xerogels was studied as a function of temperature and relative humidity (RH). High pore volume, high porosity and small pore size SiO₂ xerogels have been achieved in the calcination temperature range from 250 to 550 °C. The calcined xerogels showed microporosity or micro and mesoporosity in the whole range of calcination temperatures. By mixing sols (molar ratio: acid/base=4.8) porosities up to 54.7±0.1% are achieved, at 250 °C of firing temperature. According to EMF measurements, electrical transport is due to protons in this kind of materials. The proton conductivity of the studied xerogels increased linear with measured temperature. A S-shaped dependence of the conductivity with the RH was observed with the greatest increase noted between 58% and 81% RH. Xerogels with a low porosity (40.8±0.1%) and an average pore size less than 2.0 nm showed lower values of proton conductivity than that of xerogels with a higher porosity and a higher average pore size in the whole range of temperature and RH. When silica xerogels, with the highest conductivity, are treated at pH 1.5, that property increased from 2.84×10⁻³±5.11×10⁻⁵ S/cm to 4.0×10⁻³±7.2×10⁻⁵ S/cm, at 81% RH and 80 °C. It indicates that the surface site density of these materials has a strong effect on conductivity. Proton conductivity values achieved are less than one order of magnitude lower than that of Nafion, under the same conditions of temperature and RH.     

Effects of poly(vinyl alcohol) additions on the structure of silica xerogels

Poly(vinyl alcohol)/silica hybrid xerogels were prepared from sonohydrolysis of tetraethoxysilane (TEOS) and additions of water-solution of poly(vinyl alcohol) (PVA). The samples were studied by small-angle X-ray scattering (SAXS), nitrogen adsorption, and differential scanning calorimetry (DSC). On drying at room temperature the resulting xerogels exhibit a fairly bimodal porous structure composed by small mesopores and micropores. The pore size distribution of the mesopores was found to follow approximately a power-law with the pore size. The micropore structure was associated to an evolution at a high resolution level of the mass fractal structure of the original wet gels. The role of the PVA addition on the pore structure of the xerogels is to diminish the specific surface area and the pore volume without to change substantially the pore mean size.     

Raman spectroscopic study of bioactive silica based glasses

A study of the structure and bonding configuration of the bioactive glasses in the system Na₂O–CaO–P₂O₅–SiO₂ by Fourier transform Raman spectroscopy is presented. The assignment of the Raman lines, the changes in the Si–O–Si bond environment and the identification of the non-bridging silicon–oxygen groups (Si–O–NBO) for a wide range of silicate glasses are discussed. The frequency shifting and intensity variations of the Raman lines as a function of the bioactive glass composition are attributed to a decrease of the local symmetry originated by the addition of alkali and alkali earth oxides to the vitreous silica network. Correlation plots for the quantification of the Si–O–NBO groups as a function of the glass composition are also presented. These Raman analyses contribute to a better knowledge of the structural role of the network modifiers in the bioactive glasses and, as a consequence, improve the understanding of the bioactive process and the chemical routes of the CaP layer formation when exposed body fluids.     

Ab initio molecular dynamics simulations of oxygen-deficient centers in pure and Ge-doped silica glasses: Structure and optical properties

We have calculated the optical absorption (OA) bands of oxygen-deficient centers (ODC(I) type) in pure and germanium (Ge)-doped silica using the density functional theory-plane waves (DFT-PW) method connected to the Kubo–Greenwood formalism. A statistical approach is used in order to reproduce the complex nature of amorphous systems. This preliminary study shows that calculated locations of the OA bands of both Si-ODC and Ge-ODC accurately agree with experimental data in spite of the well-known underestimation of the gap of insulators using DFT.     

Mechanical reliability of silica optical fibers

Dynamic and static mechanical tests were implemented using a tensile test bench and a static fatigue test under uniform curve. The incidence of aging treatments at 65 and 85 °C was investigated on two standard silica optical fibers (with polyacrylate and fluorinated coatings). Microscopic observations helped the understanding of the failure mechanism. It appears that the cyclic variations of the failure stress phenomenon, with respect to the aging time, are the result of the silicate gel which migrates towards the polymer coating.     

Luminescent materials consisting of Eu(III) ions complexed in heteropolyoxometalates incorporated into silica xerogels

Luminescent materials based on the energy transfer effect consist of heteropolyoxometalates of the type K₁₃[Eu(SiMo_xW_11−xO₃₉)₂] incorporated into xerogel matrices. Results of our experiments suggest that not only the composition parameter x (where x = 1, 3 and 5) of the salts with Eu(III) complex but also the type of the matrix influence the luminescent properties of these materials. The luminescent samples were characterized by such luminescence parameters as emission intensity, luminescence lifetime and quantum yield. The highest emission intensity of Eu(III) ions was exhibited by the salt with x = 1 incorporated into a silicate modified with 3-glycidoxypropyl groups. The longest lifetime was found in the material with a methylated silicate matrix with the same salt. For the complexed Eu(III) ions in these materials there is a correlation between emission intensity changes of the ⁵D₀ → ⁷F₂ band and the quantum yield. The materials with a high organic content in matrices such as the silicates with 3-glycidoxypropyl groups (either with closed or opened epoxy cycles) are more thermally unstable and they undergo larger photochemical degradation during exposure on UV radiation than the systems with limited organic content.