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.     

Preparation of low-density xerogels from mixtures of TEOS with substituted alkoxysilanes. II. Viscosity study of the sol–gel transition

Mixtures of TEOS with substituted methoxysilanes generate low-density xerogels due to a nucleation mechanism involving the substituted alkoxysilane. The sol–gel transition of these mixtures was followed by rheological characterisation. The transition from sol to gel takes place in a few minutes at ambient temperature. For the series exhibiting nucleation by the additive, the gel time goes through a slight minimum when the ratio of additive/main reagent increases. The elastic modulus increases with increasing ratio of additive/main reagent as the particle size decreases because of the nucleation mechanism by the additive. Samples with smaller particles exhibit the highest modulus for equal silica concentrations.     

Understanding the mechanisms of reaction and release of acid–base indicators entrapped in hybrid gels

The reactivity and release of acid–base indicators entrapped in silica and hybrid silica gels were studied. Methyl orange (MO), methyl red (MR), and phenyl red (PR) were used as the indicators. Tetraethoxysilane (TEOS), n-propyltriethoxysilane (PTES), and bis(trimethoxysilyl)hexane (TSH) were used as gel precursors. HCl (0.01 M) and NaOH (0.01 M) solutions were used as the media for acid–base reaction and release in the experiments. The effects of organosilane addition were examined to understand the processes and mechanisms of reaction and release of the indicators entrapped in the gels. The experimental results suggest that adding the organosilanes lowers the pore size and pore volume of the gels, and therefore suppresses the indicator release, but does not present any obvious effects on the acid–base reactivity of the entrapped indicators. MR entrapped in the gels has no acid–base response, while MO and PR entrapped in the gels can present acid–base response without definite acid–base indicating pH ranges. Only the color change pH values can be determined, which present a ‘lag effect’ or ‘hysteresis’ compared with the cases of the free indicators in solutions. The reactivity of the entrapped indicators is suggested to be mainly determined by their chemical nature, the interactions between the indicators and the gel matrices, and the possible involvement of the indicators in the sol–gel process. In general more indicators were found to release in basic solutions than in acidic solutions. For all the entrapped indicators, in acidic solutions through the whole release process, and in basic solutions at the beginning of the release, the indicator release was found to follow the sequence of TEOS Gel > TEOS/PTES Gel > TEOS/TSH Gel. The overall release process is diffusion-controlled, and the release is mainly affected by the textural properties of the gels, the interactions between the gel matrices and the indicators, and the indicator solubility.     

Spherical and lamellar octadecylsilane hybrid silicas

Silicas bearing different contents of octadecylsilane groups were synthesized by the sol–gel method and characterized by solid-state magic angle spin ¹³C nuclear magnetic resonance spectroscopy, Raman spectroscopy, attenuated total reflectance infrared spectroscopy, small angle X-ray scattering, laser light scattering and atomic force microscopy. A structural model for such hybrid materials is proposed in which spherical or lamellar morphology, fern-like or Porod’s structure, are proposed depending on the tetraethyl orthosilicate (TEOS)/octadecylsilane (ODS) molar ratio. The effect of the ODS addition time on the chain conformation and on the particle morphology and texture was also investigated. The degree of organization lowered as the amount of ODS increased. The nanostructured xerogel obtained in the case of pure TEOS evolutes from spherical to lamellar patterns, as the amount of octadecylsilane is increased.     

Rheology of the gelation of fluorine-doped silica sols

Dynamic oscillatory shear measurements are used to probe the gelation kinetics of aqueous sols composed of either particulate silica or silicon alkoxide solution. Unlike steady shear measurements, these dynamic tests do not alter the structure and kinetics of the sol-gel process. The dynamic storage moduli of both systems show sharp transitions at the onset of gelation. However, the gelation kinetics of the two systems are very different; the modulus of alkoxide system remains unchanged until the gel point is reached, whereas that of the particulate system increases with time even below the gel point as the colloid cluster grow. Unlike the alkoxide gel, the particulate gel reverses to a sol upon shearing. The gelation kinetics of a resheared sample is slower than that of a fresh sample. The structures associated with the sol-gel transition for this particulate system have been monitored using freeze fracture microscopy and correlate with rheological observations. The gelation times for both sols are varied by changing the fluoride ion content.     

Preparation of low-density xerogels from mixtures of TEOS with substituted alkoxysilanes. I. O NMR study of the hydrolysis–condensation process

Low-density xerogels were synthesised by incorporation of an additive to base catalysed tetraethylorthosilicate (TEOS) alcogels directly during the preparation of the sol. The nucleation mechanism by the additive was established by experiments during sol–gel transition. ¹⁷O NMR spectroscopy on TEOS–ethanol–water, 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS)–ethanol–water and EDAS–TEOS–ethanol–water solutions shows that the hydrolysis–condensation of EDAS is much faster than that of TEOS. Consequently it can be assumed that EDAS forms nuclei, onto which TEOS condenses later to form the silica particles.     

Spinnablity and structure characterization of mullite fibers via sol–gel-ceramic route

Viscosity and rheology behaviors of the mullite sols prepared from aqueous solution of aluminum nitrate, aluminum isopropoxide and tetraethylorthosilicate has been investigated. Rheological measurement suggested that mullite sols exhibited good spinniabilty when the evaporation of the solvent is allowed during sol–gel process. Spinnable sols showed shear-thinning flow behavior with high viscosity to the time of gelation. By adjusting temperature, the gelation degree and viscosity of the sol could be stabilized at a certain value and the sol–gel transition could be transferred to the spinning line of a laboratory dry-spinning apparatus. Continuous mullite fibers were obtained from such sols using sol gel dry-spinning method. The final ceramic fibers had smooth surfaces with an average diameter of 50 μm. Structure evolution of mullite ceramic fibers were characterized by MAS NMR and specific surface area analyses.     

Mechanical and thermal properties of SiO2–PMMA monoliths

Organic–inorganic hybrid monoliths of cross-linked polymethylmethacrylate (PMMA) and silica (SiO₂) were prepared by sol–gel, using tetraethoxysilane (TEOS) and methylmethacrylate (MMA) as precursors and 3-(trimethoxysilyl) propylmethacrylate (TMSPM) to make compatible the organic and inorganic components. Two types of monoliths were prepared, H1-type using a molar ratio composition of 16:1:4 for water:TEOS:ethanol and 1:0.1:0.1 for TEOS:TMSPM:MMA and H2-type using 6.3:1:2.52 for water:TEOS:ethanol and 1:0.25:0.25 for TEOS:TMSPM:MMA. Semi-transparent monoliths were obtained in both cases after the gelation–solidification and drying processes, which took about seven days. Thermal properties of the samples were obtained by applying thermal lens spectroscopy and their mechanical properties were measured by depth sensing indentation and Vickers microhardness. FTIR spectroscopy measurements were performed to complement the characterization. We found that the hardness of the hybrid monoliths have values between those of commercial PMMA and a sol–gel SiO₂ monolith. This result is a consequence of the reinforcement produced by the SiO₂ component in the organic–inorganic hybrid matrix. The thermal diffusivity of the hybrid monoliths shows an appreciable increasing in the hybrid system with respect to the pure SiO₂ monolith. Mechanical behavior of monoliths is strong influenced by the SiO₂–PMMA ratio or changing the water:TEOS:ethanol contents.     

Comparative study on hydrophobic anti-reflective films from three kinds of methyl-modified silica sols

Methyl-modified silica sols were prepared with the polymer of methyltriethoxysilane (MTES), the polymer of dimethyldiethoxysiloxane (DDS), and hexamethydisizane (HMDS) as mono-, di-, and tri-methyl modifiers respectively. By comparing the size and the shape of clusters in three different methyl-modified silica sols, the special nature of the sol was found to be the key to the property of films. Different modal modification of methyl to silica particles led to different cluster fractal structures that influenced the morphology, the porosity of films and consequently the anti-reflection characteristics. The contribution of methyls in or on clusters directly determined the hydrophobicity. Using mono- or di-methyl-modified silica sol, the film/water contact angles were less than 120°. But it could reach 165° when using tri-methyl-modified silica sol. The lowest reflectivity of film could reach 0.0% for all the three modified sols. As a result of the comparative study, tri-methyl-modified silica sol was more suitable to prepare hydrophobic anti-reflective film with required high optical performance.     

Fabrication of mesoporous polymer/silica hybrid using surfactant-mediated sol–gel method

A mesoporous polymer/silica hybrid was fabricated by a surfactant-mediated sol–gel method. Under our experimental conditions, acrylonitrile (AN) monomer was located at the exterior of micelles and the sol–gel reaction of tetraethoxyorthosilicate (TEOS) proceeded concurrently with the polymerization reaction of the AN monomer. In other words, the micelle/polyacrylonitrile/silica precursor was synthesized through the radical polymerization accompanied with a hydrolysis/condensation single reaction in a reaction system. This is a unique characteristic of our methodology, which embraces the concept of ‘micelle templating’. The pore diameter of the mesoporous polymer/silica hybrid could be tuned by varying the spacer length and concentration of surfactants. Furthermore, compared with conventional mesoporous carbons, the carbonized mesoporous polymer/silica hybrids displayed an enhanced electrical performance favorable for use as a supercapacitor.     

Organic–inorganic hybrid coatings for corrosion protection

The corrosion resistance of sol–gel derived, organic–inorganic, silica-based hybrid coatings with various amounts of organic content was studied. Hybrid sols were prepared by copolymerizing tetraethylorthosilicate (TEOS) and 3-methacryloxypropyltrimethoxysilane (MPS) with a two-step acid-catalyst process. Hybrid coatings were dip-coated on 304 stainless steel substrates and annealed at 300 °C for 30 min. Such prepared hybrid coatings were found to be relatively dense, uniform and defect free. The adhesion and flexibility of the coatings were characterized. The influences of the amount of organic component incorporated into the coatings and the aging of sols on corrosion protection were studied. Electrochemical analyses showed that the relatively dense hybrid coatings provided excellent corrosion protection by forming a physical barrier, which effectively separated the anode from the cathode. Some preliminary biocompatibility tests were also conducted on the hybrid coatings.     

Effect of acid,water and alcohol ratios on sol-gel preparation of antireflective amorphous SiO2 coatings

Varying amounts of nitric acid catalyst, water and ethyl alcohol were used in the preparation of SiO₂ sols by hydrolysis and condensation reactions of tetraethyl orthosilicate in a one step acid catalysis process. Hydrolysis of TEOS was followed by FT-IR analyses. Size of SiO₂ particles was seen to vary in 8–41 nm range with respect to changing HNO₃ and water amounts in the sols. Gelation occurred in some systems. Surfaces of films were examined by FESEM and AFM, after coating on glass substrates by dip coating. Thicknesses of the films were measured to be in the range of 80–120 nm. 5.6 ± 0.2% point increase in light transmittance was obtained when HNO₃/TEOS (mol/mol) ratio of 4.74 × 10^? 4 and H₂O/TEOS (mol/mol) ratio of 9.08 were utilized. Sols were found to be stable for months and coatings prepared after 45 days still provided 5.2 ± 0.2% point increase in light transmittance.     

Effect of synthesis conditions on the fractal structure of yttrium-stabilized zirconium dioxide

The effect of synthesis conditions (sequential precipitation, coprecipitation, and sol–gel method) on the fractal structure of yttrium-stabilized zirconium dioxide has been investigated. It has been shown that the difference between methods manifests itself clearly in xerogel nanostructures, viz. by the number of fractal aggregation levels (three levels in hydroxide precipitates and two in sol–gel) and the absence of mass-fractal aggregation in sol–gel. It is been determined that the mass-fractal aggregation of precursors contributes to good filtration of hydroxide precipitates and allows preparation of oxides with soft, readily destructible aggregates, and that surface-fractal aggregation makes for good pressability of oxides.     

Light scattering dynamic study of the gelation process

Light scattering is a choice method for studying the dynamic properties of fractal structures and growth processes. We present preliminary light scattering measurements to characterize the formation of small silica particles prepared in highly basic TMOS/methanol/water sols and the transformation of sol into gel employing digital clipped autocorrelation spectroscopy. The light scattering is due to gel clusters.     

Investigation of inorganic network formation in photopatternable hybrid sol-gel films by Si liquid NMR

Liquid ²⁹Si NMR spectroscopy was used to investigate the sol-gel process of methacryloxypropyltrimethoxysilane. This hybrid precursor was involved in the realization of optical elements in laminated crack-free thick films (up to 100 μm), through spatially controlled photopolymerization. Understanding the formation of the inorganic network was of first importance to insure the creation of crack-free photopatterned thick films in a laminated configuration. Material preparation required evaporation of the volatile solvents released during the sol-gel process and limitation of the condensation degree. Both conditions were achieved by a drying process at room temperature. The structure and the composition of the sols were investigated and compared to non-dried sols. Evolution of inorganic species distribution was also studied under increasing aging time or storage temperature. NMR peak fitting of T1 species gave fruitful information on the sol structure evolution during the sol-gel process. It pointed out the presence of a large variety of oligomers in the sol. The study also allowed the identification of more constraint cyclic species in dried sols stored at room temperature. Their amount significantly decreased when increasing the storage temperature and was attributed to ring opening of cyclic species. Consequently, the structure of the dried sol will depend both of the aging time and of the storage temperature. All these results have to be taken into account when the degree of condensation has to be limited to achieved photopatternable hybrid layers for specific optical applications.     

Sol–gel simulation—II: Mechanical response

A novel computational procedure is proposed to predict the outstanding mechanical properties of sol–gel structures. An aggregation algorithm incorporating Brownian motion and chemical reactions is used to recreate the sol–gel structures at molecular scale. Just like in the physical colloidal aggregation process, the computational aggregation process produces structures with fractal features. Such fractal character leads to a recursion algorithm for calculating mechanical properties at any scale using a recursive multiscale approach. The mechanical properties are then predicted at each scale by calculating the effective properties using the Finite Element Method. It is shown that Young's modulus naturally follows a power law relationship with density, whereas Poisson's ratio displays more complicated behavior. Also, it is shown that Young's modulus and Poisson's ratio depend on a) the mass distribution of the structure, which is influenced by the Brownian motion and chemical reactivity during the aggregation process, and b) the connectivity, which is also influenced by additional processes as sintering and/or aging. Finally, it is shown that the Young's modulus and Poisson's ratio can be correlated to scattering intensity of sintered and/or aged structures.     

Periodic alignment of sol–gel derived,monodisperse phenylsilsesquioxane particles on a pregrooved substrate

Preparation and alignment of monodisperse inorganic–organic hybrid particles have become one of the most attractive research topics in recent years. In this study, phenylsilsesquioxane (PhSiO_3/2) particles were synthesized from phenyltriethoxysilane (PhSi(OEt)₃), ethanol (EtOH), hydrochloric acid for hydrolysis and ammonia water for polycondensation via sol–gel process. Spherical PhSiO_3/2 particles of 0.1–5.0 μm in diameter were obtained by varying the composition of the sols. The particle size distribution decreased with increasing the amounts of catalyst-containing H₂O and EtOH, and increasing the concentrations of catalysts. Monodisperse particles obtained were aligned on a pregrooved glass substrate using an ascending liquid flow and an attractive capillary force between the particles.     

Influence of the precursor nature and deposition mode on the oxygen sensing properties of Ru(II) complex immobilized in a SiO2-based matrix

Films are deposited by dip- or spin-coating in gels produced from tetraethoxysilane (TEOS), octyltriethoxysilane (OtEOS) or mixtures of TEOS and OtEOS (mole ratio1:1) as well as TEOS and methyltriethoxysilane (MtEOS) (mole ratio 1:3). The Stern–Volmer constants and the response to oxygen of Ru(II)-tris(4,7-diphenyl-1,10-phenathroline) dichloride immobilized in the produced matrices are calculated. The oxygen sensitivity of the film depends on the chemical nature of the precursors, the Ru(II) concentration, the film deposition technology, and the film thickness. The uncertainty of the results obtained with films of different production batches varies between 1.5% and 3% of the measured value for oxygen content for gas mixtures between 10% and 90% oxygen content. The sonication of the sol has a strong positive effect on the sensitivity of the films and the linearity of the calibration curves.     

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.     

Effects of progressive changes in organoalkoxysilane structure on the gelation and pore structure of templated and non-templated sol–gel materials

We study how progressive changes in silane structure affect the synthesis and properties of organosilicas. Tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMS), bis(trimethoxysilyl)ethane (BTMSE), bis(trimethoxysilyl)hexane (BTMSH), and bis(trimethoxysilylpropyl)amine (BTMSPA) are used as precursors in non-templated base and acid-catalyzed sol–gel processes, and in templated processes with cetyltrimethylammonium bromide (CTAB) and polyoxyethylene 10 lauryl ether (C₁₂E₁₀). The gel time of materials made without templates is mainly controlled by the structure of the silane rather than its reactivity. For instance, a dangling methyl group (MTMS) inhibits gelation, while a short bridging chain (BTMSE) promotes gelation. In basic conditions, mesoporous materials are obtained with TEOS and TMOS, while microporous materials are obtained with organically modified silanes without added amine. Dipropylamine, originally added as a catalyst, in fact templates mesopores in BTMSH-derived organosilica. In acidic conditions without pore templates, all products are microporous. In the presence of CTAB, mesopore templating occurs with TEOS, TMOS, BTMSE, and BTMSPA. With C₁₂E₁₀, mesopore templating occurs with TEOS, TMOS, and BTMSE. Surprisingly, the BTMSE-based material has the most uniform mesopores of all samples in the C₁₂E₁₀ series. Mesopore templating fails when a dangling organic limits the formation of stable pore walls (MTMS) or a large hydrophobic chain disrupts the formation surfactant micelles (BTMSH).