Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths

Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol–gel reaction. Starting from AlCl₃·6H₂O and poly(ethylene oxide) (PEO) dissolved in a H₂O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H₂O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H₂O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO.     

Graphene oxide-gelatin aerogels as wound dressings with improved hemostatic properties

Graphene oxide (GO)-gelatin (G) aerogels were synthesized via the physical interactions between GO-oxygenated groups and G amine groups to obtain potential hemostatic devices. The influence of the aerogel synthesis conditions—acid and basic GO suspensions—was used to evaluate their clotting performance. These materials were also characterized by Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, and thermogravimetric analysis, and their properties of absorption, stiffness, porosity, surface charge, and pore size were measured and compared. The clotting activity of the materials was evaluated by prothrombin time, activated partial thromboplastin time, soluble human P-selectin, and in vitro dynamic clotting assays, as well as their cytotoxicity. GO-G aerogels presented heterogeneous microporous structures with porosities higher than 90% and a high PBS absorption capacity, 49.6 ± 3.8 g_PBS/g_aerogel for positively charged aerogels (15.63 ± 0.5 mV) and 42.75 ± 2.38 g_PBS/g_aerogel for negatively charged aerogels (?20.53 ± 1.07 mV). Comparatively, positively charged aerogels had superior structural properties to negatively charged aerogels, such as stiffness, porosities, and pore sizes, because they promote H bonding. In regard to hemostatic activity, negatively charged aerogels had higher clotting performance, reaching 95.6% clotted blood, and therefore provide a suitable structure for the coagulation process and promote clot formation without using common mechanisms. In addition, negatively charged aerogels were not cytotoxic and promoted fibroblast proliferation. Therefore, negatively charged GO-G aerogels may be a potential hemostatic device that can be used as a wound dressing.     

Ambient pressure dried tetrapropoxysilane-based silica aerogels with high specific surface area

In the present paper, we report the synthesis of tetrapropoxysilane (TPOS)-based silica aerogels with high surface area and large pore volume. The silica aerogels were prepared by a two-step sol-gel process followed by surface modification via a simple ambient pressure drying approach. In order to minimize drying shrinkage and obtain hydrophobic aerogels, the surface of the alcogels was modified using trichloromethylsilane as a silylating agent. The effect of the sol-gel compositional parameters on the polymerization of aerogels prepared by TPOS, one of the precursors belonging to the Si(OR)₄ family, was reported for the first time. The oxalic acid and NH₄OH concentrations were adjusted to achieve good-quality aerogels with high surface area, low density, and high transparency. Controlling the hydrolysis and condensation reactions of the TPOS precursor turned out to be the most important factor to determine the pore characteristics of the aerogel. Highly transparent aerogels with high specific surface area (938 m²/g) and low density (0.047 g/cm³) could be obtained using an optimized TPOS/MeOH molar ratio with appropriate concentrations of oxalic acid and NH₄OH.     

Optically transparent silica aerogels based on sodium silicate by a two step sol–gel process and ambient pressure drying

Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol–gel process. To produce optically transparent silica aerogel granules, NH₄F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol–gel process-based aerogels is higher than the conventional single step aerogels. The N₂ adsorption–desorption analysis depicts that the two step sol–gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of ∼0.125 g/cc, low thermal conductivity of ∼0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of ∼425 m²/g were obtained by using NH₄F (1 M), HCl (4 M), and a molar ratio of Na₂SiO₃::H₂O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 °C.     

Synthesis of transparent silica aerogels using tetraalkylammonium fluoride catalysts

Tetraalkylammonium fluoride salts have been employed as catalysts for the synthesis of silica aerogels by a two-step, sol–gel method. Aerogel materials were characterized by N₂ physisorption and SEM. The effect of the type of catalyst on the optical transparency of obtained aerogels has been examined. It has been found that such compounds allow the synthesis of silica aerogels with the highest optical transmittance ever reported for such materials. The optimal catalysts are tetrabutyl and tetraoctyl ammonium fluoride, with which aerogels with transparency as high as 96% and extinction coefficient as low as 3.5 m⁻¹ can be prepared.     

The synthesis and characterization of zinc ferrite aerogels prepared by epoxide addition

Over the past decade sol–gel methods have become increasingly popular alternatives to the solid state synthesis of metal oxides. In many cases sol–gel synthesis is preferred due to desirable physical properties such as high surface area, high porosity, and small crystallite size. Monolithic zinc ferrite aerogels were produced by the epoxide addition sol–gel method. It was observed that addition of propylene oxide to 2-propanol solution of either the hydrated metal nitrate salts or the hydrated metal chloride salts resulted in the formation of stable red–brown gels. Aerogels were characterized using powder X-ray diffraction, high resolution scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption/desorption analysis. The metal salt used in the synthesis was found to significantly influence the properties of the aerogel. All aerogels synthesized exhibited low densities and high surface areas (>340 m²/g). Annealing of the aerogel at relatively low temperatures (below 450 °C) yielded a highly crystalline porous material which is composed of nanometer sized particles.     

High surface area alumina-supported nickel (II) oxide aerogels using epoxide addition method

Nickel (II) oxide aerogels with an amorphous alumina support were synthesized by the expoxide addition method. The monoliths were obtained by adding propylene oxide to an alcoholic solution of hydrated metal nitrate salts. The wet gels were dried by supercritical extraction to produce porous monolithic aerogels. The as-synthesized aerogels were amorphous containing aluminum and nickel hydroxides. Annealing of the as-synthesized aerogels at 400 °C yields crystalline nickel oxide materials which retain the high surface areas (>160 m²/g) and porosities of the original aerogels. The resultant aerogel materials were characterized using powder X-ray diffraction, thermo-gravimetric analysis, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, and nitrogen adsorption/desorption analysis.     

Polyurea based aerogel for a high performance thermal insulation material

Less fragile lightweight nanostructured polyurea based organic aerogels were prepared via a simple sol–gel processing and supercritical drying method. The uniform polyurea wet gels were first prepared at room temperature and atmospheric pressure by reacting different isocyanates with polyamines using a tertiary amine (triethylamine) catalyst. Gelation kinetics, uniformity of wet gel, and properties of aerogel products were significantly affected by both target density (i.e., solid content) and equivalent weight (EW) ratio of the isocyanate resin and polyamine hardener. A supercritical carbon dioxide (CO₂) drying method was used to extract solvent from wet polyurea gels to afford nanoporous aerogels. The thermal conductivity values of polyurea based aerogel were measured at pressures from ambient to 0.075 torr and at temperatures from room temperature to −120 °C under a pressure of 8 torr. The polyurea based aerogel samples demonstrated high porosities, low thermal conductivity values, hydrophobicity properties, relatively high thermal decomposition temperature (~270 °C) and low degassing property and were less dusty than silica aerogels. We found that the low thermal conductivities of polyurea based aerogels were associated with their small pore sizes. These polyurea based aerogels are very promising candidates for cryogenic insulation applications and as a thermal insulation component of spacesuits.     

New technology for rapid processing and moulding of silica aerogel materials in prescribed shapes and sizes and their characterization

The new technology has been developed for rapid processing of silica aerogels at a rate 40 times faster than the conventional method and is reported in the present paper. Silica gels were prepared using precursor chemicals Tetramethoxysilane: Methanol: Water (0.05 mol L⁻¹ NH₄OH): Methyltrimethoxysilane at a molar ratio of 1: 12: 4: 0.8, respectively. Mixture of these precursors was stirred and transferred to metallic moulds. These metallic moulds were kept in a 3-liter autoclave with an additional 400 mL of methanol. The autoclave was pre-pressurized to 4 MPa with N₂ gas and heated rapidly at the rate of 8 ^°C/min. to reach supercritical state of methanol. After 10 min, the vessel was depressurized at a rate of 0.45 MPa/min. The sol in the mould when heated rapidly gets transformed to aerogel directly and the crucial alcogel handling step is skipped. We successfully synthesized 50 mm diameter and 15 mm thick disc shaped and 40 mm diameter hemispherical monolithic aerogels using the rapid process method. Retrieved aerogels were characterized for their density, surface area, optical transmittance, fractal size, surface morphology and microstructure using SEM, TEM and SAXS. Aerogels synthesized by rapid process technology shows improved physical properties than the aerogels derived by conventional method.     

Synthesis of MWCNTs doped sodium silicate based aerogels by ambient pressure drying

The successful incorporation of multiwalled carbon nanotubes (MWCNTs) into silica aerogels prepared by sol–gel method is reported herein. Pure silica aerogels prepared using sodium silicate precursor by ambient pressure drying are so fragile that they cannot be used easily. MWCNTs were used as reinforcements to improve the mechanical properties of silica aerogels. Results show that inserting small amounts of MWCNTs in the gels causes enhanced dimensional stability of silica aerogels. The silica aerogels were prepared by doping MWCNTs in silica matrix before gelation. The influence of MWCNTs on some microstructural aspects of silica matrix has been studied using nitrogen adsorption–desorption isotherms. From SEM study it is confirmed that the silica particles get capped on the surface of MWCNTs suggesting an enhanced toughness. Further, FTIR, Raman, EDAX, thermal conductivity and hydrophobicity studies of these doped aerogels were carried out. By addition of MWCNTs, silica aerogels were formed with 706 m²/g BET and 1,200 m²/g Langmuir surface areas and 149^o contact angle. Low density (0.052 g/cc) and low thermal conductivity (0.067 W/m K) MWCNTs doped silica aerogels were obtained for the molar ratio of Na₂SiO₃::H₂O::MWCNTs::citric acid::TMCS at 1::146.67::2.5 × 10⁻³::0.54::9.46 respectively with improved mechanical strength.     

Iron-Cobalt-Silica Aerogel Nanocomposite Materials

Iron-Cobalt-Silica nanocomposites were prepared in form of aerogels. X-ray diffraction, transmission electron microscopy and N₂ physisorption at 77 K were used to investigate the structure, size and dispersion of the nanocrystals and the porous structure in the aerogels and in the final composites. The variation of the supercritical drying conditions gives rise to differences in the morphological features of the aerogels. These differences influence the size of the cobalt oxide nanoparticles in the aerogels. On the other hand, after the reduction treatment the average size of the alloy nanoparticles is the same in all the aerogel nanocomposites. The effect of reduction temperature on alloy formation and particle size is also discussed.     

Elastic organic–inorganic hybrid aerogels and xerogels

Novel aerogels and xerogels with methylsilsesquioxane (MSQ, CH₃SiO_1.5) networks have been prepared by a modified sol–gel process using surfactant and urea as a phase-separation inhibitor and as an accelerator for the condensation reaction, respectively. Optimized aerogels dried under a supercritical condition not only showed the similar properties as conventional pure silica aerogels such as high transparency and porosity etc, but also demonstrated outstanding mechanical strength against compression; the aerogel drastically shrank upon loading and then recovered when unloaded, which is called a “spring-back” behavior. On ambient pressure drying, the wet gel also exhibited the similar response against compression stress originated from the capillary pressure, and thus xerogels with the comparative structure and properties to those of corresponding aerogels have also been obtained. This unusual mechanical behavior is attributed to the trifunctional flexible networks of MSQ, low silanol concentration which prevents the irreversible shrinkage, and high concentration of a hydrophobic methyl group directly attached to every silicon atom which helps re-expansion after the temporal shrinkage.     

Electrosynthesis of polythiophene nanowires via mesoporous silica thin film templates

Early electrosynthesized polythiophene nanowires were prepared employing a mesoporous silica template, which was also electrochemically produced. A cathodic potential step was applied to a fluorine doped tin oxide conducting glass electrode in a cationic surfactant and silicate reagent medium to deposit highly ordered mesoporous silica films. To evaluate the pores order and, consequently, optimal deposition potential, the electrochemical response of the electrodes was studied using ferrocene as redox probe. The modified electrodes were used to accomplish polythiophene electrodeposits employing 0.6 mM thiophene and 0.1 M tetrabutylammonium hexafluorophosphate in anhydrous CH₃CN as working solution. Transmission electron microscope images of the deposits revealed the presence of polythiophene nanowires of about 6 nm in diameter arranged normal to the electrode surface. The unprecedented small size and arrangement of the obtained nanowires place this work as the first study that successfully accomplished the formation of nanoscale electrochemically synthesized conducting polymer nanowires.     

Effect of iron acetylacetonate on physico-chemical properties of waterglass based aerogels by ambient pressure drying

The effect of iron acetylacetonate on the physico-chemical properties of waterglass based silica aerogels by ambient pressure drying has been investigated. Doping the gels with iron acetylacetonat (FeAA) facilitates in the diminution of the density of the aerogels. The well established silica network provides effective confinement of FeAA nanoparticles which resists the collapse of silica network during ambient pressure drying. Therefore, in the present paper, the effects of FeAA on the physico-chemical properties of the aerogels have been studied by varying the FeAA:Na₂SiO₃ molar ratio from 3 × 10⁻⁴ to 6 × 10⁻⁴. The aerogels were prepared via ambient pressure drying and characterized by the bulk density, thermal conductivity and water contact angle. The aerogel’s surface morphology, elemental analysis and pore structure were characterized by means of EDAX and FTIR, TEM and N₂ adsorption- desorption analyzer. The high temperature hydrophobicity of these aerogels was checked by heating them in temperature controlled furnace. Silica aerogels with low density ~0.050 g/cc have been obtained using the molar ratio of Na₂SiO₃:H₂O:FeAA:Citric acid:TMCS at 1:146.67:3 × 10⁻⁴:0.54:9.46, respectively. EDAX and FTIR studies show that the iron species are entrapped in the mesoporous framework and not took part in the bonding with silica.     

Physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids

Experimental results on the physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids are reported. The aerogels were prepared by hydrolysis and polycondensation of sodium silicate followed by subsequent washings, surface chemical modification and ambient pressure drying using 10 various acid catalysts consisting of strong and weak acids. The strength and concentration of acids have the major effect on the gelation of sol and hence the physico-chemical properties of the silica aerogels. Strong acids such as HCl, HNO₃ and H₂SO₄ resulted in shrunk (70–95%) aerogels whereas weak acids such as citric and tartaric acids resulted in less shrunk (34–50%) aerogels. The physical properties of silica aerogels were studied by measuring bulk density, volume shrinkage (%), porosity (%), pore volume, thermal conductivity, contact angle with water, Transmission Electron Microscopy (TEM), Atomic Absorption Spectroscopy (AAS), Fourier Transform Infrared Spectroscopy (FTIR), Thermo Gravimetric-Differential Thermal (TG-DT) analyses and N₂ adsorption–desorption BET surface analyzer. The best quality silica aerogels in terms of low density (0.086 g/cm³), low volume shrinkage (34%), high porosity (95%), low thermal conductivity (0.09 W/m K) and hydrophobic (148°) were obtained for molar ratio of Na₂SiO₃:H₂O:citric acid:TMCS at 1:146.67:0.72:9.46 with 20 min gelation time. The resulting aerogels exhibited the thermal stability up to around 420 °C.     

3D hierarchical MnO2 aerogels with superhydrophobicity for selective oil–water separation

Inorganic nanowire aerogel with low density, high specific surface area and high porosity has received increasing attention in the field of materials physics and chemistry because of not only the unique structural and physical features of metallic oxide but also low cost, environmental friendliness and earth abundant of precursor materials. In this work, MnO₂ nanowire aerogels (MNA) with ultralow density, and stable 3D hierarchical structures was successfully fabricated by freeze‐drying processes using MnO₂ nanowire as building blocks. The length of MnO₂ nanowires exceeds 100 μm, making it easier to cross‐link and self‐assemble into a 3D network of aerogels, and the acid and alkali resistance of MnO₂ enables it to adapt to extreme environments. Simultaneously, the monodispersed MnO₂ nanowire was prepared by the hydrothermal method, followed by acid treatment. To obtain superhydrophobic properties and achieve selective oil adsorption, the surfaces of nanowire aerogels were grafted the hydrophobic groups with low surface energy via vapor deposition. It is indicated that the obtained 3D hierarchical MNA show both superhydrophobic and super‐lipophilic properties simultaneously with a high‐water contact angle of 156°  ±  2° and an oil contact angle of 0°. And the MNA exhibited a high oil adsorption capacity of 85–140 g/g, thereby indicating its potential applications in oil/water separation. More importantly, the resulting MNA can be recycled ten cycles without loss of oil absorption capacity (more than 120 g/g). The results presented in this work demonstrate that the as‐prepared nanowire aerogel may find applications in chemical separation and environmental remediation for large‐scale absorption of oils from water.     

Silica–titania aerogel monoliths with large pore volume and surface area by ambient pressure drying

Ambient pressure drying has been carried out for the synthesis of silica–titania aerogel monoliths. The prepared aerogels show densities in the range 0.34–0.38 g/cm³. The surface area and pore volume of these mixed oxide aerogels are comparable to those of the supercritically dried ones. The surface area for 5wt% titania aerogel has been found to be as high as 685 m²/g with a pore volume of 2.34 cm³/g and the 10wt% titania aerogel has a surface area of 620 m²/g with a pore volume of 2.36 cm³/g. Some gels were also made hydrophobic by a surface treatment with methyltrimethoxysilane and trimethylchlorosilane. The surface modified aerogels possess high surface areas in the range of 540–640 m²/g, and are thermally stable in terms of retaining hydrophobicity up to a temperature of 520 °C. The pore size distribution of the aerogels clearly indicates the preservation of the aerogel structure. High Resolution Transmission Electron microscopy has been employed to characterise the aerogels and Fourier Transform infrared spectroscopy to study the effect of titania addition to silica and the surface modification. X-ray diffraction patterns were recorded to verify the molecular homogeneity of the aerogel.     

Highly Uniform Nano and Mesostructures of Silica Obtained by Reverse Micellar and Hydrothermal Methods

Silica nanoparticles with controlled size and morphology and a high degree of monodispersity have been synthesized using single and double microemulsion systems employing a cationic surfactant (CTAB) and a non-ionic surfactant (Tergitol). Depending on the type of surfactant aggregate acting as templates, very different morphologies were formed. Nanospindles of silica of ~200–300 nm in length and nanofibres could be obtained by suitably controlling the reverse micellar methodology. The hydrothermal method resulted in highly porous and uniform spheres of diameter ~300 nm which appears to be formed from aggregates of small silica nanoparticles of size ~10 nm. The surface area of the nanoparticles (119 m²/g) was found to be much higher than the corresponding bulk SiO₂ (500–600 nm) which had a surface area of 22 m²/g. The hydrothermally obtained spheres of size ~300 nm show a surface area of 35 m²/g. In honor of Prof. C.N.R. Rao, FRS, on his 75th birth anniversary.     

Sol–gel synthesis of macro–mesoporous titania monoliths and their applications to chromatographic separation media for organophosphate compounds

We have developed a method of independently tailoring the macro- and mesoporous structures in titania (TiO₂) monoliths in order to achieve liquid chromatographic separations of phosphorous-containing compounds. Anatase TiO₂ monolithic gels with well-defined bicontinuous macropores and microstructured skeletons are obtained via the sol–gel process in strongly acidic conditions using poly(ethylene oxide) as a phase separator and N-methylformamide as a proton scavenger. Aging treatment of the wet gels in the mother liquor at temperatures of 100–200 °C and subsequent heat treatment at 400 °C allow the formation and control of mesoporous structures with uniform pore size distributions in the gel skeletons, without disturbing the preformed macroporous morphology. The monolithic TiO₂ rod columns with bimodal macro–mesoporous structures possess the phospho-sensitivity and exhibit excellent chromatographic separations of phosphorus-containing compounds.     

Fine tuning of gadolinium doped ceria electrolyte nanoparticles via reverse microemulsion process

Gadolinium doped ceria (Gd–CeO₂) nanoparticles have been synthesized by an reverse microemulsion system using cyclohexane as the oil phase, a non-ionic surfactant Igepal CO 520 and their mixed aqueous solutions of gadolinium III nitrate hexahydrate and cerium III nitrate hexahydrate as the water phase. The control of particle size was achieved by varying the water to surfactant molar ratio. The synthesized and calcined powders were characterized by thermogravimetry-differential thermal analysis (TGA-DTA), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The XRD results show that all the samples calcined at 700 °C were single phase cubic fluorite structure. The average size of the particle was found to increase with increase in water to surfactant molar ratio (R). The mean diameter of the particle for various value of R varies between 8–15 nm (SEM) and 7.5–11 nm (TEM), respectively. EDS confirm the presence of gadolinia and ceria phase in the nanopowder calcined at 700 °C. FTIR analysis was carried to monitor the elimination of residual oil and surfactant phases from the microemulsion-derived precursor and calcined powder. Raman spectroscopy and DTA evidenced the formation of a solid solution of gadolinium doped ceria at room temperature.