Fabrication and characterization of superhydrophobic silica nanotrees

Superhydrophobic silica nanotrees were obtained by sol–gel method with hybrid silica sol and jelly-like resorcinol formaldehyde resin. Rough surfaces were obtained by removing the organic polymer at high temperature. After the films with rough surface were modified by trimethylchlorosilane (TMCS), the wettability of the film changed from superhydrophilic to superhydrophobic. The surface roughness of the silica nanotrees film is about 20 nm, and it is transparent and superhydrophobic with a water contact angle higher than 150°.     

聚氧化丙烯醚对SiO2减反膜性质的影响

采用溶胶-凝胶制备的SiO2减反膜具有优良的光学特性和高损伤阈值,可以用做高反膜和减反膜的低折射率匹配层.在高功率激光领域具有十分诱人的应用前景.     

Recoverable and thermally stable superhydrophobic silica coating

A facile route to methyltrimethoxysilane (MTMS) based recoverable superhydrophobic silica coatings with dual-scale roughness obtained through the single step base catalyst sol–gel process. Superhydrophobic silica coatings have shown static water contact angle near about 170 ± 1° and dynamic water contact angle up to 2 ± 1°. Superhydrophobic-superhydrophilic switching feature also achieved by alternating heat treatment and bath surface modification with Trimethylchlorosilane (TMCS) at room temperature (26 °C). Furthermore, the superhydrophobic state could be transformed into superhydrophilic state by slow rate heat treatment. These studies present a very simple strategy for the fabrication of recoverable superhydrophobic surfaces.     

Focus on moisture-resistance and hydrophobicity of SiO<Subscript>2</Subscript> antireflective film improved by poly(isopropylene oxide) glycerolether

The poly(isopropylene oxide) glycerolether (PO) modified silica antireflective (AR) coating films were prepared by sol–gel method. The properties of the silica sols and AR films were characterized by particle size analyzer, transmission electron microscope, UV–Vis spectro-photometry, spectroscopic ellipsometer and contact angle measurement. The results show that PO addition has inconspicuous effect on the transmittance of AR coatings; the maximum transmittance reached 99.5%. Thicknesses of films increase and refractive index decrease with increase of PO concentration. The contact angle with water for the silica films with and without PO was 68.5^o and 28.5^o, respectively, which indicates an increase of hydrophobicity of the modified coating. When the films were placed in 95% relative humidity at room temperature for 30 days, transmittance loss for the unmodified and modified silica films were 2.5 and 0.6% respectively. Moisture-resistance of the film is greatly improved by PO addition.     

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.     

Wettability study of surface modified silica aerogels with different silylating agents

In wettability study, surface free energy interactions are of crucial importance for silica aerogels in which absorption of organic liquids and transportation of chemicals carried out for chemical and biotechnological applications. In present study, we have used Lifshitz–van der Waals/acid–base approach for calculation of surface free energy of aerogel sample. We have investigated that the surface free energy values of aerogels are 45.95, 51.42 and 45.69 mJ/m² by modifying their surfaces using 7 % chlorotrimethylsilane (TMCS), dimethyldichlorosilane (DMDCS) and hexamethyldisilazane (HMDZ) silylating reagents with solvent, respectively. The alcogels were prepared by two step acid–base catalyzed process where the molar ratio of precursors tetraethoxysilane:methanol:oxalic acid:NH₄OH:NH₄F was kept at optimal value of 1:16.5:0.71:0.58:0.60:0.98, respectively. To modify gel surfaces, TMCS, DMDCS and HMDZ concentration have been varied from 5 to 12 % and such alcogels were dried at ambient pressure. The aerogels have been characterized by fourier transform infrared spectroscopy, scanning electron microscopy, thermo-gravimetric and differential thermal analysis and Wetting properties of silica aerogel surfaces was studied by contact angle measurements. The surface chemical composition of DMDCS modified silica aerogels was studied by using X-ray photoelectron spectroscopy. As there is not any direct method, we have used Lifshitz–van der Waals/acid–base approach which gives, polar and non-polar components of aerogels surface free energy.     

ITO films on glass substrate by sol–gel technique: synthesis,characterization and optical properties

The present paper extensively demonstrates synthesis, characterization and optical properties of semiconductor indium tin oxide (ITO) thin films on glass substrate using sol–gel technique for gas sensor applications. Turbidity, pH values, wettability and rheological properties of the prepared solutions were measured to determine solution characteristics by turbidimeter, pH meter, contact angle goniometer and rheometer machines prior to coating process. Thermal, structural, microstructural, mechanical and optical properties of the coatings were characterized by differential thermal analysis–thermogravimetry (DTA/TG), fourier transform infrarared, X-ray diffraction (XRD), scanning electron microscopy, scratch tester, refractometer and spectrophotometer. Four different solutions were prepared by changing solvent concentration. Turbidity, pH, contact angle and viscosity values of the solutions were convenient for coating process. Glass substrates were coated using the solutions of InCl₃, SnCl₂, methanol and glacial acetic acid. The obtained gel films were dried at 300 °C for 10 min and subsequently heat-treated at 500 °C for 10 min in air. The oxide thin films were annealed at 600 °C for 60 min in air. DTA/TG results revealed that endothermic and exothermic reactions are observed at temperature between 70 and 560 °C due to solvent removal, combustion of carbon based materials and oxidation of Sn and In. The spectrum of ITO precursor film annealed at 500–600 °C shows an absence of absorption bands corresponding to organics and hydroxyls. In₂Sn₂O_7−x phase was dominantly found as well as SnO₂ with low intensity from XRD patterns. It was found that surface morphologies of the film change from coating island with homogeneous structures to regular surface and thinner film structures with increasing solvent concentration. The films prepared from the solutions with 8 mL methanol have better adhesion strength to the glass substrate among other coatings. Refractive index, thickness and band gap of ITO thin films were determined to be 1.3171, 0.625 μm and 3.67, respectively.     

Preparation of superhydrophobic silica films with honeycomb-like structure by emulsion method

Silica films with honeycomb-like structure were successfully obtained by emulsion method. Emulsion films prepared by the Dip-Withdrawing method were dried at 180 °C for 2 h and sintered at 500 °C, the films turned from superhydrophilic to superhydrophobic after being modified by octyltrimethoxysilane (OTMS) to form a self-assembled monolayer (SAM) with low surface energy. The surface structures and the thickness of the silica emulsion films were observed by scanning electron microscopy (SEM), and the results showed that the emulsion method had a similar effect to the phase separation one on producing the honeycomb-like structure that highly influenced the wettability of solid surface.     

Production of low-density sodium silicate-based hydrophobic silica aerogel beads by a novel fast gelation process and ambient pressure drying process

We report a method to synthesize low-density transparent mesoporous silica aerogel beads by ambient pressure drying (APD). The beads were prepared by acid–base sol–gel polymerization of sodium silicate in aqueous ammonia solution via the ball dropping method (BDM). To minimize shrinkage during drying, wet silica beads were initially prepared; their surfaces were then modified using trimethylchlorosilane (TMCS) via simultaneous solvent exchange and surface modification. The effects of the volume percentage (%V) of TMCS on the physical and textural properties of the beads were investigated. The specific surface area and cumulative pore volume of the silica aerogel beads increased with an increase in the %V of TMCS. Silica aerogel beads with low packing bed density (0.081 g/cm³), high surface area (917 m²/g), and large cumulative pore volume (2.8 cm³/g) was obtained when 10%V TMCS was used. Properties of the final product were examined by FE-SEM, TEM, BET, and TG–DT analyses. Surface chemical modifications were confirmed by FTIR spectroscopy. The hydrophobic silica aerogel beads were thermally stable up to 411 °C. We discuss our results and compare our findings for modified versus unmodified silica beads.     

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.     

Properties of cellulose films prepared from NaOH/urea/zincate aqueous solution at low temperature

Cellulose films were successfully prepared from NaOH/urea/zincate aqueous solution pre-cooled to −13 °C by coagulating with 5% H₂SO₄. The cellulose solution and regenerated cellulose films were characterized with dynamic rheology, ultraviolet–visible spectroscope, scanning electron microscopy, wide angle X-ray diffraction, Fourier transform infrared (FT-IR) spectrometer, thermogravimetry and tensile testing. The results indicated that at higher temperature (above 65 °C) or lower temperature (below −10 °C) or for longer storage time, gels could form in the cellulose dope. However, the cellulose solution remained a liquid state for a long time at 0–10 °C. Moreover, there was an irreversible gelation in the cellulose solution system. The films with cellulose II exhibited better optical transmittance, high thermal stability and tensile strength than that prepared by NaOH/urea aqueous solution without zincate. Therefore, the addition of zincate in the NaOH/urea aqueous system could enhance the cellulose solubility and improve the structure and properties of the regenerated cellulose films.     

Nano silver coated patterned silica thin film by sol–gel based soft lithography technique

We report the fabrication of nano silver coated patterned silica thin film by sol–gel based soft lithography technique. Initially, silica gel film on soda lime silica glass was prepared by dipping technique from a silica sol of moderate silica concentration. A PolydimethylSiloxane elastomeric stamp containing the negative replica of the patterns of commercially available compact disc was used for embossing the film and the embossed film was cured up to 450 °C in pure oxygen atmosphere for oxide film. Finally, a precursor solution of AgNO₃ in water containing polyvinyl alcohol as an organic binder was made and used for coating on the patterned silica film by dipping technique and cured the sample up to 450 °C in reducing gas atmosphere to obtain nano silver layer. The formation of only cubic silver (~4.0 nm) and both cubic silver (~5.2 nm) and silver oxide (~3.6 nm) crystallites at 350 and 450 °C film curing temperatures respectively were confirmed by XRD measurements. The % of nano silver metal and silver oxide were 75.4 and 24.6 respectively. The nano-structured surface feature was visualized by FESEM whereas AFM revealed the high fidelity grating structure of the films. Presence of both spherical and rectangular structure (aspect ratio, 2.37) of nano silver/silver oxide was confirmed by TEM. The films were also characterized by UV–Vis spectral study. The patterned film may find application in chemical sensor devices.     

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.     

Low thermalconductive,transparent and hydrophobic ambient pressure dried silica aerogels with various preparation conditions using sodium silicate solutions

The experimental results on the preparation of low thermal conductivity and transparent ambient pressure dried silica aerogels with the sodium silicate solution, TMCS silylating agent with methanol, isopropyl alcohol, hexane and xylene solvents, are reported. This study is focussed on the effect of preparation conditions such as varying the number of preparation steps, pH of the hydrosol and hydrogel ageing temperature, for the production of the low thermal conductive silica aerogels and the results are analysed. Density, thermal conductivity, % of optical transmission and contact angle of the aerogels were measured. The Fourier Transform Infrared Spectroscopy (FTIR) studies revealed the presence of Si–C and C–H along with the Si–O–Si and OH bonds and their intensities strongly depend on the processing steps, pH of the hydrosol and gel ageing temperature. The UV–Visible spectra indicated the % of optical transmission of the aerogels decreased with increasing the number of processing steps, increase in the pH of the hydrosol from 3 to 8 and decreased for ageing temperature up to 50 °C. Further increase in temperature >50 °C, the % of optical transmission of the aerogels increased. The TGA-DTA data showed the thermal stability of the aerogels with respect to hydrophobicity is 325 °C. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) analyses revealed the nanostructure of the aerogels. The porosity of the aerogels was studied using the pore size distribution. Silica aerogels with low density (0.051 g/cc), low thermal conductivity (0.049 W/m K), optical transmission (65%), high hydrophobicity (159°) and resistance to humid atmosphere >1 year was obtained in the present studies.     

Silica-containing polyetherimide hybrid films based on methyltriethoxysilane as precursor of inorganic network

Polyetherimide hybrid films containing 5–40% silica were prepared through a sol-gel process and thermal imidization by using methyltriethoxysilane as precursor of the inorganic network and a poly(amic acid) resulting from polycondensation reaction of 2,2-bis[4-(4-aminophenoxy)phenyl]propane with 2,2-bis[(3,4-dicarboxyphenoxy)phenyl]propane dianhydride. The properties of these films, morphology, water vapor sorption capacity, free surface energy, mechanical, thermal and electrical characteristics, were studied. The films exhibited good thermal stability, having an initial decomposition temperature above 470 °C and glass transition temperature in the range of 187–200 °C. They showed low dielectric constant and low dielectric loss in a large frequency field. Gas permeation tests using small molecules (He, N₂, O₂ and CO₂) at 6 bar and 30 °C indicated that the hybrid films containing higher silica content showed higher permeability for all the tested gases.     

The effect of different substrate temperatures on the structure and luminescence properties of Y2O3:Bi3+ thin films

Y₂O₃:Bi³⁺ phosphor thin films were prepared by pulsed laser deposition in the presence of oxygen (O₂) gas. The microstructure and photoluminescence (PL) of these films were found to be highly dependent on the substrate temperature. X-ray diffraction analysis showed that the Y₂O₃:Bi³⁺ films transformed from amorphous to cubic and monoclinic phases when the substrate temperature was increased up to 600 °C. At the higher substrate temperature of 600 °C, the cubic phase became dominant. The crystallinity of the thin films, therefore, increased with increasing substrate temperatures. Surface morphology results obtained by atomic force microscopy showed a decrease in the surface roughness with an increase in substrate temperature. The increase in the PL intensities was attributed to the crystallinity improvement and surface roughness decrease. The main PL emission peak position of the thin films prepared at substrate temperatures of 450 °C and 600 °C showed a shift to shorter wavelengths of 460 and 480 nm respectively, if compared to the main PL peak position of the powder at 495 nm. The shift was attributed to a different Bi³⁺ ion environment in the monoclinic and cubic phases.     

Formation of hybrid nano-crystals in organic–inorganic films from a basic sol

Organic–inorganic films containing hybrid nanocrystals have been prepared by sol–gel processing in controlled conditions. We have systematically changed the temperature and the aging time of a precursor sol containing an organically modified alkoxide bearing an epoxy group, 3-glycidoxypropyltrimethoxysilane, to obtain a controlled crystallization of hybrid layered structures in hybrid films. The precursor sol has been aged at different temperatures, from 5 to 60 °C, and for 1, 2 or 3 days; the films have been deposited from the aged sol and immediately after characterized by X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. We have observed that the formation of the hybrid crystals can be obtained only when at least 50% of the epoxies are opened and a larger silica condensation is achieved. These conditions are reached after aging at 60 °C for 1 day, or at longer aging times when the sol is aged at lower temperatures. Transmission electron microscopy and optical polarized images have confirmed the formation of the hybrid crystals.     

Microstructure,dielectric properties and optical band gap control on the photoluminescence behavior of Ba[Zr<Subscript>0.25</Subscript>Ti<Subscript>0.75</Subscript>]O<Subscript>3</Subscript> thin films

Ba[Zr_0.25Ti_0.75]O₃ (BZT) thin films were synthesized by the complex polymerization method and heat treated at 400 °C for different times and at 700 °C for 2 h. These thin films were analyzed by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, field emission gun-scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM), Ultraviolet–visible (UV–vis) absorption spectroscopy, electrical and photoluminescence (PL) measurements. FEG-SEM and AFM micrographs showed that the microstructure and thickness of BZT thin films can be influenced by the processing times. Dielectric constant and dielectric loss of BZT thin films heat treated at 700 °C were approximately 148 and 0.08 at 1 MHz, respectively. UV–vis absorption spectra suggested the presence of intermediary energy levels (shallow and deep holes) within the band gap of BZT thin films. PL behavior was explained through the optical band gap values associated to the visible light emission components.     

Study of thermal conductivity and effect of humidity on HMDZ modified TEOS based aerogel dried at ambient pressure

The experimental results on the study of thermal conductivity and effect of humidity on HMDZ modified TEOS based aerogels dried at ambient pressure, are reported. Silica sol was prepared by keeping the MeOH/TEOS molar ratio, Acidic water (Oxalic acid) and basic water (NH₄OH) concentrations constant at 16.5, 0.001 and 1 M, respectively throughout the experiments and the HMDZ/TEOS molar ratio (h) was varied from 0.34 to 2.1. Finally, the surface modified wet gels were dried at an ambient pressure. The thermal conductivity of the aerogel samples was measured. Further, the humidity study was carried out in 80% humid surrounding at 30 °C temperature over 80 days. The best quality aerogels in terms of low bulk density, thermal conductivity and durability (no moisture absorption) with an only 2% of weight gain were obtained for TEOS: MeOH: Acidic H₂O: Basic H₂O: HMDZ molar ratio at 1:16.5:0.81:0.50:0.681, respectively. The thermal stability and hydrophobicity of the aerogel have been confirmed with Thermo gravimetric and Differential Thermal (TG–DT) analyses and Fourier Transform Infrared Spectroscopy (FTIR), respectively. Microstructural studies were carried out by Scanning Electron microscopy (SEM).     

Thermal and hydrothermal stability of flame hydrolytically synthesized SiO2/TiO2 mixed oxides

The phase stability of the two TiO₂ modifications (anatase and rutile) in fumed SiO₂/TiO₂ nano-composites (0–24.8 wt-% silica) under thermal and hydrothermal conditions was investigated by X-ray powder diffraction, transmission electron microscopy (TEM) and gas adsorption methods (BET). The results show that the phase transformation from anatase to rutile type of structure and the growth of anatase crystallites are significantly retarded by mixing small amounts of SiO₂ into TiO₂, while the specific surface area is maintained. The SiO₂/TiO₂-composites reveal a remarkable shift in the anatase to rutile transformation temperature from approx. 500 °C (pure TiO₂) to approx. 1000 °C (samples with SiO₂ contents of more than 10%). The rate of phase transformation from anatase to rutile is enhanced under hydrothermal conditions compared to conventional thermal treatment, e.g. pure titania (AEROXIDE^® TiO₂ P25) annealed under hydrothermal conditions (100 g/m³ absolute humidity, 4 h at 600 °C) had a rutile content of 85%, while the same specimens annealed in absence of humidity contained only 46% rutile. However, the difference in rate of phase transformation became less pronounced when the silica content in SiO₂/TiO₂-composites was further increased.TEM results showed that the surface of the anatase crystallites was covered with silica. This averts coalescence of anatase crystallites and keeps them under a critical size during the annealing process. When the crystal domains grew larger, a rapid conversion to rutile took place. The critical size of anatase crystallites for the phase transformation was estimated to be 15–20 nm.