Ultrahydrophobic silica films by sol–gel process

Wettability of solid surfaces is a crucial concern in our daily life as well as in engineering and science. The present research work describes the room temperature (27 °C) synthesis of adherent and water repellent silica films on glass substrates using vinyltrimethoxysilane (VTMS) as a hydrophobic reagent by a single step sol–gel process. The silica sol was prepared by keeping the molar ratio of tetraethoxysilane (TEOS), methanol (MeOH), water (H₂O) constant at 1:14.69:5, respectively, with 0.01 M NH₄F throughout the experiments and the VTMS/TEOS molar ratio (M) was varied from 0 to 0.97. The effects of M on the surface structure and hydrophobicity have been researched. The static water contact angle as high as 144° and water sliding angle as low as 14° was obtained for silica film prepared from M = 0.97. The hydrophobic silica films retained their hydrophobicity up to a temperature of 255 °C and above this temperature the films became superhydrophilic. The prepared silica films were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared (FT-IR) spectroscopy, humidity test and static and dynamic water contact angle measurements.     

Synthesis of silica nanoparticles from Vietnamese rice husk by sol–gel method

Silica powder at nanoscale was obtained by heat treatment of Vietnamese rice husk following the sol–gel method. The rice husk ash (RHA) is synthesized using rice husk which was thermally treated at optimal condition at 600°C for 4 h. The silica from RHA was extracted using sodium hydroxide solution to produce a sodium silicate solution and then precipitated by adding H₂SO₄ at pH = 4 in the mixture of water/butanol with cationic presence. In order to identify the optimal condition for producing the homogenous silica nanoparticles, the effects of surfactant surface coverage, aging temperature, and aging time were investigated. By analysis of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the silica product obtained was amorphous and the uniformity of the nanosized sample was observed at an average size of 3 nm, and the BET result showed that the highest specific surface of the sample was about 340 m²/g. The results obtained in the mentioned method prove that the rice husk from agricultural wastes can be used for the production of silica nanoparticles.     

Characterization of wollastonite coatings prepared by sol–gel on Ti substrate

Wollastonite coatings were prepared by sol–gel on Ti substrate and their microstructures have been studied. The phase compositions and the surface morphologies of these coatings were examined by X-ray diffraction and scanning electron microscopy. Thermal behavior of dried gel was examined by differential scanning calorimetry (DSC) and thermogravimetry (TG). There are many cracks among coatings and particles with size about 200–300 nm distributing inside cracks. DSC and TG results show that the glass transformation temperature of dried gel is about 850°C. After calcined at temperature 900°C, the phase of coatings consists of wollastonite, SiO₂, and CaSi₂O₅.     

Deposition of alumina coatings on monocrystalline diamonds by sol–gel techniques

Machining of steel or iron-based alloys with diamond tools leads to rapid tool failure — probably due to chemical wear. The use of monocrystalline diamond tools has, up to now, been obligatory for precision machining. Coating the diamonds with a thin but hard and chemically inert alumina film may overcome the problem. Alumina coatings were deposited by sol–gel techniques. It was shown that a very thin TiN intermediate layer, deposited by reactive sputtering, results in a good adhesion of the alumina coatings to the monocrystalline diamonds. The microstructure of the coatings was characterized by field-emission scanning electron microscopy (FE-SEM) and by transmission electron microscopy (TEM). The deposited coatings showed a nanocrystalline, dense microstructure. The hardness of the coatings was investigated by ultramicrohardness measurements (UMH).     

Highly conductive coatings of carbon black/silica composites obtained by a sol–gel process

Conductive submicronic coatings of carbon black (CB)/silica composites have been prepared by a sol–gel process and deposited by spray-coating on glazed porcelain tiles. Stable CB dispersions with surfactant were rheologically characterized to determine the optimum CB-surfactant ratio. The composites were analyzed by Differential Thermal and Thermogravimetric Analysis and Hg-Porosimetry. Thin coatings were thermally treated in the temperature range of 300–500 °C in air atmosphere. The microstructure of the coatings was determined by scanning electron microscopy and the structure evaluated by confocal Raman spectroscopy. The electrical characterization of the samples was carried out using dc intensity–voltage curves. The coatings exhibit good adhesion, high density and homogeneous distribution of the conductive filler (CB) in the insulate matrix (silica) that protects against the thermal degradation of the CB nanoparticles during the sintering process. As consequence, the composite coatings show the lowest resistivity values for CB-based films reported in the literature, with values of ～7 × 10^?5 Ωm.     

Corrosion resistance performance of cerium doped silica sol–gel coatings on 304L stainless steel

The aim of this work is the synthesis and investigation of silane based organic–inorganic hybrid coatings, which can be used to improve the corrosion performance of steel structures subjected to a marine environment. The silane based sol–gel coatings were prepared by dip coating 304L stainless steel in a solution of organically modified silica sol made through hydrolysis and condensation of 3-glycidoxypropyl-trimethoxysilane (GPTMS) as precursor and bisphenol A (BPA) as a cross-linking agent in an acid catalyzed condition. The influence of the addition of cerium and the use of bisphenol A as a cross-linking agent on the microscopic features and morphology as well as on the corrosion resistance of the coatings were examined using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), neutral salt spray tests, potentiodynamic polarization and electrochemical impedance techniques. Results show that cerium modified nano-hybrid coatings exhibit a superior corrosion inhibition performance to that displayed by silica hybrid coatings. Additionally, data showed that the bisphenol A as a cross-linking agent has a significant effect on the morphology and corrosion resistance of the cerium doped silica coating. Omitting the use of bisphenol A causes the creation of defects/cracks in the coating, thereby promoting diffusion of the aggressive electrolyte toward the substrate and decreasing the corrosion resistance of the coating.     

Size-controlled synthesis of mesoporous silica nanoparticles using rice husk by microwave-assisted sol–gel method

Mesoporous silica nanoparticles with distinct characteristics like particle size, tunable pores, and high surface area have received much interest for environmental remediation, energy conversion, and biological applications. In this work, we synthesized spherical silica nanoparticles with tunable particle size and mesoporous properties using a low-cost silica source (rice husk) and polyethylene glycol (PEG) via microwave-assisted sol–gel synthesis. The formation of an amorphous silica structure was found using XRD and FTIR analysis. FESEM analysis showed that altering the PEG concentration from .01 to .005 M produced spherical silica nanoparticles with 100–500 nm in size. Nitrogen adsorption–desorption demonstrated that silica nanoparticles obtained with .005, .007, and .01 M of PEG had unique pore sizes and distributions, with specific surface areas of 51.475, 62.367, and 84.251 m²/g, respectively. These results might be due to PEG molecules’ capping effect, which acts as a soft template to regulate particle size, pore size, and dispersion by interacting with sodium silicate precursor. Hence, this approach can be a facile and cost-effective method to prepare mesoporous silica nanoparticles with controllable nanoscale characteristics for suitable applications.     

Weatherability of hybrid organic–inorganic silica protective coatings on glass

Currently, there is a growing interest in the application of silicon-based technologies for the development of advanced hybrid organic–inorganic coatings with strong weatherability. In this study, the sol–gel process is used to prepare such coatings on glass and their resistance to weathering effects is assessed afterwards. Various sols were prepared by mixing a silica-based inorganic matrix (tetraethyl orthosilicate) with different quantities of silica alkoxides functionalised with various organic groups. Subsequently, the sols were dip-coated onto glass samples at low temperatures without any heat treatment. The coatings prepared were analysed before and after three model ageing tests simulating various weathering parameters. After ageing, the best performing coatings showed good overall homogeneity and transparency (optical microscopy, SEM), improved water repellency and adhesion to the glass substrate (static contact angle measurements, cross-cut tape tests) and no colour or chemical composition changes (UV–VIS, FTIR). Compared with commercial hybrid silica products, the alkyl- and methacryloxy-functionalised silica coatings particularly displayed improved homogeneity, elasticity and barrier properties. Thus, these low temperature coatings, easily applicable to thin films, appear to fulfil the main requirements for the protection of the glass exposed to weathering phenomena.     

Structure and optical properties of transparent Er3+-doped CaF2–silica glass ceramic prepared by controllable sol–gel method

Transparent Er³⁺-doped CaF₂–silica glass ceramics were prepared by the direct physical introduction of Er³⁺ doped CaF₂ nanocrystals into acid-catalyzed sol–gel silica glass. The physical methods of ball milling, ultrasonic baths, and stirring were investigated to disperse Er³⁺ doped CaF₂ nanocrystals in the silica sols. The CaF₂–silica sol mixture went through gelation and heat-treatment to form Er³⁺-doped CaF₂–silica glass ceramics. The morphology of Er³⁺ doped CaF₂ in silica glass did not change after heat-treatment at 600 °C for 10 h. The experimental results showed that Er³⁺ doped CaF₂ in the glass ceramic prepared with the assistance of ball milling possesses the best dispersity and homogeneity. The highest in-line transmittance of the glass ceramic reached up to 85% in visible region. Glass ceramic exhibits efficient up-conversion emissions corresponding to the Er³⁺:⁴F_9/2→⁴I_15/2 transition and long lifetime of ⁴F_9/2 level (1.73 ms) under 980 nm excitation.     

Photoluminescence of ZnO layer on commercial glass substrate prepared by sol–gel process

Amorphous ZnO thin film on soda–lime–silica glass substrate was prepared by the sol–gel process at low-temperature processing, i.e., 100 °C. No distinct grain structure was observable in the surface of the film. The photoluminescence spectrum of the ZnO thin film with an intense near band edge emission was observed while the defect-related broad green emission was nearly quenched.     

Corrosion protection properties of silica coatings formed by sol–gel method on Al: The effects of acidity,withdrawal speed,and annealing temperature

In this research, silica films were coated onto aluminum sheets using the sol–gel dip-coating method from acidic and basic solutions containing (C₂H₅O)₄Si, C₂H₅OH, H₂O, and catalysts. NH₃ and HNO₃ were used as catalysts in the acidic and basic solutions, respectively. The characteristics of the films were investigated as a function of the sol catalyst, withdrawal rate, and heat treatment temperature. Morphology of the coatings was studied by scanning electron microscopy (SEM). Corrosion behavior of coated and uncoated aluminum sheets was measured in 0.1 M NaCl. Findings indicated that SiO₂ coatings can offer proper protective properties against corrosive environments. Results also showed that conditions used to prepare the coatings significantly affect the morphology and thus, the corrosion protection of the silica films.     

Modification of the glass surface by titanium dioxide films synthesized through the sol–gel method

The glass surface has been modified by titanium dioxide films synthesized using the sol-gel method. The sol-gel synthesis has been optimized by the experimental design with the use of the Latin square method. The film composition has been evaluated using X-ray powder diffraction analysis and optical measurements. The photoinduced hydrophilicity of the modified glass has been investigated.     

Preparation of hybrid silica sol–gel coatings on mild steel surfaces and evaluation of their corrosion resistance

Hybrid silica sol–gel coatings were prepared on mild steel substrate by dip coating technique. The coatings were subsequently heat treated at 200 °C in order to improve their corrosion properties. The coating sols were synthesized using Glycidoxypropyltrimethoxysilane (glymo) and Aminopropylethoxysilane (ameo) as precursor materials. Potentiodynamic polarization curves were derived and Electrochemical Impedance Spectroscopy (EIS) measurements were made in NaCl solution. The surface and cross-section morphology of coated specimens were characterized by scanning electron microscopy (SEM). Fourier transformed infrared (FTIR) analysis was used to identify the presence of various functional groups in the coating solutions. A comparison of the corrosion resistance of the coated and uncoated mild steel was presented. The results indicated that the corrosion resistance of the coated mild steel was improved considerably.     

Transparent semiconductor zinc oxide thin films deposited on glass substrates by sol–gel process

Transparent semiconductor ZnO thin films were spin-coated onto alkali-free glass substrates by a sol–gel process. The influence of ZnO sols synthesized via different solvents (2-ME, EtOH or IPA) on the surface morphologies, microstructures, optical properties and resistivities of the obtained films were investigated. The as-coated films were annealed in ambient air at 500 °C for 1 h. X-ray diffraction results showed all polycrystalline ZnO thin films to have preferred orientation along the (0 0 2) plane. The surface morphologies, optical transmittances and resistivity values of the sol–gel derived ZnO thin films depended on the solvent used. The ZnO thin films synthesized with IPA as the solvent exhibited the highest average transmittance 92.2%, an RMS roughness of 4.52 nm and a resistivity of 1.5 × 10⁵ Ω cm.     

Antireflective coatings prepared by sol–gel processing: Principles and applications

Sol–gel processing is a powerful tool to prepare antireflective (AR) coatings on optical surfaces. In this paper the different strategies to obtain antireflective properties are reviewed: porous λ/4 layers, multilayer interference-type films and index-gradient materials such as “moth eye” structures. The processing of the respective films is described and evaluated; references to respective commercial products on glass substrates are given.AR coatings may have a particularly high importance for transparent ceramics as their index of refraction is significantly higher than that of common glass types. Reflective losses therefore are higher which is especially unpleasant for materials with a yet improvable intrinsic transparency.Recent studies indicate that specific porous λ/4 layers may exhibit pronounced anti-soiling features. Laboratory experiments as well as outdoor exposure tests were used to demonstrate the dust-repellant properties.     

Non-fluorinated,room temperature curable hydrophobic coatings by sol–gel process

Non-fluorinated hydrophobic silica surfaces were generated on soda lime glass (SLG) substrates using hexamethyldisilazane (HMDS) as a surface modifying agent. Silica coatings were fabricated by dip coating of a sol derived from base catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS). Two methodologies were adopted to generate the hydrophobic surface; one where the hydrophilic silica coated surface was treated by immersion into different concentrations of alcoholic solutions of HMDS varying from 2.5 wt% to 15 wt%. In the other method, HMDS was directly added to a mixture of TEOS, water, ethanol, and ammonium hydroxide and coatings were deposited using this sol by dip coating and spray coating. Water contact angles (WCA) were measured to study the effect of HMDS treatment times and concentrations on hydrophobicity in the first case, and in the second case, WCA were measured for dip and spray coated samples. UV–visible transmission, scratch resistance, and thermal stability of the coatings were determined. The WCA increased from 66 ± 2° to 125 ± 4° after the treatment of the silica coatings with HMDS. In case of coatings generated from direct addition of HMDS to silica sol, WCA varied from 145 ± 2° to 166 ± 4° for dip and spray coated surfaces respectively. Surface morphology was studied to explain the difference in hydrophobicity of coatings generated using the two methods.     

Preparation of scratch resistant superhydrophobic hybrid coatings by sol–gel process

Organic–inorganic hybrid coatings on glass substrates with superhydrophobic properties and with improved scratch resistance were obtained by means of applying a multilayer approach including multiple sol–gel processes. The coatings exhibited a water contact angle (WCA) higher than 150°. Ultraviolet (UV)-curable vinyl ester resins and vinyltriethoxysilane (VTEOS) as coupling agent were employed to increase the adhesion between substrate and the inorganic layers. The surfaces were characterized by means of dynamic contact angle and roughness measurements. Indeed, the occurrence of superhydrophobic behavior was observed. The scratch resistance of the hybrid coatings was tested to evaluate the adhesion of the coatings to the glass substrate. The proposed preparation method for scratch resistant, mechanically stable, superhydrophobic coatings is simple and can be applied on large areas of different kinds of substrates.     

Thermally and mechanically superior hybrid epoxy–silica polymer films via sol–gel method

Hybrid organic–inorganic polymer films composed of an epoxy resin crosslinked with a flexible diamine hardener, and a silica reinforcing phase were produced and their thermo-mechanical properties were determined. Two types of hybrid epoxy–silica polymer films, named EAS-1 and EAS-2, were obtained by hydrolysis and condensation of various amounts of tetraethoxysilane within epoxy network matrix. In EAS-2 hybrids, minor amounts of an amine silane coupling agent were added to enhance interfacial compatibility. FTIR spectroscopy confirmed the formation of organic and inorganic networks. The grafting of amine silane on to the epoxy resin influenced the size and distribution of hyper-branched clusters of silica as indicated by transmission electron microscopy (TEM). The dynamic mechanical and thermal analysis (DMTA) and thermo-gravimetric analysis (TGA) results showed an increase in the storage modulus, the glass-transition temperature, and the thermal stability of hybrid polymer films as compared to the neat matrix. The integration of amine silane coupling agent produced smaller, effectively dispersed silica nanoparticles and consequently improved the ultimate properties of polymer films.     

Re-crystallization of silica-based calcium phosphate glass prepared by sol–gel technique

Bioactive glass (BG) is a potential material for treating dentin hypersensitivity owing to its high solubility. In this study, we synthesized 80S-BG bioactive glass samples using a sol–gel technique and mixed with various hardening agents. The obtained material could be used in human dentinal dentinal tubule occlusions. X-ray diffraction (XRD), scanning electronic microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) measurements were employed to investigate the physiochemical properties and dentinal dentinal tubule occlusion efficiency by mixing the 80S bioactive glass (80S-BG) with various hardening agents.The major crystallite phase obtained on mixing 80S-BG with phosphoric acid (PA) was Ca(H₂PO₄)₂·H₂O. The mixture of 80S-BG powders and 20, 30, or 40 wt% PA acted as a hardening agent and achieved a dentinal tubule penetration depth of 30.7–62.6 µm.80S-BG on mixing with suitable PA agents exhibited a short reaction time and good operability, making it feasible for use in occluding dentinal tubules. 80S-BG mixed with hardening agents exhibited a greater potential for treating dentin hypersensitivity as compared to the 80S-BG not mixed with any hardening agents.     

Hybrid nanocomposite coating by sol–gel method: a review

The development of environmentally friendly process for pretreatments of metallic substrates is a field of growing research due to the ban against chromates used as protective pretreatments. Among the possible candidates for environmentally friendly pretreatments of aluminum alloys are the silica-based sol–gel coatings. Such coatings are able to form an Si–O–Al conversion layer providing a stable alumina/sol–gel film interface, which inhibits the onset of corrosion. Sol–gel technology offers a wide range of chemical mechanisms and exhibits high potential substitutes for the environmentally unfriendly chromate metal-surface pretreatment. Sol–gel derived organo-silicate hybrid coatings, preloaded with organic corrosion inhibitors, have been developed to provide active corrosion protection when integrity of the coating is compromised. The incorporation of organic corrosion inhibitors into hybrid coatings has been achieved as a result of physical entrapment of the inhibitor within the coating material at the stage of film formation and cross-linking. Sol–gel derived coatings, especially the hybrid films, provide a dense barrier against electrolyte uptake, and offer a wide range of applications as corrosion protective, hydrophilic coatings, hydrophobic anti-reflective coatings, migration barriers against liquid and volatile compounds, antibacterial modification of textiles and water-repellent antistatic textiles. In this paper, the novel applications of the sol–gel derived coatings are presented and discussed.