Superhydrophobic Silicon Nanocrystal-Silica Aerogel Hybrid Materials: Synthesis, Properties, and Sensing Application

One-Pot Sol-Gel Synthesis of Highly Insulative Hybrid P(AAm-CO-AAc)-Silica Aerogels with Improved Mechanical and Thermal Properties

Silica aerogels and their derivatives have outstanding thermal properties with exceptional values in the thermal insulation industry. However, their brittle nature restricts their large-scale commercialization. Thus, enhancing their mechanical strength without affecting their thermal insulating properties is essential. Therefore, for the first time, highly thermally stable poly(acrylamide-co-acrylic acid) partial sodium salt is used as a reinforcing polymer to synthesize hybrid P(AAm-CO-AAc)-silica aerogels via epoxy ring-opening polymerization in the present study. Functional groups in P(AAm-CO-AAc) partial sodium salts, such as CONH2 and COOH, acted as nucleophiles for the epoxy ring-opening reaction with (3-glycidyloxypropyl)trimethoxysilane, which resulted in a seven-fold enhancement in mechanical strength compared to that of pristine silica aerogel while maintaining thermal conductivity at less than 30.6 mW/mK and porosity of more than 93.68%. Moreover, the hybrid P(AAm-CO-AAc)-silica aerogel demonstrated improved thermal stability up to 343 °C, owing to the synergetic effect between the P(AAm-CO-AAc) and the silica aerogel, corresponding to the thermal stability and strong covalent bonding among them. These excellent results illustrate that this new synthetic approach for producing hybrid P(AAm-CO-AAc)-silica aerogels is useful for enhancing the mechanical strength of pristine silica aerogel without impairing its thermal insulating property and shows potential as an industrial heat insulation material.

Thermal Properties of Porous Silicon Nanomaterials

The thermal properties, including the heat capacity, thermal conductivity, effusivity, diffusivity, and phonon density of states of silicon-based nanomaterials are analyzed using a molecular dynamics calculation. These quantities are calculated in more detail for bulk silicon, porous silicon, and a silicon aerocrystal (aerogel), including the passivation of the porous internal surfaces with hydrogen, hydroxide, and oxygen ions. It is found that the heat capacity of these materials increases monotonically by up to 30% with an increase in the area of the porous inner surface and upon its passivation with these ions. This phenomenon is explained by a shift of the phonon density of states of the materials under study to the low-frequency region. In addition, it is shown that the thermal conductivity of the investigated materials depends on the degree of their porosity and can be changed significantly upon the passivation of their inner surface with different ions. It is demonstrated that, in the various simulated types of porous silicon, the thermal conductivity changes by 1-2 orders of magnitude compared with the value for bulk silicon. At the same time, it is found that the nature of the passivation of the internal nanosilicon surfaces affects the thermal conductivity. For example, the passivation of the surfaces with hydrogen does not significantly change this parameter, whereas a passivation with oxygen ions reduces it by a factor of two on average, and passivation with hydroxyl ions increases the thermal conductivity by a factor of 2-3. Similar trends are observed for the thermal effusivities and diffusivities of all the types of nanoporous silicon under passivation, but, in that case, the changes are weaker (by a factor of 1.5-2). The ways of tuning the thermal properties of the new nanostructured materials are outlined, which is important for their application.

Super Pressure-Resistant Superhydrophobic Fabrics with Real Self-Cleaning Performance

Detergents are extensively used for laundry, causing significant negative impacts on water bodies, plants and animals. Superhydrophobic fabrics are promising to reduce detergent consumption but suffer from low pressure resistance. Here, we report super pressure-resistant superhydrophobic fabrics prepared using polysiloxane modified SiO₂ nanoparticles with epoxy groups. The fabrics show real self-cleaning performance, essentially different from the conventional self-cleaning property of solid particles loosely placed on superhydrophobic surfaces. The contaminated fabrics by various stains can be completely cleaned by home machine laundering without using any detergent whereas the traditional superhydrophobic fabrics cannot. This is owing to excellent abrasion and washing durability, low liquid adhesion force, superior pressure-resistance and vapor-resistance of the fabrics, originating from the low surface energy and dense micro-/nanostructure. Moreover, the superhydrophobic fabrics can be scaled up using the conventional fabric finishing line with low cost. The superhydrophobic fabrics will help significantly reduce the global detergent consumption.

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Superhydrophobic fabrics with real self-cleaning performance are prepared

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The fabrics show high durability and pressure-resistance, low liquid adhesion force

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The contaminated fabrics can be cleaned by home machine laundering without detergent

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The fabrics can be scaled up using the conventional fabric finishing line

Switchable wettability of grain-stacked filter layers from polyurethane plastic waste for oil/water separation

HYPOTHESIS

Polyurethane plastic waste (PUPW), a port-abundant solid waste, is difficult to degrade naturally and poses a severe threat to the environment. Hence, the effective recycling of PUPW remains a challenge.

EXPERIMENTS

Herein, a strategy of converting PUPW into stacked oil/water filtration layer grain through a layer-by-layer (LBL) assembly process is investigated. Notably, such PU-based, grain-stacked, and switchable wettability of the oil/water filter layer is first reported.

FINDINGS

The grain-stacked filter layers are flexible for separating immiscible oil/water mixtures, water-in-oil emulsions (WOE), and oil-in-water emulsions (OWE) under gravity over 10 cycle-usages. They can withstand strong acid/alkali solutions (pH = 1-14) and salt solutions over 12 h. Besides, 100-times scale-up experiments have indicated that the obtained filter layers exhibit an upper to 98.2 % separation efficiency for 10 L real industrial oil/water emulsion in the 24 h continuous operation. The demulsification mechanism for emulsions is that the electrostatic interaction along with adsorption between emulsion droplets and grains leads to the uneven distribution of surfactants on the interface film of the emulsion droplets, increasing the probability of tiny droplets colliding and coalescing into large droplets to achieve oil/water separation. This work proposes an effective and economical method of abundant plastic waste for industrial-scale oil-water separation rather than just on the laboratory-scale.

Synthesis, structures and properties of hydrophobic Alkyltrimethoxysilane-Polyvinyltrimethoxysilane hybrid aerogels with different alkyl chain lengths

HYPOTHESIS

Alkyltrimethoxysilane (ATMS) is among most widely used silane coupling agents. These commercially available, reasonably priced chemicals are often utilized to improve the compatibility of inorganic surfaces with organic coatings. With three hydrolysable moieties, ATMS is an outstanding candidate for solving the hydrophilicity, moisture sensitivity and high cost of silica aerogels. However, ATMS has a non-hydrolysable alkyl chain that undergoes cyclization reactions. The alkyl chain prevents ATMS from being incorporated in aerogel structures. Polyvinyltrimethoxysilane (PVTMS) is a silica precursor that offers two types of crosslinking to the final aerogel product. This strong doubly-crosslinked network can potentially suppress the cyclization reactions of ATMS and include it in aerogel structure.

EXPERIMENTS

PVTMS was used with ATMS having different alkyl lengths (3-16 carbons) and loadings (25 or 50 wt%) as the silica precursors. Acid and base catalysts were used to perform hydrolysis and condensation reactions on the mixture and ATMS:PVTMS aerogels were obtained via supercritical drying.

FINDINGS

The incorporation of ATMS in the aerogels was approved by different characterization methods. Results showed that ATMS:PVTMS aerogels possess hydrophobicity (θ ∼ 130°), moisture resistance, varying surface area (44-916 m²·g^-1), meso/microporous structure and thermal insulation properties (λ ∼ 0.03 W·m^-1K^-1). These samples also showed excellent performance in oil and organic solvent adsorption.

Robust and Eco-Friendly Superhydrophobic Starch Nanohybrid Materials with Engineered Lotus Leaf Mimetic Multiscale Hierarchical Structures

The use of superhydrophobic surfaces in a broad range of applications is receiving a great deal of attention due to their numerous functionalities. However, fabricating these surfaces using low-cost raw materials through green and fluorine-free routes has been a bottleneck in their industrial deployment. This work presents a facile and environmentally friendly strategy to prepare mechanically robust superhydrophobic surfaces with engineered lotus leaf mimetic multiscale hierarchical structures via a hybrid route combining soft imprinting and spin-coating. Direct soft-imprinting lithography onto starch/polyhydroxyurethane/cellulose nanocrystal (SPC) films formed micro-scaled features resembling the pillar architecture of lotus leaf. Spin-coating was then used to assemble a thin layer of low-surface-energy poly(dimethylsiloxane) (PDMS) over these microstructures. Silica nanoparticles (SNPs) were grafted with vinyltriethoxysilane (VTES) to form functional silica nanoparticles (V-SNPs) and subsequently used for the fabrication of superhydrophobic coatings. A further modification of PDMS@SPC film with V-SNPs enabled the interlocking of V-SNPs microparticles within the cross-linked PDMS network. The simultaneous introduction of hierarchical microscale surface topography, the low surface tension of the PDMS layer, and the nanoscale roughness induced by V-SNPs contributed to the fabrication of a superhydrophobic interface with a water contact angle (WCA) of ∼150° and a sliding angle (SA) of <10°. The PDMS/V-SNP@SPC films showed an ∼52% reduction in water vapor transmission rate compared to that of uncoated films. These results indicated that the coating served as an excellent moisture barrier and imparted good hydrophobicity to the film substrate. The coated film surfaces were able to withstand extensive knife scratches, finger-rubbing, jet-water impact, a sandpaper-abrasion test for 20 cycles, and a tape-peeling test for ∼10 repetitions without losing superhydrophobicity, suggesting superior mechanical durability. Self-cleaning behavior was also demonstrated when the surfaces were cleared of artificial dust and various food liquids. The green and innovative approach presented in the current study can potentially serve as an attractive new tool for the development of robust superhydrophobic surfaces without adverse environmental consequences.

Silica Aerogels with Self-Reinforced Microstructure for Bioinspired Hydrogels

Aerogel is a kind of high-performance lightweight open-porous solids with ultralow density, high specific surface area, and broad application in many emerging fields including biotechnology, energy, environment, aerospace, etc. A giant challenge remains in preventing of the hydrophilic aerogel framework shrinkage when replacing of solvent with air in its extremely abundant nanosized pores during its fabrication process in ambient conditions. In this work, started from a linear polymeric precursor with further condensation reaction, superhydrophilic silica aerogels with self-reinforced microstructure and the least volume shrinkage have been successfully obtained via ambient pressure drying process without use of any additives in the presence of a low surface tension solvent. The resulting superhydrophilic silica aerogels possess specific surface area up to 1065 m²/g, pore volume up to 2.17 cm³/g and density down to 84 mg/cm³, and these values are comparable to those of their counterparts obtained by supercritical CO₂ drying process. Moreover, as an application demonstration, the bioinspired hydrogels with desirable mechanical flexibility and adhesive performance at extremely harsh environment (e.g., below -50 °C) have been successfully synthesized by mimicking carrier of a functional bioagent with the resulting superhydrophilic silica aerogel microparticles. Our work has made a significant step forward for future high-performance hydrophilic aerogels with self-enhanced microstructures and the resulting superhydrophilic aerogels have shown great potentials in making functional hydrogels with bionic properties.

Design and Application of High Optical Quality YAG:Ce Nanocrystal-Loaded Silica Aerogels

The intrinsic properties of silica aerogels make them well suited for applications requiring high surface area. Therefore, the dispersion of functional nanoparticles (NPs) in these highly porous structures gives access to materials for wide range of applications such as catalysis, energy storage or sensing. The last one is particularly interesting if such composites possess good optical quality. Herein, the synthesis of monolithic and transparent silica aerogels highly loaded with Y₃Al₅O₁₂:Ce nanocrystals (NCs) (up to 50 wt %) is reported. The developed composite aerogels can be impregnated with liquids, contrary to most of existing aerogels, which crack because of the strong capillary forces. Therefore, this system is designed as a novel concept of 3D porous scintillator, using the efficient photoluminescent and scintillating properties of Y₃Al₅O₁₂:Ce. The investigated fluid containing low-energetic ionizing radiation emitters impregnates the material, which assures the efficient harvesting of radiation because of highly developed surface area. Such composites prove to be efficient new-type detectors of low-energy beta radiation both in liquids and gases.

Preparation and Characterization of Polyimide Aerogels with a Uniform Nanoporous Framework

Polyimide (PI) aerogels were successfully synthesized via a straightforward sol-gel process under room temperature along with the supercritical CO₂ drying method using 4-amino- N-methylbenzamide and 3,3',4,4'-biphenyltetracarboxylic dianhydride. 1,3,5-Triaminophenoxybenzene was used as the cross-linker. The chemical structure, pore structure, morphology, thermal performance, CO₂ adsorption, and mechanical performance of PI aerogels were investigated. The as-prepared PI aerogels had low bulk densities (0.091-0.167 g/cm³), low shrinkages (9.73-17.36%), low thermal conductivities (0.0307-0.0341 W/m·K), high specific surface areas (449.76-538.19 m²/g), small pore diameter (10.37-22.41 nm), high thermal stability (onset of decomposition above 560 °C), and excellent mechanical property. The CO₂ adsorption capacities of PI aerogels were substantially higher than the values of the previous porous materials reported under the similar conditions, and the CO₂ uptake capacity was as high as 31.19 cm³/g at 25 °C and 1.0 bar. The resulting PI aerogels could be potentially used as thermal insulators and CO₂ adsorbents.