Recent Progress on Asymmetric Carbon- and Silica-Based Nanomaterials: From Synthetic Strategies to Their Applications

HIGHLIGHTS

The synthetic strategies and fundamental mechanisms of various asymmetric carbon- and silica-based nanomaterials were systematically summarized. The advantages of asymmetric structure on their related applications were clarified by some representative applications of asymmetric carbon- and silica-based nanomaterials. The future development prospects and challenges of asymmetric carbon- and silica-based nanomaterials were proposed.

ABSTRACT

Carbon- and silica-based nanomaterials possess a set of merits including large surface area, good structural stability, diversified morphology, adjustable structure, and biocompatibility. These outstanding features make them widely applied in different fields. However, limited by the surface free energy effect, the current studies mainly focus on the symmetric structures, such as nanospheres, nanoflowers, nanowires, nanosheets, and core–shell structured composites. By comparison, the asymmetric structure with ingenious adjustability not only exhibits a larger effective surface area accompanied with more active sites, but also enables each component to work independently or corporately to harness their own merits, thus showing the unusual performances in some specific applications. The current review mainly focuses on the recent progress of design principles and synthesis methods of asymmetric carbon- and silica-based nanomaterials, and their applications in energy storage, catalysis, and biomedicine. Particularly, we provide some deep insights into their unique advantages in related fields from the perspective of materials’ structure–performance relationship. Furthermore, the challenges and development prospects on the synthesis and applications of asymmetric carbon- and silica-based nanomaterials are also presented and highlighted. [Image: see text]

Transparent Hydrophobic Hybrid Silica Films by Green and Chemical Surfactants

Monodispersed and transparent hybrid silica wires were synthesized by the sol-gel method using the chemical surfactant trimethoxyoctylsilane (C8TMOS or C11H26O3Si) and, for the first time, by green surfactants (Nelumbo nucifera/lotus leaf extract). The purpose was to introduce a less toxic, cost-effective, and one-step easy approach to get superhydrophobic silica films. Each of the surfactants was used at two different concentrations to investigate hydrophobicity of the films. Assembly of silica wires was obtained by dip-coating and vacuum filtration methods on glass and cellulose acetate filter paper as substrates, respectively. The water contact angle (CA) up to 154° was measured for hybrid silica films on filter paper, which revealed their superhydrophobicity as compared to hydrophobic behavior of those films coated on a glass substrate with CA up to 135°. Chemical, optical, and structural properties of prepared films were characterized by Fourier transform infrared spectroscopy, UV-vis spectroscopy, scanning electron microscopy, thermogravimetry, and differential scanning calorimetry. The hybrid silica wires prepared displayed good transparency, low surface energy, and superhydrophobicity. These silica assemblies can create outstanding and multifunctional structures with superhydrophobic coatings for waterproof electronic devices, military uniforms, self-cleaning surfaces, etc.

Janus-Type Gold/Polythiophene Composites Formed via Redox Reaction at the Ionic Liquid|Water Interface

Janus-type Au/polythiophene (PT) composites have been prepared by utilizing the liquid/liquid interface between water (W) and a hydrophobic ionic liquid (IL) as the redox reaction site. AuCl₄^- is reductively deposited, and terthiophene is oxidatively polymerized spacio-selectively at the IL|W interface, leading to the formation of the Au/PT composites. The composites are Janus-type Au-attached PT plates with two surface morphologies, flat surface and flowerlike surface at the W and IL sides of the plates at the IL|W interface, respectively. Not only surface morphologies but also attached Au structures are different at the two surfaces; Au microurchins on the flat surface and dendritic Au nanofibers on the flowerlike surface. Optical and scanning electron microscopic observations have revealed that nanofibers and microurchins are formed at the early and later stage of the reaction, respectively. Electrochemistry at the IL|W interface has illustrated that electron transfer across the IL|W interface during the formation of the Janus-type Au/PT composites is coupled with ion transfer of AuCl₄^- to compensate for the charge unbalance in the two liquid phases. AuCl₄^- transferred into IL is found to be the source of the dendritic Au nanofibers formed at the IL side of the PT plates.

Vapor-based synthesis and micropatterning of Janus thin films with distinct surface wettability and mechanical robustness

Janus polymer thin films with distinct surface wettability and mechanical robustness were synthesized using a facile, one-step, vapor-deposition process.

Oriented perylene incorporated optically anisotropic 2D silica films

Structurally oriented, optically anisotropic silica films prepared at the liquid–liquid interface.

Janus ultrathin film from multi-level self-assembly at air-water interfaces

Ultrathin free-standing Janus films were fabricated at air-water interfaces using azopyridine derivatives and poly(acrylic acid) via multi-level self-assembly on molecular and microscopic scales, which showed distinct asymmetric water wetting abilities on different surfaces.

Highly transparent, flexible, and thermally stable superhydrophobic ORMOSIL aerogel thin films

We report preparation of highly transparent, flexible, and thermally stable superhydrophobic organically modified silica (ORMOSIL) aerogel thin films from colloidal dispersions at ambient conditions. The prepared dispersions are suitable for large area processing with ease of coating and being directly applicable without requiring any pre- or post-treatment on a variety of surfaces including glass, wood, and plastics. ORMOSIL films exhibit and retain superhydrophobic behavior up to 500 °C and even on bent flexible substrates. The surface of the films can be converted from superhydrophobic (contact angle of 179.9°) to superhydrophilic (contact angle of <5°) by calcination at high temperatures. The wettability of the coatings can be changed by tuning the calcination temperature and duration. The prepared films also exhibit low refractive index and high porosity making them suitable as multifunctional coatings for many application fields including solar cells, flexible electronics, and lab on papers.

Photoluminescent silica nanotubes and nanodisks prepared by the reverse micelle sol-gel method

The reverse micelle sol-gel method was used earlier to prepare silica nanotubes, in aerosol OT/n-heptane/water microemulsions containing FeCl(3). The present communication reports the remarkable effect of the amount of water in the microemulsions on the shape, size, and spectral properties of the silica nanostructures formed. Nanotubes are formed, as expected, at lower water contents. However, for higher water contents, nanodisks form in predominance. This rather surprising observation indicates the formation of flat, disklike water pools in this medium. Notably, a phase separation occurs at higher water contents, and this appears to be essential for the formation of the disklike nanostructures. Hence, we propose that flat water pools form at the interface of the two liquid phases. The nanotubes and nanodisks exhibit blue photoluminescence. The photoluminescence of the nanotubes is more susceptible to quenching by moisture than that of the nanodisks. Luminescence is restored by heating or purging nitrogen or oxygen. Time-resolved photoluminescence studies conform to a model in which the luminescence is ascribed to a particular kind of defect center, with some contribution from surface-associated defects.