AbstractA key challenge in advancing the design of hybrid nanostructures (HNs) lies in the difficulty in mastering the principle of selected hybrid formation, which is complicated not only by the size and shape variations of nanoparticles but also by the interfacial phenomena associated with surface ligands. Here this study elaborates the formation mechanism of HNs by a combined experimental and theoretical study employing multiscale simulations and shows how molecular information encoded on particle surface can be transferred into distinct composite patterns. The emergence of different HNs is found to be not only related to ligand binding strengths affecting the reaction kinetics but also the ligand–ligand interactions responsible for phase segregation. Unexpectedly, the sulfidation of Ag nanoparticles co-stabilized by citrate/gallic acid with different molar ratios constantly produces heterodimers with faster reaction rate than the formation of core–shell structures when they are solely coated by citrate or gallic acid. The surprising result originates from the phase separation of two short surface ligands with large contrast in binding strengths as indicated by photoluminescence spectra and supported by the dissipative particle dynamics simulations. Hierarchical HNs consisting of a heterodimer shell with built-in hot spots can be further synthesized using Au@Ag core–shell particles with mixed surface layers.

Coaxial fibers are the key elements in many optical, electrical, and biomedical applications. Recent success in materials synthesis has provided versatile choices for the core part, but the search of high-performance sheath materials remains much less productive. These surface coatings are however as important as the core for their role as protection layers and interaction medium with the externals, thereby critically affecting the real performance of coaxial fibers. Here it is shown that aramid nanofibers (ANFs) with exceptional environmental stability and mechanical properties can be advanced coating materials for both wet- and dry-spun carbon nanotube (CNT) wires. Co-wet-spinning ANFs with CNT aqueous dispersion can produce coaxial fibers with a compact sheath comprised of aligned ANFs, showing much enhanced mechanical properties by transferring stress to the sheath without sacrificing the conductivity. On the other hand, an immersion-precipitation process is used to prepare a porous sheath made from randomly distributed nanofibers on dry-spun CNT wires, which can be combined with ionic conductive gel electrolyte as a strong packaging layer for flexible solid-state supercapacitors. The excellent intrinsic characteristics as well as variable ways of structural organizations make ANF-based coatings an attractive tool for the design of multifunctional high-performance hybrid materials.

Self-healing ability and the elastic modulus of polymeric materials may seem conflicting because of their opposite dependence on chain mobility. Here, we show that boron nitride (BN) nanoplatelets can simultaneously enhance these seemingly contradictory properties in exponentially layer-by-layer-assembled nanocomposites as both surface coatings and free-standing films. On one hand, embedding hard BN nanoplatelets into a soft hydrogen bonding network can enhance the elastic modulus and ultimate strength through effective load transfer strengthened by the incorporation of interfacial covalent bonding; on the other hand, during a water-enabled self-healing process, these two-dimensional flakes induce an anisotropic diffusion, maintain the overall diffusion ability of polymers at low loadings, and can be “sealing” agents to retard the out-of-plane diffusion, thereby hampering polymer release into the solution. A detailed mechanism study supported by a theoretical model reveals the critical parameters for achieving a complete self-healing process. The insights gained from this work may be used for the design of high-performance smart materials based on other two-dimensional fillers.Keywords: BN nanoplatelets; layer-by-layer assembly; mechanical properties; nanocomposites; self-healing

Elastomers such as polyurethanes usually possess low stiffness, and the addition of traditional fillers typically results in a moderate improvement. Aramid nanofibers (ANFs) represent one of the most promising nanoscale building blocks for high-performance nanocomposites. In this work, waterborne polyurethanes (PUs) have been reinforced with ANFs using two solution processing methods, namely, layer-by-layer (LBL) assembly technique and the vacuum-assisted flocculation (VAF) method. Record-high modulus of 5.275 GPa and ultimate strength of 98.02 MPa are obtained among all the reported PU based nanocomposites. We attribute such achievement to the similar molecular structures of ANFs with PUs which ensures a high affinity made possible by the manifold interfacial interactions. The formation of multiple hydrogen bonds due to the presence of amide groups with appropriate spacing in both components is confirmed by the computer simulation. Compared with the VAF method, it is found that LBL assembly allows a better load transfer, resulting in higher ultimate strength and stiffness. The VAF method shows advantages in improving the ultimate strength at low loadings of ANFs. We believe our work may not only lead to a new practical combination within the field of composite materials but also provide important implications for the future design of nanocomposites based on the innovative nanofillers.

Nanoscale materials having size- and shape-dependent interactions with light provide flexible opportunities for harvesting solar energy. Photocatalysts based on semiconductor nanoparticles (NPs) have been the most effective materials for the conversion of light into chemical energy, the efficiency of which can be further enhanced by the incorporation of metallic NPs forming hybrid nanostructures. The structural parameters of not only constituent components but also the resultant hybrid nanostructures are critical for the optimization of photocatalytic performance of composite catalysts. Here we demonstrated the successful size control over ZnO hexagonal pyramids (HPs) for the first time. The smallest HPs showing the best photocatalytic properties were used for further Au attachment. Interestingly, we found that most of the Au NPs preferred to grow on the apexes of the basal plane. Very occasionally, Au NPs at the tip of ZnO HPs can be observed. The role of light in promoting the reduction of gold salt by sodium citrate was also revealed. Quantum mechanical calculations were used to explain the site-specific growth of Au on the surface of ZnO HPs. Enhanced degradation rates over organic dyes were found for Au/ZnO hybrids under both UV and visible light irradiation.

Developing a simple and efficient method to organize nanoscale building blocks into ordered superstructures, understanding the mechanism for self-assembly and revealing the essential collective properties are crucial steps toward the practical use of nanostructures in nanotechnology-based applications. In this study, we showed that the high-yield formation of ZnO nanoparticle chains with micrometer length can be readily achieved by the variation of solvents from methanol to water. Spectroscopic studies confirmed the solvent effect on the surface properties of ZnO nanoparticles, which were found to be critical for the formation of anisotropic assemblies. Quantum mechanical calculations and all atom molecular dynamic simulations indicated the contribution of hydrogen bonding for stabilizing the structure in water. Dissipative particle dynamics further revealed the importance of solvent–nanoparticle interactions for promoting one-dimensional self-assembly. The branching of chains was found upon aging, resulting in the size increase of the ensembles and network formation. Steady-state and time-resolved luminescent spectroscopes, which probed the variation of defect-related emission, revealed stronger Forster resonance energy transfer (FRET) between nanoparticles when the chain networks were formed. The high efficiency of FRET quenching can be ascribed to the presence of multiple energy transfer channels, as well as the short internanoparticle distances and the dipole alignment.Keywords: anisotropic structure; chains; hydrogen bonding; molecular dynamics; nanoparticles; one-dimensional; self-assembly; ZnO;

Small but strong carbon nanotubes (CNTs) are fillers of choice for composite reinforcement owing to their extraordinary modulus and strength. However, the mechanical properties of the nanocomposites are still much below those for mechanical parameters of individual nanotubes. The gap between the expectation and experimental results arises not only from imperfect dispersion and poor load transfer but also from the unavailability of strong polymers that can be effectively utilized within the composites of nanotubes. Aramid nanofibers (ANFs) with analogous morphological features to nanotubes represent a potential choice to complement nanotubes given their intrinsic high mechanical performance and the dispersible nature, which enables solvent-based processing methods. In this work, we showed that composite films made from ANFs and multiwalled CNTs (MWCNTs) by vacuum-assisted flocculation and vacuum-assisted layer-by-layer assembly exhibited high ultimate strength of up to 383 MPa and Young’s modulus (stiffness) of up to 35 GPa, which represent the highest values among all the reported random CNT nanocomposites. Detailed studies using different imaging and spectroscopic characterizations suggested that the multiple interfacial interactions between nanotubes and ANFs including hydrogen bonding and π–π stacking are likely the key parameters responsible for the observed mechanical improvement. Importantly, our studies further revealed the attractive thermomechanical characteristics of these nanocomposites with high thermal stability (up to 520 °C) and ultralow coefficients of thermal expansion (2–6 ppm·K–1). Our results indicated that ANFs are promising nanoscale building blocks for functional ultrastrong and stiff materials potentially extendable to nanocomposites based on other nanoscale fillers.Keywords: aramid nanofibers; CNTs; CTE; layer-by-layer assembly; LBL; mechanical properties; nanocomposites; nanotubes; stiffness; strength; vacuum-assisted flocculation;