Self-Assembled Ag-MXA Superclusters with Structure-Dependent Mechanical Properties

Co-Assembly of Soft and Hard Nanoparticles into Macroscopic Colloidal Composites with Tailored Mechanical Property and Processability

Colloidal composites, translating the great potential of nanoscale building bricks into macroscopic dimensions, have emerged as an appealing candidate for new materials with applications in optics, energy storage, and biomedicines. However, it remains a key challenge to bridge the size regimes from nanoscopic colloidal particles to macroscale composites possessing mechanical robustness. Herein, a bottom-up approach is demonstrated to manufacture colloidal composites with customized macroscopic forms by virtue of the co-assembly of nanosized soft polymeric micelles and hard inorganic nanoparticles. Upon association, the hairy micellar corona can bind with the hard nanoparticles, linking individual hard constituents together in a soft-hard alternating manner to form a collective entity. This permits the integration of block copolymer micelles with controlled amounts of hard nanoparticles into macroscopic colloidal composites featuring diverse internal microstructures. The resultant composites showed tunable microscale mechanical strength in a range of 90-270 MPa and macroscale mechanical strength in a range of 7-42 MPa for compression and 2-24 MPa for bending. Notably, the incorporation of soft polymeric micelles also imparts time- and temperature-dependent dynamic deformability and versatile capacity to the resulting composites, allowing their application in the low-temperature plastic processing for functional fused silica glass.

Self-Assembly-Driven Nanomechanics in Porous Covalent Organic Framework Thin Films

Nanomechanics signifies a key tool to interpret the macroscopic mechanical properties of a porous solid in the context of molecular-level structure. However, establishing such a correlation has proved to be significantly challenging in porous covalent organic frameworks (COFs). Structural defects or packing faults within the porous matrix, poor understanding of the crystalline assembly, and surface roughness are critical factors that contribute to this difficulty. In this regard, we have fabricated two distinct types of COF thin films by controlling the internal order and self-assembly of the same building blocks. Interestingly, the defect density and the nature of supramolecular interactions played a significant role in determining the corresponding thin films' stress-strain behavior. Thin films assembled from nanofibers (∼1-2 μm) underwent large deformation on the application of small external stress (Tp-Azo_fiber film: E ≈ 1.46 GPa; H ≈ 23 MPa) due to weak internal forces. On the other hand, thin films threaded with nanospheres (∼600 nm) exhibit a much stiffer and harder mechanical response (Tp-Azo_sphere film: E ≈ 15.3 GPa; H ≈ 66 MPa) due to strong covalent interactions and higher crystallinity. These porous COF films further exhibited a significant elastic recovery (∼80%), ideal for applications dealing with shock-resistant materials. This work provides in-depth insight into the fabrication of industrially relevant crystalline porous thin films and membranes by addressing the previously unanswered questions about the mechanical constraints in COFs.

Nanoassembled Interface for Dynamics Tailoring

The properties and performance of solid nanomaterials in heterogeneous chemical reactions are significantly influenced by the interface between the nanomaterial and environment. Oriented tailoring of interfacial dynamics, that is, modifying the shared boundary for mass and energy exchange has become a common goal for scientists. Although researchers have designed and constructed an abundance of nanomaterials with excellent performances for the tailoring of reaction dynamics, a complete understanding of the mechanism of nanomaterial-environment interfacial interaction still remains elusive. To predictively understand the nanomaterial-environment relationship over a wide range of time scale, a deep and dynamic insight is required urgently. In this Account, our recent works including advances in the design and construction of nanoassembled interfaces and understanding the dynamic interaction mechanisms between different combinations of nanoparticle (NP) assembly environment interfaces for tailoring the reaction dynamics.NP assemblies with well-defined structures and compositions are inherently suitable for replacing bulk-type nanomaterials for the research on interfaces. We primarily introduced two most relevant nanoassembled surfaces that were fabricated in our laboratory, namely, ordered self-assembly interface and animate nanoassembled interface. The disordered nanoparticles can be arranged into an ordered superlattice based on the self-assembly method and patterned-assembly method. In addition, we used NPs with flexible properties to construct three-dimensional (3D) animate assemblies. On the basis of a thorough understanding of the structure-property correlation, a series of nanoassembled interfaces with various structures have been developed for practice. In comparison with traditional nanomaterial-environment interfaces, the nanoassembled interfaces can change the mode of contact between the nanomaterial and environment, thereby maximizing the number of active sites and driving interferent/product off the nanoassembled interface. The geometry, porosity, and deformable/motional properties in the nanoassembled interface can be applied to enhance the mass transfer dynamics in the chemical reaction. Moreover, the nanoassembled interface can be used to strengthen the affinity between the NP assemblies and targets, thereby enhancing the adsorption efficiency. As shown in these examples, the nanoassembled interface can effectively change the speed, intensity, and mode of interactions between the NP assemblies and environment in spatiotemporal scales.The overall performance of the interfacial dynamics can be improved by the nanoassembled interface, thereby facilitating practical application in flowing systems. We have extended the applications of nanoassembled interfaces from simple adsorption to complex reactions in flowing systems, including in vivo magnetic resonance imaging, electrocatalytic gas evolution reaction, bacterial capture, sensing of exhaled volatile organic compounds, and heterogeneous catalysis. Our current endeavors to explore the applicability of animate nanoassembled interfaces for dynamic tailoring have widened the scope of research, and attempts to construct intelligent interfaces for applications are underway.

Mechanical and Tribological Performances Enhanced by Self-Assembled Structures

Taking inspiration from natural materials, composite materials can be reinforced by creating matrix architectures that can better accommodate and control internal stresses. Despite the recent success in the synthesis of artificial assemblies for local reinforcement through the introduction of oriented fibers and plates into host multilayered composites, there is a lack of fundamental understanding of the factors that determine mechanical properties. Moreover, designing building blocks and interfaces that facilitate higher resistance and energy dissipation is highly challenging. When the intrinsic material is fixed, the mechanical and tribological properties can be further adjusted. In this study, europium oxide nanosheets are arranged in interlocked-junction superstructures that resist sliding at junction points, thereby enhancing the mechanical properties of the nanosheet assemblies compared to those of the conventional face-to-face superstructures formed by parallel nanosheets. Furthermore, the crystalline origin of building blocks is revealed by demonstrating that faulty crystal nanosheets adopting an amorphous structure are different from single-crystal nanosheets, with the former exhibiting superior mechanical reinforcement and improved abrasive resistance.

Self-assembly of anisotropic nanoparticles into functional superstructures

Self-assembly of colloidal nanoparticles (NPs) into superstructures offers a flexible and promising pathway to manipulate the nanometer-sized particles and thus make full use of their unique properties. This bottom-up strategy builds a bridge between the NP regime and a new class of transformative materials across multiple length scales for technological applications. In this field, anisotropic NPs with size- and shape-dependent physical properties as self-assembly building blocks have long fascinated scientists. Self-assembly of anisotropic NPs not only opens up exciting opportunities to engineer a variety of intriguing and complex superlattice architectures, but also provides access to discover emergent collective properties that stem from their ordered arrangement. Thus, this has stimulated enormous research interests in both fundamental science and technological applications. This present review comprehensively summarizes the latest advances in this area, and highlights their rich packing behaviors from the viewpoint of NP shape. We provide the basics of the experimental techniques to produce NP superstructures and structural characterization tools, and detail the delicate assembled structures. Then the current understanding of the assembly dynamics is discussed with the assistance of in situ studies, followed by emergent collective properties from these NP assemblies. Finally, we end this article with the remaining challenges and outlook, hoping to encourage further research in this field.

Effect of structure: A new insight into nanoparticle assemblies from inanimate to animate

Nanoparticle (NP) assemblies are among the foremost achievements of nanoscience and nanotechnology because their interparticle interactions overcome the weaknesses displayed by individual NPs. However, previous studies have considered NP assemblies as inanimate, which had led to their dynamic properties being overlooked. Animate properties, i.e., those mimicking biological properties, endow NP ensembles with unique and unexpected functionalities for practical applications. In this critical review, we highlight recent advances in our understanding of the properties of NP assemblies, particularly their animate properties. Key examples are used to illustrate critical concepts, and special emphasis is placed on animate property-dependent applications. Last, we discuss the barriers to further advances in this field.

Shape-directed self-assembly of nanodumbbells into superstructure polymorphs

Self-assembly of colloidal nanoparticles into ordered superstructures provides a promising route to create novel/enhanced functional materials. Much progress has been made in self-assembly of anisotropic nanoparticles, but the complexity and tunability of superstructures remain restricted by their available geometries. Here we report the controlled packing of nanodumbbells (NDs) with two spherical lobes connected by one rod-like middle bar into varied superstructure polymorphs. When assembled into two-dimensional (2D) monolayer assemblies, such NDs with specific shape parameters could form orientationally ordered degenerate crystals with a 6-fold symmetry, in which these NDs possess no translational order but three allowed orientations with a rotational symmetry of 120 degrees. Detailed analyses identify the distinct roles of subunits in the ND assembly: the spherical lobes direct NDs to closely assemble together into a hexagonal pattern, and the rod-like connection between the lobes endows NDs with this specific orientational order. Such intralayer assembly features are well maintained in the two-layer superstructures of NDs; however, the interlayer stackings could be adjusted to produce stable bilayer superstructures and a series of metastable moiré patterns. Moreover, in addition to horizontal alignment, these NDs could gradually stand up to form tilted or even vertical packing based on the delicate control over the liquid-liquid interface and ND dimensions. This study provides novel insights into creating superstructures by controlling geometric features of nanoscale building blocks and may spur their novel applications.

Modulating the Mechanical Properties of Supercrystalline Nanocomposite Materials via Solvent-Ligand Interactions

Supercrystalline nanocomposite materials with micromechanical properties approaching those of nacre or similar structural biomaterials can be produced by self-assembly of organically modified nanoparticles and further strengthened by cross-linking. The strengthening of these nanocomposites is controlled via thermal treatment, which promotes the formation of covalent bonds between interdigitated ligands on the nanoparticle surface. In this work, it is shown how the extent of the mechanical properties enhancement can be controlled by the solvent used during the self-assembly step. We find that the resulting mechanical properties correlate with the Hansen solubility parameters of the solvents and ligands used for the supercrystal assembly: the hardness and elastic modulus decrease as the Hansen solubility parameter of the solvent approaches the Hansen solubility parameter of the ligands that stabilize the nanoparticles. Moreover, it is shown that self-assembled supercrystals that are subsequently uniaxially pressed can deform up to 6 %. The extent of this deformation is also closely related to the solvent used during the self-assembly step. These results indicate that the conformation and arrangement of the organic ligands on the nanoparticle surface not only control the self-assembly itself but also influence the mechanical properties of the resulting supercrystalline material. The Hansen solubility parameters may therefore serve as a tool to predict what solvents and ligands should be used to obtain supercrystalline materials with good mechanical properties.

Hierarchical supercrystalline nanocomposites through the self-assembly of organically-modified ceramic nanoparticles

Biomaterials often display outstanding combinations of mechanical properties thanks to their hierarchical structuring, which occurs through a dynamically and biologically controlled growth and self-assembly of their main constituents, typically mineral and protein. However, it is still challenging to obtain this ordered multiscale structural organization in synthetic 3D-nanocomposite materials. Herein, we report a new bottom-up approach for the synthesis of macroscale hierarchical nanocomposite materials in a single step. By controlling the content of organic phase during the self-assembly of monodisperse organically-modified nanoparticles (iron oxide with oleyl phosphate), either purely supercrystalline or hierarchically structured supercrystalline nanocomposite materials are obtained. Beyond a critical concentration of organic phase, a hierarchical material is consistently formed. In such a hierarchical material, individual organically-modified ceramic nanoparticles (Level 0) self-assemble into supercrystals in face-centered cubic superlattices (Level 1), which in turn form granules of up to hundreds of micrometers (Level 2). These micrometric granules are the constituents of the final mm-sized material. This approach demonstrates that the local concentration of organic phase and nano-building blocks during self-assembly controls the final material's microstructure, and thus enables the fine-tuning of inorganic-organic nanocomposites' mechanical behavior, paving the way towards the design of novel high-performance structural materials.