Quantitative Prediction of Position and Orientation for Platonic Nanoparticles at Liquid/Liquid Interfaces

Effect of Particle Size on the Orientation and Order of Assemblies of Functionalized Microscale Cubes Formed at the Water/Air Interface

The impact of the particle size and wettability on the orientation and order of assemblies obtained by self-organization of functionalized microscale polystyrene cubes at the water/air interface is reported. An increase in the hydrophobicity of 10- and 5-μm-sized self-assembled monolayer-functionalized polystyrene cubes, as assessed by independent water contact angle measurements, led to a change of the preferred orientation of the assembled cubes at the water/air interface from face-up to edge-up and further to vertex-up, irrespective of microcube size. This tendency is consistent with our previous studies with 30-μm-sized cubes. However, the transitions among these orientations and the capillary force-induced structures, which change from flat plate to tilted linear and further to close-packed hexagonal arrangements, were observed to shift to larger contact angles for smaller cube size. Likewise, the order of the formed aggregates decreased significantly with decreasing cube size, which is tentatively attributed to the small ratio of inertial force to capillary force for smaller cubes in disordered aggregates, which results in more difficulties to reorient in the stirring process. Experiments with small fractions of larger cubes added to the water/air interface increased the order of smaller homo-aggregates to values similar to neat 30 μm cube assemblies. Hence, collisions of larger cubes or aggregates are shown to play a decisive role in breaking metastable structures to approach a global energy minimum assembly.

Investigation of the Orientation and Assembly of Functionalized Microcubes at the Oil-Water Interface

The dependence of the preferred orientation of polystyrene microcubes on surface hydrophobicity at the water/hexadecane interface is reported. Similar to the water/air interfaces, the microcubes were shown to reside at the water/hexadecane interface with three distinct orientations: face-up, edge-up, and vertex-up. Concomitantly, ordered aggregates with flat plate, tilted linear, and close-packed hexagonal structures were formed, driven by capillary force. With increasing the hydrophobicity of five sides of the cubes, the preferential microcube orientation at the water/hexadecane interface changed sequentially from face-up to edge-up, to vertex-up, then back to edge-up, and to face-up. This dependence of the preferential microcube orientation on surface hydrophobicity at the water/hexadecane interface differs from that observed at the water/air interface, where the preferential orientation changed only from face-up to edge-up, then to vertex-up, as surface hydrophobicity increased. In addition, preformed microcube assemblies at the water/air interface could be dynamically reconfigured by replacing the air phase with hexadecane under stirring.

Graphene Oxide-Mediated Synthesis of Ultrathin Co-MOL for CO2 Photoreduction

Metal-organic framework (MOF) materials have broad application prospects in catalysis because of their ordered structure and molecular adjustability. However, the large volume of bulky MOF usually leads to insufficient exposure of the active sites and the obstruction of charge/mass transfer, which greatly limits their catalytic performance. Herein, we developed a simple graphene oxide (GO) template method to fabricate ultrathin Co-metal-organic layer (2.0 nm) on reduced GO (Co-MOL@r-GO). The as-synthesized hybrid material Co-MOL@r-GO-2 exhibits highly efficient photocatalytic performance for CO₂ reduction, and the CO yield can reach as high as 25,442 μmol/g_Co-MOL, which is over 20 times higher than that of the bulky Co-MOF. Systematic investigations demonstrate that GO can act as a template for the synthesis of the ultrathin Co-MOL with more active sites and can be used as the electron transport medium between the photosensitizer and the Co-MOL to enhance the catalytic activity for CO₂ photoreduction.

Tunable Orientation and Assembly of Polymer-Grafted Nanocubes at Fluid-Fluid Interfaces

Self-assembly of faceted nanoparticles is a promising route for fabricating nanomaterials; however, achieving low-dimensional assemblies of particles with tunable orientations is challenging. Here, we demonstrate that trapping surface-functionalized faceted nanoparticles at fluid-fluid interfaces is a viable approach for controlling particle orientation and facilitating their assembly into unique one- and two-dimensional superstructures. Using molecular dynamics simulations of polymer-grafted nanocubes in a polymer bilayer along with a particle-orientation classification method we developed, we show that the nanocubes can be induced into face-up, edge-up, or vertex-up orientations by tuning the graft density and differences in their miscibility with the two polymer layers. The orientational preference of the nanocubes is found to be governed by an interplay between the interfacial area occluded by the particle, the difference in interactions of the grafts with the two layers, and the stretching and intercalation of grafts at the interface. The resulting orientationally constrained nanocubes are then shown to assemble into a variety of unusual architectures, such as rectilinear strings, close-packed sheets, bilayer ribbons, and perforated sheets, which are difficult to obtain using other assembly methods. Our work thus demonstrates a versatile strategy for assembling freestanding arrays of faceted nanoparticles with possible applications in plasmonics, optics, catalysis, and membranes, where precise control over particle orientation and position is required.

Modulating Orientational Order to Organize Polyhedral Nanoparticles into Plastic Crystals and Uniform Metacrystals

In nanoparticle self-assembly, the current lack of strategy to modulate orientational order creates challenges in isolating large-area plastic crystals. Here, we achieve two orientationally distinct supercrystals using one nanoparticle shape, including plastic crystals and uniform metacrystals. Our approach integrates multi-faceted Archimedean polyhedra with molecular-level surface polymeric interactions to tune nanoparticle orientational order during self-assembly. Experiments and simulations show that coiled surface polymer chains limit interparticle interactions, creating various geometrical configurations among Archimedean polyhedra to form plastic crystals. In contrast, brush-like polymer chains enable molecular interdigitation between neighboring particles, favoring consistent particle configurations and result in uniform metacrystals. Our strategy enhances supercrystal diversity for polyhedra comprising multiple nondegenerate facets.

Novel Conjugated Side Chain Fluorinated Polymers Based on Fluorene for Light-Emitting and Ternary Flash Memory Devices

Three novel conjugated polymers based on 9,9'-dioctylfluorene unit and isoindolo[2,1-a]benzimidazol-11-one with different fluorine substituents (0, 2 and 4) were synthesized. PLED and resistive memory devices based on these polymers were prepared consequently. PLED based on four-fluorinated polymer showed the highest maximum brightness of 3192 cd m-2 with almost 5-fold increase of current efficiency 8-fold increase of external quantum efficiency compared to that of the other two, and all the PLEDs exhibited good emission stability with no noticeable change of electroluminescence even under high voltage of 10 V. The memory device of doubly-fluorinated polymer exhibited ternary flash behavior with threshold voltages below -2.5 V, while device of four-fluorinated polymer possessed ON/OFF current ratio above 104. Impact of fluorine substitutions on the performance of devices were briefly investigated. The results revealed that the improvement of device performance might not scale with the increasing number of fluorine substitutions, and the four-fluorine-substituted polymer and doubly-fluorinated polymer could be encouraging materials for applications of PLED and resistive memory device and worth of further design of other new polymer systems.

Control of Orientation, Formation of Ordered Structures, and Self-Sorting of Surface-Functionalized Microcubes at the Air-Water Interface

The dependence of the orientation of microscale PS cubes, which are surface functionalized on only five faces, at the water/air interface and the ordered aggregates formed by capillary force assembly are reported. Depending on the wettability of the faces, the cubes were shown to adopt a preferred orientation that changes with decreasing wettability from face up to edge up and further to vertex up. Concomitantly, stable aggregates with different structures were formed by capillary force self-assembly. The unmodified bottom face of the cubes was localized by fluorescence labeling. Finally, self-sorting of differently surface functionalized microcubes was realized for the first time, due to the stronger capillary interactions of quadrupole-quadrupole and hexapole-hexapole interactions compared to quadrupole-hexapole interaction.

Shape-Anisotropic Colloids at Interfaces

Research in the 1980s demonstrated the formation of monolayers of particles achieved by interfacial particle trapping as a model system for investigating colloids in two dimensions. Since then, microscopy visualization of two-dimensional particle monolayers and quantification of the microstructure have led to significant fundamental understanding of a number of phenomena such as crystallization, freezing and melting transitions, dislocation dynamics, aggregation kinetics, and others. On the application front, particles at curved interfaces, as often the case in particle-stabilized emulsions and foams, have received considerable attention in the last few decades. The growing interest in the search for novel particles and new strategies to effect emulsion stabilization stems from their application in several disciplines. Moreover, particle-stabilized Pickering emulsions and foams can also be used to derive a number of advanced functional materials. Compared to several accounts of research on spherical colloids at fluid-fluid interfaces, investigations of the behavior of shape-anisotropic particles at interfaces, albeit receiving considerable attention in recent years, are still in a nascent stage. The objective of this feature article is to highlight our recent work in this area. In particular, the adsorption of shape-anisotropic particles to interfaces, wetting behavior, interfacial self-assembly, the response of nonspherical-particle-coated interfaces to compression and shear, and their ability to stabilize emulsions are discussed.