Anisotropic Particles: Preparation and Study.

摘要

Anisotropic particles have received significant attention in self-assembly for the large scale fabrication of hierarchical structures. Janus particles, a specific class of anisotropic particles, have two hemispheres with different materials. Due to the anisotropic nature of the particle shape and interactions, Janus particles have demonstrated interesting properties in interfacial assembly, switchable devices, cargo transport, and optical sensing. The objective of this research is to fabricate novel anisotropic Janus particles and explore their potential unique properties.;One of the driving forces arises from the previous work of bimetallic nanorods and their autonomous motion. The bimetallic nanorod systems undergo chemically powered non- Brownian motion due to the asymmetric distribution of catalytic source for a chemical fuel solution. However, the approach used to prepare the bimetallic nanorods is rather complex. The original design of bimetallic Janus particles is based on a general physical vapor deposition technique -- electron beam evaporation. The resulting bimetallic Janus particles are colloidal silica spheres coated with two differing metals on each hemisphere. This approach allows fabricating bimetallic Janus particles with various combinations of metals that are available for electron beam evaporation.;Chemical transformation of bimetallic Janus particles into other species provides an opportunity to expand the scope of anisotropic particles. The metals on the Janus particles are possible to convert to their corresponding metal oxides and metal sulfides through solid-gas heterogeneous reactions, and therefore, the chemical transformation of the parent bimetallic Janus particles produces a wide array of previously unavailable Janus particle types, including metal/metal oxide, metal/metal sulfide, metal oxide/metal oxide, metal sulfide/metal sulfide, and metal oxide/metal sulfide, which allows tuning their optical, electronic, magnetic and catalytic properties. This vast library of anisotropic particulate building blocks provides a powerful arsenal for engineering the assembly of specific targeted structures and systems.;Autonomous motion is distinctive from Brownian motion. Platinum half-coated Janus particles undergo self-propelled motion, which is induced by the catalytic decomposition of hydrogen peroxide. The average speed of the self-propelled Pt-SiO2 Janus particles increases with increasing the concentration of hydrogen peroxide. Motion direction analyses show that the probability for the Janus particles continuing to travel in nearly same direction goes higher in higher concentrations of hydrogen peroxide. Microscopic observation of the particle motion demonstrates that these Janus particles move, on average, with the platinum-coated region oriented opposite to the direction of motion. The trajectories of the autonomous motion exhibit a directed motion at short time scale but with an overall random behavior at long time scales. Huge benefit can be garnered by taking advantage of the self-propulsion component in the system. The control of the motion of the magnetic Janus particles in solutions of hydrogen peroxide is demonstrated using the external magnetic field. The magnetic Janus particles orient themselves with the equatorial plane parallel to the applied field and the motion direction is perpendicular to the field. The directed motion has a more distinct preferred direction compared to the case in the absence of magnetic field, and the applied field is verified to control the orientation, not influence the speed of the particle motion.;Anisotropic particles are unique building blocks to assemble complex structures. The surface functionalized Janus particles with alkanethiols are adsorbed at the interfaces of liquid-air and liquid-liquid, forming monolayers with metal hemispheres pointing to the same direction. By changing the liquid oil phase, the orientation of the Janus particles can be manipulated, which provides an opportunity to selectively modify the surface in either phase. The preferential orientation in the same direction at interfaces allows for direct transfer of the Janus particles while the desired faces remain in either a "face-down" or "face-up" configuration. An external intervention, magnetic field, is also sought to direct the assembly of the magnetic Janus particles. In the presence of uniform magnetic field, the magnetic Janus particles form staggered chain structures with the chain direction parallel to the direction of the applied field. These chain structures are destroyed due to the capillary force during solvent evaporation. However, these soft structures are successfully locked in place after the solution dries by the addition of ammonium carbonate to the solution, which suggests a promising way to achieve 2D or 3D super structures for the fabrication of photonic crystals and photonic devices.