Abstract
Core−shell nanoparticles are highly functional materials with modified properties by changing either the constituting materials or the core to shell ratio. Various core−shell nanostructures have been synthesized such as Au−Cu2O, Au−Ag, Au−Cu, Pt−Pd, and Au−Pd core−shell nanoparticles. Among them, Au−Pd core−shell nanostructures have efficient catalytic properties for a variety of reactions and plasmonic gas sensing upon exposure to H2 as reversible H2 uptake from the Pd shell occurs. Furthermore, synthesis of well-defined Au−Pd core−shell nanocrystals with systematic shape evolution is still challenging by virtue of long reaction time. In Chapter 1, we have developed a facile aqueous solution method to synthesize Au–Pd core–shell nanocrystals with systematic shape evolution from cubic to octahedral structures in just 0.5–2 h at 50 ºC. Octahedral gold nanocrystals were used as cores. Since an important purpose of this work is to systematically examine the plasmonic properties of these particles as a function of particle size, shape, and shell thickness, octahedral Au nanocrystal cores with sizes of 35, 45, 74, and 92 nm have been employed as the templating cores. Au–Pd core–shell cubes, truncated cubes, cuboctahedra, truncated octahedra, and octahedra with precisely tuned particle morphology and shell thickness have been achieved, allowing a thorough and detailed analysis of the plasmonic band appearance and shifts of these core–shell nanocrystals for the first time. Nanoparticles with uniformly thin shell thicknesses, particularly the core–shell octahedra, exhibit the most pronounced plasmonic band derived from the gold cores. In Chapter 2, we employed Au–Pd core–shell THH particles, octahedra, and nanocubes as hydrogen sensing materials. The nanocrystals were dispersed in an aqueous solution and hydrogen gas was introduced into a 10-mL flask through a syringe-attached balloon. Presence of dissolved hydrogen was detected. All of these nanocrystals were found to be excellent plasmonic hydrogen sensors producing very large spectral red-shifts after hydrogen absorption. THH nanocrystals exposing high-index facets displayed the largest spectral shifts. All these particles are highly selective to hydrogen. The spectral shifts are almost fully reversible with successive hydrogen absorption and desorption cycles. For smaller core–shell octahedra, the spectral changes can be visually observed. Larger particles with thicker Pd shells give the largest spectral red-shift. With all these advantages, these Au–Pd core–shell nanocrystals should find broad applications in which simple detection of hydrogen presence is desirable. In Chapter 3, we utilized gold, gold-palladium, gold-silver, and lead sulfide nanocrystals with cubic and octahedral structures as building blocks to fabricate supercrystals by solvent evaporation and surfactant diffusional methods. The supercrystals prepared were characterized by SEM, TEM, and XRD techniques. The microstructures were studied by small-angle X-ray scattering (SAXS) technique. The growth process was investigated. Shapes and sizes of various supercrystals were also controlled. These supercrystals are considered novel superstructures and may show interesting optical and electrical properties which may be used for the fabrication of metamaterials and photonic devices. In these works, we have developed a facile aqueous solution method to synthesize Au–Pd core–shell nanocrystals with systematic shape evolution from cubic to octahedral structures, allowing a thorough and detailed analysis of the plasmonic band appearance and shifts of these core–shell nanocrystals. We also employed polyhedral Au–Pd core–shell nanocrystals as hydrogen sensing materials. All these particles are highly responsive and reusable hydrogen sensors in aqueous solution. Furthermore, we utilized polyhedral Au–Pd core–shell nanocrystals as building blocks to fabricate 3D supercrystals, which are considered novel superstructures and may have opportunities for the fabrication of metamaterials and photonic devices.