Abstract
Very few papers have discussed about core–shell nanocrystals with ultralarge lattice mismatches. Generally, for synthesis of core–shell structures the two materials should have a lattice constant mismatch below 5%. In this dissertation, we present a synthetic method for Au@Cu nanocrystals despite the large lattice mismatch between Au and Cu at 11.4%. Then we have used these Au@Cu core–shell nanocrystals to catalyze click reactions for efficient synthesis of diverse triazoles. Finally, we used these Au@Cu core–shell nanocubes for the synthesis of Au@Cu–Cu2O core–shell nanocrystals with cubic, octahedral and rhombic dodecahedral structures and tunable sizes for facet-dependent optical property examination. In Chapter 2, copper nanocubes with tunable edge lengths over the range from 49 to 136 nm and ultrasmall octahedra with opposite corner distances of 45, 51, and 58 nm have been synthesized in aqueous solutions by reducing CuCl2 or copper acetate with ascorbic acid in the presence of octahedral gold nanocrystal cores and hexadecylamine (HDA) at 100 ºC for 45 min to 1.5 h. Addition of HDA increases the solution pH and acts as a coordinating ligand to the copper ions to facilitate controlled copper shell growth. Due to ultralarge lattice mismatch between Au and Cu, non-uniform copper deposition yields cubes and octahedra with noncentrally located gold cores. The Au–Cu octahedra show little shift in the plasmonic band with increasing particle size. For Au–Cu nanocubes, the degree of absorption band red-shift gets smaller as cube size increases. The Au–Cu nanocubes have shown reasonable reactivity toward 4-nitrophenol reduction at 40 ºC. In Chapter 3, Au@Cu cubes and octahedra were employed to catalyze 1,3-dipolar cycloaddition reaction between phenylacetylene and benzyl azide in water at 50 ºC for 3 h. Interestingly, the nanocubes were far more efficient in catalyzing this reaction, giving 91% yield of exclusively 1,4-triazole product, while octahedra only recorded 46% yield. The Au‒Cu nanocubes were subsequently employed to catalyze the click reaction between benzyl azide and a broad range of aromatic and aliphatic alkynes. The product yields ranged 78 to 99%. Clearly the Au‒Cu cubes exposing {100} surfaces are an excellent and green catalyst for click reactions. In Chapter 4, 50 nm Au@Cu cubic cores were used to fabricate Au@Cu–Cu2O core–shell cubes, octahedra, and rhombic dodecahedra with tunable sizes. Despite the unprecedented lattice mismatch of 15.1% between Cu and Cu2O, fine adjustment in the volumes of reagents introduced allows the formation of these heterostructures. To relieve the lattice strain, the metal cores are essentially never found to locate at the particle center, and slight lattice spacing shifts have been recorded. Although efforts have been made to reduce the heterostructure sizes, the Cu2O shells are generally too thick to reveal surface plasmon resonance (SPR) absorption band from the metal cores. Only the Au@Cu–Cu2O cubes with many cores located near the particle corners show observable SPR band red shift, but UV–vis spectra of all particle shapes are still dominated by Cu2O absorption and light scattering bands. Au@Cu–Cu2O cubes consistently show the most red-shifted absorption bands than those of octahedra resulting from the optical facet effects.