Surface modification of metal nanoparticles for organic catalysts and organic-inorganic hybrids

Abstract

Nanoparticles, which have a large surface area, high surface energy, and unique optical properties, can be widely applied in various fields, such as catalysis and sensor. The physical and chemical properties of the nanoparticles can be finely tailored by tuning their chemical composition, size, morphology, and interface. These properties can be classified into two - intrinsic and extrinsic properties. The intrinsic properties hardly change after the structure formation since they are majorly determined during the synthesis step. On the other hand, the extrinsic properties are essentially affected by the surface environment

thus, surface modification can easily control them. Therefore, surface modification plays a critical role in nanoparticle research. For instance, desired functional groups inserted by surface modification can increase stability. Combining hetero-materials is also plausible to construct new hybrid materials. In addition, the electronic states or band gaps of nanoparticles can be precisely modified by controlling the surface charge, which is beneficial for increasing the utilization of heterogeneous catalysts in numerous catalytic reactions. In the present study, we focus on the surface modification of metal nanoparticles and their use for two purposes: tailoring catalytic properties and the direct utilization for catalytic organic transformations and preparing organic-inorganic hybrid materials and their manipulation of structural assembly and physicochemical properties. In chapter I, we introduce significant concepts comprehensively and discuss the importance of the surface modification of nanoparticles. In chapter II, we explain noble metal nanoparticles as heterogeneous catalysts for C-H functionalization by controlling oxidation states using organic oxidants. By [Ph2I]BF4, the hypervalent iodine-based weak oxidant, Pd(II) species was made on the surface of Pd nanoparticles. In the arylation of hetero-aromatic compounds, such as indole, benzofuran, and benzothiophene, the Pd(II) nanocatalysts performed the reactions in high reactivity and selectivity. In addition, Pd(IV) species were successfully generated by Cl+ oxidants such as PhICl2. The resulting Pd(IV) nanocatalysts carried out asymmetric C-H chlorination with chiral ligands. The active species, Pd(II) and Pd(IV), and their catalytic processes were investigated by various spectroscopic and imaging techniques. In chapter III, we prepare bottlebrush polymers anchored on the gold nanoparticles by surface ligand exchange. These bottlebrush polymer-gold nanoparticle conjugates are essential building blocks for organic-inorganic hybrid materials in two ways: first, the polystyrene-based bottlebrush polymers on the gold nanoparticles were used as a mold in a nanometer scale. By increasing the water/DMF ratio in the solvent, we could yield various bimetallic hetero-nanostructures, including core-shell, rough core-shell, multi-island, and Janus. Second, monovalent polymer-gold nanoparticle conjugates were prepared with varying binding block lengths on the bottlebrush polymer. The monovalent conjugates generated self-assembled layers on the substrate, where gold units formed linear chains. The resulting substrates exhibited intense refractive colors, primarily tuned by adjusting the polymer backbone and side chain lengths.