Nanospace-Confined High-Temperature Solid-State Reactions: Versatile Synthetic Route for High-Diversity Pool of Catalytic Nanocrystals

Abstract

The present study proposes a methodology to extend the utility of solid-state reactions to a synthetic route for producing a high-diversity pool of nanocrystals (NCs) by circumventing the problematic sintering of nanoparticles at high temperatures. For this purpose, nanometer-scale-confined NC formations/transformations were investigated using specifically designed SiO2 nanospheres with a radially differentiated core@shell structure as a reaction medium. The core of the SiO2 medium was modified by aminosilanes, thus providing binding sites for the metal ions and enabling, pore creation through thermochemical treatment. This hindered the outward diffusion of fast-moving tiny species initially generated within the tens of nanometers-sized medium. The entire evolution process of Pd NCs during the high temperature reaction was confined within the SiO2 nanosphere, which transforms from the amine containing nanosphere, Pd2+/SiO2(NH2), to a hollow nanosphere, Pd@h-SiO2. This nanoscale confinement strategy was applicable to the first -row transition metals with high diffusivity in SiO2 medium. Accordingly, M2+/SiO2(NH2)(2) (M = Co, Ni, or Cu) and (Pd2+/M2+)/SiO2(NH2)(2) were thermally developed into M@h-SiO2 and MPd@h-SiO2, producing single crystalline NCs of metals and their Pd alloys, respectively. The variability of the product was further enhanced by subsequent thermal treatment of MPd@h-SiO(2)s in the presence of air, which led to the conversion of MPd alloy NCs into Pd/MxOy (MxPy = NiO or Co3O4) and Pd-0.64.Cu-0.36/CuOx hybrid NCs with metal/metal-oxide heterojunction interfaces while being confined into the SiO2 nanosphere. Moreover, the evolution process of the PdCo NCs was examined under nanoscale confinement in the rarely explored high-temperature and solid-state environments. The findings revealed that the belatedly reduced Co atoms were incorporated gradually into the lattice of the preformed Pd NC with increasing annealing temperature, developing into a random alloyed PdCo phase. The comparative investigation of catalytic activities of the pool of Pd-based NCs produced by the nanospace-confined solid-state reactions revealed that the Pd/Co3O4 NC, with a strong metal support-interaction effect at the heterojunction interface, exhibits the highest performance in catalyzing the CO oxidation reaction.