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

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 SiO 2 nanospheres with a radially differentiated core@shell structure as a reaction medium. The core of the SiO 2 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 SiO 2 nanosphere, which transforms from the amine-containing nanosphere, Pd 2+ /SiO 2 (NH 2 ), to a hollow nanosphere, Pd@ h -SiO 2 . This nanoscale confinement strategy was applicable to the first-row transition metals with high diffusivity in SiO 2 medium. Accordingly, M 2+ /SiO 2 (NH 2 ) 2 (M = Co, Ni, or Cu) and (Pd 2+ /M 2+ )/SiO 2 (NH 2 ) 2 were thermally developed into M@ h -SiO 2 and MPd@ h -SiO 2, 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/M x O y (M x O y = NiO or Co 3 O 4 ) and Pd 0.64 Cu 0.36 /CuO x hybrid NCs with metal/metal-oxide heterojunction interfaces while being confined into the SiO 2 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/Co 3 O 4 NC, with a strong metal–support-interaction effect at the heterojunction interface, exhibits the highest performance in catalyzing the CO oxidation reaction.