Size-selective synthesis and stabilization of small silver nanoparticles on TiO₂ partially masked by SiO₂

Controlling metal nanoparticle size is one of the principle challenges in developing new supported catalysts. Typical methods where a metal salt is deposited and reduced can result in a polydisperse mixture of metal nanoparticles, especially at higher loading. Polydispersity can exacerbate the already significant challenge of controlling sintering at high temperatures, which decreases catalytic surface area. Here, we demonstrate the size-selective photoreduction of Ag nanoparticles on TiO₂ whose surface has been partially masked with a thin SiO₂ layer. To synthesize this layered oxide material, TiO₂ particles are grafted with tert-butylcalix[4]arene molecular templates (∼2 nm in diameter) at surface densities of 0.05-0.17 templates.nm^-2, overcoated with ∼2 nm of SiO₂ through repeated condensation cycles of limiting amounts of tetraethoxysilane (TEOS), and the templates are removed oxidatively. Ag photodeposition results in uniform nanoparticle diameters ≤3.5 nm (by transmission electron microscopy (TEM)) on the partially masked TiO₂, whereas Ag nanoparticles deposited on the unmodified TiO₂ are larger and more polydisperse (4.7 ± 2.7 nm by TEM). Furthermore, Ag nanoparticles on the partially masked TiO₂ do not sinter after heating at 450 °C for 3 h, while nanoparticles on the control surfaces sinter and grow by at least 30%, as is typical. Overall, this new synthesis approach controls metal nanoparticle dispersion and enhances thermal stability, and this facile synthesis procedure is generalizable to other TiO₂-supported nanoparticles and sizes and may find use in the synthesis of new catalytic materials.