(369f) Development of Methods for Precise, Multifactor Tuning of Shell Morphology on Silica-Encapsulated Gold Core-Shell Nanoparticles

A seeded encapsulation synthesis method was developed to facilitate precise, multifactor tuning of mesoporous silica-encapsulated gold core-shell nanoparticles (Au@SiO₂ CSNPs). The new seeded method prevents the Stöber synthesis portion of the reaction from impacting the gold core diameter. Altering the Stöber synthesis conditions thusly results in core-shell nanoparticles with different shell morphologies, without affecting the gold active phase. Because of this, remarkably simple catalytic studies can be performed to investigate the specific impact of pore morphology on catalytic performance. Both silica pore diameter and shell thickness (corresponding to pore length) were varied. It was found increasing ethanol concentration resulted in higher pore diameters, while reducing total solvent volume resulted in high shell thicknesses.

Rotational diffusion NMR studies using d-toluene revealed anisotropic reduction in rotational diffusion within the silica mesopores: rotation about the major molecular axis was reduced less than orthogonal rotation, indicating species tended to orient themselves beneficially within the pores. It was found the difference in rotation between axes was lessened as pore diameter increased. When catalyzing solvent-free benzyl alcohol oxidation, CSNPs with narrower pores demonstrated higher catalytic activity. CSNPs with longer pores also demonstrated higher activity, suggesting longer diffusion paths can increase the extent of beneficial orientation. Results from developing this method not only demonstrate clearly the value of mesoporous encapsulation, but also provide a simple, tunable platform for investigating separately the impacts of diffusion and reaction on catalysis.