(509cf) Development of Methods for Consistently Tuning Silica-Encapsulated Gold Core-Shell Nanoparticle Morphology

There is a consistent demand for industry-ready heterogeneous catalysts that are both active and selective. To efficiently design simultaneously active and selective catalysts, it is important to understand precisely how certain fundamental aspects of a catalyst influence catalytic performance. This in turn requires methods for tuning these properties individually, with minimal influence on the others. Finding these methods can be especially challenging for one-pot, multistep syntheses.

Previous research has shown that encapsulation of gold nanoparticles within sufficiently thin mesoporous silica shells allowed for active, selective oxidation of benzyl alcohol. Importantly, it was found that mesoporous silica shells do not inhibit diffusion of reactants to the gold surface, due to the nanoscale diffusion path. To fully understand what aspects of these core-shell nanoparticles (CSNPs) improve catalytic performance, methods for precisely tuning individual catalyst properties were developed.

Surprisingly, higher silica precursor concentration did not produce thicker shells. Instead, gold core diameter and silica shell thickness were both easily tuned by reducing gold precursor concentration. In order decouple the parameters, alternative synthesis methods were pursued. Ultimately the one-pot synthesis was split into two phases: synthesizing colloidal CTAB-stabilized gold nanoparticles, then using these nanoparticles in place of the original in situ gold core reduction.

This allowed for individual tuning of gold core diameter and shell thickness. The method was confirmed via transmission electron microscopy to yield the same core-shell morphology. The final structure shows higher gold core diameters than the pre-synthesized colloidal nanoparticles, indicating some nanoparticle agglomeration occurs during the second phase. Importantly, this growth is independent from silica shell thickness, indicating the steps had been decoupled successfully. By avoiding nanoparticle reduction in the harsher reaction environment required to form the silica shell, researchers were able to tune nanoparticle polydispersity without altering the conditions necessary for consistent silica shell thickness and pore size.