Porous Silica-Coated Gold Sponges with High Thermal and Catalytic Stability

A method to fabricate porous silica-coated Au sponges that show high thermal and catalytic stability has been developed for the first time. The method involves dense surface functionalization of Au sponges (made by self-assembly of Au nanoparticles) with thiolated poly(ethylene glycol) (SH-PEG), which provides binding and condensation sites for silica precursors. The silica coating thickness can be controlled by using SH-PEG of different molecular weights. The silica-coated Au sponge prepared by using 5 kDa SH-PEG maintains its morphology at temperature as high as 700 °C. The calcination removes all organic molecules, resulting in porous silica-coated Au sponges, which contain hierarchically connected micro- and mesopores. The hierarchical pore structures provide an efficient pathway for reactant molecules to access the surface of Au sponges. The porous silica-coated Au sponges show an excellent catalytic recyclability, maintaining the catalytic conversion percentage of 4-nitrophenol by NaBH₄ to 4-aminophenol as high as 93% even after 10 catalytic cycles. The method may be applicable for other porous metals, which are of great interests for catalyst, fuel cell, and sensor applications.