An "Ion pairing" strategy for facile fabrication of densely packed gold nanostructures in ordered mesoporous silicas

Templated fabrication using ordered mesoporous silicas (OMSs) as hard templates is versatile to obtain nanostructured Au with controlled size, shape, and morphology. However, the confined Au species have low affinities with silica and exhibit high mobility even at room temperature, and tedious surface modification of the hard templates is often required to stabilize them in the nanospace. Here, a simple yet effective "ion paring" strategy was developed to fabricate densely packed Au nanostructures in OMSs. A nitrate salt of a metal cation with high charge density, such as ferric nitrate, was co-impregnated with chloroauric acid to retard the diffusion of chloroauric anions and the reduced Au species during fabrication processes to result in fully confined formation of nanostructured Au. The interactions between the ferric cations and chloroauric anions and the reduced Au species were studied. The strategy was successfully applied to OMSs with a confined dimension of as small as 3 nm. The results demonstrated the broad applicability of the simple strategy to fabricate uniform-sized and densely packed Au nanostructures in OMS templates.