Symmetry-Breaking Plasmonic Mesoporous Gold Nanoparticles with Large Pores
Creating free-standing gold nanoparticles (Au NPs) with large pores is desirable because the exterior and interior voids can enhance electrocatalytic activity, mass transport, and optical extinction properties. However, the high mobility and significant positive reduction potential of Au precursors make it challenging to create Au NPs with pores of sufficient size to strongly interact with light. We demonstrate a method to synthesize mesoporous Au NPs with large, tunable pores. l-Cysteine acts as a metallogelator to form a dense, less mobile Au(I)-thiolate precursor that traps aggregated block copolymer micelles and facilitates the reduction of mesoporous Au NPs. Electron tomography measurements showed that the pores were distributed throughout the interior and exterior of the particle. Electrochemical methods were used to estimate the chemical reactivity of the surface active sites and estimate the accessible surface area of the pores to ensure that the metal surfaces were maximally accessible to the environment. The 3D models generated by tomography were then used to simulate their optical properties. Mesoporous Au NPs support multipolar plasmon resonances that penetrate deep into the interior pores of the NP. A simple model indicates that porosity affects the local optical conductivity of the NP by subdividing it into tiny nanoscale junctions that redshift the plasmon modes without changing the overall size or shape of the NPs. Large pores promote symmetry breaking, causing the quadrupolar and dipolar modes to overlap and form strongly hybridized plasmon modes. In the context of photocatalysis, porosity-induced symmetry breaking is advantageous because strong electric fields of the plasmon are colocalized along concave/convex features where step-edges and kinks in the atomic structure generate numerous catalytic active sites. Plasmon-enhanced photodegradation of metanil yellow was used to demonstrate the superior photocatalytic properties of meso Au NPs versus nonporous Au NPs.