Au@TiO₂ Core–Shell Nanoparticles with Nanometer-Controlled Shell Thickness for Balancing Stability and Field Enhancement in Plasmon-Enhanced Photocatalysis

Plasmonic core-shell nanostructures can make photocatalysis more efficient for several reasons. The shell imparts stability to the nanoparticles, light absorption is expanded, and electron-hole pairs can be separated more effectively, thus reducing recombination losses. The synthesis of metal@TiO₂ core-shell nanoparticles with nanometer control over the shell thickness and understanding its effect on the resulting photocatalytic efficiency still remains challenging. In the present study, a synthesis method is presented for preparing Au@TiO₂ core-shell nanoparticles with ultrathin shells that can be accurately tuned in the range of 2-12 nm, based on the controlled slow hydrolysis of a titanium precursor. Electromagnetic simulations combined with comprehensive characterization of the opto-physical bulk properties, as well as energy electron loss spectroscopy and electron tomography reconstructions at the nanoscale, aid in understanding the crucial role of the shell in improving both the activity as well as the stability in a photocatalytic reaction. Ultrathin shells in the order of 2 nm do not suffice to prevent sintering of the nanoparticles upon annealing, with a consequent loss of plasmonic properties. After reaching an optimum for a shell of 4 nm, further increasing the shell thickness again reduces the plasmonic properties by a weakened plasmonic coupling. This trend is confirmed by photocatalytic hydrogen evolution experiments, as well as stearic acid degradation tests. With this study, we prove and emphasize the crucial importance of carefully controlling the shell thickness in plasmonic core-shell structures, so their maximum application potential may be unlocked.