Au@TiO<sub>2</sub> Core–Shell Nanoparticles\nwith Nanometer-Controlled Shell Thickness for Balancing Stability\nand Field Enhancement in Plasmon-Enhanced Photocatalysis

Plasmonic core–shell nanostructures can make photocatalysis\nmore efficient for several reasons. The shell imparts stability to\nthe nanoparticles, light absorption is expanded, and electron–hole\npairs can be separated more effectively, thus reducing recombination\nlosses. The synthesis of metal@TiO<sub>2</sub> core–shell nanoparticles\nwith nanometer control over the shell thickness and understanding\nits effect on the resulting photocatalytic efficiency still remains\nchallenging. In the present study, a synthesis method is presented\nfor preparing Au@TiO<sub>2</sub> core–shell nanoparticles with\nultrathin shells that can be accurately tuned in the range of 2–12\nnm, based on the controlled slow hydrolysis of a titanium precursor.\nElectromagnetic simulations combined with comprehensive characterization\nof the opto-physical bulk properties, as well as energy electron loss\nspectroscopy and electron tomography reconstructions at the nanoscale,\naid in understanding the crucial role of the shell in improving both\nthe activity as well as the stability in a photocatalytic reaction.\nUltrathin shells in the order of 2 nm do not suffice to prevent sintering\nof the nanoparticles upon annealing, with a consequent loss of plasmonic\nproperties. After reaching an optimum for a shell of 4 nm, further\nincreasing the shell thickness again reduces the plasmonic properties\nby a weakened plasmonic coupling. This trend is confirmed by photocatalytic\nhydrogen evolution experiments, as well as stearic acid degradation\ntests. With this study, we prove and emphasize the crucial importance\nof carefully controlling the shell thickness in plasmonic core–shell\nstructures, so their maximum application potential may be unlocked.