Plasmonic Core-Shell Materials: Synthesis, Spectroscopic Characterization, and Photocatalytic Applications

The ability to concentrate light at the nanoscale and produce extremely high local electromagnetic (EM) fields makes plasmonics a promising and rapidly developing research area. In the region with high EM field intensity (usually called the "hot spot"), various processes can be significantly enhanced, including spectroscopy, luminescence, catalysis, etc. However, only coinage metals ( material limitation) with nanoscale roughness (morphological limitation) exhibit significant plasmonic effects under the visible light region, which greatly hinders wider and further applications of plasmonics. Constructing plasmonic core-shell materials by coating a second material onto the surface of a plasmonic metal core is a potential solution to these limitations. The plasmonic core can amplify the signals and/or accelerate the processes of the shell materials or other substrates of interest, making plasmonic research on nonplasmonic materials possible, thus expanding the applications of plasmonics. Besides, through controllable synthesis, the size and composition of both the core and the shell can be tuned simultaneously and precisely. This offers huge possibilities to study and tune plasmonic structure- performance effects at the (sub)nanometer level, which would otherwise not be feasible. This Account summarizes recent work of our research group on plasmonic core-shell nanomaterials, including synthesis, spectroscopic characterization and photocatalysis, in terms of different compositions of the core and shell. To begin with, we introduce methods to directly coat different kinds of chemically "active" components on a plasmonic core to form Au/Ag@X (X = Pt, PtNi alloy, and MoS2) structures, including the principles of plasmonic core-shell nanomaterials and the "borrowing" strategy. Next, for general surface/interface characterization and analysis, "inert" Au/Ag@SiO2 and shell-isolated enhanced spectroscopy, including Raman, luminescence and nonlinear spectroscopies, are discussed. Then, core-shell satellite structures, Au@SiO2/M, where M includes nanoparticles (NPs) and single metal catalysts (SAC), are presented for nanocatalysis. After that, Au- semiconductor core-shell nanomaterials and hot-electron (HE)-induced photocatalysis are reviewed. Finally, we summarize the remaining challenges and opportunities in plasmonics based on core-shell materials. We believe this Account will be informative in designing novel plasmonic core-shell structures for various applications and will offer new directions in plasmonic materials and applications, not limited to core-shell nanomaterials, spectroscopy or catalysis.