Synthesis and optical properties of gold/silver nanocomposites prepared on multi-walled carbon nanotubes via galvanic replacement of silver nanoparticles

We report in this article a simple route for synthesizing small-sized silver/gold core–shell (Ag/Au) on multi-walled carbon nanotube (MWCNT) surfaces via galvanic replacement of Ag nanoparticles (Ag NPs). The Raman response of MWCNT decorated with Ag/Au was investigated under surface-enhanced Raman scattering. A relatively weak Raman signal enhancement of the tube was observed due to the large interparticle distance between neighboring small-sized nanostructures. Ag/Au gives better enhancing capability than the starting Ag because of the synergistic effect between the localized electric field of the Ag core, and the Au shell separated with a hollow space formed during the galvanic replacement reaction. Furthermore, the Ag/Au was removed from the CNT surfaces via sonication with 1-octanethiol (OT), releasing unreplaced Ag NPs and Au nanobowls (Au NBs) with 1.3 and 7.6 nm of mean diameter sizes, respectively. The production of these fine-sized nanocomposites (Au/Ag NCs) allowed us to investigate their luminescent property. Interestingly, the separated Au/Ag NCs (i.e., the mixture of Au NBs and unreplaced Ag NPs) exhibit fluorescence behavior that may be useful for single-molecule detection. Our technique provides the synthesis of smallest dimension of Au NBs so far simply achieved by wet-chemical process using MWCNTs as templates.