Colloidal Black Gold with Broadband Absorption for Plasmon-Induced Dimerization of 4-Nitrothiophenol and Cross-Linking of Thiolated Diazonium Compound

Broadband light absorbers are very attractive for manyapplications,including solar energy conversion, photothermal therapy, and plasmonicnanocatalysis. Black gold nanoparticles are an excellent example ofbroadband light absorbers in the visible and near-infrared (NIR)-ranges;however, their synthesis typically requires multistep deposition orhigh temperatures. Herein, we report the synthesis of black gold viaa facile, one-step green method using commonly known precursors (chloroauricacid and sodium citrate) performed at room temperature. The formationof the black gold particles is driven by the self-assembly of in situ-formedsmall nanoparticles (similar to 5 nm) followed by a fusion step creatingextensive networks of nanowires. These assemblies include intensehotspots for enhancing the electric field and the local temperature.Thus, the nanowires exhibit a strong photothermal effect and SERSperformance. The plasmonic reactivity of the black gold is first testedusing the dimerization reaction of 4-nitro thiophenol. In addition,we used the strong SERS signal enhancement to monitor the kineticsof plasmon-induced cross-linking of designed thiolated benzene diazoniummolecules. The analysis of the reaction performed with external heatingin the dark and under light irradiation confirmed the dominant mechanismof the charge-transfer effect (i.e., hot electrons). Our work offersa new possibility to design efficient light-absorbing materials toachieve good solar-to-chemical/thermal energy conversion.