Colloidal black gold with broadband absorption for plasmon-assisted crosslinking of thiolated diazonium compound

Broadband light absorbers are very attractive for many applications, including solar energy conversion, photothermal therapy, and plasmonic nanocatalysis. Black gold nanoparticles are an excellent example of broadband light absorbers in the visible and near-infrared (NIR)ranges; however, their synthesis typically requires multi-step deposition and/or high temperatures. Herein, we report the synthesis of black gold via a facile, one-step green method using commonly known precursors (chloroauric acid and sodium citrate) performed at room temperature. The formation of the black gold particles is driven by self-assembly of in-situ formed small nanoparticles (~ 5 nm) followed by a fusion step forming extensive networks of nanowires. These assemblies form intense hotspots for enhancing the electric field as well as the local temperature. Thus, the nanowires exhibit a strong photothermal effect and SERS performance. The high SERS signal enhancement is used to monitor the kinetics of plasmon-assisted de-nitrogenation and cross-linking of designed thiolated benzenediazonium molecules. Analysis of the reaction performed with external heating in dark and under light irradiation confirmed the charge transfer effect (i.e., hot electrons) to be the dominant mechanism. Our work offers new possibility to design efficient light absorbing materials to achieve good solar-to-chemical/thermal energy conversion.