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 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 multistep deposition 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 the self-assembly of in situ-formed small nanoparticles (∼5 nm) followed by a fusion step creating extensive networks of nanowires. These assemblies include intense hotspots for enhancing the electric field and the local temperature. Thus, the nanowires exhibit a strong photothermal effect and SERS performance. The plasmonic reactivity of the black gold is first tested using the dimerization reaction of 4-nitro thiophenol. In addition, we used the strong SERS signal enhancement to monitor the kinetics of plasmon-induced cross-linking of designed thiolated benzene diazonium molecules. The analysis of the reaction performed with external heating in the dark and under light irradiation confirmed the dominant mechanism of the charge-transfer effect (i.e., hot electrons). Our work offers a new possibility to design efficient light-absorbing materials to achieve good solar-to-chemical/thermal energy conversion.