Au Quantum Clusters and Plasmonic Quantum Nanoparticles Synthesized under Femtosecond-Pulse Laser Irradiation in Aqueous Solution and in ZIF-8 for Catalytic Reduction of 4-Nitrophenol

Gold quantum clusters and plasmonic quantum nanoparticles were synthesized under femtosecond pulse laser irradiation in an aqueous gold precursor solution. The method produced gold clusters with average sizes of 0.6–1.6 nm after irradiation for 1–3 min, where a hydrated electron reacts as a reducer. As the irradiation is continued longer, plasmonic nanoparticles with larger sizes were produced. Both clusters and plasmonic nanoparticles exhibit fluorescence, indicating a quantum property, although it is not usually expected from conventional plasmonic gold nanoparticles. Thus, the femtosecond laser irradiation procedure can produce gold nanomaterials with the quantum and surface plasmon properties, being different from the chemical reduction. Zeolitic imidazolate framework-8 (ZIF-8) was incorporated as a holder during the synthesis of gold metal. The product, gold-encapsulated ZIF-8 (Au@ZIF-8), exhibits lesser plasmonic properties but stronger fluorescence than free particles due to rich quantum properties. Au@ZIF-8 has shown good catalytic properties on the reduction of 4-nitrophenol, and 100% catalytic reaction was obtained within 4 min for Au:ZIF-8 (0.4:1 wt/wt) catalyst, indicating that the larger amount of smaller-sized gold cluster formed in ZIF-8 is the better catalyst. Thus, the present study demonstrates a promising green synthesis approach in a capsule of the metal–organic framework, which is a physical bottom-up processing without additional chemicals and procedures and produces the stabilized quantum clusters with efficient catalytic performance.