Plasmonic Catalysis of C–C Coupling Reaction in Water Probed via In Situ SERS: Bimetallic Ag–Cu as a Better Catalyst over Ag–Au and Pure Ag

Abstract

Plasmonic materials have emerged as very promising in investigating a wider range of catalytic reactions. In this work, we fabricated monometallic pure Ag substrates and bimetallic substrates by alloying Ag with Cu/Au and subsequently compared their catalytic efficiency using liquid-state in situ surface-enhanced Raman scattering (SERS). The liquid-state measurement annihilated any plasmon-induced thermal effect and thus provided insights into promoting plasmonic catalysis using this approach. Substrate fabrication was carried out using simple thermolysis of metal alkyl ammonium halide precursors (MToABr, where M = Ag, Au, and Cu) on glass coverslips and was thoroughly characterized. Two different laser excitation sources of 532 and 632.8 nm were used to inspect the C–C coupling reaction of the reactant 4-bromo-thiophenol (BTP) in water, and the rates of the reactions were monitored in kinetic mode at definite time intervals. Formation and time-dependent gradual increase of the peak at 1587 cm^–1 of the desired product 4,4′-biphenyldithiol (BPDT) and gradual decrease of the peak at 1560 cm^–1 of BTP indicated the reaction degree of the C–C coupling reaction. We also investigated the role of the hot carriers on our plasmonic substrates by selectively quenching the hot electrons or hot holes, using suitable scavenger solutions, and thereby proposed a suitable mechanism for the C–C coupling reaction. The bimetallic Ag–Cu substrate demonstrated almost a 5 times faster rate of catalysis for the C–C coupling reaction of 4-BTP than the bimetallic Ag–Au and pure Ag substrates when performed with 532 nm excitation.