Spectrophotometric Analysis and Optimization of 2D Gold Nanosheet Formation

Free-standing, 2D gold nanosheets (AuNS) offer broad potential applications from computing to biosensing and healthcare. Such applications, however, require improved control of material growth. We recently reported the synthesis of AuNS only ∼0.47 nm (two atoms) thick, which exhibited very high catalytic activity. The synthesis is a one-pot, seedless procedure in which chloroauric acid is reduced by sodium citrate in the presence of methyl orange (MO). In this study, we use spectrophotometric analysis and TEM imaging to probe AuNS formation and optimize the procedure. Previously, we suggested that MO acted as the confining agent, directing two-dimensional growth of the gold. Here, we provide the first reported analysis of the HAuCl₄ and MO reaction. We show that MO is rapidly oxidized to give 4-diazobenzenesulfonic acid, indicating that a complex interplay between HAuCl₄, MO, and other reaction products leads to AuNS formation. Time-resolved studies indicate that synthesis time can be significantly reduced from over 12 to 2-3 h. Decreasing the reaction temperature from 20 to 4 °C improved AuNS yield by 16-fold, and the catalytic activity of the optimized material matches that obtained previously. Our elucidation of AuNS formation mechanisms has opened avenues to further improve and tune the synthesis, enhancing the potential applications of ultrathin AuNS.