Heating or Cooling: Temperature Effects on the Synthesis of Atomically Precise Gold Nanoclusters

Developing an efficient, well-controlled synthesis strategy for gold nanoclusters (Au NCs) is crucial for delivering their expected applications in many fields; and such development requires fundamental understandings on the synthetic chemistry. The synthesis of Au NCs typically consists of a pair of reversible reactions: a fast reduction-growth reaction and a slow size-focusing reaction. Here we demonstrate that the above two reactions can be well-balanced while accelerated in a heated synthesis protocol, thus providing an efficient and scalable synthesis method to obtain thermodynamically favorable Au₂₅(SR)₁₈ NCs (SR denotes thiolate ligand) with high yield (>95% on gold atom basis) and fast kinetics. By investigating the Au NC formation behavior at different temperature, we identified the endothermic nature of the reductive formation of Au₂₅(SR)₁₈ NCs from Au­(I)-thiolate complex precursors. More interestingly, if overheated, after the formation of Au₂₅(SR)₁₈, there exists an irreversible first-order reaction, which could transform Au₂₅(SR)₁₈ into Au NCs of mixed sizes. As a result, 40 °C is identified as the optimal temperature to synthesize Au₂₅(SR)₁₈ in aqueous solution, as the half-life of the transformation reaction (67.8 h) is much longer than the time needed to obtain high yield Au₂₅(SR)₁₈. The detailed understandings on the temperature effects of Au NC synthesis would facilitate the development of efficient synthesis strategies for atomically precise Au NCs with predesigned size, composition and structure.