The beauty of binary phases: A facile strategy for synthesis, processing, functionalization, and application of ultrasmall metal nanoclusters

[Display omitted] •An overview of the two-phase strategies for ultrasmall metal nanoclusters.•The two-phase synthesis, processing, functionalization, and application of metal nanoclusters.•The challenges and prospects related to the two-phase strategies for metal nanoclusters. Ultrasmall metal nanoclusters (NCs) are a sub-class of metal nanoparticles (NPs) with a metal core size of ≤3 nm, and display interesting molecule-like properties. Atomically precise metal NCs can be synthesized by many solution chemistry methods in polar and non-polar reaction environments. Among these methods, two-phase based strategies involving a water-oil binary reaction system have emerged as a facile means to synthesize, process, functionalize, and apply metal NCs. This Review firstly presents an overview of the two-phase based strategies in the synthesis of hydrophobic and hydrophilic metal NCs, and aims to elucidate the underlying working principles of these strategies. Then, how a two-phase system could process metal NCs for size/composition tailoring, luminescence generation, sorting and purification is discussed. Subsequently, several examples that utilizing a two-phase system to efficiently functionalize metal NCs with different surface chemistry are highlighted. Finally, two-phase based applications of metal NCs in catalysis and self-assembly are also reviewed. The fundamentals and the advances of the two-phase based strategies in the synthesis, processing, functionalization, and application of metal NCs summarized in this Review may increase the acceptance of two-phase based strategy in the metal NC field.