C. T. Lin, Tan Zhang
Hydrothermal reduction of Cu(II) ions is a convenient method to produce Cu nanocrystals, but the reason for the dominant use of CuCl 2 as the Cu source is unknown. This work demonstrates that the formation of oxides during the redox reaction is due to the attack of the OH – ions. Cl – ions, along with other halide ions, can form a stable complex with Cu ions and alkylamine ligands, which repels OH – ions effectively in an aqueous environment. The involvement of the counterions through hydrogen bonding leads to oxide formation. Simulations confirmed the stability of these complexes with different counterions. Cl – ions also act as catalysts to enhance the ligand-to-metal charge transfer and lower the energy required for Cu ion reduction. In the presence of additional Cl – ions, the temperature for Cu(II)–Cu(0) reduction was lowered, converting a hydrothermal reaction to a relatively safe one at ambient pressure. Upon Cu seed formation, Cl – ions are intrinsically present on the Cu facets to regulate the subsequent growth of the Cu seeds. Thinner Cu nanowires were produced in the presence of additional Cl – ions at lower temperatures. Cl – ions can be a cost-effective catalyst for the energy-efficient and safer synthesis of Cu nanocrystals.