How Chloride Ions Catalyze the Hydrothermal Synthesis of Copper Nanocrystals

Hydrothermal reduction of Cu(II) ions is a convenient method to produce Cu nanocrystals, but the reason for the dominant use of CuCl₂ 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.