Synthesis of metallic Cu nanoparticles by controlling Cu complexes in aqueous solution

• Cu NPs can be synthesized by using citric acid as a complexing agent.
• An easy synthesis method of Cu NPs at 25 °C in aqueous solution.
• The control of metal complex is found to be crucial for synthesis of Cu NPs.

We report an easy synthesis method of copper nanoparticles (NPs) through controlling Cu complexes by using citric acid in an aqueous solution under atmospheric condition at room temperature. By controlling the Cu complexes based on the calculation of pH dependent metal complex concentrations in the aqueous solution and analyzing the redox potential of Cu complexes electrochemically, the relationships between Cu complexes formed at each pH and the size of Cu NPs were elucidated. Although the Cu–citric acid complexes were expected to be the same in the pH range from 11 to 13, linear sweep voltammogram results strongly indicate that Cu–citric acid complexes at pH 13 are quite different from those at pH 11 and 12. ESI–TOFMS results represent that hydroxide ions bond to the Cu citrate acid complex at pH 13 and consequently (Cu2+)(C6H4O74-)(OH−)2 is formed. The Cu complex at pH 13 is easily reduced to Cu NPs compared with Cu–citric acid complex formed at pH 11 and 12. As a result, the size of Cu NPs at pH 13 is smaller than that of Cu NPs at pH 11 and 12. In summary, the control of metal complex is found to be crucial for synthesis of Cu NPs in the aqueous solution.

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