Cu nanoparticles were synthesized by a cavitation bubble plasma with a pair of Cu rod electrodes in deionized water. The cavitation bubble plasma was generated by a bipolar high voltage pulse with a voltage of 10 kV, a pulse width 1.0 μs and a repetition rate 200 kHz in deionized water in which many cavitation bubbles produced by a high-speed rotor were feeding. The shape and size of Cu nanoparticles were observed by TEM. It was found that the particle shape was spherical. The median diameter and span of the Cu nanoparticles increased from 4.0 to 9.4 nm and from 2.9 to 6.1 nm with increasing processing time from 1 to 20 min, respectively. The observed increase in the particle size of Cu nanoparticles should be ascribed to Ostwald ripening. These results suggested that the particle size of Cu nanoparticles could be controlled by the processing time.
