Synthesis of Cu@ZnO core–shell nanoparticles by spark discharges in liquid nitrogen

Abstract

Core–shell nanoparticles are synthesized by nanosecond-pulsed discharges in liquid nitrogen using a two-step process. In a first step, copper nanoparticles (resp. zinc nanosheets) are synthesized by eroding pure copper (resp. zinc) electrodes. In a second step, copper (resp. zinc) electrodes are removed and replaced by zinc (resp. copper) electrodes in the liquid loaded with copper nanoparticles (resp. zinc nanosheets). After erosion and air oxidation, once nitrogen has evaporated, Cu@ZnO core–shell nanoparticles are obtained in both configurations. The shell is always ZnO, because of the unusual formation of zinc nanosheets instead of zinc nanoparticles. When Cu electrodes are used first, copper nanoparticles are wrapped in ZnO nanosheets. When Zn electrodes are used first, copper nanoparticles hit zinc nanosheets and get coated to form also Cu@ZnO. In this latter case, Cu2O@ZnO are sometimes encountered too but to a much lesser extent. Copper oxidation is then attributed to a failure in the zinc protective shell. Time-resolved optical emission spectroscopy measurements reveal that Cu lines and Zn lines are never emitted simultaneously, the element in the liquid remaining outside the discharge area. If zinc nanosheets are synthesized first, N II lines are exclusively observed during the first 200 ns of discharges with copper electrodes, revealing for this process the possibility of highly energetic processes.