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.

A simple method to synthesize hexagonal sheets of lead, which belong to the class of twodimensional materials, is presented. These objects can be collected on a substrate located under two lead electrodes between which nanosecond-pulsed spark discharges in liquid nitrogen are ignited. The hexagonal sheets are single crystals produced by gas phase condensation. Once nitrogen is fully evaporated, they are oxidized in the air and turned into PbO2. The oxidation process induces stress that may pleat the uppermost sheets or open cracks in the centre of the structure. The thickness of individual objects varies typically from 4 to 20 nm. If the number of discharges exceeds about 2000, two types of PbO2 sticks start being observed: bundles made of nanosticks (5 μm in length and 50 nm of diameter) and isolated stick (20 μm in length and 500 nm in diameter) in addition to sheets. These new nanostructures are mainly due to the way lead electrodes are eroded by the discharge. At the beginning, the anisotropic erosion driven by the orientation of crystallographic planes of lead crystals produces octahedra and nanosticks, these latter growing longer and longer on electrode surfaces as discharges proceed. After about 2000 discharges, nanosticks are long enough to be easily broken, likely by mechanical stress, and fall onto the underlying substrate.