A plasma-generating device includes an anode plate and a cathode plate spaced apart from the anode plate. The cathode plate includes a substrate and a hybrid diamond layer formed on the substrate. The hybrid diamond layer includes ultra-nanocrystalline diamond grains, an amorphous carbon disposed among and bonded to the ultra-nanocrystalline diamond grains, micro-crystalline diamond grains disposed among the ultra-nanocrystalline diamond grains, and a graphite phase disposed among the ultra-nanocrystalline diamond grains. Each of the micro-crystalline diamond grains is surrounded by the graphite phase.