The invention relates to a method for the carbon coating of metallic nanoparticles. The metallic nanoparticles, which are produced using the metal-salt hydrogen-reduction method, can be coated with carbon by adding a hydrocarbon (for example, ethylene, ethane, or acetylene) to the hydrogen using in the synthesis. The carbon layer protects the metallic particles from oxidation, which greatly facilitates the handling and further processing of the particles. By altering the concentration of the hydrocarbon, it is possible, in addition, to influence the size of the metallic particles created, because the coating takes place simultaneously with the creation of the particles, thus stopping the growth process. A carbon coating at most two graphene layers thick behaves like a semiconductor. As a thicker layer, the coating is a conductor. If the hydrocarbon concentration is further increased, a metal-CNT composite material is formed in the process. The composite materials developed are in themselves suitable as the raw materials of, for example, metallic inks and sensor materials.