Arc discharge sputtering of NiO-C electrodes: Structure and magnetic properties of synthesized materials

Plasma-chemical processes occurring during arc discharge sputtering of electrodes consisting of graphite and nickel oxide lead to the formation of a composite nanomaterial containing core–shell Ni@C nanoparticles. The structure of the formed material depends on the initial pressure in the reactor chamber. The smallest nickel nanoparticles with an average size of 3 nm are formed at a pressure of 12 Torr. The use of both lower 3 Torr and higher 100 Torr pressures results in the formation of larger nickel nanoparticles with average sizes of 5 and 9 nm, respectively. The proposed mechanism for the synthesis of Ni@C structures in the process of the arc discharge, including the stages of formation of a gas-plasma mixture, its outflow into the reactor chamber and cooling, processes of condensation, crystallization, and formation of nanoparticles, is considered. The structure of the material formed on a graphite substrate at various distances from the arc discharge axis was studied. It was established that larger nickel nanoparticles were formed at a closer distance, while the carbon component took the form of a graphene structure. With distance from the discharge, the size of the formed nickel nanoparticles decreased, and the graphene structure acquired a larger number of defects. Studies have showed that the synthesized materials, in terms of the mass content of nickel, had a specific magnetic susceptibility that exceeded the value for bulk metallic nickel. It was established that a decrease in the size of nickel nanoparticles led to an increase in the specific magnetic susceptibility of the material.