XPS study on ultrasonic nitridation of graphite using high-frequency (1.64 MHz) sound wave

Nitrogen-doped graphene (N-doped graphene) has attracted tremendous attention as a metal-free and low-cost catalyst for the oxygen reduction reaction; however, its synthesis involves energy intensive processes and complex chemical reactions. This study is the first attempt for nitriding graphite powder through one-pot ultrasonication to develop a process for the mass production of nitrogen-doped (N-doped) graphene. Graphite powder was irradiated with high-frequency (1.64 MHz) ultrasonic wave in 10 % ammonia solution. The volume concentration of fine graphite in the solution after the irradiation process increased approximately three times compared to the solution containing graphite powder without irradiation. The XPS analysis revealed that graphite powder was directly nitrided by ultrasonic irradiation in spite of the fact that the nitrogen‑carbon concentration N/C was less than 0.1. Pyridinic-N bonding configuration was formed. The N/C ratio increased with increasing the irradiation time from 15 to 90 min. Increasing the voltage amplitude of ultrasonic wave accelerated the nitridation. The oxidation of graphite proceeds simultaneously with the nitridation process.