Order effect of nitrogen and phosphorus co-doping carbon nanofibers for enhancing electromagnetic wave absorption

In this work, N, P co-doped carbon nanofibers (CNF-NP and CNF-PN) were successfully synthesized by a two-step pyrolysis, and the effect of doping orders on microwave absorption (MA) was investigated for the first time. It was found that graphitic-N produced from the first N-doping suppressed the formation of C3PO due to the competition of doping sites for CNF-NP. Therefore, extra P atoms combined with oxygen-containing groups formed lots of C-OPO3 during the subsequent P-doping. However, for CNF-PN large amounts of C3PO were easily formed when P atoms were first doped, which limited the production of graphitic-N and resulted in the ratio improvement of pyrrolic-N and pyridinic-N. It was found that the ratios of different doping sites obviously influenced the MA performance. CNF-PN exhibited the minimum reflection loss (RLmin) value of −55.0 dB at 2.3 mm and effective bandwidth (EAB) of 6.6 GHz, attributing to the dielectric loss increased by an appropriate ratio of C-OPO3 and a high ratio of pyrrolic-N and pyridinic-N. In contrast, RLmin of CNF-NP was only −30.8 dB at 1.7 mm with the EAB of 3.3 GHz, due to the impedance mismatching from too much C-OPO3. This work sheds new light on adjusting MA properties by doping orders, which is of great significance for the development of novel absorbers based on carbon nanofibers.