Nitrogen-doped carbon nanosheets homogeneously embedded with Co nanoparticles via biostructure confinement as highly efficient microwave absorbers

2D/quasi-2D nanomaterial (e.g., graphene, MXene, carbon nanosheet) based composites are efficient absorbers of the ever-growing pollution of electromagnetic wave (EMW) due to their large aspect ratios, ample functional groups, and tunable electric and magnetic properties. However, the nanocomposites' cost-effective and straightforward synthesis process is still elusive. Thus, we demonstrated a simple swelling and carbonization process of exfoliated cellulose fibril to synthesize nitrogen-doped carbon nanosheets with homogeneously embedded Co nanoparticles (Co/NCN). Co/NCN with a carbonization temperature of 800 degrees C exhibited a minimum reflection loss (RLmin) of -65.7 dB at 13.9 GHz and an effective absorption bandwidth of 5.8 GHz with RL < -10 dB at a thickness of 1.7 mm. In addition, the microwave absorption characteristics of the composite can be further adjusted by the carbonization temperature. Those values are superior to many nanocomposites based on graphene and MXene. The doped nitrogen heteroatoms were essential for maintaining the morphology of carbon nanosheets, increasing the interfacial polarization loss, and improving the impedance matching. The environment-friendly and low-cost process proposed here can be extended to synthesizing many other metal/NCN nanocomposites as high-performance EMW absorbers.