Facile fabrication of Co@C nanoparticles with different carbon-shell thicknesses: high-performance microwave absorber and efficient catalyst for the reduction of 4-nitrophenol

Core@shell-structured Co@C nanoparticles (NPs) were designed in this work. The shell thickness of carbon at 2.0, 3.2, 3.6, 4.0, and 4.7 nm was tuned by changing the weight ratio (Co : glucose = 2 : 1, 1 : 1, 1 : 2, 1 : 4, 1 : 10) of glucose in the reaction process. The influences of the carbon-shell thickness on the electromagnetic parameters and catalytic activity of the nanocrystalline Co were studied. Among the five samples, the Co@C-21 NPs (carbon-shell thickness of 2.0 nm) mixed with paraffin wax (PW) showed the best reflection loss (RL) of -42.5 dB at 16.1 GHz, while the Co@C-11-PW composite (carbon shell thickness of 3.2 nm) exhibited the best absorption bandwidth for RL < -10 dB of 9.6 GHz (8.4-18.0 GHz) with a matching thickness of 2.6 mm. The natural/exchange resonances, interfacial polarization, and uniformly dispersed Co NPs led to a high microwave absorption performance of the Co@C absorber with good impedance matching. Moreover, as a catalyst for the model removal reaction of 4-nitrophenol (4-NP), the Co@C-110 NPs with a shell thickness of 4.7 nm showed the best kinetic rate constant of 0.31 min(-1), 4.3 times higher than that of the Co@C-21 NPs. The Co@C NPs are a potential microwave absorber and effective catalyst for the reduction of 4-NP.