Second, we prepare the Co/C composites by laser irradiation. ZIF-67 powder is uniformly spread on a platform, which could move at two dimensions (X-Y) with a moving rate of 1 cm/s. By irradiating with a 1064 nm laser, the ZIF-67 could be pyrolyzed by absorbing the light energy and converting to thermal energy. The laser power is set at 15 mW, 25 mW, 35 mW, 45 mW, and 55 mW respectively, and Co/C composite materials are prepared in just a few seconds at air atmosphere.

Excessive electromagnetic radiation not only affects the normal operation of precision electronic equipment but also poses a potential threat to human health. Traditional microwave absorbers such as iron powder possess high magnetic permeability for effectively dissipating electromagnetic waves, but the high density restricts the applications. Although carbon materials have lower density and better conductivity, the electromagnetic wave attenuation performance needs further improvement for unfavored impedance matching. Aiming at developing the microwave absorber with light weight, high efficiency, and wide band, recently metal/carbon composites prepared by pyrolyzing metal-organic framework(MOFs) for microwave absorption have become research hotspots for their light weight and high efficiency. However, the pyrolysis process is usually performed in muffle or tube furnaces, and the slow heating rate usually leads to aggregated metal nanocrystals, thus resulting in weak interface polarization. Additionally, the preparation takes a long time (a few hours) and the pyrolysis atmosphere is usually inert gases such as argon, which makes it difficult to achieve rapid preparation. Therefore, we take laser beams as the thermal heat resource instead of traditional heat resources to achieve rapid preparation, and the micromorphology could be precisely controlled by the fast heating-quenching process, with metal nanocrystal size at nanoscale of 5-20 nm. Finally, the dielectric loss ability is enhanced with effective electromagnetic wave absorption.

First,we prepare the ZIF-67 precursor. 0.24 g Co (NO₃)₂·6H₂O is dissolved into 30 mL water as solution A. 1.64 g 2-methylimidazole is dissolved in 30 mL water as solution B. Solution B is dropped into solution A dropwide with the reaction for 24 h. By centrifugation for three times, the purple powder ZIF-67 is obtained.

Firstly, laser beam irradiation is introduced as a thermal heat resource to pyrolyze ZIF-67, and Co/C composites are successfully prepared in just a few seconds. The micromorphology of Co/C could be precisely controlled just by modulating the irradiation power, with Co nanocrystal size of about 5-20 nm by 45-55 mW laser irradiation, as shown in Fig. 1(e)-(f). The Co nanocrystal deposited on thin carbon film guarantees the improved polarization loss ability. XRD spectra and XPS in Fig. 2 further confirm the composites phase of Co and CoO deposited on carbon. The laser power could influence the micromorphology and phase of the Co/C composites, thus affecting the complex permittivity and permeability, as shown in Fig. 3. The dielectric loss ability is improved by the higher power laser irradiation, but exorbitant laser power would lead to carbon oxidation and lowered dielectric loss. With benefits from the fast heating-quenching process, the nano-sized Co deposited on thin carbon film is prepared with satisfied microwave absorption ability (Fig. 4). The reflection loss of Co/C composites prepared by 35 mW, 45 mW, and 55 mW laser irradiation could reach -23.5 dB, -53.9 dB, and -46.3 dB respectively, which indicates excellent microwave absorption ability. Specially, the working band of Co/C composites by 45 mW laser irradiation could cover the X band (8.8-13 GHz) just at the thickness of 2.6 mm to exhibit the light weight and wide absorption properties. The microwave absorption mechanisms by Co/C composites have been discussed in Figs. 5-6, which confirms that rich electromagnetic loss mechanisms (polarization loss, conductive loss, ferromagnetic resonance, and eddy current) synergized with good impedance matching induce favorable microwave absorption.

We introduce laser beams as the heat source to pyrolyze ZIF-67 and the nano-sized Co/C composites are successfully prepared in just a few seconds. The prepared Co/C shows excellent microwave absorbing properties with the effective absorption band covering X to Ku band. The improvement of electromagnetic wave loss ability is mainly attributed to the synergistic effect of dielectric loss (interface polarization, dipole polarization, and conductive loss) and magnetic loss (resonance loss and eddy current loss) by Co/C composites. Compared with the traditional thermal treatment process, the laser irradiation process exhibits low request for facility and environmental, making it feasible for industrial production and ushering in a new way for preparing the lightweight and wide-band microwave absorbers with MOFs as the precursor.