Ostwald ripening of hollow MOFs regulated by solvent proticity for enhanced microwave absorption performance

Hollow structures have received great attention in high-performance electromagnetic (EM) absorption due to their lightweight characteristics and easily tunable EM response. However, the controllable synthesis of hollow-structured materials remains a significant challenge. The root cause is that the morphological evolution mechanism during the synthesis process is not unambiguous, making effective regulation difficult to achieve. During the crystallization of MOFs, the proticity of the solvent system holds key potential for regulating the hollowing evolution mechanism, but this aspect has long been overlooked. Therefore, a solvent- proticity-regulated Ostwald ripening strategy is proposed: by adjusting the DMF/water/ethanol ratio, the hollowing evolution process of Ni-MOFs was regulated under template-free conditions, enabling the transformation of samples from small-volume solid spheres to large-volume hollow spheres. This strategy endowed the material with a moderately graphitized conductive carbon skeleton, abundant defects, and heterogeneous interfaces, thus significantly enhancing its dielectric loss performance. In addition, the investigation of Ostwald ripening (OR) time successfully revealed the effect of the time scale on the hollowing degree of the precursors and effectively improved the material's impedance matching. The resulting Ni@C-3 composite achieved an ultra-wide effective absorption bandwidth (EAB) of 6.6 GHz at a thickness of only 2.3 mm, with a minimum reflection loss (RL_min) reaching -62.6 dB. This work confirmed the feasibility of dominating OR kinetics via solvent proticity to precisely tune the EM response, providing a new strategy for the rational design of high-performance microwave absorption materials.