Amorphous–crystalline transition-induced hollowing of covalent organic frameworks: a structural evolution boosting uranium adsorption

Hollow covalent organic frameworks (COFs) have broad application prospects due to their special structure. However, the controllable synthesis of uniform and stable hollow COFs remains a challenge. Here we propose a self-template method for preparing hollow COFs through the Ostwald ripening mechanism under environmental conditions. This method avoids most of the shortcomings of the commonly used hard template and soft template methods. Time-resolved structural analyses revealed that TD-COP undergoes a morphological evolution from an initial solid-flower architecture to a hollow-flower configuration, driven by Ostwald ripening-mediated progressive inward-outward material redistribution. The synthesized hierarchical hollow-flower H-COF demonstrates remarkable crystallinity, an exceptional specific surface area (534.99 m² g^-1), and robust structural stability. This structural hierarchy facilitates enhanced exposure of phosphate active sites through its radially porous architecture, thereby enabling the phosphorylated derivative (PA-H-COF) to exhibit superior uranium adsorption performance compared to conventional PA-COP. The optimized configuration achieves a maximum uranium uptake capacity of 296.7 mg g^-1, representing a 41.2% enhancement over the non-hollow counterpart. In the actual seawater test, the adsorption capacity reached 7.38 mg g^-1 within 20 days. This work proposes a feasible strategy for constructing a hollow phosphate functionalized COF for uranium removal.