Anisotropic polydopamine capsules with an ellipsoidal shape that can tolerate harsh conditions: efficient adsorbents for organic dyes and precursors for ellipsoidal hollow carbon particles

Abstract

Polymeric capsules often buckle, collapse or even break when being processed in the dried state into other materials under high temperature and pressure due to moderate mechanical rigidity. In the case of non-spherical capsules, to keep their precious anisotropic morphology intact under harsh conditions is even more challenging since the whole surface of such kinds of capsules does not experience the same stress or strain due to the different surface curvatures. In the current work, we reported a strategy to prepare polydopamine (PDA) capsules with an ellipsoidal shape and enhanced mechanical rigidity using polystyrene ellipsoids as the sacrificial anisotropic templates. Bio-inspired oxidation induced self-polymerization of dopamine can form conformal PDA coatings on polystyrene ellipsoids of various aspect ratios and sizes. Several strategies have been exploited to increase the thickness of the PDA shell, among which, iterating PDA coating produces ellipsoidal PDA capsules with a thick and robust shell. These ellipsoidal PDA capsules can survive carbonization at temperatures as high as 800 °C and were directly turned into N-doped carbon capsules with a well-defined ellipsoidal shape, excluding the necessity of removing the sacrificial templates after carbonation. Furthermore, the rigid PDA ellipsoidal capsules are efficient adsorbents for organic dyes in contaminated water and have impressive adsorption efficiencies as high as 200 mg g⁻¹.