Template-free fabrication and growth mechanism of uniform (BiO)₂CO₃hierarchical hollow microspheres with outstanding photocatalytic activities under both UV and visible light irradiation

The morphology-controlled fabrication of nano-/microstructured functional materials has opened up new possibilities to enhance their physical and chemical properties and remains a great challenge. This work represents a one-pot template-free fabrication and growth mechanism of novel rose-like uniform (BiO)2CO3hierarchical hollow microspheres, which are self-assembled by single-crystal nanosheets. The observation of time-dependent evolution of crystal structure and morphology revealed that the growth mechanism of such a novel structure might involve a unique multistep pathway. First, an amorphous particle was formed during a nucleation and aggregation process. Then, the intermediate (BiO)4CO3(OH)2of embryonic stacked buds with attached particles were produced due to Ostwald ripening. The driving force for the formation of such embryonic structure is the intrinsic dipole field introduced by the nanosheets as a result of selective adsorption of the citrate ions on some polar surfaces of the nanoparticles. Subsequently, all the particles were consumed and (BiO)4CO3(OH)2crystals started to transform to (BiO)2CO3phase by means of repeated reaction-dissolution-recrystallization process in a homocentric layer-by-layer growth style, where carbonate ions substituted OH-groups. Monodisperse buds were then generated and the size of the hollow in the center becomes smaller to reduce surface energy. Finally, all (BiO)4CO3(OH)2transformed to (BiO)2CO3phase and uniform monodisperse (BiO)2CO3roses were produced through layers splitting driven by the OH-group deintercalating from the interlayer spaces of (BiO)4CO3(OH)2. More interestingly, the novel (BiO)2CO3microspheres exhibited outstanding activities under both UV and visible light irradiation for indoor NO removal, far exceeding that of commercial P25, synthetic C-doped TiO2and (BiO)2CO3with particle morphology due to the special hierarchical morphology and band gap structure.