Growth mechanism and photocatalytic activity of self-organized N-doped (BiO)2CO3 hierarchical nanosheet microspheres from bismuth citrate and urea

Synthesis of nano-/microstructured functional materials with 3D hierarchical microspheres structure has provided new opportunities for optimizing their physical and chemical properties. This work revealed a new growth mechanism of self-organized N-doped (BiO)(2)CO3 hierarchical microspheres which were fabricated by hydrothermal treatment of bismuth citrate and urea without an additive. Based on time-dependent observation, several evolution processes were believed to account for the formation of the self-organized N-doped (BiO)(2)CO3 hierarchical microspheres. Initially, crystallized (BiO) 4CO3(OH)(2) particles were formed during the nucleation and crystallization processes. Subsequently, the intermediate (BiO)(4)CO3(OH)(2) reacted with CO32- to generate (BiO)(2)CO3 growth nuclei on the surface of the CO2 bubbles which can act as heterogeneous nucleation centers. Next, the (BiO)(2)CO3 growth nuclei aggregated together after the consumption of CO2 bubbles with the increased concentration of OH-and further grew to be nanosheets. The microspheres constructed by small nanosheets further grew with the consumption of small particles. Finally, all (BiO)(4)CO3(OH)(2) transformed to the (BiO)(2)CO3 phase, accompanied by the doping of N element into the lattice of (BiO)(2)CO3, and thereby, the well-defined N-doped (BiO)(2)CO3 hierarchical microspheres were shaped. Depending on the distance between neighboring CO2 bubbles, the resulting microspheres can be linked or dispersed. Besides, the gradual release of CO2 bubbles and CO32- played a crucial role in controlling the nucleation and growth process, resulting in different sizes of microspheres. The fabricated N-doped (BiO)(2)CO3 hierarchical microspheres displayed admirably efficient and durable photocatalytic activity under both UV and visible light towards removal of NO, which is mainly attributed to the introduction of N element and the special hierarchical structure. This work provides new insights into the controlled synthesis of photocatalytic nano/microstructures for potential environmental and energetic applications.