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

Abstract

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)₂CO₃ 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)₂CO₃ hierarchical microspheres. Initially, crystallized (BiO)₄CO₃(OH)₂ particles were formed during the nucleation and crystallization processes. Subsequently, the intermediate (BiO)₄CO₃(OH)₂ reacted with CO₃²⁻ to generate (BiO)₂CO₃ growth nuclei on the surface of the CO₂ bubbles which can act as heterogeneous nucleation centers. Next, the (BiO)₂CO₃ 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)₄CO₃(OH)₂ transformed to the (BiO)₂CO₃ phase, accompanied by the doping of N element into the lattice of (BiO)₂CO₃, and thereby, the well-defined N-doped (BiO)₂CO₃ 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)₂CO₃ 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.