Two-dimensional (2D) materials have been recognized as one of the promising materials for various applications due to their unique characteristics. However, the formation and transformation mechanisms behind the 2D structure still largely remain unknown, which is significant for synthesis controlling and property tuning. In this scope, in situ microscopy characterization of two-dimensional (2D) materials under atmospheric reaction conditions offers a powerful tool for understanding their structural evolution at the atomic scale, which, however, has been seldom reported. Here, taking the 2D CoO as the model material which is the promising electro-/photoelectro-catalyst, we report real-time visualization of the structural transition of 2D Co(OH)2 nanosheets to CoO using in situ electron microscopy. Three intermediate phases, including one pseudo-Co(OH)2 phase, one transition phase, and one pseudo-CoO phase, are identified and characterized during the transition process. The detailed transition pathways and mechanisms are discussed based on the combined in situ STEM and FTIR data. The transition starts with the rapid dehydration process followed by two rearrangement periods and one relaxation process, respectively. The complete transition process is as follows: Co(OH)2 → (dehydration) → Co(OH)2,p → (rearrangement) → transition phase → (rearrangement) → CoOp → (relaxation) → CoO.
The in situ techniques demonstrated in this study can also be extended to many other material transition systems, which offer a novel opportunity to get insightful understanding of 2D materials and develop ideal materials with excellent performance.