Formation of two-dimensional transition metal oxide nanosheets with nanoparticles as intermediates

Juan Yang, Zhiyuan Zeng, Jun Kang, Sophia Betzler, Cory Czarnik, Xiaowei Zhang, Colin Ophus, Chang Yu, Karen Bustillo, Ming Pan, Jieshan Qiu, Lin-Wang Wang & Haimei Zheng
Nature Materials (2019) | Download Citation

[Abstract]
Two-dimensional (2D) materials have attracted significant interest because of their large surface-to-volume ratios and electron confinement. Compared to common 2D materials such as graphene or metal hydroxides, with their intrinsic layered atomic structures, the formation mechanisms of 2D metal oxides with a rocksalt structure are not well understood. Here, we report the formation process for 2D cobalt oxide and cobalt nickel oxide nanosheets, after analysis by in situ liquid-phase transmission electron microscopy. Our observations reveal that three-dimensional (3D) nanoparticles are initially formed from the molecular precursor solution and then transform into 2D nanosheets. Ab initio calculations show that a small nanocrystal is dominated by positive edge energy, but when it grows to a certain size, the negative surface energy becomes dominant, driving the transformation of the 3D nanocrystal into a 2D structure. Uncovering these growth pathways, including the 3D-to-2D transition, provides opportunities for future material design and synthesis in solution.

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