2, XiAnJiaoTong University, XiAn, , China
Two-dimensional (2D) nanomaterials have shown superior performance in catalysis, sensing, and many other surface-enhanced applications. Although a lot of 2D materials have been made, the formation mechanisms especially those of non-layered 2D structures in solution processes are still unclear. We study the formation of 2D cobalt oxide and cobalt nickel oxide nanosheets using liquid phase transmission electron microscopy (TEM). Our direct observation reveals the two-step growth: 1) 3D nanoparticles are formed from the molecular precursor solution; 2) transformation of 3D nanoparticle into 2D nanosheets. Ab initio calculations show that a small nanocrystal with dominant edge energy transforms into a 2D structure when the nanocrystal grows further and consequently surface energy becomes dominant. Revealing of such a 3D-to-2D growth pathway and the competition between the negative surface energy and positive edge energy provides opportunities to the controlled synthesis of novel materials in solution.