Two-dimensional metal oxide nanosheets: novel building blocks for advanced functional materials

Two-dimensional (2D) nanosheets, because of their ultrasmall thickness in atomic or molecular range and infinite lateral dimensions, have been widely considered as the thinnest functional materials [1]. The original work of this 2D materials can be dated back to the 1950s, when some clay minerals dispersed well in water, and a colloidal suspension was yielded due to the spontaneous exfoliation [2,3]. Shortly after that, with the appropriate selection of interlayer cations and solvents, a variety of inorganic layered compounds have been delaminated successfully, including metal chalcogenides [4-6], metal phosphates and phosphonates [7,8], metal oxides [1,9-17], and hydroxides [18-24]. Different from the spontaneous exfoliation of the clay minerals, these layered compounds need an extra modification of the chemical compositions or interlayer species displacement to weaken the interlayer charge density before the delamination. Particularly, in the past decades, some layered perovskite-type oxides, titanium oxide, and manganese oxides have been exfoliated successfully into 2D nanosheets with new chemical, physical, and/or optical properties [1,12-17]