Two-Dimensional Metal Oxide and Metal Hydroxide Nanosheets: Synthesis, Controlled Assembly and Applications in Energy Conversion and Storage

The developments and state of the art in the research on two-dimensional nanosheets derived from layered metal oxides and layered metal hydroxides are reviewed in this paper, with emphasis on their promising applications in various new energy technologies, i.e., as supercapacitor electrodes, lithium ion battery electrodes, solar cell components, photocatalysts, and as functional elements in fuel cells and piezoelectric energy harvesters. Nanosheets are defined by their molecular thickness and large lateral dimensions, which typically differ by 2-5 orders of magnitude. They exhibit a wide range of interesting properties, e.g., flexibility, transparency, semiconductivity, photosensitivity, photocatalytic activity, redox properties, a large specific surface area and a high chemical stability. A large and growing library of metal oxide and hydroxide nanosheet compositions is available from the literature. Moreover, the easy control over nanosheet composition via modification of the layered parent compound provides many opportunities for introducing and optimizing their functional properties. This review focuses on the synthesis of oxide and hydroxide nanosheet colloids, their controlled assembly into nanocomposites, thin films, multilayers, superlattices and spheres, and the utilization of these structures in energy applications.