Thin Oxides, Thick Prospects
By re-conceiving synthesis strategies, researchers at the Energy Materials Center at Cornell, shorthanded as EMC2, have demonstrated a novel and easily scalable synthesis route that produces very long sodium cobalt oxide (NaxCoO2) nanosheets. This class of flat or two-dimensional ceramic nanomaterials holds great potential for devices capable of turning waste heat into electricity, improved batteries or more efficient solar cells.
Thermoelectric Devices: Waste Heat into Electricity: The thermoelectric effect is a characteristic of certain metal oxides, which generate electricity when placed between two media at different temperatures. These materials can recover and convert waste heat from power plants and vehicles into electricity, thereby reducing the consumption of fossil fuels. Mixed metal oxides such as NaxCoO2 are particularly promising as components for these thermoelectric devices thanks not only to their rare ability to be thermally insulating but also to their ability to conduct electricity. Fine control of the shape and size of the oxide crystals, for instance by synthesizing flat nanosheets, is essential to bring about unique thermoelectric functionalities.
Metal Oxide Nanosheets: Pushing the Envelope: The classical approach to obtain nanosheets of oxide materials involves chemical exfoliation from massive crystals. However, this aggressive chemical “peeling” generally yields small quantities of freestanding oxide sheets, with limited lateral dimensions (width and length) that can lose their crystalline structure and functionality. The researchers at EMC2 are taking a step forward to tackle the limitations of the classical top-down exfoliation approaches by developing a novel strategy that allows the synthesis of NaxCoO2 nanosheets with unprecedented dimensions and uniformity. This new route allows them to prepare, at once, up to 40,000 sheets with lateral dimensions as large as 2 millimeters and thicknesses of only about 20 nanometers (5,000 times thinner than a human hair!). Remarkably, the oxide nanosheets can be gently pulled apart by a simple exfoliation treatment in water.
"Our aim is to synthesize two-dimensional oxide nanostructures to improve their efficiency in converting heat to electricity, but classical chemical exfoliation approaches do not work for the NaxCoO2 material that we are studying, which is particularly difficult to synthesize as nanosheets," remarks Richard Robinson, assistant professor at Cornell University and leading researcher of the study. The new synthesis procedure involves a kinetic-demixing process in an electric field, followed by high-temperature annealing and exfoliation in water under ultrasound.
"We saw a rapid structural change as the electron beam hit our materials in the electron microscope, and this led us to the idea of deliberately exposing them to an electrical field. It turns out this is the critical step for growing the nanosheets," explains Robinson.
The novel synthesis provides a precise, yet easily scalable, route to free-standing oxide nanosheets with millimeter lateral dimensions, which had remained elusive in the absence of a solid support. Aware that such flat nano-oxides are highly desirable in several applications other than thermoelectrics, Robinson points out the broader potential of their discovery. "The free-standing NaxCoO2 nanosheets are flexible, optically transparent and electrically conductive; therefore we expect them to have potential applications in solar energy harvesting and as conductive supports for electrocatalytic reactions that occur in fuel cells or batteries."
This research on conductive metal oxide nanosheets was fully supported as part of the Energy Materials Center at Cornell (EMC2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Graduate student Mahmut Aksit was partially funded by DOE, Office of Science, Office of Basic Energy Sciences and partially funded by the National Science Foundation.
Aksit M, DP Toledo, and RD Robinson. 2012. “Scalable Nanomanufacturing of Millimeter Length 2D NaxCoO2 Nanosheets.” Journal of Materials Chemistry 22(13):5936-5944. DOI:10.1039/C2JM15550H