(703a) Scalable Nanomanufacturing of Millimeter Length 2D Nanosheets of Thermoelectric Na0.7CoO2

A scalable nanomanufacturing technique is reported for batch fabrication of electrically-conducting 2D metal-oxide nanosheets of Na_0.7CoO₂.  We report a sol-gel based, high temperature bottom-up synthesis which is a cost-effective route capable of producing tens of thousands of nanosheet layers packed into a macro-scale pellet.  The synthetic procedure consists of sol-gel coordination of metal ions, auto-combustion, pressurized pellet formation, kinetic demixing, and calcination. The nanosheets are uniform in length and shape with highly anisotropic dimensions of nanometer sheet thickness and millimeter lateral lengths (10^-5:1:1), and are readily delaminated into free-standing nanosheets. According to GIXRD, the nanosheet stacking has turbostratic arrangement with nanosheets misaligned rotationally with respect to stacking axis. Na_0.7CoO₂, in bulk form, is already a surprisingly good thermoelectric material with a thermoelectric (TE) power factor as high as the industry standard Bi₂Te₃ (50 vs. 40 µW/K²cm respectively). Also, it has been reported in the literature that 2D nanosheets are observed to reduce thermal conductivity of materials with a factor ~10² due to phonon confinement, which provides a significant increase in the thermoelectric efficiency. Therefore, in conjunction with low fabrication cost, the reported millimeter length Na_0.7CoO₂ nanosheets are expected to provide high thermoelectric efficiency with practical importance. Preliminary thermoelectric measurements have been obtained for turbostratically aranged millimeter-length nanosheet stackings.