Grain boundary re-crystallization and sub-nano regions leading to high plateau figure of merit for Bi2Te3 nanoflakes

Abstract

Nanoengineering is an effective strategy to strengthen phonon scattering, reduce lattice thermal conductivity and boost thermoelectric material performance. However, nanostructure features are generally in the size of ∼10 nm, and the fine control of nanostructure characteristics down to the sub-nano level (below several nanometers) remains as a key challenge. Here, we demonstrate that solvothermally synthesized Bi₂Te₃ can re-crystallize preferentially at the grain boundaries to form sub-nano boundary regions with width <2 nm via the optimization of sintering conditions. The optimized formation process of these sub-nano boundary regions can induce synergistic effects, including strengthened mid- to short-wavelength phonon scattering, weakened lattice/carrier scattering, carrier concentration optimization, weakened band degeneracy, as well as the optimized bipolar effect. Finally, a wide plateau figure of merit (zT) of >1.2 (from ∼323 to ∼423 K) and a high average zT of ∼1.18 (from 303 to 473 K) have been achieved in the Bi₂Te₃ pellet sintered at 593 K. This study not only reveals the formation mechanism of sub-nano boundary regions but also demonstrates that these sub-nano boundary regions and their formation process can effectively induce synergistic effects contributing to high thermoelectric performance, and guide the design of high-performance thermoelectric materials.