Into the Void: Single Nanopore in Colloidally Synthesized BiTe Nanoplates with Ultralow Lattice Thermal Conductivity.

BiTe is a well-known thermoelectric material that was first investigated in the 1960s, optimized over decades, and is now one of the highest performing room-temperature thermoelectric materials to-date. Herein, we report on the colloidal synthesis, growth mechanism, and thermoelectric properties of BiTe nanoplates with a single nanopore in the center. Analysis of the reaction products during the colloidal synthesis reveals that the reaction progresses via a two-step nucleation and epitaxial growth: first of elemental Te nanorods and then the binary BiTe nanoplate growth. The rates of epitaxial growth can be controlled during the reaction, thus allowing the formation of a single nanopore in the center of the BiTe nanoplates. The size of the nanopore can be controlled by changing the pH of the reaction solution, where larger pores with diameter of ∼50 nm are formed at higher pH and smaller pores with diameter of ∼16 nm are formed at lower pH. We propose that the formation of the single nanopore is mediated by the Kirkendall effect and thus the reaction conditions allow for the selective control over pore size. Nanoplates have well-defined hexagonal facets as seen in the scanning and transmission electron microscopy images. The single nanopores have a thin amorphous layer at the edge, revealed by transmission electron microscopy. Thermoelectric properties of the pristine and single-nanopore BiTe nanoplates were measured in the parallel and perpendicular directions. These properties reveal strong anisotropy with a significant reduction to thermal conductivity and increased electrical resistivity in the perpendicular direction due to the higher number of nanoplate and nanopore interfaces. Furthermore, BiTe nanoplates with a single nanopore exhibit ultralow lattice thermal conductivity values, reaching ∼0.21 WmK in the perpendicular direction. The lattice thermal conductivity was found to be systematically lowered with pore size, allowing for the realization of a thermoelectric figure of merit, zT of 0.75 at 425 K for the largest pore size.