Development of high-performance bismuth telluride-based thermoelectric nanomaterials through structure manipulating and band engineering

Abstract

Thermoelectrics enable the direct conversion between heat and electricity, offering an alternative opportunity to address the environmental problems and the upcoming energy crisis. The prominent advantages of thermoelectric energy conversion, for instance, without moving part, zero emission, and long working life endow thermoelectric devices with promising prospect of wide applications. In this regard, massive efforts have been dedicated to enhancing the conversion efficiency, evaluated by the dimensionless figure-of-merit (ZT), which is proportional to the power factor (S2s) and inversely proportional to the thermal conductivity (k). Bi2Te3 and the family of similar compounds potentially satisfying the criteria of large S2s, and low k are the dominant thermoelectric candidates for low temperature (200 K - 400 K) power-generation and refrigeration applications. Compared with bulk counterparts, nanostructuring provides extra possibilities to manipulate thermoelectric properties. Because of the quantum confinement effect, the band gap of nanomaterials can be enlarged by reducing the dimension, which secures a high S2s at relatively high temperature due to the suppressed bipolar conduction. In addition, phonon scatterings for nanomaterials are significantly strengthened; therefore, an ultra-low k is secured. For these reasons, we employed nanostructuring to enhance the thermoelectric performance of Bi2Te3 systems. Nevertheless, there are at least five issues impeding the substantive applications of the nanostructured Bi2Te3-based materials. (1) The conventional solvothermal method for synthesizing nanostructures is time-consuming, and the product yield is low. (2) Their ZT values deteriorate severely at temperature over 450 K, predominately due to the bipolar conduction. (3) For most of thermoelectric semiconductors, acoustic phonons dominate the charge carrier scattering, which yield the rapid decrease in carrier mobility. (4) It is necessary to clarify the underlying reason for the reversely coupling relationship between S and s for further enhancing S2s. (5) How to further reduce k on the basis that grain boundary phonon scattering has already been greatly strengthened through nanostructuring. In order to solve these issues, the research in this PhD project has been conducted in the following steps. i. We developed a rapid microwave assisted solvothermal method to fabricate Bi2Te3-based nanomaterials with high product yield through effective growth controlling. From which, high quality Bi2Se3 nanosheets, Te/Bi2Te3 hierarchical nanostructures, and BixSb2-xTe3 nanoplates, Bi2Te3-xSex nanoplates were fabricated. ii. Large-scale Bi2Se3 nanosheets with controllable thickness have been synthesized and their thermoelectric performance has been detailed investigated by experiments and fundamental nonparabolic Kane models. A significantly reduced thermal conductivity (only 0.41 W/mK), and enhanced power factor (4.71t10-4 W/mK2 with a Seebeck coefficient of -155.32 mV/K and an electrical conductivity of 1.96t104 S/m) are observed in the pellet composed of single-layered Bi2Se3 nanosheets. Such an enhanced thermoelectric performance is ascribed to the broadened band gap and optimized Fermi level in ultrathin Bi2Se3 nanosheets. iii. To further reduce the thermal conductivity, Te/Bi2Te3 hierarchical nanostructures assembled with well-aligned Bi2Te3 nanoplates are designed and fabricated by using Te nanotubes as templates. From the comparison of the thermoelectric performance and theoretical calculations with simple Bi2Te3 nanostructures, it has been found that Te/Bi2Te3 hierarchical nanostructures exhibits higher figure-of-merit due to the optimized reduced Fermi level and enhanced phonon scattering, as well as suppressed the bipolar conduction. iv. High quality ternary BixSb2-xTe3 nanoplates exhibited a peak ZT of 1.2, caused by the obtained high power-factor of 28.3t10-4 Wm-1K-2 and ultra-low thermal conductivity of 0.7 Wm-1K-1. Based on the single Kane band model with a newly introduced variable (lEdef m the dimensionless l representing the square root of ratio between the initial effective mass and the free electron mass, and Edef representing the deformation potential) to serve as the decoupling factor, BixSb2-xTe3 nanoplates with tunable compositions can decrease lEdef and simultaneously optimize the reduced Fermi level to ultimately enhance the power-factor. Moreover, detailed structural characterizations reveal dense grain boundaries and dislocations in our nanostructures. These two phonon scattering sources in conjunction with the inherently existed Bi-Sb lattice disorders lead to a strong wide-frequency phonon scattering, and consequently result in a significantly decreased thermal conductivity. v. High-quality n-type Bi2Te3-xSex nanoplates exhibited a high ZT of 1.23 at 480 K. By detailed electron microscopy investigations, coupled with theoretical analysis on phonon transports, we propose that the achieved k reduction is attributed to the strong wide-frequency phonon scattering. The shifting of peak S2s to high temperature is due to the weakened temperature dependent transport properties governed by the synergistic carrier scattering and the suppressed bipolar effects by enlarging the band gap. Overall, aiming at the issues of hindering the thermoelectric applications, we proposed some new concepts, which were realized in our massive experimental studies. To fundamentally understand the effects of the proposed concepts, we also employed simulation studies on electronic transport using Kane band model or parabolic band model, and on phonon transport using Callaway model with various phonon scattering mechanisms.