UC Davis

Two-dimensional (2D) nanomaterials are of relevance to many energy and technologically-related fields of research due to their remarkable physical and electronic properties. Solution colloidal synthesis offers the ability to precisely tune the structure, composition, and morphology of these 2D nanomaterials for desired applications, including thermoelectric energy conversion. Many 2D nanomaterials have proven to be high-performing thermoelectric materials, including the tetradymite structures, Sb2Te3 and Bi2Te3. During my Ph.D., I have carried out various research studies involving the synthesis, characterization, and thermoelectric property measurements of Sb2Te3 and Bi2Te3 nanoplate composites, Bi2Te3 nanoplates with a single nanopore, and Cu-intercalated Sb2Te3 nanoplates.Chapter 1 outlines the recent advances in colloidal polyol synthesis of 2D nanomaterials and provides perspectives on the similarities and differences in various syntheses. Various materials classes are presented and discussed, including metals, oxides, chalcogenides, and halides, that can be synthesized as 2D nanomaterials via a polyol process. This work has resulted in one perspective article that has been submitted to Dalton Transactions. Chapter 2 discusses the thermoelectric properties of Sb2Te3 and Bi2Te3 nanoplate composites that were synthesized in solution and consolidated via spark plasma sintering. The as-synthesized Sb2Te3 and Bi2Te3 vary drastically from one another in their lateral and vertical dimensions as revealed by scanning electron microscopy and atomic force microscopy. Thermoelectric properties in the parallel and perpendicular directions were measured, revealing strong anisotropy with a significant reduction to thermal conductivity in the perpendicular direction due to increased phonon scattering at nanoplate interfaces. The Seebeck coefficient is also increased dramatically in the nanocomposites, the highest reaching 210 μV/K for 15% Bi2Te3. The increase in Seebeck is attributed to energy carrier filtering at the nanoplate interfaces. Overall, these enhanced thermoelectric properties lead to a drastic increase in the thermoelectric performance in the perpendicular direction, with zT ~1.26, for the 15% Bi2Te3 nanoplate composite at 450 K. These groundbreaking results are demonstrated to be one of the highest for this system, where many similar materials have zT ~1. This work has resulted in one publication in ACS Applied Electronic Materials. Chapter 3 presents the synthesis, growth mechanism, and thermoelectric properties of 2D Bi2Te3 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, then the binary Bi2Te3 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 Bi2Te3 nanoplates. Thermoelectric properties 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, Bi2Te3 nanoplates with a single nanopore exhibit ultralow lattice thermal conductivity values, reaching values of ~ 0.21 Wm-1K-1. The lattice thermal conductivity was found to be systematically lowered with pore size, allowing for the realization of a thermoelectric figure of merit, zT ~ 0.75 at 425 K for the largest pore size. Although this is a relatively low figure of merit, this system demonstrates an enhancement through nanoscale voids on the thermoelectric performance, and can be further optimized via doping. This work has resulted in one article that has been accepted for publication in Chemistry of Materials. Chapter 4 discusses the synthesis and solution route intercalation of zerovalent copper into Sb2Te3 nanoplates. The copper intercalant is homogeneously distributed throughout the nanoplates, confirmed by scanning transmission electron microscopy coupled to energy dispersive x-ray spectroscopy. The copper composition was shown to be 2.6 at. % by x-ray photoelectron spectroscopy. There is a slight shift to lower binding energies for the Sb and Te 3d photoelectron peaks, which may indicate higher electron density within the Sb2Te3 nanoplates due to electron donation from copper. Powder x-ray diffraction shows good agreement with the Sb2Te3 bulk phase, with no structural transitions observed upon copper intercalation. Selected area electron diffraction displays the appearance of superlattice reflections in the Cu-intercalated Sb2Te3 nanoplates, which is indicative of copper ordering within the Van der Waals gaps of the nanoplates. Preliminary electronic transport measurements of single nanoplate devices show high conductance.