Facile Hydrothermal Synthesis And Formation Mechanisms Of Bi₂Te₃, Sb₂Te₃ And Bi₂Te₃-Sb₂Te₃ Nanowires

Recently, there is the strong push in environment-friendly and recyclable products. Thermoelectric material as a green and non-pollution energy conversion material, has attracted more and more attention. Recently, theoretical and experimental studies show that thermoelectric materials with a low-dimensional structure like quantum wells, quantum wires, quantum dots have a high thermoelectric performance compared with bulk materials. Hydrothermal synthesis is a facile method to synthesize nanomaterials in recent years due to low experimental temperature, short reaction time, high purity product, small size, good crystallinity, and morphology controllable product and other advantages. In this paper,（Bi,Sb）₂Te₃ material, which has the best thermoelectric properties among the thermoelectric materials at room temperature, was systematically studied. Low deminsional Bi₂Te₃ and Sb₂Te₃ and Bi₂Te₃-Sb₂Te₃ thermoelectric materials were prepared under non-toxic and pollution-free glucose as a reducing agent by hydrothermal method in different experimental conditions. We have systematically studied the effects of process parameters on the phase, structure and morphology of the products. The different mechanisms were proposed for the formation of（Bi,Sb）₂Te₃ nanowires. Specific contents are as follows:（1） Using Bi Cl3, K2 Te O3 as raw materials, glucose as reductant, HCl as p H adjusting agent, Bi₂Te₃ single crystal nanowires with diameters of about 10-20 nm were prepared by hydrothermal synthesis. The sample composition, structure and morphology were characterized and analyzed by X-ray diffraction（XRD）, scanning electron microscopy（SEM） and high resolution transmission electron microscopy（HRTEM）.（2） The influences of process parameters like solvent, p H adjusting agent, reducing agent, reaction time, and surfactant on the phase composition and microstructure were studied. Generally, due to the layered crystal structure, the main growth direction of Bi₂Te₃ is normally perpendicular to c axis. So that, most of the synthesized Bi₂Te₃ nanomaterials come into sheet-like stucture such as nanoplates. In the present study, uniform Bi₂Te₃ nanowires are synthesized by a simple hydrothermal method. The formation and growth mechanismes of Bi₂Te₃ nanowires were studied. Initially, Te nanowires are formed. Subsequetly, Bi₂Te₃ nanowires are formed using intermediate Te nanowires as a soft template. The as-prepared Bi₂Te₃ nanowires have different wire axes that two different mechanisms have been proposed: structural preference growth mechanism and coherent growth mechanism.（3）Single phase of Sb₂Te₃ can not be obtained under acidic condition due to the slow kinetics of the reduction reaction even at elevated reaction temperatures. Based on the formation mechanism of Bi₂Te₃ nanowires, a two-step hydrothermal synthesis is proposed for the fabrication of Sb₂Te₃ and Bi₂Te₃-Sb₂Te₃ nanowires, using Bi Cl3, Sb Cl3, K2 Te O3 as raw materials and glucose as reductant. Te nanowires are first formed with HCl additive at 150 °C and Sb₂Te₃ and Bi₂Te₃-Sb₂Te₃ nanowires are then fabricated with Na OH additive at 220 °C. The influence of process parameters like reaction time, addition order of raw materials, temperature and p H on the phase composition and microstructure were studied.Besides the enhanced phonon scattering and quatum confinement effect, the structure and composition fluctuation in the Bi₂Te₃-Sb₂Te₃ nanowires may further adjust the energy band structure and improve the thermoelectric properties.