Minute-Made, High-Efficiency Nanostructured Bi2Te3 via High-Throughput Green Solution Chemical Synthesis

Abstract

Scalable synthetic strategies for high-quality and reproducible thermoelectric (TE) materials is an essential step for advancing the TE technology. We present here very rapid and effective methods for the synthesis of nanostructured bismuth telluride materials with promising TE performance. The methodology is based on an effective volume heating using microwaves, leading to highly crystalline nanostructured powders, in a reaction duration of two minutes. As the solvents, we demonstrate that water with a high dielectric constant is as good a solvent as ethylene glycol (EG) for the synthetic process, providing a greener reaction media. Crystal structure, crystallinity, morphology, microstructure and surface chemistry of these materials were evaluated using XRD, SEM/TEM, XPS and zeta potential characterization techniques. Nanostructured particles with hexagonal platelet morphology were observed in both systems. Surfaces show various degrees of oxidation, and signatures of the precursors used. Thermoelectric transport properties were evaluated using electrical conductivity, Seebeck coefficient and thermal conductivity measurements to estimate the TE figure-of-merit, ZT. Low thermal conductivity values were obtained, mainly due to the increased density of boundaries via materials nanostructuring. The estimated ZT values of 0.8-0.9 was reached in the 300-375 K temperature range for the hydrothermally synthesized sample, while 0.9-1 was reached in the 425-525 K temperature range for the polyol (EG) sample. Considering the energy and time efficiency of the synthetic processes developed in this work, these are rather promising ZT values paving the way for a wider impact of these strategic materials with a minimum environmental impact.

Keywords: ZT; bismuth telluride; colloidal synthesis; green chemistry; nanocharacterization; nanochemistry; nanoparticles; thermal conductivity; thermoelectric; thermoelectric figure-of-merit.

Figure 1

Schematic of the MW-assisted hydrothermal, and polyol synthesis process.

Figure 2

X-ray powder diffraction patterns (XRPD) (normalized for the intensity of the most intense 015 peak) of as-synthesized and spark plasma sintered Bi₂Te₃ samples synthesized through MW-assisted heating for (a) hydro-Bi₂Te₃, and (b) polyol-Bi₂Te₃ sample. The major crystalline phases are indexed to Bi₂Te₃ (ICDD: 01-089-2009) with rhombohedral crystal structure (a = b = 4.3860 Å, c = 30.4970 Å; α = β = 90°, γ = 120°).

Figure 3

X-ray photoelectron spectroscopy (XPS) survey scan (a), and C 1s (b₁,c₁), Bi 4f (b₂,c₂) and Te 3d (b₃,c₃) spectra for Bi₂Te₃ samples synthesized through MW-assisted hydrothermal (b), and polyol (c) routes.

Figure 4

ξ-potential analysis of as-made Bi₂Te₃ samples synthesized through MW-assisted hydrothermal, and polyol routes.

Figure 5

Scanning electron microscopy (SEM) micrographs of as-made Bi₂Te₃ samples at different magnifications; (a,b) hydro-Bi₂Te₃ and (e,f) polyol-Bi₂Te₃. (c,g) Transmission electron microscopy (TEM) micrographs and (d,h) selected-area electron diffraction (SAED) patterns of hydro-Bi₂Te₃ and polyol-Bi₂Te₃ samples, respectively.

Figure 6

SEM micrographs of SPS sintered Bi₂Te₃ pellets at different magnifications for materials synthesized through MW-assisted hydrothermal (a,b) and polyol (c,d) routes.

Figure 7

Temperature-dependent electronic transport properties of SPS sintered Bi₂Te₃ pellets for Bi₂Te₃ materials synthesized through MW-assisted hydrothermal and polyol routes; (a) electronic conductivity, (b) Seebeck coefficient, and (c) power factor.

Figure 8

Total thermal conductivity (a), along with the electronic and thermal contributions to the thermal conductivity (b) for SPS compacted pellets for hydrothermal and polyol synthesized Bi₂Te₃.

Figure 9

Temperature dependent thermoelectric (TE) figure of merit (ZT), of SPS compacted pellets for Bi₂Te₃ materials synthesized through MW-assisted hydrothermal and polyol routes.