Synthesis and characterization of metal chalcogenide nanostructured materials for thermoelectric applications (2011)

The synthesis and characterization of metal chalcogenide nanostructured materials with the potential to exhibit enhanced thermoelectric (TE) properties is reported. The tellurides, specifically PbTe and Bi⊂2⊂Te⊂3⊂ (Bi⊂2⊂Te⊂3⊂–Sb⊂2⊂Te⊂3⊂ alloys) were selected as these are well established TE materials in bulk form. The approaches used for nanostructuring are (1) sol-gel assembly of discrete nanoparticles of PbTe, Bi⊂2⊂Te⊂3⊂, and Bi⊂2⊂Te⊂3⊂–⊂2⊂Te⊂3⊂ alloys, and (2) formation of heterostructured nanocomposites of PbTe nanoparticles in bulk Bi⊂2⊂Te⊂3⊂ (p– or n–doped) matrices. Aerogels and xerogels of PbTe nanoparticles were synthesized by sol–gel assembly of discrete nanoparticles, leading to interconnected networks of nanoparticles and pores. The PbTe aerogel exhibited a high surface area and was more thermally stable to sublimation than the precursor nanoparticles, as indicated by in–situ heating experiments in transmission electron microscopy (conducted under vacuum). In contrast, both the nanoparticles and aerogels of PbTe melted at the same temperature as the bulk material, as indicated by differential scanning calorimetry experiments. The oxidative sol-gel assembly, when employed to thiolate capped Bi⊂2⊂Te⊂3⊂and Bi⊂2⊂Te⊂3⊂–Sb⊂2⊂Te⊂3⊂ alloy nanoparticles, led to aerogels with similar microscale morphological features to the PbTe aerogel network, but Bi⊂2⊂Te⊂3⊂ aerogels were more robust than the PbTe aerogels, yielding self-supporting monoliths. TE property measurement on hot-pressed pellets of Bi⊂2⊂Te⊂3⊂ and Bi⊂x⊂Sb⊂2–x⊂Te⊂3⊂ aerogels showed degraded electronic properties (Seebeck coefficient (S), electrical conductivity (Σ) and power factor (S⊃2⊃Σ)) as compared to the nanoparticles, suggesting a doping effect in these materials. The nanoparticles themselves showed degraded electronic properties when compared to the bulk counterparts. In contrast, the lattice thermal conductivities (κ⊂l⊂) of the nanoparticles and aerogels of Bi⊂2⊂Te⊂3⊂ had improved as compared to the bulk counterparts showing that nanostructuring was effective in scattering heat carrying phonons, thereby reducing the κ⊂l⊂. In contrast, in Bi⊂x⊂Sb⊂2–⊂Te⊂3⊂ , there is little change in κ⊂l⊂, presumably because it is already so low due to the disorder on the cationic lattice. Overall, the degraded electronic properties made the efficiency of these materials (ZT) lower than the bulk counterparts. Heterostructured nanocomposites of PbTe nanoparticles (different weight %) inside n– or p–doped bulk Bi⊂2⊂Te⊂3⊂ matrices were prepared using an incipient wetness impregnation approach. However, the thermoelectric properties of these nanocomposites again indicated a significant doping effect associated with incorporation of PbTe nanoparticles inside the bulk Bi⊂2⊂Te⊂3⊂ matrices. This doping effect causes an increase in hole concentration in p–type nanocomposites, leading to a decrease in power factor, S⊃2⊃Σ as compared to the bulk p–type matrix, whereas the same doping effect in n–type nanocomposites resulted in compensation of the majority charge carriers (electrons), again resulting in a decrease in power factor, S⊃2⊃Σ. However, at least for p–type nanocomposites,the lattice thermal conductivities were found to be reduced relative to bulk counterparts, suggesting that incorporation of PbTe nanoparticles does scatter the heat carrying phonons effectively, reducing the κ⊂l⊂ of the system. The overall lower ZT in these nanocomposites as compared to the bulk counterparts highlights the need for proper optimization of carrier concentration in nanostructured thermoelectrics.