Biomolecule-assisted hydrothermal synthesis and self-assembly of Bi ₂Te₃ nanostring-cluster hierarchical structure

A simple biomolecule-assisted hydrothermal approach has been developed for the fabrication of Bi₂Te₃ thermoelectric nanomaterials. The product has a nanostring-cluster hierarchical structure which is composed of ordered and aligned platelet-like crystals. The platelets are ∼ 100 nm in diameter and only ∼ 10 nm thick even though a high reaction temperature of 220 °C and a long reaction time of 24 h were applied to prepare the sample. The growth of the Bi₂Te₃ hierarchical structure appears to be a self-assembly process. Initially, Te nanorods are formed using alginic acid as both reductant and template. Subsequently, Bi₂Te₃ grows in a certain direction on the surface of the Te rods, resulting in the nanostring structure. The nanostrings further recombine side-by-side with each other to achieve the ordered nanostring clusters. The particle size and morphology can be controlled by adjusting the concentration of NaOH, which plays a crucial role on the formation mechanism of Bi₂Te₃. An even smaller polycrystalline Bi₂Te₃ superstructure composed of polycrystalline nanorods with some nanoplatelets attached to the nanorods is achieved at lower NaOH concentration. The room temperature thermoelectric properties have been evaluated with an average Seebeck coefficient of -172 μV K^-1, an electrical resistivity of 1.97 × 10^-3 Ωm, and a thermal conductivity of 0.29 W m ^-1 K^-1.