Solvothermal synthesis of n-type Bi2(SexTe1-x)3 nanoplates for high-performance thermoelectric thin films on flexible substrates

High thermoelectric performance of flexible nanocomposite films based on Bi2Te3 nanoplates and carbon nanotubes selected using ultracentrifugation

Thermoelectric generators with flexibility and high performance near 300 K have the potential to be employed in self-supporting power supplies for Internet of Things (IoT) devices. Bismuth telluride (Bi₂Te₃) exhibits high thermoelectric performance, and single-walled carbon nanotubes (SWCNTs) show excellent flexibility. Therefore, composites of Bi₂Te₃ and SWCNTs should exhibit an optimal structure and high performance. In this study, flexible nanocomposite films based on Bi₂Te₃ nanoplates and SWCNTs were prepared by drop casting on a flexible sheet, followed by thermal annealing. Bi₂Te₃ nanoplates were synthesized using the solvothermal method, and SWCNTs were synthesized using the super-growth method. To improve the thermoelectric properties of the SWCNTs, ultracentrifugation with a surfactant was performed to selectively obtain suitable SWCNTs. This process selects thin and long SWCNTs but does not consider the crystallinity, chirality distribution, and diameters. A film consisting of Bi₂Te₃ nanoplates and the thin and long SWCNTs exhibited high electrical conductivity, which was six times higher than that of a film with SWCNTs obtained without ultracentrifugation; this is because the SWCNTs uniformly connected the surrounding nanoplates. The power factor was 6.3 μW/(cm K²), revealing that this is one of the best-performing flexible nanocomposite films. The findings of this study can support the application of flexible nanocomposite films in thermoelectric generators to provide self-supporting power supplies for IoT devices.

Energy-Saving Pathways for Thermoelectric Nanomaterial Synthesis: Hydrothermal/Solvothermal, Microwave-Assisted, Solution-Based, and Powder Processing

The pillars of Green Chemistry necessitate the development of new chemical methodologies and processes that can benefit chemical synthesis in terms of energy efficiency, conservation of resources, product selectivity, operational simplicity and, crucially, health, safety, and environmental impact. Implementation of green principles whenever possible can spur the growth of benign scientific technologies by considering environmental, economical, and societal sustainability in parallel. These principles seem especially important in the context of the manufacture of materials for sustainable energy and environmental applications. In this review, the production of energy conversion materials is taken as an exemplar, by examining the recent growth in the energy-efficient synthesis of thermoelectric nanomaterials for use in devices for thermal energy harvesting. Specifically, "soft chemistry" techniques such as solution-based, solvothermal, microwave-assisted, and mechanochemical (ball-milling) methods as viable and sustainable alternatives to processes performed at high temperature and/or pressure are focused. How some of these new approaches are also considered to thermoelectric materials fabrication can influence the properties and performance of the nanomaterials so-produced and the prospects of developing such techniques further.

Flexible thermoelectric films formed using integrated nanocomposites with single-wall carbon nanotubes and Bi2Te3 nanoplates via solvothermal synthesis

Single-wall carbon nanotubes (SWCNTs) and Bi₂Te₃ nanoplates are very promising thermoelectric materials for energy harvesting. When these two materials are combined, the resulting nanocomposites exhibit high thermoelectric performance and excellent flexibility. However, simple mixing of these materials is not effective in realizing high performance. Therefore, we fabricated integrated nanocomposites by adding SWCNTs during solvothermal synthesis for the crystallization of Bi₂Te₃ nanoplates and prepared flexible integrated nanocomposite films by drop-casting. The integrated nanocomposite films exhibited high electrical conductivity and an n-type Seebeck coefficient owing to the low contact resistance between the nanoplates and SWCNTs. The maximum power factor was 1.38 μW/(cm K²), which was 23 times higher than that of a simple nanocomposite film formed by mixing SWCNTs during drop-casting, but excluding solvothermal synthesis. Moreover, the integrated nanocomposite films maintained their thermoelectric properties through 500 bending cycles.