Abstract
Single-walled carbon nanotubes (SWCNTs) are promising thermoelectric materials for use as sustainable power sources for the Internet of Things technology due to their flexibility and excellent thermoelectric properties near 300 K. One of the most important challenges in the development of SWCNTs is achieving n-type thermoelectric properties with long air stability. Here, we investigated the correlation between the air stability of the n-type property and the defects of SWCNTs using two types of SWCNTs with different defect densities. SWCNT films with anionic surfactants were prepared using drop-casting, followed by heat treatment. Both types of SWCNT films exhibited approximately the same n-type Seebeck coefficient values at appropriate heat treatment temperatures. The SWCNT films with low defect density exhibited high electrical conductivity, but the n-type Seebeck coefficient was converted into a p-type one at 14 d. Conversely, the SWCNT films with high defect density exhibited low electrical conductivities but maintained the n-type Seebeck coefficient for 35 d. Therefore, the defect density of SWCNTs impacted the air-stability of the thermoelectric properties. This phenomenon possibly indicates that SWCNT films with high defect density were homogeneously coated with surfactants, thus preventing oxygen atoms from adhering to the film.
Highlights
Radial breathing mode (RBM) peaks were observed in the range 100 cm−1–300 cm−1, indicating the existence of Single-walled carbon nanotubes (SWCNTs) in the low frequency region of the Raman spectra for the two types of SWCNTs,[35,36] as shown in the inset of Fig. 1
The n-type Seebeck coefficient was achieved by preparing SWCNT films with an anionic surfactant (SDBS) using a drop-casting method followed by heat treatment
According to the SEM analyses, the surfactant was homogeneously coated with the type A SWCNT films, whereas the crystallized nanoparticles were scattered on the surface of the type B SWCNT films
Summary
Hata et al fabricated high-purity SWCNTs using an efficient CVD method, the super-growth (SG) method, where the activity and lifetime of the catalysts were enhanced by introducing small amounts of water vapor in the carrier gas.[31]. Saito et al fabricated high-quality SWCNTs by the enhanced direct injection pyrolysis synthesis (eDIPS) method, where two types of carbon sources with different decomposition properties are introduced into the CVD reactor.[32]. The structural properties of SWCNTs, including the diameter and defect density, depend on the type of CVD techniques. The n-type thermoelectric property of the SWCNT films was determined by the sign of the Seebeck coefficient, and the stability was evaluated by the change in the Seebeck coefficient with time. The chronological change in the electrical conductivity and the power factor of the SWCNT films were measured
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