Facile Gram‐Scale Synthesis of the First n‐Type CuFeS₂ Nanocrystals for Thermoelectric Applications

Abstract

The described method enables facile gram‐scale preparation of CuFeS2 nanocrystals exhibiting interesting thermoelectric properties from simple and readily available precursors. Exchange of primary organic ligands for inorganic ones using either (NH4)2S or triethyloxonium tetrafuoroborate (Meerwein's salt) resulted in nanocrystals from which n‐type bulk thermoelectric materials were obtained through sintering under pressure. The measured physical properties of the fabricated bulk thermoelectric materials depend on the type of inorganic ligands used for the exchange. In particular, materials that were surface‐modified with Meerwein's salt have a higher Seebeck coefficient (|S| = 238 µV K–1) as compared to those modified with (NH4)2S, whereas the latter exhibit higher electrical conductivity (8500 S m–1) and lower thermal conductivity (0.5 W m–1 K–1), both of which are favorable for thermoelectric applications.

Doping and Surface Effects of CuFeS2 Nanocrystals Used in Thermoelectric Nanocomposites

“…[24b-e] To the contrast, in colloidal chalcopyrite NC nanocomposites a decrease of the charge carrier concentration was reported when increasing the C/A charge ratio. [13] We observe a similar behavior here with a large increase of S and a one order of magnitude drop of s for the sulfur deficient sample. Thus, additional effects related to the surface or to the core/shell structure (vide supra) have to be taken into account in the case of pellets processed from colloidal NCs.…”

“…In the literature, an increase of charge carrier concentration was obtained by increasing the cation/anion (C/A) charge ratio in the alloyed‐SPS chalcopyrite nanocomposites, either via introducing S deficiency,, or by means of Fe or Zn addition –. To the contrast, in colloidal chalcopyrite NC nanocomposites a decrease of the charge carrier concentration was reported when increasing the C/A charge ratio . We observe a similar behavior here with a large increase of S and a one order of magnitude drop of σ for the sulfur deficient sample.…”

“…The latter improves all thermoelectric parameters of the obtained pellets, leading to an increase of ZT from 0.06 to 0.18 at 400 °C. This represents a significant enhancement of the highest value reported so far for n‐type CuFeS 2 NCs (ZT=0.13 at 343 °C) …”

“…Very recently, Gabka et al. reported first thermoelectric measurements on n‐type chalcopyrite NCs, yielding a maximum ZT of 0.13 at 616 K . In this case, the initial surface ligands (oleylamine) had been exchanged by treatment with inorganic salts (ammonium sulfide or Meerwein's salt) to increase electrical conductivity within the solid materials processed from the NCs.…”

“…[7,12] Concerning chalcopyrite NCs for thermoelectric applications, very few reports exist. Liang et al described the synthesis of 6.4 nm nanocrystals with a composition of CuFe 0.99 S 2.08 , exhibiting p-type behavior and a high Seebeck coefficient (> 800 mV/K at 500 K), leading to ZT values of 0.22-0.26 at 500 K. [7] Very recently, Gabka et al reported first thermoelectric measurements on n-type chalcopyrite NCs, yielding a maximum ZT of 0.13 at 616 K. [13] In this case, the initial surface ligands (oleylamine) had been exchanged by treatment with inorganic salts (ammonium sulfide or Meerwein's salt) to increase electrical conductivity within the solid materials processed from the NCs.…”