Effect of the Annealing Atmosphere on Crystal Phase and Thermoelectric Properties of Copper Sulfide

Abstract

Cu2-xS has become one of the most promising thermoelectric materials for application in the middle-high temperature range. Its advantages include the abundance, low cost and safety of its elements and a high performance at high temperature. However, stability issues limit its implementation in real devices, restraining their operation current and temperature, which calls for the optimization of the material performance in the middle temperature range. Here, we present a synthetic protocol for large scale production of covellite CuS nanoparticles at ambient temperature and atmosphere, and using water as solvent. The crystal phase and stoichiometry of the particles is afterward tuned through annealing at a moderate temperature under an inert or reducing atmosphere. While the annealing in an Ar atmosphere results in Cu1.8S nanopowder with a rhombohedral crystal phase, the annealing in a reducing atmosphere leads to tetragonal Cu1.96S. High temperature x-ray diffraction analysis show the material annealed in argon to transform to the cubic phase at ca. 400 K, while the material annealed in the presence of hydrogen undergoes two phase transitions, first to hexagonal and then to a cubic structure. The annealing atmosphere, temperature and air allows adjusting the density of copper vacancies and thus controlling the material transport properties. In this direction, the material annealed under Ar shows higher electrical conductivities but lower Seebeck coefficients than the material annealed in the presence of hydrogen. By optimizing the annealing time, Cu2-xS with record figures of merit in the middle temperature range, up to 1.31 at 710 K, are obtained. We finally demonstrate that this strategy, based on low cost solution synthesis process, is also suitable for the production of high performance Cu2-xS layers using high throughput and cost effective printing technologies.

The Importance of Surface Adsorbates in Solution‐Processed Thermoelectric Materials: The Case of SnSe

“…[16][17][18] Alternatively, surfactants can be exchanged with volatile compounds [19] or even inorganic species [20] that can further tune material properties. [12,18,21] Conversely, the presence of surface adsorbates is usually neglected in the case of the so-called surfactant-free methods, [4,[22][23][24][25][26][27][28][29][30][31][32][33][34] the most widely used to produce thermoelectric powders in solution. The vast majority of reports dealing with surfactant-free synthesis do not perform any surface treatments since the particles are considered "naked".…”

Enhanced Thermoelectric Performance of n-Type Bi2Se3 Nanosheets through Sn Doping

“…Bi 4f, Se 3d and Sn 3d high-resolution XPS spectra of Bi 1.93 Sn 0.07 Se 3 particles are displayed in Figure 7 c. The high-resolution Bi 4f spectrum was fitted with two doublets, associated with Bi 3+ within a Bi 2 Se 3 chemical environment (Bi 4f 7/2 at 158.2 eV) and Bi within a more electronegative environment, as it could be Bi 2 O 3 or Bi 2 SeO 2 (Bi 4f 7/2 at 159.4 eV) [ 30 ]. This oxidation of the particles surface is related to their transport and handling in the air [ 4 ]. The high-resolution Se 3d XPS spectrum was fitted with three doublets, corresponding to Se within the Bi 2 Se 3 (Se 3d 5/2 at 53.6 eV), selenium in an elemental or Bi 2 SeO 2 environment (Se 3d 5/2 at 55.2 eV), probably arising from the partial oxidation of the material surface, and Se in a SeO 2 chemical environment (Se 3d 5/2 at 58.6 eV) [ 30 , 31 ].…”

“…Thermoelectric (TE) devices that directly and reversibly convert heat into electricity find unlimited applications [ 1 , 2 , 3 , 4 , 5 , 6 ], but their real implementation is hampered by their low cost-effectiveness. The efficiency of energy conversion of a TE device is in part determined by the transport properties of the TE material: Seebeck coefficient S , electrical conductivity σ , and thermal conductivity κ .…”

Two-dimensional copper based colloidal nanocrystals: synthesis and applications

“…192,193 However, Cu 2− x S faces a major problem of S volatilization upon annealing leading to S loss and formation of phases with low Cu deficiency resulting in a low carrier concentration. 194,195 Zhang et al showcased that 0.3% doping of Pb could stabilize the Cu deficient Cu 1.8 S phase up to 880 K during Pb:Cu 2− x S phase transition enabling a high carrier concentration compared to Cu 2− x S (Fig. 12d).…”